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Old 28th Mar 2023, 12:19 pm   #1
vishalk
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Default Help with a HHSCOTT LK-72

Hi All

So it's been a while and the tube bug got hold of me.

I've been searching for one these amps for ages and I finally managed to get one!

It was US voltage but a member on here "Ed Dinning" kindly rewound it for me to work at 240V.

Now when I got the amp it was quickly powered up to be shown working (using a step down transformer) after that I didn't really do anything to it.

It uses 6L6GC tubes the original tubes were 7591.

I have just refurbished the amp externally, replaced the Filter Capacitors and installed the rewound transformer.

This is where I need help, it's been along time since I've worked on Tube amps.

I can safely measure voltages, check what I need to (one hand in my pocket) I have a Variac and dim bulb tester etc.

I just need help on what to check for without tubes and with tubes.

What has thrown me is that there are lots of part changes and things added, I'm guessing whoever done the work modified it to run 6L6GC tubes.

Once I can clarify this and all is safe I can then put the tubes in and get it running.

I then need to figure out how to balance and bias it to.

Please see attached pictures.
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Old 28th Mar 2023, 12:28 pm   #2
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Default Re: Help with a HHSCOTT LK-72

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I put new decals, polished and cleaned everything. Just got to sort the volume decal out (common to rub away).
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Old 28th Mar 2023, 12:31 pm   #3
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Default Re: Help with a HHSCOTT LK-72

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Old 28th Mar 2023, 12:34 pm   #4
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Default Re: Help with a HHSCOTT LK-72

hfe_hh_scott_lk-72b_schematic.pdf

please see attached schematic, there are two types i believe lk72A and lk72B.

I think this one i have has been modified similar to lk72b?
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Old 28th Mar 2023, 1:52 pm   #5
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Default Re: Help with a HHSCOTT LK-72

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Something doesn't seem right here, I've switched the amp on without tubes (no pops or smoke).

Checking the rectifier I get 75v DC.

This then goes to the C1 capacitor. Surely that voltage is too high?

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From what I read there is supposed to be a 33ohm 5w dropper resistor. From the rectifier it's wired to the capacitor. You will also notice the bias adjustment pots (2 x 100k trimmers).
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Old 28th Mar 2023, 2:02 pm   #6
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Default Re: Help with a HHSCOTT LK-72

Quote:
Originally Posted by vishalk View Post
Checking the rectifier I get 75v DC.
If you're referring to the DC supply heater supply, the voltage will be high with no valves fitted as there'll be no load on the supply. ie no volt drop across R206 through to R209.
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Old 28th Mar 2023, 2:30 pm   #7
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Default Re: Help with a HHSCOTT LK-72

Quote:
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Quote:
Originally Posted by vishalk View Post
Checking the rectifier I get 75v DC.
If you're referring to the DC supply heater supply, the voltage will be high with no valves fitted as there'll be no load on the supply. ie no volt drop across R206 through to R209.

Yes I got that, just the way it's wired is different to the schematic and instructions. Also from research I've read that changing the original selenium rectifier makes the voltage increase so a dropper resistor is needed.

Just want to make sure what the voltages should look like before fitting valves.
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Old 28th Mar 2023, 2:45 pm   #8
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Default Re: Help with a HHSCOTT LK-72

You mention C1 which as far as I can see is nowhere near the heater power supply. Do you mean C203?

You could insert the valves then use your Variac to slowly increase the mains voltage while you monitor the heater voltages of the valves fed from the DC supply.
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Old 28th Mar 2023, 4:09 pm   #9
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Default Re: Help with a HHSCOTT LK-72

hfe_hh_scott_lk-72_schematic.pdf


I've also attached this schematic as it seems to be the original to what I have, I think the prior tech installed these extra 100k trimmers to bias the valves.
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Old 28th Mar 2023, 5:46 pm   #10
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Default Re: Help with a HHSCOTT LK-72

One diagram is dated 10/18/60 and shows fixed bias. The other is dated 8/28/63 and shows variable bias.

As you say yours is probably the earlier version modified to match the later diagram.

DC heater voltage is shown as 43VDC across the four heaters in series. As I said you could supply the amp with a Variac and check that the heater voltage rises to 43 Volts as the input voltage is slowly increased to the correct value.
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Old 28th Mar 2023, 8:45 pm   #11
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Default Re: Help with a HHSCOTT LK-72

Blimey, over 450v on the anodes of the 6L6s. We're into EL34 territory. It obviously worked ok back then though.

Good luck with it.

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Old 28th Mar 2023, 8:46 pm   #12
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Default Re: Help with a HHSCOTT LK-72

Ok so i out my DMM on pins 4&5 on the 12ax7s, all of them are in.

I used my variac and as suspected at 189vac I get 6.3v on the heaters! I dare not increase the voltage any more because clearly this is not right.
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Old 28th Mar 2023, 8:51 pm   #13
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Default Re: Help with a HHSCOTT LK-72

Have you checked the resistors in the series chain?
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Old 28th Mar 2023, 9:00 pm   #14
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Default Re: Help with a HHSCOTT LK-72

Shouldn't there be 12.6 volts across the heaters with the two sections wired in series? Pin 9 is the centre tap.

That would fit in with the 43V shown on the circuit diagram, with the heaters being under run.
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Old 28th Mar 2023, 9:34 pm   #15
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Default Re: Help with a HHSCOTT LK-72

I think you maybe right? Sorry I just checked all of them are in series.

Either way after rectification the voltage is 75vdc.

I should be seeing 43vdc. So something isn't right.

I'm a little over my head on this one, going to have to take a step back. Go over the whole thing and check it over.
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Old 28th Mar 2023, 9:42 pm   #16
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Default Re: Help with a HHSCOTT LK-72

Actually anyone in the north london area or I don't mind travelling willing to help out, happy to pay just to have someone look over it and teach me a few things.

In the meantime I need go over the schematic and check the wiring, also because it has 6l6 tubes surely the pin outs have pin changed to suit?
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Old 28th Mar 2023, 9:48 pm   #17
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Default Re: Help with a HHSCOTT LK-72

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Either way after rectification the voltage is 75vdc.
That's because the supply is unloaded. We've been here before.

12AX7's can be wired with the heaters in parallel which requires 6.3V, or with the heaters in series which requires 12.6V. If there's no connection to pin 9 the heaters are in series and there should be 12.6V across pins 4 and 5.

Parallel strap pin 4 to pin 5 and apply 6.3V between pins 4 and 9.

Series apply 12.6V between pins 4 and 5 leaving pin 9 unconnected.
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Old 29th Mar 2023, 9:03 am   #18
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Default Re: Help with a HHSCOTT LK-72

Hi Graham

All four 12ax7s are wired in series, I had the tubes in so it was loaded.

Measuring at pins 4&5 at 243vac I get 9.63volts. I just assumed it should be around 6.3 volts.
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Old 29th Mar 2023, 9:28 am   #19
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Default Re: Help with a HHSCOTT LK-72

I too initially thought that you should see 6.3V, but then I realised this didn't fit in with the 43V shown on the diagram. This suggested that the two halves of the heater should be wired in series. In total there are 8 heater halves in series across the DC heater supply along with some series resistors.

You did well to be cautious.
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Old 30th Mar 2023, 9:53 am   #20
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Default Re: Help with a HHSCOTT LK-72

9.63 VDC on the heaters seems a bit low. It should be more like 43/4 VDC which is close to 11 VDC.

You mention changing the rectifier to a silicon type and adding a series resistor to compensate for the reduced forward volt drop of silicon. If this mod has been carried out it's possible the series resistor has too high a value. The value needed is best determined by trial and error.

If the mod hasn't been carried out it's possible one of the series resistors has changed value.
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