Erratum: I said in the video that I thought the arcing rectifier had caused the choke to fail. I misspoke about that; it's more likely that a failed filter capacitor, failed output tube (in the past), or the failing caps in the bias circuit killed the choke. The choke was burned up by excessive current, and this would not have been caused by a failing rectifier. I suspect the bias supply was the culprit, because its 60+ year-old filter caps were most definitely bad, and this could have resulted in an inadequate bias voltage for the output tubes, causing them to draw way too much plate current, and of course that plate current goes through the choke. My mistake.
Wonderful Scott, I know very little of what you were doing there but every time I see one of these educational videos you do I learn something. Keep doing these I love watching them.
Great concise information on the rectifier circuit of this amp. Glad you mentioned the slow warmup of the high voltage and how it save the life of the tubes. Too many don't know this and just bypass the tube with solid state rectifiers. Some at least add a delay turn on circuit, but it's still snapping on at full voltage.
If you like this kind of video where I discuss units I am repairing at my shop, please hit the like button and comment, and I'll make more of these as time allows. Thanks!
The ST70 is hard on the rectifier with current demand and I wouldn’t use anything less than a NOS or used. Fortunately, Mullards have been rebranded by the other manufacturers of the day and are still out there. Speaking of 5AR4’s, RCA came out with the “big bottle” type which is quite sturdy but I’ve always wondered about the GE design which is the size of a 5Y3GT. I’ve used them, not on a ST70 though. I really haven’t seen anybody mention much about those.
I would just go with a full solid state rectifier without the 5AR4. The drawback is that it does have big surge current during the startup. I have been running like that for years and it seems fine.
I've had multiple problem child 5AR4/GZ34 of Chinese (or early Russian) origin, myself. One pretty good solution is to use a 5V4GA (straight sided, not an old coke-bottle ST type) which will drop 15 volts more than a 5AR4. Not terrible, and you can buy NOS US made ones under $20. It would be nice to discover a huge stash of 5AR4/GZ34's. You could retire if you could find enough of them.
No. It uses a slow turn on rectifier (the 5AR4) to slowly ramp up the current when first switched on. The 5AR4 uses indirectly-heated cathodes, which take about 15-20 seconds to warm up, as opposed to say, a 5U4, whose directly-heated cathodes heat up in 3-5 seconds. Solid-state rectifiers, of course, come on instantly.
Erratum: I said in the video that I thought the arcing rectifier had caused the choke to fail. I misspoke about that; it's more likely that a failed filter capacitor, failed output tube (in the past), or the failing caps in the bias circuit killed the choke. The choke was burned up by excessive current, and this would not have been caused by a failing rectifier. I suspect the bias supply was the culprit, because its 60+ year-old filter caps were most definitely bad, and this could have resulted in an inadequate bias voltage for the output tubes, causing them to draw way too much plate current, and of course that plate current goes through the choke. My mistake.
Wonderful Scott, I know very little of what you were doing there but every time I see one of these educational videos you do I learn something. Keep doing these I love watching them.
Thanks! There's going to be more as soon as I have time to make them.
@@ScottGrammer TIME, that’s a word that keeps coming up in everything I do too. Lol. Looking forward to it Scott.
Great concise information on the rectifier circuit of this amp. Glad you mentioned the slow warmup of the high voltage and how it save the life of the tubes. Too many don't know this and just bypass the tube with solid state rectifiers. Some at least add a delay turn on circuit, but it's still snapping on at full voltage.
Thanks for your kind words.
If you like this kind of video where I discuss units I am repairing at my shop, please hit the like button and comment, and I'll make more of these as time allows. Thanks!
Great job Scott!! Keep up the great work!!
Thank you. I hope you enjoy that amplifier.
really nice description of the yellow sheet mod
Thanks!
The ST70 is hard on the rectifier with current demand and I wouldn’t use anything less than a NOS or used. Fortunately, Mullards have been rebranded by the other manufacturers of the day and are still out there. Speaking of 5AR4’s, RCA came out with the “big bottle” type which is quite sturdy but I’ve always wondered about the GE design which is the size of a 5Y3GT. I’ve used them, not on a ST70 though. I really haven’t seen anybody mention much about those.
I would just go with a full solid state rectifier without the 5AR4. The drawback is that it does have big surge current during the startup. I have been running like that for years and it seems fine.
If it were mine, that's what I'd do as well. But, the client wanted to keep the 5AR4, so that's what we did.
One day, I would like to get two of these.
Why two?
@@ScottGrammer I have Thiel CS1.5, 2.2, and 3.6. These are low impedance and medium efficiency speakers.
mono blocks? supposed to work well, and you won't need to run the furnace in the winter
I've had multiple problem child 5AR4/GZ34 of Chinese (or early Russian) origin, myself. One pretty good solution is to use a 5V4GA (straight sided, not an old coke-bottle ST type) which will drop 15 volts more than a 5AR4. Not terrible, and you can buy NOS US made ones under $20. It would be nice to discover a huge stash of 5AR4/GZ34's. You could retire if you could find enough of them.
@@alanpecherer5705 I know that's right!
Does the ST70 have a thermistor?
No. It uses a slow turn on rectifier (the 5AR4) to slowly ramp up the current when first switched on. The 5AR4 uses indirectly-heated cathodes, which take about 15-20 seconds to warm up, as opposed to say, a 5U4, whose directly-heated cathodes heat up in 3-5 seconds. Solid-state rectifiers, of course, come on instantly.