Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: terminalgs on November 15, 2011, 07:34:57 pm
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My friend's Reverend Hellhound needs some work. He was playing at full volume (guitar straight into amp), volume dropped and now it only fizzes at a low volume. he swapped power tubes and it was the same.
We pulled the 6L6s and did a basic ohm test from both plates to the CT of the OT. It was 6ohm on one side and 82ohm on the other side. sounds like the OT is fried.
* the screen resistors measure 470ohm okay (486v on the screen)
* the negative bias seems okay, tho I don't know exactly what it should be (-52v at the grid)
* in 60w mode, that cathode switch is open and the 47ohm resistor is the cathode resistor (instead of short to ground), it measures +46v.
* his speaker isn't open, or shorted (measures 6.6ohm).
So I can't tell what might have caused the OT to fry? Any ideas? He has a Merc Mag OT on order, and I'd really like to have a smoking gun as to what might have caused the original OT to die. Its possible that it was always riding at the edge of what current it could handle and gave up the ghost (after all, we reasoned that the original OTs must die if Merc makes replacements, and they only made about 1000 Reverend amps all together,, plus failed OTs seem to appear in forums when Hellhounds are discussed.)
installing 2W or 3w screen resistors is probably a good idea in general (Kager and Naylor used the same design on later amps and bumped it to 2W on those designs), but these measure ok, so I don't think they caused a problem.
Schematics aren't available, so I drew this one (its very close to the Kingsnake FWIW).
http://i.imgur.com/73beY.jpg (http://i.imgur.com/73beY.jpg)
(http://i.imgur.com/73beYl.jpg)
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...He was playing at full volume...
So I can't tell what might have caused the OT to fry? Any ideas?
Playing hard is always a valid possibility.
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Does the amp have an additional HT/B+ fuse? If not, I'd keep the screen resistors at 1W. Although it's not perfect engineering, in case of a screen fault the resistors should 'fuse' to prevent total meltdown.
As far as the OT is concerned, I don't know. Could the speaker cable have been intermittent, or wiggled loose? I don't agree with Sluckey that an OT will fail from being run flat out alone, especially if no tubes fail in the process. I'm confident the Hellhound's OT was not underrated as well.
I don't think you've confirmed that the old tubes are okay, and you should do that before replacing the OT in case there something else wrong that'll make the OT go down again.
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Does the amp have an additional HT/B+ fuse? If not, I'd keep the screen resistors at 1W. Although it's not perfect engineering, in case of a screen fault the resistors should 'fuse' to prevent total meltdown.
No additional B+ fuse. so, you mean the resistor over-currents, exceeds 1W and fails open?
Could the speaker cable have been intermittent, or wiggled loose?
Its in a combo with a hellatone 60, I don't *think* there is an issue with the speaker wire and connection, but I will double check that.
Old tubes are not going back in the amp. we'll get new ones...
here's a gut shot and a look at the OT. anyone see an OT like that?
http://i.imgur.com/NspRh.jpg (http://i.imgur.com/NspRh.jpg)
(http://i.imgur.com/NspRhl.jpg)
http://i.imgur.com/FmDZm.jpg (http://i.imgur.com/FmDZm.jpg)
(http://i.imgur.com/FmDZml.jpg)
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Are you certain the OT is fried? I've seen fairly wild DCR readings before, especially if the speaker was still connected. I would test it properly by disconnecting the speaker (making sure it's not a shorting jack) and injecting a small AC signal into the primaries. Measure Vin and Vout to calculate the turns ratio, then calculate the impedance ratio which is TR^2*Rl. Did you check for proper AC signal at the grids of the outputs? I fixed a Bassman that did what you described. It was a busted ground wire to the LTPI.
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"..We pulled the 6L6s and did a basic ohm test from both plates to the CT of the OT. It was 6ohm on one side and 82ohm on the other side. sounds like the OT is fried. ""
guilty as [dis]charged, IMHO. I support all your thought processes as you've explained them. Maybe the Chinese OT just gave it up.
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> 47ohm resistor is the cathode resistor (instead of short to ground), it measures +46v.
This stands-out as very odd.
46V across 47 ohms is One Ampere. The combined current should be more like 1/10th of that. Meter-reader error? Or am I not understanding where the 46V is?
It is remotely possible that a pair of zero-bias 6L6 could pull 1 Amp at these voltages. However this is also 46 Watts in the cathode resistor, and I do not see any 46 watt resistor in there, so there ought to be smoke.
Four-point-six volts would be a very reasonable reading on the 47r resistor.
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No additional B+ fuse. so, you mean the resistor over-currents, exceeds 1W and fails open?
That's correct, and if none of them are open it likely means there was no screen failure. If they don't open and there is a screen failure and no B+ fuse then it's usually by-bye to the PT and smoke. Not your case, it seems.
Its in a combo with a hellatone 60, I don't *think* there is an issue with the speaker wire and connection, but I will double check that.
I was just trying to think of what could kill an OT with an amp run full bore if there was no tube failure... Do the plates (grey structure inside the tube) look burned at all? Have any of the tubes lost the silver appearence and turned white on top (lost vacuum). I'd really try to rule out a few things before sticking new tubes in that bad motha.
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here's a gut shot and a look at the OT. anyone see an OT like that?
Double C-core. Surprising in a MI amp...
I've seen Ten Pao low grade iron in some linear power supplies, didn't know they also made OTs.
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> 47ohm resistor is the cathode resistor (instead of short to ground), it measures +46v.
This stands-out as very odd.
voltage read error!!!
it is more like 1.5v !!! in the other position,, when the switch is closed, I measured 46.4 mV DC !! (1) I didn't realize the switch was closed, and (2) I didn't notice the little "m" appear.
good call, thanks.
As for the tubes, they look okay. no strange discoloration. I've got some known good but used tubes I'll try before i try new tubes.
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>6ohm on one side and 82ohm on the other side.
Was the speaker disconnect and the secondary not shorted? If the speaker was still wired in, those readings might be just fine. DCR is not the best way to test an inductor of any type.
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>6ohm on one side and 82ohm on the other side.
Was the speaker disconnect and the secondary not shorted? If the speaker was still wired in, those readings might be just fine. DCR is not the best way to test an inductor of any type.
with tubes pulled, OT CT lifted from B+, and a speaker plugged in. consistely 6ohm and 82ohm.
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Did you check for proper AC signal at the grids of the outputs? I fixed a Bassman that did what you described. It was a busted ground wire to the LTPI.
good idea.
This evening I fed a signal to the input jack, and followed a clean signal (with o-scope) all the way to both grids of the 6L6 sockets. signals at 6L6 grids were appropriate size and phase.
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What about the "nose" test?
does the tranny smell like the smoke was let out?
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replacement transformer from Mercury was installed and fixed the problem. Thanks to all that commented on the thread and provided advice and assistance!!