Ampeg V4 distortion model

[methodofcontrol]

running 2x 6550's for a while w/ no problem. my buddy asked me to fix this for him. recently, as soon as you take it off standby the power tubes red plate real bad and the amp makes a loud hum. i found a schematic for the distortion model, but it doesn't exactly line up w/ what i'm looking at.
are there different versions of the distortion model V4? (the one on my bench has a white rocker switch for power and a translucent red rocker for standby. no polarity switch)
i measured the voltage on both sides of the 5.1 ohm 10w'ers, and it's feeding about 570v into the power tubes.
this amp doesn't appear to have EVER had a bias mod, even though it's been running 6550's fine for some time now.
help?

[g1]

Is this the schem. you are working from?
http://www.ampegv4.com/images/VT22_Schem2.jpg
If you can read the fine print note #6 says you can sub in 6550's for performance upgrade. This is the only model I have seen that note on, so I assume it is due to the separate power supply winding for the screens with lower screen and bias voltages (that is the only major difference from the other V4's that I can see).
Note the fuse connecting the bias winding to the bias circuit. What kind of bias voltage are you reading on the grids?

[methodofcontrol]

first off, g-one. thanks for the schematic! i don't have the amp right in front of me
but a few key pieces already make more sense than the schematic i've been trying to use.

i'll measure the bias voltages ASAP and post again for more insight. thanks!

[g1]

Oops, I did assume you had already verified it was not the power tubes that were the problem?

[methodofcontrol]

ok, so all of my voltages are low. B+ is about 568v. i can't find any negative voltage on the power tube sockets at all. bias voltage is supposed to be -28.5v or so, i'm reading between 1.25-2.5v.
this is with power tubes pulled.

[rf7]

i can't find any negative voltage on the power tube sockets at all. bias voltage is supposed to be -28.5v or so, i'm reading between 1.25-2.5v.
this is with power tubes pulled.

Look into the bias supply. Check that .15 amp fuse and if it's OK check D205. You need to have that negative bias voltage there. There is something going on there which is not letting you develop a negative voltage.

[methodofcontrol]

that fuse checks out, as does the diode.
here are my voltages compared to the schematic voltages found here: http://www.ampegv4.com/images/VT22_Schem2.jpg

A- 567v
B- 461v
C- 232v
D- 213v
E- 362v
F- 1.25v

my voltages are WAY off. any other thoughts?

[Alex R]

Everything is ok here except you have no bias voltage at F which is NOT ok! Did you check the fuse as rf7 suggested?

[methodofcontrol]

i get continuity through the fuse. maybe the solder joint is just messed up?
what else could be causing a lack of negative bias voltage?

[rf7]

Do resistors R215 and R216 measure OK? Is there AC voltage getting to the diode from the transformer? If so, lift R211 and R212 to isolate the bias circuit (with power tubes removed, of course), and see if you get a negative voltage at where R215 AND R216 meet. If not, maybe one of the caps (C207 & 208) is shorted. You should see maybe -79Vdc where the diode meets R216.

[g1]

Isolating the bias circuit as rf7 suggested should get the bias circuit sorted out.
You mentioned your voltages are "way off". I am suspicious of "C". With tubes pulled and possibly more than 120VAC from the wall I expect the voltages to read high. You have low readings at c,d,&f. Something pulling down c is probably causing the low reading at d. Possibly leaky coupling caps C204 or C205 would cause a low reading at C,D, & F.

[methodofcontrol]

wow... you guys are great! thanks for sticking with me and handing out the advice!

here's some more info regarding the latest examination-
i've got 48v AC from the green/yellow wire on the tranny and i've got 115v AC off the orange wire, both of these readings seemed kind of inconsistent. could've just been bad connection with the meter probes, but at various points i was reading 10v AC or so off of either one of those taps.
i lifted R215 and R216 and measured out of circuit, both OK.

upon lifting R211 and R212, i've got......
-53v DC @ R216/D205
-17 v DC (and climbing) @ R215/R216
185v DC @ R211/C204
0v @ R212/C205
185v @ pin 1 of 6AN8A
56v DC @ pin 3 of 6AN8A (note here: unlike the schematic, C205 goes directly to pin 3 NOT through a 1k first. there is a 1k off of pin 3 as well.)

[g1]

C204 seems bad. With bias lifted you should have 0V like C205.
Also check the "ugly diode no one likes" shown beside the standby switch, can't read it well but it looks to be called D205. The orange wire where you measure 115AC should be grounding through that diode. Could also be why your bias reading "climbing", but while you are in there you should probably just replace C206 & C207 anyway.

[rf7]

C204 is shorted. Replace it. Replace C207 and C208, and see if your bias voltage goes more negative.

[methodofcontrol]

thanks again guys!
i made a list of resistors and caps that are bad or clearly heat damaged. replaced all of the resistors a few hours ago. i'll order up the caps today and put them in next week. i'll keep you posted.