Heater Transformer Upgrade

[soulminer]

Hi Andrew. Thanks for the reply.

Yes - I will still try the separate heater txs just to see if they reduce the temperature of the mains tx.

In answer to your latest questions:

They have always done this since I have built them.

There is no feedback at all.

All of the PSU caps are fine as far as I know.

I have no scope to test with unfortunately. Only a DVM.

The hum is the same "tone" on both amps, but at differing levels as previously posted.

The Jupiter blocking/coupling caps from the first stage to the splitter (ECC82) have "blown" and gone short circuit twice in the past (one about three years ago and one about three weeks ago), and I have recently replaced all of these with Soniqs and had no problems since (nice caps by the way ).

I am confused now. It does look like a problem with the ECC82 stage in both amps.

Maybe I will start a new thread, as this now seems to have gone off on a tangent with a different problem.

Cheers all,

[NickG]

Have you stood the indirectly heated valve heater voltage up or down from their cathodes?

It could be the heater cathode insulation in the valves breaking doen over time with heat (just a wild guess).

[soulminer]

If you mean the ECC82 heaters Nick, I think they are floating.

They come straight from the mains transformer. 3.15 - 0 - 3.15 with centre tap going to ground and two outers going to ECC82 heaters.

The ECC82 is a cathode follower with 195v Va and -195v Vk with 33K 3W cathode resistors.

The grids of the 300Bs are directly connected to the cathodes of the ECC82. They are held at about -35v DC and the grid of the ECC82 is fed from the 6AU6 via the blocking/coupling cap.

The circuit is bog standard as supplied by WAD and Andy Grove.

[NickG]

If you mean the ECC82 heaters Nick, I think they are floating.

Ok, if the center tap is connected to ground, they are not floating, thay are at 0v, better than floating, but you should find less hum from ID valves is the heater is 30v or so from their cathodes. if the ecc82 cathodes are at -35v, that should be ok, maybe its worth stepping it up 20v or so and see if that reduces the hum from the first stage.

Try constructing a potential divider between the input stage B+ and ground, that provides about 20v, add a 10uf cap across that, and remove the center tap from the heater supply from ground and take it to the 20v.

It may not work, but its worth a try.

[soulminer]

Hi Nick.

That sounds like it makes sense, but I would like to think that with the heaters at 0v and the cathodes at -35v, the potential difference is quite close to your "heaters 30v from cathodes" statement ? Wouldn't bringing the centre tap up to 20v lower the potential difference to 15v between heater and cathode taking it further away from that 30v sweet spot?

I have continued this thread on this new thread "300B PSE Hum!!", where you will see that the hum seems to be related to the type of 300B I use. It still wont go down past 4mV but that is actually within specification with no feedback fitted IIRC.

Cheers