I have an original ST-35 from the 1960s and a Dynakit version I built in 2012. All was going well. The old one is in storage and the Dynakit is the daily use. Upon power up the Dynakit one went quiet. I could see one tube was out. I decided to check out the reason (besides 13 year old tubes). Upon opening it up I see a burnt resistor. I was an early adopter of the EFB modification. I made my own board for it, but I no longer have those plans. The 100 Ohm resistor used across the tube pins 8 and 9 was fried for the dead tube. By the way, I have the 2012 version of the Audio Regenesis PC-13 boards in the amp. This is basically my issue - I can't find the original plans covering the EFB mod, the resistor was fried. I don't remember how to check this thing out or how to set the EFB bias pot.
I bought some new 100 Ohm 0.5W 1% resistors to replace the four used in the original EFB mod at each tube. I turned it back on and the tube is still toast (dimly lit and no heat generation). No sound comes out of either side. I also have a matched set of replacement tubes, but I do not want to put them in if I have another issue. Everything else looks fine visually.
A couple of observations:
I see on the one schematic I found of the old EFB mod that there is a 5 Ohm resistor between pins 3 of the tubes on each side. I see TP on either side of those resistors. I suspect that the bias is to be measured there and then adjusted. I do not have 5 Ohm resistors there, just a wire connecting the two pin 3s. Were the 5 Ohm resistors a temporary install to set bias or were they to remain?
Where else should I troubleshoot? Any help is greatly appreciated - I miss the sound. I am using the 1960s one now, but am a little concerned by its age and lack of use.
Any help would be appreciated!
I bought some new 100 Ohm 0.5W 1% resistors to replace the four used in the original EFB mod at each tube. I turned it back on and the tube is still toast (dimly lit and no heat generation). No sound comes out of either side. I also have a matched set of replacement tubes, but I do not want to put them in if I have another issue. Everything else looks fine visually.
A couple of observations:
I see on the one schematic I found of the old EFB mod that there is a 5 Ohm resistor between pins 3 of the tubes on each side. I see TP on either side of those resistors. I suspect that the bias is to be measured there and then adjusted. I do not have 5 Ohm resistors there, just a wire connecting the two pin 3s. Were the 5 Ohm resistors a temporary install to set bias or were they to remain?
Where else should I troubleshoot? Any help is greatly appreciated - I miss the sound. I am using the 1960s one now, but am a little concerned by its age and lack of use.
Any help would be appreciated!
For questions about the original EFB board you might want to contact Dave Gillespie directly. He can be reached here:
Dave's Store
Dave's Store
Maybe you should post over at Audiokarma where Dave Gillespie could offer some help.
I tried logging in there but had issues after issue. Lots of pop-up ads...
I may have to reregister there and try again. I did send an email to Dave through his store.
I may have to reregister there and try again. I did send an email to Dave through his store.
I also sent off an email to Dave through his store. I woke up to find a very detailed response from him today. Per his suggestion I checked the 50 Ohm power supply resistor and it is open...hence no sound out of either channel. I did find the 5 Ohm resistors as I had mounted them on my custom EFB board. All other sisters checked out within spec.
I went ahead and ordered Dave's newer power supply with EFB board and all new tubes. I might as well upgrade this unit for another 10+ years of pleasant sound (through vintage KLH Model 6s).
I went ahead and ordered Dave's newer power supply with EFB board and all new tubes. I might as well upgrade this unit for another 10+ years of pleasant sound (through vintage KLH Model 6s).