Hoffman DR AB763 : low b+, hum, etc.

[Wil]

Hello everyone,
It's been a while ! I'm glad to see MEF is bigger and bigger every year ! As my guitar amp problems are, haha !
I rebuild my Deluxe to Hoffman's AB763 specs and have some problem.
It's an unusual design because I'm absolutely terrified by hum and noises and decided to shield every AC wire with copper tubes, haha. A bit radical as you can see.
Symptoms :
-Low b+ : 290V. Despite the low b+, the bias readings are fine : 23mV. (I use 6v6).
-Hums. Loudly. No guitar sound can be heard.
-Hums persists when the preamp tubes are removed
-I randomly measured some test point to ground and read 390VDC from red rectifier winding to ground.
-I get continuity between PT bias wire (blue and red) and ground. I desoldered it and measured and there's still continuity (don't know if that's normal...).
Then I resoldered it and got some higher-pitched "physical" (no speaker plugged) hum.
-It just fried my multimeter when I tried to measure the plate voltage after I resoldered the red/blue wire from the PT.
-I get 50ohm from heaters to ground.
-I double check every connection and I'm pretty sure everything is ok and at the right place (3rd Hoffman Deluxe Build).







I double-checked the PT and OT wiring and it's all good.
Changing the power tubes or the rectifier doesn't change anything.
Could this be the PT ? It's a Mercury Magnetics Axiom that was installed by the previous owner.
But the amp stood on my bench for 8 months without being touched so it would surprise me.

If you have any clue what the problem could be, please shine in :-)
Thank you once again.

[loudthud]

The 50 Ohm reading on the heater supply means you have a virtual center tap with two 100 Ohm resistors to ground. That's good.

The low B+ and hum with preamp tubes (does that include the phase inverter?) pulled strongly suggests something is pulling lots of current. What are the Voltages on the power tube socket pins? Does the B+ pop up to around 450V if the amp is powered up with only the rectifier tube and the preamp tubes?

[mort]

Anytime I fire up a new build and get major unwanted noise I try swapping the OT primaries.

[überfuzz]

If you measured a B+ of 290 VDC and 390 VDC on the rectifier you should take a close look at the filter wiring. You can control the filter by measuring AC voltage at B+, A, C, D or whatever you call them, measure the rectifier too. You shouldn't see much AC, often referred to as ripple, there at all.

Good luck and be careful with those live measurements.

[überfuzz]

I looked at the attached pictures. It's very hard to see the routing of the wires, but the filter caps does not seem to be placed in the same direction. Is this intentional? I take it you know about the plus and negative side and how the capacitors should be placed in the filter?

Any yes, those copper tubes, that would be the flux-capacitor, right? ;-)

[loudthud]

The OP started another thread in Maintenance, Troubleshooting and Repair where it got more response.

http://music-electronics-forum.com/t38207/

[Leo_Gnardo]

The OP started another thread in Maintenance, Troubleshooting and Repair where it got more response.

http://music-electronics-forum.com/t38207/

Hundreds and hundreds and hundreds of postings since Sept 2012, the ongoing travesty of Wil and his mysteriously disheveled DRRI. Let's start a betting pool right now. It's going to get repaired to perfect satisfaction within N days (fat chance) or this & the parallel thread will run on way past Christmas, hundreds more frustrating postings and no end to the fiasco.

[überfuzz]

Hundreds and hundreds and hundreds of postings since Sept 2012, the ongoing travesty of Wil and his mysteriously disheveled DRRI. Let's start a betting pool right now. It's going to get repaired to perfect satisfaction within N days (fat chance) or this & the parallel thread will run on way past Christmas, hundreds more frustrating postings and no end to the fiasco.

Mhm.. I feel the frustration you bear. However, I don't see the benefit of bashing him.

[Leo_Gnardo]

Mhm.. I feel the frustration you bear. However, I don't see the benefit of bashing him.

You will, after another couple hundred posts and no results. Nobody has that much patience to burn. If you don't believe me read all the posts to date on this amp then you might have another think. FWIW I'll check out of this one and its parallel, and observe from the sideline. Let the circus continure. Carry on chaps!

[überfuzz]

You will, after another couple hundred posts and no results. Nobody has that much patience to burn. If you don't believe me read all the posts to date on this amp then you might have another think. FWIW I'll check out of this one and its parallel, and observe from the sideline. Let the circus continure. Carry on chaps!

Dude, I'm familiar with the 116.9, I've seen the threads over the years. Don't post in his threads if the sole purpose is to talk down on him. I'm pretty sure you got better things to do. ;-)

[Wil]

I can't blame Leo, haha.