Thinking about a clone of the WAD 300B PSE monoblocks

kennyk · Post by **kennyk** » Mon Jul 15, 2013 11:14 am

Nick wrote:
I think that the 425v operating point is probably too close to the design limits anyway
May be saying what you know, but remember to remove the cathode voltage from B+ before you check the operating conditions the valve is working under.

Good point, it's a cathode bias circuit. that'll teach me to post at 2 in the morning without checking everything!

Greg · Post by **Greg** » Mon Jul 15, 2013 11:39 am

When studying the data, schematics etc, be mindful that what WAD originally published in the magazine article was not necessarily acurate and the more reliable information will be had from the circuit etc published with the build instructions that came with the kit. WAD had a habit of publishing the magazine article based on the prototype build which they tweaked or changed before final production. They didn't later publish corrections in the magazine.

kennyk · Post by **kennyk** » Mon Jul 15, 2013 12:42 pm

Thanks, Greg. It's not something that I'd thought about, until I started looking at this in a bit more depth! I suspect it will become more and more a consideration as this progresses. I don't actually have copies of the the original supplements that dealt with the 300B PSE kit, other than no 58 with the 'Tuning the 300BPSE' article which covers the Jenson / Black gate mods and the DC heaters for the 300B. Maybe that's a good thing, but I hope to be able to get a chance to read the relevant articles at some point. However, they don't seem to be available on the current W-D shop at the moment.

At the moment I'm fairly open minded about how to approach the project. Given that it's unlikely that I'm going to be able to source original chassis and complete iron sets, it gives me a bit of leeway; Presumably a different OPTX primary inductance will change the operating point of the 300Bs and if it means an off the shelf component instead of a custom wound one-off set then it's something I'd consider doing to save on cost, as long as it's not going to sacrifice the lifespan of the valves or the sound quality.

Ali Tait · Post by **Ali Tait** » Mon Jul 15, 2013 1:10 pm

Some bought OPT's from the likes of Sowter back in the day, for upgrading purposes. Sowter may still have the correct specs.

Ali Tait · Post by **Ali Tait** » Mon Jul 15, 2013 1:11 pm

Some bought OPT's from the likes of Sowter back in the day, for upgrading purposes. Sowter may still have the correct specs.

kennyk · Post by **kennyk** » Mon Jul 15, 2013 3:00 pm

Ali Tait wrote:Some bought OPT's from the likes of Sowter back in the day, for upgrading purposes. Sowter may still have the correct specs.

Always good to know, Ali, thanks. I should be able to get some suitable iron for the project somehow; whether its within a reasonable budget remains to be seen!

pre65 · Post by **pre65** » Tue Jul 16, 2013 11:24 am

Hi Kennyk

can I ask what prompted you to choose the 300b PSE as your future project ?

kennyk · Post by **kennyk** » Tue Jul 16, 2013 12:19 pm

pre65 wrote:Hi Kennyk

can I ask what prompted you to choose the 300b PSE as your future project ?

It's a good question. There's a number of reasons. When I first got into the whole DIY hifi thing in 2005, the WAD kits were my main point of reference, and I built the PreII set before that incarnation of WAD fizzled out. I wanted to go for the 'flagship' 300B monos but as a novice at valve amps I didn't feel I had the knowledge to complete a point-to-point build. and then, ultimately Peter took over WAD and I decided to build the HD83 as a 'next step' since it was marginally more complex than the Pre and phonos I'd already built. Once my confidence had grown enough to have a go at a power amp, the 300B PSE monos simply weren't available and at the time I still really fancied a set; To me they looked (and still look) the business. So I went for a KEL84 amp, partly because it was available, but also because it was a low-cost option; I spent around £350 on mine at the time. I thoroughly enjoyed building it, and have enjoyed using it ever since.
I still have a very warm affection for the WAD range of kits, and I got the chance to borrow a pair of the 300B PSEs around 3 years ago and was really taken with them at the time. Tempted though I was, I get as much satisfaction, if not more, from building as listening!

So I suppose the 300B PSE has been an ambition for a number of years. Having now acquired a whole range of new skills in my two years of guitar making at college, I've got a new outlook and approach to things, and most of all, some confidence! Its going to be a long term project, purely because funds are limited, I'll be buying bits as and when I can afford them. but bits are starting to arrive; it might only be a couple of B7G bases but it is a start. won a pair of NOS 6AU6 valves for 99p as well! but in the main I'm concentrating on getting generic bits; valve bases, tag board, switches, plugs and sockets etc, building up a stock of parts.
Given that its a proven design I'm confident of getting a working pair of monoblocks. Ultimately they probably won't look identical to the originals, purely because getting the original chassis, or identical clones is likely to be cost prohibitive; also any transformers I have to get made might not be the same dimensions as the originals, in which case an original chassis might not be useable. But I plan to build something that looks pretty, and not too home-made. and sounds good obviously.

Paul Barker · Post by **Paul Barker** » Tue Jul 16, 2013 1:23 pm

OK Kenny, your downfall in this project is today the cost of the components far outweighs the original kit price.

You don't seem to have sufficient money unless you find a second hand one going cheap.

I trashed a Chinese 845 which sounded poor for the parts.

On examination the B+ was achieved by voltage doublers (a big non no in my world) But the output transformers were outstandingly good. They are 4k5 (so unfortunately not what you need) and they have byfilare secondary with silver bi wound with copper. The sound is exceptionally good. The best sounding amp I ever made used them. It was an 801a amp.

Here is the exact amp on eBay I just hope they have still got the same transformer winder.

Chinese 845 amp

I don't recommend the amp as a whole. Spoiled by voltage doubler power supply and horrible production quality valves.

But I am throwing out concepts for you.

Ali Tait · Post by **Ali Tait** » Tue Jul 16, 2013 2:03 pm

Mike may be able to supply you a full set of trannies for the amps at a good price.

Ali Tait · Post by **Ali Tait** » Tue Jul 16, 2013 2:08 pm

Mike may be able to supply you a full set of trannies for the amps at a good price.

Mike H · Post by **Mike H** » Wed Jul 17, 2013 1:57 pm

The SE OPT's may be a problem, for the 'stock' ones the inductance is on the low side. Depends how fussy you are.

steve s · Post by **steve s** » Wed Jul 17, 2013 2:50 pm

Greg has a pair of my output transformers that he has the option to buy, which i 'm sure he does not want.. They are sowters for the pse2a3, but am sure they are close enough for the 300b ...
Personally i always use transformers with a higher impedance than specified.i'm not just not sure about the current they will take though...

kennyk · Post by **kennyk** » Wed Jul 17, 2013 4:23 pm

steve s wrote:Greg has a pair of my output transformers that he has the option to buy, which i 'm sure he does not want.. They are sowters for the pse2a3, but am sure they are close enough for the 300b ...
Personally i always use transformers with a higher impedance than specified.i'm not just not sure about the current they will take though...

Just been having a look at the sowter site; if it's the off-the-shelf ones (2.5k primary), then there's two listed. though I've no idea why the 15w version (SE05) is rated at 120mA quiescent and the 20w (SE07) is rated at only 100mA. seems bizarre!

Greg · Post by **Greg** » Wed Jul 17, 2013 4:48 pm

These are 3.5K. We know that because they were ordered originally from Brian Sowter by Tony Moore. Sowter's sent me the wiring diagram but didn't send the rest of the spec as requested, so a follow up would be needed. Steve is asking a mates price for them but you will need to talk to him if you're interested. If you decide to have them, I will be happy to send to you on behalf of Steve.

audio-talk

Thinking about a clone of the WAD 300B PSE monoblocks

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