Comments for page: EFB in a Dynaco ST-35

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Posted by Art Grannell October 10, 2018 - 07:59 pm

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

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

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.

Posted by Art Grannell October 08, 2018 - 05:35 pm

Thanks for your inquiry! Assuming that the new cathode resistors are very closely matched, the most probable cause for the difference between the two channels is the output tubes are not well matched. It may be worthwhile, if you feel comfortable doing this, to check the B+ voltage at pin 7 (to ground) of each output tube, with the input jacks shorted. The voltage may be a little higher or lower than Dyna's specified value of 370VDC, depending on your AC line voltage. All four tubes however, should measure within a few volts of the average reading. This should eliminate the possibility of a wiring problem in the B+ supply, or at the tube socket.
One thing you can do, before ordering new tubes, is to try to get a better match by switching the positions of the individual output tubes, i.e., tube number 2 and 3 may more closely match numbers 1 and 4, etc.
It seems that some tube sellers do a far better job of matching tubes correctly than others. Both Dave and I have found Jim McShane's tubes to be very well matched.

Posted by Brian October 07, 2018 - 07:26 pm
Hi - I see this comment section hasn't had much activity for a while, but here goes anyway:

I recently installed the EFB modification in a ST-35 that my dad built in 1963. The 6BQ5s are Realistic so they were changed at some point, but they still seem to be working OK.

My question is in setting the bias correctly. As I understand the procedure, the average voltage measured should be 0.27 VDC. I was able to achieve that average, however to get that average I ended up measuring 0.175 on the LC and 0.362 on the RC. Although it averages to the required 0.27, there's quite a difference on each channel. Is this acceptable, or should I be looking at replacement tubes?

Any thoughts or feedback would be appreciated!

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