Simms Watt 100w Head

[josegrad]

So, you did replace that last, black cap? it's not the same one that was in there? And, if yes,..

I think we need a shot of the rest of the board. There should still be a couple of power supply capacitors in there somewhere.

Yes, I did replace the black cap, again.

Hard to get a full board picture.
This one is when it had the old caps:



Then 2 more pictures taken from the opposite side:

The jack connectors at the left are the ones for Channel One btw.





There's 3 more capacitors of 25uF, which I have but haven't changed yet. Maybe I should replace those now.




Cheers.

[josegrad]

I think that 1 ohm cathode current sensing resistors are more useful in this scenario; as the amp chassis is out of its cab, the idling point of all 4 tubes can be quickly checked without all the messing around of moving the probe from one tube to the next. ie to verify that the matching is reasonable.
The probe may be handy in future for checking a tube without the hassle of taking the chassis out.

The probe I have to assemble it again, it didn't go well enough yesterday. I bought a kit and it is actually harder to assemble than I thought. My soldering tools are not precision ones.

How/where should I measure to make sure the bias is more or less ok if I don't need to use the probe?

---

Btw I'm a bit surprised no one asked yet for a new set of valves, those are 20 years old at least. And based on what I've read lately they can also be responsible for any kind of cracks and odd noises.

[eschertron]

Channel two, as you said, works decently. No clicks, clacks, or pops? Verify absolutely that this is a true statement. Not mostly or sometimes true.

Then I'd turn my attention to parts of the amp that aren't used by channel two. Any capacitors, valves etc. that are in the signal chain from ch2 input to the speakers have passed inspection, and not likely at all to be involved with remaining issues.

[josegrad]

Channel two, as you said, works decently. No clicks, clacks, or pops? Verify absolutely that this is a true statement. Not mostly or sometimes true.

You are right. There's pops, clicks and what not in both channels. Channel Two is a bit better but not stable at all. Volume low and suddenly huge volume, then off, then pops. After those HUGE pops I can put all volumes to 0 but still the guitar comes through. Very sensitive when I hit the amp with a stick. I did hit the valves and all cause pops, the first power valve caused so huge pops. Luckily I have a cheap cabinet connected to it.

The first channel the same, you can get some sound but lots of pops everywhere, often after touching a pot just a bit.

I guess I'll end up having to replace almost every single component there.

My intuition tells me to replace first those 3 25uF caps I didn't change yet. Then every valve. Then all pots. Checking in between how it goes.
Luckily I'm not in a hurry with it.

I think can't see all the pots in the schematics though.

[Chuck H]

Have you tried cleaning the pots, jacks and tube sockets? And there may be cold solder joints that are also showing now if things have become oxidized and grimy. So also re-flow solder on iffy looking joints. I've "repaired" a lot of amps just by cleaning them up.

[josegrad]

Have you tried cleaning the pots, jacks and tube sockets? And there may be cold solder joints that are also showing now if things have become oxidized and grimy. So also re-flow solder on iffy looking joints. I've "repaired" a lot of amps just by cleaning them up.

I have tried to clean the pot to some extent by pouring contact cleaner spray inside and moving them. But I should clean the jacks and sockets.

Solder looks dirty and old. Here's some examples:





Any recommendation/good method to clear those parts? Other than pouring contact cleaner?

But besides that. Do you really think the valves could be ok after so many years?

[Chuck H]

I have tried to clean the pot to some extent by pouring contact cleaner spray inside and moving them. But I should clean the jacks and sockets.

It's best to blow out debris before cleaning. But I think it may be the tube sockets that are the problem. Anything that isn't a soldered connection and is exposed to the air will oxidize on it's contact surface. So that would include tube pins and the socket holes. The symptoms you report are common with dirty contacts.

Solder looks dirty and old. Here's some examples:

Well, solder just looks like that It actually looks like pretty good solder work. By "iffy" joints I mean any that show a lack of flow on any of the intended connected parts. You can often see it. It'll look like some sort of surface tension kept the solder from flowing onto and sticking to the surface of the component lead or turret.

Besides that. Do you really think the valves could be ok after so many years?

That only depends on how much use they've had. Since we don't know when they might have been replaced relative to use hours they could well be fine. Or horribly old. We don't know.

[eschertron]

Hearing the effects of cleaning the tube sockets is a quick and easy thing to do. Definitely make that a priority. A little bit of cleaner wiped or sprayed onto tube pins, then a little of the old 'in and out'. Let the cleaner finish evaporating then re-insert the tubes.

[pdf64]

Best to avoid lubricating type cleaner on tube pins / sockets.

[pdf64]

...How/where should I measure to make sure the bias is more or less ok if I don't need to use the probe?...

Each power tube cathode should have a 1 ohm resistor fitted between the cathode socket terminal and 0V.
On this amp, the bus bar that seems to link all the power tube terminals 1 and 8 (presumably to Chassis 0V) will need to be removed.
The '8' terminals can be linked to terminal 1, or to 0V common.
The mV across the 1 ohm resistors at idle will equal the mA cathode current.

[josegrad]

Hi again,

I cleaned all the valve sockets well and jack inputs and...
I was 10 minutes trying it out and no single pop or clicks out of the amp :-)

Now it is better. I gently hit the valves and around the amp and also no pops or anything.

So it starts to look better.

I guess I should move into checking the bias setting.
I'll need to assemble that bias tool and hope I can do it better this time.

Cheers.

[josegrad]

Each power tube cathode should have a 1 ohm resistor fitted between the cathode socket terminal and 0V.
On this amp, the bus bar that seems to link all the power tube terminals 1 and 8 (presumably to Chassis 0V) will need to be removed.
The '8' terminals can be linked to terminal 1, or to 0V common.
The mV across the 1 ohm resistors at idle will equal the mA cathode current.

Hmm I don't know pdf64, sounds a bit complex for my skills. Since I bought that BIAS tool shouldn't I try with that?

[pdf64]

What aspect is complex for you?
As I mentioned, the bias tool may be useful. But checking all 4 tubes is beneficial, and it will end up being a pain in the arse using it; there's a reason that 4 way bias probes are on the market https://www.tubeampdoctor.com/en/sho...System_BM4_491

[josegrad]

What aspect is complex for you?

Lets see if I get the idea.

Each power tube cathode should have a 1 ohm resistor fitted between the cathode socket terminal and 0V.

Ok. Cathode socket terminal in my understanding the terminal 8.
0V is 0V Chassis?

So should I place a ohm resistor between each terminal 8 and Chassis for each tube?

On this amp, the bus bar that seems to link all the power tube terminals 1 and 8 (presumably to Chassis 0V) will need to be removed.
The '8' terminals can be linked to terminal 1, or to 0V common.

Yes, that is true. 1 and 8 are all linked together and they go to the Chassis.
But "need to be removed" is not clear to me. You you mean they should not linked to the bus, making each tube independent from the others?

So I would still leave terminals 8 and 1 connected to each other, but not connected to the other tubes. And that's it, no need to connect them to 0V common (In case I was 100% where 0V common is located...)

The mV across the 1 ohm resistors at idle will equal the mA cathode current.

So I should start the amp, with all the valves in place.
Then measure the mV accross the 1 ohm resistors.
Then measure the current with the probes in terminal 8 and Chassis 0V (?)
And adjust the bias resistor to match them?

-----

So by complex that's what I mean. Many silly questions.

UPDATE:

I guess something like this procedure.

https://insigniahifi.com/2016/01/22/...bias-your-amp/

---

Another thing that is not clear to me.
There are two bias adjustable resistors. I lack general understanding on this amp. So there's 2 channels and 4 valves. Does it mean each channel uses 2 valves? Hence 2 bias adjustable resistors, one per channel?

Complex, very complex indeed for non tech people like me.

[g1]

So should I place a ohm resistor between each terminal 8 and Chassis for each tube?

Yes

So I would still leave terminals 8 and 1 connected to each other, but not connected to the other tubes. And that's it, no need to connect them to 0V common (In case I was 100% where 0V common is located...)

Right. They are now connected through the 1 ohm resistors.

Each bias trim pot will control 2 power tubes. It does not have to do with the amp channels, but it makes it easier to match up a set of tubes. Once you have the resistors installed, and start measuring the idle conditions, it will make more sense.