Simms Watt 100w Head

[pdf64]

The readings are 589, 587, 589 and 589 VDC.

I think those should be the readings at pins 3, 4 or 6?
Pin 5 (or either leg of the 10k grid stoppers) should be about -40Vdc.

[josegrad]

I think those should be the readings at pins 3, 4 or 6?
Pin 5 (or either leg of the 10k grid stoppers) should be about -40Vdc.

Good catch pdf64, yes that was Pin6 then.
When reading at Pin 5 the values are:

With mains ON:

-48.2 -48.2 -47.2 -47.2

With mains ON and Stand by ON:

-47.2 -47.3 -46.2 -46.2

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Tomorrow I could maybe go and get a new tube and see if it makes any differences. In case that one tube is bad.

[Chuck H]

Can you please, with the standby switch in the play mode (closed) measure voltage at pin 8 of any of the power tube sockets.

[josegrad]

Can you please, with the standby switch in the play mode (closed) measure voltage at pin 8 of any of the power tube sockets.

Pins 1 and 8 are interconnected to the bus that goes to ground.
I can put the red probe in the Pin 8 but where does the black probe goes?

[josegrad]

I went and purchased a new set of matches valves, all four.

Adjusting the bias I did set all four to be around 25mA using the bias probe.
But I'm not sure what's the correct mA I should aim for.

Hum is very low now when all valves are so even.

I didn't play it yet, until that apparently high voltage issue is discussed.
But apparently the other valves were not in optimal condition.

[eschertron]

But apparently the other valves were not in optimal condition.

You can say that again!
Hum is reduced (cancelled) by 'matching' the pairs of output tubes. So what you've reported is good. Try NickB's web page http://bmamps.com/ivds.html to get an idea of what the idle current should be. Please note that colder is better until you have a grasp of all the operating conditions involved.

[Chuck H]

Wow! All valves but one failing to draw significant current and that one that does was very low. Those tubes must have ten thousand of hours on them. Oh well. No need to look for voltage at the cathode now.

Ok... With the tubes drawing 25mA a piece in play mode please measure the voltage at pin 3 of the power tubes.

You can take that number and multiply it by the current (25mA, that's "milli"amps so .025) to find the watt dissipation at idle for the tubes.

[josegrad]

...You can take that number and multiply it by the current (25mA, that's "milli"amps so .025) to find the watt dissipation at idle for the tubes.

Ok, so after swapping tubes around for a while I managed to leave them like this:

	Tube 1	Tube 2	Tube 3	Tube 4
Bias probe reading	23.5 mA	25.4 mA	25.0 mA	25.0 mA
Voltage at Pin 3	576 Vdc	576 Vdc	575 Vdc	575 Vdc
Watt dissipation	13.536	14.630	14.375	14.375

[eschertron]

Since I've never worked with a tapped OT, I'm curious where the screen (pin 4) sits with these tubes. Can you share? Thanks.

[Helmholtz]

Since I've never worked with a tapped OT, I'm curious where the screen (pin 4) sits with these tubes. Can you share? Thanks.

What do you expect? The voltage at the screen taps is more or less the same as at the plates, at idle.

[eschertron]

What do you expect? The voltage at the screen taps is more or less the same as at the plates.

And would think so too, at least at idle. With the high B+ voltage All I see from the simulator I used is garbage. Way over dissipated under load. I'm guessing the tapped voltage looks much lower at AC with a signal applied. Something for which the simulator does not account.

[Helmholtz]

And would think so too, at least at idle. With the high B+ voltage All I see from the simulator I used is garbage. Way over dissipated under load. I'm guessing the tapped voltage looks much lower at AC with a signal applied. Something for which the simulator does not account.

By transformer action the instantaneous AC voltage at the taps is forced to follow to plate voltage at a percentage given by the turns ratio. This way momentary screen voltage always stays below momentary plate voltage, other than in non-UL designs. For this reason manufacturers allow higher screen voltages in UL designs for some tubes. E.g. KT88. Not sure about EL34s, though.

[josegrad]

Since I've never worked with a tapped OT, I'm curious where the screen (pin 4) sits with these tubes. Can you share? Thanks.

The reading at pin 4 is 574 Vdc

[josegrad]

Now the amp is very sensible (some clicks and pops) to any kicks, even hitting the wall in a nearby room causes some clicks. That's after the new valves.

In any case the valves sit quite tight in the sockets, the old ones were are bit tighter though, but I wouldn't say the new ones are loose once they are in the sockets.

But the socket terminals are a bit/quite loose, with and without the valves in place. I'm thinking if I should replace them.

Of course there might be some bad solder around, which I'm checking again, but those can also be very well hidden.

---

By the way, now that I was able to calculate the watt dissipation, should I use that number to recalculate the bias setting?

[pdf64]

The female contacts in the tube sockets are, necessarily / by design, fairly loose in the base. Otherwise they might put excessive mechanical stress on to a slightly misaligned tube pin.
What's important is for the female contact to grip tightly on to the tube pin, and for the mating surfaces of both pin and socket contact to be reasonably clean and oxide free.
Generally, if the grip of the female contact is tight, the wiping action of inserting / removing the tube a few times acts to establish sufficient electrical contact between the mating surfaces.
It is often feasible to retension female socket contacts, eg using a dentist's pick tool; try a google search for more detail.