Lowering Bias with a SS rectifier AB763

[deuce42]

Hi folks

I have another novice question about lowering bias I'm wondering if I could confirm.

I am making a low watt AB763 based upon a Princeton Reverb and I've also plugged in a generic solid state rectifier and some cheaper Russian 6p6s tubes (equivalents of 6v6 I understand but seem to have lower plate voltage rating).

I can't seem to get the bias low enough even at maximum/minimum turning of the bias pot. At its lowest I seem to get 24ma and there is an oscillating squealing sound coming from the transformer.

I have this idea it is possibly because I am using an SS rectifier and maybe the B+ is therefore much higher? If this is my problem, am I right that changing either R2 or R28 on the attached schematic would allow me to bias it lower (eg to about 18ma) ?? I note the first node at c28 and r58 seems to be measuring 440vdc which seems right in the schematic so Im not sure if the SS rectifier is the reason for my bias problem? I also note D4 is measuring -41.5vdc which I would have thought was correct as well.

Also, if the above is the issue, would it be better to change R2 or R28?

Please let me know if I'm doing something else wrong that is affecting why the bias won't go lower enough

I appreciate your help:)

[Stevem]

Hi!
First off how are you planing on making this a low watt PR, because how much power any amp can output is first determined by the level of DC V+ voltage and current that can be applied / used by your choice of output tube(s) which must of course be able to live at the wattage output your looking for.

Your going with a SS recto has kicked up the V+ by a minimum of 25 volts I would say and without knowing the secondary side specs of your chosen PT it's hard to make a call as to how to bias the output tubes.

Install a SS recto in a amp that a tube one when your looking to lower the amps wattage is fully counter productive!

Since you likely need at most only 4 more volts of negative bias voltage I would lower the value of R28 by 30% for starters and see where that gets you.

Do not reinstall your output tubes until you have this needed bias voltage increase as seen on pin 5 of each output tube socket!

[deuce42]

Thanks Stevem

I'm using a Hammond 15watt output transformer and these 14 watt Russian tubes. My power transformer has 325v@100ma specs and rectified with with the SS version and cap/resistor node comes out at about 440vdc on the first node, and about 405vdc on the second power node.

I realise the SS rectifier is normally counter productive to a tube version if making a low wattage amp, however I like the stiffer feel of SS rectifiers and so the reason to use silicon rather than a tube was based on tone/feel rather than just low power.

I'll lower R28 a bit and see how that transpires

[pdf64]

As we don’t know how the 14W anode limit of the power valves was derived, we can’t read much into it, just deal with them as you would 6V6.
So with the power valves out, do you measure at least -40VDC at TP 19 and TP13?

[deuce42]

Thanks guys, lowering the resistor did the trick and I'm not biasing at 19ma cleanly.