Is my OT blown ?

[hamfist]

I am on a quest of finishing my restoration/modification of a 100W Selmer PA - converting it into a two channel Marshall 2203 plus extra gain stage type amp.
I've got the two separate pre-amp's both working a treat - these can be linked out of the FX loop send of the amp, to the FX loop return of my other amp (a tube-rectified 2204 semi-clone). So I know the pre-amps and the FX loop of the Selmer are working OK.


However, if I play the Selmer's pre-amps into it's own PI and power section (which is now in 2203 spec), the volume is very low, as if it's a 1-2 Watt amp. I can easily play the amp at full volume in my front room. The volume does not vary, except with different MV settings.
This also happens if I play the 2204's pre-amp into the FX loop return of the Selmer.......so problem is with the PI and/or power section of the Slmer.

I've tried everything I know. Checked all the wiring a million times. tried new PI tubes & new power tubes. I've checked the impedance across the OT's windings. The primary winding reads about 18 ohms between the centre tap and each outer wire. The secondary windings all read about 0.4-0.5 ohms between each tap and the common ground wire. I know my DMM is not very accurate at those low levels, but there's definately no direct shorts.
All leads (primary and secondary) have a high impedance to the amp's ground, when the transformer is disconnected, so shorts of any winding to the transformer core.

THe only other thing I am not sure about is the grid stopper resistors, which are each 10K, instead of the standard Marshall's 5.6K, although I am sure all this does is dampen high end frequencies a little bit.

Oh, I've also either checked or replaced every component associated with the PI, from the FX loop return to the power section. No difference.

Here are the relevant voltages of the PI and power tubes (all power tubes have pretty much identical voltages).

PI

pin1 169V
pin2 0.045V
pin3 2V
pin6 167V
pin7 0.045V
pin8 2V


Power tubes

pin3 458V
pin4 454V
pin5 -33V

Any suggestions ? is there anything it seems that I might have missed, or is my OT toast ?

I don't feel confident testing my OT further, but was wondering if it would be safe to jumper the signal from the PI outputs (pins 1 & 6) of the Selmer (and ground, of couse), into the same point in my other amp.
if I kept the power supply to the Selmer's PI intact, and put the output signal from pins 1 & 6 straight into the PI coupling caps of my other amp, that would keep the bias supply of the "receiving" amp intact, right ? The PI coupling caps would keep all DC isolated between the amps.
Is this madness ? If it were safe and possible, it would at least identify if the problem lies in the PI circuitry or power tube circuitry onwards.

[Rosewood]

I wouldn't patch in a OT just yet. If you plug a guitar into the fx return is the sound clean, distorted? Do you have a scope and generator?

[skipm45]

If you think you may have a shorted output transformer,here's a very quick test .

First, you'll need a small 6.3 V filament transformer, this can have an output of 3A or higher. 3 Amp units are cheap, so don't worry. Next, you'll need a "current limiter" made up of a 100 watt light bulb and socket. This light bulb must be wired in "series" with one of the AC "input" leads of your 6.3 volt filament transformer. If this little 6.3 V transformer has a center tap, just cut it short, and shrink wrap it. You won't be using it for any of these tests.

Next, you'll want to add longer leads to the 6.3 V filament transformer "secondary."

Best thing is to solder them, and use shrink wrap. Add INSULATED alligator clips to the other end of these leads.

Now, you have a test rig for checking output transformers, and checking an "unknown" power transformer for lead specs.

TESTING THE TRANSFORMER:

It is not necessary to unwire the output transformer for this test, just pull the output tubes. Here's what you do; Take your little 6.3 V filament test transformer, and clip one alligator clip to the output transformer secondary tap.

This can be the 4, 8, or 16 ohm tap. It doesn't make any difference which one you use. Next, you'll take your other test clip, and install it to the output transformer "common," or "ground" wire. MAKE SURE you have the current limiting light bulb in series with "one" of the filament transformer primary leads before plugging it into the wall. If the output transformer is shorted, the light bulb will light up, case closed, replace that transformer. To confirm you have a good connection, you can use an AC voltmeter, and take a reading on the secondary plate leads. The voltage will vary, but you should see around 120 V AC on a 50 watt Marshall output transformer (plate to ground). Your other secondary taps will have voltage on them too. These readings will be like 4-15 volts between the secondary taps. Using an AC voltmeter here, you'll need to touch chassis ground with one of the probes.

Be Careful!

HTH

Skip
www.skipzcircuits.com

[Blue Strat]

Check the bias on your power tubes and report back. This is like using a stethoscope on the human body...it can tell you a lot of things.

[paolojm]

You didn't mention if you'd checked the resistance between the primary and secondary? These may be shorted and causing problems.
I think the test you mentioned is ok but you will have ground loop hum, not a problem for testing though.
Be careful.

[hamfist]

Originally Posted by Blue Strat

Check the bias on your power tubes and report back. This is like using a stethoscope on the human body...it can tell you a lot of things.

Two quads of EL34's (one quad being brand new), both biased at 70% of max dissipation have had the same effect. THe bias is definately spot on.

[hamfist]

Originally Posted by paolojm

You didn't mention if you'd checked the resistance between the primary and secondary? These may be shorted and causing problems.
I think the test you mentioned is ok but you will have ground loop hum, not a problem for testing though.
Be careful.

Yes, I did check that. I found no evidence of shorting at all between primary and secondary coils.

Thanks for your comments on the test I suggested. I think it's my next approach.

[hamfist]

Originally Posted by skipm45

If you think you may have a shorted output transformer,here's a very quick test .

First, you'll need a small 6.3 V filament transformer, this can have an output of 3A or higher. 3 Amp units are cheap, so don't worry. Next, you'll need a "current limiter" made up of a 100 watt light bulb and socket. This light bulb must be wired in "series" with one of the AC "input" leads of your 6.3 volt filament transformer. If this little 6.3 V transformer has a center tap, just cut it short, and shrink wrap it. You won't be using it for any of these tests.

Next, you'll want to add longer leads to the 6.3 V filament transformer "secondary."

Best thing is to solder them, and use shrink wrap. Add INSULATED alligator clips to the other end of these leads.

Now, you have a test rig for checking output transformers, and checking an "unknown" power transformer for lead specs.

TESTING THE TRANSFORMER:

It is not necessary to unwire the output transformer for this test, just pull the output tubes. Here's what you do; Take your little 6.3 V filament test transformer, and clip one alligator clip to the output transformer secondary tap.

This can be the 4, 8, or 16 ohm tap. It doesn't make any difference which one you use. Next, you'll take your other test clip, and install it to the output transformer "common," or "ground" wire. MAKE SURE you have the current limiting light bulb in series with "one" of the filament transformer primary leads before plugging it into the wall. If the output transformer is shorted, the light bulb will light up, case closed, replace that transformer. To confirm you have a good connection, you can use an AC voltmeter, and take a reading on the secondary plate leads. The voltage will vary, but you should see around 120 V AC on a 50 watt Marshall output transformer (plate to ground). Your other secondary taps will have voltage on them too. These readings will be like 4-15 volts between the secondary taps. Using an AC voltmeter here, you'll need to touch chassis ground with one of the probes.

Be Careful!

HTH

Skip
www.skipzcircuits.com

THankyou for your suggestion. However, I just think that testing in the way you have suggested is beyond my abilities at this time. I am still fairly new at all this. I don't feel confident to be able to safely and effectively carry out the tests you are suggesting. Is there a picture of the setup anywhere on the www that you could link to ? if I could see a picture of it all, I might be able to get my head around it rather better.

[Toggle]

I don't know the amp of the top of my head--what kind of PI stage is in there? A long-tailed pair? If so, you're voltage on the PI don't look right. I'd expect a considerably higher cathode voltage for starters.

-Tom

[skipm45]

Sorry about the unnecessary complexity.
It's really a pretty simple method. My long winded post makes it sound more complicated than it is. I was trying to emphasize safety and made it sound like "build your own fusion reactor"
Anyway, pull the output tubes. amp powered off and unplugged. speaker disconnected
Connect a 6v AC source to the OT secondary (any tap) and ground. any old wall wart will do if the output is AC
Measure across the OT primary leads at the tube sockets. should read about 120VAC if good.


HTH

Skip
www.skipzcircuits.com

[hamfist]

Originally Posted by skipm45

Sorry about the unnecessary complexity.
It's really a pretty simple method. My long winded post makes it sound more complicated than it is. I was trying to emphasize safety and made it sound like "build your own fusion reactor"
Anyway, pull the output tubes. amp powered off and unplugged. speaker disconnected
Connect a 6v AC source to the OT secondary (any tap) and ground. any old wall wart will do if the output is AC
Measure across the OT primary leads at the tube sockets. should read about 120VAC if good.


HTH

Skip
www.skipzcircuits.com

THanks Skip, that sounds much more straightforward. I could definately manage that.

[hamfist]

Originally Posted by Toggle

I don't know the amp of the top of my head--what kind of PI stage is in there? A long-tailed pair? If so, you're voltage on the PI don't look right. I'd expect a considerably higher cathode voltage for starters.

-Tom

You are spot on there Tom. Got it solved last night. Someone else pointed out those low voltages.
After a bit of further investigation i found I had put a 470K onto pin 8 of the PI (long-tailed pair), not a 470 ohm !!! So a thousand times too much resistance does make a difference !!

"Doofus of the year" award goes to .........me !

a 470 ohm was duly substituted, and volume levels are back to normal ! Yay !!!!!

THanks for all your comments guys.

[phsyconoodler]

You just found out how to attenuate your amp:AOK

[hamfist]

Originally Posted by phsyconoodler

You just found out how to attenuate your amp:AOK

And I spent all that money on an Ultimate Attenuator