EFB bias adjustment and speaker impedance

Rockyhill

Rockyhill

AK Subscriber
Subscriber
Q: Does the speaker impedance impact the bias adjustment for the EFB mod on a ST-35?

I'll be checking the bias today. I normally run 4 ohm (nominal) Infinity RSb's with the amp but it would be more convenient to use a pair of small bookshelf sized speakers where I'll be working on the amp. They are labeled "4-6 ohms average".

Thanks
 
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The bookshelf speakers will work fine. No adjustment is needed. Bias adjustment should be done one time and then maybe 6 months later to see if the bias has drifted.

Just curious as I am not familiar with the ST-35 but does it have adjustable bias?
 
I went ahead and used the small speakers anyway. Since the measurement and adjustment is done under quiescent conditions (no input signal) I figured the load wasn't a factor. I haven't checked the bias for probably a year (it's all under the hood so it takes about an hour to break down the amp, move and set up to measure then put back in place... plus the inevitable dusting!). It had dropped from the desired 270mv to about 245mv. I also added a heatsink topside for the LM337 which is mounted on the underside of the chassis. I wanted to enhance the heat transfer from the chassis.

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That should be fine. If the chassis couldn't dissipate the heat its not likely the heatsink would be the answer (but no harm). You should be good to go. I'd bet my eyeteeth Dave determined 270ma was the optimum operating point and the efb makes everything run at a lower temperature and will extend tube life. I was just wondering why you backed off the bias?
 
nj pheonix said:
That should be fine. If the chassis couldn't dissipate the heat its not likely the heatsink would be the answer (but no harm). You should be good to go. I'd bet my eyeteeth Dave determined 270ma was the optimum operating point and the efb makes everything run at a lower temperature and will extend tube life. I was just wondering why you backed off the bias?
Click to expand...

Yeah, that little heatsink was more to make me feel better about the dissapation than anything else.

Not sure I understand "I was just wondering why you backed off the bias?". I always set it to .270v as Dave advises. It had drifted to the lower value. Is that due to the 6BQ5's aging?


8. The bias control should be set for an average of 0.27 vdc across the 5.0 ohm cathode resistors
under quiescent conditions. This will produce a current flow of 27 mA through each output tube,
and a plate dissipation of just under 10 watts.
 
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