Echolette M40 problem

[gluubzhe]

Hello, i got some pretty strange issue with my Echolette m40. When i turn it and play everything is good, but after about 15 minutes, more or less, the volume goes down and sound becomes thiner. And in a couple of minutes it sounds like broken fuzz pedal with low volume. And it only resets after i turn off Echolette and let it cool down. So what's wrong with it, is it some tube or could it be for example the cabinet, what's causing problems?

[pompeiisneaks]

Looking at what looks like the schematic, it uses 4xECC83/12AX7 You may need to replace one or all of them, they could be going bad. Also there are 4xEL84 output tubes that could, as well, be going out.

echolette_m40_sch.jpg

If new tubes don't help, you may need to check any components along the signal path for failure.

Here's a schematic of it 'supposedly' per a search. Hope that helps?

~Phil

[Stevem]

From your post and the comment you made on the cabinet possibily being a issue I do not think you have enough technical ability to trouble shoot this issue even with our help here, so the amp I would say needs to go to a repair shop.
If I am wrong then please accept my apology in advance!

[Lauri]

Common problem with Klemt amps is that coupling caps go bad. If changing tubes doesn't help, I would start by checking if any of the caps in the signal path are leaking dc or shorting.

[gluubzhe]

From your post and the comment you made on the cabinet possibily being a issue I do not think you have enough technical ability to trouble shoot this issue even with our help here, so the amp I would say needs to go to a repair shop.
If I am wrong then please accept my apology in advance!

You are right about the thing, that i'm not techician and could not fix it myself, but my assumption about cabinet was because, one day i brought it to master . He repaired other problems and replaced inputs, i played some time on his 4 speaker cabinet and everything was fine, but when i came home later and played it with 1 speaker cab - the problem occured. So the best way is to give Echolette once again to the master?

[TUBEDUDE]

Yes, give it to a competent tech. It may be losing bias.

[gluubzhe]

Hello again, everyone. I still didn’t repair it. I already have it to technician and he changed those two capacitors, and ez81 tubes near them. Also he changed el84 tubes, but still the same problem. So I noticed that it happens because of el84 overheat. So they are getting so hot.. I tried to pull two of them (1st and 3rd) and it helped a bit: so now volume goes down after about two hours, not after 20 minutes like it was before.. but still it is not working properly. So what else can it be? My technician told me that he can’t find solution. What can I advise him to check? I read here about some bias, so where is it and how to repair it?

[turbofeedus]

Sounds like a bias problem, maybe the coupling caps after the PI? Are the EL84 redplating?

Hello again, everyone. I still didn’t repair it. I already have it to technician and he changed those two capacitors, and ez81 tubes near them. Also he changed el84 tubes, but still the same problem. So I noticed that it happens because of el84 overheat. So they are getting so hot.. I tried to pull two of them (1st and 3rd) and it helped a bit: so now volume goes down after about two hours, not after 20 minutes like it was before.. but still it is not working properly. So what else can it be? My technician told me that he can’t find solution. What can I advise him to check? I read here about some bias, so where is it and how to repair it?

What caps did the tech change exactly? C20 and C21 are the coupling caps that are suspect. They're not "near" the EZ81 rectifier tubes. Also need to check all the components around the bias circuit.

[gluubzhe]

Sounds like a bias problem, maybe the coupling caps after the PI? Are the EL84 redplating?

Hello again, everyone. I still didn’t repair it. I already have it to technician and he changed those two capacitors, and ez81 tubes near them. Also he changed el84 tubes, but still the same problem. So I noticed that it happens because of el84 overheat. So they are getting so hot.. I tried to pull two of them (1st and 3rd) and it helped a bit: so now volume goes down after about two hours, not after 20 minutes like it was before.. but still it is not working properly. So what else can it be? My technician told me that he can’t find solution. What can I advise him to check? I read here about some bias, so where is it and how to repair it?

What caps did the tech change exactly? C20 and C21 are the coupling caps that are suspect. They're not "near" the EZ81 rectifier tubes. Also need to check all the components around the bias circuit.

Yes they turn red inside and very hot, if you say about this.
He changed exactly two tubes near ez81 between two transformators. And you are talking about cap that is to the left of el84’s? Tech said it will be very hard to find the same cap. So maybe you can advise analog cap?
And where is the bias situated?

[Stevem]

With what you discribe you need to replace C25 and the resistor to its right which by the way is marked wrong on the schematic since it's labeled as 50k ( 50,000) but should be only 50 ohms.
The replacement for C25 should have its voltage rating increased to 50 volts and to run the tubes a little cooler I would use a 50 ohm and a 10 ohm resistor in series for a total of 60 ohms.
The 50 ohm resistor should be a 10 watt and the 10 ohm a 5 watt, both being a wire wound type .

You should do whatever you can to keep those two resistors as far away from that C25 as you can as that level of heat from those resistors will shorten the caps life greatly!

If you have room and can get one using a 15 watt resistor for the 50 ohm one will be a great idea, also with the 10% to 20% tolerence on these type of wire wound resistors( which is common ) if you get a few you might find a 50 ohm one that test close to 60 ohms anyway.

Also note that since you have been Red plating those output tubes that one or more of them may be very weak now , or bad , but not shorted.

[martin manning]

With what you discribe you need to replace C25 and the resistor to its right which by the way is marked wrong on the schematic since it's labeled as 50k ( 50,000) but should be only 50 ohms.
The replacement for C25 should have its voltage rating increased to 50 volts and to run the tubes a little cooler I would use a 50 ohm and a 10 ohm resistor in series for a total of 60 ohms.
The 50 ohm resistor should be a 10 watt and the 10 ohm a 5 watt, both being a wire wound type.

I agree that C25 is suspect, but why do you say the value of the resistor next to it (R81) is wrong? Note this is a fixed bias amp.

[Stevem]

Your so right, I missed that!
So yes that 50k resistor is correct but the that negative supply needs to be looked into!