Help! Sunn 300t Bias Problem?

[ManRay]

Hi,
I am having a balance problem with a Sunn 300t that I just got my hands on.
The Bias voltage will set to the proper level but the Balance pot will have no effect and the voltage will climb and climb until the tubes on the right hand side of each pair start glowing red.

I have opened her up and looked at all of the solder pads and traces on the power tube's PCB and everything looks fine. Nothing is obviously burnt out anywhere else on the board and I don't see any loose wires.

I ran the Voltmeter from pin 5 to ground in standby and I am showing a reading of 60DCV on all of the right sides of the pairs (which are running hot and glowing red on full power) and a reading of 52 DCV on all of the left sides of the pairs.

Any other suggestions?

Thanks,
Mr.

[Twist]

One bad tube on the right side can affect both tubes making it appear that there is a common problem. Pull out the two known good tubes on the left side and try them in the sockets in question. You don't need any tubes on the left side until you resolve what's going on. In fact you can plug one tube at a time on the side in question and monitor the current draw. Make sure you have a full load on the amp when your finalizing the repair and setting the bias.

[ManRay]

Not a bad tube?

Ok,
This amp has 3 push/pull pairs of 6550's, so I pulled all of the right hand tubes (the ones that were heating up earliest) and switched on leaving only the 3 left hand sides of the push/pull pairs. The balance reading held steady at -50mV.

So I took those same 3 tubes and put them on the right hand side. This time I got a balance reading of 200mv and climbing. Does this tell you anything that I don't see?

This doesn't seem like a bad tube to me. It would appear that I have a deeper issue with my bias.

Are there any other tests that I could run which would help me solve the problem?

Thanks,
Mr.

[52 Bill]

It's been a while since I've had one on the bench, but I seem to recall that the bias on that amp was not a straightforward voltage divider system.

Check the 12AT7 that sits between the outputs and the phase inverter.

[ManRay]

New 12AT7, Old Problem.

Thanks for the idea, but I dropped a new 12AT7 in and got the same issue.

Are there any measurements I could take at that tube socket that would help? Do I need to procure a Schematic at this point before I delve in any deeper?

Any more help would be greatly appreciated!

Thanks,
Mr.

[52 Bill]

Yes, having the schematic will always help.

Something is causing the bias to drop on one side only, so check everything from the tubes sockets to the coupling caps on that side of the amp. Compare them with the working side and see if there are any signs of problem components.

[ManRay]

Schematic!

Ok,
Under visual inspection I don't see any cracked solder pads or traces on the board that holds the power tubes.

I do not know which caps are the coupling caps, but I assume they are in one of the groups of big electrolytics that I have on the board here. Could you perhaps narrow it down for me?

I have included a .pdf copy of the service manual for the amp. If you have any trouble viewing it then please let me know.

Thanks,
Mr.

[Twist]

It may be only one tube socket and associated components on the side in question that causing the issue. Have you tried one tube at a time in each socket? Also have you tried to adjust the balance control and monitor pin 5 of each socket with all the output tubes out? Also check the 47 ohm grid resistors and associated diodes.
I'm sure someone will chime in here soon that's worked on several of these. I think I've only seen one in my shop and that was a long time ago.

[ManRay]

Not a bad tube.

I tried it one tube socket at a time with just one tube. Same results...left side is stable and the right side climbs in every pair down the board.

I just finished taking the readings on the 4&5 pins with all of the power tubes out and the power full on:

Pin 5:
V5=-44DCV
V6=.3V

V7=-44V
V8=.3V

V9=-44V
V10=.3V

Pin 4:
V5=395DCV
V6=325V

V7=394V
V8=312V

V9=394V
V10=335V

...So as per your final suggestion, I unscrewed the power tube PCB from the chassis and found that R367 and, possibly R363 had burnt up and there was a burn spot on the board in between both of those 47 Ohm resistors.

Is there anything you guys think I should know before I pop some new resistors in there and plug her in?

Thanks for helping me get to the bottom of this! I will keep you updated on the repairs.

Thanks,
Mr.

[52 Bill]

The burnt resistors are the screen grid resistors that may have been damaged by a shorted tube. Replace them and try and clean up the burned board by scraping away the carbonized material. Check the tube sockets for any carbon traces as well.

The real problem here is why the bias voltage on pin 5 of half the tubes is missing. All pin 5's should have a negative voltage. Check all of the resistors and diodes on that side of the amp and see if any have become open or shorted.

If you can safely turn on the amp and check voltages, follow the bias supply back from the bias and balance controls and see where it dies out.

[ManRay]

Update: Thanks All!

After all of your help I decided that I was in dark waters and took the amp to my local tech. He told me that several of my screen resistors were blown and he got me on my way. Some new tubes and a quick bias job later I am sounding good.

Thanks again for all of the guidance in helping me get to the bottom of this!

Much appreciated,
Mr.

[xjbear]

Sorry to add to an old thread but ive had some experience with these and one of the problems is the 1/4watt 390k resistors in the bias supply, r333/r353. These resistors go open very easily and have been updated in the 300Pro to 2watt 330k. Youll still see some lower bias voltage feeding from the pi tap but you'll not have any adjustment if one or both has failed