EL509 OTL Amplifier

[Cressy Snr]

Hi Nick

Sometimes the best way to learn a lesson is the hard way.
You have to hand it to Bruce Rozenblit though, He really does seem to have made an OTL that is user friendly and does not blow up your speakers.

If a numpty like me can build one of his designs without blowing himself up then it must be good. Time and again Rozenblit's safety features have kept the amp safe even during severe motorboating.

OK even he he couldn't do anything about the blown valves but that is just one of those things. I've learned, in future, not to test amps on end.

When I can afford some new valves, it should be a brilliant listening experience. Meanwhile I'll have to think of something else for Whitham.
Maybe at the next Owstfest it will be running. Though I can't use the amp at the moment I'm quite pleased with myself really

Thanks for the support during the build.

Steve

[Andrew]

A pity Steve, but on the positive side you now have a working amp, so when you are able to get some new valves you'll have cracked it and will have a beautiful looking and sounding amp.

If I were to build a push pull then an OTL would have to be it.

A 6C33C using a cricuit based upon Bruce's could be quite fun, be a bugger to heat tho, be interesting to see how Nick cracks that on his SE 6C33C.

cheers,

-- Andrew

[Toppsy]

Steve,

glad to hear you have sorted your amp out but a pity about the valves. Will it be ready for Withamfest end of Sept? I would reaaly like to listen to it and see if it can drive my ESLs.

I'm a little hampered in getting both monoblocks finished as Guido at Tentlabs has indicated he will repair my duff heater module rather than send me a new unit. So I could be waiting a couple of weeks at the least, pending our wonderful and efficient postal service as it is at the moment.

Colin

[Andrew]

I meant to ask you both, what you're using the Tent heaters for 300B or GM70 and if they are clearly better for the 300B than an LM317-based current source?

cheers,

-- Andrew

[Toppsy]

Andrew,

I am using seperate Tentlabs heater modules for both the 300b and gm70 valves on my monoblocs. Not sure if they are better than LM-317 based current source. perhaps Nick or one of the other guru's can comment here?

In Nicks design for my amp it was intended from the start to keep all the heaters seperate via a dedicated TX with multi windings.

The use of the Tentlabs was at the suggestion of Nick as being a better way of heating these valves.

I believe Nick first used them on his 211 build to great effect?
I'd check out their web site http://www.tentlabs.com/Products/Tubeam ... index.html

To be honest I'm not too sure whether we will hear any benefit sonically as the gm70 amp in breadboard form projected a huge soundstage way beyond the bounds of the speakers and this was without the Tentlabs. We won't be able to assess the difference until I get back the duff unit and have finished the wiring up of the second amp.

Colin

[Cressy Snr]

Steve,

glad to hear you have sorted your amp out but a pity about the valves. Will it be ready for Withamfest end of Sept? I would reaaly like to listen to it and see if it can drive my ESLs.

I'm a little hampered in getting both monoblocks finished as Guido at Tentlabs has indicated he will repair my duff heater module rather than send me a new unit. So I could be waiting a couple of weeks at the least, pending our wonderful and efficient postal service as it is at the moment.

Colin

Hi Colin

I don't think I'll be ready for Whitham. It is going to be at least October before I can get it going again. I love the job but the only problem with being a supply teacher is that if I don't work then I don't get paid.

As you can imagine this time of year is pretty lean cashwise. I have had to build up, over the academic yea,r a contingency fund for the Summer holidays, which has to see us through to October at least before the money starts coming in again.

However because I used a modular construction method for the amp. I have been able to take out the OTL output stage sub assembly and have cut and punched out another sub chassis to fit the box. This will house a 6AS7 push pull driver and output stage module.

All the power supplies in the amp apart from the bias supply, which is not needed at the moment, can still be used to power this. In fact the 550V power supply in there is just the thing I need for the drivers to the 6AS7s. The Rozenblit OTL driver stage can be left in situ and covered over on the top with a decorative plate until it is needed again.

So I'll still have something for Whitham...just not what I'd planned.

Steve

[Nick]

and if they are clearly better for the 300B than an LM317-based current source

I haven't done a direct comparison, but my feeling with them is they ar enot any better (or if so not by much) but they avoid the hassle of needing to adjust the supply every time a valve is swapped.

[Cressy Snr]

Now then Nick
Hows this for a rat's nest .



As everyone knows the OTL works but is short of valves at the mo.
So not being one to give up, and given the fact that Melanie and I love the retro casework of the Jetson OTL; yesterday I wired up the 6AS7/6080 ppamp that I was going to do but never got around to and shoehorned it into the OTL chassis.

The photo shows it hacked into the OTL's power supplies with clip leads.
It is playing nicely and it worked first time. It is also very loud!

The amp consists of a Sowter 3575 input transformer phase splitter on the front, followed by a 6SN7 differential voltage amp driving another 6SN7 differential driver stage to a lovely looking pair of Svetlana winged C 6AS7 output valves courtesy of Andrew Ivimey.


The 530V power supply feeding the 6SN7s is allowing them to swing tons of volts and these seem to have really kicked the low mu output stage into life.


Output trannies are Phillip Pre65s Hammond 125Ds that he lent me to get the pp amp going and see if I liked the push pull sound, before spending on fancy output devices.

As for the sound quality, the fact that the thing works is enough at the moment. It'll pass muster for Whitham I think.

And lesson learned, it is being tested upside down, on books. Thanks for that tip Nick. Should have had more sense in the first place but there you go.


Clip leads rock!

Steve.

[andrew Ivimey]

Well done Steve!