The Amp Garage

And your using a new correct spec tank for this build?

Personally if your looking for the sweater sounding even order harmonics out of a class A PP stage then your barking up the wrong tree, because you want a SE class A amp in that case.

Yup do the caps and look close for any signs of arcing on the output tube sockets.

Yes, lifting r37 unpowered V5A and the phase splitter T3, but then C9 still could also be the source of the hum if it’s passing dc voltage out of it and into V5A. The other possibility is that socket v5 is leaking some heater voltage . With v5 out of the socket you should read an open between pins 4...

Good we’re making progress!

Now let’s see if we can narrow things down even more.

Reconnect r37.

Let’s move on to V4B.

Off of pin 1 there is C9.

Disconnect its down stream end.

Is the hum still present?

If not then C9 is leaky and should be replaced.

This from Kevin O’conners book does 50 rms from four 6550s in SE, so to me this proves that your not getting 50 rms from two 88s even if you where able to use up to there 800 volt rating of there plate.

Might be able to use just 3 and use them up faster, also note it’s not cathode biased.

And on the cars charging system you would need a 5” diameter by 1.5 ft long 1 farad cap to sustain that !

Ditto!

I too maybe twice have had to deal with new yet bad filter caps .

Well whatever it can produce in single ended class A is exactly half of what it can produce in push pull class A.

Since a EL34 typically only does 11 watts in SE class A, I too can’t see how two 88s in PP class A outputs that stated wattage.

I would think you’re ok where you are now. 2.8 volts is 8 % closer to the needed 3 volts needed while 3.6 volts is 19% from the needed 3 volts . If there are no precision tolerance resistors in that local circuit just 10% ones then I can’t see why being within 10% of that needed 3 volts would be a i...

Try this.

Leave all the tubes in and unsolder the down stream end of r37 which is the 1K 1 watt after the choke .

Do you still have a hum from the speaker?

Do you know the history of this board?

I have come across 2 boards, one in a 67 Bandmaster and another in a 65 SR and they both had become conductive.

The SR was a real PIA to narrow down where the leakage dc was coming from.

Do you know the history of this board?

I have come across 2 boards, one in a 67 Bandmaster and another in a 65 SR and they both had become conductive.

The SR was a real PIA to narrow down where the leakage dc was coming from.

How much dc is on the OT? To block that and just get the sine way Clip a 630 or better yet 800 vdc rated coupling cap to your probes tip and use the other end of the cap as your new probe. Be sure to shrink tube insulate this end so that only a short end of the lead is left exposed to make contact w...