Lockdown Valve Amp

[zebbo]

Should be safe 5 minutes after being disconnected. The caps are supplying op amps, so at most +- 20v. The op amps will keep pulling power when it disconnected until its discharged. There are no transformer in that box so no mains voltages to worry about.

Great, thanks. So it will still discharge if the head unit is unplugged from the power supplies yes?

[Lurcher300b]

Yes

[zebbo]

Thank you.

[SteveTheShadow]

Been playing about all last week with different rectifiers, which is most unfortunate, as I have far, far too many of the bloody things and I’m not about to make a boring list of them, then run down chapter and verse on their sonic differences. And despite the tweakery that follows, the KT77 single -ended remains the best non OTL amp I’ve built to date, whatever rectifier is in use. It’s just been a matter of a nip here, a tuck there in order to nudge the tonal balance into a state that will give long term musical satisfaction.

I finally settled, after much auditioning, on the relatively cheap 1960s, Fivre 5R4GY direct-heated rectifier, as it; together with a nice, equally cheap 1970s, RTF EZ80 on the input stage supply, provided the right DC conditions, power supply impedances and what have you, to make the amp put its best foot forward.

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It’s not what I expected; expectation being the stumbling block in this instance. An excess of expectation bias and poor sonic memory was leading a merry dance, with this rectifier rolling lark and I got pissed off with it. I have Mullard GZ34, GZ37, GZ33, and even a Brimar U52 ferchrissakes; all very expensive, especially the U52. Round and round and round, I went; rolling these expensive rects in and out until the ears just got tired of it.

Eventually I gave up with the drinking by the label, put them away, and admitted that none of the posh rectifiers was providing the right electrical conditions for long term satisfaction with the sonics of the amplifier. They all allowed impressive results in their different ways but after a couple of hours, some issue or other would invariably come up concerning the tone, something that became an irritant. It was this nonsense that had (a couple of years ago) caused me to decide that I could no longer be arsed with valves. But...there’s just something about their way with music, that pushes all the right buttons. And once heard, as they say.

All is well.

[Fretless]

Keep up the good work Steve.
Entertaining write-up, that.

[SteveTheShadow]

A bit more tinkering today.
The 100 + 100uF, 500V JJ can cap at the rear has been replaced by a 50 + 50uF F&T can cap, to good sonic effect. A decent balance has been struck, I feel, between decent hum levels and power supply speed. In fact, hum levels have barely changed at all, the thing is still virtually silent. The top end detail is very nice indeed. It’ll never be as ‘fast’ as a well executed direct-heated triode power stage, but it’s close.

Whilst I was under the hood, I also fitted another two red 4mm speaker sockets, so that 4 and 8R speakers are now catered for. The 4R setting will work nicely with my mate Steve’s speakers, if we ever get out of this Covid-19 nightmare.

On a whim, I also tried out some modern production 6L6s I had knocking about, from my push-pull pentode days and they biased up excellent. It was only a small step from that to a pair of Osram, grey glass KT66, out of a quad I bought off Steve a couple of years ago. These, being the British variant of the American 6L6, biased up exactly the same, and they are being given an airing below. They don’t half sound good.

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In the amps I’ve built around EL34s, the grey glass KT66s have TBH sounded a bit meh, as has anything else based on the 6L6. But...in an amp purpose built for KT77s, the venerable old KT66 sound glorious, as do the 6L6 variants I have. So, we now have the bonus of being able to use 6L6 and KT66 as well as the KT77, without any penalties in terms of the overall sound quality. I’d call that a result.

[SteveTheShadow]

As well as our Ant, getting the Dynavector MC cartridge, in the big box Nigel (Shaman) sent to my place. I got a really nice bunch of valves he was clearing out.

Aside from the British 4V valves, this stash of tubes was virtually identical to the stash that went to Steve57’s a couple of years ago when I’d lost interest. Funny how things come round. I now have enough 6922s to build half a dozen valve phono stages

There was a couple of wartime Sylvania 6SL7 in the box, which will come in very handy for the Lockdown amp, and a nice bunch of used but good rectifiers, plus a few nice NOS rects.

I fitted a couple of 50K Allen Bradley mono on/off/volume, switched carbon track pots from the 1960s. The older 250K Allen Bradleys were getting a bit crackly, as they have been knocking around collecting dust for over ten years; at least since I had them in my 6B4G monoblocks.

Below is the Lockdown sporting the aforementioned Sylvania 6SL7s and a huge old indirect-heated rectifier, plus a little EZ80: all from Nigel’s box.


With this little lot in situ, the Lockdown gives that classic tubey sound I’ve sought since I started building valve amplifiers. Once you get out of that audiophile mindset, the music becomes much easier to enjoy.

It might not be high-end but in combination with the Fane speakers, it has soul in spades. It’s nothing like the DHT, the OTL, or the solid state sound. There’s no trace of any kind of forensic analysis or unforgiving characteristics in the presentation.

Without a time machine, I can never know for sure, but what I think I’ve ended up creating, in terms of the amp and the big, efficient, 12in single-driver speakers, is my idea of how a mid ‘50s high quality domestic system would have sounded if fed with modern sources.

“I say Mr Cholmondley-Warner, your new steerio gramophone system has a spiffing tone!”
“It certainly does Mr Grayson, it certainly does”

[SteveTheShadow]

There have been quite a few changes to this amp since I last wrote about it. I’ve now got it running on KT120s, with a Russian Soviet era, 6N1P double triode driver stage, one side feeds one output valve and the other side feeds the other. The power valves have been moved to the front to prevent them from transferring excessive heat to the output transformers.

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I put the KT120s into ultra-linear mode last evening and the results were pretty good. I’ve only ever used KT120s in triode and pentode mode, and like pentode mode, I wasn’t expecting ultralinear mode to be that viable unless feedback was employed, but it seems to work fine with my Fanes without it.

I thought I’d try SEUL (Single-Ended Ultralinear) after having had a look at Michael Abdellah’s single-ended ultralinear KT88 amp, on the DIY Audio Projects website. His SEUL amp uses a 6N1P driver and he doesn’t use any feedback, so I thought, why not try it out? The connections were there on the Hammond 1627SE output transformers, so it wasn’t exactly a big leap to give it a go.

The best feature of the KT120s in UL mode is in the portrayal of voices. They appear in the room between the speakers, large as life. This was also a characteristic of my pentode amps, when I was into push-pull. Plenty of shimmer to cymbals too. The amp is now giving 17W per channel. Very useful.

This amp has great sound and real world power levels. The Hammond 1627SE output transformers are also stonkingly good in ultralinear configuration, as are KT120s, though that last bit shouldn’t be all that surprising, as that is how they are designed to operate.

Our Ant is currently building me a chassis in afrormosia. This will house a permanent build of this amplifier.