[Amps] GU-84B Conversion

Tue Apr 12 20:51:55 PDT 2011

On 4/12/2011 11:09 PM, Jeff Blaine wrote:
> GI7B?

GI46 would be another choice with a higher plate dissipation rating.  
I've wanted to build up a pair of those, but I have far too many 
projects already under way and too many things that need fixing.

Both could have the anode cooler replaced with one machined for 
transverse cooling.
Although I think they are passable and "I think" the GI-46 is better 
than the GI-7B I don't know what the specific IM is for either tube.

73

Roger (K8RI)

> 73, Jeff ACØC
> www.ac0c.com
>
> -----Original Message-----
> From: Roger (sub1)
> Sent: Tuesday, April 12, 2011 10:02 PM
> To: amps at contesting.com
> Subject: Re: [Amps] GU-84B Conversion
>
> 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 at yahoo.com>
>> To:<amps at 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
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
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