Laney Pro Tube Lead 100w head loud hum low output

[RTR]

I have a late 89's Laney Pro Tube Lead A.O.R. 100w head that's giving me trouble. Fired it up last week and all seemed ok except the the gain level was down quite a bit. I was plugged into the hi input and had the AOR boost engaged which should have put me in Death Metal territory but was WAY weaker. After 5 mins or so I saw smoke and the output faded. I opened it up and discovered a yellow wire on the preamp 1 volume was off its lug and I saw a burnt spot on the board. Reconnect the yellow wire and replaced a 1k resistor at the burnt spot which turned out to be an old repair. Turned it on and had a very loud hum and the two el34's on the left (v5 & v6) started redplating. I put in a new quartet of tubes and the hum is still there. I have 480vdc on pin 3 of all power tube sockets and around -40vdc on pin 5 of them as well.

Sorry for the long post. Any help pointing me where I need to look next is greatly appreciated. Links to things I need to read would help too. I don't want to give up on this amp.

[eleventeen]

It doesn't seem like any sort of preamp funnyness would upset the output section to the extent that the output tubes would redplate. Redplating is pretty much a direct 1:1 indicator of insufficient (or missing) bias. Yet you say you find -40 volts (this is the bias) on all (all?) the pin 5's of those tubes which is close to what the schemo says it should be.


So respectfully, I question the implication that the preamp-related fault you found is connected to the redplate issue. That would be very unusual, borderline impossible. Because the bias supply is absolutely separate from anything/everything else and does nothing else other than to "gate down" the output tubes. In standby, you should be able to adjust the bias volts using the adjustment pot. See if that works, both sides. While servicing the amp, any amp, I usually crank the bias to a somewhat MORE NEGATIVE voltage level so you are running the amp cool/cold during service. Crank it back to whatever level you like when you are satisfied that all other issues are resolved.


Looking at the schematic, if ONLY ONE pair of EL34s is redplating, it would have to be either R67 or R68 in front of the EL34's which are 220K resistors. Are you sure you measured ALL FOUR of the EL34's for your -40 volt bias? You should be able to check for that bias in standby with no B+ applied to the plates of the EL34s, so you don't have to be in a hurry or be alarmed that you are cooking anything while you carefully measure bias volts.

[RTR]

Yes, absolutely positive all 4 power tubes are at -40vdc.

I just turned it on with the old tubes and v7 & v8 ( I wrongly stated 5&6 before ) got very hot.

The wire I found isn't the issue, I think it was unhooked on purpose as the L.E.D for the pull boost stays on all the time with no footswitch. This is a replacement pot that doesn't have a pull boost.

[RTR]

Just checked the trim pot and can adjust voltage down to -34vdc

[eleventeen]

"can adjust voltage down to -34" <<---just to be clear, this would be HOTTER bias; technically you would be speaking about adjusting voltage UP [from -40 v to -34v] If I was unclear, I apologize. A greater number of neg volts shuts the tubes off more.   


While on a bench, I like to run the amp cooler / cold until I am sure the output section (if it's having a problem) is working right.


Can you get the bias to -50 volts on the EL34 pin 5's?


You might check for AC on your bias supply. That supply has only 2 qty 10 uf caps for filtering. I would want more...


And I will change my prior comment about the bias supply being only for the output tubes. No, it also (through R47, a 470K) feeds the "bass boost" circuit. So...maybe there *is* a connection. But if all your EL34's are getting -40 volts, that would seem to be enough neg bias to prevent redplating. A little hotter than I would like (meaning, I would probably run between -45 to -50 volts) but that's a matter of taste.

[RTR]

I didn't try the bias pot in the other direction, I will when I go back to it. I noticed on the schematic that it doesn't show pull boost for mid and treble, probably because it isn't the exact schematic for my model.

I've been afraid to put the new tubes in and let it run for more than a few seconds because of the hum and the cost of the tubes! 

If it were only power tubes this would have been easy.

Is there a way to check a tube in any way without a tube tester?

[RTR]

The bias pot will adjust to -43

I checked R67 & R68 and both are within tolerance and went as far a checking voltage at both resistors and they measure what I have at pin 5.

The preamp things I found initially are non issues, the wire is disconnected on purpose because it's not a pull pot and as I stated before the resistor was good that I removed.

I'm questioning now whether or not the tubes redplated at all. If not, my symptoms are I had a fuse blow and after replacing it I have a loud hum. Could this be a transformer issue?

[eleventeen]

Probably not.


If you're now saying your tubes may not have redplated, only you can tell this.


Either you are biased too hot (I consider -42 or -43 volts of bias to be pretty hot, more negative volts is desirable, at least in my book) or a voltage surge came along and whacked your fuse and possibly your caps, or they just failed out of old age.


You're kind of at a multi-decision point. Are your caps bad? Or getting bad?


You kind of have to test these things.


What I would do:


1: Open up the connections from the EL34 cathodes to ground and place 1 ohm 1% resistors in there, 1 for each tube. This enables measurement of tube current, per tube, by reading millivolts on your meter which will equal milliamps of tube current, again, per tube. If you want to bring those measurement points out to an external connector, that's a matter of whether you want to drill a hole or holes in your chassis and how religious you want to get about being able to measure bias easily, from the outside of the amp.


2: I would modify the circuit to allow for a greater range of bias volts. I have much more experience with 6L6 outputs over EL34, but a Twin Reverb usually takes about -52 volts and when I am ready to power up a 6L6-fired amp build for the first time, I very much like to know I have as much as -65 volts on the grids, just so I know the 6L6's are seriously throttled back and cannot go overcurrent. You can do this by inserting a 15K or 22K underneath the bias adjust pot. Or you could change R78 to 1K to 5K. We see that the bias circuit seems to take 1 mil, because R78, a 15K, has -58 volts on one side and -42 volts on its other side. So if 15K drops 16 volts, it's passing 1 mil, close enough. Change that R to a smaller value, every 1K you drop its value adds a [negative] volt to the bias range. If that R loses 10K, you would gain 10 more negative volts of range. That would make me happier, if it were my amp. If I can avoid it, I don't like adjustment pots to be thrown all the way to either end of their adjustment range.

[RTR]

At this point I'm unsure if they redplated. Why? Because I haven't found a failed component to point me to that. As you have pointed out, and I have read in my researching the issue, something should have failed and so far I haven't found that. Since all voltages on pin 5 move the same amount and is within the tolerance range it would seem all is well in that area.

I will get some 1ohm 1% resistors like you have suggested and see what I get.

The biggest thing bothering me at this point is the hum.

Eleventeen, thank you for your help so far, I really appreciate it!

[eleventeen]

Let me bring up one more consideration: Not all EL34s (or any other tube) bias at the exact same point, or voltage level. Everything in tubes is +/- 10%.


You might throw a set of Sylvania tubes in there and not touch your bias control, and find that 30 ma per tube (again, +/- 10%) flows through each. Very happy. Not too hot, not too cold. Then you swap in some JJs and they show 24 ma. They want to be biased hotter to achieve that 30 ma average flow. And then, some RCAs want a different level.


Point 1 is, different manufacturers' tubes (and, perhaps even different era of the same mfr) may want different bias volts.


Point 2 is, this is one reason why there's a bias control in the first place.

[RTR]

Haven't been able to put the 1ohm resistors on since Radio Shack has been out of them at my location.

With new tubes in and bias supply as far negative as it will go I still have a very loud hum. The electrolytic caps all measure well above 15k ohm and should be good but I suspect they might be the problem.

I plugged another head into the cab and it's quiet so I know it isn't an input jack problem on the cab producing hum.

I just don't know. Every component that I have put my meter on reads in spec and all components pass a visual inspection. Very discouraged right now.

[HotBluePlates]

The electrolytic caps all measure well above 15k ohm and should be good but I suspect they might be the problem.

Caps should measure "infinity ohms" (though with many uF's it may take several seconds to charge up to that reading). Any cap that doesn't get to infinity ohms (on a resistance range) with the power off would be suspect.

With new tubes in and bias supply as far negative as it will go I still have a very loud hum.

Yank all tubes and turn on. Any hum?

If no, replace only output tubes. Keep the bias pot all the way to the end giving -43v. Turn the amp on. Any hum/redplating?

The schematic shows a bias of -42v with a plate & screen voltage of ~460vdc. You have 480vdc, so I'd expect the bias to be more like -45v (or more negative) for happy output tubes. Could be even more than that depending on the specific tube.

I'd also interpret the -42v on the schematic as occurring at about the midpoint of the bias pot. So I'd suspect the electrolytic caps in the bias supply as failing. If you get hum but no redplating when you put back only the output tubes, I'd again suspect the bias supply caps as having failed.

[RTR]

If I pull the power tubes I have no hum. The 15k ohms I was referring to was on a link at the too of this sub forum page. To the best of my memory I know two of the 4 that are together measured in the high meg ohms and the other two were in the high hundreds of K ohms.

The amp has always been noisy but this loud hum only happened after the fuse smoked. I suppose it's time for a cap job.

[HotBluePlates]

If I pull the power tubes I have no hum. The 15k ohms I was referring to was on a link at the too of this sub forum page.

Measuring a cap in-circuit is not diagnostic (meaning it very likely tells you nothing of value).

If I pull the power tubes I have no hum.

Now if only the output tubes are in the amp, does it hum?

[RTR]

I will try it with only the power tubes in a bit.

Thank you for your help!

[RTR]

No hum with just preamp tubes pulled.   

[HotBluePlates]

Then the hum is not originating in the output tubes or bias supply. It must be at the phase inverter or an earlier stage.


Does the hum volume change when you fiddle the volume control?

[RTR]

I don't think it changes with volume...I need to check that again. I remember having the volume up and having the hum and then turning all volumes and preamp levels to zero and still having the hum but don't recall having a level drop.

[RTR]

Hum does not change with volume.

[Ritchie200]

Check your unused input contacts to make sure they are grounding, effects loop contacts, etc.  Check the easy stuff now.  Good luck!

Jim

[RTR]

I will check them tonight.

[HotBluePlates]

Hum does not change with volume.

Does it change when you fiddle the master volume control?


Point being, I'm trying to cut down the number of places to look for a cause of hum.

[RTR]

No, hum doesn't change when I turn the master volume.

[HotBluePlates]

No, hum doesn't change when I turn the master volume.

That's valuable info. You now know the hum is after the master volume but before the output tubes. You pulled the preamp tubes before, but try pulling just the phase inverter (preamp tube closest to the output tubes). Based on your earlier report, I'd guess the hum would stop.


That would place the hum between the master volume and phase inverter tube, which is your Effects Loop opamps and jacks and line out, as well as the channel switching circuitry.


C47 on the schematic (10uF 150v) is the output of the fairly cheesy -15v supply for the opamps. You might try measuring for a.c. volts at that cap (ideally 0v a.c.; I'm not sure what the upper limit would be before hum becomes an issue). You could also measure for a.c. volts at the opamp outputs to isolate where the hum is. Keep all volumes on zero for this test, and check for a.c. at C25 (output of 1st opamp) and C32 (output of 2nd opamp).


Also, Ritchie's suggestion about checking the jacks of the effects loop would be wise.

[RTR]

I checked the jacks and they measure in the .4ohm or less range. The hum does go away with the PI tube removed (good call HBP) as far as measuring the caps for AC goes, c25 and c32 have less than 1vac. C47 I haven't been able to identify yet. I decided to check for the -15vdc on the orange wires of the PT and at the board I don't get a reading. I'm probably doing something wrong.

On a whim, I jumpered the fx loop and the loud hum is gone, it's still noisy but these amps are known for that.

There might be some differences in the schematic we are looking at and my amp...mine doesn't have a line out and my fx loop is +8dBv.







The last pic shows what I think are C25 & C32