Leak TL12 point one

[maninashed]

I'll have to do this in several parts,so please bear with me. Its always a bit of a dilemma with a restoration just how far to go, or rather how far to take it back. I'm of the opinion, that if a thing looks fairly original and 'honest' it then looks more 'period' and therfore in keeping with its age. I aquired this Leak amplifier in pretty rough and abused condition. Some hack of a tech had 'had a go' at 'fixing it up'. The KT66s had been replaced with Chinese EL34s and the valve associated passive components altered to suit the EL34s. The original reservoir/smoothing caps are all confined to one large metal box on the chassis and this had leaked badly covering the entire chassis and under-chassis in a horrible greasy mess, the only plus side being that it had protected it from rust! At some point there had been a burn up under the chassis and the valve bases for the KT66 had been replaced. From the information I had, the amp hadn't been powered up for about 25yrs so I didn't know that the transformers and choke were still OK, as it happened, fortunately, they were all fine...phew!
I decided to rebuild her, but not go over-board, so this is how I went about it. Attached are some photos of the amp before the repair, it doesn't look that bad in the photos, but believe me, it was bad!
The previous tech had disconnected the reservoir/smoothing capacitor block which on this amp are non polarised capacitors (non electrolytic) with electrolytics, which were beginning to bulge. I decided to reverse this and restore it to as near original as possible. Next part to follow...

[maninashed]

Part II. The first job was to remove the electrolytic capacitors and then the leaking black capacitor can, then followed a good clean up and degrease using white spirit and solvent degreaser with a toothbrush and kitchen towel. Someone in the past had used a more powerful solvent, probably acetone based and damaged the laquer in places but it wasn't too bad. I then set about dismantling the capacitor can and removing the contents. The seal on the underside was neatly cut open with a Dremmel and cutting disc this revealed the contents! (see photo). The connecting wires were cut and the contents prised out with a screwdriver after first warming it up with a hot-air gun.This has the effect of melting the wax, the contents are then much easier to remove and the chance of damage reduced. The can was washed out with solvent degreaser. I decided to use non-polarised capacitors rather than electrolytics, although they are physically huge compared to electrolytics of the same value!. The main advantages are that they are more reliable, longer-lasting and don't dry out like electrolytics. My choice was non-polarised motor-run capacitors, very robust and high AC voltage rating. The capacitor can is quite large 4.5"x4"x3.5" so hiding these was not going to be a serious problem. They were packed in using foam and hot melt glue, this has the effect of sound deadening and the can now 'sounds the same' when you tap it, and the contents can't vibrate. (The +/- marks on the caps was just to assist me when wiring up) A paxolin panel was used to help with insulation from the contacts when the can was inverted. I kept the reservoir capacitor the same value as the original 4uF, but increased the smoothing capacitor from 8uF to 20uF. The edges of the can were then cleaned up and tinned, and the lid with the terminals then re-soldered into place. After another degrease the can was repainted with a sympathetic finish.

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Some more photos

[maninashed]

The next stage was to refit the capacitor can and then have a look at the rest of the amp components. All the TCC capacitors were replaced as they were all leaky to various degrees. They were replaced with good quality polyester 1kv caps to provide reliability particularly to the output valves. Someone had fitted 680R cathode resistors instead of the original 600R ones so these were replaced along with the cathode bypass electrolytics and any out of value resistors.
The output valves were replaced with 2x KT66's, (one of the EL34s was short unfortunately) I decided to use a couple of JJ branded ones as these are reasonably priced compared with originals and they will 'do' until I can find some NOS British ones (it's always better when you are not in a rush to find these things!) A replacement GZ32 was also sought and acquired from a kind forum member. The EF37 and ECC33 both tested fine and after a thorough check over, I decided to power her up and I was astounded by the results. The amp, although mono, sounds absolutely superb and so stunningly clear and precise. there is absolutely no discernable hum or distortion. (OK, I dont have the necessary equipment to test this amp to specification, but to my ears, it sounds wonderful). A lovely piece of British audio history!
Bill

[Aub]

Nice job Bill. You just need another one now to go stereo.


Aub

[maninashed]

Thanks Aub, yes I know!

Bill

[John_BS]

Looks really very neat, Bill. Good restoration.

John

[HamishBoxer]

Very nice,i have that pre amp but not a point one.

[GrimJosef]

Originally Posted by Aub

Nice job Bill. You just need another one now to go stereo.

Or you need to sell this one and buy a very nice stereo amp as a replacement. You might get a second hand Radford STA15 for the price of this.

I'm currently trying to sort out low-level hum on the output of a TL/12.1 that I have. It's at 100Hz so it's coming from the HT supply and it's at least 10dB above the spec (I do have a spectrum analyser so I can measure it). In a quiet workshop I can hear it through my Goodmans test speaker. What I don't know is if the level of ripple on the HT is the whole cause of the problem, or just part of it, or if in fact it's a distraction i.e. it's always at the few volt level and there's something more subtle causing it to affect the amp's output more than usual.

The sliver lining I suppose is that by working my way through this I should at least end up with a better understanding. And a quiet amp of course .

Cheers,

GJ

[maninashed]

GJ, the 100hz hum you are experiencing is probably eminating from the main smoothing can, have you checked the ESR of the capacitors within? Disconnect all the wires first, to get a true reading, anything more than 'didly squat' is bad!
This amp is now silent, no hum whatsoever, Im using a matched pair of KT66.
Just something to check first though, make sure that one of the output valves isn't going low emmision, as you are probably aware, any inbalance in the output pair wont cancel the 50/100hz derived hum.
Thanks everyone for your comments, I appreciate it, good or bad. To me its a case of I dont want to go overboard with a restoration and make it look pristine and totally false, its a 60yr old amplifier, which in the day, was never intended to be 'on display', but stuck in the back of a cabinet controlled by a pre-amp. However I would quite happily have this on display in my living room now(esp if I had two!) Im using a Leak Stereo 20 which I adore.
Regards to everyone
Bill

[GrimJosef]

Thanks Bill, I have to say my money's on the smoothing block too. It's the original one which in this case is one of the rarer TCC 'lipped' variety. If I strap a brand new 16uF across the 8uF smoother I can roughly halve the HT ripple. But actually that still doesn't get the hum into spec. So I'm wondering if the 4uF reservoir is in a poor state too. I don't want to risk a 500V electrolytic on that so I'll have to find a reliable 600V or 800V paper-in-oil or maybe a handful of 0.47uF 630V polyprops in parallel before trying a more careful test later today.

I have gone to some trouble to match up the KT66s. The voltages across the cathode resistors are 38.3V and 39.1V so assuming the resistors are the same value (note to self - go and check) the valves are running total currents which differ by just a couple of percent. However in the literature Harold Leak does boast that the TL/12.1 can be got within spec without having to match the valves. Of course that was in the days when valves generally came new out of the box. So they were likely to be reasonably closely matched in any case.

Thanks for the advice,

Cheers,

GJ

[maninashed]

GJ, it will probably be the 4uf reservoir cap, just disconnect it and temporarily wire in a 4uf motor-run cap, they are rated at 630v dc, it will prove the point at little cost. When preparing to restore mine, I checked the esr of the original set of capacitors, even though they had been disconnected, The esr of the 8uf was about 1.5R, whereas the same value motor-run cap hardly registered. It doesn't sound a lot, but the values of the caps in this amp are very low, and anything under par will give hum. The original 4uf reservoir on mine was o/c. It's a big decision to open the can up, but in my case it was leaking badly, so that made my mind up really.
Bill

[GrimJosef]

OK, I disconnected the old 4uF reservoir and 8uF smoothing capacitors and replaced them with modern ones (4.2uF polypropylenes and 16uF electrolytic respectively - I didn't have any spare motor-runs available). It has helped the hum very considerably and it is now in spec although still not by much (spec is 74-84dB below 10W and I'm measuring 76dB below). I still have to replace the final 4uF smoother though.

Anyway I thought I'd mention the distortion while I'm about it. The spec says that the THD will be less than 0.1% at 12W output. And indeed it is. The attached poor quality (sorry) pic of the output spectrum shows the fundamental at 1kHz, the 2nd harmonic below 0.02% (so my output valves really are well-matched) and the 3rd harmonic just above 0.03%. But the 100Hz power supply noise is still making its unwelcome presence felt. You can clearly see 100Hz cross-modulation satellites either side of the 1kHz fundamental and they are 10dB above the worst of the harmonic peaks ! So the total distortion is not less than 0.1% despite the fact that the total harmonic distortion is.

Cheers,

GJ

[maninashed]

GJ, glad you are making progress, I bet the 1st stage electrolytic is high esr also. Might be worth trying a different GZ32, or a GZ34( higher current rating, lower heater current) just in case the rectifier is struggling a bit. Have you replaced the grid coupling caps on the KT66s ? Both mine were very leaky.
Bill