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Old 24th March 2007, 09:45 PM   #1
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Default Leak Stereo 20 clone? possibly with 6V6?

Has anyone ever cloned a Leak Stereo 20? I've been looking to build an easy stereo push-pull amp in the 10-20 watt range using Hammond iron for OTs and everything I have read about vintage Leak gear has been positive. I originally wanted to clone a Quad II or Leak TL/12.1 but decided against monoblocks because I don't want to buy two power transformers, and because KT66s are expensive! I am looking to use the Hammon 1609(10k primary), 1615(5k) or 1620(6.6k) output transformer as they are all very close in terms of price, hopefully they sound good too.

Would a Stereo 20 be a good choice or should I go with something else? There does not appear to be a choke in the power supply(unlike the monoblock TL/12+) but I think I would like to use one in place of the first resistor in the PS if there is likely to be a significant impact on ripple. I've also thought about using 6V6s instead of EL84.

here is a schematic for the Stereo 20

http://ampslab.com/SCHEMATICS/LeakStereo20.gif

also, i'm interested in this Mullard EL34 schematic as an alternative.

http://bonavolta.ch/hobby/en/audio/el34_1.htm

thanks
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Old 24th March 2007, 10:15 PM   #2
phn is offline phn  Sweden
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Scrap the idea of a Leak clone. It's a good amp, but not a great amp. You can do better.

What output tubes you use, EL84 or 6V6, isn't very relevant. The EL84 is probably superior, but the 6V6 looks nicer. But before you decide on tube, first decide how much power you need.
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Old 24th March 2007, 10:20 PM   #3
EC8010 is offline EC8010  United Kingdom
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The Stereo 20 (like all Leak amplifiers) is ludicrously over-sensitive at 120mV for full power. To get that gain, it uses more coupling capacitors (thereby compromising LF stability) than is ideal. It was a good design for its time and sounded better than a Quad II, but without identical output transformers you'd be hard-pushed to equal its performance, let alone match a latter-day design that doesn't try to screw every last dB of gain out of the valves.
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Old 24th March 2007, 10:40 PM   #4
qwad is offline qwad  Australia
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hi, l'm no expert but l agree with another poster who says forget it, the sound of these classic amps depended bon the o/p trannies more than anything modern equivelants will more than likely not give you anything like the sound ofthe originals, if you want somethig to build that sounds terrifi try a pp 6aq5 amp the 6aq5 is a6v6 in a miniature envelope, from all accounts they make great sounding amp do a search on audio asylum for info on them......
cheers TC
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Old 24th March 2007, 10:56 PM   #5
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Yes, I 'cloned' one half of a Leak Stereo 20 back in 1962. I didn't want two channels, because mono was still the mainstream for radio and records in those days. Anyway, as a schoolboy, I didn't have the money for two OP transformers. I chose a high quality OPT, though, one made by Parmeko for the Mullard 5-10. I also built one half of the Leak Varislope preamp, which I simplified to meet my needs. It all seemed to work OK, at least to my inexperienced ears.

However, the Leak Stereo 20 has a number of design weaknesses, as I've come to realize since then, including:

* AC coupling between the first stage and the splitter, which can cause LF instability in the negative feedback loop, particularly if you use a rather ordinary OP transformer.

* the use of 12AX7 for both the first (voltage amp) and second (splitter/driver) stages; it's inclined to accentuate the non-linearities that are peculiar to that particular tube type, which is not all that linear to start with, and it might have sounded better if Leak had chosen a different, preferably lower impedance tube for the splitter/driver .

* no constant current sink (CCS) in the tail of the LTP splitter, which would be very much better than the resistor Leak used.

* the use of a resistor instead of a choke for PS smoothing, weakening the bass response and causing higher ripple.

* very heavy negative feedback, which is excessive for an ultralinear design (they believed that there was no such thing as too much NFB in those days).

* high input sensitivity.

In the 1960s, the public was not very discriminating and something like the Leak Stereo 20 was many orders of magnitude better than the run-of-the-mill radio sets, record players and radiograms on the market at that time.

Leak was cost-cutting when he decided to use a smoothing resistor instead of a choke. You are only building one amp for personal use, not to make a profit, so you don't need to make such 'economies' and a choke would be a much better idea.

I prefer the Mullard 5-10 or 5-20, which is the other alternative you mention, but it shares many of the weaknesses of the Leak Stereo 20. Its open loop gain, negative feedback and input sensitivity are all excessive - even worse than Leak's! But at least the Mullard circuit has direct coupling between the first and second stages, so it is inherently more stable.

I believe the Mullard 5-10 or 5-20 would be a better place to start than the Leak, but it should be improved by fixing all the weaknesses it has in common with the Stereo 20, including using a choke in the power supply and a lower impedance medium mu double triode as the splitter/driver (e.g. 6CG7 or 6FQ7). This would give excellent balance, despite its lower mu, if you used a CCS instead of a resistor in the tail. I would also omit the cathode bypass cap from the EF86 but I wouldn't triode-strap it, as some people do, because the gain would already be reduced a lot by using a lower mu splitter tube.

I wouldn't advise you to use a Hammond OP transformer, since this is an ultralinear design and requires a special quality OPT to work well. Hammonds are not bad but IMHO there are better transformers for your purposes.
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Old 24th March 2007, 11:07 PM   #6
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Bikehorn,

Hammond 1650Es will work in an UltraLinear wired "El Cheapo". The 'AQ5 called for is a 6V6 in a 7 pin mini package. IMO, the DynaClone Z565 is better than Hammond "iron", but it's all too easy to spend somebody else's money.
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Old 25th March 2007, 01:20 AM   #7
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I've probably made at least 30 different versions of the Stereo 20, and I'm still not satisfied! I agree with the others - build something else. I'd suggest Allen Wright's PP1 amp on his website www.vacuumstate.com in the schematics. It's simple, effective, and will teach you some useful techniques like constant current sinks and having a balanced input option. You can go on and change the output stage to other valves like 2a3, 6B4G, 300b at a later date since the input stage is quite versatile.
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Old 25th March 2007, 08:51 AM   #8
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Well I guess the Stereo 20's out...

I've definitely been interested in the 6AQ5, in fact if i had built the Leak with 6V6s I probably would have gone with those. NOS 6AQ5s sell for way less than new production 6V6s. I also thought about building a Dynaco ST35 clone per DIYtube but the available clone chassis seem to be awkward and clunky

As transformers go, I need something that won't break the bank, or else I'll have to build gainclones. I will not need a lot of power, 15 watts would probably be enough as most of the time I will be using efficient speakers(at least 96-97 dB). Even if they aren't the best, on a relative scale i'm sure they'll handily outperform the mid 70's solid state Sony stereo receiver I have What other options are available for outputs? The Dynaclones look good. Will it be easy to include a triode/pentode switch? can i run the 6AQ5s in the El Cheapo in pentode mode just by connecting the screen grid taps on the transformer? i've seen this feature on some guitar amps...obviously the purpose would be a little different on a hifi amp.
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Old 25th March 2007, 10:09 AM   #9
phn is offline phn  Sweden
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5W is more than enough for 96db. So triode-connected 6V6 will give you all the power you need. Triode-connected because it tends to be easier to get good result.

This trafo should be enough:

HAMMOND 1620, 20W, 6,600 ct, 6AQ, 6L6, 6V6

Costs $45.17 at Parts Connexion.
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Old 25th March 2007, 10:29 AM   #10
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You don't need a terribly expensive OP transformer, just a good one! Your power requirements are modest, so cost should be reasonable unless you choose an especially expensive brand. I expect you can get plenty of helpful suggestions from others here.

From a data sheet I've seen, a pair of 6AQ5s in pentode push-pull can give up to 10 watts output (quite respectable for such a tiny tube!) with 10k plate-to-plate (P-P) load. I don't know what load impedance they would need in triode-mode but, in any case, you'd probably find the available power inadequate, maybe 2.5 watts.

If you want to try switching between pentode and triode, it can possibly be done in the way you suggest, but you have to consider the required P-P load in each case. If compromise is necessary, it would be better to base the OP transformer on the needs of pentode mode, since this is quite critical. Triode-mode is less fussy, and is particularly forgiving if the P-P load is a higher impedance than the ideal load - you just get less power and less distortion.
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