New OTL project

[Firebottle]

Just trying out different configurations for a low power OTL. I've had the parts for quite a few years now but the performance of the 'Firebottle' I made is so good that I haven't had any urge to change. The current 'Firebottle' amp is a Trancendent design with some minor modifications.
Here is a pic of the rats nest on the test bench:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/P1020832.jpg

This is just really for interest, using different (cheap) valves. I hope it will put out about 5W into the ESL57s.
I'm using an ECL80 for the gain/phase splitter, 6J6s for the driver stages and 6080s for the output stage. The heater transformer is a bit overloaded but hey-ho.
I've had music through it so at least I know it works :mental:

Cheers, Alan

[Reid Malenfant]

:eek:

Better you than me... Though I must admit to often doing very similar, it tends to be with much lower voltages as I play about with solid state stuff :)

Then again, given the output power you are looking at & the fact it's an OTL, I doubt you using silly voltages for the PSU anyway :scratch:


Good luck with the project!

[Firebottle]

Many thanks :)
The 6080 output valves are rather high impedance, the output stage is on +/- 100V but the driver stages are across +300 and -150V :eek:

Cheers, Alan

[Reid Malenfant]

Actually, thinking about it I'll soon be messing with +/- 155V DC in a solid state amp. 100V + rms output swing.

So not too much difference at the end of the day, it can still kill & it's far worse than AC as I'm sure you are more than aware :mental:


Please be careful, I know I'll try my best to :D

[Firebottle]

Please be careful, I know I'll try my best to :D

Will do, I've been around valves long enough to develop a healthy respect for the voltages involved plus safe working practices.

Anyway some small developments for the OTL. I now have a reasonable case to build it into and have been trying out the valve layout on the top. More for the aesthetics at the moment, here's a pic:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/P1020874.jpg

:cool:

[loo]

:clapclapclap:
thats looking great love to know how it sounds ,and how the 57's perform driven by 5 watts .
I use a otl with quad 57's and 63's and find it a wonderful combination ,
my amp puts out about 18 watts.

[Firebottle]

Hi Paul, thanks for the praise.
The amp isn't actually built yet, the pic shows an empty case with the valves just stood on top to work out the layout.
The rats nest on the workbench was only tested with an old speaker in the workshop, not the 57s.

I have since increased the supply rails on the output valves and should achieve 15W into the 57s. I am very much looking forward to hearing what it's like when finished, will probably be a couple of months yet.

What OTL are you using?

:cool: Alan

[loo]

Hi Alan, it was the Rats nest I was commenting on:lol:
no seiously though will look pretty good if thats the lay out you go for:)
I use a Graaf GM20 I used to have 2 bridged into monoblocks but big lecky bills and an un sympathetic partner led me to sell one:(
propbably did not need it as one is still sublime , but I still miss her:wah:
OTL's and ESL's a match made in heven
dont forget to let me know how it sounds when finished
regards Paul

[DSJR]

I remember the Croft OTL I tried to use (two quads of 509 tubes I seem to remember). Brilliantly made and very simple, it still kept blowing one of the four line fuses. The resulting lightning effect was not something i wanted to see again...

Good luck and hope your amp will be happy with the odd loading of the 57's.

[Reid Malenfant]

Nice looking layout :) With a polished top cover that'd look quite fetching with those bottles glowing nicely :D

[Firebottle]

I've just been tinkering with the circuit, only on paper, so yet to test it.

I have designed out a few capacitors from the signal path, particularly cathode bypass caps. I've got rid of 4 caps from each channel so it's got to be good :)

That's one big benefit from specifying multiple taps on the custom mains transformer, I can change the supply and bias rails quite easily.

I've put a couple of tweaks in as well to balance the drive to the totem pole O/P stage, I've not seen this exact combination before in the same circuit. I hope it does what I think it should :scratch:

Will report back when I've tested it,
:cool: Alan

[Firebottle]

Hmmm, didn't quite work out as I was expecting (not such a guru then).

I have now completely re-vamped the circuit, out has gone the pentode input stage, out is the cathodyne phase splitter, I am now following the KISS principle.

I now have a simple circuit as far as the signal path is concerned, all triode and no overall feedback. I'm quite keen myself now to find out how it sounds when it's built. It looks good on the scope so here's hoping it's as good with music into the ESL57s.

I let you know, when it's done in the new year.

Seasons greeting all, Alan

[Reid Malenfant]

I now have a simple circuit as far as the signal path is concerned, all triode and no overall feedback.
:thumbsup: The most linear type of valve, hopefully it'll sound rather nice :)

The simplest circuit often can do! As long as you don't mind some even harmonics (which are musical anyway) then you could be onto a winner.

[daytona600]

just recieved my 1st OTL amp , two box with 6c33c 16wpc SET
with remote control , cheated a bit not DIY

[Firebottle]

Hey Scott it doesn't have to be diy :yay:
When I looked at the picture I thought it was Gangnam :lol:

Be very interested how you get on with it and of course how it sounds.

:cool: Alan

[lurcher]

OTL's and ESL's a match made in heaven

Sonically, yes they can be, but the falling with frequency impedance of the typical ESL is a nightmare for any OTL. If you are simulating the amp, make sure you see how it behaves with a capacitive load.

http://www.audiomisc.co.uk/57and303/interact.html

http://www.audiomisc.co.uk/57and303/57imp.gif

[Firebottle]

Nick many thanks for that link, I was aware of the big dip in impedance at HF.

I have loaded the amp with a 2.2uF capacitor on the output and the stability is excellent.
However, as I think you're suggesting, the output level does fall above about 10KHz due to the finite (err - big :eek:) impedance of the output valves.

My hearing stops above 12KHz due to age so for me it's not going to make a lot of difference.
I must get around to to testing the Transcendent Sound OTL copy and see how that actually looks with 2.2uF on the output :scratch:

Cheers, Alan

[SPS]

A friend brought OTL his around to my house a few weeks ago, it was a diy job

it sounded really good, It highlighted the damage caused by transformers to me, it was great as we swapped out my px4 se amp, so was a good learning session for me.

Steve's amp was good, but unfortunatley the valves i prefer to use for audio are not really suitable for an otl amp. i have heard a few otl's over the years, this was the first one that that for me had a real wow factor.

[Firebottle]

That's intriguing Steve, what valves were in this friends OTL?

What design was it based on? Also
:worthless:

I've been doing a little of the mechanical work on mine, here is the front panel layout
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/P1020877.jpg

Bias and idle adjustment controls on the left, volume control in the middle and meter and On/off on the right.
:cool: Alan

[loo]

just recieved my 1st OTL amp , two box with 6c33c 16wpc SET
with remote control , cheated a bit not DIY

Hi according to their web site the Gaanam is not an OTL amp

[loo]

Sonically, yes they can be, but the falling with frequency impedance of the typical ESL is a nightmare for any OTL. If you are simulating the amp, make sure you see how it behaves with a capacitive load

Hi yes I must admit the Graaf eats 6C33cs when its driving the 57's I find 63's are an easier load and I get much longer tube life:)
regards loo

[loo]

That's intriguing Steve, what valves were in this friends OTL?

What design was it based on? Also
:worthless:

I've been doing a little of the mechanical work on mine, here is the front panel layout
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/P1020877.jpg

Bias and idle adjustment controls on the left, volume control in the middle and meter and On/off on the right.
:cool: Alan

Hi Alan that panel looks nice not sure about the meter though.
I love the idea of those bias controls :idea:
wish my amp had something like that for bias and offset adjustment

[Thermionic Idler]

That's intriguing Steve, what valves were in this friends OTL?

That would have been Steve C's scratch built OTL wouldn't it? Bruce Rozenblit's latest 15W OTL design called "Son of Beast".

It was designed for JJ EL509's, but Steve had adapted it to use miscellaneous PL519's.

All OTL's image like nothing else I've heard. The speakers pretty much vanish.

[Firebottle]

Well that is interesting because I've got a scratch built original 25W Bruce Rozenblit's OTL using EL519s.

I called my build a 'Firebottle' hence the forum name.

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/Firebottlelrg.jpg

I'm really keen to hear if there is any or much difference between the Bruce Rozenblit OTL and the one I'm building.
It is using totally different valves using a much simpler circuit :scratch:

I'd be interested to see the circuit for 'Son of Beast'

:cool: Alan

[Thermionic Idler]

I'd be interested to see the circuit for 'Son of Beast'

Can be found in this book...

http://www.amazon.co.uk/Tubes-Circuits-Bruce-Rozenblit/dp/1477532862

[Firebottle]

Do you have a copy Dave, I've only got his first book.

:cool: Alan

[Thermionic Idler]

Do you have a copy Dave, I've only got his first book.

I have the book and obviously bought and built the kit as well --- cannot recommend it highly enough --- but it is a patented and copyrighted circuit, and out of respect to the designer who has made what I feel to be a reference quality design available at a very reasonable price, I wouldn't feel at all comfortable about distributing copies of it.

[Firebottle]

Dave I appreciate your sentiment, fair play.

I do own a copy of his first book that has the original patented circuit in it, again like yourself I built a copy and was mightily impressed.
I know Bruce Rozenblit has made some changes, he has made the circuity simpler and has incorporated choke filtering of at least one of the supply rails.
I was just wondering on the details that was all.

:cool: Alan

[Firebottle]

I did have a small doubt about this posting but I stand by my convictions.

I have now studied the 15W 'Son of Beast' circuit and whilst it is a fine circuit, if Mr Rozenblit is implying it to be covered by his patent then he is taking the pi$$.

The circuit diagram has the patent number of his original patent in the title, but the patent is very specific as to the design of the circuit, that being the original 80 and 25W OTL designs. The only technique used in the 'Son of Beast' is bootstrapping which has been used in a valve OTL circuit previously.

:cool: Alan

[Firebottle]

I'm now back on the construction and have positioned the valve bases.
I've taken into account the internal layout, the size of the valve cage and the aesthetics.
Also shown is the cover I have for the mains transformer at the back of the chassis.

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/OuTLaw.jpg

More pics as I progress some more,

:cool: Alan

[Firebottle]

I've now completed the amplifier sub-chassis that has the complete amp circuit apart from the power supply.

Looks like more components than I expected but then it is both channels, I have only been experimenting with one channel.

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/P1020969.jpg

:cool: Alan

[Firebottle]

Been constructing the front panel today, here is the wiring for the volume and bias controls:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/P1020968.jpg

I've made good progress fitting the sub-chassis and case together this afternoon, will have some more photos tommorrow,

:cool: Alan

[loo]

Hi Alan
looking good I really like the new layout Especially that tranny cover .
Looks like I might be getting that review with the Quads sooner than I thought. Keep it up Sir :respect:
regards Paul

[Firebottle]

IT LIVES :)

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/P1020970.jpg?t=1358423910

Photo of the amp in the lounge, driving the ESL57s.
:wow: what clarity and depth to the soundstage. I've been playing individually selected tracks and have never heard them so good, finished off with Comfortably Numb by Scissor Sisters :wowzer:

BUT.... as soon as I played the first track I thought 'what incredible bass' then gave then sound a good listening to (a bit like a good talking to but with your ears :D)

The bass is overblown at the 100Hz region and I've figured out why. The OTL without overall feedback is totally unsuitable for driving speakers with large changes in impedance (ring any bells? -- ESL57s).

The 57 impedance peaks at 100Hz and the amp output is also peaking at 100Hz because of the high output impedance. The output signal has to be referenced back to the input signal using feedback
so that the level remains correct over the full frequency range given a varying load.

So it's back to the workbench to rewire with feedback over the whole circuit, :hmm: hope I don't lose the soundstage,

:cool: Alan

[The Grand Wazoo]

Very nice.
Did you get that volume knob from a 50's Chrysler? All that's missing is the fins!

(NB - piccy resize would be good if you can manage it)

[Firebottle]

I was wondering when the first comment would be about the knob. I think it came off an old cooker, I can't be sure as it had been in the spares box for years:scratch:

I've resized the photos but the last one doesn't want to play for some reason :rolleyes:

[Firebottle]

Tonal balance is restored, and the soundstage remains :hifive:

That's a relief, I did have worries about the detail diminishing when I applied overall feedback.
It still sounds stunning, I've been going through more of the music collection this morning. Every track I have put on has been more detailed, ....importantly more enjoyable :drool:. The performers have tangible dimensions :)

I like triodes, no ....I love triodes.
Thinking back I bought the valves and tranny a long time ago, after building the Bruce Rozenblit T8 copy with the express thought of building and trying an OTL with inexpensive valves.

Now I have completed it, to say I'm well pleased is an understatement.
The 6J6 triodes now cost $1 each and the 6080 triodes cost $7 each, all available from vacuumtubes.net

I'm now going to build the remote control interface and then finish the top plate and cage supports. Photo to follow when finished,

:cool: Alan

Postscript:
This afternoon I improved the power supply filtering to remove the very slight hum I had, I then listened to more various tracks and am absolutely blown away.
It's such a thrill to realise that you have achieved a step change in resolution and subsequent enjoyment of the music. I can really empathize with those of you that eulogise about achieving improvements in your own system. To have done it with some kit that I have designed myself is doubly satisfying.
If anyone wants a copy of the circuit that I have used just shout out.

A very happy bunny, Alan

[Firebottle]

For those that like to study the pictures, here is the shot of the internals:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/OuTLawinternal.jpg

[Greg2010]

Hi Alan, glad it's performing well, I'd be interested in having a look at the final schematic.

Regards
Greg.

[Firebottle]

Hi Greg, I've kept the signal path as simple as possible, here it is:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/6080OTL.jpg

:cool: Alan

[Macca]

IT LIVES :)

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/P1020970.jpg?t=1358423910



Fabulous! Like something out of Buck Rogers. Really impressive build that, Alan.

[Greg2010]

Many thanks Alan,


Hi Greg, I've kept the signal path as simple as possible, here it is
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/6080OTL.jpg


Regards
Greg

[Firebottle]

It's now finished. The top plate has had the clearance holes for the valves cut, it's a bit of a task cutting 43mm holes, even in aluminium.

The cage has been cut down in height to suit, it was one I had made a while back. The finish is rather good, it is pickled and polished stainless steel.

For Chris (The Grand Wazoo) the cage support posts at the rear were the nearest I could get to fins :doh:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/OuTLawfini.jpg

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/OuTLawfinished.jpg

:cool: Alan

[The Grand Wazoo]

For Chris (The Grand Wazoo) the cage support posts at the rear were the nearest I could get to fins :doh:


Hehehe...fantastic!

[Firebottle]

I thought the sound coming out of the OTL couldn't get any better......but it has :yay:

I've made a small modification to the driver stage circuitry and the bass has tightened up, not to say it was in anyway loose before.

For those of a technical bent or those wanting to learn here's the theory:
All OTLs suffer from the same problem; because there is no such animal as a reverse polarity valve the output stage is always unbalanced.
The top valve is operating as a cathode follower (can also be called a 'common anode') and the bottom valve is operating as a common cathode.
The cathode follower has no gain whereas the common anode configuration has gain, but not a lot with the typical valves used in OTLs.

There have been various approaches to adjust the drive to the output stage, usually by reducing the drive to the bottom valve but there is always some imbalance.
Ultimately the negative feedback 'cleans up' the signal and things sound good.

I recently came upon an approach that adjusts the drive level to both top and bottom valves, increasing the drive to the top valve and reducing the drive to the bottom valve at the same time. It seems so obvious I wonder why I didn't think of it before :doh:

Here is the updated schematic

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Valve%20amp/Outlawcircuit.jpg

Comparing to the previous schematic you will see that both sections of the driver stage are fed through a common resistor, bootstrapped to the output.
The gain of the bottom 6080 output valves is about x2, the ratio of the 6K8 and 22K resistors gives a larger drive signal to the top valves and a corresponding smaller drive signal to the bottom valves.
Previously with the feedback disconnected the negative of the output would always go into clipping before the positive, now the clipping is symmetrical.

They say that negative feedback can cover a multitude of sins, so with one less sin to deal with the output has to be purer, right :scratch:

Sounding good :D

Edit: This modification could be applied to the Bruce Rosenblit 'Son of Beast' very easily, perhaps one of you will try it.
Change R7 & R10 to 12K, and R11 to 5K6 to start with. Move the top end of R7 to the junction of R10 & 11. If you have a 'scope adjust the 5K6 value with the feedback disconnected (lift R31) until the output clips symmetrically :)