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angelfj 20th January 2007 05:08 PM

304TL based Class A
I am new to the forum but I have been reading about the 833A based SE amps. I have a supply of 304TL tubes. Would this tube be suitable for a class A SE? Any suggestions for a circuit? Would the driver and output transformer selection be similar to the 833A?

Thanks in advance.


ulibub 24th January 2007 10:16 AM

Hi Frank!

After my big 6C33CB-Circlotron, I am starting now with a 833A-amp. SE-design, class A2, driven (just as my 6C33C-tubes) by a mu-follower, this time only much more powerful. I am planning to use a PL 519 as a driver tube, it can provide easily much more than half an Ampere of current for the grid. And pentode operation assures me lowest possible output impedance. Plate voltage of the 833A should be around 1 - 1.2 kV - everything still higher gets difficult to manage for an amateur. Heater supply (10 V, 10 A) DC stabilised, I've read somewhere (and would guess from my experience) that AC heating inevitably causes hum. Plate circuit will be Parafeed. Plate voltage stabilised also by a (bigger than usual...) standard three-tube stabiliser (glow tube reference (OB2, 108 V), amplifier tube (PL 81), regulator tube (EL 519)). Such a stabilisation also is the easiest way to get the hum out - if you don't want to use lots of heavyweight and expensive chokes. I don't like much of iron in an amp, and be it only for the reasons of weight.... For this reason, I also would like to do without an interstage xfmr.

If you are interested, I would keep you informed about the evolution of the project.

For your 304 TL tubes, these are still more powerful (if I'm right) and still look more impressive ;) than the 833A. But I think, just for evaluation, the data for the 833A could be taken as minimum requirements for such a project. And - since data for such an operation of these tubes are hardly to obtain, building a test circuit should be the best way to see, if it works satisfactorily or not. The crucial point, I think is, if the tube still can work with comparatively low voltages in the 1 kV-range.... Everything higher will get a REAL challenge and also will make your budget explode.....

Good luck....

Here's a pic of my 6C33CB-Circlotron....

Dave Cigna 24th January 2007 09:59 PM

I had the pleasure of listening to a pair of 304TL SE amps and was impressed. They are definitely suitable for SE use if you can manage the tremendous power supply requirements. One very nice thing about them is that they have a high enough mu that you can pretty easily build a two stage amp. The one I listened to (Jeffrey Jackson's) ran 1500V on the plates, and needed (IIRC) about -15 or -20V bias to get the plates a nice cherry color.

There are two challenges:

(1) lighting the filaments is not trivial. I don't recall exactly, but for some reason 5V@20A comes to mind. So, for DC you would be looking for transformers something like 10V@30A. You'll need two. And some good filter chokes as well. And a good fraction of a Farad of capacitance. Good luck with that. :)

(2) SE output transformers for this beast are not trivial. The good news is that you do not need a very high load as with some transmitting triodes. People are running 20k on 75TL's. The 304TL is basically four 75TL's in parallel, so a 5k load is fine. The hard part is handling all of the DC current (200mA? I don't remember what Jeffrey was running.) That and 50W capability means it will be large.

Jeffrey uses transformers with an extra 'cancel' winding. The DC filament current is run through the extra winding to cancel some or all of the DC flux from SE operation. Parafeed is another option and might work well. Of course you would still need mammoth plate chokes or CCS's.

Don't need much of a schematic if you're imaginative. Assuming you build all the power supplies you just need a fairly meaty driver; there is a LOT of miller capacitance. But, as I said, it doesn't need to deliver a whole lot of volts, so it's do-able. Any big transmitting tube wants a low DC resistance at its grid, so either interstage coupling or direct coupling with a choke loaded driver is pretty much mandatory.

-- Dave

angelfj 24th January 2007 11:31 PM

Dave: many thanks. this sounds like quite a challenge, but I'm up to it, if not too crazy. I attached a basic schematic from Alum, but I don't know if this is accurate. What do you think?


nhuwar 24th January 2007 11:57 PM

You are heading do approximately the same road I am but I have changed to push pull. The one piece of advice that everyone gave me and I took them up on it is to join the yahoo group . They are a yahoo group dedicated to the design and building of tube audio amp using large transmitter tubes.

And the other piece of advice I what to give you is if you are convinced on devoting the large quantity of time and money to this amp then dont let anyone change your mind so stick to it and good luck.


angelfj 31st January 2007 09:11 PM

Nick: yes, I did join the boatanchor group. My problem is that it has been too many years since my technical education, and I don't know the theory part. I do have fairly good construction skills.

Question: is there a simple way to find the "operating point" for a tube, in my case , the 304 TL? I have been told that this is important, but I do not understand what this means. For example, I have heard people say that they use a plate voltage of 2000 vdc and the output transformer with a ratio of 5000 ohms on the tube side and 4 or 8 ohms on the speaker side. If the plate voltage is selected and the output transformer is known, what other variables remain? Does it have anything to do with the tube bias setting? I thought that class A uses zero bias?

Thanks in advance, Frank

nhuwar 31st January 2007 11:14 PM

Well I'm assuming by the way you are talking is that you dont have the spec sheet for the tube so here you go.

But unfortunately I am kind of in the same boat as you that is I know rf amps but not audio and the suggestion that I was given by alot of the people on this forum is to buy Morgan Jones book (valve amplifiers) it's a 600+ page bible on tube audio.

I have mine coming in tomorrow :D :D :D :D

As for your bias point there isn't one for class A that I see on the spec sheet so you would probably have to play around with the bias at lowered plate voltage to find the right voltage so at max signal input you don't draw to much current through the plate.

As I have learned about tube audio it's not a easy undertaking to build a amp thats never been built before It's a lot of trial and error oh and time.

Hope this help a little


nhuwar 31st January 2007 11:27 PM

Oh shoot the link I posted for the spec sheet is wrong here is the right one.

I honestly think the spec sheet should answer alot of you question and if there is something on it that you dont know just ask us on this forum and we will help you out.

The big problem is the tube isn't really made for class A if you have 4 it would be a whole lot easier to do push pull class ab1.
But dont think I'm trying to change your mind just telling you what I learned.


Dave Cigna 1st February 2007 12:07 AM


Originally posted by angelfj
Question: is there a simple way to find the "operating point" for a tube, in my case , the 304 TL?
Usually it's done by either:

1) Obtain plate curves for the tube (showing plate current vs. plate voltage for a variety of different grid voltages.) Look at the curves and draw tentative load lines until you find something you like.

2) Pick something from the manufacturer's data sheets.

3) Pick something recommended by someone else.

As far as I know, plate curves are not available for these tubes, at least not ones that are suitable for class A operation.

The data sheets definitely don't offer any suggested operating points for this kind of application.

If you're lucky, you might find someone that can tell you how they ran theirs ...

But all is not lost. These tubes like (need) to be run with their plates cherry hot, so what you do is pick a plate voltage (1500, 2000, you might try running lower) then bias up the tubes until the plates glow visibly in a well lit room. You'll know it when you see it. Apply signal to the grid and enjoy!

Seriously, once you pick a load - say 5k in this case - there's only two things left to adjust: the plate voltage and the grid bias. Make both adjustable and you can decide for yourself what sounds best. You need them both to be adjustable anyway so that you can bring the amp up slowly. Variacs are your friends. Surround yourself with them.

-- Dave

nhuwar 1st February 2007 12:22 AM

[QUOTE]Originally posted by Dave Cigna

1) Obtain plate curves for the tube (showing plate current vs. plate voltage for a variety of different grid voltages.) Look at the curves and draw tentative load lines until you find something you like.

The spec sheet in the link I posted has a awesome plate current to grid voltage curve practically
8 1\2 by 11 it's huge.


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