Thinking about a clone of the WAD 300B PSE monoblocks

[Greg]

Just to remind you that John Caswell on the WD forum still has a pair of WAD 300B PSE monoblock mains Tx's available for £120.

[kennyk]

Just to remind you that John Caswell on the WD forum still has a pair of WAD 300B PSE monoblock mains Tx's available for £120.

Thanks, Greg. I'm waiting for John to get back to me.

with an i/p impedance of 3.5k I'm not sure how compatible the sowters are with a parallel 300B; maybe for a single 300B. but the WAD info seemed to indicate 1.5k. Obviously I'm less experienced in these matters so I'll bow to guidance on what effect a much higher impedance will have. it'd be like running a single 300B into a 7k load I'd have thought? I've got the 300B datasheet and the highest figure in the charts for loading is 6.5k at 450v with a plate current of 50mA (9.0W o/p with 30 and 45 db for the 2nd and 3rd harmonics) and a grid bias of -102v. Though this is in the 'maximum operating conditions section'

Thoughts anyone?

[Greg]

I have a pair (which I use) of WAD 300B PSE O/P transformers and they are spec'd at 2.5K.

[IslandPink]

I'd have though you'd want to run something like 1.5k for a pair of 300B's if you want the power , but the sound will not suffer with 2.5 or 3.5k for the pair, may get better . One thing you gain is output impedance, as the speakers see a lower driving impedance if the transformer step-down ratio is increased.

[steve s]

Thanks mark, and thanks greg, the transformers are there if needed then , and again the price would be reasonable.

[kennyk]

I managed to get in touch with Sowter and they've identified the specs as follows:

15W Single Ended Transformer type number 9583
Primary impedance 2000 ohms Secondary impedance 8 Ohms
LF cut off -3.0dB at 20Hz Source 400 Ohms DC Current 100mA


So probably not got the current handling required for 2 300Bs.
Also I'm confused about the primary impedance quoted; Greg's asserted that 3.5k was specified, yet the pdf I received from Sowter with all the info clearly says 2k.

[Greg]

Hi Kenny,

I was thinking about your situation and the difficulty of getting the ironwork to replicate the WAD 300B PSE.

Have you thought about this offering from SP Wound? Full schematic and design description and all the iron readily available from SP Wound themselves.

http://www.spwoundcomponents.co.uk/300b-docs/pse001.pdf

Greg

[Andrew]

The 5687 is a nice sounding valve. The only thing I can see "after a quick glance" is that I would add a relay that shuts off the HT if the bias voltage goes phutt!

Andrew

[kennyk]

Hi Kenny,

I was thinking about your situation and the difficulty of getting the ironwork to replicate the WAD 300B PSE.

Have you thought about this offering from SP Wound? Full schematic and design description and all the iron readily available from SP Wound themselves.

http://www.spwoundcomponents.co.uk/300b-docs/pse001.pdf

Greg

Yeah, I've looked at that one. I'm a bit put off by the fixed bias for my first scratch build just in case I badly muck it up.

However, is there any reason why those o/p trannies wouldn't work in the WAD circuit? the impedance is a bit lower but not by much. also SP offer identical sized ones in 1.5k and 1.75k primary impedance, so perhaps that might be the solution for o/p trannies? from what I can see the 1.25k trannies are £235 for the pair, so I'd assume that the others might be of a similar price. I've had SP in mind for trannies since I started looking at this project; at the moment the hardest part seems to be getting a reliable spec of the originals.

[Andrew]

I imagine those outputs would be fine for a WAD type build. Or you could build that circuit and use cathode bias. The choice of bias impacts more of the power TX choice than what outputs you use. For cathode bias simply add 70v to the required B+ and use a 1K cathode resistor with a suitably large capacitor across the 1k, you can then eliminate the bias circuit.

I would guess given my singled ended 300B uses a nominal 2k75 that something in the region of 1k25 for PSE would be about right, you could even go up a bit more; less distortion but slightly less power.

Andrew

[Paul Barker]

use a 1K cathode resistor.

For each valve or 500 ohm for a pair.

incorporate means of equalising current to each valve and so on and so forth.
matched valves blah blah.

[Andrew]

For each valve or 500 ohm for a pair.

Yes, thanks for pointing that out Paul, forgot that, half for a pair; twice as much current.

Andrew

[kennyk]

that's interesting;
The wad circuit has 470 on each cathode, presumably to raise the cathode to only 38v because the grid of each 300B is at -35v from the cathodes of the ECC82? so that will bias the grid at -73v w.r.t. the cathode?

[Greg]

Kenny, James has now kindly tested the O/P Tx's and we can confirm that as Sowter told you, they are 2K impedence. I've posted the full test results on this thread.
http://www.audio-talk.co.uk/phpBB2/viewtopic.php?t=5013

[kennyk]

Thanks for that Greg. I'm pleased the mystery has finally been cleared up!

If these were originally specc'ed for fitting to the 2A3, then I have a feeling that they won't be able to handle the current draw of 2 300Bs in parallel. The Sowter site seems to indicate that 100mA is the spec for their 2A3 OPTX. 2 300Bs in parallel would draw nearly double that, and at 2k Primary they probably wouldn't suit a single 300B; 3.5k would be more what I'd expect.