WAD 300B PP Rebuild Advice Needed.

[ZakB]

Hi All,

After many happy years with a WAD K5881, I've just acquired a WAD 300B PP from a nice chap on this forum .

The amp wasn't working when I bought it and has been extensively modified. I've got it functioning, but I've decided to give it a complete rebuild over Christmas. However, I'd really appreciate the thoughts of those who know these amplifiers on the effect of some of the mods that have been carried out and whether I should incorporate them into the rebuild.

Having made no significant performance measurements yet, the two mods I'm mainly concerned about are shown in red on the attached circuit diagram:

1) The addition of a 15H 30mA Hammond choke to the input stage HT of both channels.

2) The addition of an 820R 50W wirewound resistor to each line of the choke input supply.

Do any of you know what the thinking was behind these mods and if they offer any real improvement? Should I keep them as part of the rebuild?

Your thoughts are much appreciated,

Rgds,

Zak

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

[Richard]

Hi Zak, Rather than us trying to second-guess it might be better to ask the chap who sold you the amp

[ZakB]

Morning Richard,

I didn't ask the chap who sold it, as I believe (although I may be wrong) that the mods we are discussing were done prior to his ownership so he may not have a view on their effectiveness.

I was also hoping to draw on the experience on this forum to discuss the engineering thinking behind the mods. I detailed the changes on the circuit diagram attached to the post so hopefully no second guessing is needed.

Rgds,

Zak

[Ali Tait]

As I recall, one good thing to do was to reduce dissipation on the 300b's if not already done.

[ZakB]

Hi Ali,

Thanks for your reply. That's already been done and I'll keep that modification in the rebuild.

Rgds,

Zak

[Richard]

Hi Zak,

Well, the 820R resistors will drop the HT.

The HT is 565V nominal and as standard each pair of 300Bs draws 160mA so they'll drop approx 130V (= 21W) each. There'll be some sea-sawing of the voltages if you're using 1K5 instead of 1K2 at the cathodes and the net plate voltage (HTV less cathode V) will probably end up a little over 300V. Original spec 300B were rated 400V plate and typically 300V or 350V in use. Modern 300B such as Svet were rated 450V. Mesh plates such as TJ Sophia were 300V.

The chokes will have an HT smoothing effect working with C2 of 160/(160+9425) = 0.0167 at 100Hz. Voltage drop will depend on the choke's DCR but certainly lower than using a resistor.

Mine was reliable with modern 300B with just the change of cathode value to 1K5 by snipping out the 6K8. That put plate dissipation of my otherwise standard amp nearer 30W, stopped the red spots, and a set lasted going on a year of regular use. If you have mesh plates or more expensive types you might want to keep the mods as shown and have a listen and check voltages and plate dissipation