Harmony H400A- Series Filament Nightmare

[nihonlabtiger]

hey.  First post here.

I don't expect most of you saw my intro post, but I've only been into tube amps in a serious way for a few months now.  I'm not totally unfamiliar with electronics though, I've built some pedals and been an Uncle Doug watcher for a couple years.

Since January I've brought a '61 6V6 Reverberocket back to life and built the Sluckey PeeWee Hammond, but the project that I haven't been able to tackle is a Harmony H400A.  Seems like it should be easy since it's the simplest amp with the lowest parts count that I've messed with yet.

When I got it, it had been effed with already.  Going by the schematic it was stock, besides new electrolytics- one of which was installed backwards.

I installed a Triad N-51X isolation transformer, and placed the volume pot after the first preamp stage.  The problem I'm having now is a sort of ring-mod sounding distortion at all volume levels. 

I've read that this can be caused by DC leaking into the signal path and being modulated by the guitar signal, which to my noob brain would indicate a bad filter cap (even though they're new.)  But!  After reading the specs on the isolation trafo I've installed, it sounds like it might not be up to the task.  I believe it was rated for only 35 VA.  Maybe that's the source of my problems?  It doesn't seem to be getting hot, though... hardly even warm.

What's the first thing you guys would look at?

[PRR]

The factory schematic claims 25 Watts. A quick figuring says 22 Watts. Anyway if the PT was too small it *would* work, until the smoke came out.

Hate to state the obvious, but: voltages!!

The 400's grounding was complex to cheat the safety regs. How much of this has been undone?

[nihonlabtiger]

voltages.  right.

here's a schematic I've been referring to: https://karaslittlebrother.tumblr.com/harmonyh400a

I didn't do any of the mods that he did; just inserted the isolation transformer in the mains path.

wall voltage on the primary- 125.0 AC
secondary-                          110.6 AC

35W4 pin 3-                         94.4 AC
               4-                        55.5 AC
               5-                        109.5
               7-                        114.6

12AU6 pin 1-                       -0.6                                  50C5 pin 1-             1.8
                2-                        0                                                   2-             0
                3-                        11.1 AC                                         3-              55.4 AC
                4-                        0                                                   4-             2.2 AC
                5-                        57.9                                              5-              0
                6-                        20.7                                              6-              90.1
                7-                        0                                                   7-              109.

B+1-                 114.6
B+2-                 90.0


Like I said, I'm a noob at this. I could inundate you all with questions about these numbers, but I'll hold off for now in case the problem is obvious.  The only other thing I will state is that where the link above shows -8V at pin 1 of the output tube, I have 1.8V and -11.3 on the other side of the cap+resistor to ground.

As far as changes to grounding, the only thing I've done is remove the parallel cap/resistor that separates the cathode and suppressor grounds of the preamp tube from the main ground bus.


               
               

[PRR]

Power tube cathode voltage is so wrong I suspect typo or mis-read.

[nihonlabtiger]

mine or the one from the schematic?  haha

I mean I'm not exactly merlinblencowe dot valvewizard dot uk, but; pretty sure the cathode is supposed to have a positive voltage compared to the grid.

um... right?

[PRR]

"1.8V" does not seem right for a 50C5 cathode. 7.5V sticks in my mind, similar to the "8" on the plan.

So is it 1.8V or ???

[nihonlabtiger]

yep, just got home from work and double checked it.  1.93V.

I'll check the cap+resistor to pin 1 at some point tomorrow- i work in a hospital lab and all this covid bullsh*t has creamed my crackers. I'm going to bed.

Anything else I should look at while I'm in there?

[nihonlabtiger]

p.s. the schematic looks like it says negative 8V; that's what was messing with my head.

[DummyLoad]

by data sheets as PRR suggested, 7-8 volts there would be expected - a measurement of 1.9V is a problem. have you tried a new 50C5? i believe that the schematic is actually stating 8V - the modified schema reflects that fact and 9V is stated at the cathode of 50C5 - see attached. check yer output tube, the bypass cap, or any wiring changed.


--pete

[nihonlabtiger]

I've only got one other 50C5 at the moment, but it gives a similar k voltage.  Both are same brand and vintage so could be that they're both bad.

I'll check everything you suggest when i get outta work tonight.

[nihonlabtiger]

took the cathode resistor and cap out and tested them; they tested fine.  33uF and 150.5 ohm.  I don't know how reliable my cheapo Banggood transistor tester is, but it gives an ESR of 0.89 ohm and VLoss of 1% on the cap.

I'm thinking next steps are to do a full redraw of the schematic and order up another 50C5.

 Ugh, I don't enjoy spending this much time on such a stupid little amp, but I guess this is a good one to learn on.  Glad nothing like this happened on the Reverberocket, haha