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Hi folks,

I just finish my ST35. No incident, the amp works OK and sound good so far.
I measure the B+ and it's abit high on 415V. The power kine is 245VAC (i'm in Australia) and the power trans is Edcor XPWR005-120/240. Secondary spec of the trans rated at 660V CT 180mA. Also using Hammond 156R inductor rated at 1.5H, 200mA, 56 Ohm.
I think this a bit hard on the tubes. I tried to dial the bias, but it can't get lower than 35mA. My calc the tubes dissipate at 14W which is higher than 12W rating for EL84.
Anyone know how to lower the B+ ?

Thanks in advance

Get a variac.

Thanks,
I do not have variac, but I think it would not work.
The filament supply are already pretty much spot on at 6.4V.
If my calc is correct, I need to reduce AC input by ~8% to get B+ from 415V to 380V, this will take the filament under 6V.

How about another choke with higher DCR / adding an RC filter? 35 volts is a lot to dump though.
I would convert to a Vacuum tube rectifier personally or replace the Power Transformer.

Andrew

I would disconnect the first capacitor of the power supply LC ladder, C9 (the one between the rectifier and the choke),
to see if less smoothing of the rectified wave form before the choke can drop the B+ voltage enough.

If it does, you can re-connect C9 after the choke.
If it does not, you could try replacing the choke with a one with higher inductance, such as 157R or 159S.

Good luck.

A trick we use in guitar amps with high B+ is to put a zener in series with the B+ line of about the drop you need to make.

Try changing your bias sense resistors from 10 ohms to 100 ohms to get a lower bias - say 30mA. Such a scheme won't be much different than what Heathkit did in their PP EL84 amp.

Shannon

Thanks guys,

I'll try to put 360ohm/5W resistors in each B+ first, and maybe change the cathode resistor if needed.

Cheers

Just to update,
The resistors are installed, B+ now 395V, bias at 33-34mA. My calc the total dissipation is around 13W (plate and grid2) so I guess it's fine
The amp sounds very nice with good bass and treble, but mid a bit stiff/glassy. Maybe needs break in.

Cheers

The very popular EFB fixed bias mod posted on here and on the Tronola Electronics site should help you with tube life and sonics. You can take out the added resistors in the B+ which surely won't help the sonics. In fact a lower DCR choke (around 20 ohms or less) from Edcor may further help the sonics. Look at the Eico HF-81 PS.

High anode voltage, within reason, will not harm EL84 tubes. Some are rated by the manufacturer to 450 volts and one in particular lists 500 volts. The ratings listed by RCA and followed by other companies in their “spec” sheets are very conservative. To determine the effective plate voltage, the cathode voltage can be subtracted from the anode voltage as measured to ground. Ex: 415 VDC -13VDC (cathode voltage) = 402 Volts (plate cathode differential).

Lowering quiescent plate and screen dissipation will however, increase tube life.
I think the recommendations to either implement EFB bias, or Shannon’s recommendation to correct the bias level are both better solutions, since adding series resistors in the B+ supply, hurts power supply regulation, and conversely, reducing it (one of the big benefits of using a choke), improves regulation.