Tube Amps / Music Electronics
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|3/21/2006 10:46 AM|
||Sockets gone bad?|
I know I've posted on here numerous times in regards to the V4 I've been tinkering with in my free time from school. In short, amp was purchased and working and soon after fried a resistor in the power amp section (plate if my memory serves me correct). Amp was taken back to the store where new resistor was installed but they neglected to notice the lack of volume and glowing tubes. The two closest to the center (1 half of the PP) glow red if you start playing. Voltages were all good. From the phase inverter up I've replaced all the tubes and componants on the board and added some grid stoppers thinking it might be parasitics. That left the transformer. Talking to a tube guy he said I could swap the leads for a second to see if it went to the other sockets. I did that and disconnected the NF lead but it's still those tubes that glow. I'm looking towards the socket since I seem to have done just about everything short of buying a new amp. Could they be the culprits? Thanks for your patience and wealth of knowledge.
Young and still trying to learn,
|3/22/2006 7:59 PM|
Since what's left of my memory doesn't include any of your previous threads some of this will probably be repeats.
There are no plate resistors in the power amp - well, not on the output tubes, anyway, and that's what I think you're talking about.
It sounds like the inner pair of power tubes has lost bias. This could be from a loose grid pin socket, a failure in the bais network or a bad coupling cap from the phase inverter.
If by 'two closest to the center,' you mean closest to the center of the amp, I'd look at the 100k bias feed resistor (measure bias voltage before and after) or the PI coupling cap.
If, instead, you mean center of the quartet (middle two power tubes), then I'd look at the socket pin tension.
Then there's the pinout difference between 7027 tubes (which use pin 1 for something) and more common types that don't use pin 1, prompting some mfrs and service people to use it as a convenient tie point.
What output tubes are you using?
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|3/23/2006 12:41 AM|
Actually the V4s do have plate resistors in the output. Like 10 ohms or something between the plate and the OT primary.
|3/23/2006 7:08 PM|
... I must need new glasses - they're right there.
|3/23/2006 1:31 PM|
It's half of the output, either the Push or the Pull. I replaced the coupling capacitors and the resistors to the grids (I don't have it infront of me but I think they're all getting the juice from one trace on the board prior to the and my logic, which may be flawed, says that anything prior to that would throw the bias on all the tubes). There are plate resistors like Enzo said. I'm in school right now so I don't have it infront of me but it was either one of those or a screen resistor the popped. I remember reading about carbon deposits in tube sockets but I wasn't sure if that could make the bias take a dump. It does have 7027A's in it. Thanks.
|3/23/2006 7:08 PM|
Your logic is fine. Those particular resistors, caps and their solder joints, on the other hand ... the trace may also be cracked. Are the sockets on the PCB or are they chassis-mounts?
It's good those plate resistors were there - could have been a lot more expensive to fix.
|3/25/2006 10:22 PM|
Everything but the output are PCB mounted but the octals are chassis. I'm at a loss. At idle they all read the same voltage wise but when you apply a signal the two that glow don't shift as negative as the others. Granted, I'm not extremely knowledgable and at 22 I didn't grow up around this stuff, but it seems like I covered most of my bases as best I could with my skill level.
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