Thinking about a clone of the WAD 300B PSE monoblocks

[kennyk]

I've been thinking about building a set of the 300B PSE monos over the next few months, and I joined up here with the intention of trying to find out the original chassis supplier. from what I've found from searching and reading various threads, it seems that they're unlikely still to have drawings, so I'm on my own. That being the case (pun unintended) it gives me free reign to have a bit of license.
At the moment, I'm thinking that some of the wooden surround 17" x 10" cases from Bluebell might be the perfect thing, and get some top plates milled from Shaeffer.

I'm quite handy with wood, so I was thinking about modifying the wooden surrounds with recessed front and back panels for the switch / inputs, outputs, humbucker pots etc , that are screwed to the inside of the wooden sides.

For the Iron, I'm hoping that I am correct in assuming that SP Wound were the original transformer manufacturers and that they'd be able to supply me with similar units.

And that's as far as I've got, other than a circuit diagram.

Please feel free to pitch in or dissuade me from attempting this...

[Greg]

I don't think SP Wound made the transformers for this amp. I believe it was Morite who sadly went bust a few years ago and the company that took over their business had no interest in continuing to produce their previous products. Not good news I'm afraid although there are still 300B PSE's that come up on the secondhand market occasionally.

[Ali Tait]

I was lucky enough to get a mostly complete pair for £200 recently. Greg is correct that Morite made the trannies. John Caswell on WD should have the details of the trannies, and may even have some originals for sale. I'm not selling mine, but if you need any info, feel free to shout. I have a full set of instructions/diagrams too.

[Ali Tait]

I was lucky enough to get a mostly complete pair for £200 recently. Greg is correct that Morite made the trannies. John Caswell on WD should have the details of the trannies, and may even have some originals for sale. I'm not selling mine, but if you need any info, feel free to shout. I have a full set of instructions/diagrams too.

[kennyk]

I don't think SP Wound made the transformers for this amp. I believe it was Morite who sadly went bust a few years ago and the company that took over their business had no interest in continuing to produce their previous products. Not good news I'm afraid although there are still 300B PSE's that come up on the secondhand market occasionally.

Thanks for the information, Greg. No doubt, I could get Geoff at SP to wind some for me to the original specification, but obviously there comes a price premium.

I'm not planning to get started right away, so I can always watch to see what happens in terms of stuff coming up second-hand in the short term. but for me I'd rather build it - there's more satisfaction that way - than plug and play. Having recently completed 2 years at college learning guitar making, I'm a lot more confident in my abilities with casework, following drawings etc.
Longer term, I'm hoping to put these in my (as yet unbuilt) guitar-making shed where hopefully they'll double as an extra heat source in the winter months....

[kennyk]

I was lucky enough to get a mostly complete pair for £200 recently. Greg is correct that Morite made the trannies. John Caswell on WD should have the details of the trannies, and may even have some originals for sale. I'm not selling mine, but if you need any info, feel free to shout. I have a full set of instructions/diagrams too.

Good shout, Ali. I think I have all the diags/instructions courtesty of the the internet way-back-machine and the old WAD site from 2005 I've got PDFS of the circuit, layout, text and tag layouts, so hopefully it's all there.

If I'm going to have to do my own chassis, I'm toying with the idea of doing a mirror-image pair at the moment.

oh, and btw, you lucky s*d! getting a bargain like that.

in terms of Iron, just the three, mains, OPT and 10H choke? is that the the lot?

[Ali Tait]

Yep.

[Ali Tait]

Had to buy chokes for them as they were missing, speak to Mike here, who can supply you a pair.

[kennyk]

Thanks, Ali. Presumably that's MikeH on here ?

I'm starting to work out some of the things that I'll need to source for the chassis at the moment, such as nuts, bolts, capacitor clips etc, as well as mains switch and IEC socket. I've been searching on eBay for some of the bits and so far there seems to be some useful stuff. I think I'll start getting bits here and there as I can and have as much of the parts to hand, before I go ahead and get the chassis plates, so I can be sure that I've not forgotten any mounting holes or standoffs.

[Ali Tait]

Aye, MikeH. Modushop is good for chassis, hifi collective is good for components.

[Greg]

Kenny, can I suggest that before you do anything else, you research whether you can get the transformers you need replicated. I don't know the full spec for these (other than O/P Tx is 2.5K impedence. The rest is on the schematic) but I do suggest an initial consultation with John Caswell first as he may well be able to help you. My recollection is that when Peter Comeau took over WD, he was supplied with little information on the 300B PSE. That being the case, unless he has other sources, Matthew Snell (current WD) will be in a similar position. If you can be certain of acquiring all the iron you need, you are then clear to go ahead and plan everything else.

[kennyk]

That's a fair point, Greg. I'm under the impression that the primary of the OP T/x is 1.5k - at least this is what is written in Gary Devon's circuit description that I managed to retrieve using the internet 'way-back machine' from the old WAD site. is 2.5k perhaps for an SE design, and a 1.5k for PSE?

Anyway, indirectly, it's now thrown up a question mark regarding consistency in my research. I've got 2 versions of the circuit diagram - one shows the secondary HT windings as 380 - 0 - 380 and the other 395 - 0 - 395. the difference ultimately means that the connection to the opt tx (Point 'A' on the diagram) is at 400 or 425v. (However it seems to leave the supply to the 6AU6 the same (130v) regardless of whether the point B voltage is 350 or 360. )
I know from what background reading I've done that when Andy Grove was designing the PP monoblocks, the brief was to be able to use the same Power TX and choke in case anyone wanted to convert to PP from PSE. I've now tracked down the PP monoblock circuit, and the secondaries are 380 - 0 - 380 on the version I have, so for the moment I'm going to assume that is correct. Having looked at the 300B datasheet, I think that the 425v operating point is probably too close to the design limits anyway.

But you're right, I probably need to have a chat with John Caswell to see what I can find out!

[pre65]

John Caswell was advertising a pair of mains transformers for those amps, so best contact him sooner than later.

[Paul Barker]

You won't want to pay this but just so you have some data.

Audio Note use 1.25k so your 1.5k is to be expected.

Their cheapest for the job are £526 each.

I haven't looked at Sowter prices.

[Nick]

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.