WAD 300B PP Rebuild Advice Needed.

[Richard]

Hi Zak,

Very nice!

A couple of points/questions,
my 2mV hum was simply measured with a multimeter at the speakers with the amp idling in system. The meter is quite decent and quotes 45Hz - 500Hz response for ACV so it will be a broadband of all the "hum" noises. The humbuckers were set in system by ear but fine tuned with the meter.

Did you phase the 300B heaters?

Did you settle on DC heating for either/both of the small valves or are they AC? NealG had already done mine and I can't remember but think it would have been both valves linked as per the build sheets. Neal or Greg may be able to comment on the difference in hum with DC heating.

[ZakB]

My apologies for the ridiculous reply time and thanks for all your kind comments and suggestions.

Richard, the 300b heaters are phased, but the input board has AC heaters. I did try a DC supply, but noticed very little change in the hum. I suspect the amp will benefit from DC supplies once the the main contributing culprit is found.

John, thanks for the very useful suggestion on introducing the caps to help determine where the hum is originating from. I'll try that along with moving the interstage Tx wires. The 6D22S idea is good to hear as I was wondering what i'll do when the GZ37 and my spare expires.

To be honest, I've not done any hum hunting over the last few days, but have been doing some performance testing for fun, although it got pretty tedious and warm after a while. Though there is no detectable difference to my (not so excellent) ears, I think something is amiss with the left channel. I'm quite new to looking at the performance of valve amps in any detail, so have no feel for what to expect in terms of acceptable levels of channel to channel variation. However, the sub 200Hz performance difference between the channels (-5dB at 20Hz) seems far higher that would normally be explained by natural component variation surely? A total valve swap has not cured the issue, so this must lie either in a capacitor or inductor somewhere? Your opinions on this are very welcome.

I hope everyone is staying sane. More time to play with Hifi is a great unintended outcome of these crazy times

[IMG]300b Rebuilt Bode Plot by Zak Barrett, on Flickr[/IMG]

[Richard]

Hi Zak,

Ironic but I suppose many folk would have built it, not tested, and been happy. Manufacturers would have shifted 0 dB down 1dB and stated response as 40Hz-40KHz +/-3dB which sounds quite impressive really. This doesn’t help much though does it.

I agree it’s not likely to be a cap or resistor at fault unless one is incorrect value or hopefully there’s a dry joint somewhere. It looks a bit like an impedance match issue. Did you use the same load for both channels when testing? Double check the OPTX connections – I had one once which had the 0, 4, 8, 16 taps connected wrongly on one channel. Check the primary connections too back to the respective valves. The amps are shown built as mirror in the instructions but they aren’t always constructed that way.

Did you have an eye and ear on the valves as you wound up level? A different amp was fine up to 19W then burst into HF oscillation on one valve of one channel. It showed on the scope trace and the valve glowed blue.

After that it might be a case of tagging the R channel inter-TX into the L channel and seeing if the result is better. Then the OPTX. A bit of a bugger really but you should be able to check just the L channel that way with minimum disturbance.

[ZakB]

Hi Richard,

You're right, many would have built it and used it. Unfortunately, I'm an engineer (although not an electronic one) which can sometimes be an affliction!

Thanks for all your ideas. I'm going to swap the interstage transformer over first. When I tested all the components individually before I put it back together, the left transformer had a significantly different primary inductance when measured on my LCR meter at 100Hz. At the time I didn't have confidence in the measurement, but now it looks like a good place to start.

As ever, thanks and I'll report back.

Zak

[Richard]

I think you're right, the reponse is tilted. Are yours from SP Wound? They have brighter bells than mine, perhaps Greg might recognise them if you don't have the history.

[Greg.]

Here is a picture of mine although I don’t think it helps a lot. Although I built a clone, I and several others obtained all the transformers and chokes from the original manufacturer, SP Wound, which, until recently advertised these products on its website as still available. However, SP Wound have a new website which appears to be under construction and I can find no reference to the WAD 300B PP ironwork. There are contact details and Geoff Budden still appears to be the MD and certainly the person to talk to if you are looking to obtain a replacement or rewound interstage transformer.

[ZakB]

Thanks both for the information.

I'm not sure if they are from SP wound. I'll take a closer look when I swop the channels over early next week.

Many thanks,

Zak