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Old 17th June 2012, 10:38 AM   #1
DeeLuxx is offline DeeLuxx  New Zealand
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Default Transmitter tube hybrid - Because why not?

Lets face it, big tubes look really, REALLY cool (even though they are red hot) when they are running and sitting on top of an amp!

Unfortunately, they are ungodly to drive (depending on how big) and provide outputs that require AC transmission line sized transformers to be made useful, and among other things dont have very suitable characteristics for audio use in general...

But this got me thinking! There must be a way to make it work! - And not just work, work nicely and provide beautiful sound.

So first thoughts of course were to try and do it the normal method, and just construct a welding-spec power supply and have a super massive OPT - Obviously though this would realistically be unfeasible.

So how about OTL??? Although not the obvious choice it is more achievable I think. Simply going the old school route of running many tubes in parallel is not ideal because the amp would function more or less as a low efficiency 4 kilo watt room heater so another method must be taken.

Circlotron maybe?! Sounds promising at first but again doing this set up with tubes would require many stages to provide a useable output and many tubes all adding distortion etc... - not ideal.


So this leave me with my final (and better so far I think) idea to have a minimal voltage gain stage using a single transmitter tube running class A full time (or maybe push pull?) followed by a solid state mosfet current gain stage (also running class A).

Thoughts on this so far;

- The tube gain stage can happily run in class A full time and at low efficiency as they are high output devices so give plenty of headroom for what is being asked of them.

- It is possible to set up a circlotron output stage using MOSFETs also, not sure on this though because of the power supply requirements and also differential input ect ect

Specifications -

Would ideally like to have ~20 watt output (per channel into 8 Ohms)

Would like to have as few gain stages as possible as to have THD as low as possible (super super low)


Wanting to have super low THD brings up the issue of which config to run?

So thats what i'm asking - any ideas are helpfull - So far im leaning towards a more single design as i wish to use this as a headphone amp aswell (with lower gain of course!!) and single ended will make this easier.


Pictures below graphically demonstrating the sexyness of large tubes

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Old 17th June 2012, 01:24 PM   #2
DF96 is offline DF96  England
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Quote:
Would like to have as few gain stages as possible as to have THD as low as possible (super super low)
Few gain stages normally guarantees high THD, as there is insufficient gain for any feedback and decent amounts of feedback are the way to get "super super low" THD. I make no comment on whether "super super low" THD is a desirable aim in itself.

Personally, I would rather see a transmitter valve in a transmitter doing what it was designed to do rather than just being a piece of audiophile jewelery.
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Old 17th June 2012, 02:10 PM   #3
DeeLuxx is offline DeeLuxx  New Zealand
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Thanks for the reply!

Yea sorry forgot to mention that hopefully with using a transmitter tube you will be able to run with high enough gain (because it has alot of headroom) to be able to run a decent amount of negative feedback and then still have plenty of gain left over for the solid state stage.

I will hopefully experiment and see what sounds the best in my ears (that's what really matters at the end of the day right?) but for the time I would like to head in the direction of low THD.

I guess it would probably be smarter to go for push pull in both stages I guess to get lower distortion and tbh, I dont mind the idea of having both input and output as differential (no phase splitters) even though this means there will have to be a phase splitter upstream of the amp if i'm not using a balanced source device.

Any thoughts would be much appreciated!

Thanks again!
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Old 17th June 2012, 05:57 PM   #4
Funker is offline Funker  Germany
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Originally Posted by DeeLuxx View Post
Thanks for the reply!

Yea sorry forgot to mention that hopefully with using a transmitter tube you will be able to run with high enough gain (because it has alot of headroom) to be able to run a decent amount of negative feedback and then still have plenty of gain left over for the solid state stage.

I will hopefully experiment and see what sounds the best in my ears (that's what really matters at the end of the day right?) but for the time I would like to head in the direction of low THD.

I guess it would probably be smarter to go for push pull in both stages I guess to get lower distortion and tbh, I dont mind the idea of having both input and output as differential (no phase splitters) even though this means there will have to be a phase splitter upstream of the amp if i'm not using a balanced source device.

Any thoughts would be much appreciated!

Thanks again!
Hi ,
the most of transmitter tubes are also designed to use in audio applications, but not in that way as you like. Unless you are in need of an 5 kilowatt amp to blow your neighbourhood up.
If you want to employ a transmitter tube which is designed to operate on 2500V plate on say 300V they would perform bad.

If you need only 20-30 watts choose tubes which are designed to this power level.
Yes there where some guys which build single ended 50W amps with huge transmitting tubes . But there are only few types of tubes which are suitable for this and the guys are quite seasoned in their business. And on the other hand they didnīt told us how many transformers they have rewound, how many tubes are blown and how many times they fiddled about their driver circuits before their development was an acceptable result . These guys are freaks , they like to do so and never care about the waisted money.

And so the financial point is also subject to reconsider before you start to bulid such an amp.

If you are new in tube diy start simple objects to gain experience . One day you will realize that a project which seem simple in the beginning are impossible to realize the more you think about it.

I also like huge transmitting tubes but I really can hold myself back to start such an Intension.

73
Wolfgang
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Old 17th June 2012, 07:25 PM   #5
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Quote:
Originally Posted by DF96 View Post
Personally, I would rather see a transmitter valve in a transmitter doing what it was designed to do rather than just being a piece of audiophile jewelery.

I don't agree jewlery is for adornment which implies absence of function. Also if you need high power levels and either do not want to go SS or venture into PPP territory then high power tubes are the only solution.

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Old 17th June 2012, 07:39 PM   #6
7N7 is offline 7N7  United Kingdom
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Apparently only 20 watts required; so a big transmitter valve would/ must have an element of jewelry about it.

I make no secret of my love of these devices. On the other hand for 20W it does seem a lot of trouble. There will be plenty of distortion in a SE amplifier; push-pull adds greatly to the cost of course with big valves.

I built a p-p amplifier using triode-strapped 813s; it made about 40W and sounded very nice. 813 as a triode is easy to drive: at 850V and 100mA bias was -75V. But oh my! the cost...


He could get an easy 30W at low voltage using triode-strapped 13E1s; nice looking and pricey. I built one; HT was only 275V and it made 33W at 1kHz, 1.9% THD (no feedback). AT 10W THD was 0.28%; these are good figures for a zero-feedback amplifier - it depends what he means by low distortion in this connection.

Whatever he does, it won't be cheap!

Paul
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Old 17th June 2012, 07:55 PM   #7
Koifarm is offline Koifarm  Netherlands
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I have also 813 (fu13)tubes, they are juwels which also perform the best.

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Old 17th June 2012, 08:13 PM   #8
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Nice tubes, you can hide couple of running Gu-50 behind them, to get both Audiophile look and great sound!
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Old 17th June 2012, 08:26 PM   #9
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Tubes are not SS. For every additional Watt of output power your costs quadruple. Easy as that.

If you have the resources then everything is possibile. I just sold two GM100 to a guy who wants to build a PP amp. I have more large tubes and people are starting to contact me for custom transformers and large tubes.
The only thing is the projected cost. Once you get over it you are half way there
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Old 17th June 2012, 09:15 PM   #10
fpitas is offline fpitas  United States
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I'll preface this by saying I'm not a tube guy, although I fooled with them as a kid. I'd try using the transmitting tubes as the top part (common grid stage) of a cascode arrangement, with a small triode or even a JFET as the bottom transconductance element.
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