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Bias doubler
Posted: Tue Jun 09, 2020 1:26 pm
by mhuss
I finally got around to designing a PCB for the oft-needed bias doubler mod for DR103 amps. It mounts in the chassis using the existing holes for the "missing tube" socket.
- Bias1-1.jpg (273.43 KiB) Viewed 6249 times
Re: Bias doubler
Posted: Wed Jun 10, 2020 6:32 pm
by Dr.HI-TONE
very nice!
Re: Bias doubler
Posted: Fri Jun 12, 2020 2:56 pm
by gtomax
How do you know if you need a bias doubler?
Re: Bias doubler
Posted: Sat Jun 13, 2020 1:48 am
by mhuss
If the stock bias is running the tubes too hot, a doubler is the best solution. You have to actually measure the output tube current to determine this (unless it's way out of whack, in which case the output tubes will start red-plating.
) About 34mA per tube is appropriate given current tubes and a typical 510-520v B+ using modern mains; I'd definitely recommend keeping it under 40mA.
Re: Bias doubler
Posted: Sat Sep 12, 2020 8:13 am
by Stratocasphere
Hi
Quite in trouble with my DR103 1975.
I have tried to change my 4 EL34 tubes.
I measured with a Tube Amp Doctor Bias device the current on the tubes
With Svetlana tubes i find
T1 37.7 T2 40.3 T3 39.9 T4 37.2
With Mullard Reissue i find
T1 39.7 T2 53 T3 43 T4 40
With Svetlana C wings
T1 56 T2 50.5 T3 50.2 T4 49.4
With Siemens RFT
T1 39 T2 39.2 T3 60.4 T4 61.2
The tubes are supposed to be matched...
Any help? Recommendations? Explanations? Thanks in advance,
Re: Bias doubler
Posted: Mon Sep 14, 2020 3:31 pm
by mhuss
"Matched" when it comes to power tubes is a somewhat nebulous concept.
1) It's unlikely the vendor matched the tubes at the same plate voltage as your amp.
2) Modern tubes are of such variable quality, and NOS tubes are getting harder to source; I suspect the acceptance window for "matched" is getting wider and wider.
3) Tube characteristics will vary slightly after a burn-in period.
These days, I usually make sure the variation is not way too wide, and just bias the amp so the dissipation is OK for the 'hottest' tube.