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Hi Kevin,I've used some of your Lundahl interstage transformers with great success in a DIY project. Recently, New Year's eve, a friend dropped a bone stock ST-70 on my lap, so with my previous experience with your transformers I figure I'll investigate your ST-70 plan. Now I can't quit thinking of what to do with it. Do you have a suggested upgrade plan or path using your improved components if I can't buy it all at once? Should I buy the circuit board first, then the shunt regulators, then etc.? You get the idea. I didn't find a thread like this. My amp doesn't have any tubes with it, so I'm not constrained by a specific tube but I have some matched quadsof the Russian 6P3s-e tubes from Jim McShane that have worked fine in ST-70's of friends. I can start with them.
I also wonder how would a person calculate the maximum bias current for a given output transformer? The old Dynaco catalog doesn't specify.
Thanks,
John
De gustibus non est disputandum
Edits: 01/26/14 01/26/14Follow Ups:
There is no thread like the one you suggest, until now....
Yes, you can do the upgrade in the order you outline. The Main Board Kit will work well to start, and the shunt regulators can be added at any time after that. Ultimately, it's also possible to improve the sound quite a bit by adding Lundahl output transformers. Operated using the original power transformer and 50mA of bias current, the 6P3S-E tubes sound very good. I was using them in mine until I increased the voltage by replacing the power transformer with my SST-70 power transformer. The increased voltage was simply too much at that point and I had to switch to either 6550s or KT-120s.
Kevin Carter
K&K Audio
www.kandkaudio.com
Thanks Kevin.
Do you have any idea of how much bias current the original output transformers can stand? I like class A and know the improvements it can make to sound quality, but a given primary winding can only be pushed so far.
John
De gustibus non est disputandum
The output transformers will take far more bias current than the original power transformer will deliver and remain whole. However, the real limitaiton is the tube's capacity. The tubes you suggested have a pretty well-defined 21 watt max dissipation rating and the 430VDC raw supply less the 40VDC of cathode voltage puts 390V across the tube. With a bias current of 50mA, that's 19.5 watts...
Kevin Carter
K&K Audio
www.kandkaudio.com
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