Finished and working - 12 valve all DHT PP 26-71a-45

If they glow, this is the place to be
User avatar
Nick
Site Admin
Posts: 11264
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Nick » Mon Aug 06, 2018 12:16 pm

Not sure that would stop it working as long as the transformers have sufficient max isolation voltage. On the other hand, not sure what it would gain. But try it and see would be the way I guess. Personally I would at least link to ground with a resistor and bypassed cap, but thats just me.
Resistance isn't futile it's V / I.

simon
Social outcast
Posts: 3953
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by simon » Mon Aug 06, 2018 5:20 pm

My GM70 amp has separate HT txs for the driver and output, and I have separate signal grounds for each which are tied to safety earth by slightly different resistors i.e. slightly different signal ground potentials. It seemed to work well I recall, but it was a long time ago when I did it so memory's not so great!

RhythMick
User
Posts: 166
Joined: Fri Jun 23, 2017 12:24 pm

Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Tue Aug 07, 2018 9:51 am

Simon, where in the PSU circuit for your output valve do you ground it?

My circuit joins all the PSU to ground at the same point, which is the bottom of the LTP, effectively the bottom of the 2nd cap.

The signal circuit for each stage involves the preceding stage transformer secondary, valve grids, cathodes and part of the LTP tail.

simon
Social outcast
Posts: 3953
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by simon » Tue Aug 07, 2018 5:04 pm

I'm a bear of simple brain and build SE amps ;-).

From the CT mains trafos I have 2 damper diodes followed by a pure choke input supply IIRC so 10H choke, first main cap (really guessing now, 22uF???), then a split to 2 10H chokes, 1 per channel, and a final 80uF motor run per channel which I use as the local star earth for each channel of the output to minimise current loop.

I do similar for the driver but with a small first tuning cap.

I'm struggling to remember exactly how I earth the local star earths to chassis ground now and I'm away from home at the mo. But I think I ground at the first main cap via ground lift resistors. The idea is to have the driver and output grounds at slightly different potentials to improve isolation. It seems to work anyway.

Post Reply

Who is online

Users browsing this forum: No registered users and 3 guests