Brighten tone of Ampeg B-15 "Big M" amp

[Paul Lebow]

I have an early 1960's Ampeg B-15 amp. I've done some minor mods and it seems to be working fine. It has a very strong bass output even with tone cranked for full treble. Where would the best place in the simple tone stack to brighten up the tone? The coupling cap to the stack is .02 uf. Would increasing that value block some of the base? ( see attached schematic)

Thanks!

Paul

Ampeg M15 Schematic 2.pdf

[maxkracht]

You could reduce the cathode bypass cap on v1, currently 100uf, going below 20uf will start to roll off some bass on both channels. Reducing the .02uf cap would also cut bass, increasing would add bass. You can compare to the other channel with a much larger .1uf coupling cap.

[Stevem]

You might try lifting one end of the cathode bypass cap on the output tubes.

[sluckey]

Please clarify. Do you have a B-15 or a M-15? Big difference. The B-15 is a BASS amp and changing to a speaker voiced for guitar would be the most likely thing to do. The M-15 has a guitar channel and bass channel as well as bias vary tremolo. I don't know about the speaker.

[Paul Lebow]

Whoops - it the M-15 as in the schematic.

[Paul Lebow]

Thanks very much for the suggestions. The M-15 was apparently meant to address accordions as well as guitars.( I've attached the actual manual from the Ampeg archives if anyone is interested - its quaint

As a follow-up question, I would like to see if I can increase the gain in one of the stages to get some increased distortion at higher volume settings. I did put a 10k pot in the feedback circuit from the output to the phase inverter input. This increases feedback resistance above the 6k to 10k range shown in the schematic to as much as 15k. This definitely increased gain and provided some crunch. I don't know if increasing further would improve or cause things to go unstable. Any other suggestions would be appreciated.

M-15 Manual.pdf

[maxkracht]

did put a 10k pot in the feedback circuit from the output to the phase inverter input. This increases feedback resistance above the 6k to 10k range shown in the schematic to as much as 15k. This definitely increased gain and provided some crunch. I don't know if increasing further would improve or cause things to go unstable.

You can lift one side of that resistor, meaning no feedback. This would be the most output you can get by messing with the feedback loop. I doubt it would make the amp unstable, but no real harm experimenting. A larger pot in place of the 10k would do essentially the same thing.

[Phil_S]

As a follow-up question, I would like to see if I can increase the gain in one of the stages to get some increased distortion at higher volume settings.

Are you aware that the owner of Ampeg was convinced that distortion was a very bad thing? The amps are designed for clean tone. You might consult chart #5 in the attached to see if you want to modify the plate load resistor(s) and/or the cathode resistor(s) to see if you can get more gain -- gain that you like -- from the 6SL7's.

[Paul Lebow]

Great resource - thanks.

Yes, I agree the M-15 seems very much geared to clean tone. (Who wants a crunchy accordion? Trem is smooth and still has original square magnet Ampeg 15 inch speaker.

On the advice of a couple of folks here I tried with no feedback to the cathode of the inverter. I had replaced the 6k fixed feedback resistor with a 5k + 5k pot with some improvement. Turns out I had forgotten that pot also had an on/off feature which allowed me to disconnect feedback altogether. Major difference. Begins to distort nicely with Strat set to 7 and pretty crunchy when at 10. Don't know if the inverter is distorting or the power tubes, or both, but great tone. It's a different amp now.

[B Ingram]

I have an early 1960's Ampeg ... amp. ... It has a very strong bass output even with tone cranked for full treble. Where would the best place in the simple tone stack to brighten up the tone? ...
Ampeg M15 Schematic 2.pdf

For the schematic you posted:

• Accordian Channel C8 is 4-10 times bigger than it needs to be for guitar. Fender would have used 500pF in that spot.

• We can say the same of C7 in the Guitar Channel: it is 0.002µF (2000pF) and about 4 times bigger than it needs to be. 500pF would work here.

• Ampeg added extra treble-shave in the phase inverter. You may wish to keep this after you shrink the Tone control caps, but C15 (500pF) is rolling off treble in the 2nd half of the paraphase inverter.

You can shrink the cathode bypass cap at the input stage (V2), but it's helping prevent hum from tubes with heater-to-cathode leakage. Therefore, perhaps we leave this cap alone.

The point of the capacitors I mentioned (C7, C8) at the Tone controls is they act like "bright caps, bypassing the Volume control" when the Tone control is turned towards the treble-end of its sweep. Make those too-large, and mids come along for the ride, keeping the Tone control from making the amp "brighter."

[Paul Lebow]

Just what I was looking for, thanks. I'll try the C7 "brighter cap" swap first. It's pretty accessible right off the volume pot. ( Kind of a rats nest compared to later Ampegs)

Question - At full volume isn't C7 completely bypassed?

[Paul Lebow]

As per suggestion by B. Ingram, I changed the filter cap, C7 from 2000pf to around 600pf. Significant change in adding more sparkle but only with volume turned down half-way. Seems the volume control essentially shorts out that capacitor when at full volume as one can see in the schematic. Definite improvement. Don't know if there is a way to keep the vol and tone independent with a single tone control.

Thanks all.

[Stevem]

Since this is a old amp if it has its original speaker and especially if it's been played a lot then it will provide more bottom then it should because the spider and the cone edges are likely fatigued.

With this condition the cone moves a lot for a little amount of low's pumped in it.

So the question is have your played the amp thru a different speaker with a known good suspension?

[Paul Lebow]

Good point. The speaker and cone look to be original - big square magnet with "Ampeg" on it. It makes sense that cone surround would soften up over time. I am playing through an OX box cabinet simulator and listening through headphones. Actually the direct speaker output sounds less boomy.

[martin manning]

I am playing through an OX box cabinet simulator and listening through headphones. Actually the direct speaker output sounds less boomy.

It's a good idea to explain the situation completely up front ;^) That tone control did not seem bass-heavy to me.