The 7242 triode

[Opcom]

I know this tube has been discussed before, but don't find it. Anyway, the 7242 triode is a 400V 900mA regulator with 100W plate, a low mu triode. A 6336 on steroids.

Its CCS current rating is 900mA in DC regulator service with . This is apparently limited by the cathodes.
There are three, to be used in parallel, one grid, and one 100W plate.

We know that in an amplifier, the peak plate current can be several times the average. Looking at any tube the peak current is always much more than the average current at full power output. For example the 811 average plate current as a modulator is 350mA max per pair, in the RCA book.

So the 7242 is rated 900mA DC. What would be a good guess at its peak current value in audio service?
How would I judge the tube for its own peak cathode current limitation to avoid ruining it in AB service?

How do I find out peak current of a tube in push-pull AB1 service into a given load from tube curves? Better for me to do it myself than just get an answer but any explanation is welcome. I could use the 812 curves to study as an example because that tube needs bias just as the 7242 does. The 812 has audio data with it so i could try to work it out..

7242 datasheet attached (2322.pdf).

[kb3ouk]

According to the datasheet that I read, the grids are also seperate, even though the schematic of the tube only shows one. If you need more current handling capability, the 7241 is pretty much the same tube, but with max plate current rating of 1.2 amps. I wonder what would limit either of them from being used for RF? With the 7241, you could probably run about 300 volts at 1 amp on the plate, get 225 watts (assuming it would do it at 75% efficiency), The 7242 could probably do 300 volts at 750 mA, get 170 watts out.

[WBear2GCR]

Patrick,

The thing here is that the plate Z is going to be super low.
This would have been the holy grail tube for a big OTL amplifier!!

Compare to the Ruskie 6C33C for starters.

You'll need a suitable low Z primary transformer to run them.
For these the Z of the cathode is going to be darn close to the plate.
The gain is low too...

How many of these do you have there??

You want to make an audio amp or a modulator?

The good news is that getting wide bandwidth from the iron ought to be not difficult.

[Opcom]

The 6C33 and 7242 circuits are almost alike. I have maybe 5-7 of them, but the goal is to use two. The structure is three flat cathodes and three flat-ish grids, and a plate on each side, the plates connected together. And the grids connected together also.

I don't have in mind a specific amplifier, but desirable options for lab use would be capable of pulse and linear operation with a bandwidth from 5Hz up to at least 100KHz, large voltage swing +/-100V or more, as well as high current, and resistant to destruction by transients.

A very desirable function would be a current amplifier, meaning that an example voltage of 1V p-p square wave input would cause an output square wave current of 2A-3A p-p, regardless of the load impedance, within reasonable limits. Such a function could be used to control the current in an inductor regardless of its resistance to change, within reason. The example of a goal would be a 5-10mH inductor going from 0 to 1A in 0.1us. The voltage would have to be increased instantly, just long enough to get the current up to the value determined by the current feedback*input voltage. Instead of taking voltage feedback from the speaker output terminal, a voltage proportional to the current is taken from a small non-inductive resistor between the load return and ground. This works with solid state well. Unfortunately I have been destroying transistors. They do not like being partially turned off when in series with an inductor. -the transition from 8V@1A to 400V@1A voltage drop across the transistor when an inductor is in series with it. There is a nice DC amplifier circuit here which could make a good basis for driving a pair of these tubes once its 40Hz HF cutoffs are removed.

Optionally a traditional push pull amp could be built for audio, but I don't have any transformers for that. The anode-grid spacing inside is quite close, like a 6080 or 6336. I didn't have an audio amp specifically in mind.

The attached OTL-7242.jpg circuit looks OK, but with these things there is always some question of what the best load resistance would be. They all imply to drive a speaker load, some 8, some 16 or 32 Ohms, and they just don't say what the load is supposed to be. Those are impositions based on what speakers are around, I think not really the best load for efficiency, highest power, and low distortion, which may be higher value than a speaker.

In one of the older dc coupled tube amps used commercially, an aircraft version I got from a Branniff enginer, used 400Hz, making +/-150V via doubler on three 6080's, and was to make 16W into a 15-32 Ohm load. It is very similar to the RCAOTL.GIF diagram.

Anyway, the plate voltage is generous, and the tube can pass 1A at 100V, so the peak current is a limiting factor. I'm not committed to a project just exploring the peak current capacity. About using it for RF, the capacitance may be high.

[Opcom]

This Sanborn DC amplifier only goes to 40Hz, but it could be a basis for driving the 7242 tubes.
The Sanborn DC amp full manual is on bunkerofdoom.
This goes back toe the current feedback system, see R341 near the power supply return.

[WBear2GCR]

Patrick,

I didn't yet look at the schematics you posted...

But I want to comment before the Swizz Cheeze solidifies in my head...
A reverse didoe (fast Shottky) across those transistors with the inductive load may keep them from becoming silicon fuses or wires.

Not sure what ur trying to do with this amplifier??
make sound or drive an industrial type load?

                       _-_-

[Opcom]

The main purpose is an industrial load. It's mainly inductive with variable inductance, series resistance, and parallel capacitance, depending on other factors.

[WBear2GCR]

Looking at the Sanborn amp, I didn't see any obvious reason that the circuit itself limited at 40Hz. Maybe you meant limited to 0Hz to 40Hz?

I guess the tubes would be more forgiving of strangely reactive loads.
You are planning a transformer?
What's the frequency range?
If it's not going too high, suggest considering one of those toroids that are popular for use when reversed for external mod iron? Getting a highish power LF transformer with sufficient self inductance (to go low) and also the requisite turns ratio ought to be fairly easy. Assuming you can maintain the DC balance, you ought to be good to go that way.

Seems to me that OTL is not a good option if stability and reliability is a concern.

Send pix when the project is complete!!

[WD5JKO]

Here is a bunch of new research and development concerning OTL tube amplifiers. This is by Patrick Turner of Turner Audio. Patrick doesn't do anything half way...This guy is good.

http://www.turneraudio.com.au/OTL-amps-pros-cons.html

Jim
Wd5JKO

[Opcom]

I'm not planning a transformer, this is a general purpose DC-100KHz (or better) lab amplifier plan. It's a piece of test equipment, not purposely for audio. It just happens that it will be able to pass audio, and it needs to be driven by a reasonable voltage, operate linearly in the voltage or current output domain. It won't be for low impedance loads like speakers. The load will depend on how much signal I need, but 200 Ohms is do-able. It's not decided which circuit is to be used yet.

That article has usable technical information of value but it's rather a chore to read the first part. I've run into that personality type before, some officious hams are that way so it's par, but insulting audio hobbyists as being lazy and ignorant does not really belong in an article like that. I just scanned to the technical details and confirmed what I wanted. The article mainly addresses overloaded audio OTL amp shortcomings and transformer band-aids. The better details are as stated above, the load resistance and power limitations, etc.

[Opcom]

Looking at the Sanborn amp, I didn't see any obvious reason that the circuit itself limited at 40Hz. Maybe you meant limited to 0Hz to 40Hz?

I guess the tubes would be more forgiving of strangely reactive loads.
You are planning a transformer?
What's the frequency range?
If it's not going too high, suggest considering one of those toroids that are popular for use when reversed for external mod iron? Getting a highish power LF transformer with sufficient self inductance (to go low) and also the requisite turns ratio ought to be fairly easy. Assuming you can maintain the DC balance, you ought to be good to go that way.

Seems to me that OTL is not a good option if stability and reliability is a concern.

Send pix when the project is complete!!

I forgot about this but a change in the project rendered it moot. I doubt I will want to build an audio amp with the 7242s though. As big as they are I have enough >100W tube amps (if that is possible) to last my whole hi-fi requirement.