300B PSE battery bias on 6AU6

[Cyprus Martin]

I've just been trying to eliminate a hum problem but can't check the wiring connections post battery bias mod as I can't find the circuit diagram/instructions for this. Can anyone (Max?) supply them for me as the WAD BB is no longer available.
Martin

[soulminer]

I'm going to "bump" this back up, as I am seriously considering doing the battery bias mod to my PSEs too. What I would like is a circuit diagram, because as Martin states, the old one is no longer accessible (not that I could find anyway). I think I have it in my mind what needs to be done, but would rather be 100% sure.
I would also like some advice, opinions and experiences of anyone that has actually implemented battery bias. Pros, cons, effects on sound, etc. What happens if I don't use my amp for three or four weeks for example.. do I have to charge the batteries before use, or will they retain their charge for ages ?? I have read something posted by Dr John on this subject, and he suggested fitting it in place of cathode resistors and by-pass caps, so it must be quite good if it comes with a recommendation from the Dr himself.
I am also interested in something posted by Max I think, about changing the grid capacitors (?) on the 6AU6 for slightly higher values to increase the bass response. Any info on that would be great too.
Thanks as usual chaps.

[Greg.]

Hi Lee,

You'll find some of your answers in this thread from about post 12.

http://www.world-designs.co.uk/forum...ead.php?t=1372

Assuming the circuit remained the same as originally printed in the supplement, remove R4 and C1. Connect the positive end of the battery to cathode pin 7 on the 6AU6 and the negative end to ground where C1 would have been grounded. My only concern is that you are looking for approximately 2 volts, so I'm not sure what batteries were used by Max and others who have done this.

I can't comment on the effect on 300B PSE because I've not heard it, but generally I find it brings a cleaner more detailed sound with better controlled bass and possibly with a bit of gain. That was the effect on both my 2A3PSE and my original WAD 300B PP clone.

Best wishes,

Greg

[soulminer]

Cheers Greg. That was one of the posts I read initially that set me thinking about it. It seemed to be quite a good mod a few years ago on the old WAD BB, but then it all went quiet and no-one mentions it much any more. It obviously works, but I was wondering if the bother of having batteries would outweigh the performance gains. I sometimes don't listen to my amps for three or four weeks on the run now, due to other commitments, and would not want to be bothered having to spend three hours charging the bias batteries up for a quick thirty minute listening session, if they had gone flat over the four week period. Any comments anyone ?

[John Caswell]

Hi Lee,
it is most unlikely that the batteries lose charge at all over a 4 week period, after all useable shelf life is often quoted in years.
Think about it, when the amp is switched off where is the path for the battery to discharge? The valve is not working, so effectively you have a complete open circuit with no path to the other terminal of the battery, there is not even capacitor leakage to take into account

John aka Dr John

[soulminer]

Thnks John. As usual I am probaby worrying over nothing. I will start sourcing the parts to get this little mod on the go, ASAP. Many thanks for the info and I will probably be back with more questions nearer the time.

[russell allen]

This is probably the best "bang for the buck" mod I did on my 300BPSE's.
I used BG bypass caps on the 300b's, jensen PIO's for C2 and C4, telefunken 6AU6,s and mullard ECC82's but the battery bias was probably the most noticable.
The feedback circuit has to be removed/disconnected as well as the 6AU6 cathode bypass cap and cathode resistor if I remember correctly. I haven't used or opened up my PSE's for a few years now, I used the the green lozenge shaped batteries from RS (I can find the part number if anyone is interested). I may be putting them on the for sale board and/or e-bay soon I think, they are just gathering dust at the moment.

Regards

Russell Allen

[soulminer]

Originally Posted by russell allen

I used the the green lozenge shaped batteries from RS (I can find the part number if anyone is interested).

Yes please Russell. That would be great. BTW, when you say "best bang for buck" mod you did, what kind of "bang" was it ? Were your findings the same as Greg's ? Or did you get a different set of results ?

I have never had feedback on my amps, so have just wired the bottom of the feedback circuit straight to earth. I must get the circuit diagram out again tomorrow and have a good look over it. It's been almost two years.

[Richard]

Lee,

Battery charge life will depend on cathode or grid battery bias. With grid bias it will not charge in use and will discharge due to internal leakage. NiMh go down in a few months, NiCads stay up longer. With cathode bias they should charge in use but if they are left to go truly flat they may allow the valve to over-run (no bias) and I'm not sure whether the valve would expire before the bias voltage was re-established.

In any event try and arrange the same 2v the stage is set up for at the moment as distortion may rise considerably if you change bias on a pentode without other parameters and this may be more sensitive to changes than a small triode stage which often runs around 1.5v. I'd expect a fully charged NiCad in cathode bias to maybe have a bit of surface charge in regular use and run around 1.3v.

Rich

[Greg.]

Hi Lee,

Prevoiusly Tony Moore and I experimented between the 'green lozenge type NiMh and Nicads. We preferred the sound of Ni-Cads, finding the MiMh brought a sharp glassy edge to the sound, particularly noticable in upper treble. As said in the other thread, some fitted batteries without pre-charge with no ill effects on the valve.

Best wishes,

Greg