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Old 5th May 2007, 09:55 PM   #1
G is offline G  United States
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Default Driving a 6B4G with a single 5687/7044/E182CC?

I have a SE EL34 amplifier that I built about 4 years ago that I would like to modify into a 6B4G amplifier. I used only a single grounded cathode 5687 input stage to drive the EL34s running at around 12mAs. Will that single stage be enough to drive a 2A3/6B4G? If I use a 12B4A preamp with say a gain of 3 or 4 in to drive the amp would that be enough? In my thinking 5v (the gain after the 12B4A preamp) multiplied by 12 (the estimated gain of the input stage) would give me more than the 45v needed to drive the 6B4G to full output. Am I looking at this wrong?

The EL34s are only running at around 55mA because I am using a pair of TFA204 output transformers (designed for 2A3s) that are limited to 60mA before saturation so minimum modifications are needed to switch it over if I have enough drive.
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Old 6th May 2007, 03:28 PM   #2
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Gavin,

IMO, your thinking is essentially correct, but there are some "gotchas" lurking.

If the 2 stage power amp is driven by 5 VRMS from the line stage, the bias on the 5687's grid has to be at least -7.1 VDC. Clipping at the I/P grid is a NO-NO.

AC heating the 6B4s rates to yield unacceptably high residual hum levels. FWIW, current regulated DC on DHT filaments gets good "press".
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Old 6th May 2007, 11:29 PM   #3
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Thank you for the reply Eli. I have since changed my mind. I'm going to just build a new amp and leave the SE EL34 amp as it is. I think a 2A3 driven by either a 417A or a cascoded 2C22 front end.
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