Mr Steve Bench and his parallel valves...

[Andrew]

Hi Gents,

On Steve's site I read that his measurements have shown that, in general, parallel valves are a bad idea, this would seem to correlate with experience of those with WAD 2A3 amps and the valve pulling experiments. However, Steve firmly believes, to the point of using it in his design for the RIAA5, and putting his money where his mouth is that where un-bypassed cathodes resistors are employed paralleling valves can actually reduce distortion.

I like constant current sources on my signal valves, so my question is how would a CCS change the results on unbypassed parallel valves. I was erring on the side of building such an amp when it occured to me that a CCS fixes the current and so might change things...

Any thoughts?

cheers,

-- Andrew

[Greg.]

Hi Andrew,

I can't answer you specific question but not everyone who tried the valve pulling experiment on WAD 2A3PSE found it to be a good option. I can understand how some have been beguiled into thinking it's an improvement, but with my experiments, it's only good with very simplistic music. It'll rapidly run out of steam with anything even moderately challenging music wise and of course, the whole balance of the amp is upset because it wasn't designed to work with just on set of 2A3 valves.

Best wishes,

Greg

[soulminer]

This is exactly what I found with my 300B PSE.

http://www.world-designs.co.uk/forum...hread.php?t=71

[NickG]

IMHO it seems clear that if the amp is being driven near the limit of two valves then expecting it to improve with one is asking a lot, and doing that while playing complex music is also expecting a lot.

But there may be a indicator here that if the amp is well suted to the speakers and room, and if the material is not asking more of the amp than can be delivered (true of any amp), then there may be a hint that a single output device can provide better results.

[soulminer]

So what would happen if I removed the cathode by-pass capacitors on my 300B PSE ??

[Paul Barker]

You would get no base.

[Ray P]

It has been recommended to me that increasing the value of the cathode caps in my Bottlehead (2A3 SET) amps by an order of magnitude will improve bass perfromance - I've acquired some suitable caps and aim to do some experimenting when I can find a few hours to spare.

Ray

[Andrew]

Originally Posted by Paul Barker

You would get no base.

Ah but what about small signal valves?

-- Andrew

[Les Reed]

Using un-bypassed cathode resistors on small signal valves results in lower harmonic distortion and subjectively better sound due to local feedback. The only problem is that the gain is significantly reduced.

[Paul Barker]

Yes i was discussing in context of original post, output stage. My answer is irrelevasnt to va and driver stages, where base is unaffected except in the case of inductive loads. sound is normally improved, but at the cost of gain and hf perfoirmance in some extreme cases.