Ampeg VT-40 Redplating Solution?

[neddyboy]

I'm another fool trying to revive a well-loved VT-40 top loader. I've seen a few threads on this subject, but they're to advanced for me so I'm requesting a dumbed-down version.

The power tubes redplate after a minute of playing. I've tried different sets of tubes: the Sylvania 6L6s that were in it, and a new pair of EH KT88s. Both redplated. I haven't replaced any of the filter caps as the amp is pretty quiet as it stands. I will when I can afford all those can caps.

After reading some suggestions online, I've replaced the cap that feeds the bias (C15). I already had the parts, so I also replaced the grid limiting resistors (R36 & R37) and the two 47k resistors off pin 5. All the new parts matched the original specs with the exception of C15. It was .047 and I replaced with .05. Still replating.

My next step will be to replace the electrolytic next to C15, 80mf 100v. I can't find it either on the parts layout or the schematic, but it's def. original to the amp. I assume it's part of the bias circuit. The forum I saw also suggested replacing the bias filters (C14 & C21), but I can't find them on the actual board. Is it possible the single 80mfd cap is a substitute for the two 10mfd 100v C14 and C21? There's a link to a schematic below.

Do any of you learned gentlemen have any further suggestions?

EDIT: better schematic. From the front load version, but seems to match the earlier model.
http://drtube.com/schematics/ampeg/vt40pwr-jp.gif

[g1]

Don't install power tubes til you get the proper bias at pin 5 of the power tube sockets.
Have you checked the bias diode? What do you get for DC voltage coming out of the bias diode (anode end)?

Schematic:

[52 Bill]

Yes, please stop changing parts in hopes of stumbling upon a solution.

Use your meter and read the voltage on pin 5 of the power tube sockets. If you don't have a solid (minus) -50 volts or so investigate why it isn't there. Is the voltage at the bias supply there?

[neddyboy]

I have not checked the bias diode. Good idea. FYI your attached V2.pdf link goes to a blank page, at least for me. Is it different from the one I posted?

I have another question. On the schematic, the bias circuit includes two 10mf 100v caps, 2 resistors, a diode, and an .047 cap. My board has the same components, but instead of the two 10mf 100v caps there's one 80mf 100v cap. Does that make sense? Seems weird to me. I have an original service manual for this thing, and the parts layouts don't match my amp. Anyway, I'm going to replace the 80mf cap. I assume a 50 year old electrolytic would screw up the bias too.

[neddyboy]

You're right of course. In my defense, the resistors I replaced were in rough, overheated condition. I would have replaced them anyway. Replacing the capacitors? Sure, total shot in the dark.

[Chuck H]

I'll back the points on accessing bias voltage before continuing. Maybe you need to use the amp? I dunno.?. But if you run it without stable bias voltage to take care of your power tubes you run the risk of turning the need to pay for new filters into the need to pay for new filters and power tubes! Maybe more too.

80uf would be higher than I usually see for a bias cap. It's possible someone put it in there because it's what they had or because they thought slamming a shit ton of uf's on the supply would stabilize it. DO return it to a stock circuit. But be absolutely sure that your circuit topography matches the one you're basing your parts selection on. If it doesn't, find a schematic that DOES match and use the component values shown there.

I wouldn't plug power tubes into the amp before this is accomplished and stable bias voltage is witnessed.

[neddyboy]

I'll be measuring voltages this weekend, but the power amp PCB in my amp doesn't match the PCB diagram in the service manual I have. All the schematics available online also show a pair of 10mf 100v caps in the bias circuit. Mine is different. The board has no space for the two 10mf caps C14 and C21. There is no spot for C21 and C14 is the 80mf cap I mentioned above. I looked closely at the solders and the age of the cap and it seems original. I think I must have a different rev than the Rev D schematic that's online.

[Chuck H]

I think I must have a different rev than the Rev D schematic that's online.

Very possible. There are forums dedicated to Ampeg. Maybe someone here would recognize your board, but I know someone on one of those forums would. If you post a gut shot here and on an Ampeg forum maybe we could identify your amp circuit.

[neddyboy]

I feel like I should apologize for this, but replacing that 80mf 100v cap in the bias circuit fixed it. I have KT88s in it now and it's louder than god with no glowing plates. I'll still measure the bias voltage tho just to be sure. Thanks for the help guys!

[Leo_Gnardo]

I feel like I should apologize for this, but replacing that 80mf 100v cap in the bias circuit fixed it. I have KT88s in it now and it's louder than god with no glowing plates. I'll still measure the bias voltage tho just to be sure. Thanks for the help guys!

Good news on that easy fix. What's the old quote from the radical 60's? "Don't trust any (capacitor) over 40 years old." something like that... KT88's, what a hoot! Do you have a power reading on your VT-40 with those behemoths in there? Also, measuring the bias current tells us more than the bias voltage. I'd expect about 40 to 45 mA apiece ought to do right for '88's. FWIW I usually use 7027A at 30 to 35 mA, and get 55 watts at clip.

[neddyboy]

I'll be doing all the measurements this weekend, but I'll let you know. The KT88s are just placeholders since I had a set lying around. When I can afford a set of nice US tubes I'll switch back to 6L6s. The 88s are likely running cool since I haven't re-biased. It's loud, but not insanely loud and I don't like KT88s for guitar. They're spares for my V4B. which has been re-biased.

Yes, bias current, slip of the keyboard!

[Chuck H]

I might fact check that 80uf value anyway.?. Excessively large bias caps take time to charge and are reported to be less than ideal for other reasons too. Trace your bias circuit topography. Is it the same as other VT-40 schems? Do you have a 10uf cap in there for one of the two bias circuit caps? If so I might change that 80uf to a 10uf.

[neddyboy]

I didn't have an 80mfd cap so I put in a 40mfd. I was thinking a lower value would be better, but that 80 was definitely original so I didn't want to go down to 10 right away. What would the consequence of too high a value look like? The 88s are definitely throwing a lot of heat! I don't have much basis for comparison since I've never run them in a guitar amp.

[The Dude]

Have you measured bias current since you got the bias supply circuit working?

[neddyboy]

Not yet, too much actual job work, but I'll be working on it this weekend. I'll keep you updated.