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Compactron characteristic curves?

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I was looking into doing a Compactron amplifier.

The 6LU8 was the highest power one I could find. However, it has a max current of 75mA. Check out 75mA on the curves, it's in a very non-linear region. So I came to the conclusion it was only suitable for a push-pull amp. Don't know how the curves look for triode strapped though, maybe better.

If you want to go sigle-ended I found the dual triode 6GF7A is the only one that may work well. It's rated at 11W and i'm not sure if it's actually a compactron as it only has 9 pins, but it has the compactron look. It's equivalent to one of those popular octal's that are used for single tube, single ended amps. The number escapes me 6em7, I think :confused:
 
My above post got posted before I had added that the 6MY8 is rated at 16W, as mentioned above.

14W pentode section tubes are 6LR8, 6LU8.
12W 6MF8, 6T9
and a variety of 10W tubes, 6AG9, 6AH9, 6AK9, 6AL9, 6HC8 (11w).

I've used 6LR8s and over driven them to about 18Ws for a few days so I can't say anthing about tube life.
Matt
 
Re: compactrons

engels said:
I wonder why do you need triode-pentodes? There are many more higher rated compactrons which are output tubes only and you have those triple triodes as well to compliment them.

As I'm planning to do something monstrous with a quad of 21GY5 (18W pentodes) I'm interested in your developments.

Plan is to build something like a Crowhurst twin-coupled, as revisited by John Stewart. The purpose of this query was to investigate any higher powered tubes that might work -- Stewart successfully used 6LU8's. A prior article by the same author mentions a 6MV8 @ 16W, but I can't find any substantial data, like a real curve. Stewart liked the 6LU8 for a couple of reasons - inexpensive, performed well, and he uses the triode to avoid a "blocking" condition as described by Crowhurst.

One change I'm pondering on Stewart's effort is to substitute the Hammond 125E for a 1620 -- wider bandwidth for $20 more. But I need to analyze the operating point he selected, as I suspect that the impedance of this transformer won't work.

There, it's all out in the open now.
 
I'm thinking of building something similar myself - the fact that I have about 50 NIB 6LU8s might have something to do with it... I am thinking of using Antek power toroids (at $12 each) as outputs. They should go down to 30 Hz at 15W or better in this circuit.

You want transformers with about 1/4 the "usual" impedance - i.e 2 - 2.5K CT for EL84s, 1 - 1.6K CT for 6L6s. The "universal" transformers are at their best in this circuit, as it's a low impedance and you use all of the secondary.
 
Tom Bavis said:
I'm thinking of building something similar myself - the fact that I have about 50 NIB 6LU8s might have something to do with it

Would you be interested in selling, oh 10 of them?

I am thinking of using Antek power toroids (at $12 each) as outputs. They should go down to 30 Hz at 15W or better in this circuit.

Thanks for the tip -- I'll give these a look.
 
Tom Bavis said:
You want transformers with about 1/4 the "usual" impedance - i.e 2 - 2.5K CT for EL84s, 1 - 1.6K CT for 6L6s. The "universal" transformers are at their best in this circuit, as it's a low impedance and you use all of the secondary.

I'm sure you're correct, but I don't understand why, yet. In his article, Crowhurst indicated, "...the primary of each transformer presents half the normal plate-to-plate load for these tubes." The plate-to-plate impedance for the 6LU8 is given as 12,000 ohms from the spec sheet. Doing some quick math, I get 6,000 ohms as the desired transformer impedance.

However, in his article Stewart has setup Hammond 125E's to present a load impedance of 3,000 ohms, just like you said above. What is it that I'm missing? Should the tubes in a PP configuration be considered to be in parallel? Or did Stewart chose a different operating point for the 6LU8?

Finally, what do you think of a Lundahl LL1664 in this application? It's about 3x the cost of the Hammond that Stewart wrote about.
 
weinstro said:


I'm sure you're correct
I'm not sure though... it would be 1/4 of the "normal" impedance per winding on the SAME transformer, but with two separate (uncoupled) transformers, I'm thinking it should be HALF. By the way, the plate resistance has little relation to load impedance with pentodes - load impedance depends mostly on supply voltage/current.


weinstro said:

Finally, what do you think of a Lundahl LL1664 in this application? It's about 3x the cost of the Hammond that Stewart wrote about.

The whole point of the Crowhurst design was good performance from cheap iron. For the cost of four Lundahls, you could get some decent custom parts made...
 
Tom Bavis said:
The whole point of the Crowhurst design was good performance from cheap iron. For the cost of four Lundahls, you could get some decent custom parts made...

Yeah, I know....

Assuming that the transformer impedance does indeed need to be 3,000 ohms, following Stewart's design, how close to that figure do I need to get? For example, if a transformer were 3,300 ohms, is that good enough, or do I lose 20% of the power rating (1.1^2 = 1.2)?

A Hammond 1620 might work. It costs ~$50.
 
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