[Amps] 8171

[R.Measures]

On Jan 27, 2005, at 10:52 AM, David G4FTC wrote:

> Barrie,
>
> I'm the builder of the 6 metre 8171 amp referenced in Steve G8GSQ's 
> posting.
>
> Although the output capacitance of 23pF may be considered high for a 
> tube
> operating at VHF, when I built my 8171 amp I didn't find this to be 
> the main
> problem - in fact I used a very conventional PI output but with 
> flappers for
> both tuning and loading simply as an experiment to see if it was 
> possible to
> build a "tubes with handles" amp without needing to resort to vacuum
> variables. The flappers really do work fine.
>
> I built my amplifier in a grounded cathode configuration and I didn't 
> fully
> appreciate the losses that I would find with a tube having a 120pF 
> input
> capacitance.

Hello, David -- What is the grid termination's resistance?
Is this amplifier neutralized?

> Consequently the drive requirements were over and above what I
> had expected that I would need. IMHO it is the input capacitance which 
> will
> cause you many more problems than the output capacitance.
>
> The SK300A tube base isn't really ideal at VHF, and the tube 
> manufacturers
> recommend the SK360 for frequencies above 30MHz. I understand the SK360
> allows better RF grounding for the filament with a lower inherent 
> inductance
> at VHF. However, these are expensive bases compared to the SK300A.
>
> An alternative to the SK300A may be the SK300. The SK300A was 
> developed from
> the SK300 to provide improved cooling to the tube but this resulted in
> cutting out the "skirt" from around the base. This increases the 
> inductance
> and makes providing an effective RF ground somewhat more difficult,

I have used both and I can not see why the SK300 should have less 
inductance than a SK-300A.

> and
> results in less that optimum gain.  However even with the SK300A base 
> which
> I used, I'm still getting 16dB of gain at 50MHz which in reality isn't 
> too
> bad.

How much anode-V and screen-V?
>
> Currently I'm researching building a new amp for 2 metres, and at this
> frequency I feel that the 120pF input capacity for a grid driven
> configuration would be a real show stopper.

In days of yore, the 8170 and 8171 were used in high-band VHF TV 
transmitters up to 216MHz.

> However, the input capacitance
> of a 8170 in grounded grid drops to about 56pF - not much more that 
> the 42pF
> of the 8877, and I really feel that this is the way forward.  However 
> to get
> really low inductance grounds for the screen and grid will require a 
> certain
> amount of metalwork where I will remove the collets from a SK300A, 
> discard
> the frame, and mount the collects on individual plates. These plates 
> will be
> chassis mounted with PTFE insulation to provide RF decoupling and also 
> the
> DC feeds for the screen/grid biases.

A less inductive way to go is:  direct-ground the screen with 8 Cu 
straps, ground the screen PS-pos., connect the screen PS-neg and the 
anode-PS neg. to the fil CT (cathode).  With this configuration, the 
anode-PS/cathode neg./bias-PS+ floats at the screen-potential above 
ground.
••  photo showing the 8 screen-grounding straps on an SK-300: 
http://www.somis.org/pb_grid.gif

> ...
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org