TSL601 - No Sound

[Enzo]

Many things are possible. The way to solve it is not to go down a list of possibles. We systematically determine where the problem lies. Once we know that, the fix is usually obvious. How do we do that? We check that we have all power supplies and that the power is getting to all stages. We make sure every stage has tubes that are conducting current. We then inject a signal at the input, and trace it through the circuit. Does it reach the first stage? The second?, the third? Does it reach the power tube grids? You trace until youfind a point the signal goes no further. Alternatively, inject some signal, even noise, into the grids of the power tubes. Does it come out the speaker? move back to the phase inverter and inject there, hear it? Keep moving back until you find a point it no longer passes. And when you find that point, it could be a bad part, could be a bad solder joint, could be a broken wire or loose connector. And so on.

[g1]

I tested both old tubes one at a time and they both tested around 18.8 that was only with a few minutes of warm up, so like you say they are probably around 20mv. So they are still working. Will the lower bias affect the sound?? I’ve never been quite happy with the sound of this amp and bought pedals to compensate. So knowing both tubes are working but under biased, Do you think it’s safe to try in the new tubes and bias them to 80mv?

Yes it sounds like they are both good and it was just set for low idle current. Could have been in error, could be someone wanted to deal with the heat that alexradium mentioned above. You can play around with it and see if it has much impact on the tone, I think it will be minor. My guess is that something in the middle around 60mV will probably be a good compromise between tone and tube life.

In regard to the other issue, the loss of sound completely, could it be a dry solder joint? It’s only ever happened that one time. The only thing I did between it not working and coming back to life was remove the tubes and chassis, then put it back together, I didn’t nothing else. Could it be possible there is a dry joint on one of the tube sockets that when I’ve removed and replaced them it’s reconnected something? Or is that unlikely.

With board mounted tube sockets it's not uncommon for the pins to need resoldering. Your theory that you may have restored a bad connection by removing and re-installing a tube is quite possible.

It is preferable to locate and correct the fault as Enzo mentioned. Inspection of the socket solder connections with a magnifier will often show obvious solder faults.

[Stevo6791]

Yes it sounds like they are both good and it was just set for low idle current. Could have been in error, could be someone wanted to deal with the heat that alexradium mentioned above. You can play around with it and see if it has much impact on the tone, I think it will be minor. My guess is that something in the middle around 60mV will probably be a good compromise between tone and tube life.


With board mounted tube sockets it's not uncommon for the pins to need resoldering. Your theory that you may have restored a bad connection by removing and re-installing a tube is quite possible.

It is preferable to locate and correct the fault as Enzo mentioned. Inspection of the socket solder connections with a magnifier will often show obvious solder faults.

Hi

So I put the new tubes in and set the bias to 60mv then monitored it for 30 minutes or so, it did move around a little. But it’s more or less now steady at 60mv, it’s drifting up to around 60.7, then down to 60.3 but it’s not going any further than that.

I have to say, on first play this amp has never sounded how it now does! it’s like I have a new amp. It’s much cleaner and crisper on channel one, and channel 3 which I’ve very rarely use because it’s always sounded really dark and muddy now sounds close to the pedal I use. I wonder if the store I sent it to all those years ago didn’t really know what they were doing and maybe even put in a second hand set of power tubes.

I am now considering replacing the preamp tubes as well with JJ’s, the original Marshall ones will be close to 20 years old now.

The loss of sound Issue, seeing as it hasn’t happened again yet, I’ll give it a week or so to see if happens again. If it does I’ll check the solder and other options given here.

I’ll run it in over the next week and see what happens.

Thank you everyone for all your suggestions, most appreciated.

Steve

[Stevo6791]

Hi,

So its been about a week and the Amp is still alive, I have not lost sound again and its the best its sounded as long as I can remember, however I do not think all is well.

This Amp has always had a hum when the standby switch is on and its never really been annoying enough to notice when playing. But that has increased now and can be heard when playing, I assume it should not hum. Also when turning on the standby switch I'm getting a loud thump from the speaker. I don't know if having the bias set correctly now has increased the hum somehow, its always been there its now just a lot louder, the thump was not there before.

I did a bit of a search online and found a few people with JMC2000 who had hum and a thump when switching on the standby switch they traced it to C26 and C27 22pf 500v ceramic capacitor that are connected to the EL34's. Apparently they are underrated for the job they do and its quite common on JMC2000 series amps. They replaced these with 22pf 1000v and apparently that solved the issue. Anyone have any experience of this?

I took the chassis out to have a look at C26 and C27 and noted an oddity on one of the resistors next to C27, R49 which is also connected to the EL34, it seems to have a hole in the outside coating? is this normal for this type of resistor? it still reads exactly 1K, or is this not a good sign? The other one that doesn't have a hole in it R48 reads 0.987K

I have attached some photos of said resistor.



Thanks for any advice

Steve

[Stevo6791]

TSL601 - Hum and standby thud (fixed)

Ok, so I bit the bullet and just went ahead and changed out those two caps C26 and C27 myself with 22pf 1000v replacements and it worked, hum and pop are no more. It is absolutely silent now, and louder. I was having the master volume at 5 or 6 to get a reasonable volume but now it’s more 3.

First off taking the board out was a hell of an excursion! Having never done it before.....

One of the caps I took out had a burn above one of its legs and seemed to read 11K ohms on my meter, the new Caps and the other cap I took out didn’t read anything at all. So from that I can only assume one of them was bad the other probably was ok?? I’m only guessing.

The hole in the coating of the resistor in my last post can’t be causing an issue and must be just a manufacturing defect, a bubble that popped maybe.

The bias is rock steady now, but it did require setting again as it had gone up to 72mv from the 60 I set it at previously. It was going from about 59.8 to 60.2.

Just thought I’d post this here it may help someone else.

Steve