LMEricsson 6761

[roren]

Hi,
I have a box with 10 of these power pentodes made by LM Ericsson. Here's a link to a datasheet.
http://frank.pocnet.net/sheets/185/6/6761.pdf

I'm curious to here what you guys think of this tube. I'm not an expert on this but it seems to me that it is a rugged tube. Long
life. Also it is made for relatively low plate voltage. High gm and cathode current.
Here's an Ericsson document where they tell a bit more about the development of the '6761'
Ericsson 6761

Tanks for watching. All input are welcome.

Rolf

[kheper]

At 6-10W audio output in PP, they should produce a nice, compact amp for a small room. In PPP (8 tubes total for stereo) - at 16-20W, the amp would still be a reasonable size. They recommend a rather low input impedance, 2.2-3.5kohms, however.

At $18 a pop, they are not the cheapest tube around:
http://www.sphere.bc.ca/test/tubes3.html

[Geek]

At $18 a pop, they are not the cheapest tube around:
http://www.sphere.bc.ca/test/tubes3.html

Doooooooood.... using Sphere as a price ref is like using Sotheby's ... there are cheaper!

[roren]

kheper,
Well, 6-10W could be enough for me. I understand that it can handle relatively low plate load impedance. But how do you
figure out the input impedance? 2.2-3-5 Ohm, is that for the PPP variant?

Rolf

[kheper]

At $18 a pop, they are not the cheapest tube around:
http://www.sphere.bc.ca/test/tubes3.html

Doooooooood.... using Sphere as a price ref is like using Sotheby's ... there are cheaper!

Yep, but $12 a pop is not dirt cheap, either. It's in line with most new production el84s, however.
https://www.tubedepot.com/products/6761

[kheper]

kheper,
Well, 6-10W could be enough for me. I understand that it can handle relatively low plate load impedance. But how do you
figure out the input impedance? 2.2-3-5 Ohm, is that for the PPP variant?

Rolf

The specs from the article you posted recommended a 2.2-3.5kohm primary input impedance for the output transformer in PP mode. You don't need to figure it out. You just purchase an output transformer in that range. If you cannot find a 2.2-3.5k output transformer, a 5k ohm will work.

[roren]

Okey I see, I got confused when you said "input" it made me think of the grid. As I said, I'm no expert
on this. So I easily get confused . I don't think that there will be that hard to find a suitable OPT. By the
way, isn't it in a way a good thing this low plate load impedance? Makes the OP need less impedance ratio?

[kheper]

By the way, isn't it in a way a good thing this low plate load impedance? Makes the OP need less impedance ratio?

If you mean the internal plate resistance, Rp, the 6761 isn't really that low at 23k. 807s have around 25k, and many recommend a very high, 10k, input impedance for the output transformer in PP. The recommended input impedance depends on many factors, especially the noise performance. In some cases, it probably occurred by designers listening to various transformers, then deciding: "Well, this one sounds good.".

[kheper]

By the way, isn't it in a way a good thing this low plate load impedance? Makes the OP need less impedance ratio?

If you mean the internal plate resistance, Rp, the 6761's isn't really that low at 23k. 807s have around 25k, and many recommend a very high, 10k, input impedance for the output transformer in PP. The recommended input impedance depends on many factors, especially the noise performance. In some cases, it probably occurred by designers listening to various transformers, then deciding: "Well, this one sounds good.".

[roren]

If you mean the internal plate resistance, Rp

No, I meant the input impedance on the ouput transformer. I have a pair of PP output transformers (Hammond 125E)
that are multi tapped. Making an amp with those. I can try different impedance's. Maybe I learn something. I know
that the 125E transformer is not Hi-Fi but it could be fun to start with.

[kheper]

If you mean the internal plate resistance, Rp

No, I meant the input impedance on the ouput transformer. I have a pair of PP output transformers (Hammond 125E)
that are multi tapped. Making an amp with those. I can try different impedance's. Maybe I learn something. I know
that the 125E transformer is not Hi-Fi but it could be fun to start with.

The Hammond 125E will work. If you want to upgrade to a reasonably priced hi-fi transformer, the Edcor GXPP10-8-2.5K is another option.

https://www.edcorusa.com/gxpp10-8-2_5k

[roren]

I have already looked at that Edcor
Thank you kheper.