Leak TL10 power supply modification

[Herald1360]

Originally Posted by snowman_al

Good, glad it is near normal!
Do not worry about the voltages being higher than on the service sheet. They are quoted using a meter with a resistance of 1000 ohms per volt. (Today's digital meters are in meg ohms per volt range.) When measuring round the EF86 their meter would be drawing more current from the supply than the valve. And yes you have reduced that slightly too with the revised values.

Not megohms per volt (even the hi-z option on some of the classier bench DMMs' lower FSD ranges still only offer fixed loading in excess of 50meg or so) but a fixed input resistance of 1meg (typical Chinese cheapy) to 10meg (any half decent meter).

An AVO 8 on its 500V range is as good as a decent DMM for checking HT level voltages as far as loading goes. (It may not be as accurate or precise.)

[qualityten]

Thanks again Alan. I keep forgetting to get out the Avo.

It looks like the noise is coming from the EF86 area, which still has mostly original components.

The internal speaker connections are original, as you describe. When pairing the amp to test with another mono Heathkit MA12, I found that the phase needed to be changed.

I’m going to move on with tidying up the wiring, trying cable lacing for the first time, and first wondering how to repaint the chassis, which has deepish scratches on top and some corrosion at the back.

Looking around B&Q and Halfords I haven’t yet found the right colour.

[qualityten]

Thanks Chris. I didn't see your post while replying on my phone. I have an AVO 8 and need to keep it in sight!

[qualityten]

Here is an oldish thread which mentions the same Leak amp bring used in a Bal Ami jukebox: https://www.vintage-radio.net/forum/...ad.php?t=27749

[Valvepower]

Hi Qualityten,

Thanks for the link to the other TL10 thread on the forum.

When I restored my TL10’s I used two separate cathode resistors – it made sense as I had to replace the resistor(s) and capacitor(s) anyway.

Regards the speaker terminal I could take a picture of the wiring of speaker terminals on one of my TL10’s. I use the 8 Ohm tap with modern speakers.

Finally, it looks as if the TL12.1 could be an early juke box amplifier, but I suppose I’ll never know for sure. I did ask an engineer friend who worked for Leak in the early to mid-60’s and he couldn’t help either. He told me stories of TL12.1’s in the Pirelli Tyre factory PA system and TL25+’s in Abbey Road, but couldn’t identify my TL12.1

Regards
Terry

[qualityten]

Looking more closely at my unit, I think that the holes are an aftermarket modification, but done for Bal Ami. The original 117V transformer suggests the jukebox was originally used in the US.

The way they crammed the volume control, preamp and tone controls into the tiny space where one octal socket was is amazing. Ironically, I think the amp may have been set aside. because the RCA input socket broke. If only these old amplifiers could tell their own stories.

[snowman_al]

Where Terry's TL12 was clearly drilled 'after' manufacture, I wonder if your chassis has paint or signs of paint inside the extra holes? That might indicate it was made for the purpose. (As has been mentioned, Leak made amplifiers for different markets and the chassis were usually manufactured to suit.)
You would best take it to a motor respray firm to match the colour. I am not aware of anything off the shelf that comes close to the original gold paint.

I am not up on jukeboxes, but aren't a lot of them or the mechanisms, 117 volts even in the UK?

Alan

[qualityten]

Alan, looking at the chassis in artificial light last night suggests that the holes on mine were drilled after manufacture. I'll have a look in daylight (when me and the chassis next meet in the daylight!) and double check.

I have no first hand knowledge of jukeboxes at all....

[Radio Wrangler]

Originally Posted by qualityten

Looking around B&Q and Halfords I haven’t yet found the right colour.

Just over the Pennines from you, I use Wood Auto Factors in Colne Road Huddersfield for paint mixed to match or mixed to spec. I've used them for over 50 years. They also make up aerosols. They do cellulose strictly for vintage vehicle restoration, of course. also acrylics and some 2-packs.

David

[qualityten]

Thank you for the steer David!

[qualityten]

I've finished my restoration of this amplifer. I used gold hammerite paint, so I'm not sure whether it may be considered sympathetically done or not! Wood Auto Factors were unable to supply matched paint with a posted sample.

I have one further request of any forum member who owns a TL/10: please can you help me with the physical grounding locations. As I received this amplifier with a non-standard power transformer, the wiring loom had already been disturbed. In wiring it up, with some replacement wire, I have ended up with one additional black ground wire, and a very soft hum. With the thick Hammerite paint, I only have the chassis grounded at a chassis point between the input RCA socket and the EF86, with a connection going underneath the tag-board from there to the first of the four filter capacitors.

So, if anyone with a TL/10 could describe for me the 'as original' grounding points, that would be very helpful. Thanks.

[qualityten]

Revisiting this build, I noticed that there are two different values given in different places for the coupling capacitors C7 and C11.

Please can I have advice on this. Should they be 0.1uF (as per the nice clear circuilt published by the Australian Radiotronics journal, Vol 20 Dec 1955, No 12), or 0.04uF, as per the more common circuit, also found on the 44bx.com Leak website?

And, is the difference likely to be audible either way?

I'd also be interested to hear any others' experience of replacing the original carbon composition resistors with metal film ones. I know it's a well-trodden subject, but I found replacing them with cheap Chinese metal film ones made the amplifier sound a bit brighter/harsher.

[snowman_al]

Originally Posted by qualityten

Revisiting this build, I noticed that there are two different values given in different places for the coupling capacitors C7 and C11.

Please can I have advice on this. Should they be 0.1uF (as per the nice clear circuilt published by the Australian Radiotronics journal, Vol 20 Dec 1955, No 12), or 0.04uF, as per the more common circuit, also found on the 44bx.com Leak website?

And, is the difference likely to be audible either way?

I'd also be interested to hear any others' experience of replacing the original carbon composition resistors with metal film ones. I know it's a well-trodden subject, but I found replacing them with cheap Chinese metal film ones made the amplifier sound a bit brighter/harsher.

C7/11 should be 0.1uF (as is C5).

Not sure I have noticed such a marked difference with MF vs carbon resistors. Though they are a little brighter. I always use metal film to the anodes to keep noise down. Interesting to hear other opinions.

[PJL]

Old carbon composition resistors drift in value, are intrinsically noisy and crackle and pop so there is no knowing how they might be behaving, so any audible differences after replacing resistors is likely to be because the new metal film resistors are working as the designer intended.

[qualityten]

Thanks, Alan and PJL, for your replies. Was Leak known for modifying circuits? The H H Scott amplifiers from the USA are often confusing because of circuit variations for the same model number.