AC30 build driving me crazy

[rcrmck]

I've posted here before about a tone cabinet conversion to guitar amp that went very very well, but I couldn't remember the username I used. Anyway, I've been trying to build a Hoffman AC-30 using mostly new parts, with some recycled transformers/sockets/etc. At first test I was using the field coils of 2 old electrodynamic speakers as a filter coil. The results were thumping, loud hum on warm-up, no signal I could hear, and it all happened regardless of the volume. I swapped in an actual filter coil for the second test, and that stopped the thumping, but there is still a loud hum (except in standby it's quiet) and also I noticed I could hear a faint signal through the speaker that is almost drowned-out by the hum. I tried pulling tubes and there was no change by pulling preamp tubes or individual power tubes, but it was silent when I pulled the all power tubes. I was checking grounds and found the negative side of the speaker was open to ground! I hope I didn't kill the amp with an open speaker ground. I grounded it and checked again and it's still the same.

To sum it up:
1. Loud hum comes on as the tubes warm up regardless of volume setting or if there is a guitar connected. So bad I have to shut it off within a few seconds.
2. I can hear a faint guitar sound with the volume up, which may be slightly distorted, but its hard to tell because the hum is so loud.

And now my soldering iron died too

Help

[jhadhar65]

You'll need to provide more information on your exact build.  Detailed hi-res pictures would be nice, an accurate schematic is a must, and posting your voltages is the absolute first step.

[rcrmck]

You'll need to provide more information on your exact build.  Detailed hi-res pictures would be nice, an accurate schematic is a must, and posting your voltages is the absolute first step.

I'm using the Hoffman AC-30 schematic I got off the 'Tube Amp Library of Information' section of this site. I would love to get some voltages, so I can compare to examples I found, but because I'm afraid to let the loud hum go more than a couple seconds, I can't get anything other than the voltage of the heater circuit with it in standby. Maybe if I pulled the power tubes (hum stops when they're all out) I could at least get pin voltages from the preamp tubes. But would'nt having those tubes out of the circuit throw off the voltages anyway? Also, it seems that it would be in the power section, so I expect the voltages of the preamp tubes would be good anyway. Is there another way to get those voltages?

[plexi50]

With power tubes in: Pull the PI tube. Does the amp still have the loud deep hum?

[jhadhar65]

>Is there another way to get those voltages?

After you check plexi50's question, pull all the tubes and leave them out until you're sure the circuits are sound.

Drain the caps and start with simple continuity checks - grounds, power rails, etc.  Make triple sure your actual layout is exactly what it's supposed to be and matches the schematic you're working with.  Hoffman's layout is certified hum free.  Did you follow the layout plan or just the circuit schematic?

When you're sure all that looks right, use a lamp limiter and see if you have voltaqe where you should have it and not where you shouldn't.  Be safe and use a load (a speaker is fine).

Following that, put the tubes back in, keeping it limited, and try voltages again.  You're still not worried about absolute readings at this point - just big voltages where big voltages are expected, little voltages where little voltages are expected, and no voltages... you get the idea.

After that and ASSuming all is either fixed or checks out well, unlimit it and run voltages again.  This time you want to record them and you are interested in seeing that they're in the expected ballpark this time, i.e. +/- 20% or so.  Still have the hum?

Do some searching here looking for start up recommendations.  You'll find one or a few threads that will give you pointers and safety precautions you should observe.  It's possible you've cooked something already, but probably not.  Quality components available these days (such as you would obtain from that Hoffman Amps guy at the bottom top of the page) are pretty forgiving.  Just take your time and work through it a step at a time.

One last question - did you stick with the original power amp plan or did you add a negative feedback circuit?

[plexi50]

Any luck yet with the hum issue? I had a Deluxe reverb 2 months ago that had this loud hum even with the PI tube pulled. I swapped out the Fender branded tubes for some old Sylvania 6V6 tubes and the hum was completly gone. Very strange. I think i read some where that the PT was causing an oscillation and that the power tubes were interacting making the hum. It really does depend on the preamp or (power tubes) in your case that you are using

If all your PS caps and wiring is done correctly then you have to look else where
If changing the power tubes does not stop the hum then i would wonder if there is something internally wrong with the PT