jdsalinger
Super Member
I realize I'm resurrecting an old thread, but I didn't have sufficient contribution for a new thread and part of the reason I'm posting is to share my experience with my recent restoration. I refered to this thread when building, so I presume that others will reference it in years to come.
I found that I needed to use a 270V dropping resistor (8w min. I used a 25w Arcol) after diode CR3 to get the appropriate B+.
R126 - 3K 1w
R123 - 6.8K 1w
All my voltages are within 2% (even with 62 year old resistors!)
But that brings up a question. I'm an engineer and I take maximum ratings on parts seriously. Why is it OK that Fisher operates at 10% over the 6BQ5's maximum plate voltage and 28% over the maximum screen voltage? Is it because tubes were so commonplace and plentiful in those days?
Originally, I used a 1K dropper to operate within 6BQ5 tube specs, but then decided that it really wasn't a Fisher if I didn't have the same operating point, so the above is where I finally landed.
Wonderful sounding amp, by the way. Different, but as good as my JE Labs EL34SE. It has extreme detail and great separation between instruments/voices. It allows me to hear instruments that I never noticed before. After being an SE snob & mostly scoffing at PP, I must admit that I love the el84/6BQ5 sound.
I have been waiting for a Fisher tube amp for 15 years and finally found my first.
I found that I needed to use a 270V dropping resistor (8w min. I used a 25w Arcol) after diode CR3 to get the appropriate B+.
R126 - 3K 1w
R123 - 6.8K 1w
All my voltages are within 2% (even with 62 year old resistors!)
But that brings up a question. I'm an engineer and I take maximum ratings on parts seriously. Why is it OK that Fisher operates at 10% over the 6BQ5's maximum plate voltage and 28% over the maximum screen voltage? Is it because tubes were so commonplace and plentiful in those days?
Originally, I used a 1K dropper to operate within 6BQ5 tube specs, but then decided that it really wasn't a Fisher if I didn't have the same operating point, so the above is where I finally landed.
Wonderful sounding amp, by the way. Different, but as good as my JE Labs EL34SE. It has extreme detail and great separation between instruments/voices. It allows me to hear instruments that I never noticed before. After being an SE snob & mostly scoffing at PP, I must admit that I love the el84/6BQ5 sound.
I have been waiting for a Fisher tube amp for 15 years and finally found my first.