Comments for page: EFB in a Dynaco ST-35

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Posted by Art Grannell October 17, 2018 - 10:00 pm

The purpose of C4 and C5 in the ST-35 is to pass AC signals without allowing DC to appear on the grids of the output tubes, The left side of each of these capacitors is connected to pins 1 and 3 of the 7247, which operate at 215VDC and 112VDC respectively. The right side is connected to the 6BQ5 control grids which need to operate at 0 VDC in this circuit. All capacitors will allow some DC current to pass through them, but with the type used in this application, it should be so little that it isn't measurable, except under laboratory conditions. This is called "leakage", and will sometimes increase from the effects of time, heat and use, eventually causing the problems that you've seen.

All of this is generally independent of the value of the capacitor, which in this case is in microfarads. From your measurements your capacitors are within 15% of the specified .1mfd value, and I would not consider this to be unusual for "vintage" capacitors made in the 1960's. Because of the way that capacitors are constructed, it is more difficult to maintain close tolerances than it is with resistors. Modern materials and manufacturing methods, however have improved both, and I'd guess that your new parts will measure closer to the desired value. The only effect of this slightly lower value would be at very, very low frequencies, and probably could not be heard with any "normal" loudspeakers.

Posted by Brian October 17, 2018 - 12:24 am

Once again, thank you so much for sharing your expertise with me!

I took your advice and removed C4, C5, and E6 from both channels. I ended up removing all the capacitors from the left channel, actually (the problem side). Not only did all the resistors measure within their respective tolerance (5% or 10%) but C4 and C5 for both channels measured 84 - 86 nf, or 0.085 ish uf. That is below the 0.1 uf specified, but it seems odd that only that one output tubes would "run away" if all four of the capacitors are measuring low, right?

Or does that explain why it was somewhat intermittent? In other words, why, with the old tubes, was I able to swap the RC and LC and get it all biased and operating? Were all the output tubes just teetering on having the same issue, based on all of them measuring 15% low? Or perhaps the one problematic C5 was being affected by heat more than the others?

Regardless, the take away is the resistors all seem to be fine (and in non-cracked condition), so when the capacitors show up I'll install them and see how it all goes.

Thank you again, Art, for all your time and effort helping me troubleshoot! I can't thank you enough!

Posted by Art Grannell October 14, 2018 - 10:04 pm
I hope that the new capacitors will solve the issue for you.

Now, while you are waiting for parts, would be a good time to check the PC-13 resistors. With power off, the tubes out, and C4 and C5 removed, it should be a quick and easy job. I'd take a close look at R8 and R9 and replace them if they don't measure close to the original value of 470 Kohms. They could be involved in the issue you have. Although they would not be suspected of contributing to your problem, the more critical ones are R3, R5, R6, and R7 and should check within 5% of their original value.

The remaining 10% tolerance resistors are less critical, and almost certainly would not be involved with your issue but are worth a quick check. To get an accurate measurement of R10, you will need to temporarily remove the feedback wire from the 16 ohm output tap of the output transformer. This step will also improve the accuracy of the measurement of R3. You can either lift the lead at eyelet 6 or at the speaker terminal strip. Also, visually inspect each resistor and replace any that show charring or cracking of the resistor body.

Good luck!

Posted by Brian October 13, 2018 - 12:36 am

Thank for you sharing the photos of your beautiful rebuild! Looks really nice, and I'm sure it sounds even better than it looks.

And thank you for helping me troubleshoot my ST-35. I have a sentimental attachment to it, as I have a photo of my dad building it in 1963. I also have the original manuals with his hand-written notes, and the serial numbers on the manuals match the PAS-3, FM-3, and ST-35. My goal is more to rehabilitate the set rather than rebuild entirely... make it last long enough to pass on to one of my children.

I agree that the age of the components is worrisome, especially since I measured the pin 2 voltage on the sockets tonight and confirmed your suspicion - when cold, the voltage from pin 2 to ground was actually negative, but after warming up the voltage climbed and perhaps 30 seconds in to the measurement it was at 60mV and continuing to climb, so I unplugged the amp.

If it were any other amp, I would do a complete PC13 replacement and try to make my amp look as good as yours! Given my desire to keep whatever I can, I did place an order with Mouser this evening for a complete set of capacitors. I'm hoping the original resistor values are still holding.

The cloth-covered wire from the output transformers isn't in great condition. Getting the wire moved from pin 9 to 8 as part of the EFB mod was dicey, and I had to use shrink tubing over some of the crumbling insulation that was closest to the output tube. If there are future problems and I have to swap the sockets, I'll have to plan on repairing the transformers, too - either sending them to a place that rebuilds transformers, or attempting the repair myself, in order to get new leads put on.

I probably won't get the parts until later next week, so I'll report back how things go once I get them swapped in.

Again, I really appreciate all your help and sharing of expertise on helping me troubleshoot!

- Brian

Posted by Art Grannell October 12, 2018 - 01:09 pm
Before changing tube sockets, you should check the grid voltage at the output tube sockets to eliminate the possibility of leaky coupling caps, which can cause the issue you describe. You can remove the output tubes (only) from the sockets for this test to prevent further tube damage, and again short the inputs. If you have a Variac to reduce the line voltage to 100 VAC for this test, it would be helpful, otherwise work reasonably quickly, since the B+ voltage will be high.

The DC voltage measured between ground and pin 2 of each output tube should read very close to "0". Anything more than a few hundredths of a volt would indicate leaky coupling capacitors (C4 and C5). If one is found to be bad, I'd suggest replacing all four.

If all is okay there and you decide to replace a tube socket, I'd also recommend replacing all four. I prefer Beltron sockets to the more common white ceramic sockets. The Beltrons are widely available from a number of vendors.

Another possibility to seriously consider would be to replace the PC-13 circuit boards with new ones and new components. The original 50+ year old boards are certainly in poor condition by now and will cause additional problems in the future.

A couple years ago, the ST-35 pictured in this article developed elusive issues, and rather than troubleshoot a number of aging components, I decided to replace them. I used Dave Gillespie's PC-13 boards, and made few other changes, including replacing my original EFB board.

It's my opinion the ST-35 with EFB produces extraordinarily good sound, and should be well worth your investment for many years to come.

Photos of the revisions that I made using Dave's boards are attached:

Posted by Brian October 11, 2018 - 12:39 am

Thank you once again for all your help and feedback. I am typing this now more perplexed than ever. When I first installed the EFB mod and attempted to set the bias, the Realistic tube (physically) in the back of the left channel "ran away." I swapped that tube with the one on the right, and the problem disappeared. I set the bias at that point, but had gone ahead and placed an order for new tubes.

Then I discovered your ST-35 article and left my first question, which led us to testing the B+ voltage. The new tubes showed up today, which was timely from your recommendation to not run the amp until the tubes were replaced.

I swapped them in, and attempted to set the bias, but quickly saw the bias voltage rising to above 1 volt on the left channel. The plate in the tube on the same problem spot (left channel, rear) was glowing hot lava. So I unplugged the amp and let things cool. On a whim I swapped the tube from the right side, but the problem remained tied to the socket location itself. The former RC tube, now LC tube, started running away and the voltage was 1.2 vdc when I unplugged the amplifier.

I double checked the resistor modifications I made as part of the EFB board installation and it all appears to check out. And the fact that I was able to set the bias which held for a few days of use indicates an intermittent problem. And since the original tube "run away" was solved by swapping tubes, I'm wondering if it isn't a corroded tube socket? Or loose? My dad told me a new tube socket would have a ton of resistance, and all the 6BQ5 tubes are pretty loose, but the one "running away" seems to be the loosest.

The condition of the original cloth covered wire leaves a lot to be desired, but all the sockets the rear ones seem to be in the best condition to enable a clean socket-swap.

If you had any additional thoughts or comments I would certainly appreciate the feedback. Thank you again so much for all your help, Art!

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