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Tubes Asylum: REVIEW: Leak St20 Amplifier (Tube) by Timbo in Oz

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REVIEW: Leak St20 Amplifier (Tube)

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Model: St20
Category: Amplifier (Tube)
Suggested Retail Price: whatever you can agree to
Description: small pp EL84 stereo 'power' amplifier ideal restoration /tweak target
Manufacturer URL: Not Available
Manufacturer URL: Not Available

Review by Timbo in Oz ( A ) on August 28, 2003 at 00:05:22
IP Address: 144.138.154.32
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for the St20


Two Leak Stereo 20’s - modified. Stage 4/5


The Leak Stereo 20 modified.

All work carried out, over several iterations and discussions, by Merlin Audio here in the Australian Capital Territory. I have noted all the mods on a circuit diagram, but this is a bit rough and won't scan well.

All the replaced passive circuit components are commercially available audiophile quality, ie not Solens or MIT's Wundercaps.

Component choice was done to balance tonal quality, and to retain the wonderful midrange purity, speed and believability of tubes in up-to-date circuits. The total cost including labour and learning was not small, (about $Aus 2,000, or $US 1,300) but this is still a lot less than two new small tube amps would have cost me.

Two (2) rebuilt / modified Leak Stereo 20's, as a WIP they are now at Stage V. One is in pentode (not UL) and one in triode mode. So the originality types hate me!

Swapped for a Leak Stereo 60 which I was given and did not like compared to Radfords. Zero cost to that point!


PSU Modification;

Cost can be the only reason why HJ LEAK p/l dropped a choke from the TL12 Plus / Stereo 20 chassis, the value of using a tube rectifier is pretty low without one IMO, and they could have used metal diodes instead. This would have been in the same vein, being cheaper still! Maybe not!?

I tried to stay original - Anyway, three out of 6 replacement original type caps (NoS) were no better than the crappiest I had (leaky after a few hours) ; all good GZ34’s are NoS and were very expensive and ‘no serious guarantee’.

Back then I wanted all the power I could = max HT, so I didn’t put in a choke and stay with a tube psu. Instead I went the ‘big cap SS’ route. Hate me if you must.

The PSU uses an array of fast soft recovery diodes (a TPC rec’dn) then big 470 uf 400 volt Ni(ppon)ch(emi)con filter capacitors, at least they fit the can and recifier holes, there are some coupling and bleed resistors in there too!

3 of these caps in the triode amp, and just lately 4 in the pentode. HT is 320 volts (B+!), giving > 20 joules in both amps, and there is good decoupling. Ripple is > 20 db down on Leaks spec!

Each amp has one cap where the GZ 34 rectifier tube usually sits, just for a laugh! The next two are in the usual can pos'ns, and the 4th goes underneath.

Each is bypassed with 1 or more WIMA MKP and MKS. There is also an NTC thermistor on the AC in on both chassis.

The AC leads are hard wired.

They are not tweaky, hmmmm here comes a DIY braided lead experiment?


Gives higher HT volts, and a little more power, energy storage 24 times larger! [[(470+)*4]*[(320)*(320)]/2] c/f [[(32*3)]*[(240)*(240)]/2]. Storage being 1/2 the capacitance times the voltage squared. Ripple is now insignificant.

Yes, the transformer is pulsed a bit harder, but each pulse is for a lot shorter period, the 20 didn't come with a choke, so the original pulsed the TX as well! Overall the trannie runs cooler than before, because there's no valve rectifier pulling 2 amps from the heater windings!

Thermistor surge protection / soft start, cleaned fuse sockets and new fuses - all cleaned and polished. Don't laugh this is audible, try it!

Can't find the power resistor (used as a slow blow fuse by HJL). I asked Bill and he said the new circuit and the fuses were all we needed.


Audio stage mods:

For each stereo chassis – 1 X 6DJ8 (shared) – 2 of 6CG7 (LTP and a fet CCS) - 2 of 2 X EL84M.

The input twin triode (with DC heaters and new ceramic sockets - NoS Mullard. It is flexible enough that I can use a few other tubes straight in eg. 6H30 and 6N1P.

Phase splitter/driver triodes are AWA and AWV Australian made NoS, and very good too, they’ve been in there > 2 years now. 2 x Selected NOS Australian mfd (AWA/AWV) 6CG7 driver / phase splitter tubes, 1 per channel, replacing the 2 X 12AX7's. Bill has applied a FET Constant Current source circuit to this tube to hold it and the output tubes in balance. NB it is a differential stage and would need two CCS otherwise.

Note: This an extension of Mellotone (Haden Boardman's) 'Perfect Phase' rebuild (UK in HFW 2 years back), which use 12AU7 / ECC82's in both roles. (2 per amp). [The 6CG7 is used by Audio Research, Conrad Johnson et al, and is the Noval / 9-pin miniature equivalent of the famous Octal 6SN7).

Yes, I know this is probably overkill, but all three small signal tubes are just idling, which is nice. :-)}.

Global feedback caps and resistors replaced with high stability precision bulk foil resistors and polystyrene and teflon caps.

All the coupling caps are Wima MKP's, the main circuit resistors are now 1% MFs, Beyschlag and Philips, and the cathode/bias resistors are 270 ohm non inductive power resistors (dark blue), I forget the brand.

Such caps and resistors tend to 'cross-compensate' the tempco effects and help the amps to settle stably once warmed up. Output power; 16 watts both channels into 8 ohm. Distortion? A lot less than 0.1%, ie 0.05%, but at 16 watts not 12 watts!. Watts @20 hz and at 20 khz is 16. Output impedance was also down good bit.


Feed back around the input and driver stages is lower due to using a 6dj8 input and a SCG7/6FQ7 phase splitter/driver. Took advantage of lower total and cathode feed-back in first and second triode stages, ie it's implicitly lower with the different tubes. Overall, total feed back is maybe a bit lower and sensitivity still well below 200 millivolts for full output.

I wanted to keep 6BQ5's but have replaced them with Sovtek EL84M's. I did use up a set of eight 7189's JAN NoS (I think they may have been made in Japan though, National brand?).

I was lucky to get these for the price I did, they are not cheap now. While the EL84M Sovteks are, and are quite OK in this role. My memory says that the Sovtek EL84 M’s are not quite as nice as NoS 7189’s!

I replaced the RCA's with good ones (Teflon ins'n). New multi-way binding posts at 3/4 inch banana / pomona centres. One chassis has nickel plated brass and the other has gold plated brass but, I still use bare wires which I dip in cleaning fluid every few months.

“It isn’t a Leak anymore.” Or also “It doesn't sound like a Leak St20!

Well, maybe not, but it does sound good. Quiet too.


Other issues

Tapping: You should try two tapping options ie if it’s a nominal 8ohm spkr, dropping to 3 or even 2 ohms in places, it may perform better on the 4-ohm tap, if it’s a high impedance spkr, 16 ohms may be best but some times the 8-ohm tap is best, definitely so into a 9-11 ohm spkr. You may lose some power into a lower tap, but sometimes the bass is a little tighter.

Note: Make sure the global feedback loop is connected to the tap you're using, otherwise you'll not get all the potential benefits.


Chassis:
Dark gold / bronze. They now have matching steel mesh and sheet covers. They sit on white concrete pavers, 2 blocks, one on top, bottom one is spiked to the floor, all under a little occasional table each, ie inside the four legs.


History / Background:

The rebuild was a careful, thoughtful and iterative process, to audibly and measurably improve the basic circuit's achievable sound quality, and within the design's limits. I don't agree that modern components, or even different tubes, is the wrong way to go about things. The result is more reliable, sounds better in every way and gets jaw-dropping reactions from listener's.

I had listened to them in original form; and then, Stage I, overhauled with original value parts and with new tubes, and found them (one at a time) very nice and surprisingly loud, BUT…… hissy, a bit heavy and slow in the bass and dull treble, and hummy and buzzy still.

Stage II - Sovtek EL84's and 12AX7's, much quieter but GZ34's kept failing, and good ones - ie NOS Mullards or Brimars were not cheap! Some original filter cap's measured very ripply and leaky.

Further, some original, NOS replacement filter caps that measured well went hummy and farty within a few weeks.

I agree with Steve, the HJLeak - Kiewa Valley site's builder, that Stereo 20's mains transformers run hot, even hotter if you use lower quality GZ34's, 5AR4's, or even 5AS4's. The 5AS4 is a directly heated tube and has a very high heater draw current!

BUT with 'any' off or cheap new tube rectifier the heater current drawn can run right up against the transformer's original design limits. My two are the later ones with upgraded T’s, but they still ran hot then, with the tube rectifiers.

I think the original circuit draw is close to or over 2 amps with any of these. Way over with a 5as4. The 'slow blow fuse' (actually a resistor), used by HJL in the 20 to protect the transformer, confirms this limitation, at least for me. I mentioned it earlier.

The GZ32 and 37 are often suggested as an alternative, but I recall that heater current draw is higher than a GZ34 and I think filter capacitance limits are lower still than the GZ34 family. Yes, they are rugged but I don't have room for two on each chassis!

My hum and noise problems with both my Leak ST20's were first due to the condition they were in when exchanged for a Stereo 60, I had actually been given, see details above! It took a while to get it back from Allen Wright, Joe Rasmussen finally found it for me. Good swap eh!?

Causes? poor rectifier tubes, serially - including expensive NoS, and Chinese tubes. These latter died so quickly it was like watching the fuel gauge for a Ford Cleveland 351 V8 GT motor, just more colour and movement, like watching a mini big-bang.

This did put me off them. Though I already knew that tubeophiles, like Qvortrop et al, had 'issues' with SS PSU's. Second were the filter caps, again a serial experience of leaky and noisy OEM and then NoS 'originalist' replacements, farting, buzz and hum.

I was already very wary of tube amps that farted like that. ? - barrup, BurRP baRARR... BARRA... BABOOM!!! ONE STA25 house filled with nasty smelling smoke etc, burnt out OTX and output stage, tubes, R's and C's.

[a friend had lent me his Radford amps, a pre and one each of the two different STA power amps, 15 and 25. He turned me on to valves / tubes.

One of them blew up very early AM. I had left them on! And that chassis also had farted occasionally that week. NB This was not enough to put me off tubes. Nasty and scary though it was!]

Soooo, …. , out the GZ34’s went.

BUT, I did use an array of quiet fast soft recovery diodes, snubbed, an inrush NTC thermistor, and 3 (or 4) big caps, each bypassed with a ladder of Wima MKS and MKP's.

THE SOUND!?

Ahhh .... Kvwyet they each voz, schmulik.

Bass went Bang Bang and decayed and bloomed, and shook the room, when it should. Hard headed timp sticks in HIP perf's very sudden and real. Tube bass guitar heaven, and - quiet - too.

Cymbals, well you can tell rim shotteds from without, and you can hear brushes circling on Chesky's CD's.

Now I finally could do it - biamp. One stereo amp. chassis per speaker.

Tried one channel in triode, for each tweeter, less hash, clearer and sweeter, together at the same time?!

BUTTTTT, and damn it, back came the hum, buzz, and hash, and loss of grip at LOUD!

So, thought, asked around, stopped using vertical, and went to 'horizontal'. DC heaters, using the spare 5 volt winding, and a good ceramic socket - both being for the input twin triode also helped.

Horizontal biamping ?= One amp does stereo duty in the mid/bass, now in pentode (4 filter caps) 20wpc, yes, 20 to 20k. The other is in triode mode and does stereo tweeter duty only.

Much, much better. Theories? Less recirculation of 'earthy' currents/volts. Less overload for each leg?

NB I did run a single wire between each amp, use one neg. spkr post on each amp, it helped too, try it if you are biamping.

Next? a proper zero volt reference ground plane, in the PSU for the preamp, was another big help.

If properly earthed, they don't hum now.

NB Any high impedance high gain circuit is prone to hums and buzzes from stray capacitance alone, so a bit of maintenance on leads and earth posts helps.

A real exterior ground system signal earth point is a big help too. Use thick 'earth wire' and a copper coated (at least) earth post. OZ 'soils' can be very hard so solid copper earth rods are like hen's teeth. I have three coated steel ones banged in.

BUT What do they sound like? TIMBO? eh?

Just using one amp? very much more powerful than one expected, real clarity, 'they disappear the spheres / speakers', just like AR valve amps are s'posed to do, even to Roger's models 7 and Studio1A.

Listening to good ensemble is ravishingly concentrating, try the 3 Kuijken brothers playing Haydn Trios on Accent. On Ry Cooder's Jazz LP some times there are player sitting on my right way in front of the spkrs and Ry is behind and to the outside of the Left spkr.

Depth is as deep as the venue, try Westminster Abbey and the flat trumpets and drums in the music for Queen Mary's funeral on Hyperion.

With two, biamped, as now? well the filagree stuff and nuance and clarity just get better and there is plenty of power, most of the time!

Little Feat or Led Zepp can shake the room, and the music is much clearer than ever before, and even Mahler sort of works.

The pair is sort of like a 50 watt Class A SS amp (a redone STAX DA80) but just about twice as detailed and clear and lovely, all at once.

NB I have owned the spherical speakers for a long time, in much the same pos'ns, in the same room.

Happy? Oh yes.


Timbo


Product Weakness: The transfomers may run hot.

Limited power output.

No base to stop silly fingers. No mesh guard for young fingers.

Product Strengths: Cheap and worthwhile target for tweakitus complusivus types like YT, perhaps not so available in USA?

ALL tubes are available but better ones can be used. GOOD OTXers.
The mains transformer is OK, but do watch the 5 volt heater current draw, if you do keep the tube rectifier.

In hot climates do watch them in summer! Get rid of the silly resistor fuse and replace it with some better idea. A house fire is not a good thing.


Associated Equipment for this Review:
Amplifier: LEAK Stereo 20's modified
Preamplifier (or None if Integrated): FVP based on an AWE design modded, Quad 22 box
Sources (CDP/Turntable): TT is a td150 SME3009, VdH's Denon 103D - Rotel 965 Ltd Edition Disctre
Speakers: Audiosphere Model 3's
Cables/Interconnects: mixture, VdH and short Supra spkr cable
Music Used (Genre/Selections): Catholic tastes
Room Size (LxWxH): I forget L sahped a bastard and smallish x x
Room Comments/Treatments: sides front, rear and ceiling
Time Period/Length of Audition: many years
Other (Power Conditioner etc.): 2 separate AC lines
Type of Audition/Review: Product Owner




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Topic - REVIEW: Leak St20 Amplifier (Tube) - Timbo in Oz 00:05:22 08/28/03 ( 6)