45 SE

[Cressy Snr]

That was quick

Yes and I won’t do that again. My eyes were on stalks by the time I’d done.
“Pace yourself laddie” will be the mantra going forward.

I have a few loose ends to tie up today.
I’m going to change the mains transformer for a 1-1 isolation toroid. This’ll lower the rectifier output a bit and bring the HT right into middle of the correct range. At the moment I’m having to slug down the HT with resistors and I don’t like having to do that. It just smacks of bodging and wastes heat.
The transformer transplant is easy enough, as it is plumbed in with high temperature electrical screw terminals, so no soldering involved. It’ll give peace of mind.

[Ray P]

You put me to shame Steve, my rate of project progression is positively glacial in comparison.

Good to see you're enthusiasm with the outcome shining through.

[Paul Barker]

I imagine you’ll spend today
enjoying it before you do too much.

[Nick]

At the moment I’m having to slug down the HT with resistors and I don’t like having to do that.

Picking the correct rectifier can help with that. Or maybe its a suggestion for choke input? 5Y3 and 5R4G maybe.

[Cressy Snr]

At the moment I’m having to slug down the HT with resistors and I don’t like having to do that.

Picking the correct rectifier can help with that. Or maybe its a suggestion for choke input? 5Y3 and 5R4G maybe.

Unfortunately, the mains TX I have falls in that difficult position of being slightly too high for cap input and too low for choke input.
However the 230V isolating transformer looks a bit better. I should get 310V out of it as it’s a 200VA job.

[Paul Barker]

Thats the set of compromises I always end up with.

Dont forget try between 1 and 2 microfarrad before the choke for a hybrid. Adjust on test.

Conversley the benefit of choke input is regulation; but if you cant lift choke input to the voltage you want, use a simple series pass valve reg; get the regulation as a bonus. You have el34’s just one would cope with 70mA, youre 45’s will use 70mA as se triode ,which it is. the voltage drop across el34 is low so dissipation no stress. Any small high gain triode to control el34 grid. Put black gate 1/2 before choke 1/2 on output transformers.

[JamesD]

Another option is to put a rectifier in series with the psu before the filter element and used it as a forward biased diode - depending on the choice of rectifier you can get up to 30V voltage drop across it and this handles the heat dissipation better than series resistors. If you use a TV dampener it gives you a slow start psu too.

[Paul Barker]

Thats the set of compromises I always end up with.

Dont forget try between 1 and 2 microfarrad before the choke for a hybrid. Adjust on test.

Conversley the benefit of choke input is regulation; but if you cant lift choke input to the voltage you want, use a simple series pass valve reg; get the regulation as a bonus. You have el34’s just one would cope with 70mA, youre 45’s will use 70mA as se triode ,which it is. the voltage drop across el34 is low so dissipation no stress. Any small high gain triode to control el34 grid. Put black gate 1/2 before choke 1/2 on output transformers.

sorry i forgot 20mA for ef84. you probs have bigger valves like 6550 kt100 etc

[Cressy Snr]

Thanks chaps.
It's all good fun, as they say.

[Nick]

depending on the choice of rectifier you can get up to 30V voltage drop across it

I refer the reader to my earlier post. A 5y4g will make a nice 60v radiation cooled voltage drop.

[JamesD]

depending on the choice of rectifier you can get up to 30V voltage drop across it
I refer the reader to my earlier post. A 5y4g will make a nice 60v radiation cooled voltage drop.

Sorry Nick I took your comment to mean changing out the original rectifier for a different one whilst I was suggesting adding a rectifier as a forward biased diode in series with the original rectifier just to get the voltage drop...

Your suggestion is better than adding another device.

[Nick]

It was, yours is still valid, I was just pointing to those rectifiers with intentional high voltage drop.

[Cressy Snr]

Well, I ended up changing out the mains transformer. It ended up being the most pragmatic and practical solution to the overvoltage situation.
I also did a load of other work, which included setting myself up with a centre tapped 6V winding on the outside of the toroid, so that the EF80s could get their heater power from somewhere. There is only a 115V + 115V secondary on the toroid. I also found a small 6V/5A transformer, which I fitted inside the black box, alongside the heater transformer feeding the 45s.

This transformer provides the heating for what are now my rectifiers of choice in this amp: a pair of 6CJ3 TV damper diodes. Using them meant fitting a pair of novar sockets in the holes formerly occupied by the octal bases. It also meant that the interior near the blue can cap was clear of the transformer gubbins that had previously been next to it.

EE714914-C518-4547-81C8-E61709BA0BEA.jpeg (150.67 KiB) Viewed 3296 times

Once everything was powered up, I got some nice readings for the HT and the bias for the 45s. Red meter shows the bias, Blue meter shows the anode voltage:

2E066CED-720F-4170-AE7D-3680D0202AC5.jpeg (169.17 KiB) Viewed 3296 times

These conditions have the 45s operating at 89% anode dissipation.

And here it is playing some nice tunes:

F19EFE9D-AE10-412F-B4C1-2FAB985B6DD0.jpeg (96.47 KiB) Viewed 3296 times

Thomas Mayer often uses 4 TV diodes in a bridge rectifier setup on most of his amps. My two dampers are configured as a hybrid bridge. I’ve got plenty of them, mostly 6D22S and EY500A, but in this instance I think the top cap less 6CJ3s look a bit more streamlined on this particular chassis, and keep the visual attention on the 45s themselves.

So no more resistors apart from those that are part of the RC filters. It’ll do nicely. The sound is clean, lively, even across the frequency range. The tone is gorgeous and the famous 45 midrange magic is present and correct. I now know and appreciate what the fuss is about concerning these little triodes. Wow!

And even with my 100dB Fane single drivers, the background is silent. Couldn’t ask for much more.
I’m a convert.

[IslandPink]

Impressive work in such a short space of time. Glad you are enjoying it.
I love my 45's for the treble.

[Paul Barker]

Impressive work in such a short space of time.

I’m dumbstruck how quick.