hifi engine

Overheating EL84 - Leak TL/12Plus

Firstly, many thanks to HiFi Engine for the wiring diagram for this amp.

Valve V4 of this mono amp is overheating. Following a suggestion by GeoffH about another valve amp with a similar problem, I checked the grid resistor (it's OK) and I have replaced the cathode electrolytic but with no success. I then set about swapping components between V3 and V4 to see if I could switch the problem from V4 to V3 and thus isolate it. Again, no success. The voltage at the V3 cathode is around 15v but it gets up towards 40v on V4. (On a second TL/12Plus I have, there is also a discrepancy but it's not as marked - about 15v vs.25v.)

Any suggestions would be appreciated.



krokkor's picture

Hi, sorry but i need to

Hi,

sorry but i need to require you to check again some resistors and particulary (pls see diagram on the manual you can download from this site) R18, R16, R19; this measurement should be done with no power and with the end stage tube pulled out from the socket (you can compare your results with the matched resistors of V3 - R21, R17, R20). Then if all resistors are OK as show in the diagram, check if the node between R16 and R19 is good connected to the ground (i think that this point should to be connected to the chassis ground). Reinstall the end stage tube(s), power on the amp and with a VTVM check if on the grid of V4 is present a voltage (in normal condition this point should to be at the same potential of the ground, so 0 volts). If a voltage appears on this electrode may be the capacitor C10 of the phase splitter stage is leaked and need to be replaced. I don't think that V4 was shorted or have some problems on his internal insulation 'cause you say that swapping V3 and V4 the problem stays again in the same circuit's section (if i've correctly understood).
Another problem you can have (i hope not) is that the screen grid connection of the Output transformer is shorted with the plate connection (this way the screen will increase of 20-25% his voltage and causes a current augmentation inside V4).
Sorry for the "Italienish" (i hope you understand about the procedure for fixing).

Have a nice hunt.

Krokkor

Follow up

Krokor

Many thanks for your suggestions which I can understand fine. I should be able to go through the checks you suggest tomorrow (28 July) so will let you know what happens after that.

Best regards

sjp

krokkor's picture

Follow up

Another little thing: if C10 will be found defective (voltage present on G1 of V4) you can check the end stage simply disconnecting the two input capacitors of the phase splitter from the grids of the end stage tubes. After the disconnection of this 2 capacitors the situation on V4 shall to return normal and the 2 EL84 will have the same voltage on theirs electrodes. The capacitors disconnection don't causes damages to the amp (simply you'll have interrupted the signal to the end stage).

Regards

Krokkor

Success!

Well, it was a leaky C10. In fact, C11 was also leaky to some degree, as were the same caps on my other TL/12 Plus, although less so. Not sure why switching the caps didn't switch the problem to the other valve. May be there is something else that made the V4 side particularly sensitive to the leak?

Anyway, I have been running it for an hour now and all is well - only heater glows to be seen! Many thanks for you help - much appreciated.

By the way, I notice that R16 and R17 are 1M (as in the schematic) in the older TL/12 Plus but 470K in the later model (which is exactly like the one on the home page - black and with a side mounted fuse). The 470Ks are clearly original equipment, judging by the soldering. You wouldn't happen to know why Leak made this change? Is it worth modifying the earlier amp if it provided some sound or stability benefit.

Thanks, once again for the advice.

Best regards

sjp

krokkor's picture

Success

I'm happy for you and your fine amp. The reasons of the difference of the grid resistors between the previous model are known just to Leak technicians but in both cases the low pass frequency of the end stage input is lower than 2 Hz.
About the difference after switching the caps i think that heating the capacitors with the iron something changes inside the components and this may causes the different way to show the fault after the swapping.
I would to recommend you to replace both capacitors with a matched pairs components (or just identical in brand, model and characteristics on the body).

Enjoy your amp.

Cheers from Italy

Krokkor