R4 (100 Ohm) might only give 0.42dB of feedback, but it will lower the PSRR of that stage by a lot more than that by virtue of the increased anode resistance. I've always felt that minimising that effect was the only real justification for including the electrolytic etc, rather than simply applying the feedback directly to the cathode and leaving Rk completely unbypassed.
Bulgin,
In case there is a misunderstanding: It is not the output transformer that gets hot.
It is reportedly the power transformer that gets hot, as is often the case, when all the ancillaries (pre-amp and radio) are hooked up. Don't ask why, to me it is simple inefficient design. These fellows know that they might be exporting to countries that have a higher average temperature - that is not difficult to work out if your factory is in the UK. That is why I am a little surprised to find such shortcomings in the likes of Leak and Quad II.
Also, while I am revved up
, it is not acceptable that the mere replacement of say a rectifier tube with a 3A heater instead of 2A should ruin a transformer. That is only 5W extra (OK, 6W if you want to be pedantic about efficiency). What happened to safety factors? I am not advocating replacement by non-specified items; but it simply shows that the design was marginal to begin with.
_______________________________________
I would still like to hear from others regarding the off-optimum performance that I experienced (reason for this thread) - do I have things wrong somewhere? It does appear that Leaks are not that popular.
Regards.
In case there is a misunderstanding: It is not the output transformer that gets hot.
It is reportedly the power transformer that gets hot, as is often the case, when all the ancillaries (pre-amp and radio) are hooked up. Don't ask why, to me it is simple inefficient design. These fellows know that they might be exporting to countries that have a higher average temperature - that is not difficult to work out if your factory is in the UK. That is why I am a little surprised to find such shortcomings in the likes of Leak and Quad II.
Also, while I am revved up
, it is not acceptable that the mere replacement of say a rectifier tube with a 3A heater instead of 2A should ruin a transformer. That is only 5W extra (OK, 6W if you want to be pedantic about efficiency). What happened to safety factors? I am not advocating replacement by non-specified items; but it simply shows that the design was marginal to begin with._______________________________________
I would still like to hear from others regarding the off-optimum performance that I experienced (reason for this thread) - do I have things wrong somewhere? It does appear that Leaks are not that popular.
Regards.
I'm just pulling this out of thin air, but... do you suppose that the design was optimized with an insufficient universe of tubes, that the circuit might be more susceptible than most to tube variation?
It's been decades since I rebuilt one. I only vaguely remember the circuit, just that at the time I thought it was bizarrely overcomplicated and replaced it with a much simpler and more linear driver. I'll have to go have another look at it.
edit: I dearly wish I still had the transformers to measure. They were physically big but didn't deliver the goods on the bass end.
It's been decades since I rebuilt one. I only vaguely remember the circuit, just that at the time I thought it was bizarrely overcomplicated and replaced it with a much simpler and more linear driver. I'll have to go have another look at it.
edit: I dearly wish I still had the transformers to measure. They were physically big but didn't deliver the goods on the bass end.
SY said:
Yes, SY, the input triode (half ECC83) has a 100K Rl and a bypassed 2K2 bypassed cathode resistor, going to common through a further 100 ohms to which global NFB is fed.
I take it that by your question you mean the complete cathode resistor unbypassed (e.g. Williamson). I seemed to find that Kirchoff's Laws work here: Currents work independently through common resistors. Thus the input stage gain would depend on the degeneration provided by the (unbypassed) cathode resistor. A part advantage of a completely unbypassed cathode resistor is the avoidance of another l.f. pole, although that gets out of the way again further down (sometimes to advantage!).
Regards
SY said:
We posted simultaneously - I just caught your edit.
First sentence: I thought about that. Also, having the input triode anode that high up might not be so serious, as it outputs only about 1Vpp for maximum output. But when one builds that stage separately and experiments, it soon becomes clear that it is simply better (less critical) with more Ia - I could not see any reason why not. Also, the main problem I found with the overcompensated lead, was also there originally. It was just simply out. (I must try to post just a hand-drawn square wave of what I found.)
Transformers: I did measure the output transformer when I had the last Stereo 20, but I am getting another for the same purpose soon, and will again measure and post. (It was not outstanding, though sufficient.)
Edit: Typo
Those output transformers can manage 10W at 50Hz. Drop the frequency and the distortion rises catastrophically.
Leak and Quad mains transformers get hot in Blighty without any additional load. It occurs to me that if their core material had deteriorated over the years, the primary inductance would fall and that would cause increased magnetising current (and raise the LF limit for output transformers).
Leak and Quad mains transformers get hot in Blighty without any additional load. It occurs to me that if their core material had deteriorated over the years, the primary inductance would fall and that would cause increased magnetising current (and raise the LF limit for output transformers).
Hi ED8010,
Interesting - that did not occur to me.
I just confirmed with a learned friend about transformer cores. Apparently if in continuous use at over 75 degrees C temperature or so, changes can occur, but also not expected to be more than a few percent over years.
Others with better experience?
As said the Leaks I had did get hot (but not finger-burning) after >1 hour, but not so the Quads, although they all oozed. But I would presume that was more from the adjacent choke with the "oven" (KT66 cathode resistor) right underneath. After years it was difficult to detect where the leakage actually started. Temperature was still touchable after several hours - and I don't have "mechanic's fingertips". I am not sure whether a design change may have occurred somewhere along the line.
Regards
Interesting - that did not occur to me.
I just confirmed with a learned friend about transformer cores. Apparently if in continuous use at over 75 degrees C temperature or so, changes can occur, but also not expected to be more than a few percent over years.
Others with better experience?
As said the Leaks I had did get hot (but not finger-burning) after >1 hour, but not so the Quads, although they all oozed. But I would presume that was more from the adjacent choke with the "oven" (KT66 cathode resistor) right underneath. After years it was difficult to detect where the leakage actually started. Temperature was still touchable after several hours - and I don't have "mechanic's fingertips". I am not sure whether a design change may have occurred somewhere along the line.
Regards
Leak Stereo 20 - Johan's...
Hi Johan & All
My S/20 was acquired about 12 years ago, together with other Leak equipment. None in good working order. The seller (now deceased), owned a large electronics store and also offered repairs done by himself and assistants.
I negotiated the sale for 'repaired goods, with limited guarantee on parts and workmanship' and paid extra for that.
I was at this time informed that the mains transformer (of the S/20) required a rewind.
After months of waiting, I finally received the repaired items, which included new valves.
At this time, I noticed koki writing of the seller's business name on the mains transformer, so I could assume that it had been removed and re-wound.
Prior discussion with the seller (before I paid him the money), raised some expectations that I was about to enter 'tube heaven'.
After the seller's repairs, I asked to hear the equipment in his store and it was a huge disappointment - totally gutless and barely able to drive 2 small monitors.
I left the stuff there and so, more months went by hearing Mr X was still working on it.
Finally, after another 2 months, I was called to collect the items.
It WAS better, but still not what I expected. The seller then waived his 'repaircosts' and I gave him the money - punchdrunk after waiting almost 5 months.
At home, I inspected the underside carefully and found very little evidence of replacement parts or fresh soldering.
A friend then gave me the name of someone else from which he had good service for his Mac and AR stuff and I took my things there.
Two weeks later, I collected my properly repaired things and it has been magic ever since. In the years since, I have fitted new binding posts (from Maplin), replaced the 2 caps on top and fitted the correct bulgin (hehe) mains socket and plug.
The mains transformer only gets mildly warm - even after a long session at highish volumes if I need that.
bulgin
Hi Johan & All
My S/20 was acquired about 12 years ago, together with other Leak equipment. None in good working order. The seller (now deceased), owned a large electronics store and also offered repairs done by himself and assistants.
I negotiated the sale for 'repaired goods, with limited guarantee on parts and workmanship' and paid extra for that.
I was at this time informed that the mains transformer (of the S/20) required a rewind.
After months of waiting, I finally received the repaired items, which included new valves.
At this time, I noticed koki writing of the seller's business name on the mains transformer, so I could assume that it had been removed and re-wound.
Prior discussion with the seller (before I paid him the money), raised some expectations that I was about to enter 'tube heaven'.
After the seller's repairs, I asked to hear the equipment in his store and it was a huge disappointment - totally gutless and barely able to drive 2 small monitors.
I left the stuff there and so, more months went by hearing Mr X was still working on it.
Finally, after another 2 months, I was called to collect the items.
It WAS better, but still not what I expected. The seller then waived his 'repaircosts' and I gave him the money - punchdrunk after waiting almost 5 months.
At home, I inspected the underside carefully and found very little evidence of replacement parts or fresh soldering.
A friend then gave me the name of someone else from which he had good service for his Mac and AR stuff and I took my things there.
Two weeks later, I collected my properly repaired things and it has been magic ever since. In the years since, I have fitted new binding posts (from Maplin), replaced the 2 caps on top and fitted the correct bulgin (hehe) mains socket and plug.
The mains transformer only gets mildly warm - even after a long session at highish volumes if I need that.
bulgin
Bulgin,
Yes, I should perhaps bring balance by saying don't dispair regarding my findings. A lot of folks like the Stereo 20, and in the end those anomalies might not be very noticable. I remarked on them mainly because of the unexpected deviation from normal design goals and the ease of correcting - 2 resistors and a capacitor per channel, plus an input tube change from ECC83 to ECC81 if you are really serious. The incorrect impression may have been created that this was a definite poor design - not so.
Regards.
Yes, I should perhaps bring balance by saying don't dispair regarding my findings. A lot of folks like the Stereo 20, and in the end those anomalies might not be very noticable. I remarked on them mainly because of the unexpected deviation from normal design goals and the ease of correcting - 2 resistors and a capacitor per channel, plus an input tube change from ECC83 to ECC81 if you are really serious. The incorrect impression may have been created that this was a definite poor design - not so.
Regards.
EC8010,
An aside just for the record: I think it was to you that I mentioned on another thread weeks ago that I thought Peter Walker was awarded an honourary doctorate. I may in fact have confused that with the fact that he was awarded an OBE for the ELS design. Thus Sir Peter, not Dr Peter.
An aside just for the record: I think it was to you that I mentioned on another thread weeks ago that I thought Peter Walker was awarded an honourary doctorate. I may in fact have confused that with the fact that he was awarded an OBE for the ELS design. Thus Sir Peter, not Dr Peter.
Leak Stereo 20 - Johan's...
Hello Johan, EC8010 & All
I'm using the Leak for what it is. I admit, I'm out of my depth when it comes to valve theory and it's time to acquire some knowledge.
I hope to 'hang around' here and perhaps something will penetrate the lack of matter between my ears.
What I DO know, it is immensely satisfying if you build something yourself and getting it right.
Regards
bulgin
Hello Johan, EC8010 & All
I'm using the Leak for what it is. I admit, I'm out of my depth when it comes to valve theory and it's time to acquire some knowledge.
I hope to 'hang around' here and perhaps something will penetrate the lack of matter between my ears.
What I DO know, it is immensely satisfying if you build something yourself and getting it right.
Regards
bulgin
Ooohhh, Ray doesn't like Leaks much does he?
Have got to chip in with my findings, is the Leak perfect, nope!
is it as bad as Ray makes out , not a chance.
I've made my living working for a well known Broadcasting corporation for 20+ years, and have heard a lot of kit as well as a lot of live music in that time,
I use a Stereo 20 with a Border Patrol supply at home, no circuit changes apart from the supply, but lots of better quality components fitted.
I've owned and borrowed dozens of amps , valve and transistor,
if I had to keep just one it would be the Leak.
It likes passive preamps, Mullard tubes and Snell type J and quad 57 speakers, everyone who visits the house is gobsmacked with the sound, perhaps they're all mad?
Jules
Have got to chip in with my findings, is the Leak perfect, nope!
is it as bad as Ray makes out , not a chance.
I've made my living working for a well known Broadcasting corporation for 20+ years, and have heard a lot of kit as well as a lot of live music in that time,
I use a Stereo 20 with a Border Patrol supply at home, no circuit changes apart from the supply, but lots of better quality components fitted.
I've owned and borrowed dozens of amps , valve and transistor,
if I had to keep just one it would be the Leak.
It likes passive preamps, Mullard tubes and Snell type J and quad 57 speakers, everyone who visits the house is gobsmacked with the sound, perhaps they're all mad?
Jules
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