Its not that 250TH's etc dont work but they need a lot more drive due to the
low mu, a hefty regulated bias supply voltage wise, and arent much beyond
sweep tube for linearity.
Ive been sitting on a NIB pair of 5868 aka TB4/1250 for the same reason.
I never tried any of the low mu triodes in AB1 which might be an all around
answer but reduce the output. I run the 304TL modulators in AB1 which
simplifies the driving chain a lot but until I added bias shift (called EBS
these days but used since the 50's) there was a lot of pure heat waste.
Im sure a lot of the vintage tubes can be used at least to beat the high
cost of modern stuff. GG 4-125A, 4-250A, 4-400A, 4-1000A are still viable
choices and the 4-400 is an easy swap into most 3-400Z/3-500Z amps, and the
4-1000 to replace the 3-1000Z.
In the mid 60's I built a QST pair of 100TH's for 6M SSB/CW using tuned
lines. Decent output but also got Worked All TV awards; no idea about
linearity. Thats when I converted a NCL-2000 prototype to 6M monoband and
still use it. About 3X the power in a fraction of the real estate.
Ive never owned a Zahl tube but have read a lot about their capability in
WW2 radars.
A NOS pair of QB5/1750's are in another modulator and a NOS pair of GU-81M's
in another which are like 813's on steroids. I enjoy playing with oddball
tubes, rarely seen in ham gear at least in NA. When #2 son was still in the
USAF and stationed in Germany, Georgia, Moscow, Kiev, etc he got great deals
on all sorts of parts and shipped them home via APO or DPO rates which is
the same as NH to NYC. Now retired and stuck with the German postal system
(talk about sticker shock!) unless he can hand them off to somone with
embassy creds when he is near one.
Carl
KM1H
----- Original Message -----
I once built a pair of 813's as a linear grounded grid amplifier driven
off my
pair of 6146B. Worked great, I then tried a pair of 250th in GG and they
worked fine
as well.
I always wanted to play with some VT-127's, they are neat looking tubes.
EIMAC actually
made a ion vacuum gauge tube using that tube.
I have a VT-158 but wondered if there was some way to suppress its VHF
oscillation and
use it as a HF amplifier tube but with the VHF hairpin's build in I doubt
it.
73
Bill wa4lav
________________________________________
From: Carl [km1h@jeremy.qozzy.com]
Sent: Wednesday, December 23, 2015 5:23 PM
To: Fuqua, Bill L; Tom Osborne; amps@contesting.com
Subject: Re: [Amps] 813's
I save my glow bottles for Class C AM use and as modulators. A couple of
304TL's can really light up the room as can HK-354C's, and 250TH's. Some
long years ago I ran a pair of VT-127A's (aka 100TS after WW2) on 40 and 20
CW only at close to 1200W out, the white plates never seemed to weaken them
but I did use plenty of air.
I also have a pair of 6C21's (pulse 450TL's running way overvoltage on the
filaments as short life wasnt important) and havent run them anywhere near
their best glow as regular 450TL's.....yet.
Too bad none of those oldies are good linear amp tubes.
Carl
KM1H
----- Original Message -----
From: "Fuqua, Bill L" <wlfuqu00@uky.edu>
To: "Carl" <km1h@jeremy.qozzy.com>; "Tom Osborne" <w7why@frontier.com>;
<amps@contesting.com>
Sent: Tuesday, December 22, 2015 11:28 PM
Subject: RE: [Amps] 813's
One problem was that most articles stated that a pair of 813's would work
operate
at 2kW PEP or about 1200 or 1300 Watts PEP output. I can see why they seemed
"unclean" since
many hams built these amplifiers from magazine articles and pushed the pair
of 813's to the 1 kW average
plate input power limit on SSB. But four good 813's should do today's legal
limit just fine at 2.5 or 3 kV.
That would be like running four 4-125A's or even better 6155 and you can
watch the plates glow too.
Which brings up a question I have, anyone ever try 4E27A's in grounded grid?
73
Bill wa4lav
________________________________________
From: Carl [km1h@jeremy.qozzy.com]
Sent: Tuesday, December 22, 2015 6:05 PM
To: Fuqua, Bill L; Tom Osborne; amps@contesting.com
Subject: Re: [Amps] 813's
While the 813 was rated at 125 PD it was a very conservative CCS and over
years of use Id rate them in the 200-225W range for ICAS service. Compare
the size with the Cetron 572B anode and you can see why; Svetlana and
Chinese are only 125W but the new ones may be higher. This is only for the
graphite anode versions; the later sheet metal version used would melt down
as fast as an 811A in a AL-811 series amp.
With a good tube final exciter such as the Kenwood hybrids the IMD is in the
low -30's real dB; that is about -37 ARRL dB's.
With an SS exciter and an untuned input there is no flywheel effect loading
on the exciter so IMD is typically 3-5dB worse.
The best IMD with a tuned input is at around 2700V but it varies little from
2500 to 3000V when output remains the same; 4 full emission tubes will
easily do 1300-1500W with a little help from a fan.
Carl
KM1H
----- Original Message -----
From: "Fuqua, Bill L" <wlfuqu00@uky.edu>
To: "Tom Osborne" <w7why@frontier.com>; <amps@contesting.com>
Sent: Tuesday, December 22, 2015 1:15 PM
Subject: Re: [Amps] 813's
Graphite is always messy, I have gotten it on hands clothes and it is
hard to wash out.
I think perhaps they should not have earned such a reputation. I believe
the problem stems from the
fact that most 813 amplifiers in the early days had untuned inputs and
that they were pushed
harder than they should have been. Mind you these tubes in triode
connection are about like 572B's.
They have even less plate dissipation than 572B's but I suspect the idea
of pushing them to
2kW PEP plate input was just too much. Also, most used 813's I have gotten
had very little
emission left. That is an indication that they were used beyond the point
where if pushed to
get full gallon output they were probably producing a very dirty signal. I
think if you use
a good pair, with tuned input and don't try to push them too hard they
will be just fine. Treat them
as if they were a pair of 575B's.
73
Bill wa4lav
________________________________________
From: Amps [amps-bounces@contesting.com] on behalf of Tom Osborne
[w7why@frontier.com]
Sent: Monday, December 21, 2015 1:57 PM
To: amps@contesting.com
Subject: [Amps] 813's
Why are 813 tubes considered 'dirty'?
I have an old amp in the attic I haven't used for a long time that has a
pair of 813's in it I have read somewhere that they are a very 'dirty'
tube. I'd assume that means lots of spurs or something.
Just wonder why that would be. 73
Tom W7WHY
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