crate palomino 32 212 crazy scifi noises update

[Chuck H]

The resistance of the resistor shou!dnt change during operation, should it? Maybe something tricked the meter? I better change it out and check the capacitor too. The tubes have been super stressed too. I better bench test those resistors they could be fakes...
thanks for the schematic dude. I can t upload stuff

The way the amp is arranged physically the resistors may already be suffering heat before their own heating due to dissipation occurs. There may also be peak currents not captured or taken into account (common in testing with constant signal which is not really how amps are used). Some resistors are less up to the task than others, but all derate and/or increase resistance with heating. Move the cathode resistor and choose resistors with a good spec for any expected derate for the area heated above the power tubes. There's not much else to be done. The amp is arranged badly from a physical/mechanical perspective. Hard to guess how that happened considering how many players like them. You'd think it wouldn't have ended up an abandoned design and they would have come up with a more reliable layout to accomodate the actual circuit. But no such luck in this case. Think of it like trying to keep an old karmann ghia running. Sure it has some problems with how it's built, but some are correctible and others can be made tolerable. In the end you do it because of what you get back... Or not.

[Helmholtz]

...cathode resistor is 81ohms but when warmed up is 94ohms...

How do you measure hot resistance?

Do the resistors recover to nominal after cooling down?

[georage]

yes Helmoltz, they do. i believe the resiatances i was reading were a trick of some residual voltage on the cathode messing with the meter.
i bench tested the resistors against a few know good ones by heating them up with a bench power supply until they were about 160 degrees f. then i would quickly unclip the power supply and clip in the ohm meter. the new resistors would generally read 1ohm less while hot and the recover once cooled. the control resistors would all increase one ohm while hot and then return to normal after cooling.

[georage]

i ran 12vdc through the 88ohm,15 watt resistor and the 82ohm 10watt resistor and observed the 15watt resistor running cooler. on the bench supply. so i figured the homemade 88ohm reaistor would be ok..
i played guitar through the amp for about 30 minutes at all different volumes and channels and viewed the cathode resistor with a thermal imaging camera. the resistor got to exactly 180 degrees f and stayed there. the tube sockets got to about 200 and the power tubes got to 400...which is cooler than they did when they used to fail...same tubes.

[Helmholtz]

yes Helmoltz, they do. i believe the resiatances i was reading were a trick of some residual voltage on the cathode messing with the meter.
i bench tested the resistors against a few know good ones by heating them up with a bench power supply until they were about 160 degrees f. then i would quickly unclip the power supply and clip in the ohm meter. the new resistors would generally read 1ohm less while hot and the recover once cooled. the control resistors would all increase one ohm while hot and then return to normal after cooling.

I guess that means that the large increase in resistance wasn't real. 1R +/- doesn't matter.

Question is what made the cathode voltage increase.
Might be a bad tube or a leaky coupling cap.

[georage]

btw using 88ohms for the cathode reaistor did affect the tone of the amp. it lost its sparkle and sounded a little deader, but it did have more thumpy metal overdrive and less overall clean sound...but i wanted the amp to run at the recomended wattage..since it previously would break down. the 82 ohm resistor would probably sound better and run the tubes at around 100 percent.
using the reverb added back a little magic to the sound.
running the amp through the dimbulb limiter gave it a really cool distorted tone. i think lowering the b+ voltage would help this amp sound better, but, for now i'll give it a good full 2 hour jam and see if it survives.

[georage]

thats agood point Helmoltz. i never saw that increase in voltage happen again...i assumed it was due to the increase in resistance causing the voltage to go up. maybe i soldered the connection better or something...i did add a bit of 12gauge copper wire to the assembly as a tiny heat sink, but it was really insignificant.