Confusion over capacitors

[MartinH]

I have a WAD 2A3PSE which is currently in standard spec. However, I have been taken by the urge to try some tweaking. One supposedly simple modification I would like to try is adding additional capacitors to the power supply tag board that follows the valve rectification and choke. This was described by 'Greg' on the WAD forum. The capacitors that were supplied with the kit were 100uF 450V types which were polarity marked and therefore suitable for the DC voltage in this circuit. The modification suggests bypassing these with various caps, for example 'C13 is bypassed by 2 x 0.22uF Arcotronic polyprops (RS order no. 114-610) in parallel, and also paralleled with 2 x Rifa PHE428 2200pF polyprop (240-5621)'. I guess by bypassed and paralleled this means that the caps also connected between the live and grounded part of the circuit, similar to the 100uF 450V caps.

So, what confuses me is that the caps referred to appear to be those suitable for AC use and not polarity marked. Is it OK to put these between the live (375V) and grounded parts of the circuit? Or will my amp disappear in a fireball??

[cjnuk]

As long as the voltage rating of the capacitors you intend to use (and it is usually marked on the package) is reasonably in excess of the HT voltage then they will not be damaged. Also capacitors like these are not polarised and can be connected either way.... Although the purists out there may well disagree with that!

Good luck.

[Darren D]

You will be fine, AC caps can be used connected either way around, BUT, DC caps must observe polarity or they'll object profusely.

[MartinH]

Thanks a lot for the help..

[andrew ivimey]

Martin, what do you want to achieve?

just adding smaller caps around the existing electrolytics won't achieve a sonic improvement.

you need to get rid of the electrolytics. It was generally agreed on the old WAD forum that using Solen polypropylene caps was the way to go. Instead of the first 100mfd electrolytic, use 30-50 mfd solen. After the choke, replace the 100mfd electrolytic with as much as you can squeeze into the restricted space 100mfd or more. One of us actually used a sub chassis to build up an impressive second cap made of polyprop caps and was very impressed.

But something on the lines of 50mfd/choke/100mfd was generally agreed to be the business. If you wanted to add o.1 or o.2 'snubber' caps in parallel with the new polyprops that might help. I didn't.

I was so happy with using Solens that I always have since in power supplies unless I can afford Blackgates WKZs or been lucky to have some Cerafine electrolytic power supply caps; smaller! than the equivalent polyprops.

Looking back, much as I really liked the 2A3 amp, and using only one 2A3 instead of parallel really was better, IMHO, the weak point of the amp is the output transformers. They can be easily bettered.

[Greg.]

This will be a first. I've not disagreed (I think) with Andrew on the forum before

Anyway, Martin, my 2A3PSE and, I believe, Tony Moore's are still working well and sounding good. They've not gone up in smoke! As much as I like my WAD 300B PP clone, I also thoroughly enjoy listening to my 2A3 as well, but with certain music only. Nothing too demanding is the recipe.

The mods are also reproduced here. See

http://www.world-designs.co.uk/forum...read.php?t=143
http://www.world-designs.co.uk/forum...read.php?t=142
http://www.world-designs.co.uk/forum...read.php?t=141

I disagree with Andrew about no sonic benefit to sound from bypassing the PSU caps. Both I and Tony heard a distinct improvement when we did the mods Remember we also replaced the PSU caps being bypassed with good low ESR types. Most of that is revealed as more weight and authority to the sound but including better transparency and detail.

I further disagree that Solen's are good caps to use in PSU application. I find Solen's sound horrible in amplifiers. I like them in speakers, but never in amps. Others share my view, but it is a matter of personal preference and choice. I wouldn't argue with the claim on Blackgate WKZ's.

I do agree on his observation on the O/P Tx's, however, I've compared the sound of my amp with Tony's that is upgraded with Sowter replacements and although I acknowledge the Sowters sound better, I don't think the amount of improvement justifies the amount of cost involved. It's very much a 'splitting hairs' difference. I also realise that the spec of the original Tx's is poor and on testing they measured badly, but that doesn't account for the fact that my upgraded 2A3PSE still has a creditable sound performance which I really like. I know others with good ears who share that view. I also know others who have listened who consider it runs out of steam with certain music. I'd agree with that view also. It depends on what you want to listen to.

My speakers are 89dB sensitive albeit an easy load. Pulling one of the 2A3's per channel sounded good on the most simplistic of music, but anything slightly more challenging revealed the lack of puff the amp could then deliver.

My suggestion therefore is to weigh in the balance what might be best for you and the system you use.

Anyway, Andrew's now way past this kit amp thing. He's now a completely unbalanced mad scientist. Just look at his latest offering in the Gallery.........

Best wishes,

Greg

[andrew ivimey]

- not exactly disagreement Greg, I am sure we can find something about that later.

Solens are good for PSUs (and crossovers), not e.g. coupling caps

you are right to actually bring up the thread references

eva cassidy can sound good on weedy amps sure, and that was a fault too of a PSE2A3 as only a 2A3SE, but consider other 2A3 amps (I've done); SRPP, Mu, the infamous Loftin White, 6SJ7 (a pentode) even a simple 6SL7 as driver and it became obvious that it wasn't the nature of just one 2A3 that was weedy. It had to be something else and that only leaves the output transformers (if the PSU is sufficiently power full).

I never did 'snubbers' so that was why I said they (only) might help.

Whether its a good thing or not, that 'mess' in the gallery now has another amplifier next to it, really just to try out why I can get 807s to sound like. I am taking a much slower approach to giving 6C33s one last chance, this winter break, but you never know what I will bump into and find a use for. I have been threatened with more huge transmitter valves ... oh boy!

[MartinH]

As much as I would like to buy another valve amplifier (WD88VA ??), I think I am going to have to stick with my 2A3PSE for some time despite its faults. Currently I have soldered the polyprops across the original PSU caps. It doesn't look overly pretty and maybe sometime in the future I should build up a new PSU board to tidy things up.

I am doing a number of mods at the same time (operating point change, battery biasing and fitting the new relay switched input circuit) so it will be difficult to determine the effect of the additional polyprops in the PSU. I know this is not the best approach, but I thought I would remove the original input wiring, perform the PSU, battery bias and operating change mods and then put in the relay switched input circuit and therfore avoid a melted plastic disaster with my soldering iron (e.g. making a mess of the original, or worse, the new input wiring!!!). Hopefully when all is done there should be some improvement??