BF Twin Reverb mod suggestions

[cocobolo]

Hello all,

I'm new to building amps and was hoping someone could help me out with a BF Twin Reverb clone that I built. So far the major mod that I have done to the amp is to have reverb on both channels. There is intermittent noise from the reverb when the reverb pot is turned up. I also have the dreaded ticking when the tremolo is on. So basically I just leave the reverb tank disconnected and the trem off (using a weber 50k RA pot with switch). Sounds great that way and I'm not that big of fan of reverb or trem anyways. Any suggestions of mods I can do that would replace the reverb/trem circuits without changing the tone I have? Does the reverb even effect the tone if the tank isn't attached?

[ktritty]

Noise in Reverb: I have that too

Hello. This might go better in "troubleshooting", but I was searching Twin Reverbs and found this. I am a "relative newbie" with tube circuits, and my uncle has asked me to fix his twin reverb. It is making intermittent noise in the reverb that goes away with reverb knob set to zero. Sound is reminiscent of someone passing gas in the distance, and lasts about 0.5 second, with about 3-5 seconds between noises. Not perfect intervals, but fairly regular. Is this nose similar to yours? Does anyone have an idea what components may be bad? I am suspicious of decoupling capacitors. I am decently experienced working with many parts of a tube amp, but don't yet understand what makes a reverb circuit tick, so am clueless if the (driver or oscillator?) tube or tranny may be suspect as well.

Thanks!

[txstrat]

What kind of noise are we talking about, cocobolo? Hum, hiss, crackle...?
I assume you both have switched tubes already with known good ones?
If the tubes are good the noise could come from bad plate resistors, I only find it odd, that the noise is sort of regular.

[ktritty]

What kind of noise are we talking about, cocobolo? Hum, hiss, crackle...?

I tried the amp again today, the noise is different. Still somewhat regular, but a higher frequency. Difficult to describe, but sounds like a scratchy version of what a medium-sized gull-type bird would make. Lasts about 0.5 sec, every 3-5 sec like before. Noise independent of channel volume knob position, but dependent on master volume (no noise at zero).

There is a second noise in the reverb (dependent on reverb level knob) that sounds like distant thunder, so I will also try replacing a few basic components around V3 (reverb driver tube) such as decoupler caps and V3 cathode resistor. What are the odds that the reverb coupling transformer (T4; couples V3 to Tank) is bad? I'm assuming not very likely. Sorry I forgot to mention mine is Silver Face, schematic drawing number 010182 rev. E or F (it is hard to read revision letter).

I have not tried alternate tubes (suspect, amp was recently retubed). My uncle did not keep old tubes. I have no 12AT7 on hand, but a whole mitful of good New, NOS, and Old 12AX7.

This amp has a lot of hum, some dB of it are intermittent, so I plan to replace the old paper filter caps (need to order, will take days) and move forward with the other noises after that.

[txstrat]

I would try to find the cause of the noise first. That might result in changing the caps anyway.
You can try the 12AX7 instead of the AT7. Just keep in mind the AX7 has some more gain.
You could pull the reverb tank cables to determine which stage causes the noise. When the noise is still there after pulling the cables it's most likely the recovery stage (assuming the noise IS from the reverb stage).
I would also try new (known good) tubes in the other sockets (at least the sockets after the volume and before the master).
I can't help myself but the regularity in the noise makes me think of the vibrato...

[Rich]

"Any suggestions of mods I can do that would replace the reverb/trem circuits without changing the tone I have? "

Try running it without reverb and vibrato ie, eliminate them as a noise source by pulling v2, v3, v4 and v5 so those circuits don't come into play.

[boyer]

Yeah, sounds like you might have some bleed from your trem oscillator. Try replacing your grid lead with shielded cable, grounding at one end (I usually ground on the chassis right by the tube, but pick an end that is easy for you) and try to keep it as short as possible, and not running along side any other sensitive signal wires.

If you don't plan on using either, then yeah, pull the tubes, or wire one up as extra gain stages on one channel, and you've got a distortion channel. If you're careful with the gain, you could get a little more clean head room if you drove your tonestack with an 12AT7 or 12AY7 (lower impedance = better driver).

There are lots of things you could do with the extra stages.

Also, shouldn't be hard to sus out the reverb noise issue. Make sure you're using good quality (ie flameproof metal film), 1/2w or bigger resistors for the plate loads, and possibly shielded wire to the grids.