Simms Watt 100w Head

[Jazz P Bass]

The 270 ohm / 1 watt resistors appear to be the screen resistors.
I would replace all four of them.
(Don't be surprised when you get new ones in the difference in the size of them. They will definitely be smaller in diameter.}

For testing either one that you have on hand will function.

[josegrad]

Right. I have now replaced the bad resistor. And the light bulb limiter stays dimmed, when the valves out.

As soon as I put the valves in place the bulb limiter lights up.

Should I go and buy new valves?

[eschertron]

Right. I have now replaced the bad resistor. And the light bulb limiter stays dimmed, when the valves out.

As soon as I put the valves in place the bulb limiter lights up.

Should I go and buy new valves?

No. Please measure the voltage across the bias cap, and voltage at the output tube plates, to see if the tubes are running away because of bias voltage or something else, or.. are normal for the tests you are running. Until we have a good idea of how much current is going through the tubes, we can't say.

[josegrad]

The voltage across the bias cap is 41.5 VDC. With the valves on, which I guess don't matter.



Voltage at the output tube plates I'm not yet sure how to measure it.
I was thinking of buying this kit to measure it easily:
https://en.uraltone.com/uraltone-bia...-kit-5544.html

[g1]

Now the light bulb lights just a bit and then remains almost invisible.

But when I place all the valves again then the light bulb lights. Not full brightness but much brighter than without the valves.
I guess I should buy new valves and see what happens.

This sounds like it may just be the heater draw that is making the lamp glow. If that is the case, it may be normal.

[eschertron]

While tubes are in, there's no indication that the tubes are in thermal runaway? No Large glowing orange or red spots on the tube innards? A bright glow localized on the heater wires is normal, where the heater wires are visible.

You measured the supply voltage before, reported as 388vdc. If that's with tubes out, what is it with tubes in?

[Chuck H]

Well we don't yet know if "tubes in" only means that tubes are in.?. What position is the standby switch in during these reports?

[josegrad]

The stand by switch has been all the time off. All I've done is only with the mains ON.
The 388vcd was measured with and without the valves, same result.
The heater wires of the power valves light up a bit when the mains are ON, I would say a normal valve on.
The preamp valves show no light at all.

[pdf64]

Terminology around standby can be confusing - what exactly does it mean for standby to be on or off, as it may or may not be understood as being the inverse of the amp being able to operate, HT energised
To improve clarity, I suggest that the standby switch be referred to as being in 'standby' mode or 'operate' mode.

Where exactly is the 388Vdc being measured? It would be most helpful to do so at the 'C' HT node.

Do the preamp valves get hot?

[josegrad]

Where exactly is the 388Vdc being measured? It would be most helpful to do so at the 'C' HT node.

There was an image for that also:

[josegrad]

This post is getting almost impossible to follow with so many sub-threads.

Today I got a bit more powerful light bulb, about 116W, to see if that would make any difference.

With all the valves placed in the amp. Only mains turned ON it looked like this:



Which was quite acceptable I think.

Then, hmm I turned ON the other switch, which reads Stand By.
At that point the light bulb went much more bright and then dimmed out. Not as dimmed as the image above, but almost.
Slight hum was coming out of the speaker.

So, I know you don't like it, but I did it.

I Removed the light bulb limiter

I connected a guitar, all amp volumes set to zero.

I switched the mains ON.
Preamp valves lighted up, power valves lighted up. In my opinion normal, no flashes, red or blue lights.




Then
I turned the second switch on...

...and

Within a minute a saw smoke coming out a resistor
This one:



For a second I could actually hear the guitar, quite distorted, even with all the amp potentiometers at zero.

------

So, I guess I'll have to stop playing with it and start thinking of getting it serviced properly. Before I completely destroy it

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I'm can't wait to read your comments

[Helmholtz]

There was an image for that also:

[ATTACH=CONFIG]51122[/ATTACH]

388V HT with or without the bulb limiter? The limiter reduces all internal voltages.

[josegrad]

388V HT with or without the bulb limiter? The limiter reduces all internal voltages.

With the limiter. But I think you could check my latest post in the main thread.
Update. I switched this thread to linear mode, and actually what I though was my latest post is just before this.

[eschertron]

So what would be interesting to know is what the HT voltages do without the light bulb limiter, before and after flipping the 'standby' switch. The bulb getting bright for a moment, then dimming again is exactly what we'd expect from a healthy amp as the PSU caps charge. I think you're close. But be patient, voltage readings can tell us more than words like 'bright' 'dim' 'hum' (all amps hum to some extent).

Which resistor? After discharging the PSU caps, measure the resistor. Has it failed? Drifted far in value? If it didn't smoke or burst into flames immediately, you may still be able to power up and take voltage readings on each of its legs to ground (before it starts smoking again).

[josegrad]

So what would be interesting to know is what the HT voltages do without the light bulb limiter, before and after flipping the 'standby' switch. The bulb getting bright for a moment, then dimming again is exactly what we'd expect from a healthy amp as the PSU caps charge. I think you're close. But be patient, voltage readings can tell us more than words like 'bright' 'dim' 'hum' (all amps hum to some extent).

Did you also read this one?
https://music-electronics-forum.com/...l=1#post514186

Well, without the light bulb limiter I wouldn't dare to measure anything after flipping the "standby" switch. Things start getting fried after a minute. I just killed a resistor as mentioned in my previous post. Or well if 2 secs are enough to get the reading I guess I could do it.

Could you please define HT. Tech acronyms just pass by my brains.