1986 Laney Pro Tube Lead A050 Series YIEKS

[J Luth]

A friend bought the amp and had it shipped to him. When it arrived it had a broken input jack. He asked me to fix it, I figured an input jack no problem Opened it up and what a mess. Looks to be heavily modified as well as a few broken wires. Power trans obviously burnt up and was replaced at some point. When it burnt up it took out the entire power section on the PC board. Everything in that area has been replaced. It was not done very well but works so far. I fixed the jack and that’s when all hell broke loose. Fired it up on the limiter no problem, Fixed a couple of things then fired it up without the limiter, horrible sounding and all kinds of noise. I told him to send it back and get his money back but wants to keep it. It's actually worse than what I listed. Well now the fun begins.


A question, please correct me if I am wrong.
During the voltage check I am finding that the supply voltage for the bias is 90V. More than likely the wrong power trans installed. Would using a voltage divider be the way to go to drop the voltage to around 50V? Maybe using a 220K 1W and a 330K 1W (not sure of the Watt rating) for the divider. The bias for the EL34’s can only be set between -59V to -66V.

[m1989jmp]

A friend bought the amp and had it shipped to him. When it arrived it had a broken input jack. He asked me to fix it, I figured an input jack no problem Opened it up and what a mess. Looks to be heavily modified as well as a few broken wires. Power trans obviously burnt up and was replaced at some point. When it burnt up it took out the entire power section on the PC board. Everything in that area has been replaced. It was not done very well but works so far. I fixed the jack and that’s when all hell broke loose. Fired it up on the limiter no problem, Fixed a couple of things then fired it up without the limiter, horrible sounding and all kinds of noise. I told him to send it back and get his money back but wants to keep it. It's actually worse than what I listed. Well now the fun begins.


A question, please correct me if I am wrong.
During the voltage check I am finding that the supply voltage for the bias is 90V. More than likely the wrong power trans installed. Would using a voltage divider be the way to go to drop the voltage to around 50V? Maybe using a 220K 1W and a 330K 1W (not sure of the Watt rating) for the divider. The bias for the EL34’s can only be set between -59V to -66V.

Your best bet is to find a schematic first. With bias voltage that high, you can use 6L6 and similar tubes.

Sure, you can form a voltage divider and 1W is more than fine, no significant current passes through the bias network.

[J Luth]

I have a schematic it calls for -48V but I will calculate the voltage to run them probably between 60 - 70% as requested.

[Shawnobi]

Is this the AOR series? Hunt down a guy called Oli Foxen in the UK. Has an Ampstack page on Facebook. He knows these things backwards.

[Chuck H]

As mentioned above, you would want to build a bias supply and not simply voltage divide the existing bias supplies voltage. There are a ton of adjustable bias circuits you could copy and they all include a voltage divider. You would simply need to identify the voltage division in the circuit and tweak the circuit values for the output voltage range you want.

[J Luth]

Yes it's an AOR.
Thanks, I'll take a look around.

[J Luth]

Thanks Chuck. This thing is a mess.

Just trying to save some time on the bias issue, that's why I asked about the divider. I figured it would be quicker to use a divider for now just to get the voltage down. It still has a lot of other issues that need addressing.

Who ever modified it and or repaired it should be keelhauled.

Granted I am still a newbie to all this and learning all the time but even I know better than a lot of what has been done to this.

I have spent a lot of time just trying to clean up what was done never mind trying to fix it.

Good thing the owner is a good friend. I keep telling him to send it back get his money back since the amp was miss-represented, broken when he got it and unusable when the jack was fixed and the wires reconnected.

[Chuck H]

Who ever modified it and or repaired it should be keelhauled.

That's funny because I've said the exact same thing here. Another time I suggested that the modder should be "peeled". Either should be suitably awful

The reason you shouldn't simply voltage divide is that you really do want an AC bypass (cap across the divider load) at the junction of the bias resistors to support other circuits requirements. And if you trouble about managing that then you may as well build a useful bias supply. Which typically includes that feature. The impedance of the bias supply is important and DC resistance IS impedance in the absence of a bypass cap.

[J Luth]

For right now I am just trying to calm this thing down so it can be checked without having to pop out the circuit board. Can't lift it enough to get under it without disconnecting a lot of wires.

It has an adjustable bias circuit on the board and all the associated resistors and caps appear to be within spec but it only allows adjustment of -59 to -66V. My thought was to try a simple voltage divider on bias supply wire off the transformer between the transformer and where it connects to the board to bring the voltage coming out of the transformer down so that the current bias circuit can be adjusted lower. Not putting anything in the actual bias circuit on the board.

[Chuck H]

I don't think you should do that yet. The resistance right off of the bias winding already goes through a voltage division to create the LV for the channel switching. Altering voltage off the winding will goofball that if the last guy to "mod" it has already set up the correct series resistance. So, assuming this is an after market PT and is basically working, you should start by checking the actual bias voltage at the tube sockets and the current through the tubes at idle I think.

[J Luth]

When I checked the Bias voltage at the tubes it is -66V and can be adjusted between -66 and -59. I have not checked it using a 1ohm resistor off the cathode to ground. There isn't much out there for schematics for this that I have found. I am using the one he gave me which is close. The bias circuit on the board looks correct as far as values and locations of the components. still need to check them. There were and are a few things the previous person did such as putting a jumper across a couple of resistors, one appears to be to repair a possible broken trace and another is just jumping out the resistor which I removed. After I check everything I maybe I'll try changing the values on the divider on the board if nothing else works.

[The Dude]

......I have not checked it using a 1ohm resistor off the cathode to ground......

Well, I'd do that first. The bias voltage is somewhat meaningless. If bias current is in the ballpark, no modification may be required.

[g1]

I don't think you should do that yet. The resistance right off of the bias winding already goes through a voltage division to create the LV for the channel switching.

I'm missing something. Is there a schematic you are referring to? He said in the first post that the replacement PT is giving 90V bias source, I assumed this was a dedicated bias winding.
It is also feeding the channels switching supply? This would have also required mods to get to the appropriate voltage?
If it's just a dedicated bias winding, the standard voltage divider arrangement should be the simplest.

[Chuck H]

The forum won't let me upload the file from my poot.

I'll try linking it directly. Hope you all get it here. Not real legible I'm afraid.

[g1]

Thanks Chuck. Now I see the switching.
Here is another that has 90V marked on that winding. Maybe JLuth can check where the difference lies and why the bias voltage is so negative.