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PYE PF91 / HF25

In conversation with Brecklandboy, it appears that PYE updated the HF25 during production without re-labelling the model. As I have never seen this mentioned anywhere else, perhaps this is a good place to document the changes? The modifications concern removal of the grid stoppers for the KT66’s and the HF filters replaced by a single capacitor between the two grids with the values of numbered components remaining as originally published.
HF25 original published circuit:

and the modded version:

Can anyone shed any light on what these changes accomplish?
 
simonpyeman said:
I’m currently restoring a pair of PYE PF91’s and wondered if there is anyone out there who has specific knowledge of them? They appear to be an evolution of the Leak TL12.1 but with some interesting differences. It seems to be very difficult to find much about them on the web, however. The HF25 is a near identical circuit but with ultra linear KT66 pp instead of triode connection. I have the circuit diagrams for both and the complete manual for the HF25 but only an incomplete one for the PF91. Any information or interest gratefully received.
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I have a pair of PF91's which I had professionally restored many years ago. I also have a photocopied manual but it's heat bound and would be difficult to photocopy without breaking the bindings. Having said that if I can be of any help please let me know.
 
R16 & R17 are 470k and the capacitor is 150pF. My limited electronic knowledge isn’t up to properly understanding how that cap functions: it looks like that above a certain frequency the cap starts to conduct, thus cancelling the out-of-phase HF content. Since grid stoppers are also for prevention of HF oscillation, they become redundant - hence removal. How does one work out what frequency the revised arrangement starts to become effective? Apart from the obvious gain of there being fewer passive components in the signal path, is anything else of benefit achieved?
I suppose I’m asking in a roundabout kind of way whether it would be safe and of possible benefit to try this out in the PF91s - the difference being the KT66s used in triode?
 
connected365 said:
I have a pair of PF91's which I had professionally restored many years ago. I also have a photocopied manual but it's heat bound and would be difficult to photocopy without breaking the bindings. Having said that if I can be of any help please let me know.
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Thank you for the kind offer. I would be interested to know what signal path capacitors are fitted to your amps, if it’s possible to see without undue hassle?
 
Your understanding of the action of the capacitor is correct. The difficulty of calculating the exact roll-off is knowing the output impedance of the phase splitter which is also in parallel with R9 R16 or R12 R17. If we take an educated guess at 15k then a-a is 30k and -3dB freq will be approx 35kHz.

Pye obviously had trouble with stability on this amp, note the snubbers on the original version in addition to the grid stoppers.
 
You’re probably right: for 1954, a “substantially flat” frequency response from 2 - 160kHz must have been a challenge!
 
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