Adding adjustable bias to S5000ii

[charnich74]

Agreed.

Chris Hornbeck is right.
Check bias circuit parts. Missing bias is much worse than mis-adjested bias.

I am at a disadvantage, I can not see the schematic well enough to look for other items to check.

Check the 4th post in the thread. I took better pics and split the schematics into 2 parts. Its much better than the first one I uploaded. Cheers

 

[charnich74]

Over a couple of decades or more, JJ has constantly been improving all of their tubes performance and sound, and also a much improved reliability.

I have been using JJ tubes from eurotubes.com for a long time now.
They do additional testing of the tubes when they come here from Slovakia.
These are not tubes that sat on the shelf from years ago, waiting for a buyer.

Some quick ways to kill a good tube:

Put it in an amplifier that exceeds the tube rating(s).

Use coupling caps that are leaky. (I use modern 600V coupling caps in amps that have a B+ rise to 500V before the tubes warm up and draw current).
A 450V cap is no good here.

Misadjust the bias.

Try and use a Hi Fi amp to play Van Halen at max power, all the way through side one of the vinyl that has a very bad record warp at 3 Hz. The output transformer primary acts like a short.

etc.

Good advice on the cap voltage. I hadn't considered that.

 

[Chris Hornbeck]

I am at a disadvantage, I can not see the schematic well enough to look for other items to check.

What works for me, in Windows7 Pro and Firefox, is left click on the thumbnail in post #4, then right click on the image, choose View Image, a circled "+" appears, left click expands to original size. Clumsy, but it works for me. Maybe something similar would work in your setup.


Always the best fortune,
Chris

 

[wiseoldtech]

Some quick ways to kill a good tube:
Put it in an amplifier that exceeds the tube rating(s).
Use coupling caps that are leaky. (I use modern 600V coupling caps in amps that have a B+ rise to 500V before the tubes warm up and draw current).
A 450V cap is no good here.
Misadjust the bias.
Try and use a Hi Fi amp to play Van Halen at max power, all the way through side one of the vinyl that has a very bad record warp at 3 Hz. The output transformer primary acts like a short.
etc.

All that is good info for sure.
And I might add that continually blasting music with home Hi Fi equipment is simply abuse and is not designed for such operation.
Only fools would do that.

 

[charnich74]

Those pots are indeed for bias adjustment, but they adjust the relative bias of the two valves in each channel. Adjusting for minimum hum is a crude way to zero the standing DC field in the output transformers. Your plan to add 10 Ohm sensing resistors is much better.


The next step beyond this would mean two more pots, each adjusting the bias voltage for its channel. An alternative would be four separate cathode bias resistors, each with a big capacitor bypass, and no pots.


All good fortune,
Chris

Good morning Chris. So I'm thinking I don't need adjustable bias, it seems overly complicated to do so and I'm not sure why it was recommended. I just need to bias the tubes. Is bias applied at the control grid? In this case I'm looking at R41,44,81,and 84 on the schematics, the 1k resistors. Are those my bias resistors? In other words, can I just change those out with appropriate values to achieve proper bias with the new tubes? Also, I've adjusted my output tube balance pots to yield equal resistances at the grid in each tube pair of the left and right channel so then I can now measure voltage drop across the 6.8 ohm resistors and divide the calculated current/2 to determine cathode current to each tube, and have it be pretty accurate? Does that sound right to you? I feel like I'm starting to get it but please correct me if I'm wrong. Thanks again, you've been very helpful.

 

[wiseoldtech]

Charnich74, your assumptions are way off there, sorry, you're opening up a can of worms.

Those 1K resistors are grid stoppers, used to prevent tube oscillations, nothing to do with biasing.
Additionally, the "output balance" controls are only to be set with signal generator test tones and an oscilloscope, to balance the sine wave/clipping balance at the speaker outputs.
This is usually done after installing new output and/or driver tubes.
The service manual contains all the information related to that.


Messing with them from the original settings now only adds more complexity to final servicing.


If anything, the 47K resistors ultimately determine bias voltage, used as a voltage divider in conjunction with those controls.


Now, unless you've got all the proper test equipment that's needed (sig gen/scope, etc) and know how to use them, you'll wind up with less than superior performance and stability that was designed into that unit.

 

[charnich74]

Charnich74, your assumptions are way off there, sorry, you're opening up a can of worms.

Those 1K resistors are grid stoppers, used to prevent tube oscillations, nothing to do with biasing.
Additionally, the "output balance" controls are only to be set with signal generator test tones and an oscilloscope, to balance the sine wave/clipping balance at the speaker outputs.
This is usually done after installing new output and/or driver tubes.
The service manual contains all the information related to that.


Messing with them from the original settings now only adds more complexity to final servicing.


If anything, the 47K resistors ultimately determine bias voltage, used as a voltage divider in conjunction with those controls.


Now, unless you've got all the proper test equipment that's needed (sig gen/scope, etc) and know how to use them, you'll wind up with less than superior performance and stability that was designed into that unit.

Thank you for the input. I appreciate it. I don't have the specialized testing equipment. Just a man with a multi-meter. I'm the first to admit that I'm a total novice, but I purchased the project with the intention of learning, and that I am. I'm glad you set me straight on the grid stoppers. I do have another question for you, what function do the 6.8 Ohm resistors serve?
Cheers

 

[wiseoldtech]

The 6.8 ohm cathode resistors are there for measuring total cathode current when testing switch is set.


As for lacking the proper test equipment, a multimeter is not sufficient.
It's like asking a car repairman to change the spark plugs and change the oil with just a screwdriver.
It ain't gonna happen.

 

[Chris Hornbeck]

So I'm thinking I don't need adjustable bias, it seems overly complicated to do so and I'm not sure why it was recommended. I just need to bias the tubes. Is bias applied at the control grid?

Yes, and the output valves are (of course) already biased. You won't know whether you need to adjust bias with modern valves until you've measured it. Your DVM will work fine for this.

Also, I've adjusted my output tube balance pots to yield equal resistances at the grid in each tube pair of the left and right channel so then I can now measure voltage drop across the 6.8 ohm resistors and divide the calculated current/2 to determine cathode current to each tube, and have it be pretty accurate?

Voltage across the 6R8 Ohm resistors will tell you the sum of the cathode currents in each channel, but not how well they're balanced. Separate 10 Ohm resistors at each valve is the gold standard and the safest, so recommended. But cold measurements of the output transformers' winding resistances, the hot measurements of their voltage drops, and a little Ohm's Law also works, not nearly as safely but can be done. Or the factory method of transconductance balance - safe and built in; so be sure to keep it operational.


Tools: restoration of antiques really really requires a metered Variac. You'll thank me later. A contact cleaner like D5. Load resistors, nothing fancy, just a pair of 10 or so Ohms, 20 Watt rated or more. Good lighting and an open mind. You've already got the important part.


All good fortune,
Chris

 

[charnich74]

Yes, and the output valves are (of course) already biased. You won't know whether you need to adjust bias with modern valves until you've measured it. Your DVM will work fine for this.




Voltage across the 6R8 Ohm resistors will tell you the sum of the cathode currents in each channel, but not how well they're balanced. Separate 10 Ohm resistors at each valve is the gold standard and the safest, so recommended. But cold measurements of the output transformers' winding resistances, the hot measurements of their voltage drops, and a little Ohm's Law also works, not nearly as safely but can be done. Or the factory method of transconductance balance - safe and built in; so be sure to keep it operational.

Good news is that I already have the tools on your list. i don't have a signal generator, scope, distortion meter etc....etc....etc.

Now I just need to get the tubes purchased. In add'n to the 7591s this thing has 6 12ax7's all of which original and nobody nearby with a tester so I really don't know what I'm working with. I guess I'll just have to pony up and buy all 10 tubes....ouch.

Thanks for your help!

 

[Chris Hornbeck]

If your 12AX7's are original, keep them. Failures in hifi service are very rare. Not impossible, but rare. Sometimes noise issues arise, even with the best originals, but "tube testers" don't test for that. Modern replacements are generally inferior. You'll know what you'll need once you've got it fired up, so no rush.


Type 7591 was a late design valve, built with lots of know-how, and is difficult to make in the modern world. If I were buying some, I'd get a matched set of four from Jim McShane of whatever brand he currently recommends; it's a moving target and he's on the front lines.


All good fortune,
Chris

 

[charnich74]

If your 12AX7's are original, keep them. Failures in hifi service are very rare. Not impossible, but rare. Sometimes noise issues arise, even with the best originals, but "tube testers" don't test for that. Modern replacements are generally inferior. You'll know what you'll need once you've got it fired up, so no rush.


Type 7591 was a late design valve, built with lots of know-how, and is difficult to make in the modern world. If I were buying some, I'd get a matched set of four from Jim McShane of whatever brand he currently recommends; it's a moving target and he's on the front lines.


All good fortune,
Chris

If I don't need to spend $$ on the 12ax7s that's a ton of money saved! And they're all Telefunkens to boot. Jim pretty much only deals with the Tung Sol 7591's. Per his webpage there are too many issues installing the EH in vintage gear and it sounds like he really dislikes JJs. He's the one who turned me on to the need to rebias in the first place otherwise I just would have gotten a set and thrown them in...and probably regretted it.

Cheers