On 4/12/2011 9:37 PM, Carl wrote:
> Why?
>
> No room for 3-500Z's or 4-400's?
Needs the room for the HV transformer and screen supply?
The GU84B is a Tetrode while the Swan is designed around a pair of
Triodes. It Is relatively expensive tube even as NOS, sockets run around
$150 +/- 30 to 40 dollars. You are likely to have more invested in the
tube and socket than the entire amp cost new. I like the GU-84B, but
you are moving up the ladder quite a ways in tube capacity. You will
not need a tuned input and can go with a swamped grid, BUT you may need
to add some compensation to keep the SWR/reactance low across the entire
operating range.
IIRC the GU-84B has been out of production for about 17 years so you
would need at least one spare if not two that would or should be swapped
out a couple times a year. They are typically running around $500 USD
+/- at present but are trending upwards although Dr Alex has them for
under $400USD at present.
I would think the conversion from a straight forward Triode circuit to a
much more powerful Tetrode would be a major undertaking, particularly
when or if the extra protection circuitry (Tetrode control board) is
included. I like Tetrodes, but don't forget the need for a screen
supply that will handle both + and - current flow. Many Tetrodes run a
negative screen current when lightly loaded. At the legal limit output
the GU-84 is certainly lightly loaded and with a screen voltage of 375V
can run about 80 ma reverse current. *Claims* of 4 KW PEP out for the
tube are common. A transformer capable of providing enough input with
good regulation would be a real hernia builder.
OTOH the duty cycle for SSB (20%) and CW(40%) would *somewhat* negate
the need for quite that much iron. Running a Tetrode at less than half
it's rated output is not as simple as doing so with a Triode.
My guess for your plate voltage you could expect about 250 to 270 ma
idle current with proper bias and screen voltages, or roughly 800 watts
of no signal dissipation. (any time the amp is keyed unless you use
electronic bias switching -EBS) to keep the power low except when a
signal is applied. Many of the older amps would be hard pressed to run
that much power with out the EBS.
I have to assume (which on this group is never safe) that you are
planning on the legal limit output of 1500 watts SSB and CW. Digital or
AM might take a major change in approach, or reduction in power out.
How many KVA is the new transformer rated? Barring that, how much does
it weigh?
You are also going to need to add plenty of air by pressurizing the
input compartment.
As a personal opinion and all of us on here have different ones, this
seems like a a lot of work and possibly expense for the gain. OTOH I
don't know your intentions and reasons for this project. Like Carl, I'd
probably just plug in a pair of 3-500Zs and adjust the bias if necessary.
It's much like a friend asked me the other day why I didn't purchase a
crank up tower instead of building one. My reply was, because I just
wanted to build one. That and it was a really good excuse to get the
lathe and milling machine I want. <:-))
So good luck on the project for what ever reason you have chosen to go
for it.
73
Roger (K8RI)
> Carl
> KM1H
>
>
> ----- Original Message -----
> From: "Bill Smith"<ko4nrbs@yahoo.com>
> To:<amps@contesting.com>
> Sent: Tuesday, April 12, 2011 8:47 PM
> Subject: [Amps] GU-84B Conversion
>
>
>> I'm considering converting my old Swan Mark 1 (two 3-400Z tubes) to use
>> one GU-84B. Some time ago I replaced the panel bandswitch in the Swan
>> with a Radio Switch Model 86 six position switch with two wafers. At that
>> time I also replaced the tank coil with one that was made for the National
>> NCL-2000 amplifier. My new bandswitch required me to rearrange the RF
>> compartment considerably.
>>
>> The transformer was replaced with a new one from Heyborer in Michigan. At
>> that time I had them use the next larger diameter wire in the HV and
>> filament windings when they wound it. The caps, etc. were all replaced
>> with snap in type on a circuit board.
>>
>> Ok that's the history of the amp to date. It has a resting plate voltage
>> of 2900vdc and 2500vdc or so under load.
>>
>> Readings with two 3-400Z tubes:
>> Plate Current Readings on Operate HV Setting (SSB):
>> 270ma using a Tuning Pulser
>> Resting Plate Current is 190ma
>> 460ma SSB Peak (Talking in Mic)
>>
>> Plate Current readings using CW Mode(1500vdc)
>> Key down 660ma
>> Resting 80ma
>> SSB 320ma Peak
>>
>> My question is will the GU-84B perform satisfactorily with this
>> transformer? I know I'll need another filament transformer and one for
>> grid voltage.
>>
>>
>> 73,
>> Bill Smith KO4NR
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