| Posted by Art Grannell | October 12, 2018 - 01:09 pm |
| Brian, 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: http://www.tronola.com/misc/RedoneST35-A.jpg http://www.tronola.com/misc/RedoneST35-B.jpg http://www.tronola.com/misc/RedoneST35-C.jpg |
| Posted by Brian | October 11, 2018 - 12:39 am |
| Art, 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! |
| Posted by Art Grannell | October 10, 2018 - 07:59 pm |
| Brian, In the 1950's and 1960's the average line voltage in the US was about 115-117 VAC, which is the specified voltage for your Dynaco . When operated at today's more usual 120 VAC, these transformers will yield a slightly higher voltage at the secondary. This is normal and of no concern, and is probably even slightly beneficial. With solid state rectifiers, the initial voltage at the tube plates will be high, but should quickly drop when the tubes conduct, placing the normal load on the power transformer and power supply. This also is of generally little concern, although some folks, including myself, like to preheat the tube heaters which prevents this from happening. It seems that your issue is now with the output tubes themselves. From your description of the tube "runaway", it's quite likely that at least one tube is damaged, and that a new set is needed. Personally, I'd be a little reluctant to run this amplifier until they are replaced. By "shorting the inputs", I meant that the center pin of the input jack on each separate channel should temporarily be connected to ground, preventing any signal from entering. |
| Posted by Brian | October 09, 2018 - 11:07 pm |
| Art, Thank you again for your suggestion. Per your instructions, I shorted the inputs together and measured the B+. Cold, I measured 450 vdc, and as the tubes warmed up the voltage dropped to and settled about 390 vdc. It was within a few volts at each of the four tubes. For what it's worth I measured the line voltage coming in at 120 vac using the same Trms multimeter. The Dyna instructions show, as you also stated, 370 vdc is expected. Do you think 390 is high enough to warrant further investigation? Thank you again! |
| Posted by Brian | October 09, 2018 - 01:49 am |
| One last thing - just before I noticed the tube runaway, I was measuring over 1 vdc on LC. That's when I took a step back and noticed the one 6BQ5 breathing lava down the center of itself! The tubes are at least 40 years old - the entire Dyna set of pre-amp, amp, and FM tuner was in storage - hence the capacitor board replacements for the PAS3 and ST-35. The FM tuner is still working, but it's going to need a capacitor replacement, too, I'm sure. But one step at a time! Thanks again Art! |
| Posted by Brian | October 09, 2018 - 01:43 am |
| Art, Thank you so much for your feedback and assistance! The tube matching may indeed be the cause, as I moved one tube from the left channel to the right as part of troubleshooting initially. After installing the EFB cap board from Dave's Store, I was measuring the LC and trying to get the voltage down to the specified 0.27 with the trim pot. I noticed that particular tube was what I believe is called "run away" - the entire inside of the tube was glowing bright orange. So I unplugged the amp and let things cool. I moved that one tube from the left side to the right to see if it happened again. It didn't happen to the original LC tube moved to the RC; and it didn't happen to the RC tube now sitting in the LC. I left the tubes as they were and set the bias as I stated in my original message. Post installation, the trim pot was turned mostly counter clockwise, which I think had the voltage set high. To get the average to 0.27 I had to turn it nearly all the way clockwise, and I haven't seen any tube "run away" again. I don't want to attempt the measurements tonight being so late, but I will tomorrow after work. I think I'll try swapping the tube back to where they were originally and try setting the bias again. I'll pay close attention to the problem tube while doing so. I'll also take a measurement of the B+ on pin 7 and make sure it's OK. I got all the components from Mouser - they came in appropriately marked bags, and I double-checked as I installed each - so I don't suspect there being a resistor issue but I'll double-check that as well while taking the other voltage measurements. Again, thank you so much for your help. I'll post my findings, hopefully tomorrow. |