Tiny combo - el95 with 12ax7 preamp - will this work?

[j-stylez]

Hello guys,

I'm building a tiny little practice amp combo with a 6" speaker.
I have an el95 tube for the poweramp, and 12ax7 for the preamp.
I want a single tone control, so I decided to try out the "Moonlight" tone control circuit. The poweramp is pretty much copied from a SE el95 i found someone doing a report on.

1.) If I want a single volume control, is it better to leave it where I have it placed right now (after the tone control), or is it better to put a master volume control, just before the power amp section?

2.) Also, if I want to play around with the value of the cathode bypass in the preamp, is it better to do that in the first or second stage or both? I read about the "fat" switch some people use, switching the preamp cathode bypass to another value...


Any other ideas are welcome, and please say if you spot any errors! help is appreciated!

[Steve Conner]

The usual place for the volume control is between the first two stages. However, I have an old Selmer with 1x EL84 that put the volume control on the EL84 grid. It seems to work fine, but the amp can't be cleaned up when playing with high output pickups, because the second 12AX7 stage is always hit with a big signal. You can still clean it up with the guitar volume knob, of course.

Cathode bypass caps: I modified my Selmer by reducing the first stage cathode bypass cap from 25uF to 2.2. This helped to make it less flatulent when overdriven. It already had a tiny cathode bypass cap on the second stage, 10nF or something.

[pdf64]

That design should work fine, but if used with regular guitar speakers, then the circuit would likely be a bit dull / dark in tone, although Steve's idea with the cathode bypass caps may help with that. A bright cap across the volume control is another option.
However, a small speaker may be naturally more bright, so it could all match together ok.
Alternatively, the Fender Havard has an effective single tone / vol arrangement you may like to consider.
http://www.webphix.com/schematic%20h...6g10_schem.pdf
To take up from Steve's amp, you could add a second volume control at the power tube grid. However, an external boost pedal would be needed to get much overdrive from the pre-amp alone, ie the second volume at a low setting.
Pete.

[j-stylez]

Thanks for the help so far!

I will go for the Fender Havard tone circuit you mentioned. Is the tone pot supposed to be linear and the volume pot logarithmic?

[loudthud]

Any other ideas are welcome, and please say if you spot any errors! help is appreciated!

There is no blocking cap on the plate of the preamp triode. Probably need 0.022uF 400V or higher between R10 and R9. Also there needs to be a resistor to ground, perhaps 220K or 470K, at the right end of C5. Also, don't forget a fuse on the mains. Probably 0.5A 250V.

[madkatb]

You have no grid stoppers on either triode of the 12ax7. Recommended to avoid possible RF interference at the input and blocking distortion at the second triode.

[j-stylez]

Thanks to you all, Ive got it working. Ive been playing around with the cathode bypass caps, but I just cant seem to tame the distortion. Theres too much gain!

Any ideas? I can see that while the tubes are warming up and the speaker is just starting to make a sound, theres a point in time when I really like the way it sounds, nice and clean, but by the time the tubes are warmed up completely its just too loud and too much distortion.

Could I try to increase the 100k plate resistors on the triodes to a higher value?

My cathode bypass cap values for now are: 10 uF first triode, 10uF second triode, 100uF powertube.
I have 56k grid resistors on both triodes.

Now how can I reduce the gain to get some clean sounds out of it at low volumes - and make to volume pot more usable (right now its too loud after maybe 10% of turn). Could I try a tonestack with some more loss? or nfb?

[loudthud]

Now how can I reduce the gain to get some clean sounds out of it at low volumes - and make to volume pot more usable (right now its too loud after maybe 10% of turn). Could I try a tonestack with some more loss? or nfb?

The volume pot needs to be an "Audio" or "Log" taper. If you set the pot to mid rotation, from the center terminal to the ground terminal you will measure about 100K ohms From the signal side terminal to the center terminal will measure about 900K ohms. You will need to disconnect the rest of the circuit to get accurate readings. If both sides measure about the same, 500K, you have a "Linear" taper pot.

[j-stylez]

I'm already using a logarithmic pot, its A1M and I measured it. I must say though, I measure 100k from the ground to middle and not from signal when its at half... the amp is way too loud, I can barely turn it up a milimeter or so.

Maybe some voltages can help?

My b+ after recitification is 274, then first filter to 243, then 238, then finally 228.

12ax7 (a) anode 174, cathode 0,98
12ax7 (b) anode 181, cathode 0,7
el95 anode 237, grid 230, cathode 8

[tedmich]

Thanks to you all, Ive got it working. Ive been playing around with the cathode bypass caps, but I just cant seem to tame the distortion. Theres too much gain!

Any ideas? I can see that while the tubes are warming up and the speaker is just starting to make a sound, theres a point in time when I really like the way it sounds, nice and clean, but by the time the tubes are warmed up completely its just too loud and too much distortion.

Could I try to increase the 100k plate resistors on the triodes to a higher value?

My cathode bypass cap values for now are: 10 uF first triode, 10uF second triode, 100uF powertube.
I have 56k grid resistors on both triodes.

Now how can I reduce the gain to get some clean sounds out of it at low volumes - and make to volume pot more usable (right now its too loud after maybe 10% of turn). Could I try a tonestack with some more loss? or nfb?

Hmm I like distortion, you could add a dual 50k pot after the two 10uF cathode bypass caps and dial in a calmer tone. The caps should tame the pot scratch but Steve Conner found this wasn't the case, if so adust the pots while vol down, find the R value which works and add switched fixed resistors. This should tame gain /dist, maybe add master vol to lower overall vol.

[j-stylez]

I can live with the small range of the volume pot, total volume is not so much the issue as distortion is...anything else i could try? maybe add another resistor in series with the signal before the gridstopper of the first stage? Iam really liking the sound of this amp, even with the small speaker...if only it wouldnt break up so hard and so soon.
maybe a higher voltage on the triode plates? i read this could increaes headroom...or would you consider raising the cathode resisitors of the 12ax7s?

[pdf64]

Can you sketch and post your current schematic, then we can advise where best to cut things back?
I'm thinking of a potential divider in the output tube grid circuit but not sure what's there.
Pete.

[j-stylez]

thanks for your help, heres the current schematic:



The cathode caps, IVe played around with all of them. This seems to work the best so far, although far from my goal...

[loudthud]

You might try this: Cut the wire that goes from the top terminal of the Volume control to the center terminal of the Tone control. Install a 1 meg resistor in place of that wire. Reroute the wire from the coupling cap of the first 12AX7 stage to the center of the Tone control.

[j-stylez]

Allright, that helped. I put the 1m resistor where you mentioned. I think that helped a lot!
However, still not quiet where I want to be with the distortion... Could negative feedback from OT to second triode help a bit?
Or, how could I try to "smoothen" the distortion a bit, maybe I can learn to live with it...It still comes in hard and soon though.
Or maybe higher voltage on the preamp tubes for a little bit more headroom there?