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HH Scott LK48

snowman_al

snowman_al

pfm Member
So something completely different on the 'bench' at the moment.
One of HH Scott's finest kits the LK48, equivalent to their 222C or 222D. Fixed bias EL84 (7189 really) amp from the mid 60s.
No history, but the right channel output transformer is primary to secondary short and some previous owner had clipped the HT+ (or should I say B+) wiring looking for it? Also the small 20Ω 'Standee' resistor had cracked and lost its silica filling all around the chassis.





Whoever built this originally has made a super job of the wiring and component dressing, it looks better than the factory built examples I can find pictures of on the web.
 
Oooh, nice! They are allegedly very good sounding amps, quite a following over at Steve Hoffman’s site IIRC. I never realised they made kits, only ever seen pictures of factory-built ones. As you say it looks really neat, far neater than I expect from a Scott. I know people argue neatness/aesthetics “don’t matter”, it certainly makes it easier to figure out what’s where IMHO.
 
So some progress.
All the valves test okay. The 4 x 7189s are Telefunken as are the 4 x 12AX7s, and probably original. The 5AR4 (GZ34) is branded HH Scott and has date code 3563 so I assume from August 1963. The 2 x ECF82s are obviously not original.
I repaired the 'Standee' with fire cement, filled the whole inside space and formed a smooth-ish curve on the outside. Then I cooked it a 200 degrees C for an hour in the oven...




The fixed bias supply and DC heater supply for the 4 x 12AX7s comes from the 55 volt winding on the mains transformer. The selenium rectifier was very tired with only 34 volts DC at the 12AX7 filaments (they are in series pins 4 to 5 so should be getting about 48 volts.) so that came out and rather than fit a new silicon bridge rectifier, I pulled the old one apart and fitted 4 x 1N4004s back in the package. That resulted in more than 50 volts for the heaters, so I replaced the original 10Ω dropper resistor with a 22Ω replacement. Heater volts now about 48 volts.



Once the Standee was cooked, I re-fitted it and re wired the HT. I fitted a 10kΩ in series at first and let the old can caps re-form for a good hour, the I dropped a 500Ω in there for another 30 minutes. All good, nothing getting warm and time to try without. So far so good.



Time to plug in the valves and do some voltages tests...
 
I'm always impressed by your work Alan.

I like to work on my own kit but if I didn't, I'd want you to do it for me!
 
Mike P said:
I'm always impressed by your work Alan.

I like to work on my own kit but if I didn't, I'd want you to do it for me!
Click to expand...

The Stereo 20 I bought from Alan that he restored was the quietest valve amp I’ve ever owned. Quieter than my 303 or any of the Croft valve/MOSFET power amps I’ve had.
 
Thanks guys, very kind, and yes both I and the Scott survived switch on...
In fact the left channel actually worked okay-ish straight off. (There is no output transformer in the right channel until I can get it re-wound.)
Fortunately all the controls are working with no crackles or pops and feel nice as well. The switches too all work as expected - they will get a dose of Servisol later.

So there is a fair amount of rebuild information on US web sites and YouTube, they all slate the white 'Ceracap' coupling caps as unreliable, many calling them wax paper caps. Umm, in fact they are film caps, no wax and no paper. So far the 0.1uF ones have all measured perfectly and no leakage. I'm still going to replace them



There are however plenty of way out of spec resistors, 40-50% in some cases. I'm rooting them out as I go round.



So the output pairs, the phase splitters and pre-amp / tone control sections have been gone over using what ever is in the spares box at the moment. Just short of a couple of 4uF 250 volt electrolytics for the 6U8A stage so the orange original is still there.



Its been on (upside down) for over 2 hours yesterday and into scrap speakers and it actually does sound rather better. It does not even run very hot, but it is missing the right cannel output valves sorry tubes...
 
I'm waiting for several parts at the moment so have had an hour on the cosmetics this afternoon. Front panel and knobs remove and cleaned and polished. I've polished the 4 valve covers and cleaned the slide switches with Autosol.

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I've also given the chassis a better clean and given the external case a really good clean. (No picture yet) And added an Earth connection as it has a 2 pin US plug fitted, it is a 117 volt unit.



I have to say these are superbly built amplifiers. The quality of the chassis parts are top notch. It weighs nearly 17kgms! Considering it is nearly 60 years old it has faired very well.

I keep forgetting to post a link to the schematic http://hhscott.com/pdf/222D.pdf
 
Quick update here on the Scott.
The rewound output transformer has returned and fitted.
We now have a stereo amp. It does have a lot of 'punch' for an EL84 amp, really solid bass and clear high frequencies, though I haven't put it on my Dynaco 25s yet.
My 120 volt dropper transformer is too puny and drops to below 110 volts with both channels driven so I loose 20 or so volts HT...
A bigger dropper is ordered.

 
snowman_al said:
I keep forgetting to post a link to the schematic http://hhscott.com/pdf/222D.pdf
Click to expand...

Interesting how it's the 4 ohm tap that's grounded, I've never seen that done before. Means, at least for 16R, the LS output is balanced. Presumably it's been done to allow the centre speaker function which could help overcome the 'hole in the middle' problem that everyone was paranoid about in the early days of stereo.
 
This has been one of those slow renovations...
But today the Scott is back in her case and playing in the lounge into the Dynaco a25s.
Had a new 1000VA 120 volt step down transformer that does not dip below 117 volts. That showed the original output valves (4 Telefunken 7189s) would not bias up all together. One of the four is a little down and unbalanced the whole bias 'system'.
So I had to order a set of Russian 6P14P-EVs from Ukraine. Super fast delivery considering.
Installed and set the bias to 44mA per side, yes spec is 22 to 23mA per valve, they run well under their rated dissipation for a long life and, comparatively, cool running.
Then I decided to fit a proper US 3 pin mains lead in place of the contrived 2 pin with external earth.
A few days of testing, with an eye on the bias and all is well. You have to take the case and bottom cover off to monitor the bias properly, so I wanted it stable...







And how does it sound?
Stunning is a good summary. It can easily match the Radford STA15 for volume, louder in fact.
So much bass punch even at low levels and treble is sweet and not sharp.
Need a few more days to get a feel for the sound stage.
I understand why these Scott amps have such a good reputation in the States. Ooo and tone controls that work really well too.

Alan

Album here: https://ibb.co/album/vZdP16
 
Lovely job Alan.

I also bought a spare quad of 6P14P-EV from Ukraine recently. I figured it was a good time to support Ukrainian sellers. Delivery was about 2 weeks.
 
Well done Al, that's a very good job you've done there.
Rewinding the OpTx must have been fun. The primary must have been fairly high impedance (guessing10 to 12k) as It's class A/B.
 
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