Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Anticpunk on August 13, 2018, 10:31:14 am
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Hey all! I've just joined up here after finding a lot of great info in the past.
I've been working on amps for a few years now, but I just hit a wall on a HRD3 and I'm hoping maybe somebody can get me over this hurdle....
The original issue:
The amp came to me with the complaint that it periodically makes a couple of pops and then a very loud hum. Upon inspection I can see that somebody blindly swapped one of the GT 6l6gc (presumably replaced a failed tube) but didn't check screen resistors (one was blown open) and never even cracked the glue on the bias trimmer. Tested all of the tubes on my 2 tubes tester sets. All test ok.
Replaced the screen resistors, installed the bias probes and fired it up. A little hissy, but not bad.
Then I start hearing static and crackles. Sounds like plate resistors, so i swap those out and resolder the main pcb while I have it apart. Power up and start checking test points. Everything looks good against the schematic.
Then it happened.. p....p....hrrrrrrrooonnnnnnnnn.... full power 120hz groan, like holding your finger on the tip of a connected input cable with the whole amp cranked....
Bias voltage went from -44 to -56, plate voltage dropped from 408 to 312, cathode current on both tubes shoots up to 128-130ma.. do the old emergency shutdown...
No smoke, no abnormal heat, no smell....
Try it again, but this time while it's running away I pull the phase inverter tube and it settles back out to normal. Tube swap the PI, same outcome. Different powertubes, same outcome.
So now I'm figure maybe the resistors on the ltpi are getting wacky under load, so I shotgun the phase inverter (resistors and coupling caps) and desolder and resolder all of the sockets.. Still does it.
I inserted a cable into the power amp input to cut off the preamp. Same thing happens.
Scoped the b+ rail, only a tiny tiny bit of ripple (though my scope could be better, admittedly). No cap swell or leakage and then don't get but barely warm after it runs for awhile. Scoped the phase inverter section, but don't see anything odd in the waveform there.. every now and then you can hear a ticking, like the trem bug tick in an old fender, but that may be on the line in the house...
At this point I'm thinking either a parasitic oscillation or an intermittent filter cap, or an intermittent short in the opt maybe(?), but I'm looking for insight before I take any further plunge.... has anybody seen a case like this?
Any help is much appreciated!
Jay
The updates:
I also for a split second fed a 400mv rms 400hz sine wave test tone into the power amp input which causes identical runaway conditions. 128ma on both cathodes. Turn the tone down to 10mv and it settles back into 28-30ma per tube... perhaps I should revisit the opamp in that line too as I would expect that part of the circuit to be normally swinging a lot more signal than that off of the stock preamp, no?
Opt tests 86 and 106 ohms relative to CT and 35meg between primary and secondary or ground with tubes pulled but leads connected, so I can rule that out
And now:
Replaced the 6L6s with brand new matched JJs, filter caps with new F+T units, replaced the bias feed and gridstopper resistors. Doesn't blat and hum anymore, but the B+ is low. On a cold start I get 408vdc, once things get running for awhile it settles down to 390vdc. Power tubes out i get 432vdc. Bias runs fairly stable with only tiny fluctuations in cathode current with no signal input. Trying to scope 6l6 grids for possible parasitics makes the cathode current on whichever tube I'm probing run up by roughly 8ma with no real change in plate voltage. PI in or out makes no difference, so if the low B+ is due to parasitic oscillation it has to be coming from the power section itself, but there is no path for local feedback there unless its thru the rail or across the opt...
I just cant see what I could be missing here....
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resolder the main pcb
is this a re-issue pcb amp. If so many poor/bad solder things, crappy sockets
I pull the phase inverter tube
this might indicate the issue before the PI
is there a pre out jack, if so, jack some headphones or small speaker there. probably have the PI and PA tubes out, just in case.
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Shooter,
Thanks for your reply.
It's a Hotrod Deluxe 3 from 2012.
All pcb and does have a preamp out as well as the power amp input.
Plugging into the power amp jack cuts out the stock preamp, so I'm confident that the issue is between the power amp input and the speaker or in the B+ rail. There is an IC buffer/gain stage, the phase inverter and the power amp section, and other than idle test point voltages, I haven't picked thru the IC part yet.
I desoldered and resoldered every single joint in the whole amp, and it didn't solve the problem unfortunately. I may take a leap and recap it. I have to finish the job and if the recap doesn't fix this issue, it will at least avoid them being a future issue anyway...
Thanks!
Jay