Sunn Sonora - Can you rebias tubes?

[doveman]

My son's bass player has asked if I could rebias his amp for new tubes. Searches have not produced any info but I did find a schematic. Not good at reading schematics frankly ... but it doesn't this look like it has a pot for rebiasing power tubes to me? Maybe one of you can spot it.

http://music-electronics-forum.com/atta ... sorado.gif

If so, I can get him to bring it over to me next time he comes. I'm sure I could open it up and see. But trying to save them a trip if it's not an amp with bias adjustment.

Thanks in advance for any info.

[sliberty]

There is a bias pot shown in the schematic, so the short answer to your question is yes. But, having said that, I am a little concerned that you might not be prepared for the job. I mean no offense here, but if you aren't famiilar enough with amps to have read that schematic, maybe you should bring the amp to a qualified tech. Biasing involves putting your hands inside of an amp that is on and live, and this can be pretty dangerous if you are not experienced.

If I have mis-read your qualifications, my apologies. Just want to keep you safe.

[doveman]

There is a bias pot shown in the schematic, so the short answer to your question is yes. But, having said that, I am a little concerned that you might not be prepared for the job. I mean no offense here, but if you aren't famiilar enough with amps to have read that schematic, maybe you should bring the amp to a qualified tech. Biasing involves putting your hands inside of an amp that is on and live, and this can be pretty dangerous if you are not experienced.

If I have mis-read your qualifications, my apologies. Just want to keep you safe.

None taken ... in fact I appreciate your concern.

I have built a D'Lite 22/33 ... albeit from a kit and layout. I rebias the tubes on it occasionally but it has bias points on the back of the chassis. Added a loop ... added the dumble-ish relays. I used to rebias my son's Fender. Modded my Richter with VVR ... mods to the coupling caps ... mods for switchable bypass caps on V1 and V2. I have recapped my '78 Boogie Mark 1 ... so pretty aware of the dangers ... I researched them all before modding and building amps. Learned how to discharge the caps ... etc. Actually ... most of my work is posted here. Having said that I'm not an electronics guy ... never really spent time reading/learning schematics.

For example: I just replaced my 5E3 tweed tone and volume wiring with a 6G3 "brown" version. Wonderful mod ... just looking at the schematic ... I still wanted to see a picture. Got the picture ... made the mod ... super results on the Richter. Posted some recent audio clips here.

If he brings it over and I pop it open I will be able to tell ... just not good at reading schematics. Too lazy to get good at reading schematics ... don't plan on building any more amps though. I got two killer amps now ... 100% successful on my one and only build.

Just for my education ... where is it at on the schematic?

[sliberty]

Well, then, with that in mind, let me help you a little with the scematic.

(Fixed) Bias is generated in the power supply, and presented to the power tubes. So what I looked for was :

1. Is it fixed bias? Since the cathodes of the power tubes are grounded, the answer to this is yes. Look at the power tubes on the schem, and you will see a pin from each connected together, and then connected to ground (the rake looking symbol). Also, I saw that on the grids of the power tubes, there was a reference to -55V. This is the bias voltage.

2. Find the bias circuit. I looked at the portion of the schematic that represents the power supply. There, I saw the small circuit that is generating the -55V. In that circuit, there is a 10K pot shown (looks like a resistor with an arrow pointing to the center). This 10K pot is the bias pot, and is most likely a trimmer.

3. Notice the 18K resistor right next to the bias pot. If for any reason, you cannot get to -55V when adjusting the pot, this resistor can be changed to provide more or less bias voltage. This is not usually needed, but can come up sometimes due to the tubes installed.

Also remember that -55V is the factory recommendation, but you could bias based upon current instead, and you could also bias based upon tone. -55V is not a magical specification.

Be safe, and good luck!

Steve

[doveman]

Super ... thanks ... makes sense ... I see it now. Thanks for the help. At least I know he won't make the trip to bring over just to find out that it doesn't have a bias trimmer.

[rdjones]

Super ... thanks ... makes sense ... I see it now. Thanks for the help. At least I know he won't make the trip to bring over just to find out that it doesn't have a bias trimmer.

Something else to watch for on the Sunns, and that's if there's a "test point" terminal, it's just the bias voltage coming off the pot.
It's NOT a current sensing point between the cathode and ground.

I just encountered this on a Solarus and it appears to be factory.
FYI ...

rd