Dowser
Learning to bodge again..
That’s a bit harsh - why awful? Looks OK to me, even if not original - and maybe it sounds better? Or is it just sacrilegious to modify such circuits? I’m always modifying stuff, sometimes better, sometimes worse….
Leak TL/12 Plus / Point One Plus
That’s a bit harsh - why awful? Looks OK to me, even if not original - and maybe it sounds better? Or is it just sacrilegious to modify such circuits? I’m always modifying stuff, sometimes better, sometimes worse….
toprepairman
pfm Member
Have to agree with GT, looks pretty awful to me too.
Not harsh at all, just pointing out the facts. It has been poorly done, check out the original image that Tony posted and compare. If you are going to rebuild one of these amplifiers then at least do it, or give it to someone who can replicate it to the same standard as the original.
Here are some of the reasons:
1. Incorrect capacitor values...
2. Poor quality plastic caps used in the signal path...
3. The use of wire wound resistors for the cathode bias...
4. The use of cheap metal film resistors...
5. Incorrect value power supply capacitors...
6. Incorrectly fitted power supply resistor...
7. Poorly fitted power supply capacitors, one left flapping about in the chassis...
8. Mains wiring a mess and poorly soldered...
9. Components not centred in their position on the circuit board...
10. An extra hole drilled in the top of the chassis and an RCA socket fitted...
11. The RCA socket poorly fitted and wired up...
12. Circuit board tags bent and not straightened up...
13. Circuit clearly altered from standard...
14. An IEC has been fitted and just wired on to the original Bulgin mains socket...
15. Poor quality parts have been used which will cause a significant degradation in audio performance...
16. PVC wiring burnt and left bent and straggly...
I could go on...
Basically a butchered job and now worth significantly less money than if it had been kept standard, or done sympathetically by an expert.
Barrymagrec
pfm Member
I`m inclined to agree, it`s in the lash it together and make it work / serviceable build quality range, part of the pleasure of owning a quality vintage valve amplifier should surely be knowing that underneath it`s been maintained to the original standard, including any modifications that may have been made for perfectly good reasons.
Tony L
Administrator
For me much of the fun of owning these vintage classics is really learning about them and restoring them properly yourself. I’ve learned a lot, done things right, and could likely ask a high price if I ever sold them on. They’ll certainly outlive me!
The beauty of these amps is (any after-market chassis holes aside) they can be brought back assuming the transformers are good. If someone wanted to strip-back a modified amp to the empty tag-board and start again it can be done quite easily.
The beauty of these amps is (any after-market chassis holes aside) they can be brought back assuming the transformers are good. If someone wanted to strip-back a modified amp to the empty tag-board and start again it can be done quite easily.
So what can you suggest that I should do to improve? I hope the answer it's not just bin it or no hope. Imrve from what's could be there as it is will be much appreciated.
chiily
PFM Special Builder
Do you want my address and send it to me, I'll look after them?
Tony L
Administrator
There is absolutely nothing there that can’t be undone aside from the holes for the RCA plugs, which now done I’d just leave be. They certainly aren’t junk! You first need to decide if you are happy with them as-is. If not my first stage would be to reverse-engineer exactly what has been done to them. They have clearly been significantly modified to use a non-standard input stage. This needs to be understood and every other aspect of the circuit needs going over with a fine tooth comb as there are other changes too. Changes it needs more electronics expertise than I have to explain. Have you any idea who modified them? If so asking them to list the changes and the logic behind them would be a huge help. Regardless they could be brought right back to Leak spec should you wish.
PS Your grey amps were the final Leak spec, they have different mains and output transformers, so don’t copy my schematics and component choices exactly (mine are the earliest version, though I did alter some values around the EL84s (as is common to give the valves an easier life)).
Darmok
pfm Member
I so enjoyed following your restoration of your nice tidy pair of Leak TL12+ some months back in time.
You should have bought a head mount camera and recorded the process for vintage Hi-Fi posterity and post the footage on YouTube, I think you even get paid if you attract lots of viewing traffic.
Get in on the act, "TL, pfm FOLK."
I have watched and enjoyed many techies servicing and aligning CB radio handhelds and base stations on YouTube.
Aligning tuned circuit's, you do need sophisticated test gear, digital frequency counters and oscilloscopes higher than 20 khz.
I do think Arkless does have the correct test gear for aligning FM stereo tuners.
I can't get a good enough view of his test servicing bench to identify the kit he uses for his work.
Any ex / retired CB radio users and / or Ham, short wavers here?
Darmok, ducks and covers, rubber ducks! hehe.
Stuart'sradio
New Member
Good morning Tony L,
I've followed this post with interest as I have a pair of early "3921" leak monos.
I would like to put my findings out.
1. Early mains transformer ,3920. Later mains transformer fitted with 3920R (R=revision?)
2. I fitted 5V4G instead of GZ34. Gives a lower HT voltage and so much cheaper than GZ34.
Brings the HT voltage to the output transformer down by 25v or so.
3. Fit 470k grid leak resistors.
4. EF86 anode load resistor R5, 82k with C15, 15pf. Mine had 82k and 30pf (original components) the other amp had no cap ?
I fitted new silver mica 22pf in both.
In a post here you fitted the later Zobel network R23+C15 from the later design. Did you try both and if so any issues?
5. Steven Spicer's book- Firsts in High Fidelity, the history of H J Leak, page 154 gives a review of the design revisions that lead to the 3921/ 8778 changes.
best, Stuart.
I've followed this post with interest as I have a pair of early "3921" leak monos.
I would like to put my findings out.
1. Early mains transformer ,3920. Later mains transformer fitted with 3920R (R=revision?)
2. I fitted 5V4G instead of GZ34. Gives a lower HT voltage and so much cheaper than GZ34.
Brings the HT voltage to the output transformer down by 25v or so.
3. Fit 470k grid leak resistors.
4. EF86 anode load resistor R5, 82k with C15, 15pf. Mine had 82k and 30pf (original components) the other amp had no cap ?
I fitted new silver mica 22pf in both.
In a post here you fitted the later Zobel network R23+C15 from the later design. Did you try both and if so any issues?
5. Steven Spicer's book- Firsts in High Fidelity, the history of H J Leak, page 154 gives a review of the design revisions that lead to the 3921/ 8778 changes.
best, Stuart.
Tony L
Administrator
I didn’t try both. As I understood it the original design (and the first S20s) was notoriously hard on the EL84 and Leak modified any amp later returned for service. As such I just built mine with the later values as it was, to my understanding, a Leak revision to correct an error in the original design. I’ve no idea regarding sonic differences.
Stuart'sradio
New Member
Yes, early S20s and TL12+ all were run too hard with 350v HT and 1m grid leak resistors, the later revision cured this.
I feel that the addition of the Zobel network better suites the later 8778 output transformer?
I have not seen any document Leak or other wise that gives the definitive view.
Perhaps we will see on here?
I feel that the addition of the Zobel network better suites the later 8778 output transformer?
I have not seen any document Leak or other wise that gives the definitive view.
Perhaps we will see on here?
Tony L
Administrator
Apologies, I’ve only just noticed this post!
I didn’t change R23/C15. My TL12 Plus don’t have R23 at all, they predate it! C15 is untouched. My only changes are detailed on the bill of materials upthread here. To the best of my knowledge the couple of changes I made were Leak approved and purely to give the EL84s an easier life, i.e. they did this to any amps returned for service. There is very little altered here.
Tony L
Administrator
Just a small update to this thread as the 270 Ohm resistors (R19 & R20) have been bugging me for a while as I massively over-specd them on this rebuild without understanding they were designed to be sacrificial. As such the last time I ordered from HiFi Collective I bought some 1W Takman carbon film to replace the 5W Mills I’d originally fitted. They are spaced off the board by about a cm and I’ve left the baseplates off for a while so I can keep an eye on them for a while, though they should be fine. The original Leak spec was apparently 1W so they should be fine. It’s been playing on my mind for months after reading (Toprepairman I think) saying you should never use higher Wattage here, and especially not wire-wound, so 2xD’oh there. Listening to the amps now and they sound superb, but they always do.
I mentioned in the ‘interesting eBay listings’ room a few days ago that someone was making steel baseplates for vintage Leaks. Anyway, as evidenced by the picture above I bought some for the TL12 Plus and my Stereo 20 as it finishes them off, gets them a heck of a lot closer to current safety regs etc. They fit pretty well, I had to finagle the Stereo 20 one a bit (I needed to drill the holes out larger), but the TL12 Plus fitted without issue, look neat and solid, and I doubt they impede ventilation too much. I’d definitely recommend them as they protect the tag boards and components from accidental damage plus end users from nasty jolts (capacitors in a valve amp can hold some very high voltage for quite some while even if disconnected from the mains). I’d also far prefer to pack a Leak for shipping with these bases fitted. Far less prospect of damage. Here’s an eBay link.
PS I was surprised they fitted my TL12 Plus so well as the F&T dual cap I fitted (pics upthread) does hang down a mm or so, but luckily their mounting band could be nudged into one of the ventilation slots! Everything is a nice flat and solid fit here.
Are these base plates still available for the Stereo 20? The link is no longer active.
regards
Kevin
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Changing the grid resistors from 1M to 470K will have almost no effect on the amplifier. These are fitted to prevent the grid from "floating". They simply tie the grid to audio earth through each resistor, post the coupling capacitors to keep the output stage stable. They have no effect on the current draw of the EL84, unless the higher value causes the output stage to go unstable. I've not come across this. The only things that can effect the EL84 current draw is the HT voltage (higher voltage will cause the valve to draw more current) and the amount of standing current the EL84s are set to, which is set by the 270R cathode bias resistors. Lowering these resistors will cause the valves to draw more current so keep these at 270R.
Don't forget these amplifiers were originally designed to power a preamp too so that will draw current from the TL12+'s power supply. Check on the diagram as there should be 2 voltage values. One for when the preamp is connected, normally the lower value, and one where the pre-amp is not connected, which is normally the higher voltage value.
Tony L
Administrator
Not seen them listed for a while, and to be honest I can’t remember the seller. IIRC Will at @Radford Revival made some, so may be worth dropping him a PM.
toprepairman
pfm Member
Valvepower.uk is their home address, still there.
Base plates still available but may be currently out of stock.
Should be available in 3/4 weeks' time.
Base plates still available but may be currently out of stock.
Should be available in 3/4 weeks' time.
Tony L
Administrator
Thanks to a very kind pfm member (thanks Steve!) I was able to secure one of the Point One Stereo preamps that was up for grabs recently that the seller refused to post. No cracks on the faceplate or knobs, a very nice tidy example I’ve done nothing more than rob a set of rubber feet off a mono one I had knocking around and clean it up for its photo:
The inside is very tidy too.
It dates from 1959; there is a stamp on the case (I think from the case manufacturer) from April ‘59, and a sticker suggesting August ‘59 assuming I’ve interpreted it right. It appears to have been re-valved at some point as there is a pair of solid-plate Mullard EF86 from 1965 and a pair of mesh-plate from 63. I can’t see any obvious signs of any rework to the circuit.
I do certainly plan to get it up and working at some point out of curiosity. I do have a Leak octal lead somewhere. I can’t see it worrying the Audio Synthesis, but it will be fun to have the full amp working. I don’t think I’ll do anything more than replacing the worst looking electrolytic caps, I’m not going to fully rebuild it. It looks very clean in there and all the ‘mustard’ caps will be fine.