AC4tV EL84 Red Plating

[iccaros]

here is the story, bought a AC4TV off Ebay for $100. Started playing it and started to smell something.. could be dust, kind of a plastic getting hot smell.. but wanted to make sure. I had the amp open as I did not want to turn it on until I check that everything looked good after shipping. I played and it sounds good.. fun amp, but still smelled something..
I watched the tubes and the EL84 glowed more than any EL84 I have seen. Looked like a 6L6 in a fender champ.

I have a video, I used my phone, sideways to the tube with the cathode to the left and you can see the tip light up with the heater. As I play, the cathode brightens with audio. after a min the anode starts to light up.
This happens even at low volume.
The cathode cap and resister are reading correctly.
I am thinking of adding a screen resistor, say 1.2K 1 watt.

I appreciate any help, Thanks

B+ is reading at 311V

[Enzo]

Why think first of modifying the amp instead of just finding out what is wrong? If you woke up tomorrow and your foot wouldn;t fit into your shoe, would you think you needed to get your shoe modified? Or would you think maybe your foot was swollen and we needed to find out why?

If the amp needed a modification to work right, they would all need this mod right out of the box.


Most likely cause is a bad power tube, the simplest test is to substitute another in its place.

You know, the amp already has a 470 ohm screen resistor, really.

When a tube gets red hot, either ther is a defective tube, or the tube is underbiased. This is a simple amp, so if the tube is known to be OK, either the grid is getting positive, or the cathode is getting less positive. SO measure the voltage at the cathode, is it around zero (bad) or up to something like 7-10 volts? By the way, the schematic says pin 8 for cathode. Isn;t that supposed to be pin 3? Pin 8 is cathode on the larger EL34. Then pin 2 is the control grid. SHould be zero volts DC there. Is that the case?

Your cathode cap may read OK on a hand meter, but that doesn;t tell us if the thing leaks like a seive at any sort of current. And a leaky coupling cap from the previous stage - C18 - could be allowing DC on your grid.

[iccaros]

Thanks for the reply

Two things.. One, sorry for the confusion on the screen resistor, it was going to be a mod to allow me to use a 8 ohm speaker instead of the 16ohm, so had nothing to do with fixing this, the OT is reflecting 10K with the 16ohm so would be 5K @ 8 ohm. I believe to my ear the amp sounds better @ 5k on a 8ohm, but I am not starting that process until I figure this issue out first. second I have swamped out 4 tubes, so either I have 4 bad brand new el84 that do not show bad in my other amps or something else. I did see some nice blue flashes on one tube, but it stopped after I let it warm up for 10 min.
I have some Russian 6P14P-K, and they are also red plating. I do not have a tube tester, but the likely hood of 4 new tubes being bad and three 6P14P-K being bad is low, but not impossible. May have to order some more to find out


I measured Cathode voltage, you have a good point, a good reading on the LC meter does not mean its not leaking..
I get -9.8v at pin 3 (cathode 8 is not connected I think or is that shield?) to pin 2 (I did not read this to ground but should I not see a negative voltage DC? )
and 9.75v from pin 3 to ground, I will check c-18 in the morning.

[Enzo]

I was happy just trying one new tube, no I don;t think you got four bad ones. Though it is possible.

Yes it is important that the relationship between cathode and grid be what it is, I just find it easy to look for grid voltage and see if any positive potential is there where it ought no to be. 9.8v of bias sounds OK, but I recommend measure to ground from grid also, because we don;t want our technique to possibly mask something. For example a volt or so positive on the grid would increase current, which would raise cathode voltage. But the mere presence of the positive voltage on the grid means there ios a falure in the grid circuit - likely leaky cap. You would still get a reasonable looking relative voltage between those elements of hte tube, but miss the leakage voltage. I hope my clumsy description works.

But your numbers didn;t look bad.

Also, if one wants, one can pull the power tube and still check for DC on the grid pin of the socket.


I thought of but did not mention another possibility, the amp is oscillating hard at RF and working the tube hard at inaudible freqs. Far fetched but possible. A quick scoping would reveal anything like that.

[jmaf]

The 220 OHM cathode resistor is bypassed by a 220uF cap. That cap may be shorted.

Other than the cathode bypass cap, the cathode resistor itself and the grid leak resistor(edit: a bad contact or an open one, leaving the grid floating), there is little else there that would make all your tubes so hot...

[iccaros]

I was happy just trying one new tube, no I don;t think you got four bad ones. Though it is possible.

Yes it is important that the relationship between cathode and grid be what it is, I just find it easy to look for grid voltage and see if any positive potential is there where it ought no to be. 9.8v of bias sounds OK, but I recommend measure to ground from grid also, because we don;t want our technique to possibly mask something. For example a volt or so positive on the grid would increase current, which would raise cathode voltage. But the mere presence of the positive voltage on the grid means there ios a falure in the grid circuit - likely leaky cap. You would still get a reasonable looking relative voltage between those elements of hte tube, but miss the leakage voltage. I hope my clumsy description works.

But your numbers didn;t look bad.

Also, if one wants, one can pull the power tube and still check for DC on the grid pin of the socket.


I thought of but did not mention another possibility, the amp is oscillating hard at RF and working the tube hard at inaudible freqs. Far fetched but possible. A quick scoping would reveal anything like that.

Good Ideal, If I read the grid with no tube I should see zero in comparison to ground.. Thanks

[iccaros]

The 220 OHM cathode resistor is bypassed by a 220uF cap. That cap may be shorted.

Other than the cathode bypass cap, the cathode resistor itself and the grid leak resistor(edit: a bad contact or an open one, leaving the grid floating), there is little else there that would make all your tubes so hot...

the cap reads good, but I will swap it tonight to see, it may leak under pressure but it does not read as a short.
The grid leak resistor and its 22p bypass, I have not looked at yet.. This thing is a pain in my 4th point of contact to get to.


This schematic looks right minus the 2k2 off pin 9 of the EL84 this goes directly to R16 on the board.

[jmaf]

It can't hurt to try running the amp without both the grid leak bypass cap and the cathode bypass. Yes, they do behave differently under high voltage, I have many caps I'd like to use but I can't. They read perfect on my LCR meter but under high tension they leak.... Like Enzo said, even if they leak a tiny bit they'll offset the bias there.

[Enzo]

You don't have to actually replace the cathode bypass cap, just unsolder one end and lift it. All we care about at the moment is the red plating current, not tone. SO if that reveals the cap was the cause , THEN replace the cap. If it makes no difference, then just resolder the cap lead and move on.

[iccaros]

You don't have to actually replace the cathode bypass cap, just unsolder one end and lift it. All we care about at the moment is the red plating current, not tone. SO if that reveals the cap was the cause , THEN replace the cap. If it makes no difference, then just resolder the cap lead and move on.

Thanks
that is the plan..
I am also going to look for oscillation with the o-scope..

[iccaros]

latest report..
Removed C8 and C20 completely, no change, Changed out C18 because the lead broke when taking it out.. No change..

I am at a loss, unless its the power transformer.. I have a 5K I can test, but I do not see if this is putting out audio how it could be a shorted transformer,,,, could it?

[iccaros]

one more question, This is wired, I would swear that the filament is glowing brighter as I play, the metal plate surrounding the assembly is not glowing.. I see into the center and it looks like the same spiral that the heater is on or one next to it is what is glowing brighter..

A better video.. This is tonight, with a Russian tube that has brighter heaters than the other tubes I have, it make it clearer the increase in brightness when I play through the guitar. @ about 22 seconds It gets clearer.. Hard to hold the phone and play at the same time,, even crappy playing..:P

[Enzo]

Transformers can fail in increments like anything else. It can be grossly defective and not work at all, or it can have just a shorted turn so it is like driving the car down the road with the parking brake still on.

Not many wires, if you have another transformer, you can sub it into the circuit and find out.


Hard to see just what we are fighting here. Your cathode glowing inside the works will get brighter when current increases.

[iccaros]

Transformers can fail in increments like anything else. It can be grossly defective and not work at all, or it can have just a shorted turn so it is like driving the car down the road with the parking brake still on.

Not many wires, if you have another transformer, you can sub it into the circuit and find out.

Understand..

Hard to see just what we are fighting here. Your cathode glowing inside the works will get brighter when current increases.

This is my real question, am I fighting anything or is this normal for a EL84 in SE class A.. My other EL84 amps do not do this at all, they do not change brightness with audio, but they are push pull.. I can admit to being paranoid..

[jmaf]

The tube on the 2nd video seems to be doing just fine. It was harder to tell on the first video, as it looked like parts of the plate were glowing, but I think it was just out of focus.