Cooked power tranny? (Vibrolux Reverb)

[slidincharlie (Carlo P)]

Update and final (?) setting

Now the amp has the voltage selector on 240V (mains around 235VAC here), which brought the heater voltage down to specs. BUT this setup allows for lower than specs B+ (390V instead of 450V of silverface specs). With the 6L6 bias set at 40mA I am at 15.6W or 56% max plate dissipation. As long as I like the amp's tone, do the power tubes risk to be overbiased at such values? If not, I would be happy with the current setup since it sounds safer for the PT and should let the whole amp run cooler.
I'd love to hear a final comment on this current scenario.

[pdf64]

That's great, there's no danger from too much bias voltage (though your bias set up is fine).
If you're happy with the tone, go with it.
If you want to experiment, you could try a GZ34 in there to see how much more plate voltage that gives you - remember to re-bias though.

[slidincharlie (Carlo P)]

If you want to experiment, you could try a GZ34 in there to see how much more plate voltage that gives you - remember to re-bias though.

You mean like the blackface version of this amp... I have a new GZ34 here.
Isn't there any risk with a stock silverface PT, once rebiased?

[pdf64]

No, infact it will less less load on the rectifier heater winding (2A instead of 3A), saving 5 watts, though a higher B+ would draw a little more current from that winding, unless you re-bias. Peter.

[Dave Curtis, dB AudioTech]

No, infact it will less less load on the rectifier heater winding (2A instead of 3A), saving 5 watts, though a higher B+ would draw a little more current from that winding, unless you re-bias. Peter.

With the extra B+, and the static dissipation @ just over 15W with the 5U4, I'm thinking he'll have to rebias. I run a SS rectifier in my '76 Pro Reverb; no problem after a rebias.

Re: 15W dissipation: tube life will be extended. The VT22 I have came with the original Magnavox 7027s biased at around 14W; still going strong at 35 years old.

[slidincharlie (Carlo P)]

Odd behaviour from GZ34

Hey...
I put a JJ GZ34 tube in place of the 5U4GB and for starter I put an eye on the total current draw... In a few seconds the amp draw as much as 190mA and was still rising, then I shut it off.
The I turned it on again to check the plate voltage on the 6L6's and I got 430V (instead of the 390V obtained with the previous stock rectifier).
What can cause such a high current draw?
I haven't even checked the power tubes bias for fear of cooking something...
If you check my posts above, I had around 120mA total draw with the new (old actually) rectifier and the mains selector on 240V...

[pdf64]

Always turn your bias up full, ie to highest negative voltage on the pot, before fitting new power or rectifier tubes.
Then adjust bias to a suitable level.
Sorry if I took that as said, guess I should have noted it in previous post.

[slidincharlie (Carlo P)]

hmmm... I believe that something strange is going on here with the GZ34.
I rebiased the 6L6's to 40mA @ 430V on the plates, for 17W dissipation. Apparently an optimal scenario.
Yet the amp is drawing much more current (160 mA) than with the the stock 5U4GB and the power tubes biased at 40mA @ 390V on the plates.

Unless you guys can give me a possible explanation of this and redirect me to a different solution, I think I'm better off with the 5U4GB.
That said, if 400V on the plate (pin 1) of the tremolo tube are not an issue, I think I'm finished with this amp. Looking forward to road test it next Wednesday at a gig!
Thanks.

[MWJB]

"That said, if 400V on the plate (pin 1) of the tremolo tube are not an issue, I think I'm finished with this amp. Looking forward to road test it next Wednesday at a gig!" The Fender schems say measure vibrato tube voltages with effect "switched off at the footswitch & speed & intensity at full CW rotation".

If you had 0.0v at the cathodes the tube would be switched off & not working.

I don't know why 17W dissipation is "optimal", you seem to be consistently setting to the high side of normal (30-35mA, +/- 2-3mA).

Out of interest do you have paralel or series filer caps at the main B+ node, you say caps were done a few years a go, I'd doublecheck that they are wired correctly, whether it be series or parallel.

[slidincharlie (Carlo P)]

The Fender schems say measure vibrato tube voltages with effect "switched off at the footswitch & speed & intensity at full CW rotation".
If you had 0.0v at the cathodes the tube would be switched off & not working.

Vdc readings on the tremolo tube are the same irrespective of the footswitch and of knob rotation. For whatever reason today I have 360+ V on the V5 plates (40V less than the other day): maybe due to low B+ (370V instead of 390 of yesterday) and low mains supply (230V instead of 235+).
I have just re-checked the cathodes of V5 and I have 0.0V on pin 3 and 0.021V on pin 8 (unless my Fluke DMM went nuts).

I don't know why 17W dissipation is "optimal", you seem to be consistently setting to the high side of normal (30-35mA, +/- 2-3mA).

I am no tech, so I rely on techs' and experts' advice that I get on books and in reputable forums like this one. Some like you tell to stay in the 30-35mA range, others say to stay within 70% plate dissipation. With 17W and 56% of plate dissipation I thought I was in a correct setup...

Out of interest do you have paralel or series filer caps at the main B+ node, you say caps were done a few years a go, I'd doublecheck that they are wired correctly, whether it be series or parallel.

I replaced the old caps with new caps one by one, paying much attention to wiring... But who knows, I'll check again.

[MWJB]

"With 17W and 56% of plate dissipation I thought I was in a correct setup... " Well, there's no "correct" set up, zero current will stop the amp from working and too much current will burn up tubes/blow fuses...maybe damage OT. Anywhere between the 2 extremes that gives you the performance you like is "correct"

It's just a bit odd that you are, on one hand, worried about excessive current draw & on the other hnd you are biasing on the high side of normal? On a Fender with 6L6s i set to 30-35mA and have a listen...by all means if you prefer 40mA & you are within a reasonable plate dissipation, then that's fine...but bias it there because of the tone, not to meet some arbitary percentage figure. A 6L6 Fender will still sound ball park at 30mA, whether it runs 400vdc or 525vdc.

[Prairie Dawg]

Divide and conquer is the watchword for repair issues.

For power supply problems, the chief thing to be divided is problems internal to the power supply from problems external to the power supply. On tube amps, this is easy - pull all the tubes. Now put a voltmeter on a secondary and let'er sit until you either get the power turn-off or until you're tired of waiting for it to turn off and therefore conclude it was the amplifier pulling too much current.

If the transformer does not get hot and go open, the problem was past the rectifier tube.

So then put in the rectifier tube, but no others. This then adds the filter caps and choke to the loading. If the trannie now overheats and goes open, the problem is with the filter caps and possibly (I've seen this only once) a short from the choke winding to its core, or a wiring short in the power distribution network.

If the trannie does not get hot and go open, the problem was past the filter caps and choke.

Now put in the power tubes. Try it again. The power tubes or wiring to them could be bad and causing them to pull too much current. This could be a tube problem, bias problem or capacitor problem if it's emitter biased or screen component problem if it uses screen resistors and capacitors.

That set of debugging will get the vast majority of all power problems, as the preamp and PI tubes use only a tiny fraction of the amp's power, even if a tube is shorted.

If it's not one of those, you have a real rarity, and therefore an interesting problem.

That's basic troubleshooting theory, RG. I used it a lot in my former life as an aircraft engine mechanic/test technician.

The first thing to do is define the universe you're working in and work to limit its geography. In this case the problem is located in a 12x24 piece of real estate. Already things are starting to look up.

As my old crew chief always used to say, "Think 'system'" that is, visualize parts working together in a systematic way and not in isolation. Then, localize the problem within the system.

then, you can proceed to test variables and see where it leads.

Amps are nice that way because the power's organized in a linear fashion.

The 'interesting problem' is, indeed interesting, and I have found it to be almost always attributable to lack of a certain speck of knowledge.

One time it was a check valve the size of a pencil lead that is buried underneath a prop governor that appears on no schematic diagram. If a bit of crud holds it open you can't schedule forward pitch.

The other time it was a mis-drilled hole that hand been plugged with epoxy and redrilled. 700 hours later it blew out and the oil pressure dropped 30 pounds while the pilot happened to be looking at the pressure gauge. Massive freakout in the cockpit.

That one was harder to solve.

[Steve Conner]

I agree. Since I work with computers a lot, I use binary divide and conquer. That means cutting the "universe" as nearly in half as you can, to see which half the problem is in. Then cut the part containing the problem in half again, and repeat until you're down to the faulty component.

This is not so good for "systems" problems: with one of these, you might cut the system in half and discover that both halves worked just fine when tested in isolation, but when you join them back together the whole system fails. Even that tells you something, though.

[slidincharlie (Carlo P)]

It's just a bit odd that you are, on one hand, worried about excessive current draw & on the other hnd you are biasing on the high side of normal?

Actually I've never thought to an amplifier in terms of current draw... In this thread excessive current draw has been identified as an issue in this amp, possibly causing excessive heat in the PT.
To make things more complicated, voltages have varied quite a bit in the last two days within the same tube setup.
I have just tested again the two different recto setups, and this is what I got:
- with the 5U4GB = 380V B+, 35mA bias on the 6L6's and 133mA total current draw;
- with the GZ34 = 430V B+, 35mA bias on the 6L6's and 147mA total current draw.
If it was for tone I'd like the GZ34 setup better becuase the amp sounds a little louder, cleaner and bolder; in a few words I find it closer to its original silverface personality. BUT 147mA current draw I've been explained it's very high. Yet I haven't found a culprit for such high current draw. As reported in one of my earlier posts, almost all the current is drawn with only the rectifier and the power tubes installed.
Is there anything more that I can do to identify what is sucking all the current?

[MWJB]

You have checked tube sockets & power supply wiring.

How much current is drawn accross the standby with just the GZ34 in place? Then try with a SS rectifier if you have one. Do a similar test & see what is "normal" in another, similar amp. Establish a datum line. I would really worry about the current draw test if it was massively out of whack.

The vast majority will be drawn by power tube plates, preamp tubes don't draw much.

If heater voltage is OK you probably don't have anything to worry about, but 50v drop between the 2 rectifiers seems a lot (high side of "ball-park", could be down to brand/performance of individual parts).

If your PT is just a little overstressed/borderline, it might last for years & years, still run hot & the amp may compress a bit more.