The Inverted OTL

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IslandPink
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The Inverted OTL

Post by IslandPink » Sat Apr 13, 2019 6:24 pm

I was chatting to Chris V on the phone today , and he alerted me to an article I's never read before. Reading it kind-of blew my mind. The genius of Steve ( as was ) Bench, again :
http://diyaudioprojects.com/mirror/memb ... erted.html
Needs reading carefully, such a weird concept - but it all makes sense practically, for an OTL. He claims about 1.8W output and 3.4R output impedance without feedback, using 10-ff 5687 per channel.
I just wondered if anyone here from the 'OTL mob' had seen this and/or considered it ?
I have encouraged Chris to keep planning his !

( Love the final twist on the output stage - since the G1's are all at 0V nominally , he can supply and set the DC conditions using a 1H choke, so no output capacitor is required )
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

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Ray P
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Re: The Inverted OTL

Post by Ray P » Sat Apr 13, 2019 6:56 pm

On a quick read that looks interesting Mark. No output cap is attractive but I'm not convinced about all those paralleled output valves - that's why Wolfgang started exploring SE-OTL with only a single output valve. Also, might be good to find some output valves that are in current production as you'll need 20 of them!

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Re: The Inverted OTL

Post by IslandPink » Sat Apr 13, 2019 8:06 pm

It help to know there's a 5687 with higher heater voltage - plenty of those if you know what to look for ..
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

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Re: The Inverted OTL

Post by Cressy Snr » Sun Apr 14, 2019 10:07 am

IslandPink wrote:
Sat Apr 13, 2019 6:24 pm
I was chatting to Chris V on the phone today , and he alerted me to an article I's never read before. Reading it kind-of blew my mind. The genius of Steve ( as was ) Bench, again :
http://diyaudioprojects.com/mirror/memb ... erted.html
Needs reading carefully, such a weird concept - but it all makes sense practically, for an OTL. He claims about 1.8W output and 3.4R output impedance without feedback, using 10-ff 5687 per channel.
I just wondered if anyone here from the 'OTL mob' had seen this and/or considered it ?
I have encouraged Chris to keep planning his !

( Love the final twist on the output stage - since the G1's are all at 0V nominally , he can supply and set the DC conditions using a 1H choke, so no output capacitor is required )
Interesting stuff as ever from Ms Bench. "The OTL mob" I like it.
We turn up at Owston with evil looking, barely under control, multi tube creations, sporting Hulk Hogan looks and Frank Spencer sonics. :lol: :lol: :mrgreen:
....Apart of course from Dave (Thermionic Idler) who's Beast will be a slammin' piece of work when he gets it finished!
Last edited by Cressy Snr on Sun Apr 14, 2019 10:14 am, edited 1 time in total.
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Paul Barker
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Re: The Inverted OTL

Post by Paul Barker » Sun Apr 14, 2019 10:14 am

Not cheap either though Mark and 6as7 have become rediculously overpriced. A project which uses so many more valves than usual not to be taken lightly, never was, but now a rich man’s folly. The Bench 5687 otl always interested me, and once I had sufficient 5687’s. A useful element in favour: the valves don’t have to be well matched.

I recall asking a valve dealer I bought from in the past, maybe 10 years ago, “what do you think will happen to our valves?” he straight away said without hesitation, “we shall run out”. Turned out he was right. If I could go back to when I had collected the 5687’s for this project I would one day build it, but I’m not going to buy them or the 40v version at todays prices, or the 6as7 at todays prices. But I may find sufficient 6as7’s in my stocks to make a mono version one day. It’s obvious I’m not active presently, due to work and home life pressures, it’ll all have to wait until someone pays me to stay home. But thanks for the reminder?

Don’t forget to them as likes SE, vertually any 1.8 watt version sounds sublime, I know and love that sound. Only one valve per channel required, spend the money the other 18 valves would cost you on Nickel output’s once and for all, compared with twenty valves wearing out from the day you first use them.
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Re: The Inverted OTL

Post by Cressy Snr » Sun Apr 14, 2019 10:21 am

Aye, the 13E1 as a single ended OTL amplifier is just a gorgeous sounding thing. I can't imagine being without it, so I'm going to have to scrounge a few 13E1s up to keep it going for as long as possible.
“ What a load of cobblers! ”.....Said Dougal.

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Re: The Inverted OTL

Post by IslandPink » Sun Apr 14, 2019 12:50 pm

Paul Barker wrote:
Sun Apr 14, 2019 10:14 am
Not cheap either though Mark ...

Don’t forget to them as likes SE, vertually any 1.8 watt version sounds sublime, I know and love that sound. Only one valve per channel required, spend the money the other 18 valves would cost you on Nickel output’s once and for all, compared with twenty valves wearing out from the day you first use them.
Hi Paul
This is just the sort of project you would have built a few years ago, I can tell !
But I don't feel the cost is as big a deal as you suggest.
6N6P's from 1960's to 80's are excellent 5687 alternative - look, here's a batch of ten :
https://www.ebay.co.uk/itm/10-tested-6N ... SwPhRaxh1f
Even those are the last, you could buy them at £5 or £6 in pairs or 4-s.
The 40/20V heater 5687's are around £15 singly but I suspect less if you can find 2 or 4-off.

Talking to Chris, I don't think you could get the top-end of an OTL without Litz wire on the secondaries. So in that case the competition would be a normal SE amp with nickel and litz wire ( ask Dave S nicely ! ) - that would cost more than eg. £250 ( or less ) for the valves for an Inv-OTL.
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

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Re: The Inverted OTL

Post by Wolfgang » Sun Apr 14, 2019 3:16 pm

I think this amp has some potential as low power output OTL for 16R drivers with zero NFB. Four 6AS7 or EL509 could be enough per channel and for $20/$40 each this is a very good deal compared to the 300B OTLs.
I have no idea how such an amp could sound in comparison to typical OTLs but there are some indicators that this could be some kind of breakthrough regarding SQ:
No grid stoppers (which always reduce the”speed” of the amp/transients to some degree)
No NFB which has the biggest influence (good, bad, or otherwise) on the sound as I have practically found out with all my OTLs

On the other side I am a bit concerned what to do if there is some audible distortion/hum/noise and I cannot adjust the NFB. Of course I could always add some NFB but then the effort with the high voltage driver stage doesn’t make sense any more.
With the Pinnacles for example there is a clear threshold which is very close to the original 500R for the NFB resistor before the SQ gets really bad. With the SE 300B version one can use resistor values up to ten times higher than the original and it starts to sound really good. It also depends of course on the drivers/CDF.
“Balancing” 4 tubes for equal current might also be a completely different thing with the Inverted OTL compared to typical OTLs but it definitely has some clearly noticeable effect on the SQ (PP and SE).

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Paul Barker
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Re: The Inverted OTL

Post by Paul Barker » Sun Apr 14, 2019 3:31 pm

Thanks Mark for the more up to date knowledge of alternatives and prices?
I can do it right or I can do it now, but I can't do it right now.

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Re: The Inverted OTL

Post by IslandPink » Mon Apr 15, 2019 11:23 am

Cressy Snr wrote:
Sun Apr 14, 2019 10:07 am
"The OTL mob" I like it.
We turn up at Owston..
I'm thinking of you on motorbikes, with customised leather jackets.
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

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Re: The Inverted OTL

Post by Nick » Mon Apr 15, 2019 2:08 pm

IslandPink wrote:
Mon Apr 15, 2019 11:23 am
Cressy Snr wrote:
Sun Apr 14, 2019 10:07 am
"The OTL mob" I like it.
We turn up at Owston..
I'm thinking of you on motorbikes, with customised leather jackets.
They will be wearing the 1% (THD) patch :-)
Resistance isn't futile it's V / I.

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Ali Tait
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Re: The Inverted OTL

Post by Ali Tait » Mon Apr 15, 2019 7:22 pm

:mrgreen:
Image[/img]

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