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Old 31st January 2007, 12:58 AM   #21
qq is offline qq  United States
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I was thinking of going PPP 815's. Then I can play with ultra linear and triode connetion. But yes, the screen voltage would greatly limit these designs.

Ever thought of using a regulator tube for the screen? Sure would look purrdy...

I have a sofis tube curve tracer and was going to curve an 815 with both sections conneted.
Yeah, I was a little disappointed I couldn't play with ultralinear since the screens are connected together internally... Using a single 815 p-p I'm getting a little over 10 watts at <2% THD (as measured by laptop and random software, so take with grain of salt Twice that would be a pretty loud little amp..

Gas regulator tubes would indeed be cool looking, but I think you'd need two of them - with a B+ of 400v, you need to drop at least 180v to get down to a safe screen voltage for the 815, so maybe a pair of 0B3s.
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Old 31st January 2007, 03:54 AM   #22
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From your schematic and also from the tube data, which is not very clear, I gather that the screens (as you say) and the cathodes are internally strapped together. That means that if you use cathode bias it forces you into Class A because the 2 beam tetrodes in the same envelope cannot be given individual cathode resistors. It also means that you can't use UL.

However, for tetrode-mode class AB1, AB2, B1 or B2 it should work fine, with the cathodes grounded, neg. bias voltage on the grids and the screens fed from a low-impedance voltage source.

You will need to devise a method of measuring plate current, for balancing purposes, because you won't be able to use the popular and safe method of measuring voltage drop across cathode resistors, since the cathodes are strapped internally together. Be cautious, because both leads of your meter will be at high voltage!

Just a thought - the 815 is intended for transmitter use, which entails intermittent bursts of power. You might find it gets too hot under continuous use, especially with two tetrodes in the same envelope. This is one very good reason to avoid Class A. Other transmitter tubes, like the 6146, used to come to grief for the same reason in audio use.
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Old 31st January 2007, 06:42 AM   #23
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I think it's the baem plates (suppressor) and the cathode which are tied together.....

qq- I was also thinking fixed bias. That and gas regulation is not too complicated. Albeit I haven't built mine yet!

ray_moth- if one follows the 815's CCS ratings and keeps plate wattage down from maximum, shouldn't that be OK?
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Old 31st January 2007, 07:40 AM   #24
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Firstly, I realize that what I posted could be ambiguous.

What I meant to say was that, from the RCA data sheet I've seen, the screens are strapped together; in addition, the cathodes are also strapped together. (Cathode is also strapped to beam plates, as you mention, but that's always the case with beam tetrodes). I certainly did NOT mean that the screens are strapped to the cathodes!

As to the safe margin for continuous running of 815s, it's all a matter of degree. As I mentioned, the 6146, a powerful and popular tube for modulator use, did not tolerate the continuous heat when it was used in audio amps - the base eventually became loose. I don't know what level of dissipation would have been comfortable for it in audio use. The same MIGHT be true of the 815, I don't know, but the presence of two power beam tetrodes in a single envelope suggests to me that there could be problems in this regard.

For safety's sake, I would avoid Class A, which is at its hottest with no signal. Notice that, for CCS operation, the suggested quiescent plate current is only 20mA, whereas the full signal plate current is 150mA. This suggests a very cool bias setting, almost in Class B.
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Old 31st January 2007, 08:19 AM   #25
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Quote:
Originally posted by ray_moth

. . .
You will need to devise a method of measuring plate current, for balancing purposes, because you won't be able to use the popular and safe method of measuring voltage drop across cathode resistors, since the cathodes are strapped internally together. Be cautious, because both leads of your meter will be at high voltage!
. . .
A very simple (and somewhat surprising) method of measuring plate current is to attach your (well insulated !) hand meter across the respective plate winding of the OPT.
The internal resistance of the meter being much lower than the one of the winding, almost all the current flows through the meter, not trough the winding and the result is precise.

Not valid with signal applied because the tube(s) has/have no longer a correct load but sees a short circuit.

Yves.
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Old 31st January 2007, 08:27 AM   #26
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Yes, the cathodes are strapped together, the screens are strapped together, and the cathodes are strapped to the beam plates. The only separate parts are the grids and plates. Given that, I don't actually see how you could do any balancing of the tubes, other than just making sure the outputs of the phase splitter/drivers are equal in amplitude.

Also, Iunderstand that one doesn't use cathode bias if one is going to go outside of class A, but I don't intuitively understand why. Is it because if the tubes are allowed to go into cutoff, the balance between the plate currents on each tube isn't maintained, and the cathode resistor's voltage drop changes?
I.e. in class A, if tube 1 draws more current, tube 2 will be drawing proportionally less current, causing the overall draw on the shared cathode resistor to be the same.. but once one tube goes into cutoff, it can't keep decreasing as the other tube increases, so the overall current goes down, thus lowering the bias voltage?
I'm guessing that leads to crossover distortion, but that's pure speculation.

thanks for all the help.
qq
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Old 31st January 2007, 11:02 AM   #27
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That's why I will go PPP with this tube- sure there will be differences between elements. But by summing them together, inbalances are easier to to deal with.

At least I think so!
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Old 31st January 2007, 11:17 AM   #28
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The only separate parts are the grids and plates. Given that, I don't actually see how you could do any balancing of the tubes, other than just making sure the outputs of the phase splitter/drivers are equal in amplitude.
That's probably because you're thinking in terms of cathode bias. If you think of fixed bias, then you can adjust the DC balance by adjusting the individual negative voltages on the grids.

The cathodes are connected to ground and a negative voltage is applied to each grid via the 'grid leak' resistor (or via the driver, if using direct-coupled cf drivers). An arrangement of potentiometers is used, so that the bias is adjustable. You can 'dial in' the amount of negative grid bias at each grid, to obtain the required current through the respective plate and to balance them so that the current is the same for each.

Quote:
Also, I understand that one doesn't use cathode bias if one is going to go outside of class A, but I don't intuitively understand why. Is it because if the tubes are allowed to go into cutoff, the balance between the plate currents on each tube isn't maintained, and the cathode resistor's voltage drop changes?
Yes, that's right. With 'mild' class AB1 (i.e. close to class A) you can get away with cathode bias, if you use separate cathode resistors and each is bypassed by a large value capacitor (this is impossible with an 815 tube, because the cathodes are strapped together). The cap holds its charge long enough for the differing current at cutoff to have negligible effect on the bias voltage. However, it can't work for deep class AB or class B, because the excursion of the tube into cutoff lasts too long for the cap to be able to 'cover up' for it.

Quote:
I'm guessing that leads to crossover distortion, but that's pure speculation.
Not crossover distortion, but non-linearity: as the bias changes, so does the internal resistance of the tube. Crossover distortion is caused by a discontinuiity at zero volts (the crossover point), if one tube doesn't start to conduct in time before the other cuts off. It is a danger with Class B and is avoided by raising the quiescent currents of the tubes a little. Crossover distortion is particularly objectionable because it is not dependent on the signal magnitude, therefore it becomes highly noticeable with low level signals.

If you wnat to read more about this stuff (and more), I can recommend the technical articles at Aiken's site
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Old 31st January 2007, 12:08 PM   #29
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There's another trick you can do to balance the sections: the heaters are individually accessible, so varying their relative voltages a few percent (not more than that, you want the tube to operate properly) can bring things into balance if the sections aren't too badly mismatched.
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Old 31st January 2007, 08:27 PM   #30
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I'm playing with fixed bias at about 20mA per unit. Initial results are very bad - awful distortion as soon as I go past pure class A. This is with about -30v fixed bias, and feeding the grids with very clean and well matched 20v p-p drive signals.

I'm pretty sure the problem is with my screen supply. Nominally it's at 220v, but it's just a voltage divider off B+, and as soon there's any substantial signal, it sags down to like 190 or less and the signal at the plates looks like this:

http://mexico.limpoc.com/~eric/bothplates.jpg

I'm thinking of using zener diodes for the screen - DigiKey has 47v ones rated for 30mA, so a string of four would get it down to ~212v at well over the rated max screen dissipation of 4.5W. Gas tubes would look cool but I have too many tubes already.

It worked much better when using shared cathode bias, but I was barely straying out of class A then. I guess going further toward B starts demanding screen current?
Anyway, I imagine with a stiff screen supply there will be a lot more power output available.

I've been reading the articles at aikenamps.com and they've been very helpful.
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