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#1 2012-12-14 23:40:49

Redwood
Member
Registered: 2012-12-10
Posts: 19

V4 loud popping and runaway feedback

I just got done recapping and completely retubing a V4 distortion model and changed out many resistors and all the power section diodes while I was at it.   I didn't do every single capacitor in the whole amp but all the electrolytics and a good handful of the coupling caps.  I also replaced the tube socket in the position that fried since there wasn't enough left of pin 3 to solder a new 5k 5W resistor to it (I replaced all four plate resistors for good measure).

I have a Ruby Tubes bias meter so I had it installed on first startup with a dummy load and while they weren't perfectly uniform all four 7027s were running between 39 and 43 mA. I did not change out the 47K resistors in the bias network and without pulling them off the board their measurements vary about about the same percentages so once I obtain a set of four very uniform resistors for that section I will swap them out too.  I couldn't find .33 mF caps locally so didn't change the original Euros in that section either but they also appear to be within tolerance.

Everything appeared to be OK so I went ahead and plugged in a speaker to check it out with a low signal and as soon as I hit the standby switch after warmup I got a very loud pop followed by just a bit of hum.  I was able to minimize this with the hum balance pot.  Strumming at low volume was OK at first but when I turned it up past 2 I get very loud distorted popping at the pick attack.  If I keep at it for more than a few notes I get runaway low pitched but very loud feedback regardless of where the volume knob is set.  The same thing happens in both channels.

The amp was brought to me after one 7027 had gone south taking out its plate resistor with it - or more likely vice versa.  I never witnessed it in a functional state before I started working on it but I'm given to understand it was being used when that power tube failed, consequently I don't have any particular reason to suspect pots, jacks, or switches, though I've checked them out too and they all seem fine.

The brand new 6AN8 is a bit microphonic but not as bad as I've seen in some others that still worked.  None of the other preamp tubes appear microphonic at all.  The feedback does stop if I pull V3 out of the socket but that cuts the signal altogether so I'm still not sure if the problem is in the power supply or the signal path.

I'm at a loss here.  I took copious photos and even labeled wires before pulling the multisection caps.  I've double, triple, and quadruple checked my work and can't find a mistake.  I did find one resistor labeled R7 coming off pin 2 of V1 that the schematic says should be 470K but it had a 47K that looks like it's been there forever so I did not change that one but I suppose I should.

One other thing is that at some point this whole amp was clearly running hotter than hell and most of the tube sockets are full of gunk that looks like old solder resin.  I cleaned them out as well as I could but there's a chance I'm not making perfect contact at all pins.    I fail to see how that would cause the symptom I'm getting though.

Can anybody provide some troubleshooting tips?

I'm using schematic 591761 for the Distortion model by the way.  It's not a perfect match but it's very close, the main difference being simply the ID numbers of a handful of components on the smaller board don't match exactly but for the most part the values do.

Thanks very much for reading my rambling post.  What I've read of this forum so far has been very helpful though it appears my particular schematic is not the most common.

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#2 2012-12-15 01:27:00

Redwood
Member
Registered: 2012-12-10
Posts: 19

Re: V4 loud popping and runaway feedback

I just pulled the power tubes and went through the whole amp with a voltmeter to compare what I'm getting to the values I should be seeing and there's definitely some wonky stuff going on here.  No order of magnitude variances but some high and some low without a lot of rhyme or reason that I can easily discern.  I'm hoping this table will make something obvious jump out at one of you smart guys.

(note that I took the * value for B+ at point "A" off the schematic because I have PT 300725)

    schematic V    measured V    Delta        Delta %
V1-1    145        153        8        -5.5%
V1-3    8        11        3        -37.5%
V1-6    165        171        6        -3.6%
V1-8    1.4        1.45        0.05        -3.6%
V2-1    185        82        -103        55.7%
V2-3    1.6        2.5        0.9        -56.3%
V2-8    3.5        2        -1.5        42.9%
V3-1    105        99        -6        5.7%
V3-3    2.8        3.8        1        -35.7%
V3-8    0.6        4.5        3.9        -650.0%
V4-1    200        280        80        -40.0%
V4-3    45        25        -20        44.4%
V4-8    3        2        -1        33.3%
V5-1    310        320        10        -3.2%
V5-3    190        190        0        0.0%
V5-8    0.6        0.78        0.18        -30.0%
V2-1-1    242        253        11        -4.5%
V201-2    75        72.6        -2.4        3.2%
V201-3    95        95        0        0.0%
V201-6    138        160        22        -15.9%
V201-8    2        4        2        -100.0%
                           
A    545        558        -36        6.1%
B    352        320        -52        14.0%
C    347        345        -2        0.6%
D    310        347        37        -11.9%
E    338        341        3        -0.9%
F     -28.5        -31.9        -3.4        -11.9%

Last edited by Redwood (2012-12-15 01:29:12)

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#3 2012-12-15 23:36:07

Redwood
Member
Registered: 2012-12-10
Posts: 19

Re: V4 loud popping and runaway feedback

Embarrassing edit time.  After sleeping on it I went to check those voltages again and realized after testing the tubes I mixed up V2 & V4.  Fliptops shipped me units with the ECC832 for the 12DW7s and ECC83S for the 12AX7 and I just got them mixed up.  After correcting that issue I now get the following voltages, better but still wonky:

    schematic V        measured V
V1-1    145        145
V1-3    8        10
V1-6    165        161
V1-8    1.4        1.35
V2-1    185        180
V2-3    1.6        1.2
V2-8    3.5        1.76
V3-1    105        93
V3-3    2.8        3.5
V3-8    0.6        4.3
V4-1    200        195
V4-3    45        66
V4-8    3        1.8
V5-1    310        291
V5-3    190        170
V5-8    0.6        0.7
V2-1-1    242        238
V201-2    75        67
V201-3    95        88
V201-6    138        151
V201-8    2        3.7
           
A    545        539
B    352        302
C    347        325
D    310        336
E    338        331
F     -28.5        -30.8

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