[AMRadio] Difference and use of Eimac TH and TL tubes

[jeremy-ca]

What is the current feeling about the use of those old Eimac jugs? 100T,
250T, 304T, etc
If memory serves me OK the high mu TH was mostly used as a RF tube and the
low mu TL as modulators. BC-610 excepted of course. What are the trade offs
between the 2 versions as modulators besides drive requirements?

Carl
KM1H

[david knepper]

Carl, the problem is that many of these old bottles are gassy.

There are some techniques to rejuvinate them, like reduced plate voltage to
burn off the gas.

Dave, W3ST - W3CRA
Collins Radio Association
www.collinsra.com
Join the largest Collins group in the world
Nets on 7208 at 4:30 EDST every day and
Monday at 3805 at 8 PM EDST

----- Original Message -----
From: "jeremy-ca" <***@jeremy.mv.com>
To: <***@mailman.qth.net>
Sent: Tuesday, November 13, 2007 10:48 PM
Subject: [AMRadio] Difference and use of Eimac TH and TL tubes

Post by jeremy-ca
What is the current feeling about the use of those old Eimac jugs? 100T,
250T, 304T, etc
If memory serves me OK the high mu TH was mostly used as a RF tube and the
low mu TL as modulators. BC-610 excepted of course. What are the trade
offs between the 2 versions as modulators besides drive requirements?
Carl
KM1H
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[Ben Dover]

-----Original Message-----

Sent: Nov 14, 2007 7:19 AM
Subject: Re: [AMRadio] Difference and use of Eimac TH and TL tubes
Carl, the problem is that many of these old bottles are gassy.
There are some techniques to rejuvinate them, like reduced plate voltage to
burn off the gas.

Gassy? Indeed they ARE!!!

I'm a big fan of the 304TL. The vast majority of the tubes out there now are
military JAN jugs left over from WW2, and you can count on 'em being gassy.
There ain't no such thing as a perfect vacuum seal, and these bottles have
had over 60 years to leak.

I was rather amused by some of the postings about 304TLs on a "triode audio"
web group... imagine using this huge old war horse of a tube in class A to
produce 5 or 10 watts of audio!

One dude who was trying to really get serious with the 304TL warned everyone
that the MAXIMUM plate voltage used shouldn't exceed 2000 volts, otherwise
the tubes would flash over internally. Not surprising, if you haven't done a
bit of conditioning before putting 'em into service.

The whole point of conditioning is to get the tube HOT... hot enough to
activate the remains of the getter that sucked up the last of the gas when
the tube was made.

My approach in the past has been to hook up the tube to a filiment transformer,
bias supply, and a BIG plate supply with a variac on it. Thus set up, I apply
maybe 500 volts to the plate, and adjust bias for 200 or 300 MADC. Then, just let
the beast sit and cook for at least a day.

After that... start raising the plate voltage slightly to increase the plate
dissipation a bit, and let it cook some more... maybe another day, max.

Finally... on the third day, increase plate voltage until the tube is pulling
300 watts (the tube's rated plate dissipation), or if you're feeling lucky, go
perhaps 350 - 400 watts and let it sit some more... maybe 6 - 12 hours. BTW,
when you've reached this point, it's a good idea to deal with the normal, Eimac
defined tube cooling methods to keep from overheating the plate and grid seals.
Air thru the hole in the base wouldn't hurt either; the filament seals need some
cooling too.


After this, you're ready to test.

Change the bias voltage to completely cut off the tube, and keep it cut off. Now,
start increasing plate voltage SLOWLY, and in steps with a resting period when you
reach your intended step... maybe 2 or 3 minutes.

Sometimes, as the plate voltage comes up, you'll get a brief, minor flash inside the
tube. These are usually split second duration... and not a big deal. If you should
strike a solid, sustained arc in there, cut B+ and try cooking the tube some more.

If you can get to the rated Eb (3000 volts) without an arc, Congratulations! I don't
run 'em at that high a plate voltage, so I usually stop in the 2000 - 2500 VDC range.

If you can't get to your desired Eb, even after repeated cooking, Congratulations! You
now have a pretty glass paperweight/conversation piece for the shack! <<GRIN>>

Once you have the tube at the plate voltage you want, it's a good idea to adjust the
bias to bring up plate current in small steps, and let the tube cook for a while at each
step. I don't find it necessary to take it all the way to max plate dissipation; I just
go for maybe 1/4 or 1/3 of it, just to get the plate heated up good, and let it sit for
a half hour or so.


Bear in mind that when the newly zapped tube may or may NOT be good in AM service; remember
that with modulation the instantaneous plate voltage is gonna go up. The only way you can
know if the tube's OK with AM is to try using it.


Mr. T., W9LBB

[jeremy-ca]

Good info Ben, almost identical to what the old time tube companies wrote up
in the 30's when seal technology wasnt the best. I even use the process with
rx tubes such as UX 280's and similar oldies.

One thing that should be added is a HV surge resistor, around 25 Ohms 50W
and a real wirewound; no cement.
That will limit arc current to a safe value and minimize the chance of tube
damage.

Once cooked it is a good idea to hi pot them; I built my setup around a neon
sign xfmr and can variac up to 13KV. Hi potting also helps cull out bad
Chinese 3-500Z's and similar; Ive returned several back to RF Parts.

Thankfully TH/TL Eimacs were built into the 70's and maybe later and the
those havent deterioated much if at all. My NIB 810's are USA JAN Cetrons
with 1990 date codes.

Carl
KM1H



----- Original Message -----
From: "Ben Dover" <***@ix.netcom.com>
To: "Discussion of AM Radio in the Amateur Service"
<***@mailman.qth.net>; "Discussion of AM Radio in the Amateur Service"
<***@mailman.qth.net>
Sent: Wednesday, November 14, 2007 9:24 AM
Subject: Re: [AMRadio] Difference and use of Eimac TH and TL tubes

Post by Ben Dover
-----Original Message-----

Sent: Nov 14, 2007 7:19 AM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] Difference and use of Eimac TH and TL tubes
Carl, the problem is that many of these old bottles are gassy.
There are some techniques to rejuvinate them, like reduced plate voltage to
burn off the gas.

Gassy? Indeed they ARE!!!
I'm a big fan of the 304TL. The vast majority of the tubes out there now are
military JAN jugs left over from WW2, and you can count on 'em being gassy.
There ain't no such thing as a perfect vacuum seal, and these bottles have
had over 60 years to leak.
I was rather amused by some of the postings about 304TLs on a "triode audio"
web group... imagine using this huge old war horse of a tube in class A to
produce 5 or 10 watts of audio!
One dude who was trying to really get serious with the 304TL warned everyone
that the MAXIMUM plate voltage used shouldn't exceed 2000 volts, otherwise
the tubes would flash over internally. Not surprising, if you haven't done a
bit of conditioning before putting 'em into service.
The whole point of conditioning is to get the tube HOT... hot enough to
activate the remains of the getter that sucked up the last of the gas when
the tube was made.
My approach in the past has been to hook up the tube to a filiment transformer,
bias supply, and a BIG plate supply with a variac on it. Thus set up, I apply
maybe 500 volts to the plate, and adjust bias for 200 or 300 MADC. Then, just let
the beast sit and cook for at least a day.
After that... start raising the plate voltage slightly to increase the plate
dissipation a bit, and let it cook some more... maybe another day, max.
Finally... on the third day, increase plate voltage until the tube is pulling
300 watts (the tube's rated plate dissipation), or if you're feeling lucky, go
perhaps 350 - 400 watts and let it sit some more... maybe 6 - 12 hours. BTW,
when you've reached this point, it's a good idea to deal with the normal, Eimac
defined tube cooling methods to keep from overheating the plate and grid seals.
Air thru the hole in the base wouldn't hurt either; the filament seals need some
cooling too.
After this, you're ready to test.
Change the bias voltage to completely cut off the tube, and keep it cut off. Now,
start increasing plate voltage SLOWLY, and in steps with a resting period when you
reach your intended step... maybe 2 or 3 minutes.
Sometimes, as the plate voltage comes up, you'll get a brief, minor flash inside the
tube. These are usually split second duration... and not a big deal. If you should
strike a solid, sustained arc in there, cut B+ and try cooking the tube some more.
If you can get to the rated Eb (3000 volts) without an arc,
Congratulations! I don't
run 'em at that high a plate voltage, so I usually stop in the 2000 - 2500 VDC range.
If you can't get to your desired Eb, even after repeated cooking, Congratulations! You
now have a pretty glass paperweight/conversation piece for the shack!
<<GRIN>>
Once you have the tube at the plate voltage you want, it's a good idea to adjust the
bias to bring up plate current in small steps, and let the tube cook for a while at each
step. I don't find it necessary to take it all the way to max plate dissipation; I just
go for maybe 1/4 or 1/3 of it, just to get the plate heated up good, and let it sit for
a half hour or so.
Bear in mind that when the newly zapped tube may or may NOT be good in AM service; remember
that with modulation the instantaneous plate voltage is gonna go up. The only way you can
know if the tube's OK with AM is to try using it.
Mr. T., W9LBB
______________________________________________________________
Our Main Website: http://www.amfone.net
AMRadio mailing list
List Rules (must read!): http://w5ami.net/amradiofaq.html
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Help: http://mailman.qth.net/mmfaq.html
the word unsubscribe in the message body.

[D. Chester]

Post by jeremy-ca
What is the current feeling about the use of those old Eimac jugs? 100T,
250T, 304T, etc
If memory serves me OK the high mu TH was mostly used as a RF tube and
the
low mu TL as modulators. BC-610 excepted of course. What are the trade
offs between the 2 versions as modulators besides drive requirements?
Carl
KM1H

The "L" designation means "Low" as in amplification factor. The "H" means
"High".

Many tubes come in high-mu/low-mu pairs. Some examples:

811A, 812A

203-A, 211

810, 8000

100/152/250/450TH, 100/152/250/450TL

Taylor 822, 814 (different from RCA 814)

Generally, the high mu version makes the best class-B modulator. The lower
mu version is better for class-C rf service, and also better for class-A
audio service.

So usually, a pair of 211's would be modulated by a pair of 203-A's. A pair
of 8000's would be modulated by a pair of 810's. A pair of 812A's would be
modulated by a pair of 811A's. Of course the reverse could be used, since
all those tubes have plate modulated class-C rf ratings as well as class-B
audio ratings. But the most optimum performance can usually be had using
higher mu for class B and lower for class C.

For example, a pair of 810's will work as class-C plate modulated finals.
They are widely used as such in broadcast transmitters. But a pair of
8000's will operate just as well, but at about 75% of the rf driving power.
Even zero-bias tubes like the 805, 811-A, 3-500Z, etc. may be run in class-C
plate modulated service, but those are not the best tubes to use if
something else is available.

One exception, the very low mu 845, is recommended by RCA only for use in
class A or AB1 audio service. It is not recommended for use as an rf
amplifier. They make excellent class-B driver tubes (if you can find one
that the audiophools haven't already scooped up).

Don k4kyv