Remember These?

[Cressy Snr]

I would be interesting to use a variac to tweek the mains to correct for the HT difference and see if there was still any audible change.

Aye, that would be an interesting experiment. That’d leave the impedance question if there were still differences.

[Mike H]

It certainly is fab.

I'm surprised different rectifier tubes are having that much effect on the sound TBH.

I compiled a geeky list of HT voltages produced in my amp by the 2 Amp rectifiers I have:

GZ34 - 160V
5Z4G - 156V
UU4 - 152V
5R4GY - 148V
5Y3G - 141V

The direct heated rectifiers produce the largest voltage drops.

The subjective differences are likely to be down to the different HT voltages, moving the operating points of the driver valves around,

I can see how that might be possible.

I like Nick's idea if it were possible to get the HT the same for each rectifier type for a comparison.

It occured to me also if you were to shove a series pass regulator between the rectifier + reservoir combo and the ourput to the supply rail of the amp, then logically any influence made by the rectifier should be negated? It would then be down to the audible effect of the post reg supply smoothing capacitor I would have thought.

[Cressy Snr]

...It occured to me also if you were to shove a series pass regulator between the rectifier + reservoir combo and the ourput to the supply rail of the amp, then logically any influence made by the rectifier should be negated? It would then be down to the audible effect of the post reg supply smoothing capacitor I would have thought.

You mean, like my old 6B4G monosblocks? Although to be fair, they only had a single channel power supply, with the usual RC, inter-stage decoupling arrangements.

Unfortunately, the power supply end of the 13E1 OTL is now rammed solid with bits. There’s simply no more room for anything else.
To give you an idea of how rammed it is, I had toyed with the idea of choke loading the driver stage; there wasn’t even enough room for the tiny Hammond 150H chokes, no matter how much I scratched my head and stared at the chassis.

[Mike H]

Oh I do know what you mean!

[Cressy Snr]

I have been using the NVA clone for a while, but got the OTL out again as it’s turned cold up here. It’s a far better room heater than the DIY NVA.

[Cressy Snr]

Found some big old Russian 0.01uF, silver mica caps and yesterday, on a whim, tried them as bypasses across the output caps.
Niiice.
I think I got these from Simon, way back in the dim and distant past and never found a use for them. I vaguely remember trying them as bypasses on the interstage coupling caps on some amp or other I was farting about with, and being sorely disappointed at the bright edgy result.
Across the output caps on the OTL however, they work beautifully. Just goes to show it’s not what you use, it’s where you use it that counts.

[simon]

I'd forgotten about those. I got them back in the day when Russian stuff, caps and some valves, were cheaper than chips. I don't remember how much they were, probably less than a quid each.

[Cressy Snr]

I'd forgotten about those. I got them back in the day when Russian stuff, caps and some valves, were cheaper than chips. I don't remember how much they were, probably less than a quid each.

Well I’ve found good use for them at last! Not bad for £1, even if I didn’t pay for them (at least I don’t think I did)

[simon]

What's a couple of 10nF caps between friends . Glad they've worked for you, I still haven't found a use for them yet.

[simon]

They're bigguns as well ISTR. Very big.

[RushPCB]

Interesting threat.. Yes, I remember your 13E1 valves.

[Mike H]

Can still get Mica, but getting darned expensive now. I was looking at getting some a year ago. Just had another look, yeh 10nF 500V 1% on the RS site, £24.70 each.

£12.50 each if you buy 50.

[Cressy Snr]

Can still get Mica, but getting darned expensive now. I was looking at getting some a year ago. Just had another look, yeh 10nF 500V 1% on the RS site, £24.70 each.

£12.50 each if you buy 50.

Jeez!

Just thinking about the whole OTL thing. Mine, and all the rest of the single ended OTLs out there (apart from the only inverted OTL in the world) may be “output transformer less” but they are certainly not direct-connected to the speakers the way totem pole OTLs are, so the quality of the output cap will affect sound quality, just like an output transformer does, so FFS, bypass your output cap with summat decent.

Them SEOTLs are output transformer less, the same as a separate valve preamp is output transformer less.

What a bleed’n con! I’m puttin’ in a complaint!

[Ray P]

...bypass your output cap with summat decent.

My Nichicon KG caps are bypassed with Mundorf ZXs, which are themselves bypassed with polystyrenes...

[Cressy Snr]

...bypass your output cap with summat decent.

My Nichicon KG caps are bypassed with Mundorf ZXs, which are themselves bypassed with polystyrenes...

Yep, I’ve had pretty mixed results with bypassing caps, so became a bit jaded and sceptical over the years. However the silver mica caps have worked wonders on the output stage of this particular amp. The sound now bears little resemblance to what was heard at Owston. With the Steve S inspired, triple power supply upgrade and the output cap bypasses, I can safely say it’s a different animal altogether.

At the moment I’m listening to a BBC Music Magazine choral playlist and it is really rather good.