Pye Mozart HF10 stereo pair

[Arkless Electronics]

Their EL34's were a bit of a bomb, not all, but quite a lot.

Oh yes I can believe it. see my comments above about EL84's and GZ34's from some of these dubious brands... Whilst not common it was possible to get Mullard, Tesla, Tungsram etc remarked as these brands so it could work both ways... but 9 times out of 10 they were crap and it would take a dozen to find 2-3 that worked properly...

[spendorman]

7581A box
5881A-.jpg5881A.jpg

I must have got the numbers wrong previously!

Also says 7581A on the valve.

[spendorman]

Oh yes I can believe it. see my comments above about EL84's and GZ34's from some of these dubious brands... Whilst not common it was possible to get Mullard, Tesla, Tungsram etc remarked as these brands so it could work both ways... but 9 times out of 10 they were crap and it would take a dozen to find 2-3 that worked properly...

Yes, I forgotten that I bought some rubbish EL84's from Zaerux.

[Arkless Electronics]

7581A box
5881A-.jpg5881A.jpg

I must have got the numbers wrong previously!

Also says 7581A on the valve.

Very interesting! I did say earlier there are probably more variants of the 6L6 than any other valve... But I'd not heard of these! I wonder if they were the yanks answer to the British, improved version, of the 6L6, the KT66? The box markings would suggest this. I did resort to google on this for a quick search but mainly found reference to a modern "Tung-sol" version and old Ge plus Philips ecg JAN versions (later USA Sylvania in reality).

[spendorman]

I also have these.

6550A.jpg

Can't be bothered with camera at the moment, used scanner, otherwise I would have included pics of the valves.

[spendorman]

Pic of GE 7581A / KT667581A--.jpg

[helma]

I've done loads of conversions from one type to the other on guitar amps in the past and for proper operation the screen grid resistor needs changing from 470R to 1K and the bias voltage is sufficiently different for optimum operation that only in some models of amp will it come within the adjustment range of the bias pot.. on some amps a resistor needs changing as well to get it in range. The Pye is fixed bias... sort of cathode bias ish I guess (you can't adjust it IIRC) but it's an interesting design! After rebuilding and modernising a pair a few years ago I was quite surprised by their sound... At reasonable levels quite SS like in many ways (in a good way), much more detail and precision than you may expect! However... it all went a bit Pete Tong well before they reached anywhere near clipping from memory. Rated at 10W (or was it 8?) I would not want to use them much above say 4W for best results. Leak Stereo 20 trounces them on max volume and slam etc but at low volumes I recall them being at least the equal of the Leak and maybe better

I can attest to this regarding the sound of the HF10 - I was really surprised how modern they sounded the first time I hooked the pair up. And this was with all original parts. Changing the caps only highlighted that. I'm not sure what I was expecting, something way more "50s sounding" I'm sure I also agree they don't seem to keep very well together at high volumes, but to my ears this happens at levels when there's already significantly increased distortion. It's possible though the original Mullards are tired and start breaking up earlier than they should. If not pushed near their limits, there's suprisingly solid and agile bass and nice snap to percussion etc. making drum kits really punch like they are supposed to. Unfortunately that just tempts me to crank it up some more and then they run out of juice.

This leads me to my dilemma - I can just about squeeze a nice listening level out of them with my fairly inefficient speakers, but with nothing left on the table, no cranking it up when feeling like it. It's just not enough power for my speakers and room, I think I would need speakers with a sensitivity of at least 93-96dB/W. So the question then becomes if I want to get new speakers or let the Pye go. This would be a perfect excuse for experimenting with some high efficiency full ranges or something, but another money sink. Will have to try with shiny new valves before making any decisions though. I've got quite a bit of hifi kit I need to get rid off and there are some other items I'd be willing to see go before the Mozart, so hopefully I'll find a way to talk myself into keeping it Also I have a history of flipping anything I got from a deal too good to be true, it would be nice to hang onto something for a change. It's usually not how I planned it, but then someone sees what I've got and makes an offer I can't refuse.

[spendorman]

It would be nice to try some highish efficiency speakers on the Pye before parting with it. I have a Chinese single ended EL34 amp that is pretty sweet on low volumes. I don't have any really efficient speakers, possibly the highest efficiency ones that I have are Tannoy HPD315's.

[mik_rik]

No offence intended Mick. I'm just blunt as you will have realised... The 6p3s is roughly equivalent to the 6L6 but has much reduced ratings. It would be fine in a 6L6 based amp giving 20-25W but in one giving 40 it would have a life in single digit hours probably. they are though (or were) very cheap. Yes there are other rated types available that get near to USA types in rating. The e or b (Russian e or V) normally mean long life rating or harsh environment rating and are often (if you're lucky!) military NOS, which if you get the pukka thing are as good as Mullard. I wouldn't trust the "OTK" in purple on the envelope much any more as resellers/charlatans have cottoned on to it and it's easy to fake!

Philips (Dutch owners of Mullard since about 1930-ish IIRC) had a patent on the pentode well before RCA came up with their answer in the "beam power tetrode". Their "new big thing" was the 807, which was a top anode RF rated valve that did sterling service in WW2 for us and the yanks and was knocking on a bit by then... The 6l6 was a cheapened version optimised for audio amps but based closely on the 807. There are loads of variants of the 6L6! probably more so than any other valve. The KT66 is indeed a "kinkless tetrode" ie beam power tube by a different name to get round RCA's copyright. It's a further improved and higher rated SQ version of the 6L6 but British basically.. The KT77 is a GEC/Marconi/Osram answer to the EL34 and although a beam tetrode is optimised to be as pentode like.. and EL34 to be specific.. as possible, and again was intended as a higher rated better quality version of the commonly used EL34 of course.

5881 is a military spec version of 6L6 and there is a lovely Russian version of this which may be based on the 6p3s.. it has a very thin "button base" to cut down on the height of the valve... rumours are they were used in the servo of the bomb doors of Russian cold war era bombers and they had to be reduced in height to fit in the available space... possibly apocryphal! These, if pukka, are superb and I'd take them in preference to non military spec/SQ/selected Brimar, GEC (USA GEC of course. Unrelated to UK GEC AFAIK) etc..

All from memory, as google would be cheating.... and weds are pub night... hick... so if wrong I apologise

I've done loads of conversions from one type to the other on guitar amps in the past and for proper operation the screen grid resistor needs changing from 470R to 1K and the bias voltage is sufficiently different for optimum operation that only in some models of amp will it come within the adjustment range of the bias pot.. on some amps a resistor needs changing as well to get it in range. The Pye is fixed bias... sort of cathode bias ish I guess (you can't adjust it IIRC) but it's an interesting design! After rebuilding and modernising a pair a few years ago I was quite surprised by their sound... At reasonable levels quite SS like in many ways (in a good way), much more detail and precision than you may expect! However... it all went a bit Pete Tong well before they reached anywhere near clipping from memory. Rated at 10W (or was it 8?) I would not want to use them much above say 4W for best results. Leak Stereo 20 trounces them on max volume and slam etc but at low volumes I recall them being at least the equal of the Leak and maybe better

The transfer characteristics are fairly different and they therefore sound pretty different.... for good or bad depending on taste and amp model...

Hi there Jez ,... no offence taken ,just to clarify ,it may be you thought I was referring to the 6p3s .The valve I was talking about was the 6p3s-e version which is totally different in construction and performance,much better imho.This is the one with the "coin" base and is much more rugged and is a very good sounding valve[I thought they were used in ballistic missile servo's,pleased they were not used in anger].When people talk about the sound of different valve types it is easy to use exaggerated terms to describe differences to make a point,these terms become gospel and so certain makes become must have and the price soon becomes inflated.Mullard are very good but not that other classic makes are rubbish in comparison hence as you say Brimar,Mazda are at least on a par. - and a lot cheaper! Cheers Mick

[Arkless Electronics]

Hi there Jez ,... no offence taken ,just to clarify ,it may be you thought I was referring to the 6p3s .The valve I was talking about was the 6p3s-e version which is totally different in construction and performance,much better imho.This is the one with the "coin" base and is much more rugged and is a very good sounding valve[I thought they were used in ballistic missile servo's,pleased they were not used in anger].When people talk about the sound of different valve types it is easy to use exaggerated terms to describe differences to make a point,these terms become gospel and so certain makes become must have and the price soon becomes inflated.Mullard are very good but not that other classic makes are rubbish in comparison hence as you say Brimar,Mazda are at least on a par. - and a lot cheaper! Cheers Mick