latest version of the Sopht amp with a bit of theory

[tazwolf]

Stephane
My grid voltage on the triode section is -0.45 on the second one it is -0.54.
I experimented by removing the the 0.068uf cathode bypass capacitor on the second triode and it got rid of the uneven clipping (clipping only  the positive going part of the waveform) that I was talking about in my previous posts. This however results in a lower volume. It also brings the grid voltage on the second triode to the same as the first -0.45 volts.

So now I'm waiting for my 12k5 tubes to arrive so I can actually listen to it thru a speaker - I might end up adding the bypass capacitor via a switch and experimenting with a few different values. What exactly does the this capacitor do?

I was reading thru the sopht theory and I think I understand about biasing for class A operation (and so the grid voltage should be half way between 0 and -1). My question is if one had a higher voltage on the plate then the voltage swing would be higher - according to the chart for the 12U7 (if I read it correctly) at 12v on the plate the swing would be between 0 and -1.5 thus resulting in a grid voltage of -0.7. Would this be better in terms of output level? or have I completely misunderstood.

BTW The guitar going through just the two 12U7's sounds pretty nice.

[tazwolf]

I just read up on cathode bypass resistors again for the 3rd and I think I finally understand what they do  - stabalise the grid voltage!

[slajeune]

Hi Tazwolf,

First, the cathode resistor bypass cap.  You are right, it is used to stabilize the voltage on the cathode.  While doing this, it also increases the gain considerably (roughly 2x!).  That explains why without this cap, the volume is lower.  To see it's effect on gain, go back to the link I gave to calculate the gain of a triode stage.  In the cathode resistor entry, put the value of the cathode resistor if you aren't using a bypass cap or put 0 if you are using a bypass cap.  You can see it's effect on gain.

As for the question on plate voltage, I have "two" conflicting versions.  One says that we should use the voltage coming from the power supply as the "plate voltage" on the chart (in our case 12v) and one says that we should use the voltage after the plate resistor (in our case, the plate resistor is 100k) which would make our voltage 4.8v.....

The difference between both is the bias point.  If we take 4.8v as our plate voltage, the bias point is -0.5 which gives us a 7k cathode resistor.  If we take 12v as our plate voltage, the bias point would be -0.75 which would give us a cathode resistor of 12k.

For what it's worth, the latest sound clip I posted in this thread had a cathode resistor of 12.2k on the first triode and 14.7k on the second triode.  This would lead me to think that the second method (i.e. 12V on the plate) would seem to be the most appropriate way for our needs.  I still need to see the output on the oscilloscope though...

As for using a smaller plate resistor, everything that I have read in regards to space charged tubes points to using large plate resistor values.  Look at the data sheet for the 12AL8, it shows a 330k plate resistor to it's triode stage!!!!

One of my plans is to make the second bypass cap "swithcable".  Therefore, I can go from clean to dirty with a simple flick of a switch!

Puretube (or anybody that has lots of tube experience), is any of this making sense?

Thanks,
Stephane.

[puretube]

that switchable cathode-cap is exactly what a lot of old (and new) prof. amps got, named "Boost" . Often a medium value is chosen, to boost only the mids/highs. Sometimes a clever R/C combination for just mid-boost.

[tazwolf]

Stephane:
So using 12v at the plate and biasing would maximise the potential of the tube - in theory. Did you have any problems with ths - if I just remove the 100k resistors with jumpers and use trimpots for the cathode resistors I can't get the grid voltage bias to anywhere near -0.75 using the values u mentioned 12k and 14k. I get a value of 5.2k on the trimmers with -0.75 as the grid bias voltage and the tube just seems to refuse to conduct. - but I'll keep working on it.

All this is rather addictive - I have to remind myself to go to sleep cos I have to work the next morning or to take my motorbike out for a spin. Soundwise I think you have really come across a great idea with these 12v tubes Stephane even though I haven't come to the setup I am satisfied with yet I have not been this inspired by an amp sound since I played a gig with my SG and an ac-30 on full whack...but that was a long time ago.

Puretube: And there was me thinking it was another preamp stage on those amps.


/Taz - another beer another grid voltage bias

[puretube]

Puretube: And there was me thinking it was another preamp stage on those amps.

he he, that`s the 2nd (or "hot") channel  

[tazwolf]

which one usually ends up paying "hot" money for as it's an extra feature over the standard model!

/Taz

[slajeune]

Hi Tazwolf,

ok, sorry about the confusing post.  the 12 and 14k are both cathode resistors.  Don't remove the plate resistor (i.e. don't jumper the 100k resistor).  The plate resistor needs to stay at 100k, only play with the cathode resistor (for now).  You bias the tube with this resistor.  So, I would use trimpots (or simple pots) for the cathode resistors.  Again, I'm sorry I haven't had the time to try any of this oscilloscope thing yet.  Too many things, too little time.....

Thanks for all your help Taz!

Stephane.

[Alpha579]

slajeune, at the moment, how is the 12k5 biased (ie some voltage/current readings)?

[Alpha579]

BTW, on your theory page, how did you calculate the voltage drop across the 100k resistor?

[Alpha579]

Another quick question ,
What would it sound like if you took the schematic of a Marshall plexi, and replaced all preamp tubes with 12u7's, and power tubes with 12k5's, and rebiased to work on 12v/24v?

[puretube]

crap... (sorry)

[Alpha579]

lol, just a thought  

[william]

I'm getting slightly fustrated by my inablility to comprehend how to read tube charts.  I'm trying to adapt the gain stage of the 12AX7 using the 12U7's that I have.  I realize the 12AX7 is a high MU tube and the 12U7 is a med MU tube, but the desire still resides.

The gain stage is set-up as,

and is the typical configuration for Soldano Super Lead Overdrive amps, such as the actual Slo100, Mesa Boogie Dual Rectifiers, and Peavey 5150's.

How do I go about reading the charts for the 12AX7 and 12U7 to see how they're biased and get close to the results I'm looking for.  I've re-read the book I have on tube amp design, but the author used a butt load of math without explaining where he was getting the values or why.

[william]

I guess this also goes into the territory of getting a plexi/fender/matchless/Vox/insert amp here type sound as well......

[puretube]

check here:
http://www.glass-ware.com/tubecad/,
and read thecomplete http://www.tubecad.com
lots to learn! (although maybe a little advanced, sometimes....)

[slajeune]

Hi Alpha,

concerning the 12K5, I will get back to you, I haven't checked the voltages recently.

I didn't calculate the 100k voltage drop, I observed it with a DMM.  I whish I could calculate it though...

I will update the theory page.  I read a second article that confirmed that we should use the voltage comming from the power supply as our "plate voltage" and not the voltage after the plate resistor.  Therefore, the cathode resistor should be roughly 12k.  Again, this is what I currently have (ok, 12.2k and 14.7k....) and this is what was used to record the last clip.

As far as voicing is concerned, from my understanding, a good part of the tone is molded in the preamp.  Therefore, great care must be taken to "replicate" the preamp section of your favorite amp.  Having said this, the 12U7 is a different tube than say a 12AX7 or a 12AU7 and therefore, will sound different.  Also, the gain is a lot lower (20 vs 100 for the 12AX7).  Since we seem to be using larger cathode resistors, the bypass cap will need to be adjusted in order to achieve the proper voicing.  From what I gather, the cathode resistor and the bypass cap, form a filter which modifies the voicing of the amp.  This and the coupling caps would definately change the sound of the amp.  One needs to play around with them to find the right tone for them.

Cheers,
Stephane.

[Alpha579]

just on an off note, does anyone kmow where i can get a power transformer that has a variable primary for 112v/220v/ etc, and has a 24v/0v/24v @3A and a 5v @ (howerver many amps a valve rectifier sucks) secondary?

[tazwolf]

I been reading the tubecad page as suggested by puretube - it goes through triode biasing without a plate resistor. If one tries the formula for biasing with values for the 12U7 then it cmes out that it is not possible to do so (the cathode resistor would have to be negative) with a B+ of 12v.
If you try it with a B+ of 12.5 thewn it works (I haven't got a power supply greate than 12v so I can't try it in reality).

The other thing that struck me is that the plate resistance (resistance of the actual plate which is 12500 ohms according the datasheet) is used in the formula for calculating the cathode bias resistor when there is no separate plate resistor - but why is the internal plate resistance not used in the other formulas for calculating bias?

/Taz

[tazwolf]

I've been thinking.... :shock: about the bias and setup of the first triode.

In the experiments I've been doing - the grid bias has been set in order to give the maximal output swing - in this case - 0.75 without plate resistor or -0.5 with plate resistor. If I am using a guitar as an input then am I really gaining (no pun intented) anything from having the bias at such levels as a guitar input will not be able to swing the grid anywhere near + or - 0.5volts.

So is it better to bias the grid appropriately for the input coming into it? in this case bias the triode so that approx plus or minus 0.05v going into the grid will give maximal output swing. As i have it setup at the moment the first triode is givng me 4x gain from a guitar level signal with no distortion (as seen on a scope using a sine wave input) with the grid bias voltage set to -0.1.

Sorry for all the silly questions but sitting and messing with 50 year old tubes is just so much fun that it would be a shame not to try and understand how they work.

/Taz