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Old 16th November 2015, 08:15 PM   #1
nzoomed is offline nzoomed  New Zealand
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Default Eimac 35T any good for an amplifier?

I believe that the Enimac 35T is a very good sounding audio output tube, although its main use is intended as a transmitter triode.

Has anyone worked with these tubes before?

I can only find two examples of amplifiers built with them, one seemed to have a ridiculous amount of transformers and/or chokes on the chassis.


I have come across a few NOS boxes at a swapmeet so would like to put them to good use if they sound good. They need a high plate voltage i believe.
They are rated at up to 2KV at the plate and it seems people are running them around 800-900v typically.

I would be interested to find a schematic for these to use as a guide.

TIA.
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Old 16th November 2015, 08:26 PM   #2
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This will tell you all you need to know about biasing and the classes it can run in.
http://www.tubecollectors.org/eimac/...tg%2851%29.pdf
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Old 16th November 2015, 10:52 PM   #3
nzoomed is offline nzoomed  New Zealand
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Quote:
Originally Posted by JonSnell Electronic View Post
This will tell you all you need to know about biasing and the classes it can run in.
http://www.tubecollectors.org/eimac/...tg%2851%29.pdf
Yes i had been looking at that datasheet.

Im not too sure what is recommended for driving the tubes, but i believe they need alot of power to drive them, ive seen ECL82 tubes used before, but i was wondering if an EL84 would be better to drive them with rather than an ECL82 which i believe they dont have a very long life and seem to bottom out with use.

I can expect about 10W with these running in single ended mode i believe.
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Old 17th November 2015, 12:12 AM   #4
kaputt is offline kaputt  Germany
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They are rated for 2kV but you have to choose a lower plate voltage and higher current within specs not to go beyond max plate dissipation and dissipate enough to have the anodes glow. You will have to run lots of current through them to get Rp down, otherwise you will have a very hard time to get any output transformer with a half way ok frequency response. And with the say 300mA of current this means a MASSIVE core.
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Old 17th November 2015, 12:23 AM   #5
nzoomed is offline nzoomed  New Zealand
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Quote:
Originally Posted by kaputt View Post
They are rated for 2kV but you have to choose a lower plate voltage and higher current within specs not to go beyond max plate dissipation and dissipate enough to have the anodes glow. You will have to run lots of current through them to get Rp down, otherwise you will have a very hard time to get any output transformer with a half way ok frequency response. And with the say 300mA of current this means a MASSIVE core.
I was wondering about that.
If i can run them at a lower voltage that would be ideal i guess because that means i will be able to run them at higher current alot easier without using such a large transformer.

I know they say these tubes need to have a glowing plate or else it means they are not running optimally.

Perhaps this is why few are using them for audio.

It might be better for me to sell them and look at using another triode type.
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Old 17th November 2015, 12:34 AM   #6
kaputt is offline kaputt  Germany
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Eimacs (and other specialised transmitters) have no getter flashing to avoid having a conductive element without defined potential inside the tube. The tantalum anodes, when glowing, have a gettering action. These tubes sound excellent but usually need pretty high voltages in the 1-3kV range which isn't everybody's cup of tea.
Lower voltage and higher current is the same as higher voltage and lower current in regards to mains transformer size...
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Old 17th November 2015, 12:58 AM   #7
nzoomed is offline nzoomed  New Zealand
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Quote:
Originally Posted by kaputt View Post
Eimacs (and other specialised transmitters) have no getter flashing to avoid having a conductive element without defined potential inside the tube. The tantalum anodes, when glowing, have a gettering action. These tubes sound excellent but usually need pretty high voltages in the 1-3kV range which isn't everybody's cup of tea.
Lower voltage and higher current is the same as higher voltage and lower current in regards to mains transformer size...
Yes your right about transformer core size.

Correct me if im wrong, but if these tubes need 300mA to run at normal operating temperature on the plate, then running at 1600v at 300mA will almost need a transformer double that if running at 800v at 300mA

If i was running at 150mA at 1600v, then yes, your right, the transformer would be the same size as an 800v transformer rated at 300mA.

Interesting about the getter, i was wondering about that myself.
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Old 17th November 2015, 02:32 AM   #8
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The O/P transformer issue could be attacked by going to PP "finals". You can get the sonic "sweetness" back by including a resistively loaded 12AT7 section in the small signal circuitry.
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Old 17th November 2015, 02:35 AM   #9
nzoomed is offline nzoomed  New Zealand
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I just got a reply from hearinspace which is not showing here for whatever reason, that states this:

"I've been experimenting with the 35T. It has a fairly high internal impedance but also fairly high gain that can be traded off with feedback to lower the output impedance.
It's not hard to run hot enough to get the gettering action from the plate. Mine is presently running 500Vp-k / 100mA.
It sounds pretty good at low output but starts to get clipping distortion at mid power out. I'm poking around to understand what's happening but it will likely take a while. Long story short. It's a good tube but not efficient and needs a certain amount of power to make it's getter work."

Looks like it is a very tricky tube to work with, but worth it in the long run i guess if you can get it working.
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Old 17th November 2015, 07:37 AM   #10
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Beautiful build - Reinout's Silvercore 35TG

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