Amp Mods for Better Reliability & Protection

ivan H said:
Did you check the 6.3VAC (3.15VAC a side) filament supply to be sure it's in the acceptable range? Cheers
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Apparently the heater voltages are in spec. I didn't feel like removing the chassis's to connect the meter to measure the heater voltages. Instead, I pulled the V1 tube and checked the heater pins from the outside. It's not the proper way to do this, and I know the readings might be a tiny bit higher because there is one less tube drawing current from the PT, but it should be close enough. Anyways, with the thermistors in place, the Spiffy has 6.28VAC at the heaters, and the Chihuahua has 6.56VAC at the heaters!
 
syscokid said:
Apparently the heater voltages are in spec. I didn't feel like removing the chassis's to connect the meter to measure the heater voltages. Instead, I pulled the V1 tube and checked the heater pins from the outside. It's not the proper way to do this, and I know the readings might be a tiny bit higher because there is one less tube drawing current from the PT, but it should be close enough. Anyways, with the thermistors in place, the Spiffy has 6.28VAC at the heaters, and the Chihuahua has 6.56VAC at the heaters!
Click to expand...
An easy way to check the heater voltage is to leave all tubes in. As the PT's heater winding CT (or faux CT) is grounded we can simply attach the meter's common lead to the chassis & then measure each "side" of the heater supply (3.15VAC) at any valve socket. Saves trying to put two meter probes simultaneously on the sockets heater pins. We then simply combine the reading from each side of the supply to get the total voltage. Cheers
Edit:
Your readings are well within tolerance. Cheers
 
ivan H said:
An easy way to check the heater voltage is to leave all tubes in. As the PT's heater winding CT (or faux CT) is grounded we can simply attach the meter's common lead to the chassis & then measure each "side" of the heater supply (3.15VAC) at any valve socket. Saves trying to put two meter probes simultaneously on the sockets heater pins. We then simply combine the reading from each side of the supply to get the total voltage. Cheers
Edit:
Your readings are well within tolerance. Cheers
Click to expand...
Wait...! You can do this from the outside of the amp?
 
syscokid said:
Yesterday I accidentally posted the following info at another amp thread of mine. But it's still relevant on both threads:

As a follow up with the results of adding a soft-start inrush current limiter to this amp: With my meter connected, I can clearly see a much more gradual buildup of DC voltages just before the first filter cap. Without the thermistor, immediately after flipping the power switch to on, full voltage is hitting the caps. For a couple seconds, I saw the voltages of around 10% to 20% past the normal operating voltages before settling down. With the 200 ohm thermistor installed, in about a 10 second time span, there was about half the voltages initially and from there it was a linear climb to the proper operating voltages. This has to be better for the amp's internals, right?

Another result from adding a thermistor is a 2% to 4% reduction in the B+ voltages. The Spiffy amp lost about 5 to 10 vdc. I installed another thermistor, this time in my Chihuahua amp, and it lost about 15 vdc. Very minimal losses and I cannot tell the difference if the amps performance has changed any.

Update to the update:
Also, Ivan H just reminded me to check if the heater voltages are still in proper specs!
Click to expand...
If I missed this point as already discussed, apologies.
I think this would be more relevant with SS rectification as the diodes put out B+ right away; a tube rectifier output B+ ramps up as the rectifier tube filaments heat up and the tube starts conducting.
 
jtcnj said:
If I missed this point as already discussed, apologies.
I think this would be more relevant with SS rectification as the diodes put out B+ right away; a tube rectifier output B+ ramps up as the rectifier tube filaments heat up and the tube starts conducting.
Click to expand...

When the power is first turned on, the rectifier will go about 30% over voltage for the first few cycles of the sine wave input.
Then it comes down.
That's what kills the filter caps if you have 500 volt caps in a 460 volt power supply...
Now you can see why Marshall stacks 2 filter caps in series. It probably doubles the life of the filter caps....
Not so great for Fender amps. They got like zero headroom in the filter caps....
 
jtcnj said:
If I missed this point as already discussed, apologies.
I think this would be more relevant with SS rectification as the diodes put out B+ right away; a tube rectifier output B+ ramps up as the rectifier tube filaments heat up and the tube starts conducting.
Click to expand...
Yes Sir. The two amps that I installed the thermistors are both SS rectified, and they both don't have a Beer-Break switch. I might end up installing thermistors in my two JCM800-style amps, too.
 
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