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Thread: Pye Mozart HF10 stereo pair

  1. #1
    Join Date: Mar 2015

    Location: Finland

    Posts: 185
    I'm Kai.

    Default Pye Mozart HF10 stereo pair

    I already made a thread about this when I first got it back in March or so, though at the time of purchase I mistook it for HFS20. It came in a small cabinet also housing a Garrard HF4 record player. What we have here is a pair of Pye Mozart HF10. The twist is the left one has stereo pots and switches with wires going to the 2nd unit, which obviously doesn't have any controls of it's own then. It seems this was a factory job, since the amp with the faceplate has "Stereophonic" engraved on it in place of the "High Fidelity" regular of regular HF10 faceplace. Also the 2nd unit has the chassis made smaller so there's no empty space where the pots would normally be. I've yet to build a proper case for it, they've been just strapped to this piece of wooden board. On the second photo the other unit is missing one of it's preamp valves since I was taking some measurements from the socket. A couple (bad) photos:






    At the top right on each unit you see an empty valve socket - that's for an EF88 for the phonostage, this is not normal and I'm not sure if it was a Pye Factory job, or made by whoever sold this console or a later modification by someone. The date code on the valves reads D0J which I assume would mean they were made by Valvo in Hamburg in October 1960. All the original capacitors and stock Mullard valves were dated 1958, so I'm assuming this was made then and the EF88 is a later addition by someone or the company who made the cabin and bundled it all together.

    Never seen anything like this trying to dig up information on these old Pye Mozart series amps, I suppose if Pye made more like this it didn't made a whole lot, seems like it could maybe be a 'transitional' stereo setup before HFS20 was introduced. Anyone seen anything like this before? I guess I should try some forum specializing on old British valve stuff

    I've recapped them, changed all paper caps and electrolytics, replacing the smaller ones with film caps and Nichicons for the big cans. Hum was reduced and the sound cleared up, with the 60 year old stock caps the treble was a bit rolled off. Surprisingly all the changed caps still measured within spec at the small test voltage at least, but don't know how they'd behave with the high voltages they see in actual use.

    Sound is awesome to my ears, very full without getting overly warm and fuzzy and both frequency extremes are surprisingly good as well. Would like to try out some a bit more efficient speakers, I've been driving a pair of Kef 104aB and JBL 120Ti, both of which are rather ineffecient. Sounds great but don't get very loud. The Kefs are not that great at high volumes anyway, though I never realized that before I got the JBLs. Stereo imaging
    with the PYE is not the greatest, I mean not that it's horribad or anything, it's very stable but it's more a "broad brush" version as opposed to pinpoint etching. Still it has become my go to amp for most listening. Competition has been some better 80s japanese amps.

  2. #2
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 3,755
    I'm Alan.

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    Wow. Thanks for posting the details and photos.

  3. #3
    Join Date: Oct 2012

    Location: NE England

    Posts: 4,643
    I'm Jez.

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    A strange find there.... I've renovated a few of the stereo version (pre amp and stereo power amp) but never seen that set up you have. I was pleasantly surprised by the power amp. I was expecting it to be awful but indeed it sounds pretty good and surprisingly modern. A very unusual and intriguing design, it uses an EL34 pentode single ended with cathode windings as well as anode and loads of feedback.

    If you are not already doing so I would use it as a power amp only to get the best results from it.

  4. #4
    Join Date: Jan 2009

    Location: Essex

    Posts: 16,282
    I'm openingabottleofwine.

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    Quote Originally Posted by Arkless Electronics View Post
    A strange find there.... I've renovated a few of the stereo version (pre amp and stereo power amp) but never seen that set up you have. I was pleasantly surprised by the power amp. I was expecting it to be awful but indeed it sounds pretty good and surprisingly modern. A very unusual and intriguing design, it uses an EL34 pentode single ended with cathode windings as well as anode and loads of feedback.

    If you are not already doing so I would use it as a power amp only to get the best results from it.
    http://www.valve-radio.co.uk/literat...ervice-manual/

    Have you listened to this month's choice in the Album Club?

    Barry

  5. #5
    Join Date: Mar 2015

    Location: Finland

    Posts: 185
    I'm Kai.

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    I've been currently using the Pye from the tape-outs of an 80s Yamaha A-700 (with an upgraded phonostage), so the signal goes through the preamp. Power-amp inputs are in the plans, kind of waiting for putting together the enclosure which will be a good opportunity to do other work as well. With some tweaking of the bass & treble knobs the preamp gives a reasonably even response, but I can certainly imagine it being a weak link. The tone control section has the paper caps replaced, but the ceramics are still originals. I'll be changing those as well at some point.

    I'm wondering if I should just do a complete rebuild, I measured some of the resistors and they were nothing close to modern tolerances, some of the values were pretty far from spec, but on the other hand they were same for both units so I guess there was at least some effort to match the channels. Modern resistors would surely sound different though. Also the rectifiers are still the original selenium ones, I've read those to be pretty unreliable. I've got modern bridges waiting but I like listening to my kit more than working on it, so haven't got around to it.

  6. #6
    Join Date: Oct 2012

    Location: NE England

    Posts: 4,643
    I'm Jez.

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    Quote Originally Posted by helma View Post
    I've been currently using the Pye from the tape-outs of an 80s Yamaha A-700 (with an upgraded phonostage), so the signal goes through the preamp. Power-amp inputs are in the plans, kind of waiting for putting together the enclosure which will be a good opportunity to do other work as well. With some tweaking of the bass & treble knobs the preamp gives a reasonably even response, but I can certainly imagine it being a weak link. The tone control section has the paper caps replaced, but the ceramics are still originals. I'll be changing those as well at some point.

    I'm wondering if I should just do a complete rebuild, I measured some of the resistors and they were nothing close to modern tolerances, some of the values were pretty far from spec, but on the other hand they were same for both units so I guess there was at least some effort to match the channels. Modern resistors would surely sound different though. Also the rectifiers are still the original selenium ones, I've read those to be pretty unreliable. I've got modern bridges waiting but I like listening to my kit more than working on it, so haven't got around to it.
    The resistors will have drifted by roughly the same amount rather than been chosen to match!! Almost always high. I don't really buy into resistors sounding different to any great extent personally. I replaced the selenium rectifiers in ones I've renovated but be careful to add extra series resistance as silicon rectifiers will result in considerably higher output voltage.

  7. #7
    Join Date: Mar 2015

    Location: Finland

    Posts: 185
    I'm Kai.

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    Quote Originally Posted by Arkless Electronics View Post
    The resistors will have drifted by roughly the same amount rather than been chosen to match!! Almost always high. I don't really buy into resistors sounding different to any great extent personally. I replaced the selenium rectifiers in ones I've renovated but be careful to add extra series resistance as silicon rectifiers will result in considerably higher output voltage.
    I finally got around to replacing the selenium rectifiers. The reference voltages were down something like 25V from spec, getting 225 where there should be 250V etc. By contrast using modern silicone rectifiers the voltages were up by roughly similar amount. Had to add 330R series resistance after the rectifiers, now it's about spec, generally around 5V low so could maybe finetune that resistance a bit, but on the other hand my mains was a bit lower than usual last night, so maybe I'll just leave it be. Didn't hook it up yet to speakers, just checked everything with a dummy load, so don't know how much the sound was affected. At the moment the rectifiers and resistors are hooked up using screw terminals, wanted to make sure everything works as it should before I solder them and also need to figure out where to install them physically.

    I've been using the whole thing, eventually I'd like to add pre-out/main-in jacks once I figure out how to best do it, so it could be hooked either way. For now the preamp is very much in the picture. What did you find disagreeable about it and the sound and do you personally think the pre is just a poor design? Obviously the signal passes through a set of tone controls and some very old pots and switches, however these in my unit seem to be in tip-top shape and didn't really even need a clean, no symptoms what so ever. I've recapped the tone controls, but to get flat frequence response (judged by ear), treble needs to be bumped up to about +1 and bass needs to be cut back to about -2 . Dialed in right it sounds quite neutral though, albeit perhaps not the last word on audiophile qualities, however very engaging and musical nonetheless.

    Btw. most of the original caps were also in very good shape, at least measured with the tiny voltages the cheap ebay tester I have uses, but it could be they behave differently under high voltages and subjectively the sound was improved a lot by the recap. The damping adjustment pots are in a rough shape, to the point they are really not to be adjusted while the unit is powered on for all the screetches and loud noises they make, should replace those at some point.

    Also still haven't got around to ordering a new set of power tubes, the original Mullards seem fine as far as I can tell, but I can't tell much for not having a tested new pair as a reference...

  8. #8
    Join Date: Feb 2008

    Location: Middlesex, UK

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    I'm Alex.

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    Good that you got rid of the selenium rectifiers, my experience of them (decades ago) was that they were not reliable, and emitted a horrible (understatement) smell when they went wrong.

    Perhaps new EL34's, just to preserve the Mullards, I have used JJ EL34's and JJ KT77's to good effect in my Radford STA25 III.
    Spendorman

  9. #9
    Join Date: Oct 2012

    Location: NE England

    Posts: 4,643
    I'm Jez.

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    Quote Originally Posted by spendorman View Post
    Good that you got rid of the selenium rectifiers, my experience of them (decades ago) was that they were not reliable, and emitted a horrible (understatement) smell when they went wrong.

    Perhaps new EL34's, just to preserve the Mullards, I have used JJ EL34's and JJ KT77's to good effect in my Radford STA25 III.
    Foul indeed and IIRC toxic so it's open window and out of room when it's happened to me...

    I've heard rumours that JJ quality control has vastly improved in recent years but when I was dealing with loads of valved guitar amps I found JJ to be the most unreliable as regards catastrophic failures...

  10. #10
    Join Date: Feb 2008

    Location: Middlesex, UK

    Posts: 3,016
    I'm Alex.

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    Yes, JJ quality increased from the early days, it probably had to!
    Spendorman

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