Dear Marge

Cressy Snr · Post by **Cressy Snr** » Wed Sep 19, 2012 1:09 pm

Dear Marge,

Sorting my valves out yesterday, I discovered I had 16 PL519.

Just looking at them gave me the creeps. Why? Because I have now got the hankering to breadboard a Rozenblit Transcendent OTL, just to see what it is really like.

I have most of the bits, including a big pair of 40v toroids for the heaters on each bank. I am a much more experienced builder than the last time I attempted something like this. Just need a few more magnoval sockets.

The intention may be to build the 25 Watt version with four tubes per channel.
But I made such a balls-up of it last time that even looking at those beastly top-capped power tetrodes set off the butterflies.

Should I leave well alone or confront my daemons head on, risking burned fingers and public humiliation.

Hope you can sort this dilemma Marge.

Sincerely
Steve.

pre65 · Post by **pre65** » Wed Sep 19, 2012 1:12 pm

There is only one answer, especially if you have most of the bits.

GO FOR IT !!

You've come a long way since the last OTL experiment.

steve s · Post by **steve s** » Wed Sep 19, 2012 5:30 pm

well steve, was it 4 weeks without a new project ?

and the forum has been quieter without you having a project
be nice to hear an otl in novemeber.

Ali Tait · Post by **Ali Tait** » Wed Sep 19, 2012 6:42 pm

Seconded. Go for it Steve!

Cressy Snr · Post by **Cressy Snr** » Wed Sep 19, 2012 7:18 pm

steve s wrote:well steve, was it 4 weeks without a new project ?

and the forum has been quieter without you having a project
be nice to hear an otl in novemeber.

Well its not exactly one of my usual projects as it won't be documented, at least not until I am sure it is going to be viable and not burn down the house. Also there is no way it will replace the ACP/1 low power setup in my own system. The ACP/1 is far too good for that.

It is going to be more of a test bed for new ideas rather than just another amplifier. I need something to mess about with whilst the system remains a stable reference, just to keep my interest up if nothing else.
I hope to have it ready in breadboard form for Owston.

jack · Post by **jack** » Wed Sep 19, 2012 9:30 pm

"A good plan today is better than a perfect plan tomorrow"

Just do it!

Thermionic Idler · Post by **Thermionic Idler** » Wed Sep 19, 2012 9:55 pm

Hello Steve

Any Rozenblit OTL is definitely worth a shot. I have had the T16 monoblocks for eight years now and have just sent off for the new EL509 based 15W OTL kit (justification, to take up less space and put out less heat!). Planning to build it in November when I have some time off work.

What I've found is that they can be extremely revealing of what's up the line - warts and all, harshness/sharpness that I thought was the amplifier vanished when I upgraded/tweaked the source. Matched with the right speakers they can be amazing, and image like nothing else I've heard. Think "straight wire with gain". Very different to SET.

I don't know how close spec-wise the PL519 comes to the JJ EL509. Bruce has a new book out (Tubes and Circuits) which has the schematics for the new 15W OTL that uses 8 EL509's/4 12AT7's, so you may wish to consider that. It does away with the need for separate high voltage and bias transformers.

The book also has a rather mad 1W single ended OTL that uses 8x300B's if you're feeling flush.

Cressy Snr · Post by **Cressy Snr** » Wed Sep 19, 2012 10:05 pm

Hmm......the 15W OTL in the new book looks tempting. Could use the PL519s in that. The old PL509 and PL519 are the same valve as the JJ EL509, only difference is the 40V heaters rather than 6.3V and of course the top cap and socket requirement.

Steve.

Thermionic Idler · Post by **Thermionic Idler** » Wed Sep 19, 2012 10:15 pm

Yes, the design calls for a 2x12V transformer to supply heater voltages to the 12AT7's and the EL509's (4 groups of 2xEL509 heaters connected in series to give 6V across each) plus some capacitor / zener diode gubbins to supply the A- / B- bias.

So you'd need a separate 40v supply but it sounds like you have that anyway. Just need 12V and I think, 115-0-115.

Cressy Snr · Post by **Cressy Snr** » Thu Sep 20, 2012 9:30 am

Thermionic Idler wrote:Yes, the design calls for a 2x12V transformer to supply heater voltages to the 12AT7's and the EL509's (4 groups of 2xEL509 heaters connected in series to give 6V across each) plus some capacitor / zener diode gubbins to supply the A- / B- bias.

So you'd need a separate 40v supply but it sounds like you have that anyway. Just need 12V and I think, 115-0-115.

Thanks for the heads-up.
That 15W OTL seems a better bet than the T8 stereo amp, both in terms of heat output and degree of complexity. I like simple.
Just need to get the book now.There are a few people who see Bruce as an arrogant self-promoter, but TBH I like his no-nonsense style,
and if you are in business, you have to have a degree of belief in yourself, your own ability, and your products, or else, why bother?

Dave the bass · Post by **Dave the bass** » Thu Sep 20, 2012 10:50 am

Shoddy!

Oh.... hang on.... its not built yet...

hehehehehehehe

DTB

Mike H · Post by **Mike H** » Thu Sep 20, 2012 11:44 am

How many times will it be threatened with being thrown out of the window

Place your bets now....

Cressy Snr · Post by **Cressy Snr** » Thu Sep 20, 2012 2:34 pm

Mike H wrote:How many times will it be threatened with being thrown out of the window

Place your bets now....

Look! I've got to get over this block I have concerning OTL amplifiers

I'm mentally scarred by them at the moment and those demons need sending packing

Last night I ordered Bruce Rozenblit's new book and will carefully go through his design for the "Son of Beast" 15W OTL. Apparently he goes through the design step-by-step, something he did not do with the OTL in his "Audio Reality" book

As T.I. says, this new design is far far simpler at the power supply end, which is where I cocked up last time.

Re-reading the Audio Reality article, perusing the circuit diagram and thinking back, it looks like I made a major mistake with the bi-polar supply for the output stage, meaning that the negative tube banks were grossly under biased from the outset and destroyed themselves within a few seconds of power-up.

I remember, as a last resort, removing the knackered valves from both negative banks and getting music from just the positive side. The sound was very, very good but by then I had had enough, didn't know how to proceed any further, had burnt my fingers pretty badly, been zapped to death by PSU caps, and was thoroughly demoralised by the whole adventure.
It very nearly finished off my interest in scratch-building to be honest!

We learn by our mistakes though. It was all 6 years ago, and this time around I don't think there will be any major problems.
Hands together, eyes closed......

Paul Barker · Post by **Paul Barker** » Thu Sep 20, 2012 5:58 pm

At least you figured it out now. I was having a break at that time so was unaware until you just mentioned it.

Now you would be able to get the power supply right.

In theory just one pair of output valves should work as a test bed, when you know all is well add output valves in pairs.

A worthy project in any case.

Thermionic Idler · Post by **Thermionic Idler** » Thu Sep 20, 2012 6:56 pm

SteveTheShadow wrote:Apparently he goes through the design step-by-step, something he did not do with the OTL in his "Audio Reality" book.

Yep, Chapter 13, 21 pages of detail.

audio-talk

Dear Marge

#1 Dear Marge

#2

#3

#4

#5

#6

#7

#8

#9

#10

#11

#12

#13

#14

#15