Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: svejkist on March 11, 2019, 03:56:52 pm
-
I built Hoffman AC30 in the carcass of a Peavey Classic 30, using the Peavey transformers. The only thing I really changed was the rectifier, which is a solid state bridge. A local guitar player fell in love with it, and asked me to build him one. The C30 PTs are cheap and easy to come by, and supposedly have been revised to be ore robust. So I said sure and went with that tranny. The C30 uses a funky heater circuit, and it was impractical to get the voltage and current I wanted, so I added a heater tranny and went with orthodox parallel heaters, DC on the input tube. the heaters are dead quiet. the amp actually sounds fantastic, but there is a nasty 120Hz hum. Moving the volumes just introduces a slight hiss when dimed. Bass has a tiny effect when on 0 and treble a tiny effect when on 10. Cut makes a significant difference (more on that later). Removing the preamp tubes has no effect; remove the PI removes the hum entirely, which combined with the impact of the Cut, leads me to believe the issue is with the power to the PI. Swapping the PI tube didn't help. So far I have:
1) Chopsticked. No changes with any lead manipulation.
2) In the PS- Verified B+ leads and joints, and replaced all the filter caps. Verified the PS grounds. I did not replace the decoupling resistors, but I did verify their values. Bypassed the choke with a resistor.
Voltages are
B+1 310
B+2 297
B+3 269
B+4 260
B+5 265
3) Checked values in the PI.
4) Checked for resistance on the signal ground and found none. After my PS investigation found nothing, I went ahead and changed signal grounding schemes anyway. Now whole preampn is grounded through the Topboost input jack. The PS, including the tail of the bridge, and the output section, are grounded to the original star ground.
5) Pulled the heater tranny and moved it around, in case there was some kind of coupling going on.
None of this has made any difference at all. Anyone have ideas about further troubleshooting this?
Thanks in advance.
-
How sure are you that its 120hz?
How are you grounding the new filament heater transformer?
-
The tuner app on my phone says 120hz. The filament tranny is center tapped, and grounded to the same star lug as the B+ rail. I didn't mention it because I would expect that to be 60hz, but come to think of it, the DC on the first tube would b e 120hz. It isn't that though. I rewired it AC and nothing changed. I lifted the center tap then too, and could barely hear the 60 underneath the roaring 120.
-
Use a gator clip lead to ground the junction of R19, R20, and C9. Does this kill the hum?
Use a gator clip lead to ground the junction of R23, R24, R25, and R26. Does this kill the hum?
Reflow all the solder joints in the PI circuit including all pins on the tube socket. Pay close attention to the ground connection of R26 and C12. Any better?
Did you use a Hoffman board?
Show us some hi-rez pics of this build, especially the PI circuit. Need to be able to read resistor values.
-
Also layout of the connections between rectifier "-", main cap, and amplifier common ground.
-
OK, I feel dumb. Rookie mistake, that I just kept glossing over. I had grounded the tail of the rectifier to the end of the output section ground rail, instead of directly to the star point. I realized this as I was drawing a layout of the power section for PRR. I make so many power supplies, I don't bother drawing layouts. And this time, I had a brain fart. Sorry to waste all your time, and thanks for the help. Especially PRR, for making me realize what was staring me in the face. Man, I got a lot of solder sucker splatter to clean up now.