Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tommyzcat on December 11, 2022, 10:20:10 am
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1st build guidance needed:
Advance apologies.
1st tubes out turn on went well. Realized my ExTech multimeter wasn't up to the task. Hubris had me do a tubes-in test before getting a meter up to the task.
As soon as standby turned off I heard the cab make a rippling sound and was thinking //is this the "motorboating" everyone talks about when OT leads are backwards?//
I tried to switch between Tube and S.S. rectification in my blind-man's search to isolate the issue. No real change.
After about 3 minutes I saw a tendril of smoke from the area of the presence knob and as I was looking to see which component was dying there was a percussive bang and flash.
I turned off amp, let the caps drain a bit and opened the rear.
It was obvious that R36 (1k/3w) was toasted and the nearest 10uF/100V C13 had blown its guts. [YES! - all polarities of caps and diodes were (and are) correct to layout/schematics]
https://el34world.com/Hoffman/files/Hoffman_5F6A.pdf
A few things I quickly felt shame for:
1) being an over-confident idiot and not going through proper procedural tests.
2) I had forgotten to reflow R27 220k after making the under-board connection. I had meant to do it before mounting board in chassis to ensure reflow wouldn't affect that connection while it could still be visually verified (see point #1)
So now I have
A) Reflowed R27.
B) Replaced C13 &C14.
C) Replaced R36.
Without a doubt, I will proceed to tubes removed testing and further rechecking for shorts or mistakes.
My 2 questions for the forum are:
1) was bad connection to R27 or the underboard connection to bias responsible for destruction of R36 and C13? R36 was glowing like the sun before C13 exploded. My gut reaction is that something else is amiss.
2) what else could have been damaged beyond the destroyed R36 and C13? Should I worry about the bias pot and diode?
Any and All constructive comments are appreciated.
"But if I do not strive...who will?" - Lao Tzu
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Congrats on your first go!
I'm not sure I will be of any help, being a newbie as well, but I'd like to see your schematic so I'm sure which component ID's you're referring to.
Thanks!
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For ‘R36’ etc to be meaningful, we need to be ‘singing from the same hymnsheet’, ie note the footer of sluckey’s posts; please provide a schematic link.
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Sorry all.
Was referring to Hoffman 5F6A build. Should have linked the schematic earlier (it's combined with layout and turret board info as well)
https://el34world.com/Hoffman/files/Hoffman_5F6A.pdf
Also a photo of the one I printed out which may be easier for some.
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Also a look-see at the rest in the state the build currently resides:
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"Glowing like the sun" , to me, might mean lots of current flowing directly to ground. Could have been your underboard connection if it was contacting the chassis somehow. Maybe check all those downstream connections with your multimeter to make sure you're seeing the resistances you expect at each point in the bias circuit.
I assume you know you are dealing with deadly voltages - BE CERTAIN YOU KNOW HOW TO WORK IN YOUR LIVE AMP SAFELY!! :)
If you have established through power-off testing that the bias circuit seems sound, and you start it up on regular power without tubes, then it may be useful to confirm that the DC measurements at all filter nodes, bias points, and all tube pins, are comparable to expected values.
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> R36 (1k/3w) was toasted and the nearest 10uF/100V C13 had blown its guts. [YES! - all polarities of caps and diodes were (and are) correct to layout/schematics
On a 50VAC bias tap, a 1K R36 would burn like 50^2/1000 or 2.5 Watts, less than 3 Watt rating, won't die for a year.
A possible mistake is connecting R36 to an END instead of the TAP of the PT winding. C13 would charge toward 500V, and being a 100V part, will be unhappy.
Not connecting R27 properly is just careless, risks burning a $50 6L6 (or have they gone up this week?).
Yes, you may have paid tuition on more than one valuable(?) lessons.
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Yes. Maybe killed a perfect pair of Tung-Sol 6L6GC STRs. Maybe did damage to V6 Rectifier.
I went over connections (visually, not physical stress-testing) 3x before tube-out test. The root of my over-confidence :/
And Yes, I am aware of the lethality of the beast and the craft. I am following R.R.'s method of draining mains caps to ~>30V before even opening the rear (which is metal mesh tied to chassis ground). And work with gloved hands and with only one hand (getting better all the time with this practice).
Multimeter test should easily show if it's the single underboard wire that's shorting.
God help me if it is, as I will have little choice but to unwire nearly all to get that board to come out @ this point.
I did use taller standoffs than what came with the kit (as I did w/ many resistors - in favor of higher wattage) but not nearly enough to allow me to fish the wire out in favor of an over-turret route [likely much noisier as the filament windings are so near]
Replacement 10uF/100v are Sprague TE1407
PT is MM Tone clone FTBP-58 [Red lead w White Stripe is connected to "top" of the sunny resistor which is adjacent to diode and blown cap]
Thanks again for guidance thus far.
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You should be able to remove the mounting screws from the board and lift it just enough to snip the single underboard jumper as close to each turret as you can. Then reinstall the mounting screws and put that jumper on top of the board.
Where does the PT red/yellow wire connect?
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It’s a really good idea for initial power up to be via a light bulb limiter.
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Multimeter says no chassis ground on under-board jumper.
Positive connectivity from bias pot turret to jumper turret (shared w/ 220R).
PT red/yell-stripe lead goes to 500mA inline fuse then to ground point I drilled approx 1" from PT bolt.