Laney LC15R - have I got sympathetic oscillation ?

[chazpope]

Hi all,

I was so pleased with my (now fanned) LC15R as described in other threads that I did go and buy another one. This is a 2004 model and looks a bit better built - this time I was pleased to find no SMDs inside. I had some valve trouble (broke/replaced an EL84) but the amp sounds and plays OK; did a gig or two with it.

However ... it has a case of singing to itself when set to about mid gain and volume - it sounds like a clear high pitched sound same as mike feedback, independent of input or reverb but dependent on all the other knob settings, particularly gain and vol. It is not mechanical, though any chassy knocks are amplified but I guess that is normal. I have resoldered, swapped valves - same thing.

The amp is usable, but you have to watch that fb all the time and though it plays loud you can't really turn it up full wack

One thing I noticed is that my other amp has a sticky gooey foil patch over the preamp part of the pcb while this one hasn't - but looks like that is by design.

Does anyone have any ideas?

Next I think I will disconnect and short P3 and P4 (this will turn it into a LC15 no R) and see if it makes a difference.

[soundguruman]

It would help to have a schematic,
but basically, apply some negative feedback/ snubber caps...
It's not complicated.

This type of system is used in many many amps.
It stops the ringing oscillations, just about completely...

Many tube amp designs have the same problem.
It's very common in overdrive, high gain preamp circuits.

But it's hard to be more specific without the circuit diagram.
Basically, about $2 worth of parts will usually stop the problem.

[DrGonz78]

The other thread on MEF that had this schematic the link no longer works. It can be found online but just thought to upload the file again for easy access.

Link is working now...

[Mick Bailey]

Do you know that the valves are good? A valve prone to oscillation will not necessarily respond to thumps. V1 and V2 both need to be good. If either one is defective you can get the symptoms you describe.

[chazpope]

Now that is interesting - I thought if amp sounds OK valves should be OK, no idea how to check them anyway I swapped them around, no joy; but I have a spare 12AX7 I could try it

Thanks DrGonz for updated link!

[soundguruman]

Hi all,

I was so pleased with my (now fanned) LC15R as described in other threads that I did go and buy another one. This is a 2004 model and looks a bit better built - this time I was pleased to find no SMDs inside. I had some valve trouble (broke/replaced an EL84) but the amp sounds and plays OK; did a gig or two with it.

However ... it has a case of singing to itself when set to about mid gain and volume - it sounds like a clear high pitched sound same as mike feedback, independent of input or reverb but dependent on all the other knob settings, particularly gain and vol. It is not mechanical, though any chassy knocks are amplified but I guess that is normal. I have resoldered, swapped valves - same thing.

The amp is usable, but you have to watch that fb all the time and though it plays loud you can't really turn it up full wack

One thing I noticed is that my other amp has a sticky gooey foil patch over the preamp part of the pcb while this one hasn't - but looks like that is by design.

Does anyone have any ideas?

Next I think I will disconnect and short P3 and P4 (this will turn it into a LC15 no R) and see if it makes a difference.

Method One: click on attachment...
Capacitor between plate and grid of V1A to stop ringing oscillations.
There are several other methods also.
The ringing oscillation almost always originates at the first preamp stage.
Installing the 7 pf capacitor between plate and grid stops this ringing...
Method used by Fender and Marshall...

I prefer to use 7pf 1000V silver Mica Capacitor, soldered directly to the tube socket.
Fender and Marshall used 7pf 1000V ceramic Disk capacitor soldered to tube socket.

To be continued. Will show you other methods.
This above method is the one that I personally prefer to install.

Next: will show methods used by other designers.

[Mick Bailey]

Now that is interesting - I thought if amp sounds OK valves should be OK, no idea how to check them anyway I swapped them around, no joy; but I have a spare 12AX7 I could try it

Thanks DrGonz for updated link!

A valve can still perform as an amplifier and sound ok up to the point oscillation sets in, even if its mechanically unsound. The internal mechanical assembly can become loose enough to respond to a particular resonant frequency long before it goes so slack that it responds to thumps or tapping. You need enough gain/volume to initiate a mechanical feedback loop, after which it will self-sustain like any other feedback.

Your first port of call with oscillation is to eliminate the possibility of a defective valve. It's the most common cause and the easiest thing to fix. Don't assume your spare 12AX7 is ok unless you definitely know that it is. The procedure would be to swap out V1 and test. If no improvement, swap out V2.

Sometimes swapping the PI (V3) with V1 or V2 can fix the problem, as you're moving the affected valve to a less-sensitive part of the circuit. Doesn't always work, but worth a try if you're stuck.

[Jazz P Bass]

Rule out that it is not a Reverb issue.

[chazpope]

Mick it was exactly like you said - replaced V1 with a brand new Electro Harmonics one and that fixed it!

No more whistles and also no more amplified chassis knocks! This really was the tell tale sign I did not see, I will know next time !

I measured at the HT fuse at standby 340V, 39ma. At the OT primary between P1-P8-P2 voltages were 2.0V and 1.4V resp - now that probably is because my EL84s no longer match - is it a problem? I have measured the OT primary as 116ohms on another LV15R not sure if this is the same

[Mick Bailey]

If you're looking at how good your output tube match is there are a few pointers;

1. Turn the lights down and make sure neither valve is red-plating.

2. Calculate the current draw; switch the amp off and measure the resistance from plate to centre tap for each valve - P8 to P1 and P8 to P2. Make a note of the reading. It will most likely not be the same for both halves. Now switch the amp on and measure the voltage across each half of the OT primary at the same connectors. Using Ohms law you can now calculate the current draw for each valve. V/R=I. You need to multiply this by the anode voltage on P1 & P2 to give the dissipation for each valve.

3. If you have a slight mismatch it won't hurt at all so long as neither valve is running to hot. A good match is up to 5% and fair one up to 10% difference. Fender used to specify 'less than 25%' for matching on some of its 6L6 amps.

A good indicator of a reasonable match is if the amp doesn't hum too much with the volume turned right down.

If you want to keep the existing valves and one is a little on the hot side, you can re-bias to bring the pair down so that the hottest one is within limits.

Also, you can sometimes swap the output valves over to get a more even match. This is because of differences between the OT primary halves.

[chazpope]

thanks I will try that today, 2.0v to 1.4 v is exactly half the power though - it can't be good. Also on the pcb there is a Hum Adj pot - I wonder what it does

[Mick Bailey]

thanks I will try that today, 2.0v to 1.4 v is exactly half the power though - it can't be good. Also on the pcb there is a Hum Adj pot - I wonder what it does

You can't tell if it's half the power unless you know the resistance in order to work out the current. The DC resistance mismatch between OT primary halves on small amplifiers can be quite large, and so the voltage drop on each side will reflect this, even if the current draw is identical.

Is the pot marked as hum adjust? The schematic shows a 22k bias pot. Don't adjust this unless you know what you're doing.

[soundguruman]

"The schematic shows a 22k bias pot. Don't adjust this unless you know what you're doing."

Hey! You're trying to take all the FUN out of this?

[chazpope]

You can't tell if it's half the power unless you know the resistance in order to work out the current. The DC resistance mismatch between OT primary halves on small amplifiers can be quite large, and so the voltage drop on each side will reflect this, even if the current draw is identical.

Is the pot marked as hum adjust? The schematic shows a 22k bias pot. Don't adjust this unless you know what you're doing.

So the coils were 117 and 105 ohms and when I swapped valves I did not get much difference, current was 26 and 18 ma and I did not get a better power ratio still one side cooks about 50% more than the other. Sound is ok but can't really get much clean out of it but it is good enough to play.

The bias pot is marked Bias Adj and that's how I set the current to 22 (300v) and about 15 ma resp.
But there is another pot marked Hum Adj - it did not affect tubes balance and I could not find it on the schem. it is located at the heater corner of the pub and I did not want to dismantle it again to have a look from the comp side. If I turn gain to 0 and vol to 10 I do get some hum mind then if I play it would be deafening.

[Mick Bailey]

With the OT resistance you've measured and a current of 26ma and 18ma these don't resolve to the original 2v and 1.4v drop whichever combination of figures is used. Anyhow, looks like you need a matched pair of EL84s.

The hum balance will just be a pot of around 200 ohms with the wiper grounded and each end connected to the heater supply. You just find the best point for minimum hum.