Simms Watt 100w Head

[josegrad]

Ok. I still don't understand the advantage of removing the bus to Chassis, placing a 1 ohm resistor on each socket and measure VS using the bias tool I got. The first option seems even more work to me (without mentioning I need a 40 Km round trip to buy four 1 ohm resistors).

[eschertron]

I'd think a cost-benefit analysis needs to be done. I agree, saving 10 to 15 minutes of swapping a bias probe around is not offset by the hours and cost of shopping for resistors, modifying the amp, etc. However, if you'll be doing this bias measurement often, the time savings adds up. Some people are inveterate 'tweakers' and will take their amp apart whenever they hear the slightest bit of hum. With a 4-tube power section, there's increased opportunity for the amp to do that.

For me as a hobbyist/DIY-er, power tubes are plug-and-forget and there's no need to check or set bias more than once for a set of tubes that may last years. A tech/maintenance person might expect to see the amp repeatedly, or add the resistors for the 'greater good' of the the next tech who opens the amp. Especially if the amp belongs to a regularly gigging musician.

[josegrad]

I went on and started measuring with the Bias tool I bought. I'm not planning to do this frequently but I would like to set it properly after so many years.

Now I've been able to find which adjustable resistor controls each pair of tubes.
And also I can see how the tubes differ.

Apparently two of the tubes are quite well matched, and the other tubes are night and day.
But the values I get are quite low with 3 of the tubes, the last one is higher.

Without touching the adjustable bias resistors. Without light bulb limiter.

This is how I set the multimeter.



If I move the adjustable resistor that value goes up, and hum increases as well...
The current value is 3.1

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Then the other tubes, one is really low, 1.1 while the other is about 16.0.

So in fact the readings would be like this:



The two on the left are the ones that match very well. And their reading is about 3.1
The two on the right one reads 1.0 and the other reads 16.0

To make sure the difference was not caused by anything in the circuit I switched the places of the tubes and got the same results.

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Did I set the multimeter to the correct unit, is around 30 the value I should see there? That would cause a lot of hum by the way.

So this is what I'm using to measure it, moving it from socket to socket.



Any suggestions?

[pdf64]

Those numbers are rather low, but can you show a photo of how you've got the probe plugged into the meter?

[eschertron]

With those low readings, I'd wonder if there are screen resistors burned open (like maybe 3 out of the 4)?

[josegrad]

Those numbers are rather low, but can you show a photo of how you've got the probe plugged into the meter?

Well the Bias tool has two probes, so black to COM and red to V/ohms/mA
I actually purchased it assembled since I screwed the first kit I got.

[Chuck H]

Can we assume the standby switch is in play mode for these measurements?

[josegrad]

With those low readings, I'd wonder if there are screen resistors burned open (like maybe 3 out of the 4)?

Resistors look fine from the outside. I have been checking their values and replaced one a few weeks ago.
But as I mentioned, I did swap the tubes to make sure the reading was not affected by the circuit.

For example, those two tubes in the left that give me 3.1. If I take the last tube that reads 16.1 when in the last socket and place it in the first socket (where other read 3.1) the measured value is still close to 16.1. It does not drop to 3.1 because of changing the socket. I guess that should rule out any circuit issues right?

[josegrad]

Can we assume the standby switch is in play mode for these measurements?

Yes, both switches are in play mode.

[Chuck H]

For example, those two tubes in the left that give me 3.1. If I take the last tube that reads 16.1 when in the last socket and place it in the first socket (where other read 3.1) the measured value is still close to 16.1. It does not drop to 3.1 because of changing the socket. I guess that should rule out any circuit issues right?

Probably. I might guess that the bias voltage is excessive and the one tube showing significant current is actually bad and would draw excessive current under normal bias conditions. But that IS only a guess. Can you please tell us the -V reading from all power tube sockets for pin 5.

[josegrad]

Probably. I might guess that the bias voltage is excessive and the one tube showing significant current is actually bad and would draw excessive current under normal bias conditions. But that IS only a guess. Can you please tell us the -V reading from all power tube sockets for pin 5.

So black to chassis and all measurements with red at pin 5.
With all the tubes in place right?

[pdf64]

So black to chassis and all measurements with red at pin 5.
With all the tubes in place right?

Yes those conditions are fine.

[josegrad]

Yes those conditions are fine.

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

[eschertron]

So high? In post #34 you report B+ at 388vdc. Was this before major surgery on the amp?

edit: I see yes, some PSU issues corrected. Do you have any readings from point "C" after the fix?

[josegrad]

So high? In post #34 you report B+ at 388vdc. Was this before major surgery on the amp?

edit: I see yes, some PSU issues corrected. Do you have any readings from point "C" after the fix?

Reading at point C is 592 VDC.

Measured here: