Low end breaking up too early in BF Pro Reverb

[beasleybodyshop]

Hello everyone! Been a while, thought I would pick your brain on a issue that Im having.

I have a mid 60s blackface pro reverb that I have gone through and completely recapped, replaced out of spec components, etc. All electrolytics have been replaced,I have tested all the tubes, they test strong, no microphonic issues, etc. Ive added a push pull switch in the back to disonnect the normal channel and the tremolo from the circuit to get more gain, as well as added a 10KL midrange control for the vibrato channel.

When i turn the amp up past 4 or 5, the low end starts to sound really distorted and a farty and sounds like ghost noting. I have swapped in several known good matched pairs of output tubes and connected the amp to another speaker cabinet, and it still sounds really distorted in the low end. With a hotter guitar plugged in, the low end does break up much quicker, however it does it with both low output and hot pickups as well.

At this point, I dont know where else to look for issues. Any thoughts?

[Stevem]

In any BF it SF Fender amp it's the output stage that clips first and doing the mods you did to increase gain and in turn kick up the drive signal level to the output stage only serves to get them clipping and the farting of the outout stage happening all the more sonner!
The first thing I would do is to understand that with this amp one you are playing at a volume level above 4 you need to back down on that bass a lot if you normally run it open to 7 or more at lower volume levels!
If for you normal volume set playing level is in the 5 area I would change out the PI output coupling caps down to .022uf and if you keep those gain mods in place, then also try out a 12ay7 or even a 12au7 in place of the 12at7 the drives the output tubes now.
Another mod that will never hurt is to up the uf total in the first power supply node to 200.
You should first off confirm that your speakers are not in need of new surrounds and spiders by playing the amp thru known good speakers!

[cbass]

I like to use smaller bypass caps like 2.2uf or 4.7 smaller coupling caps from the pi also.

[vibratoking]

I like to use smaller bypass caps like 2.2uf or 4.7 smaller coupling caps from the pi also.

I think cbass' recommendation is a good one, but move the decimal point two places to the left - 0.022u for example.

[beasleybodyshop]

Thanks everyone. Here is where it ended up:

Guy uses an Ernie Ball guitar with hot pickups, complained that the low end was breaking up too quickly, etc. I ended up swapping out all of the .1uf caps with .022s, it definitely took some of the fartyness and distortion out of the low end, but still was present to some degree. I ended up swapping out the 100K slope resistor with a 220K that I had, and hey....low end mush is gone. Amp sounds really good with a humbucker equipped guitar now. So strange. I would never have thought to fool around with the slope resistor before. That was the magic ticket for this amp. Wierd!

[beasleybodyshop]

Also forgot to mention i went with a 12AU7 in the PI and that helped a ton.

[cbass]

I like to use smaller bypass caps like 2.2uf or 4.7 smaller coupling caps from the pi also.

I think cbass' recommendation is a good one, but move the decimal point two places to the left - 0.022u for example.

I meant 2.2 for the cathode bypass caps . .022 would b good for the coupling caps

[Teleguy61]

Those Pro transformers saturate a lot more quickly than a Bassman or Twin.
Not enough iron in them to be a "pro" amplifier.
Hmmm.....about the name......

[Stevem]

Yup, a Hammond BF Bassman replacement OT with its near 1/2 lb greater mass would allow the PI coupling caps to go back up to .047uf or better if need be!

[Roe]

I increase mains and screens filtering and decrease the coupling caps somewhat

[rooster]

i am biting my tongue.

[Structo]

I usually start with the cathode bypass caps, reducing them.

Then the coupling caps on the PI to the power tubes, if it is a .1 uF I reduce them to .05uF.

If the bass is blatty or cuts out, it's probably blocking distortion.
Or it can be crossover distortion.

This also can be cured by reducing the couplers on the PI.

Also the grid stoppers to the power tube grids can be increased to see if that helps.

Reduce value of the grid feed bias resistors (typically 220K).

Here is some good info on it.

http://www.aikenamps.com/images/Images/ ... _small.png

[jjman]

Could be parasitic oscillation. You can try adding grid-to-ground caps on the outputs as a test. 1200pf or so. That's the bandaid Fender used on the SF models.

[Reeltarded]

i am biting my tongue.

[Reeltarded]

220k oversloped all the nuts out of the preamp. The TS is lossy as hell and now it is dumping gain to aim below the failiure..

Which is in your midrange addition.