Ampeg VT-40 Redplating Solution?

[g1]

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?

The one you posted is a re-drawn unofficial schematic, which have sometimes had errors.
The one I posted is an official factory version. I'll add a working one to this post, even though it does not match your amp.

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.

Sounds like for your version they went to the VT22/V4 type bias circuit. ( http://www.ampegv4.com/images/schematics/V4.jpg )
When they use only one cap instead of the 2 with resistor, they use a higher value.
The V4 uses 100uF, I think your 80uF must be correct.
P.S. The V2's run even higher plate voltage than the V4's. In both models they get away with that by biasing quite cold. I would not recommend biasing any warmer than the stock value.

[neddyboy]

This my first time working on a whole bias circuit rather than just a couple resistors. I don't understand how that 80mfd cap influences the bias setting. Is the 40mfd cap making the tubes run hotter than the stock 80mfd?

[Leo_Gnardo]

This my first time working on a whole bias circuit rather than just a couple resistors. I don't understand how that 80mfd cap influences the bias setting. Is the 40mfd cap making the tubes run hotter than the stock 80mfd?

Ampeg at some point chose to change from the 2 stage filter, pair of 10 uF caps, to the "brute force method", one whopping big 100 uF cap. Yours being 80 uF, close enough, maybe somebody put in a fresh one years ago. And even with 40 uF, no harm except possibly hearing a little hum/buzz in your speakers.

What's probably making your KT88's run hot is you haven't adjusted the bias voltage to a point where the output tubes pass "just enough but not too much" current, while no signal is applied. That is the essence of setting bias. Ampeg, along with lots of other amp companies, left out a variable bias control but there's no reason you can't add a trim pot to your circuit so you can set bias for your output tubes now and in the future.

Additionally you could add bias current sensing resistors so you don't have to stick your meter probes on high voltage nodes. If you want to get fancy also add some test points on the chassis so you don't have to unass it from the box every time you want to check/adjust bias.

FWIW I find 7027 or 6L6 tubes in these amps to run best with a bias voltage in -52 to -58V range. I expect you may need a bias voltage -60V or a bit more to control KT88s. And the '88s are larger tubes, draw more filament current, small wonder they heat up more.

[neddyboy]

I picked up a 100mfd 100v cap on the way to work, so that'll be going in this weekend when I try to get the bias set.

[neddyboy]

I swapped in the 100mfd cap and the power tubes just started to glow before I shut it down. The 40mfd is back in for now.

[g1]

They started to glow red?

[Leo_Gnardo]

I swapped in the 100mfd cap and the power tubes just started to glow before I shut it down. The 40mfd is back in for now.

Electrolytic caps have a polarity. If you put the cap in backwards it will not sustain voltage across it. I can't imagine any other way swapping in a 100 uF cap would cause a problem, other than mounting the cap backwards. Plus, it's always a good idea to read the bias voltage after making any changes, without the output tubes in place, so you don't wreck them due to a mistake.

Have you made any bias measurements, voltage or current?

[Chuck H]

Perhaps the problem isn't the cap value? Maybe there is a cold joint elsewhere that is intermittent and playing foolies on us.

Though I can't technically say why, it seems to me that a cap coupled bias circuit might have more difficulty charging the higher value cap? Otherwise it's strange that one cap "works" and the other one doesn't.

[Chuck H]

Plus, it's always a good idea to read the bias voltage after making any changes, without the output tubes in place, so you don't wreck them due to a mistake.

THAT^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^

[neddyboy]

20 years ago my brother in law put a cap in backwards on a tube amp we were working on. It went off like an M80. Since then, I've made that mistake only once, and that was in an effect pedal running 9v. Just a little "pip".

I know I know I still need to measure the bias. I swear I'll do that this weekend. Hopefully a spot on the plate starting to a turn a bit orange won't ruin the tubes, fingers crossed.

[g1]

The reason it's more common with the bias caps is that they are supposed to go in with positive end grounded, which is 'backwards' to what people sometimes expect.
So it's usually one of the first things we look at with bias issues.
If it worked with a 40uF, nothing else changed but installing a 100uF, then maybe that new cap was a dud.

[Chuck H]

If it worked with a 40uF, nothing else changed but installing a 100uF, then maybe that new cap was a dud.

And the OLD cap was a dud. Every cap is a dud except that one 40uf cap.?. I know, I know. It's that coinkidink thing. Happens all the time. It's something we watch for as a matter of course.

[g1]

Chuck, I agree with you that some intermittent problem may be more likely what's going on here.
I hope it will manifest itself and he can get it sorted out without the tubes installed.
My point was that cap value here is not an issue as far as loss of bias. But if loss of bias is because of the cap (installed correct polarity), it's because of a bad cap, not the value.
Sorry I didn't explain that better above.

[Chuck H]

Not at all. I got that. I was trying to back your play as the "bad cop" in the routine Something is "afoot" that probably isn't the cap value. Bad cap? I know old caps can be bad caps. In fact I threw out all my old electrolytics about two weeks ago because I never use them in favor of my reliance on new caps. So these old caps have been hangin' around and it was time to discard them. My point is that if a cap like this was used it could still be the problem and the 40uf cap was simply fresh and therefor operational.?. Even buying electrolytics can be iffy. I had a run of bad Atoms from Mouser before Atoms were all the rage. My guess is that stock wasn't rotated or the (at the time) five month expiration date wasn't observed. I don't know, but I don't use Atoms anymore because they have had some other iffy press and were, after all, the common denominator. Anyway... Old, unused electrolytic caps=bad.

[neddyboy]

I'm going to measure bias today and am planning to use the cathode resistor steps on this site: How to bias your tube guitar amplifier. I've only measured bias on two amps, and they both had adjustment pots. This Ampeg doesn't, and it uses individual bias resistors for each tube. Do the steps at the link above look right to you guys? I'll admit to being a little nervous about all the voltage in the very confined space around the power tubes! I also own a V4B but I've never had to work on it thank goodness.

So, to adjust the bias as-is I'd need to try different value resistors, yes? If I want to have adjustable bias, would I have to install an adjustment pot for each tube? Would .5w trimmer resistors be appropriate?