Stereo guitar amp trouble shooting

[Magic Potion]

Hello
I've built a stereo guitar amp
20 watts a side using 7189s in pushpull (actually using Russian 6P14-EVs) these can handle higher voltages.
It's a cut and paste of fender ideas basically a couple of AB763 deluxes with a 6G16 bias vary tremelo and individual bias adjustments(off of one supply).
It's built all in a line with the two power amps at one end and the preamps at another.
The B+ runs down one side with the filter caps next to each section and B+ node they correspond to.
The preamps has it's own ground near the input jacks and everything else goes to a single ground point by the PT
This is a scheme I used on my previous builds successfully and I put up a picture from the hoffmans site which shows the method.
It's pretty successful considering it's kind of an experiment.
Everything works and it sounds really good
It has a little more hiss than my other builds but not at all unusable.
Because I'm using a Hammond organ AO-14 power transformer 375-375 I have quite high B+ up to 460 on the plates of the power tubes. Nothing has lit up yet but it is worrisome.

One thing that is strange is that if I try lifting the ground at the AC plug (checking for ground loops) the amp makes a very ugly buzz, something non of other builds do. I don't plan to lift the ground at all in practical use but it still seems like it is a symptom of something amiss in the build
I have checked all the solder joints and particularly the main grounds.
Any ideas?

[pdf64]

Maybe the PT insulation is failing.
With the mains ground lifted, is there any Vac between the amp chassis ground and the ground of the wall outlet?
Be very careful, one hand in pocket etc!

[Magic Potion]

So I put a ground lift on the AC plug at the power bar the amp is plugged into (a properly grounded 3 prong power bar)
and measured between the power bars earth ground and the amps chassis and I get 235 volts AC
turn the amp off, voltage drops, turn it back voltage returns along with loud evil buzz.

[pdf64]

How about if, rather than a complete lift, you put a 10k resistor between the amp chassis and mains ground; what is the chassis Vac then?
Take extreme care!

[Magic Potion]

Hi Before I do that i was just going to say that curiously i swapped the primaries on the PT around (they're grey and black) I put the black on the neutral and the grey on the line (usually it's just two blacks so there's not really an orientation) Strangely I now measure 127VAC on the chassis instead of 235VAC. regarding the 10K resistor will a 2 watt resistor suffice?

[Magic Potion]

OKay I tried what you said with a 10K resistor between chassis and mains ground. Meter between chassis and wall socket ground and i measured .57 VAC instead of 127VAC and the loud buzz seems to be gone.

[pdf64]

I think it's reasonable to condemn the PT as faulty, unsafe to use, as there seems to be a problem on the insulation between the primary and metalwork.
At the moment it's not actually dangerous, but insulation that's failed 'a bit' can later fail 'a bit more'.
Anything less than perfect is unacceptable. I don't think that any competent tech / builder / manufacturer would allow an amp with that PT out of their shop.
Sorry

[Magic Potion]

Ah well
It's good to know regardless and I'm not dead!
Which so far is also good!
Thanks!

[loudthud]

Can you contact the manufacturer of the transformer? Ask them if they Hipot test their products and at what Voltage. The symptoms described could just be capacitance between primary and other windings.

[Magic Potion]

You know I don't even know what hipot testing is, how curious , Ill have to look it up

I should've said its' a donor PT from a 50s Hammond M2 organ AO-14 Amplifier . It was bit of a gamble to try it I guess. I've had very good luck with other Hammond organ chassis and Transformers (actually the OTs on this build are from Hammond Amps I think both from Hammond AO-63s, Deluxe size 30-1 OTs used with EL84s and with an 8 ohm tap.)

[Magic Potion]

Well Looking at transformers on the Hammond site I can get a bandmaster/concert/pro PT for $93 Can which is about $72 US dollars, Also as they're in Canada too shipping is relatively quick
http://www.hammondmfg.com/pdf/EDB290EX.pdf
it's 330 0 330 276 ma which I think should just do the job with my 4 6P14P-EV power tubes (65ma each and 7 preamps tubes)
It will also give me a lower B+ which will be a little better for the Power tubes.

MY amp is SS rectified as I want a clean machine so This PT is also a little cheaper as it doesn't have 5V rectifier heater taps.
i thought about buying another Hammond AO-29 PT locally but though it would be only $50 it might be throwing good money after bad and also though they are quite substantial they're current rating might be a little low for my build.

[Magic Potion]

I was looking at this site regarding testing PTs
Power Transformer

in testing the primaries it says :

Using an ohmmeter, measure the resistance of the primary (usually Black - Black wires) and the resistance of both leads to the chassis. The primary should be under 1K ohms. If it is not, the transformer is dead.
Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm.
If it is shorted or less than 100K ohms, the transformer is bad. If it is between 100K and 1M ohm, unsolder the primary leads from the terminals they contact and measure again.
If it is now less than 1M ohm, the transformer is failing, and should be replaced.
If it is over 1M ohm, there is a component connected to the wiring leading to the power transformer which is leaking to the chassis that needs to be traced down.

I get inifinite resistance to chassis from the primaries
Could a leaky filter cap be giving me B+ to ground?

[eschertron]

Under voltage conditions the insulation could be breaking down. That's why the "hi-pot" (high voltage potential) testing was suggested. The test above can cull out a definitely bad PT, but may not detect all failing PTs. Sorry I did not answer your question

[Chuck H]

First, kudos to Pete for his concise and fast troubleshooting and guidance

Second, to our new member... Wow Magic Potion! That's a very ambitious build and you nailed it. Were it not for the bad PT you'd have been good to go. That's rare for an experimental build of this caliber. Welcome to the forum.

[pdf64]

...The symptoms described could just be capacitance between primary and other windings.

Yes, that a possibility, but it's difficult to be sure whether the current is due to coupling or imperfect insulation, without access to a megger or hipot tester.
As it's a vintage PT, perhaps best to play it safe and assume the worst.

...I get inifinite resistance to chassis from the primaries
Could a leaky filter cap be giving me B+ to ground?

The big problem with a regular meter is that the test voltage is only 9Vdc.
A bad bit of primary insulation might test fine at 9V, but when subjected to mains voltage it could allow mains current to leach past.
That's why high voltage testers are needed https://en.wikipedia.org/wiki/Hipot

The B+, being a secondary, doesn't have a current path back to mains ground.
Just checking, has your build got a death cap, eg about 0.047uF between the incoming mains live and ground? As that would cause these symptoms.

And yes, great work with your project, very impressive!
The plan to bring the HT down a bit is good.