JJ 7027's and high plate voltages

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g-dog

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Jul 9, 2006
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I'm currently working on an Ampeg VT22 that seems to have the original RCA labelled power valves in it. One is pulling 71mA at idle, one 34mA and the other two approx 54mA. I've swapped a couple round to make sure it's the valves and not the circuitry. Needless to say I intend to replace these valves. Has anyone had any issues with the JJ 7027's with plate voltages around 560Vdc? (Max value on data sheet is 500Vdc although the triodeelectronics page suggests that the JJ's are fine in Ampegs). All the grid resistors check fine (47K) with the bias voltage sitting at -48Vdc. I've located and changed a few dead components. Would it be best to back off the bias a bit to bring the idle current back down to approx 30mA with the JJ 7027's? This would just involve increasing the grid resistor values yes? Perhaps to 56K?
Any help appreciated.
 
This may or may not be helpful, as it's a different tube type, but...

The guitar amp techs on another forum I go to have absolutely no problem plugging JJ 6V6's into amps where they'll see 500v. I'd feelsafe plugging in the JJ 7027's in your amp considering their apparent ruggedness.
 
i used to run the jj's in my V2 head and biased just a hair cold, but not much. i have however found that a set of KT88s biased a little hotter than usual has given it a whole new sparkle that i never thought this amp could have. if you have a set laying around, i highly recomend trying it out...
 
Thanks, I'll give the JJ's a go. I was originally unsure as I put a brand new set of JJ 6L6GC's in and one instantly went up in a puff of smoke, hence my further investigation leading to a S/C rectifier. Now I have a wonderful matched triplet of 6L6's!
 
Anyone know if it's normal for these amps to have -595VDC on the grids with the power on but standby off?? Seems a bit odd.
 
[quote author="g-dog"]Anyone know if it's normal for these amps to have -595VDC on the grids with the power on but standby off?? Seems a bit odd.[/quote]

Is this the version with the "distortion" knob and the white switches? If not, there shouldn't be 595VDC ANYWHERE in that amp. The early model V4/VT-22s have about 535V on the plates and screens, the last versions have more like 590V plate, 350V screen. In either case I dont think your grids sould be seeing any voltage with the amp on standby.
 
Yes it has the distortion knob and white switches. I'm just right now replacing
components in the capacitive divider bias circuit and I'll see what sort of result I get.
Previously, when I turned standby on the -595 would drop down to a more healthy -46, which is all I really want to see on the grids. Working from a VT22 schematic with completely different designators!
 
Alas no change. The bias current just starts to creep up and up with valves installed and the amp hums like crazy.
 
No, just a schematic for what I assume to be an earlier model. It's very similar but with different valve complement. These things are so messily wired, it's a menace! I'm starting to think there may be an issue with most of, or one of the power caps.Unfortunately I have a phobia of tubular multisection caps as I tend to be a "one lead in, one lead out" kind of guy. I'll have to get over that one sooner or later. Thanks for your help Baltimore, it's much appreciated. :grin:
 
With a fixed bias amp often the output grids are tied to the bias supply via grid to bias supply resistors

Do you have access to a meter like a sencore lc102 that can measure leakage of a cap at voltage(lc 102, 1 to 999VDC in one volt steps)

You might have output tube grid coupling caps that have leakage. The other end of the caps most likely are connected to the plates of the phase inverter section.
 
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