I am planning an SE 300B amp using fixed bias. The power transformer I have has a 70V bias tap and a spare 280V tap.
I think I need about -120V or so for my -ve bias supply, is that about right? So what would be the best way of obtaing this?
Using the 70V tap would give me about -95V or so after some filtering. Would this be enough?
The 280V tap would give way too much, but I could use a zener follower to get an appropriate voltage.
Or half-wave voltage double the 70V tap?
Also, my fiddling with PSUD shows that with a couple thousand uF's of filtering the bias supply would take up to 30 sec to come up to max. Should I be delaying the B+ or DC filament supply until I have stable -ve bias?
Any other suggestions on implementing fixed bias on 300Bs are most welcome
BTW I plan on controlling the bias with DIY Hifi Supply's Autobias modules.
Rod
I think I need about -120V or so for my -ve bias supply, is that about right? So what would be the best way of obtaing this?
Using the 70V tap would give me about -95V or so after some filtering. Would this be enough?
The 280V tap would give way too much, but I could use a zener follower to get an appropriate voltage.
Or half-wave voltage double the 70V tap?
Also, my fiddling with PSUD shows that with a couple thousand uF's of filtering the bias supply would take up to 30 sec to come up to max. Should I be delaying the B+ or DC filament supply until I have stable -ve bias?
Any other suggestions on implementing fixed bias on 300Bs are most welcome
BTW I plan on controlling the bias with DIY Hifi Supply's Autobias modules.
Rod
As designed the 70V winding would work just fine for the bias supply, a minor resistor value change would be all that would be required to adjust the voltage to the required values.
No need to use a fancy bias regulator with the 300B, use good quality tubes like the JJ which are quite stable over time after the initial hours of operation. Get a set that have been burned in and matched. Bias servo is just something else to go wrong and adds little value in terms of actual performance. (I've built about 25 300B based amps both SE and PP, all of which have proven reliable and consistent with good tubes.)
Proper bias voltage needs to at worst be applied at the same time as the plate voltage, and should preferably have a much shorter time constant than the plate supply.
No need to use a fancy bias regulator with the 300B, use good quality tubes like the JJ which are quite stable over time after the initial hours of operation. Get a set that have been burned in and matched. Bias servo is just something else to go wrong and adds little value in terms of actual performance. (I've built about 25 300B based amps both SE and PP, all of which have proven reliable and consistent with good tubes.)
Proper bias voltage needs to at worst be applied at the same time as the plate voltage, and should preferably have a much shorter time constant than the plate supply.
Thanks, Kevin. That's all good news.
My B+ comes via damper diode rectification which gives a nice, slow start-up so that should allow time for the bias supply caps to charge up.
JJ tubes were not on my radar but I'll look into them also.
My B+ comes via damper diode rectification which gives a nice, slow start-up so that should allow time for the bias supply caps to charge up.
JJ tubes were not on my radar but I'll look into them also.
What is the purpose of R10 and D2 in the amp section? First off, D2 looks like it should be reversed. What is R10? I first thought it was an NTC, but the NTC in the soft-start circuit (R9 in the supply) uses a different symbol.
~Tom
~Tom
Hey Tom, I agree, and I too suspect D2 is reversed, and the S10K75 apparently is a varistor. Very odd feature, the designer is a member here and can perhaps clarify his intent.. I'd probably replace that with a zener string totaling 150V. The varistor is probably acting like a very noisy zener given the low current through it.
You're right ! D2 is reversed !
SK1075 is a varistor having a breakdown voltage approximatly 1.5 times the value noted, that is 1.5 * 75 = 112V to wich D2 adds 15V.
But no, a varistor is not noisy and have a much better tempco that a zener (specially HV ones), don't ask me why !
To be fair, I must admit that the screen voltage of the driver should be ajusted to obtain the targeted plate current 😱
That said, since the B+ is already regulated, it is probable that using a single -bypassed- serie resistor to feed the screen would be sufficient !
It's just my parano about bypass caps . . .
I promise to check the next time this amp will return on the bread board.
Yves.
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