Twin Reverb Build Problem

[Rockman]

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 4.2 m. the amp lost its sparkle and I was able to get the reverb channel 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.

[phsyconoodler]

The pictures are too small to see anything,but it sure sounds like you have a cold solder joint somewhere.
  The lead dress is sloppy and grid wires are too long.They really need shortening.All the wires look too long to me.
I'm betting a cold solder joint is the culprit.Or an incorrect value of resistor like a 470k where a 470 ohm should be,something like that.
  The 'bias dropping resistors' you refer to are actually grid stopper resistors and not intended to drop bias voltage,but to prevent oscillations.They need to be 1.5k or larger. 100k is likely too small,but that's not your issue here.

[Rockman]

Thanks for the reply. I agree that the grid wires could be shorter, but I don't think that's the problem here. I will clean up the lead dress and shorten the grid wires as soon as I can. I did check the component values and the solder joints and all seems fine. Voltages on the preamp tubes are also in spec.
The grid dropping resistors I was referring to are not the 1.5K grid stoppers that are on the power tube sockets, they are the resistors just after the PI coupling caps, usually 220K on fender. I lowered them to 100k along with changing the PI caps to .047 to help with blocking distortion, if any. Seems like the  signal is severely clipping for some reason.

[Willabe]

Hi Rockman,

The 220k's you changed to 100k's are the grid return resistors.

Don't let it get to ya, you'll find it. phsyconoodler has got a lot of amp builds and trouble shooting under his belt, I'd think about what he suggested to look at again, if it were me.    
 
Looks like a nice build!                    

                     Brad      

[punkykatt]

Build Doug`s Listening Device, all the info is on this site under cool tools. You will find the problem in a matter of seconds with this tool.

[DummyLoad]

they are the resistors just after the PI coupling caps

often referred to as a "grid leak".

why did you change the value? just curious. are you using a 12AT7 for the PI? check the values and wiring in the PI and NFB path. it's a common place for wiring errors.

--DL

[Rockman]

Thanks for all the suggestions and the kind words of encouragement. I am posting a picture, which I hope is  clearer this time.
I'm have no doubt that most, if not all of you have much more experience in amp building than I do and I really appreciate you all taking the time to help me with this. I didn't mean at all to imply that  phsyconoodler's suggestions weren't good ones and very much appreciated, it's just that I've been over the solder joints and the components at least 3 times and couldn't find anything. I guess my frustrations are getting the better of me at this point. I can always check again, sometimes it's right under your nose and you miss it.
I may be able to get my hands on an O-scope and maybe figure out where the problem is.
DummyLoad, Yes, I am using a 12AT7 for the PI, I changed the grid return or grid leak resistors from 220K to 100K and the PI coupling caps from .1 to .047  to possibly help if the problem was being caused by blocking distortion. I think this was suggested as a possible cure for blocking distortion by Randall Aiken. It didn't make any noticeable difference.
Here are the voltages on the tubes:
2nd Pre amp (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

[Rockman]

Here's another picture

[VMS]

I would twist and separate the wires coming from pots to their own groups.

Like Leo did (guts):

http://www.amparchives.com/album/Fender/Twin%20Reverb/65%20Fender%20Twin%20Reverb%20SN%20A03566%20-%20Mike%20W/slides/3245766047_c6ab2357dc_b.html

[Rockman]

Sorry there was a typo in the tube voltages.
Pin 8 of the Reverb Return (12AX7) is 2 volts, not 8.37

[Danskman]

I agree with VMS, you should not run input wires along pre-amp volume/tone wires, and those along high voltage power wires or trem control wires. Input wires coming from input jacks run at very low voltage; this could be lead to oscillation if they get preamp output signal in return, leading to some unwanted "feedback" effect.
I always dress blackface builds like Leo did them in the early 60s and never had any problem with it.
See http://s269.photobucket.com/albums/jj63/denferdan/65%20Super%20Reverb%20Repro/?action=view&current=SuperReverb-25.jpg
it's a BF SR repro I did two years ago; runs very fine, without any troubles. OK, this is not a turret board, but wiring dress can be the same.
Hope that helps, keep the good job up and best regards,
Danskman