Question re 6021 tubes, distortion, biasing

[mlabbee]

I've breadboarded a simple two stage tube preamp using a 6021 tube with a gain and master volume, no other tone controls (the topology is like the Real McTube II, but the resistor values are what I'm playing around with).

Has anyone played around with these tubes much?  Any favorite voicings?  

I've tried 30k plate resistors with 4.7k cathode resistors - nice, smooth distortion when cranked, but pretty low overall gain.  

220k plate resistors give much more gain, but the distortion has a very distinct octave effect, particularly in the higher registers.  Does the octave effect come from symetrical or assymetrical clipping? I remember seeing something about some kinds of distortions causing this effect, but I can't find the reference . . .  

I found that a 220k plate resistor on an unbypassed first stage with a 30k plate, bypassed 2nd stage gives a nice balance of sounds - with low gain, nice gritty rumble, high gain getting you to the octavey sound.  

Today, I play with cathode resitors to see what happens - any suggestions on good ranges of values?  Any other ideas for aspects to fiddle with?  Once I pin down a good voicing for each stage, I'm going to try putting in a tone control.

[zachary vex]

drop the cathode resistor value for more gain.  or bypass it with a 25V 25uF cap.

[mlabbee]

Thanks - I'll try dropping the cathode resistor today.

I've been bypassing with around 23 uF, but that tends to throw me into the "octaving" effect (assymetrical distortion?). I've thought about doing a build where you can switch the by passing on and off to flip between the two sounds.

[Paul Marossy]

I've thought about doing a build where you can switch the bypassing on and off to flip between the two sounds.

I did that with the bypass cap on the second stage of my Octal Fatness. There is a definite change in the amount of gain. It's actually a useful mod, for me. You might find the same thing for your build. YMMV.  

[ragtime8922]

Thanks - I'll try dropping the cathode resistor today.

I've been bypassing with around 23 uF, but that tends to throw me into the "octaving" effect (assymetrical distortion?). I've thought about doing a build where you can switch the by passing on and off to flip between the two sounds.

How did it go? How about posting the schematic? I have some submini's I'm dying to play with.

[mlabbee]

Ugh - it's going . . . lots of brain/ear overload listening to the difference between one set of resistors and another.  Further complicated by the fact there are two stages, and the sound varies noticeably even when you you take one set-up on one stage and transfer it to the next . . . can you say "option anxiety?"   :shock:

I'm getting closer - cathode resistors in the 1k5 - 2k2 range seem to work nicely (both bypassed and unbypassed).  Now I need to pin down the plate resistors - 220k is too much gain - you can't get it under control, so I'm playing around with 30k, 56k, and 100k now.

Once I get settled on my choices, I'll post a schematic.  It could be a while - I want to play around with adding a tone stack and I haven't even started playing with my B+ voltage (although I've been running tests around 140V, which I should be able to get with Nachbaur's Real McTube power system).  If you want to get started, the Powerman schematic is a good start (just use the pre-amp stage)http://www.harmonicappliances.com/powerman/powerman.html

Or go off the Real McTube and use resistors in the ranges I've listed (plus 22 uF for bypass and .022 coupling caps between stages.  Be sure to use high voltage caps and 1/2 watt resistors on the B+ side.

Paul - is there any reason you chose to have the switchable bypass on  the second stage?  I seem to be finding that switching the first stage is giving me a lot wider range of tone . . .

[Paul Marossy]

I was talking about the Octal Fatness.  :wink:

In this case, the absence or presence of a bypass cap on the second stage (12AX7) is much more noticeable than on the first stage (6SJ7). The bypass cap on the 6SJ7 seems to have more to do with controlling hum than it does increasing gain. (I know, it's weird... maybe it's these NOS RCA tubes that I have?)

Anyhow, on the McTube, it's a completely different story. As you say, and looking at the schematic, it appears that it would be more effective because it would drive the second stage triode harder than if it were on the second stage cathode.  :wink:

BTW, after it sitting for a while not being being played too often, I finally worked out all of the bugs and I have this excellent sounding Octal Fatness 5 watt class A amp working to my complete satisfaction. You can check it out here: http://www.diyguitarist.com/GuitarAmps/OctalFatness.htm  

[mlabbee]

Ah yes - very different topology, indeed. I haven't even started to play with pentodes yet . . . I'm sure that has something to do with it - I know they have very different behavior than triodes.

Very nice build, BTW - that Octal Fatness looks awesome.

[Paul Marossy]

Very nice build, BTW - that Octal Fatness looks awesome.

Thanks.  

It's pretty much my favorite amp at this point. Or at least a tie with my '74 Twin Reverb. They are both different amps, though.  

[mlabbee]

Gah! All that work for nothing.  I spent all that time trying to tame the gain to get a nice sound and then I add in the tone stack . . . it didn't even occur to me to consider how much gain the tone stack would suck.  So now I'm back up to 220k on the plate and 2.2 on the cathode - and there's much more control over tone, gain and everything.  D'OH!   :oops: LOL . . . you learn something new everyday.

BTW - kudos to Jack Orman on the AMZ Presence Control - very nice tone circuit.

[Paul Marossy]

Ahh... the dreaded "insertion loss"....  

[zachary vex]

for my opening stage in the nano head, i used 100k, 1.1k bypassed with 100uF.  subsequent stages have 270k plate resistors and 3k cathode resistors, with bypassing only used to tweek the frequency response.  it's pretty much like the kind of thing you'd find in a big amp.  8^)

the B+ is around 225V.

[Paul Marossy]

the B+ is around 225V.

Hmm... that is kind of an intermediate B+ voltage, at least in a regular old tube guitar amp. Just what is "the optimum" B+ voltage for 6021s anyhow?

[Peter Snowberg]

Just what is "the optimum" B+ voltage for 6021s anyhow?

http://wps.com/archives/tube-datasheets/Datasheets/CBS-Bulletin-E-290S/7.JPG
http://wps.com/archives/tube-datasheets/Datasheets/CBS-Bulletin-E-290S/6.JPG

Datasheet shows 150V MAX on the plates.  :twisted:

[Paul Marossy]

Datasheet shows 150V MAX on the plates.

Ah, I kinda figured that they were being pushed real hard to get that distortion out of them...

[zachary vex]

yeah, they're just tubes.  beat the snot out of them, i say.  8^)  plate voltage, who cares... just make sure you aren't asking them to dissipate more than 1.1 watts on the plate.  i ran a set of plates bright red for a couple of minutes this evening when i accidentally forgot to hook up the grid wires on my test bench tube setup, and they sounded fine when they cooled down.  tubes are made to be abused... abuse them!  

have fun!  play hard!  8^)

[Paul Marossy]

tubes are made to be abused... abuse them!

Yeah buddy!  :wink:

[mlabbee]

400 Volts, here I come!  

[Paul Marossy]

400 Volts, here I come!

Umm, I hope your talking about something that uses 5881s or EL34s...  :wink:

[mlabbee]

Hell no - I'm talking about my Rangemaster!  Germanium tansistors can handle 400V, right?