Pye HF25 'Provost'
Here are some quick pictures and links to circuit diagrams as I've had several requests for information about these amplifiers.

A proper description and more design history will follow.
If you have any information about the Pye Hi-Fi products, please email me at:
jeff@audiomods.co.uk

The story of the HF25s in my family spans half a century. It started when my late father-in-law bought one new at the beginning of 1958, together with the Pye HF25 corner reflex loudspeaker (Tannoy 15" silver) and a Collaro Turntable, and they remained with him for the rest of his life, passing to me in 1998. It took me six years to find a second amplifier and just as long to locate a manual.

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Right and above: One of my HF25s as they look now. When rebuilding vintage amplifiers, there is always the choice between trying to retain originality and going for modern component replacement. I've chosen to use modern equivalent parts to bring them back into running condition and then to gradually upgrade with higher-quality capacitors etc. one by one.
The latest step was to replace the two .47uf 650v caps with Jensen paper-in-oils. these proved to be a huge improvement over the little "ashcroft" Polys used to get the amps running.

Right: An HF25 in original condition, apart from replacement wirewound resistors at the top right. The components were all of the best quality available at the time - the TCC capacitors were very expensive and normally only found in military equipment. You'll find the awful Hunts ones in most contemporary hifi amplifiers. The very neat and structured wiring is again military practice, being designed to ensure that each unit had exactly the same eddy current and stray capacitance characteristics.

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The rebuild
After 50 years, both amplifiers would give an output but both were very tired, even sick. The first rebuild stage was to get them running and measuring right, for which I'm grateful to Seth Pithham who had both the knowledge and the equipment to do the basic sorting. this involved replacing, sadly, every one of the original caps and a few resistors and putting the output taps of the second amplifier back to the correct wiring. so now they were up and running and sounding good.
There was still some hum and, in one, some very low-level oscillation, so I went through each amplifier changing almost all the resistors (drifted), the input valve octals (bad contacts) and the feedback circuit components, which finally sorted the oscillation. One of the best changes I made was to replace all of the AC heater wiring with shielded cables, a big reduction in background hum.

After that I had three months of seriously good music, until one amplifier stopped working with worrying symptoms. I checked everything I could get at, swopped components and then gave up and called on Len Gregory, who I knew for his superb "Music Maker" cartridge and also by reputation as a very experienced and enthusiastic valve expert. Len found the problem (a bad ancient solder connection underneath the board, not the output transformer as we'd both feared) then went on to re-work my rather flaky supply caps, tidy up my soldering and introduce me to the current Quad branded KT66 instead of the very disappointing Golden Dragons.

The result is a pair of amplifiers with a stunning, modern performace that drive difficult speakers (Spendor SP1s, Quad '63s, even, yes. Magneplaners), are very clean and open with awsome bass, sparkling treble and yet are *very* quiet.

History

To see these amplifiers in their context, we have to remember that in the 1950s Pye were a very large company at a time when electronics in Britain had received an enormous boost from military advances in the Second World war.
The Cambridge area and Pye in particular were at the centre of that military electronics industry: centimetre-band radar, vhf and uhf communications, the vhf radio range and glideslope (which became the modern ILS system), IFF, (later to become the transponder), VDF and even PCM data communications. All done with valves.
A company that could make valve VHF direction finding equipment accurate and stable enough to locate a U-boat in the Atlantic and reliable enough to survive 16 continuous hours operation in a piston engined aircraft could easily design and build an AF amplifier with a flat response from 2Hz to 160kHz once they had decided it was necessary.

Downloads

Circuit diagram (schematic) Parts list ......Chassis layout

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The range of audio amplifiers that bore the "Pye, Cambridge" brand were each intened to be the very best products the company could manufacture in each class. The people who designed and built them had been involved with the military contracts and had the advantages of knowledge and equipment that were beyond the resources of the contemporary companies that we think of as the founders of "hi-fi". For example, Harold Leak had to send his amplifiers to the National Physical Laboratory to get them measured and he was one of the very few who bothered. At the time the NPL were testing Harold's TL12.1, they were using Pye test equipment :-)

The feedback design

The amplifiers have 26 db of Negative feedback. This sounds like a lot by modern standards but it needs to be understood as part of the combined negative/positive feedback loops. The amplifiers are set up when connected to the speakers and the positive feedback is adjusted until the amplifier is stabilised as a complete, circiuit including the loadspeaker load. To get a feedback figure we need to subtract the positive from the negative, so the actual figure is maybe halved.

The HF25 design concept
The design concept behind the HF25 outlined in the owner's manual sounds very modern today and appears to be directly linked to the thinking of Norman Crowhurst: that all distortion is not equal! They also understood that a loadspeaker coil inside a big magnet makes an effective generator that will try to drive a badly-damped amplifier right back.
The HF25 is a push-pull KT66 amplifier of Ultralinear design using an ECC35 driver and ECC33 phase splitter supplied by a GZ32 rectifier and IH filter choke. Sounds very standard for its time. Where it becomes very special is in the design philosophy of the combined negative/positive feedback circuit:

To quote the company:

On feedback:
"It is a well known fact that the frequency response of a feedback amplifier must be maintained for at least one octave above and below the limits of the useful (audio) band for every 10 db of negative feedback. In fact, to avoid any tendancy to self-oscillation and to ensure a good transient response, one more octave must be added to each end of the frequency range as a final precaution.

"The HF25 has 26 db negative feedback, which requires the frequency response to be maintained to a total of 3 1/2 octaves above 15000 cps (the design range) and below 20cps. The amplifier must, therefore, have a reasonably level output from 2cps to 160,000cps...

"...Without such an extensive frequency range, an amplifier having 26db negative feedback and a response from 5cps to 20000cps would be capable of producing full feedback over the band 56-1800cps..."

On transient response:
...a well designed amplifier will follow the transient waveform of the cymbal almost exactly, and all oscillatory excursions will be highly damped, even though the frequencies may extend for two octaves above the limit of audibility."

On Loudspeakers and damping factor:
"...(Artificially colouring the end of the musical scale) is also a form of transient distortion which arises in the loudspeaker rather than the amplifier....

"...The requirement (of an amplifier) is that the output terminals as seen by the loudspeaker should appear as a "short circuit"....(in which case) ... as soon as the audio drive ceases, the voice coil of the loudspeaker comes to rest rapidly instead of tending to oscillate at its natural frequency....

The damping factor of most good quality amplifiers is between 30 and 50. In the HF25 this value can be infinity. This remarkable effect is obtained by a combination of positive and negative feedback, the benefits of which are recognisable over the entire audible range of frequencies"

Valve choice

KT66

The HF25s run the output valves at the maximum design voltage for KT66, so they need a realy good quality tube. I've found that most of the current production tubes don't respond well to the Pye's B+ so a quality old stock tube is really needed.

These amplifiers were originally supplied with grey-glass Osram KT66 and these would still be a very good choice if you could find (or afford) them. The GEC clear-glass would be the obviouce choice but, again, these are now becoming very expensive.

The manual give 6L6GC as a direct replacement (careful - other 6L6 series won't run 450V) so I did a little reseach and found the 7581A. This is a military 6L6GC, rated at a full 500V with 35W output and is a drop-in replacement. It also has the advantage of lower heater current than KT66.

They sound really good with exceptional bass and really clean treble. I'm using closely-matched Philips "JAN" stock from the 1980's but "pink base" GEC ones from the 1960's are also available.

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PS: I've been asked about those input valve shields. Given that small-signal valves are microphonic, they're a thick-walled aluminium tube with the outside finned like a motorcycle cylinder. The inside has a classic venturi shape, Theory is that as air is sucked in below and as it heats up and rises the pressure is reduced a little so temperature and velocity drops so it can absorb and dissipate more heat than if the valve were in free air. Should work, but I can't really measure the effect accurately.

contact: jeff@audiomods.co.uk

5 Tormore Mews, Rectory Road, Deal, Kent CT14 9SX United Kingdom
Tel: +44(0)1304 379698