Hi all ,
Last few days I have been playing music through my revamped ST-70 and must say that I am very happy.
The biggest change is new OPTs that are designed by Bud Purvine of Onetics. Bud Sir has shared this with Asnain of Kolarkar audio research and he has built the new OPTs as per those designs. He also built new power transformer and a choke, both of substantially higher capacity than the originals. The power transformer has a 250V primary with a240V tap, this enables matching the supply voltage that is 240V nominal but climbs above that many a times.
The OPTs are a tad larger than the originals so we had to drill new holes in the chassis but the cover fits without any problem.
The old phenolic PCB was replaced with a new one that is based on the original schematic.
The GZ34 rectifier was treated with the "yellow sheet mod" to make life a bit easy for it. The selenium rectifier was also replaced with a silicon one.
I don't have a quad ofEL34 right now so I am running a quad of Sovtek 5881 that are sounding great.
Really enjoying this amp. Thank you Bud Sir and Asnain 🙂
Best Regards,
Last few days I have been playing music through my revamped ST-70 and must say that I am very happy.
The biggest change is new OPTs that are designed by Bud Purvine of Onetics. Bud Sir has shared this with Asnain of Kolarkar audio research and he has built the new OPTs as per those designs. He also built new power transformer and a choke, both of substantially higher capacity than the originals. The power transformer has a 250V primary with a240V tap, this enables matching the supply voltage that is 240V nominal but climbs above that many a times.
The OPTs are a tad larger than the originals so we had to drill new holes in the chassis but the cover fits without any problem.
The old phenolic PCB was replaced with a new one that is based on the original schematic.
The GZ34 rectifier was treated with the "yellow sheet mod" to make life a bit easy for it. The selenium rectifier was also replaced with a silicon one.
I don't have a quad ofEL34 right now so I am running a quad of Sovtek 5881 that are sounding great.
Really enjoying this amp. Thank you Bud Sir and Asnain 🙂
Best Regards,
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I suggest replacing all the carbon resistors with the modern metal film ones. Even they measure good, they may become open circuit when hot. I don't have any confidence in those carbon resistors.
Looks very nice. Not to be a downer, but I'd want to see the amp on a 'scope to see how well the OPTs really perform, and also to make sure the amp is stable with the replacements.
analogadikt,
Very nice!
Did the fixed adjustable bias have enough range for the larger bias requirement for the 5881, versus the higher transconductance EL34?
Very nice!
Did the fixed adjustable bias have enough range for the larger bias requirement for the 5881, versus the higher transconductance EL34?
@6A3sUMMER
Thanks..yes I had adjusted the grounding resistor for high adjustable voltage and low.
Thanks
Regards
Asnain kolarkar
Thanks..yes I had adjusted the grounding resistor for high adjustable voltage and low.
Thanks
Regards
Asnain kolarkar
Pleased to see your family memeber's outputs in an amp. Many years ago I reworked an ST 70 with this OPT design but I also converted the driver board to one designed by Audio Research. Was a delight to listen to and I only replaced it when AudioPrism became one of our customers, it was that good an amplifier.
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Thank you Bud Sir,
Even though the new driver board has identical schematic to the original , the performance as judged by my ears is much superior than the original. My late evening listening sessions are stretching to 2 AM. Asnain tells me that he is whipping up a few more of your designs like MC step up etc and I am eagerly awaiting those.
Best Regards,
Anwesh
Even though the new driver board has identical schematic to the original , the performance as judged by my ears is much superior than the original. My late evening listening sessions are stretching to 2 AM. Asnain tells me that he is whipping up a few more of your designs like MC step up etc and I am eagerly awaiting those.
Best Regards,
Anwesh
It has been an enriching and rewarding journey for us, learning from Bud Purvine Sir and putting in practice and experiencing the sonic outcomes in actual amplifier builds.
Sharing a few pictures of the builds:
First three are F2a SE, fourth is 300B SET with a 5kg output iron, 6BG6 SE and the RS1003 SE. All the iron were hand wound by Kolarkar Audio under his guidance and tested on the respective amplifier builds. Same goes with power and chokes.
The sonic experience is hard to put in mere words. I never experienced a need for sub, playing all these even on my 2 way bookshelf speakers.
Soundstage is unlimited and imaging is spot on. Over all, a very engaging experience with natural and relaxed presentation.
More builds planned, including IT coupled EL84 PP and 300B PP, no feedback.
Samir.
Sharing a few pictures of the builds:
First three are F2a SE, fourth is 300B SET with a 5kg output iron, 6BG6 SE and the RS1003 SE. All the iron were hand wound by Kolarkar Audio under his guidance and tested on the respective amplifier builds. Same goes with power and chokes.
The sonic experience is hard to put in mere words. I never experienced a need for sub, playing all these even on my 2 way bookshelf speakers.
Soundstage is unlimited and imaging is spot on. Over all, a very engaging experience with natural and relaxed presentation.
More builds planned, including IT coupled EL84 PP and 300B PP, no feedback.
Samir.
Mmmmmhhhhh...quite intching to build with the BudP outputs I axquired a bit ago... 🙂 Going to push the loading of a pair of EL84 to 6k6 a-a from a 300V supply.
Douglas
Douglas
Erm, what's that, please?The GZ34 rectifier was treated with the "yellow sheet mod"
Best regards!
Please clarify your comment.A stereo boat anchor rather than two mono boat anchors?
ray
Regards,
You have a DM.Erm, what's that, please?
Best regards!
Best Regards,
I believe Ray has carried a few of my creations. At least one end of them anyway. They have not had SS power supplies( big Iron and tubes ).Please clarify your comment.
Regards,
Douglas
For others not familiar with the "yellow sheet mod" here it is.Erm, what's that, please?
Attachments
I know that. Anyway, silicon diodes plus tube rectifier appears to be belts and braces. Either or, I'd say, and prefer "either" (Si diodes).
Best regards!
Best regards!
Except that the ST-70 is notoriously hard on rectifiers, and many new-manufacture 5AR4s will struggle in the amp. The mod is designed to relieve the strain while allowing for the slow warm-up.
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