Help! What could possibly be wrong?

[ray_moth]

Don't forget to check for bad solder joints, e.g. large blobs of solder touching something they shouldn't.

[bernification]

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!

[oshifis]

Another remark: I would not use 3.3M for R8 and R14. 1M is a more reasonable value.

[sonata149]

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.

[chrish]

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

[SY]

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.

[kevinkr]

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!

[TubeHead Johnny]

The fact that it was working a few days ago tells me that it's a cold solder joint. Reheat all the solder joints that you've worked on and check for loose wires.

[Seraph]

You said that you changed C5 and C6, what about C4, C8, C10?

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

[sonata149]

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