1986 Laney Pro Tube Lead A050 Series YIEKS

[Chuck H]

Ah, yep! Maybe. J will need to see if his amp matches your schem. Bottom line, if the switching is working correctly and the voltages on that supply check out then the problem is with the bias supply itself. And it's still not a good idea to voltage divide right off the winding because it would mess with the LV supply voltages.

[J Luth]

I have the AOR50H but I will check out the one from g1 also. Thanks.

Here's were I am right now, Cleaned up some more of the mess and installed the 1 Ohm resistors from the cathode to ground. On my cheap meter on the 200mv scale I am getting 2.5 and 4.6 across the resistor that don't look right. The voltage difference follows the tubes. He brought over 4 other tubes and they read a lot lower than the one's that are in there right now.

Maybe issues with the tubes?

No IC's in the amp.
Looks like the AOR50H is the closest.
If I am reading it correctly it calls for -68V on one side of the 22K and -48 after it. Right now it is -86V before the 22K and -73V after it.

This one may be beyond me. But we shall see.

[J Luth]

Thanks Chuck

[J Luth]

Thanks g1

[Chuck H]

Well if the PT has been replaced, as you suspect, it's possible the PT is one intended for the higher bias wind voltage. Unless compensated for at the divider for the LV supply I would have thought this could cause switching problems. It's also possible that the PT is stock and your amp is simply a crossover parts model. In which case compensation should have already been made by the manufacturer. Happens all the time. In any case...

Sounds like a tube problem to me. That doesn't mean the bias supply is correct yet. But go with the tubes that give expected and consistent results, which would seem to be one of the "other" sets brought to you.

Does your board have R#'s and C#'s printed on it?

[J Luth]

Chuck,

I have a quite long post coming soon of where I'm at.
What I am experimenting with out of frustration may be wrong but it will have the results that I am getting.

With regards to the power trans and why I suspect it. All the components that have been replace in the power supply area of the circuit board, it looks a lot newer than the others and is a completely different than the choke and the output which are the same manufacturer, the mounting of the trans is totally Mickey Mouse and the wire colors do not match any schematic.

[J Luth]

Frustration took over this is a bit long

When the amp came in it was in bad shape. I cleaned it up a bit fixed the broken wires, got the broken jack to work, hooked it to a dummy load and fired it up on the limiter which showed no shorts. Shut it down and connected it to a speaker and plug a guitar in and turned it on. Nothing but distortion and noise. Shut it down and connected it to the scope. With the Preamp volume and the Master volume on what would be about 3 the sine if you want to call it that wave is horrible. Any increase in either of the volume controls only shows very slight output increase and the sine wave gets completely messed up and distorted.

As I posted previously the voltage off the bias tap is 90+Vac. The bias voltage at the tube is -66V and the voltage across the 1 Ohm resistor is 2.5 and 4.6. Everything on the bias circuit appears to be right.

It may not be right but here’s what I did and the results.

Remember this out of frustration.

I made a voltage divider and put it on the bias wire and it dropped the voltage off the bias supply to 70Vac, checked the bias and it is at -51V. Plugged it into a speaker and it immediately squealed real bad just as if the output transformer wires were reversed. The amp does not squeal with the higher voltage but makes all kinds of other noises. So I reversed the output wires to see what happens and fired it back up. No squealing and quite.

Results of the test with and without the voltage divider and the output transformer wires swapped.

Without voltage divider.
Voltage supply 90+V
Voltage at the diode -88.8V
Voltage after the 22K -75.5V
Bias voltage -66V
Voltage across the 1 Ohm resistor with the meter set at 200Mv 2.2V on 1 tube and 4.2V on the other
Voltage in pin 3 of the power tubes 468
On the scope, nice shaped sine wave
Turning the Preamp volume up the sine wave increases until just under half way before it starts to show distortion. With the Preamp volume on 3 and turning up the Master volume up and the sine wave increases nicely until about 7 to 8.

With Voltage divider.
Voltage supply before divider 90+Vac
Voltage out of the divider 70Vac
Voltage at the diode -69
Voltage after the 22K -50
Bias voltage -53
Voltage across the 1 Ohm resistor with the volt meter on 200 Mv -21.5 on both tubes
On the scope nice sine wave was nice clean and the volume controls worked as expected. But I noticed that the output had increased.

Final test
Without the voltage divider I set the scope at 1V Per division and increase the output until the sine wave was 3 divisions peak to peak.

With the voltage divider connected and without touching the volume controls or the scope settings making sure they were at the same level, the sine wave was 6 divisions peak to peak.

Also fairly quite when plugged into a speaker, the volume controls work as they should and doesn't sound too bad with a guitar plugged in.

Thought's and opinions on this?

[Chuck H]

Well there's other things to consider, I think. Does the channel switching work? If yes, what channel are you testing with? And what is the voltage on the switching circuit?

Why would you set your meter at 200mV range to measure whole volts?

2.2V or 4.2V across a 1 ohm resistor represents 2.2A or 4.2A. That's pretty much impossible. Assuming a conservative plate voltage of 400V the 2.2A tube would be dissipating 880 watts.

Negative voltage across the 1 ohm resistor also seems impossible.

The horrible distortion you heard was likely crossover distortion and that would also explain why the power increased when you made the bias voltage less negative.

I could go on, but it wouldn't be helpful. Clearly the amp has been kludged because you've likey ID'd an aftermarket PT and the OT was wired out of phase. This is going to go well beyond a simple repair I'm sure. Maybe you're frustrated because this isn't what you signed up for. I can get behind that for sure. But that doesn't mean it's out of your depth. Much of the circuit is stock and working. The voltages can be sorted out. You'll probably need to make alterations to the dividers off the bias winding for the bias supply and/or the LV switching supply. You can do that.

What is the voltage on the LV supply for the silicooties right now? And does your board have R#'s and C#'s that match any schematic provided?

[J Luth]

Well there's other things to consider, I think. Does the channel switching work? If yes, what channel are you testing with? And what is the voltage on the switching circuit?

Only one channel in the amp and a whole lot of other things pulled out of this mess

Why would you set your meter at 200mV range to measure whole volts?

DUH, I used mV and low range but posted mV that's what I get for not re-reading first

2.2V or 4.2V across a 1 ohm resistor represents 2.2A or 4.2A. That's pretty much impossible. Assuming a conservative plate voltage of 400V the 2.2A tube would be dissipating 880 watts.

That's what I got 3 or 4 times with the wire bias wire connected

Negative voltage across the 1 ohm resistor also seems impossible.

Not negative eyeball error again

What is the voltage on the LV supply for the silicooties right now? And does your board have R#'s and C#'s that match any schematic provided?

TIf you are asking about a LV supply for the silicooties? IC's I assume, there aren't any IC's in the amp

Main reason I tried a divider on the wire is, no IC's and no LV supply that I see, and I cant lift the board enough to get to the under side to remove any of the components to make a change there.

If he wasn't a friend, I would have told him to come and get it before I even started.

[g1]

I took it that those readings across the 1 ohm resistor were in mV as you said you used the 200mV range. You just need to specify mV when you tell us the readings.
Sounds like you made good progress with flipping the OT leads. Can you get good power output?
The bias stock set-up uses a voltage divider (bias pot, R44,R45,R46). You should be adjusting those values rather than adding an additional divider, but it sounds like you are just doing that for convenience rather than getting under the board. If you leave the extra divider there permanently, you may be drawing too much current from that winding.
The only switching I can see off that negative supply is the bass boost. The voltages needed for that are shown above R29 next to the bass boost switch.
If you can get it set to -50 (as you have) at C24, the voltage to the bass boost switch should be close enough. Or you could go for -48V as the schematic shows.
Really, the only thing you should have to change on the negative supply is the R46 value. (instead of adding an extra divider)
On the + side of the LV supply, you should have the voltages shown at C17, the 2 voltages depending on the channel switch. If necessary, R33 value may have to be adjusted.

[J Luth]

Thanks g1.

First, which schematic are you referring to? The one you put up or the one from Chuck?
Beginning to look like the only thing Laney is the faceplate, chassis and a few other things. I am not familiar with these amps at all. Never been in one.

I took it that those readings across the 1 ohm resistor were in mV as you said you used the 200mV range. You just need to specify mV when you tell us the readings.
OK
Sounds like you made good progress with flipping the OT leads. Can you get good power output?
Yes, and on the scope it's a nice clean sine wave. With the preamp at about 4 the master can now be turned up more than 3/4's before it shows clipping. Of course that changed the more you turn up the preamp.

The bias stock set-up uses a voltage divider (bias pot, R44,R45,R46). You should be adjusting those values rather than adding an additional divider, but it sounds like you are just doing that for convenience rather than getting under the board. If you leave the extra divider there permanently, you may be drawing too much current from that winding.
Yes just temporary


I put it aside for a while probably until next week. I have a busy weekend coming 2 definite gigs and maybe a 3rd and I need to learn a few tunes I have never played for one of them. Oy I'm getting toooo old for 3 in a row. But hey I'm still vertical.
The main problem is the amp has been modified so much it makes no sense, well at least not to me right now that will change the more I study it. I need to study the board more, I was concentrating on the bias and the distortion along with trying to figure out the mess. There has been a lot removed from it and a ton of purple wires running everywhere. Only one channel so no switching available.

If I had a camera here that worked, I post some pictures.

[Dave H]

Without voltage divider.
Voltage supply 90+V
Voltage at the diode -88.8V
Voltage after the 22K -75.5V
Bias voltage -66V
Voltage across the 1 Ohm resistor with the meter set at 200Mv 2.2V on 1 tube and 4.2V on the other

It shouldn't need the voltage divider. I've simulated the AOR50H bias circuit in LTSpice with a 90V AC voltage supply and it gives the voltages on the schematic (-68V -48V) and -36V bias. Double check the values of R44, R45, R46 and the bias pot. I'm guessing by the voltages measured above that the load resistance is too high, someone may have tried to mod it for 6L6s by increasing resistor values.

[J Luth]

Thanks Dave

[J Luth]

Another day another adventure

Got back to the amp today after making a parts run. Broke down, bit the bullet and lifted the board. What a pain removing all the wires. Found quite a few bad solder joints, some possibly causing shorts. Didn’t bother recording where they were, too many of them. Popped out R44 and measured it, it measured 100K, so I took it out. R45 and R46 measured fine.
Tested the circuit with a 39K and a 47K in R44 location the 47K I think give the best results see below.

No extra voltage divider installed

Voltage measurements on the EL34 output tube's
Pin 3 459V
Pin 6 -41V
Pin 8 with a 1 ohm resistor to ground, .031 to .032V with meter on 2V scale.
If this should be tweaked let me know.

Also found a 1 Ohm resister on the input jack instead of a 1M and a 33K at the grid of the first tube rather than a 68K, changed both, 1M on the jack and 68K on the grid.

See what pops up next.

Question

The two schematics I have show the values for C23 and C24 differently. On the one that closely matches this amp calls for each to be 22uf, the other calls for 10uf/22uf. The amp currently has 2. 10uf caps. Any opinions on if the 10uf are OK or should I go 22uf?

[Chuck H]

[B]The two schematics I have show the values for C23 and C24 differently. On the one that closely matches this amp calls for each to be 22uf, the other calls for 10uf/22uf. The amp currently has 2. 10uf caps. Any opinions on if the 10uf are OK or should I go 22uf?

10uf is close enough to fully bypassed that I doubt you'd hear a significant difference changing to 22uf. There are other considerations but I don't think they're relevant at this time. Don't fuss over that right now. I am surprised you haven't reported more consequential symptoms with a 1 ohm resistor in place where the 1M resistor should be on the input. That should have caused significant trouble.

Since you don't have a stock amp, or anything even close to that apparently, there is no schematic for this project. Now we hear that there is a 1 ohm resistor grounding the input signal. Pretty sure this thing is a tear down/ rebuild. At best. Whomever built the circuit that's in it now clearly cannot be trusted. That means repairing it as is (without a schematic for whatever circuit is in there.?.) would be chasing an idiots folly. Your buddy has bought a project chassis. It's a chassis, knobs, transformers and tube sockets. Someone skilled at working with such parts is going to have to built an amp out of it without regard for what's wired in it now before it's useful.

Just my two cents on the matter. Unfortunate as it is.

If the board is stock with R#'s and C#'s (which has never been answered to) and you work from a schematic it may be possible to rebuild it as the stock circuit. I think this would take some considerable experience and familiarity with circuit repair.