I had a pair of Leak TL12+ UL PP amps which I modified with new components (resistors/capacitors). The EF86 first stage is triode connected - the link at the junction R6-C3 is connected to pin 6. The circuit is here: http://www.44bx.com/leak/Leak/Circuits/TL12Plus3.gif
They have been working for a couple of afternoons, but the next day were dead. I've been checking/replacing components but I'm still getting wrong dc voltages (without input signal). I'm using a common, commercial DVM. Filament voltages are within spec.
The output stage (EL84s) measures OK at the cathode. At the OPT centre-tap voltage is 322V (slightly higher than specified) and the voltage across R22 is 9V.
Now, here comes the cracker... Across R15 it is 3.04V but at junction R15-R7 to chassis it is 265V.
At pins 1 and 6 of the phase splitter (ECC81) it is 3.5V and 3.04V respectively. At the cathode pins 3-8 to chassis it is 3.7V
On the driver side (EF86 in triode mode) HT only measures 3V! and at the cathode it is 0.9V.
I used different valves, changed PS capacitors and resistors, dismantled the PCB to check connections and clean it but the problem remains. It seems the problem is around the PS but so far I haven't been able to spot it.
Can some members on this forum please help. Not too technical please. Not exactly a newbie (I've built a couple of amps and understand basic valve terms). My measuring equipment consists of a DVM and analogue multimeter!
Thanks a lot for your help.
Joe A
They have been working for a couple of afternoons, but the next day were dead. I've been checking/replacing components but I'm still getting wrong dc voltages (without input signal). I'm using a common, commercial DVM. Filament voltages are within spec.
The output stage (EL84s) measures OK at the cathode. At the OPT centre-tap voltage is 322V (slightly higher than specified) and the voltage across R22 is 9V.
Now, here comes the cracker... Across R15 it is 3.04V but at junction R15-R7 to chassis it is 265V.
At pins 1 and 6 of the phase splitter (ECC81) it is 3.5V and 3.04V respectively. At the cathode pins 3-8 to chassis it is 3.7V
On the driver side (EF86 in triode mode) HT only measures 3V! and at the cathode it is 0.9V.
I used different valves, changed PS capacitors and resistors, dismantled the PCB to check connections and clean it but the problem remains. It seems the problem is around the PS but so far I haven't been able to spot it.
Can some members on this forum please help. Not too technical please. Not exactly a newbie (I've built a couple of amps and understand basic valve terms). My measuring equipment consists of a DVM and analogue multimeter!
Thanks a lot for your help.
Joe A
Thanks oshifis and SY:
I checked the resistors with both valves removed and they all measure close except R12 (NFB) which is practically the same value as R4 (100R). But that is because (I suppose) it is connected in parallel to R4 via the OPT secondary winding. Isn't that so?
SY, I had already taken C2 off the board so the voltages I reported were taken without it.
That leaves me back to square 1. But anyway thanks a lot. I already feel relieved that I have expert friends like u to lean on.
I checked the resistors with both valves removed and they all measure close except R12 (NFB) which is practically the same value as R4 (100R). But that is because (I suppose) it is connected in parallel to R4 via the OPT secondary winding. Isn't that so?
SY, I had already taken C2 off the board so the voltages I reported were taken without it.
That leaves me back to square 1. But anyway thanks a lot. I already feel relieved that I have expert friends like u to lean on.
Hi there,
remove all the valves, except the rectifier and power up the amp. You should see about a 20% rise in voltages, but the scale to each other should be similar.
If the voltages remain as you report them, then remove r7 and see if they rise. This would indicate c3 as having gone closed. Not unlikely.
Bernie
remove all the valves, except the rectifier and power up the amp. You should see about a 20% rise in voltages, but the scale to each other should be similar.
If the voltages remain as you report them, then remove r7 and see if they rise. This would indicate c3 as having gone closed. Not unlikely.
Bernie
Removed driver (ef86) and phase-splitter (ecc81) valves and the voltages remained practically the same. OK on the supply rail but very low on the phase splitter plates (ecc81).
On the driver ef86 side, voltages at the supply rail and the plate are both around 3-4V, that on the plate is slightly lower.
These measurements were taken with C2 disconnected at the cathode. c3 has been completely removed as I'm running the ef86 in triode mode.
Joe A
On the driver ef86 side, voltages at the supply rail and the plate are both around 3-4V, that on the plate is slightly lower.
These measurements were taken with C2 disconnected at the cathode. c3 has been completely removed as I'm running the ef86 in triode mode.
Joe A
The supply rail on the phase splitter side is OK but the voltage on both plates is 3-4V. Even on the cathodes, where it should be 47V it's giving me the same 3-4 volts.
Similarly on the driver (ef86) side it's even worse coz on the supply I'm getting 3v again.
I already changed C5-C6 with another dual capacitor (reformed) and the situation was the same.
What I find curious are the voltages on R15 (10K) and R7 (100K) which are 3.15 and 1.5 volts respectively whereas these should be around 35 and 80V. The HT feeding this supply rail (i.e. at C13-R22) is with spec.
Other aspects I notice are that (a) when switching on from cold state, the ecc81 valve lights up brightly but burns normally after some seconds; (b) the speakers I have connected to the output respond positively to any prodding with the DVM. But the last time I fed it with an FM tuner signal there was no output.
Thanks SY and all concerned for your assistance.
Joe A
Similarly on the driver (ef86) side it's even worse coz on the supply I'm getting 3v again.
I already changed C5-C6 with another dual capacitor (reformed) and the situation was the same.
What I find curious are the voltages on R15 (10K) and R7 (100K) which are 3.15 and 1.5 volts respectively whereas these should be around 35 and 80V. The HT feeding this supply rail (i.e. at C13-R22) is with spec.
Other aspects I notice are that (a) when switching on from cold state, the ecc81 valve lights up brightly but burns normally after some seconds; (b) the speakers I have connected to the output respond positively to any prodding with the DVM. But the last time I fed it with an FM tuner signal there was no output.
Thanks SY and all concerned for your assistance.
Joe A
When building tube equipment, the voltage tolerance of the capacitors is usually considered, but the voltage tolerance and power handling of the resistors are sometimes overlooked. R5, R6, R9, R13 and maybe others should tolerate at least 300V. Also calculate the power on them from the circuit diagram, and use resistors with at least 5x power handling.
I don't say this is the problem, but at the beginning you wrote that you replaced some resistors and capacitors...
I don't say this is the problem, but at the beginning you wrote that you replaced some resistors and capacitors...
Hi Joe
re-read your post, and realised your talking about both amps.
From what I can understand, both amps are not working, and you are rebuilding them piece by piece. The likely-hood is that you have something in incorrectly.
You said that both worked, and then next day nothing. It is possible that the last part replaced could fail, but on both amps without drama
(fire, smoke etc) seems a little implausible.
The circuit is laid out on a tag strip. Check on the tag strip with your meter that none of the components that form the ht. line have low resistence to ground. The other amp will be the same.
I have made this mistake with guitar amps built on tag strip.
Sometimes you can't see whats on the otherside!
re-read your post, and realised your talking about both amps.
From what I can understand, both amps are not working, and you are rebuilding them piece by piece. The likely-hood is that you have something in incorrectly.
You said that both worked, and then next day nothing. It is possible that the last part replaced could fail, but on both amps without drama
(fire, smoke etc) seems a little implausible.
The circuit is laid out on a tag strip. Check on the tag strip with your meter that none of the components that form the ht. line have low resistence to ground. The other amp will be the same.
I have made this mistake with guitar amps built on tag strip.
Sometimes you can't see whats on the otherside!
Thanx for yr support. However, I still can't believe it. Bernie, I checked the resistors and capacitors to ground as u suggested and they measured OK - very high resistance (>20Meg) because each end of the resistors is connected to a smoothing capacitor.
To be true, when I started testing there was a tell-tale sign - the 10K (R15) was browning. During tests it started to smoke so I changed it to 2W but it didn't change the overall picture.
The HT+ is OK up to R15. Everything goes berserk from that point onwards: the plate voltages on the phase-splitter and all the EF86 driver section all give a reading less than 10V. The 280V at R15 is being wasted somewhere. When I touch the test probe to the plates the meter reads a high voltage but it dwindles down to a ridiculous 3-4V.
Oshi, you may be right about the values of R8, R14 but they are the stock value so I can't see them causing such a voltage drop. But I'll keep it in mind as a last resort.
To be true, when I started testing there was a tell-tale sign - the 10K (R15) was browning. During tests it started to smoke so I changed it to 2W but it didn't change the overall picture.
The HT+ is OK up to R15. Everything goes berserk from that point onwards: the plate voltages on the phase-splitter and all the EF86 driver section all give a reading less than 10V. The 280V at R15 is being wasted somewhere. When I touch the test probe to the plates the meter reads a high voltage but it dwindles down to a ridiculous 3-4V.
Oshi, you may be right about the values of R8, R14 but they are the stock value so I can't see them causing such a voltage drop. But I'll keep it in mind as a last resort.
First of all, I must qualify all that follows by saying that I am only moderately experienced...
If everything is OK up to R15, then your problem is downstream from that, as I am sure you have worked out.
If R15 was smoking, then it would appear to my limited experience that you have got a short to ground downstream of R15. That short may be a loose connection, sliver of crud from construction, a solder blob or a failed component. You said that you changed C5 and C6, what about C4, C8, C10? Probably not, but worth checking. Not sure how neat your wiring around the valve sockets, but on my last build I got close to shorting things out there a few times too. Sorry I cant offer much more help.
Any photos?
Best of luck,
Chris
If everything is OK up to R15, then your problem is downstream from that, as I am sure you have worked out.
If R15 was smoking, then it would appear to my limited experience that you have got a short to ground downstream of R15. That short may be a loose connection, sliver of crud from construction, a solder blob or a failed component. You said that you changed C5 and C6, what about C4, C8, C10? Probably not, but worth checking. Not sure how neat your wiring around the valve sockets, but on my last build I got close to shorting things out there a few times too. Sorry I cant offer much more help.
Any photos?
Best of luck,
Chris
Pardon me for being dense, but I still haven't understood what and where you're measuring from your answers. With the input and phase splitter tubes out, what is the voltage at the plate pins (i.e. at pin 6 and 1,6, respectively)? What are the voltages at the power supply rail end of the plate resistors (i.e., where the schematic is marked 200 and 280, respectively)?
In any event, replace C5 and C6, just on principle.
In any event, replace C5 and C6, just on principle.
Meter set to measure ac when you meant to measure dc?
Power supply bypass caps wrong voltage and avalanching to ground somewhere after R15? (Counterfeit, or just wrong voltage rating?)
Something common to both amplifiers seems to indicate that a replaced component has failed.
Try lifting (disconnecting) the supply bypass cap right after R15 and retest.
Don't start wholesale replacement of any remaining parts until you identify why they are currently not working!
Power supply bypass caps wrong voltage and avalanching to ground somewhere after R15? (Counterfeit, or just wrong voltage rating?)
Something common to both amplifiers seems to indicate that a replaced component has failed.
Try lifting (disconnecting) the supply bypass cap right after R15 and retest.
Don't start wholesale replacement of any remaining parts until you identify why they are currently not working!
Under-rated or improperly derated coupling caps relative to the initial maximum potential they are subjected to can gradually fail. The result would be a large voltage drop across the cap due to a large leakage current that could swamp the potential from the preceding node. If the current is large enough it can cause the output tubes upstream go cherry red and possibly runaway sometimes taking out the OT with. But you said the EL84's are fine.
I'd like to know the culprit once you've found it.
Thanks
Hello again! I wish to thank you all for your suggestions. I have today fixed the problem and as some of you asked me to report on the fix, here I am.
After almost losing all hope of getting the 2 amplifiers to work I did one last thing which I didn't write about because I considered it immaterial. At the input of the first stage (ef86) on the circuit (go to 1st post link) is a 1M resistor. I had earlier connected a 120K across it to bring close to 100K impedance which I had often seen on modern valve power amplifier circuits.
Bringing back the 1M input impedance solved the problem and now I'm listening to it again. However I still cannot understand why this should have created the problem - no sound. Can some of the more knowledgeable members of the forum explain, please.
Thanks for being with me throughout.
Joe A
After almost losing all hope of getting the 2 amplifiers to work I did one last thing which I didn't write about because I considered it immaterial. At the input of the first stage (ef86) on the circuit (go to 1st post link) is a 1M resistor. I had earlier connected a 120K across it to bring close to 100K impedance which I had often seen on modern valve power amplifier circuits.
Bringing back the 1M input impedance solved the problem and now I'm listening to it again. However I still cannot understand why this should have created the problem - no sound. Can some of the more knowledgeable members of the forum explain, please.
Thanks for being with me throughout.
Joe A
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