Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 1Ab on August 17, 2025, 08:37:24 am
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Hi all,
I am new to this forum, I registered because whenever I want to know something about valve amps I always end up here. I figured this might be the best place to ask this question.
I recently acquired a "broken" valve amp, its a 5w amp using a 12ax7 for preamp and a el84 for power amp.
There are 2 burned out resistors (burned beyond recognition) and I am currently trying to identify them.
R13 and R14 are cooked.
See attached picture and unfinished schematic I am working on.
Any help / suggestions would be greatly appreciated.
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Schematic
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Pcb
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make it less fuzzy, higher res, n show both front AND back
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make it less fuzzy, higher res, n show both front AND back
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make it less fuzzy, higher res, n show both front AND back
Hopefully these are better
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ain't got time now, here's a side-by-side, trace the foil runs for the 2 R's, using ohm meter to verify you traced the right foil, i "color" the runs so they are clear n easy to follow.
did you poke a solder pencil around on the board?? there are some really bad joints
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That amp has no brand ?
Franco
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That amp has no brand ?
Franco
It's a Herley benton mighty-5th
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Now that we know what amp you're dealing with, I'm willing to bet the circuit is very much like the Harley Benton GA5 - same tube arrangement with SS rectification.
Here's the schematic for that:
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Going by that schematic, then your R14 would be 220 Ohm and 4W, and R13 would be 2.2K (if that goes with the 12AX7 cathode).
Another question is why did those burn up to start with? I expect you'd want to prevent a recurrence.
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why did those burn up to start with?
guessing the 84 shorted, can't really prevent a tube from givin up, you can prolong it but in the end.....Techs :icon_biggrin:
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ain't got time now, here's a side-by-side, trace the foil runs for the 2 R's, using ohm meter to verify you traced the right foil, i "color" the runs so they are clear n easy to follow.
did you poke a solder pencil around on the board?? there are some really bad joints
I marked up the pictures but it will not let me add the picture no matter what I try. :BangHead:
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Now that we know what amp you're dealing with, I'm willing to bet the circuit is very much like the Harley Benton GA5 - same tube arrangement with SS rectification.
Here's the schematic for that:
Thanks for the schematic, looks promising.
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why did those burn up to start with?
guessing the 84 shorted
I checked the continuity of the pins and only the heaters have continuity so not sure if a shorted EL84 is actually the problem.
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marked up traces
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Can this be of some help ?
(https://i.imgur.com/szpF06g.jpeg)
Franco
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R14 is the cathode R for the 84, to take out that R you need copious amounts of current, the 84 should NOT draw enough current to smoke the series R without something in the series path drawing way more than design current.
R13 is the oddity, I woulda expected R12 to flame out before R13
I would replace both R's with schematic values, Verifying the values with meter, then I would OHM each of the 3 Taps A,B,C to chassis ground (NO tubes NO power)
you should expect to see "charging" ohms, starting at many K-ohms to probably infinite ohms (DO NOT use continuity, use ohms)
post results
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Can this be of some help ?
[img width=1024
Franco
That's really helpful thanks
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UPDATE
I did contact Harley benton last week and asked them what the resistor values are, knowing it was a long shot.
But today they replied and confirmed, R13 and R14 are both 220 ohm 3 watt.
I will replace them and then update what happens if they burn out again or not.
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Update 2
I replaced the resistors, powered up the amp without valves and all seemed fine no burning.
I then added the valves and as soon as I powered up the amp the el84 started arcing blue inside the tube. I assume its cooked. Going to source another el84 and try again.
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Got some new EL84s plugged one in and the amp worked straight away.
Thanks all for your help its much appreciated.