Simms Watt 100w Head

[eschertron]

Sorry. I thought I'd read 'HT' in one of your posts. HT = high tension, what the British call high voltage.

You mentioned the resistor got hot after a minute, so I thought that 5 or 10 seconds would be OK. Have you drawn out a schematic of the power supply section? Include the bias circuit. A picture with some voltages. You can leave the tubes out for the first voltage readings, standby in 'operate' mode.

I did read that post. Sounds hopeful. I don't think you're destroying anything. Well, maybe that one resistor... but let's find out why.

[Chuck H]

It's hard to be certain because neither the schematic or the photo's are very revealing, but...

I believe that resistor that is burning serves HV to the preamp circuits. That would indicate excessive current in the power supply AFTER the power amp. If I've interpreted the thread correctly I don't think you've replaced that filter/decoupling cap in the power supply. It may be misbehaving and demonstrating a partial short under high voltage conditions. That or there may be a miswire or short in the HV wiring after that resistor. Or possibly a shorted preamp tube. Which is a rarity, but a possibility I think I brought up much earlier.

[pdf64]

There was an image for that also:

[ATTACH=CONFIG]51122[/ATTACH]

Sorry it's not obvious to me what circuit node the red lead is clipped to?

Yes, reading threads is best done using linear viewing mode.

[josegrad]

Sorry it's not obvious to me what circuit node the red lead is clipped to?

How to say. The red lead is clipped to the 'Standby' switch. The switch is OFF, and by turning it ON the measured voltage would be passed to the rest of the circuit.
I hope that makes sense.

[josegrad]

You mentioned the resistor got hot after a minute, so I thought that 5 or 10 seconds would be OK. Have you drawn out a schematic of the power supply section? Include the bias circuit. A picture with some voltages. You can leave the tubes out for the first voltage readings, standby in 'operate' mode.

Yep the resistor got as hot that smoke was coming out of it.

The schematics I have are these:

[nevetslab]

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:

[ATTACH=CONFIG]51123[/ATTACH]

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.

[ATTACH=CONFIG]51125[/ATTACH]


Then
I turned the second switch on...

...and

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

[ATTACH=CONFIG]51124[/ATTACH]

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

------

I'm can't wait to read your comments

Look at the lowest picture, where the resistor that is overheating. It's connected between the (+) terminals of the Blue Cap on the right, and the other side, it's connected to the (-) terminal where a red wire is connected, while the two (+) terminals on the Left cap has a GRN wire attached. This cap is wired backwards. And, I think the Red wire is going to power other circuits, while the GRN wire is probably going to ground. Verify that GRN wire is going to ground. At any rate, this dual section filter capacitor is damaged and needs to be replaced. It should have been wired so the resistor in question went to the strapped two (+) terminals, and the (-) terminal go to ground.

[eschertron]

That ^^^

If those two caps pictured are the totem pole arrangement, then the resistor is in the wrong place. If the resistor (27k?) is correct, then one cap is installed backwards.

It's good to have the schems provided. Now draw out what's actually in the amp to compare.

[josegrad]

This cap is wired backwards

Hmm. I replaced those caps.
While looking for info about the amp I found, in Finland actually, a company that did service the same model a few years ago.
And they even have some images of the circuit, so I copied that part, to get it right.
http://uraltone.com/info/wp-content/...tts_mkii_1.jpg

I think I did the same connections.



I guess you've seen the schematics above. The section in question is this:




So are those wrongly connected then?

[nevetslab]

Hmm. I replaced those caps.
While looking for info about the amp I found, in Finland actually, a company that did service the same model a few years ago.
And they even have some images of the circuit, so I copied that part, to get it right.
http://uraltone.com/info/wp-content/...tts_mkii_1.jpg

I think I did the same connections.

[ATTACH=CONFIG]51129[/ATTACH]

I guess you've seen the schematics above. The section in question is this:

[ATTACH=CONFIG]51130[/ATTACH]


So are those wrongly connected then?

They got the bottom cap wired wrong too. The top two caps are the series-connected 2x50uF caps to yield 100uF + 100uF with the 220k ballast resistors across each cap can. You've repeated their mistake, sorry to say. That's why your 27k resistor is burning up. Shame, as this dual section cap isn't inexpensive.

[josegrad]

Shame, as this dual section cap isn't inexpensive.

Cool. I'm buying most of the components I've replaced from them. I'll ask them for a discount.
Takes me 1 hour round trip to get to their shop.

So I'll have to visit them again next week. I'll ask them remove that offensive picture from their site.

THANKS A LOT for your comments guys!
I'll report the progress as soon as I get the new components in place.

[Chuck H]

Just in case a supporting thumbsup is't enough... I puzzled and puzzled until my puzzler was sore and all I could conclude was that both images show incorrect wiring. Good catch nevetslab.

[josegrad]

Right. I have now replaced the capacitor, the resistor and now the connection looks like this:



With the valves in place and through the light bulb limiter.

Mains ON, light bulb lights and then dims.
Stand by ON, light bulb lights and then dims.
Slight hum coming from the speaker.
When I switch it off by turning off the stand by, and then mains off, a relatively loud click comes out of the speaker.

With the valves not in place and through the light bulb limiter.

Mains ON, light bulb stays off.
Stand by ON, light bulb lights and then dims to invisible.
No sounds from the speaker.

----

I got one of these tools to measure bias btw:



I didn't try anything yet without the light bulb limiter.
I can measure anything you are interested in before removing the light bulb limiter.

But in case I have to measure anything I would appreciate to know where to place the probes.
The schematics were here:
https://music-electronics-forum.com/...7&d=1542389576
https://music-electronics-forum.com/...8&d=1542389601

Cheers.

[josegrad]

Ok. So I did a test without the light bulb limiter.

No smoke this time!
Even sound. Almost no hum from the amp.

Channel Two of the amp, "Brilliant" seems to work decently. Sound with both jacks. Tone controls affect the tone, not sure how much their effect has been affected by the years but they do alter the sound.

Channel One, "Normal" is a bit problematic, from the first jack I can only got loud pops, from the second jack some guitar sound but quite some pops as well. Tone and volume controls give even more loud pops.

Switching off the amp gives also a loud pop.

---

So some progress I think. But still some work to do with it.

Comments appreciated.

[Chuck H]

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.

[pdf64]

...I got one of these tools to measure bias btw:

[ATTACH=CONFIG]51195[/ATTACH]

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.