Bad amp grounding??

[Helmholtz]

We already had a lengthy discussion about C12 being bad in the other thread and here you stated that you replaced it:
https://music-electronics-forum.com/...267#post933267

[Sea Chief]

We already had a lengthy discussion about C12 being bad in the other thread and here you stated that you replaced it:
https://music-electronics-forum.com/...267#post933267

Hi HH.

I had a thread 'vox gremlins': innitially with just 1 issue: a bad input jack connection. Then, I found the tremolo ticking badly coinciding with successfully fixing this input jack, so this was then discussed. But another sudden issue, by now at page 9, is frustrating because of so much prior information within the thread (actually nothing to do with this 3rd new issue just arisen). I just cannot cope with continuing on that thread. Its too much for me to deal with.

So this third goddam gremlin sprang from nowhere, this current bad grounding HUMM!! thing. I did one test wrong (I forgot to turn up 1 knob) & posted my findings, which in fact whichever capacitor I was testing/ shorting across I said I found faulty.. was in fact ok, I think. So I need to scrap that thread.

I decided to start afresh. A new thread. It is sensible. Please therefore, disregard the other thread: it really complicates things reverting back to it, because on it I said "bingo!! that's it!! the noise has stopped" this is entirely unhelpful information, entirely my fault, entirely incorrect findings because I tested without thinking to turn --both-- volume knobs up at the same time.

It might not be able to be understood what I did, the two volume knobs etc, but if not- it doesn't matter.

So I just need to start again, with no reference to the other thread, otherwise I just cannot proceed.

Thanks, SC

[Sea Chief]

Sea chief the sleeve of the input jack is connected directly to one side of R24, hence 0.6 ohms resistance. You said it your self it has continuity. Measuring from the sleeve to other side of R24 is essentially the same as measuring across it. That adds another 0.5 ohms to the first measurement 1.1 ohms. R24 is in parallel with C12 so you are measuring across C12 at the same time. Go measure across R12 what is the reading? 1.1 ohms?

edit: I would focus all your energy on this one issue first. Resolve it and then move on. Despair never pairs well when troubleshooting for an answer.

Hi Gonz..

believe me, I'm doing exactly what you state to do/ everything all energy into this one issue by dedicating a new thread to it. I have a week put aside to finding it but I can barely get going/ I haven't a 0.1% clue where to start.

Ok here do you mean go measure across C12 not R12? (I cannot find R12 in this general "V5 area"). Isn't that the same as measuring across R24 though, in which case surely it's been done (1.1 ohms). So I am confused as to A) where you suggest measuring, & B) if across R24.. why you suggest this, if I've only just done it within the last few posts. I'm snowed under in confusion with each reply.

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I desperately need to start afresh folks. Could someone kindly just point out a few things to test, right from the top (as simply as possible) & I can repost all measurements.

Thanks, SC

[Sea Chief]

Here's an observation. SC, haven't you had trouble with the input jack on this amp at one time or another? Has anyone yet suggested a ground fault for the input jack? Seems like the first thing to eliminate to me. With the amp off, plug a cord into the input and take a resistance reading from the sleeve to either end of R24. What is the reading?

Hi Chuck, yes indeed I did have an input jack issue. No-one suggested a ground fault with the input jack yet. So it makes sense to start afresh here, then move on. Could I ask, could you help me out by telling me how I can completely test this input socket?

I'm going to call this resistance reading @ R24 test 1.

My measurement is 8.5 Ohms (I test the other side of R24, 8.5ohms). I have redone the test 3x to make certain done correctly. I plug a cord in, the amp is off, I measure from the input jack sleeve terminal (closest to the front of chassis) to either side of R24.

(Why this is different to before.. I have no idea. If relevant: I measure directly across R24 & get a reading of 6.6ohms).

Chuck, to get me going.. could you voice an opinion on whether my 8.5 ohms reading is ballpark correct, or not?

Thanks, SC

[Sea Chief]

I just blitzed/ resoldered every single solder joint up to the V5 socket. All grounds, resistors, diodes, caps, both input socket 4x into pcb.

Nothing changed. Noise remains.

[Sea Chief]

Can anyone else offer any ideas, or by all means please ask me to test anything-?

Thanks SC

[Helmholtz]

Please therefore, disregard the other thread

Well, it's your decision but it bears the risk of losing relevant info and going around in circles.

If you want to save some time, replace C12, C4 and V5. Maybe it helps - maybe not.

[DrGonz78]

Sea chief the sleeve of the input jack is connected directly to one side of R24, hence 0.6 ohms resistance. You said it your self it has continuity. Measuring from the sleeve to other side of R24 is essentially the same as measuring across it. That adds another 0.5 ohms to the first measurement 1.1 ohms. R24 is in parallel with C12 so you are measuring across C12 at the same time. Go measure across R24 what is the reading? 1.1 ohms?

edit: I would focus all your energy on this one issue first. Resolve it and then move on. Despair never pairs well when troubleshooting for an answer.

double edit: I wrote R12 but meant measure across R24

Sorry I wrote R12. I meant R24. Yes measuring across C12 is the same measurement since it is in parallel with R24.

Post the measurement and we’ll see if it reads 1.1 ohms or less. The shorting across R24 and C12 was not to truly test C12. It was to test if it made a change to the nasty noise, which it doesn’t. However the cap reading short should still be replaced with a new cap before proceeding to other tests.

[Sea Chief]

Well, it's your decision but it bears the risk of losing relevant info and going around in circles.

If you want to save some time, replace C12, C4 and V5. Maybe it helps - maybe not.

Hi HH.

No the information on that thread is not correct. If it's used to refer to, & my results from test X are wrong due to simply forgetting to turn one knob, then it completely hampers me on here. I could edit it.. but it would take me a morning.

I said on there the noise stops when doing Y: this was not correct in retrospect. If this info is referred to.. it completely messes up my efforts afresh on here.

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I tried a new tube in V5. The noise remains. So -I think- this rules out further need to replace it. But I'm not 100% sure.

C12 & C4.. I've replaced C12 (I'm not removing it to start afresh, so it stays). The noise remains. C4? I will replace.

----

But I was hoping to start, test 1 etc, on the input jacks. It seems the logical place to start if I had noise from it before, at the start of the Gremlins thread. Chuck H asked for two resistance readings from its sleeve "to either side of R24". I posted the results. If I could just progress from here, rule out the input jacks.

Also I had a brief moment yesterday, when the amp worked using input 1. So, I'm not sure if this signifies any further focus on this area, or not, so I'm stuck in limbo if I don't know if the input jacks have been completely tested/ ruled out. It seems to suggest further tests needed on it, doesn't it-?

Thanks, SC

[Sea Chief]

Helmholtz regarding replacing C4. I don't have a 10uf 350v cap.... only a 16/475. Is this ok to use?

[Sea Chief]

I just tried something. Going back to basics, I pulled out V5 & instead of replacing with a known good 12ax7.. I swapped V1 & V5. An informed idea? no, just laziness as my spare new 12ax7 I put in my twin reverb.

The noise stops. Both inputs work. (!!)

I repeat this & get the same results. And so it seems possibly it was the PI tube originally in V1.. the culprit.

But could this be even possible? I'm so confused I have no idea. Could pulling V5, & the noise stopping (as was the very 1st thing to try), ---not neccessarily then-- rule out everything further down??

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I will have to monitor what happens today, but, if I repeated the test by swapping out/ in this 'suspect' 12ax7 & results are the same, I think it points to this tube doesn't it?

Thanks SC

[Dave H]

I just tried something. Going back to basics, I pulled out V5 & instead of replacing with a known good 12ax7.. I swapped V1 & V5. An informed idea? no, just laziness as my spare new 12ax7 I put in my twin reverb.

The noise stops. Both inputs work. (!!)

I repeat this & get the same results. And so it seems possibly it was the PI tube originally in V1.. the culprit.

I think it's more likely the tube that was originally in V5 was the culprit. It is now in the V1 slot with less amplification down stream of it so it's not causing a problem.

[Sea Chief]

I think it's more likely the tube that was originally in V5 was the culprit. It is now in the V1 slot with less amplification down stream of it so it's not causing a problem.

Hi Dave.

surely I ruled this out. I've surely boiled it down to the tube which was originally in V1. I mean it has to be, I'm 95% sure.

If V5 is back as it originally was, & the amp's sounding fine again (woohoo!).. & the only difference is I've put my new 12ax7 in for V1 then surely it has to have been V1 (a total fluke on my part, but that does happen). I can only surmise this has to be a solid fact.. but tube amps & logic seem to be incompatible bedfellows, so god only knows.

If this is correct, then some sort of tube tester is a must it seems.. or its weeks of pulling apart an amp tearing your hair out if one develops a fault. And if you've got a stupid ammount in a row (5) its a recipe for disaster. And I think tube testers are very expensive.

I'm considering an amp without preamp tubes after this. SC

[Sea Chief]

Seems like tube testers are complicated devices, even a diy one will be alot more costly than an attenuator to make..

https://valveheaven.com/2015/03/an-i...vetube-tester/

[nosaj]

Hi Dave.

surely I ruled this out. I've surely boiled it down to the tube which was originally in V1. I mean it has to be, I'm 95% sure.

If V5 is back as it originally was, & the amp's sounding fine again (woohoo!).. & the only difference is I've put my new 12ax7 in for V1 then surely it has to have been V1 (a total fluke on my part, but that does happen). I can only surmise this has to be a solid fact.. but tube amps & logic seem to be incompatible bedfellows, so god only knows.

If this is correct, then some sort of tube tester is a must it seems.. or its weeks of pulling apart an amp tearing your hair out if one develops a fault. And if you've got a stupid ammount in a row (5) its a recipe for disaster. And I think tube testers are very expensive.

I'm considering an amp without preamp tubes after this. SC

Are the tube positions labeled on the amp V1 v2, etc?
Check with a local ham radio operator testers aren't that expensive locally and used.

nosaj