Designing a Two-stage Tube Circuit With Compressive Cathode Follower.
Jeremy "Tubegeek" Epstein March 2014
This design process is directly inspired by Merlin Blencowe's analysis of the Fender Bassman-style two-stage combination in his terrific book "Designing Tube Preamps for Guitar and Bass" 2 ed. I am indebted to Merlin for his clear and thorough explanation of the design process as well as his insightful demonstration of the distortion which results from the typical Bassman two-stage circuit. All mistakes are mine; Merlin has cheerfully agreed to permit me to refer to his explanation and method. Thanks Merlin!
Some design goals and parameters:
A) To design a two-stage tube circuit including a gain stage, direct coupled into a very compressive, soft clipping cathode follower, as described in Merlin's book. Essentially to replicate the Bassman-style circuit's tonal qualities, using different tubes suitable for a very small build.
B) To use subminiature dual triodes I have on hand: 6N16b.
C) To use a small 12V:120V power transformer on hand, fed by a 120V:12V wall wart. In other words, a simple linear power supply design. The 12V will feed the heaters and then the stepped-up 120V will be rectified, for a B+ voltage of approximately 160V DC after filtering.
Note that this transformer could also be configured for 240VAC output, I expect to use the lower B+ though.
Also note that the final unit will utilize four triode sections: I have 6V-heater tubes on hand and a 12V heater supply. My expectation is that I will use one two-stage circuit to drive a tone stack, and a second two-stage circuit to make up gain and drive the output. This is still TBD.
Design steps:
1. On the standard anode curves for the 6N16b, plot grid-anode curves (shown in red, these curves are nearly vertical for a cathode follower.)
2. Choose a B+ value, and add a reversed scale for the cathode voltage (red scale.) B+ chosen to be 160V (rectified 120V minus a few volts for filtering.)
(http://i680.photobucket.com/albums/vv166/tubegeek-original/6n16bcathodefollowerwcathodecurves.jpg)
Iteration I.
3. Plot a load line in the normal fashion (blue line, Rk = 10K.)
4. Mark 0 V end of load line on both voltage scales (left and right green crosses.)
5. Locate approximate center bias point on load line (center green cross at approximately Vgk = -2.5V, Vak = 109V, Vk= 51V.)
6. Point A: Vk = 95. Point C: Vk=0. Output V = 95 V pk-pk.
7. Point A: Vga = -75. Point C: Vga = -160. Input V = 85 V pk-pk.
8. Gain = 85/95 = .895
9. Operating point at B: Vgk = -2.5, Vk = 51. Vg = 48.5 Ia = 5 mA
Direct Coupling to previous stage:
Va must be Vg for the cathode follower = 48.5 V
10. Using Ra = 10K and B+ = 160V, plot load line.
11. 2.5 / 5 ma = 500 ohms Rk, 5 mA * 10K = 50V. (Not shown, no good.)
Evaluation:
One design goal is to try the grid-current mode for the cathode follower. With 2.5 VBias, we are far away from the grid current region.
Iteration II.
Using a different bias point, at B"' we will have about .8V Vbias which should get us some grid conduction and the following other results from steps 3 and 9-11. (We will skip steps 4-8 from here on out because we aren't really interested in linearity here and because a huge output swing also isn't needed.)
Vga = 78V
Vgk = -.8V
Vk = 79V
For the gain stage, using Va = 160-79 = 81V and Ra = 17K
1.5V/4.7 mA = 320 ohm Rk
Iteration III.
Iterating again with small Vbias (hotter bias on CF.)
20K load line (blue line)
A' at 0Vgk. B' at 1Vgk, Vga about -73. Vga = 92. Vgk = -1. Vg = 91.
Direct Coupling to previous stage:
Va must be Vg for the cathode follower = 91.
Using Ra = 20K and B+ = 160V, plot load line (same blue line.)
(At E) 2.2 / 3.4 ma = 650 ohms Rk, 3.4 mA * 20K = 68V.
This looks like one good candidate.
(http://i680.photobucket.com/albums/vv166/tubegeek-original/6n16bcathodefollower123.jpg)
We will iterate again, using Merlin's suggestion of a lower value for Rk on the CF. We'll try a load line of 8K this time.
Iteration IV.
8K load line for CF
7 mA Vgk -2 (at B")
Vga 104
Vg 56
Vk 58
Va for gain stage = 56
load line = 160 / 2mA = 80K
VBias for Va = 56V = -1.5V
1.5/1.3 = 1.15K Rk
Iteration V.
8K load line for CF
(At B*)
Vgk for CF = -1
Vga = -90
Vg = 70
Vk = 71
For gain stage, Va = 70
80K load line
Ra = 80K
VBias = -2 V
-2V/1.1mA = 1.8K = Rk
(http://i680.photobucket.com/albums/vv166/tubegeek-original/6n16bcathodefollower45.jpg)
Iteration VI.
The Bassman cathode follower has equal Rk and Ra on the two stages. It looks like a load line of somewhere around 15K might get that done - let's see how that works out. 16K makes the line easy to draw...
Rk = 16K
For Vgk of -.6V at X, Vga = -65, Vg = 95
For the gain stage:
Va = 95 VBias = 2.2V Ia = 4 mA, Rk = 550 ohms.
(http://i680.photobucket.com/albums/vv166/tubegeek-original/6n16bcathodefollower6.jpg)
Using these values, here is most of the circuit. Merlin's suggested grid stoppers and build out resistors are shown here. Tone stack TBD.
(http://i680.photobucket.com/albums/vv166/tubegeek-original/6n16bcathodefollowercircuit.jpg)
Next step is to breadboard the circuit above. The 550 ohm Rk's will need to be tweaked to dial in the bias most likely, however, it's been my experience that direct-coupled circuits are nowhere near as tricky to set up as you'd think. They'll always settle in somewhere!
Comments welcome. Thanks again to Merlin Blencowe = merlinb !
Nicely done! What's the expected drive voltage into the gain stage?
Guitar levels I guess - consensus on that is less than 10V, so I suppose the input could be overdriven by high output humbuckers now that you mention it. Or do you mean the 2nd gain stage? That's dependent on the loss in the tone stack which I haven't really dealt with yet. I'd like to do Stephie Bench's inductor tone stack actually, that seems like an interesting one to try.
I meant to ask what is the signal going into the first tube on the schematic, because the "2-tube CF" stage is usually used well down the chain from the input jack, so its input signal is far from the guitar level, which is commonly only 100mV, so when you say 10V, I assume it is already been through a few gain stages, i.e., the CF before the TS is reaaa...lly over-driven. ;)
Well, the plan is for the schematic at the bottom of the first post to be in largish pedal format, and to accept guitar or another pedal as input.
I seem to remember seeing that high output HB's could give a volt or more out? Anyway, 2+ volts of bias is enough input headroom for every guitar amp I've ever looked at so that should be OK.
This design would certainly be different if it had to worry about REAL CF signal swing but I'm not really trying to do that - there's no requirement to scale the signal up to drive a power amp, certainly not with clean swing.
In Merlin's book, he explains that running the CF section in the grid-current region (his example shows .6 VBias on a 12AX7 a la Bassman and Marshall) leads to a very rounded-off distortion characteristic and a lot of signal compression. Loading the gain stage with the grid current flowing into the large cathode resistor of the CF is a medium-heavy load so you get a rounded-off clip instead of a flat-topped clip from this setup.
In the actual Bassman you don't really use it because the output stage already clips before this happens, but in a master volume setup like a Marshall (using the same two stage setup) it's an important source of sweet grunge, according to Mr. Blencowe.
His description of this distortion source is what's leading me to try and isolate that behavior and try it in a pedal.
So - just to be clear - those inputs and outputs on that draft schematic at the end of the first post are meant to represent in & out jacks and the designed circuit stands alone (just doubled up coming in and out of a tone stack.)
If I had an appropriate 6V transformer on hand I'd try just the two stage circuit with nothing else. I will set it up that way first for testing and I hope to show oscilloscope shots as soon as I get some time in my basement lair.
But since I'm going to be using a 12V transformer, I'll build up two complete stages to use two 6N16b's (this is a twin triode, so I have four triodes to work with.) And I figure I'll put a tone stack in between just because it'd be a good use of the first CF.
I guess I'd need an output trim on an actual build, I'll use Merlin's suggestions there too for driving an output with a CF, from the section on effects loops in his book.
Book Plug Time: "Designing Tube Preamps for Guitar and Bass" 2 ed. is an excellent book with a superb organizational style and outstanding explanations of tube audio circuits as well as hybrid arrangements where appropriate. If you are familiar with Kevin O'Connor's books, this is at that level and Merlin does a better job - to me - of explaining his design process thoroughly. I'll have to go back to "The Ultimate Tone" to see how O'C deals with this material but I did look through Valley & Wallman, and Reich, and even the Bible RDH4, and Merlin's explanation was the most useful by miles. Especially since he goes into the forbidden topic of this special distortion/compression resulting from grid current flow, which of course isn't even acceptable dinner-table conversation between Langford-Smith and his pals.
I see, the best way to tell is breadboard it to hear what it sounds like but I think the gain might be a bit low, here is my reasoning: without cathode bypass capacitor, stage one has a gain of ~10x, so the first CF stage's output is only 1V with an input of 100mV (the humbuckers could put out more but most tube amps are designed with input sensitivity of 100mV). Of course, this is moot if you have another pedal(s) before this one. Let us know how it turns out...
> gain might be a bit low
+1.
The 5F6a has a gain-stage and gain-control *before* this stage.
The stage you show can put out 10V-30V. Gain will be under 10. So input overload is 1V-3V. You will have to work hard to get any clipping. With additional gain=10 in front the input overload becomes 0.1V-0.3V, still hard work but feasible.
> I'd need an output trim
Your second stage also can output 10V-30V, you only want ~~1V into a guitar amp (or further pedals).
You propose gain of 10, loss of 10 (tonestack), gain of 10.
Your second stage level is no higher than your input stage level. You are doubling your hiss power.
Your input overload is awful high for guitar.
Your output level is awful high for guitar-amp input.
Re-order the stages. Gain stage. Gain pot. Gain stage. Now you have gain of 100, can boost guitar to 10V-30V and clip it. Now tone-stack, loss of 10, output level 1V-3V. Only minor trim needed to keep guitar amp input happy.
Working around 5mA means fat filter caps in B+. 1mA-1.5mA was good enough for Leo.
> acceptable dinner-table conversation
Try Valley & Wallman, Pulse-Shaping.
FWIW I used Merlins "bootstrapped" cathode follower setup in my last build it really gave me nice clipping and compression over a standered CF.
I got a chance to start building this project today.
1st pic shows the terminal strips laid out with the 6H18b Russian subminiature tubes threaded into place. This is from the bottom. The ground busses are holding the terminal strips in position.
(http://i680.photobucket.com/albums/vv166/tubegeek-original/2014-05-10225003.jpg)
2nd pic shows the same thing from the top.
(http://i680.photobucket.com/albums/vv166/tubegeek-original/2014-05-10225012.jpg)
3rd pic shows the progress I made before turning in for the night.
(http://i680.photobucket.com/albums/vv166/tubegeek-original/2014-05-11002818.jpg)
Cool ideas, I'm always interested in seeing the finished project and hearing a clip ;)
Are you running this at 160V?!?!?
(http://i680.photobucket.com/albums/vv166/tubegeek-original/2014-05-11002818.jpg)
You can get electrocuted easily!
Quote from: tca on May 15, 2014, 02:21:09 PM
Are you running this at 160V?!?!?
That's the idea!
Quote
You can get electrocuted easily!
I'd better be careful!
(It's not my first rodeo. But thanks for the concern!)
Question for Merlin:
Re: Section 6.16, Bootstrapping for More Gain, Page 130 & 131.
Doesn't the signal from the cathode follower into the junction between R1 and R2 cause the junction to have MORE signal than the anode? There should be 1/2 the signal there already, the cathode follower is adding approximately 1x the anode signal with the same polarity, so would there be an additive effect that causes the top of the resistor to have MORE than the anode?
I'm not sure how the mixing behavior would work - I guess that the low resistance output of the cathode follower would approximate a virtual earth..... ???
Or am just I looking at the current described in the last paragraph of P. 130 from a different point of view, and is this the same current that gets dumped into the low output resistance of the CF? If I understand virtual earth correctly, there still would be a mixing of the signals there though, which would give the output of the CF some additional gain as a result.
I think?
Confused!
Thanks,
-j
I know you've all been just holding your breath waiting to hear what's been going on with this project.
Well, until today, nothing for quite a while.
I made a little progress today: I replaced the broken 6N16b, which wasn't nearly as difficult as I had foreseen (I had thought that re-lacing the eight wires inbetween all the other stuff that was connecting to the terminal strips was going to be a nightmare. Naaah. Not so bad!)
The second thing I did was a bit of a one-step-forward, one-step-back kind of thing.
I had been getting too-low voltage off the high-voltage winding of the second back-to-back transformer in the power supply. If you look at the older posts, you'll see I have been blaming under-rated, junk-box parts. I'm getting about 145VDC on the first filter cap instead of the 160 that the math says I am supposed to.
I don't think that's the problem. I tried somewhat less junky, properly-rated transformers and the problem is still there, despite the fact that the power supply is not yet loaded. I think I figured out what's going on, and it's something else - I think it's related to the back-to-back trick.
Small power transformers have so-so regulation. They deliver an output voltage that's a little high under no load and they sag down to their rated voltage under load. This means that their turns ratio is a little closer than it should be for the rated voltage: instead of a 120:12 transformer having a 10:1 turns ratio, it'll be more like 10: 1.1 or 1.2 and the "12V" winding will be more like 14 to 16 no-load.
Well, if you turn this transformer around, you don't get as much out the primary as you wish you did if you drive it with the rated voltage on the secondary. The step-up ratio is not as high as the ideal 10:1 in the example, it's a little short. To get the rated primary voltage OUT, you'd have to drive it with the NO-LOAD voltage onto the secondary. Damn.
What this means is I need to either:
a) run the unit off a somewhat higher input voltage, say 18VAC, and put some resistance in series with the tube heaters to bring the heater voltages down to 12V, while driving the second transformer with this higher voltage (which may or may not actually deliver a bigger output voltage in the real world under these conditions - experimentation needed.)
b) go to a SMPS running off of the 12V power supply, which I have been avoiding because I've never built one and I thought that adding a new wrinkle would give me additional development issues and I didn't need the extra complexity. Joke's on me - sticking with the tried-and-true linear supply gave me a head-scratcher that I just figured out after about a year on the back burner. Oh, well. I know that most people reading this will say that's what I should have done in the first place - in my defense, I don't have the parts for a SMPS on hand. But I may have to step up to 2016....
In any case, I *think* if I go with option a) I may be able to go back to the smaller-sized, more pedal-sized transformer - I don't think it was the transformer's fault. We'll see once I get the circuit further along and I can load down the power supply completely. But I think I was wrong about placing the blame for the low output voltage on an under-rated transformer. I think it was the ratio.
Quote from: tubegeek on May 15, 2014, 06:25:53 PM
I'd better be careful!
(It's not my first rodeo. But thanks for the concern!)
With a name like tubegeek I would be shocked (pun intended) if you didn't know what you were doing here ;D
Never saw this thread first time around, so it's cool to see it now. Looking forward to see how it turns out!
> you've all been just holding your breath waiting
That's why I look like a blue Smurf!
> I'm getting about 145VDC on the first filter cap instead of the 160 that the math says I am supposed to.
Yeah, so?
IMHO, perhaps better for this application.
I'd sure never fret (much) about 20% variation in supply voltage on tubes.
If it works on 145V, but gives 1.1% IM distortion, where your contract calls for 1.0% IMD, try finding 160V.
If it doesn't work on 145V, it won't work on 160V.
My long-forgotten suspicion is that you want distortion and are unlikely to get a bunch. This suggests a LOWER supply voltage, not higher, to get some "strain" happening.
Quote from: PRR on February 18, 2016, 08:54:41 PM
My long-forgotten suspicion is that you want distortion and are unlikely to get a bunch.
If I could get just exactly what I want, I'd get gorgeous Bassman-style compression. If Merlin is to be believed (and I believe he is) the Bassman is a happy accident of grid current drawn from a voltage amp into a cathode follower that sounds the way it sounds.
This whole project is an experiment to see if I can do that, twice, inside a little box.
It's a specific kind of distortion I guess. Dynamic distortion instead of harmonic.
Quote from: vigilante397 on February 18, 2016, 08:20:21 PM
With a name like tubegeek I would be shocked (pun intended) if you didn't know what you were doing here ;D
Still I appreciate the concern for my safety, thank you. I always try my best to give the same high voltage warnings. Only takes one electrocution to bring everything to a halt. And I do get pinched every now and again - I am ashamed to admit how untidy I let my workspace get.
Quote from: vigilante397 on February 18, 2016, 08:20:21 PM
Never saw this thread first time around, so it's cool to see it now. Looking forward to see how it turns out!
You and me both. I made up a little testing power supply that puts a (nominal) 18V into the (nominal) 12.6V winding, we'll see what that all really means with the heaters and some compensating resistors drawing current from the low voltage side, and the rectifier feeding a simulated circuit load from the high voltage side, should be about 15 to 20 mA for the 4 tube sections. I anticipate I'll get close enough to spec to move forward with this stack of kludges in the next days. Might get a little warm, but so what?
My biggest issue now is I can't find the layout drawing I made, so I have to re-trace the schematic around the wiring I did (see the photos above) to see what connects where on the controls. I had put two jacks on already, so those are going to be good hints, I just have to figure out which is IN and which is OUT to get a good start!
Just parking this info here for myself for testing purposes:
To simulate the circuit current draw on the rectifier: 160V drawing 16 mA would require a 10K resistor for testing, 2.5 Watts dissipation on this resistor.
6N16b heaters draw 400mA, both in series will drop 12V, dropping another 4V at 400 mA for a total of 18V (which is what I expect from the power transformer under load) will require a 10 ohm resistor in series with the heaters, 1.6 Watts dissipation.
The project is built up to the extent that I got a chance to try it yesterday (power supply is still not right, waiting on parts to use SMPS instead.) Apologies to anyone who has been waiting three years for an update!
The comments about inadequate gain are spot on. Weighing two options: a JFET booster ahead of V1 (quick and dirty) or adding one more tube to the input, ahead of the circuit as drawn. Both just to get the levels where they belong to see whether the clipping is going to happen the way I want it to. Probably going to go with the JFET just for a quick trial.
More later. When later? Good question!
Quote from: tubegeek on March 21, 2014, 09:29:50 PMJeremy "Tubegeek" Epstein March 2014
Quote from: tubegeek on August 01, 2019, 02:48:52 PM....I got a chance to try it yesterday .. Apologies to anyone who has been waiting three years for an update!
5 years since you started. Just-now facing lack of overdrive. Stop painting the house and PLAY!
P, your advice is spot on, as always, brother. How are your hummingbirds doing?
Here's what I have since I last posted (a mere 2 days ago!)
I added a pi filter after the rectifier, dropping a couple of volts but knocking down a ton of humbuzz. I made a q&d 12V power supply node out of a 100K resistor in series with a 12V Zener || electro cap. That's all fed by the 140V 138V B+.
I started messing with a breadboard and a 2N5457 to get the JFET to bias somewhat well off the 12V feed. Ended up with a 6K3 load and a 680R source resistor, just shuffling through parts using the Tillman JFET Booster preamp values as a starting point. Once I had a reasonable bias point I soldered that mess in between the input jack and the old input node. 1uF tantalum cap DC blocker.
Then I played a little!
So: this is still a "preamp" with gain <1. But the JFET got the input level up so I could hear a bit of nice smooth grunge - I'd say the design CONCEPT is still promising. I'm going to have to get a little more serious - bust out the scope - to get much further and get a sense of the signal levels in each stage.
The "BONE RAY" tone stack is a bit different from the typical, but I think I like it. I'll see what happens when I get a chance to mess with it more. I should probably try bypassing it completely too, to see what it sounds like with that signal drop neutralized, so that maybe needs a switch?
Oh hey, wait a second - I didn't bypass the source resistor with a cap - so I have some "free" gain for the cost and effort of soldering in one cap. I'll do that tonight for sure.
It's good to be back.
I'll finish with a cautionary tale: the reason I had to put this project – all my projects! – on hold for so long, was, I let my workspace get so disorganized I couldn't find anything nor find a clear space to work. A nightmare. It took a visit from some German cousins to get me to spend TWO WEEKS cleaning up and sorting through mess, both in the "guest room"/workshop and the "pit of hell"/basement.
I found all sorts of half-baked idea carcasses in various stages of undevelopment along the way. I started off easy, boxing up another Gus Smalley ASDF that I had already completed the circuit board for, but my real target was this project, which I had never stopped wondering about. Still a challenge but finally making some progress!
Bypassed the JFET source with a 47uF electro. On the one hand, there is now good, usable level. On the other hand, I now really can't tell where/how the clipping happens without a scope. Played around with it a bit more and got a feel for the Bone Ray tone stack - it's weird but I think I like it.
Here are some pics. No bottom on it yet.
What do you think of the name?
UNDERVIEW:
(https://i.postimg.cc/BtTfNQ75/2019-08-05-15-07-00-copy.jpg) (https://postimg.cc/BtTfNQ75)
CIRCUIT BOARD:
(https://i.postimg.cc/9DM3WPm1/2019-08-05-15-07-06-copy.jpg) (https://postimg.cc/9DM3WPm1)
BONE RAY TONE STACK AND POWER SUPPLY:
(https://i.postimg.cc/5HYZzxFC/2019-08-05-15-07-15-copy.jpg) (https://postimg.cc/5HYZzxFC)
OVERVIEW:
(https://i.postimg.cc/XZRtcTwD/2019-08-05-15-23-34-copy.jpg) (https://postimg.cc/XZRtcTwD)
ANOTHER OVERVIEW, WITHOUT THE UNUSED DRILL HOLES PHOTOSHOPPED OUT:
(https://i.postimg.cc/bdwCKymf/2019-08-05-15-23-39-copy.jpg) (https://postimg.cc/bdwCKymf)
> I let my workspace get so disorganized I couldn't find anything nor find a clear space to work. A nightmare.
Indeed.
Gets worse when you have another project-shop 10 miles away.
Quote from: PRR on August 05, 2019, 11:49:35 PM
Gets worse when you have another project-shop 10 miles away.
In my case, four stories away, but yeah. Definitely worse!
On the positive side, I've been finding SO MANY parts as I sort and organize. I do NOT need to buy resistors for a while.
Why not just sub a 6n17b for V1? After all, nobody ever ran an actual Bassman with a 12au7 in the first or second position, AFAIK.
> nobody ever ran an actual Bassman with a 12au7 in the first or second position, AFAIK.
Bassman 5F6a specified 12AY7 in the front end.
12AY7 was introduced in late 1948 with much hoopla, but nobody seems to have stuck with it past 1960.
Quote from: PRR on August 12, 2019, 09:03:33 PM
Bassman 5F6a specified 12AY7 in the front end.
12AY7 was introduced in late 1948 with much hoopla, but nobody seems to have stuck with it past 1960.
I was aware of that (I actually thought about writing "12ay7, yes, with less than inspiring results, but 12au7, no"), but the the 12ay7, with a mu of 44, is still in the middle gain range, while the 12au7, with a mu of 20, is clearly nothing other than a current driver. The 6n16b is pretty close to that with a mu of 25, so it's really pretty hopeless to try to get any meaningful "gain" (in the imprecise way guitarists use the word), or to make up for the losses of a passive tone stack. Yeah, you can try to compensate somewhat for that by boosting the input, but why not just sub in the right tube for the job?
But tubegeek surely isn't the first to try: https://www.diystompboxes.com/smfforum/index.php?topic=83164.0 (https://www.diystompboxes.com/smfforum/index.php?topic=83164.0)
Quote from: Ben N on August 12, 2019, 03:12:46 PM
Why not just sub a 6n17b for V1?
Funny you should mention that - when I get back home from vacation there's a package with Cyrillic writing on it waiting for me ;)
I tried with the 16's first because a handful of 16's thrown in with some other Russian parts were my first submini purchase and I wanted to try using them for something. It was more an attempt to work through
merlinb's cathode follower design info than anything else. As it turns out it really wasn't worth building with 16's.
Now I'm either going to need to recalculate everything to try
merlinb's grid-current idea with some level of precision - or just run two half-17 gain stages into two half-16 cathode followers and see what happens....
I'm trying to get together an order for the SMPS parts so they will be there when I get back too. I promised my wife a Nixie clock for her office and the next one of those I build will use a SMPS. Might as well build one for this project too!
Yeah, well, I'm a big talker. I've got a few packages with Cyrillic writing on them hanging around here since... :( But along with them, I also have a board from vigilante397 for a submini Alembic F2B clone (in my regular pedal-building queue, meaning there's a fair chance I'll get to it in a matter of months rather than years), so I'm watching your work here with interest, 'cause I've heard it said that a CF is the mysterious extra tube stage that was supposedly added to Gilmour's F2B. Then all I have to figure out is whether the power supply will handle the extra tube (Nathan thinks it will), and what to do with the other half of the tube. First world problems.
Quote from: tubegeek on August 23, 2019, 12:46:08 AM
I'm trying to get together an order for the SMPS parts so they will be there when I get back too. I promised my wife a Nixie clock for her office and the next one of those I build will use a SMPS. Might as well build one for this project too!
I completed a nice SMPS board for this sucker today - haven't installed it yet but it tests quite well, a very cool addition and no more "whatever the transformers sag to" voltage selection. I may have to load down the 12V input a bit so it doesn't burn out the heaters - my 12V supply brick is running about 15 V unloaded, I'll have to put a resistor across it and see what it will actual put out with my heaters connected.
Quote from: Ben N on August 25, 2019, 04:01:53 AM
Then all I have to figure out is whether the power supply will handle the extra tube (Nathan thinks it will)
Totally will 8)
Source: I have that same SMPS running three tubes simultaneously in a different design.
Tried to test with the SMPS last night - I must have a short somewhere in the audio circuit board - the heaters lit OK when only the 12V supply was connected but when I connected the B+ the IRF740 in the SMPS board got super hot and no B+ was measurable. I don't think I killed the power supply board but I'll have some troubleshooting to do, knifing the gaps etc. Serves me right for building a high voltage tube circuit on perfboard.
Wait, what? There's something wrong with HV on perf?
Quote from: Ben N on September 10, 2019, 09:46:41 AM
Wait, what? There's something wrong with HV on perf?
Well, not a lot of room for using the insulating properties of air on perf, is all I meant.
Yeah, but OTOH you get a nice wide track for all your little electrons to meander down, instead of having to squeeze by on an ultrathin copper trace like a salad with sprouts down a supermodels gullet.
2 steps forward, one step back...
I decided I needed a better power supply so I built the NE555-based SMPS design that I've seen in several places, and that works pretty well, within limitations.
Clearly a higher-gain arrangement was needed beyond 4 sections of 6N16b so I redesigned with 1 6N17b and 1 6N16b as follows: the two halves of 6N17b as input and recovery-from-tone-stack gain stages, each DC coupled to a 6N16b cathode follower. (In the drawing further up the thread, the 1st and 3rd triode sections have been substituted with 6N17b.) Still trying to draw grid current into the cathode followers as per the original plan for smooth compression and distortion.
My first try at new parameters was a bust: the result was too much current drawn overall for the SMPS to supply (approx. 15 mA) - the voltage sagged, because the cathode followers drew too much current, it ended up overheating the SMPS and shutting it down. I calculated some new operating points based on lower current draw and a B+ voltage of 200 which seems to work if the overall current draw is much lower, about 7.5 mA.
The two 6N17b sections use -1.2V bias, 60K plate resistor, 1K6 cathode resistor self-bias, .75mA per triode. The cathode followers are designed to use -0.6 V bias and a cathode resistor of 50K and about 3.5 mA per triode. (I see I must've done something wrong here - that doesn't work to get the direct coupled operating point, with the cathode up around 150V which is what's needed.... back to the drawing board.)
With the values above the 6N17b sections are working fine, but the cathode followers are not biased properly - they seem to be at about VBias -22V (!!) I find this hard to credit but now that I've noticed that 3.5 mA across 50K is 175V not 150V that explains why they are so far off.
I connected the two units directly, omitting the tone stack (for extra gain and a quick setup) and tried it out. It sounded great! - very creamy, compressed and overdriven. Then I inadvertently left the SMPS running with no load on it for a while and it smoked. Back to work but VERY promising even with the weird operating points...got to review Merlin's book again!
Excellent report. Do you think the first CF is contributing anything to that nice creamy tone? Functionally, one would think that without the stack, there is no need for a CF, but if CFs really add compression, then maybe it does make sense. How was noise?
Noise was not an issue so fas I could judge with an unboxed amp using only partially shielded signal cable. No HF whine from the SMPS or anything like that.
If I am getting grid current compression (? not with -22 Vbias, I'm not?) in the cathode followers then sure they are adding to the tone but if you're asking, is guitar signal through one triode stage enough to overdrive the CF, dunno, haven't scoped this thing yet. Doubt it....
My next move, I think, is going to be replacing the cathode R's on the followers with pots and seeing if I can dial in the bias better. It's just not doing what the plate curves say it's supposed to be doing.
One possibility, I guess, is that I'm getting an RF oscillation that's causing weird behavior - i have the buildout and stopper resistors as per Merlin's recommendation, though. It just now occurs to me that I have the 220K grid "stopper" R's at the wrong end of the jumpers between the 1 and 3 plates and the 2 and 4 grids - should be closest to the grids and they aren't.
I wish I could build this thing on a breadboard but I don't trust the breadboard connections. So a LOT of soldering/unsoldering going on when I want to make changes.
Status: with a working SMPS, this setup (at least with the tone stack bypassed) has PLENTY of gain - too much really. I'm having trouble measuring DC conditions properly, what I'm getting isn't making sense, maybe there is some oscillation going on when it's just on my workbench. Next steps are to box it up and then scope it.
Each pair of triode sections on its own sounds pretty good too, I may have to make the topology switchable, at least maybe bypass the 3rd triode section (run the tone stack into the final cathode follower with no gain stage ahead of it.)
This is what's built right now:
(https://i.postimg.cc/bdxv2QHD/6n16b-cathode-follower-circuit-rev2.png) (https://postimg.cc/bdxv2QHD)
So a few of my design parameters have gone by the wayside:
Quote from: tubegeek on March 21, 2014, 09:29:50 PM
Some design goals and parameters:
A) To design a two-stage tube circuit including a gain stage, direct coupled into a very compressive, soft clipping cathode follower, as described in Merlin's book. Essentially to replicate the Bassman-style circuit's tonal qualities, using different tubes suitable for a very small build.
B) To use subminiature dual triodes I have on hand: 6N16b.
Well, I'm using one of those, plus one 6N17b which I went and bought.
Quote
C) To use a small 12V:120V power transformer on hand, fed by a 120V:12V wall wart. In other words, a simple linear power supply design. The 12V will feed the heaters and then the stepped-up 120V will be rectified, for a B+ voltage of approximately 160V DC after filtering.
That looks SO cool. I love the tilt+mids tone stack in the middle. Any luck on taming the gain? Puzzling that you have so much without a single cathode bypass cap and 60k plates. Is the goal a dirt box or more of a full-range preamp?
Quote from: Ben N on October 03, 2019, 04:44:39 AM
That looks SO cool. I love the tilt+mids tone stack in the middle.
Thanks! It *IS* cool! That stack is another of merlinb's developments, he calls it the "Bone Ray" tone stack, rather cryptically. I tried to get him to explain the name but he's being opaque on that one. I had a chance to play around with it a bit yesterday and it does quite a lot. I will plug it into my interface and record some stuff in a bit (maybe not today, busy-ish day) and post a sound sample. I want to do that anyway so I look at the waveforms it's outputting.
Quote
Any luck on taming the gain? Puzzling that you have so much without a single cathode bypass cap and 60k plates. Is the goal a dirt box or more of a full-range preamp?
Well, sort of inbetween. I wanted to make just a preamp that would illustrate the round-clipping effect of the Bassman arrangement, and then I found that it was a little too boisterous without any tone stack so I got One More Feature, and I also liked the sound of it without the tone stack so I might add a tone stack lift switch.... Experiencing some mission creep. With the Level control after the stack there is plenty of control of the insanity, including some pretty rough dirt (but that is probably coming from overloading my amp, not internally.)
Once I get it boxed up safely I'm going to bring it to my pal's guitar shop, let them mess with it and and see what they think of it. Right now it's all hanging off an open top plate and the noise situation is actually pretty good.