Evh 5150 plate voltage modulating?

[nevetslab]

After replacing the Quad 6L6GC pwr tubes, I'm seeing the plate voltage swinging from around 420VDC to 395VDC and back, cycling at around a 1-2 second rep rate. I'm used to running into that when there's a Tremolo circuit running, but, no such circuit in this 3-Ch EVH 5150, similar to the Peavey 5150, but this is the Creamy-white EVH 5150.

It came to the shop from our rental inventory, no output (blown HT fuse), and after replacing it, I found it was pulling high current, from one pair of power tubes that were failing. Ordered a new matched quad set of J/J 6L6GC's, and upon installing the first pair into the bias probes, I found once again the max plate current I could dial up was 22mA. Tacked in a 100k resistor across a 68k resistor in the bias circuit to increase the neg bias, now having sufficient range to set up any set of tubes.

With each pair, checking to verify the matching, I was noticing the plate voltage cycling in magnitude. I wasn't seeing any variation on the AC mains current/wattage being pulled. Only about 1mA variation in the plate current seeing on the bias probe meter..

[Chuck H]

It came to the shop from our rental inventory, no output (blown HT fuse), and after replacing it, I found it was pulling high current, from one pair of power tubes that were failing. Ordered a new matched quad set of J/J 6L6GC's, and upon installing the first pair into the bias probes, I found once again the max plate current I could dial up was 22mA. Tacked in a 100k resistor across a 68k resistor in the bias circuit to increase the neg bias,..

I'm confused. Wouldn't 22mA be a fairly small amount of current for an el34 at most sane voltages? And wouldn't increasing negative bias REDUCE current? I think there's a typo in there somewhere.?.

I didn't look at a schem to see about possible culprits for the fluctuating plate voltage. Since it does it with only one set of tubes at a time I'm guessing it's the amp and not the tubes. But it could still be the tubes. I've seen a lot of weird stuff and a coincidental "won't settle" failure in two pairs of tubes wouldn't actually surprise me. Another thing though is the oscillation timing. If it's consistent between tube pairs I think it must be an amplifier, rather than tube problem. I'll look up the schem, but I'm not a repair tech. I'll be looking for what component failure could turn the amp into an oscillator but I don't expect to make much headway with all the channel switching and variables. My first inclination would be that's it's either a switching failure combining the wrong things or a filtering failure doing the same. I don't often post on your threads because you're miles ahead of me as a repair tech. And it's probable you've already considered all of this. But I lurk your threads often (stalking? ) I'd be very proud if any of this helps.

[g1]

The power tube screens are getting their proper voltage?

[Enzo]

6L6, not EL34.

Nev, I don;t know if you do much 5150 work, but VERY cold is normal at least for the Peavey ones. I think Fender copied the 5150-2 circuit pretty faithfully, so I expect similar currents. I found typical power tube currents in the 11ma to 20ma area, maybe 16ma average. The power amp in these is meant to be loud and strong, all the tone comes from the preamp. The 5150 was set, the 5150-2 and later had an adjustment but with purposely limited range. So looking for that magical 70% bias is futile in a stock amp, and frankly to me not a needed modification.

But none of that solves your motorboating.

Pull ALL the power tubes, does the B+ still cycle? If no, forget the bias probe for now, put ONE power tube in one socket. Does that tube cycle the B+? Then move the tube to each other socket, one at a time to see if the same tube acts different in one or more sockets. If we prove the amp and all sockets are stable like that, pick ONE socket, and try each of the four tubes in that one socket alone. DO they all four act normal, and alike? Or does one or more tube act odd? Once we verify that all four tubes work individually, and all four sockets work individually, if it is stable that way, then what combinations of tubes makes it go of track? Is it just any two or more tubes? Or do they have to be in a particular set of sockets?

And I don't know why it would, but does load impedance matter?

I know I don't need to teach you troubleshooting, but I am hoping spelling it out in a different fashion may help.

[nevetslab]

Actually no, not much work on the 5150's. We have both the Peavey ones as well as the EVH ones. I don't have the service docs that match this EVH, just going off of the Peavey 5150. I hadn't read / heard they are normally biased cold like that.

I'll put the chassis back up on the bench to check thru your suggestions. I looked thru my notes when I first had it up on the bench, with one pair of 6L6GC's that would suddenly start pulling lots of plate current, so tossed those. The other pair was ok, though seemed to me weak, only being able to get them up to around 18mA. I didn't make any note of the plate voltage wandering like I'm seeing at present.

So, I'll see what I find this time around, and also see if running them cold as you've normally seen, and checking each of the sockets. And also see if there IS any difference with load (both resistive and speaker). The amp did sound fine, and didn't notice anything odd with it biased on the hot side....just hadn't seen that motorboating supply variation on these before. Will advise on the rebound.

Thanks,

Steven

[nevetslab]

I'm confused. Wouldn't 22mA be a fairly small amount of current for an el34 at most sane voltages? And wouldn't increasing negative bias REDUCE current? I think there's a typo in there somewhere.?.

My choice of words....increasing negative bias....decreasing the negative bias to increase the plate current is what I meant & did. Sorry about that.

[Enzo]

"Bias" is the voltage differential we apply between grid and cathode. The current we measure is power tube idle current. We use that current in bias adjustment and monitoring, but it is not "bias".

[Enzo]

Unless the world recently changed the EVH5150 is a Fender product, and Fender could provide the drawings. AKA 5150-3.

I have attached the file, I hope it opens.

[Helmholtz]

My choice of words....increasing negative bias....decreasing the negative bias to increase the plate current is what I meant & did. Sorry about that.

Well, actually -35V = -40V +5V. This means that -35V is +5V higher than -40V and such increase in grid voltage causes higher plate current. For this reason grid voltage and plate current are in phase. But I know....

[Enzo]

Oh please don't start up that old argument. -35 is 5v more positive than -40v, but not "higher". When we start saying things like -200v is a lower voltage than -10v, we are asking for people to injure themselves. Zero is the lowest voltage, not minus a zillion.

Popular Electronics magazine tried to put that into practice years ago, and got pounded on their editorial heads. They also tried to get us to call circuit boards "etched circuits" instead of "printed circuits", making some rationale that printed circuits are what is inside an IC.

[The Dude]

To avoid confusion and when referring to negative bias supply, I usually use the terms "more negative" and "more positive".

[Helmholtz]

Zero is the lowest voltage, not minus a zillion.

I don't agree, but I am not interested in such discussion.

[nevetslab]

OK....some days, it doesn't pay to open my mouth when my feet are anywhere near it. I'm so used to seeing 'bias adjust' on schematics and even on the back side of some of the Fender amps currently in the shop. Even on this Peavey-built EVH 5150, the silk screen with an arrow points to 'bias adjust'. My appologies in the incorrect terminology....I should know better.

At any rate, having pulled the EVH 5150 chassis back out of the cabinet and back up on the service stand, I re-adjusted the negative grid voltage to drop the plate current to 22mA per tube, and while I was still seeing this 'motorboating' Plate Voltage reading on my Compu-Bias meter, I also looked with my Fluke DMM and saw NO modulation on the plate voltage. It was sitting steady at 428VDC, with the screen voltage at 422VDC. I removed the tubes, leaving the 'bias probes' installed, powered back up, and still seeing the meter displaying modulating plate voltage readings, with the Fluke DMM reasing steady, now higher from no load from any power tubes installed.

I haven't noticed the plate voltage reading on this dual-probe meter modulating like I'm seeing connected to the EVH 5150 on the other amps I've been servicing, using it in checking and adjusting plate current. It does make me wonder if the sudden increase in plate current I saw and shut off the HT supply immediately the other day on this amp, whether that has done something to the meter. Hope not, but I guess I need one more item to worry about.

So, there ISN'T any problem with the Power Supply on this amp. Does anyone have adjustment details on this amp? I don't find anything on the schematics I have.

[Enzo]

Did the file I posted open? Page 1 lower left, see note 6. It is called by TP6 which is the bias voltage. It refers you to the 70922 drawing later in the file, and note 6 there for adjustment. It gives a recommended spec. They are looking for 20ma per tube or 80ma at the TP. Hotter than Peavey, but still pretty cool.

I don't know what is going on with your rig, but every now and then when I try to measure output plate voltage with my meter, it makes the amp oscillate or cycle. Perhaps for some odd reason this amp is reacting to your bias probe. Try making the readings without the adaptor. There is a 1 ohm resistor already in it.

[nevetslab]

Ah....I missed the attachment. Got it....many thanks! I'll look more when I get in tomorrow...gotta run