Picked this up today

ok I'll help you troubleshoot your amp.

without looking back to try to find out what this guitar amp is, 80% of them are push pull with some more or less bells and stuff.


ok lets first divide and conqure:

the output stage of your amp will have big tubes (usually 2 or 4)
the stage behind that one is the phase splitter. it takes the unbalanced connection from the preamp and buffer it, the result output is a buffered balanced connection. remove this tube with the power off.
it will be a small signal tube like a 12ax7, 12au7, 12at7, 6sn7....
anyways, power on:
does it buzz after the heaters warm up the tubes? if so, we need to look at that buzzing issue before we go any further.
 
as soon as I take it off standby it is buzzing. I am not finished replacing all the filter caps yet. There are two or three more after the main filtering of the PS. After the large can cap, which I replaced, there are some other caps in line before the voltage goes to the tubes etc, I have to replace a few of them. This is a stereo amp with two amps in it. After replacing the can cap and two others the buzz subsided quite a bit. At idle with nothing plugged in it sounds like you have the buzz from a single coil guitar plugged in. There are two amps with 2 7189As in push pull in each amp. Each amp has its own phase inverter, 12DWX7s; the preamp tubes are 7025s, the rest of the tubes are for the reverb and the vibrato.
 

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drtechno said:
ok I'll help you troubleshoot your amp.

without looking back to try to find out what this guitar amp is, 80% of them are push pull with some more or less bells and stuff.


ok lets first divide and conqure:

the output stage of your amp will have big tubes (usually 2 or 4)
the stage behind that one is the phase splitter. it takes the unbalanced connection from the preamp and buffer it, the result output is a buffered balanced connection. remove this tube with the power off.
it will be a small signal tube like a 12ax7, 12au7, 12at7, 6sn7....
anyways, power on:
does it buzz after the heaters warm up the tubes? if so, we need to look at that buzzing issue before we go any further.
Click to expand...

there is the schematic, have at it.
 
no, its not that bad. I am juggling 5 things at once.


ok, you say yove have a small buzz/hum after unplugging the 12dw7's ?

if so the heater supply winding might have small short in it. to remove that get an orange drop or some other film and foil cap , .22 600V stick it in series between the gray/violet wire (center tap for the heater) and ground.
now see if it hums.
 
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another way is to look at the heater signal.

set an oscilloscope for 2V/div x1 probe ac coupling ground your probe to chassis ground and look at the both sides of the heater. then look at it across the heater windings.( the probe's ground to one side of the heater.) it should be a clean sine wave. if there is any hash (noise) on that it will translate on the cathode.
 
I still have to replace the two 1mfd 450v caps on line g and f, going to the plates of the reverb circuit. The hum does get louder when I turn the reverb up but the reverb tank is not plugged in at the moment, I have to get a new tank.
 
dragonworks said:
I still have to replace the two 1mfd 450v caps on line g and f, going to the plates of the reverb circuit. The hum does get louder when I turn the reverb up but the reverb tank is not plugged in at the moment, I have to get a new tank.
Click to expand...

It wouldn't hurt replacing those caps. But I would investigate if your heater winding is faulty. Because that is the part that gets the most wear and tear. also you can substitue it temporarily by using a 6 volt motorcycle battery or a 5V computer supply remove the heater windings and hook up the battery. take a low ohm resistor (100 oihms) tie it on the substituted power supply ground and the other side of the resistor to the amp's chassis ground.
 
I am having a hard time with your reasoning. The 6.3 volts going to the heaters is working and the tubes are working. If the heater supply was faulty the tubes would not work?
 
well in old equipment the first thing that goes is the winding that gets the most wear and tear wich is the heater winding. insulation breakdown of the winding itself can allow noise to ride on the sine wave voltage. the noise will electromagnatically reflect onto the cathode. This phenomenon can be observed by disconnecting the high voltage from the circuit, with just the heater supply running to the tubes. take an o scope probe w AC coupling, look at the cathode to dc gound. of any small signal tube that is powered by the heater . no b+ or bias voltage. the noise will be present. on the heater, the noise will ride on the heater signal like this:
 

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dragonworks said:
I am having a hard time with your reasoning. The 6.3 volts going to the heaters is working and the tubes are working. If the heater supply was faulty the tubes would not work?
Click to expand...

I too have never come across this phenomenon.

With the HT disconnected, what is the heater winding breaking down to? The mains winding? If that were the case for grounded heaters (which should always be the case) a PAT test would show a fail.

You CAN get noise on a heater line due to HT rectifier spikes. This can be fixed with the use of very fast diodes and some limiting resistance*

Hand drawn wave forms will not do I am afraid, scope photos please.

*Blencowe. Power Supplies for Tube Amplifiers.

Dave.
 
ecc83 said:
With the HT disconnected, what is the heater winding breaking down to? The mains winding? If that were the case for grounded heaters (which should always be the case) a PAT test would show a fail.

Hand drawn wave forms will not do I am afraid, scope photos please.
Click to expand...


usually, if that strange situation happens, I find it shoring to itself. the enamel on the coil itself is breaking down. on those I find mechanical buzz that doesn't go away after tightening the elams because the coil is buzzing. this fm distortion appears on the cathode with no B+ applied to the circuit. I've only seen this come up in tube gear older than 20 years.
The last time I had one short to the B+ secondary was on a mesa boogie 50/50 the heater measured to chassis ground with the normal gounding on the heater lifted was about a volt in ac. it read near 0 ohms between the heater and dc ground with the hum balance resistors removed and no tubes installed (just in case there was a fault in a tube, I removed all of them) that one, I changed the gouding scheme on the heater so the balanced ct was going through a .22 cap with a 10K resistor in parallel to ground. that away I could shunt the noise, and still have a dc reference if the "short" becomes an "open". been working like this as a daily guitar amp for 5 years after this mod.

next time one of these strange animals comes this way, or my friend with the 50/50 wants me to work on his amp again, I'll snap some pictures.

the bad ct I've seen in cheap tube supply transformers. they just don't get the ct exactly in the center.
I 'll dig in my junk tansformer box for one, mount it up on my tube prototyping setup I have, and show you a picture.
 
First thing I am going to do is replace the two caps I mentioned and put in a new reverb tank, then go from there. I haven't been back to her for awhile and won't get to it for some time to come. A bit busy on other things lately.
 
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