Metropoulos Forum

Built a Twin Reverb, Hammond transformers, metal film resistors, Mallory and Orange Drop caps. The Normal channel sounds great. The reverb channel has this buzzing-speaker farting type of sound if the volume is turned up past 4. It sounds great at at volume of 3. I double checked the schematic and all components, everything is in spec. Rechecked the solder joints and lead dress, all OK. Amp is biased at 36ma using a Weber bias rite meter.
Changed PI coupling caps to .047 and bias dropping resistors to 100K - no change. Changed the .1 and .047 tone caps to .022. I like the way they sound, but no change on the reverb channel. The only thing that helped a little is I removed the 10pf bypass cap at the preamp/reverb signal mix and changed the 3.3m resister to 4.2m to attenuate the dry signal. The amp lost its sparkle and I was able to get the reverb channel volume up to 5 before it started that buzz/fart sound. I tried shutting off the reverb with a foot switch and then removing the reverb tank entirely, no change. As I mentioned, the normal channel sounds great. I know that Twin Reverbs are not known for their great distortion at higher volumes, but this is an unpleasant buzzing sound like a speaker cone is ripped. I am attaching some pictures with the 4.2M resistor in and the 10pf cap removed. Sorry the board is a bit of a mess, I used some liquid flux and haven't had the time to clean up the residue.
Have been building amps for several years. I am new to the forum and would really appreciate any help you guys could give me. This one is driving me nuts.

it might be some kind of cross over distortion in one of your preamp tubes. what are your voltages in the reverb channel preamp?

I had a similar problem and it dissappeared after I installed a 68k grid resistor on the reverb driver.

you need to find out which stage clips. try running straight from stage 2 of the vibrato ch. into the pi like the normal channel

Any time someone says "farting" sound related to gain I think of blocking distortion, which can usually be corrected with a grid stopper like Roe mentioned. Try like a 47k on the possible preamp grids after that channel's volume pot.

I am posting some pictures which I hope are a little better.
Really appreciate all the suggestions , many thanks to all of you for taking the time to help.
Roe, when you said run the signal straight from stage 2 to the PI should I take the signal from pin 6 of the preamp tube or right after the .02 cap and jump it over to the 220K resistor that goes to pin 2 of the PI? Where do you install the 68K resistor on the reverb driver , between pins 2 and 7, or right after pin 7?
Toner, Would I put the 47K resistor on both pin 2 and pin 6 of the 2nd channel preamp tube or just pin 6?
Here are the voltages on the tubes:
2nd Channel (12AX7)- Pin 1, 250, Pin 2, 0, Pin 3, 2, Pin 6, 254, Pin 7, 0, pin 8, 2.17
Reverb Driver (12AT7)-Pin 1, 436, Pin 2, .03, Pin 3, 8.58, Pin 6, 437, Pin 7, .03, pin 8, 8.58
Reverb return (12AX7)- Pin 1, 264, Pin 2, .04, Pin 3, 2, Pin 6, 266, Pin 7, 2, pin 8, 2
Tremolo tube-(12AX7) Pin 1, 457, Pin 2, -55, Pin 3, 0, Pin 6, 372, Pin 7 , -55, pin 8, 0
PI- (12AT7) Pin 1, 237, Pin 2, 58, Pin 3, 93, Pin 6, 235, Pin 7, 61, pin 8, 93

Rockman wrote:I am posting some pictures which I hope are a little better.
Really appreciate all the suggestions , many thanks to all of you for taking the time to help.
Roe, when you said run the signal straight from stage 2 to the PI should I take the signal from pin 6 of the preamp tube or right after the .02 cap and jump it over to the 220K resistor that goes to pin 2 of the PI? Where do you install the 68K resistor on the reverb driver , between pins 2 and 7, or right after pin 7?
Toner, Would I put the 47K resistor on both pin 2 and pin 6 of the 2nd channel preamp tube or just pin 6?
Here are the voltages on the tubes:
2nd Channel (12AX7)- Pin 1, 250, Pin 2, 0, Pin 3, 2, Pin 6, 254, Pin 7, 0, pin 8, 2.17
Reverb Driver (12AT7)-Pin 1, 436, Pin 2, .03, Pin 3, 8.58, Pin 6, 437, Pin 7, .03, pin 8, 8.58
Reverb return (12AX7)- Pin 1, 264, Pin 2, .04, Pin 3, 2, Pin 6, 266, Pin 7, 2, pin 8, 8.37
Tremolo tube-(12AX7) Pin 1, 457, Pin 2, -55, Pin 3, 0, Pin 6, 372, Pin 7 , -55, pin 8, 0
PI- (12AT7) Pin 1, 237, Pin 2, 58, Pin 3, 93, Pin 6, 235, Pin 7, 61, pin 8, 93

hi, using a cap should work - do it like the normal channel does it. you need to isolate the blocking distortion - se which gainstage it comes from or whether its the reverb circuit. I think the latter is probable. If I remember correctly, grid (input) is pinns 2, 7 and anode (output) is pins 1, 6.

Sorry there was a typo in the tube voltages.
Pin 8 of the Reverb Return (12AX&) is 2 volts, not 8.37
I corrected the previous post.

watch leaddress, twist wires going to the faceplate/pots and dont mesh all the wires together. look at a BF fender or something that is built right

Rockman wrote:Toner, Would I put the 47K resistor on both pin 2 and pin 6 of the 2nd channel preamp tube or just pin 6?

The grids are pins 2 and 7, not 6.

Sorry, I'm not too familiar with twins so I'm just giving general suggestions, which may not help. The problem won't be on the first gain stage (right after the input jacks).

Since the problem starts with the volume above 3, I doubt the grid right after the pot, which is probably pin 7, is the issue. You can try adding a large grid stopper (like 220k) there but you've already got a lot of resistance left from the volume pot.

Try adding a resistor on the input of the reverb driver, then the return from the reverb, etc. Basically just work your way through from the volume pot to the PI. Hope this helps.

Rockman wrote:Reverb return (12AX7)- Pin 1, 264, Pin 2, .04, Pin 3, 2, Pin 6, 266, Pin 7, 2, pin 8, 2

Is pin 7 really +2 VDC? That grid should have less voltage than the cathode ( pin 8 ). It should be around 0. If it's really 2 VDC, something is wrong there. Check the wiring around that half of the tube.

You may have a leaking coupling cap right before the reverb pot. You can check this by un-soldering the end of the cap that goes to the reverb pot and test for DC voltage. There shouldn't be any there.

Toner,
I'll check pin 7 again. It's possible I wrote the pin 8 voltage as pin 7 also. If it is 2 volts, I'll check the coupling cap. I'm taking a break for a day and then I'll start checking everything again and start correcting the lead dress and twisting the wires to the pots, etc.
I may be able to have a friend bring his O-scope over this weekend. I'll let you know how it all turns out.
Thanks for all the great suggestions. Sooner or later I'll get this corrected (I hope).