Plate resistor blowing

[fossilshark]

Hi, the second stage (or clean input) on my 2204 clone is the only thing giving me issues with this build. I've replaced the 100k plate resistor 3 times (although my last test was only a 1/4 watt) and it keeps burning up. 384v on it and the input stage (also on 384v) seems to be amplifying fine (signal gets through to the power amp just really really quiet).

I know it's the v1b stage because inputting a sine wave on pin 6 gets sound out at full volume but signal on pin 7 does not amplify at all. There are no shorts as far as my meter can tell and all signal connections check out.
Could it be my tube socket?

[Pietro]

bad tube, bad socket...yes.

2.7K cathode resistor present and solid connecting too ground?
The plate resistors are really 100K and not 100R by any chance?

i added the input stage schematic. if you study this you can see the expected plate voltage is near 150 and 205V. That's a different cookie then 380V!

Don.t do 0.25 watt resistors.  Thats for baby electronics. But in this case the 1/4 watt probably saved your cirquit!

pull tubes and test without them present.

[pdf64]

Yes, what is the plate voltage?
If it’s too low then the resistor will overheat, eg if 0V then it would dissipate ~1.5watts.
1/4 watt resistors are generally unsuitable for plate circuits; even if the dissipation is acceptable, their voltage limit (typically 250V or lower) may easily be exceeded.

[tubenit]

It doesn't surprise me that a 1/4 watt resistor burns up on a plate.  I have 1 watt resistors on the plates of two of my amps and the 3rd amp has 1/2 watt metal film resistors.

With respect, Tubenit

[fossilshark]

Replaced it with what I assumed was a 1/2 watt 100k. 0v on the side connected to the tube still with no shorts to ground.

Measuring B+ right off rectifier (no tubes) is 512V, then after the first 10K its 466V, then after the second its 412V. This power transformer isn't exact to the schematic so mabey its output is too high?

My processing issue is that if the B+ supply was wired wrong why is it only the second stage of the 1st 12ax7 having an issue?

And I swapped tubes with my other amplifier. It is not a bad tube.

[shooter]

0v on the side connected to the tube still with no shorts to ground.

0 is not a good number.  try again, this time measure the top of the resistor, then measure the bottom of the resistor,  something like 283vdc top, 175vdc bottom (BS #'s, good example:)

[fossilshark]

Sorry I ended up getting sidetracked (again). I went out and bought some 2 watt 100ks and there is still 0v on top of resistor connected to pin 6 with a high B+ on the bottom. The 100k is not blown just hot, no shorts to ground or terminals around it, and the line has continuity to pin 6.

Pulling my hair out lol

[sluckey]

and the line has continuity to pin 6.

When you say "line" are you talking about the 120VAC line? If so, that should be very easy to find out how.

[fossilshark]

No, I was referring to the wire connecting the resistor to pin 6

[sluckey]

Turn the amp off and wait for caps to drain. Set your meter to read ohms. Connect one probe to chassis. Connect the other probe to pin 6 (directly on the socket). What resistance do you measure?

Post a hi-rez pic that clearly shows pin 6 on the tube socket. Post another hi-rez pic that clearly shows the 100K on the board.

[fossilshark]

5+M ohm between 6 and chassis. Mind the burnt up wires and crappy joints, if anyone has any suggestions soldering shielded cable that would be great. Also most shielded lines I've replaced at least once in this picture.

https://drive.google.com/folderview?id=18A8KCO3--dVLe2FkerP1S95GDYFcMGO0

[sluckey]

Don't have permission to see your pics. 

[fossilshark]

This one one should work. Pictures from my phone are apparently are too large for the forum.

https://drive.google.com/folderview?id=1gdCg64LC483vr37RevxRPIwn0x2bELta

[DummyLoad]

coax is shorting conductor to shield intermittently to ground - replace the coax segment from 100K to pin6 with a piece of wire.


--pete

[shooter]

Pete you're no fun 
where were you when I wanted the easy answers on solving quadratic equations 

[DummyLoad]

Pete you're no fun 
where were you when I wanted the easy answers on solving quadratic equations 

in hiding. to me i'm funny. :p


--pete

[fossilshark]

Replacing that shielded wiring with normal wire didn't change anything. I tested continuity to the chassis before and after the wire was installed and theres no short.

[shooter]

so is the R still pop-in or just no sound?

if it's still smoking cut where the red X's are and verify the vdc in blue, if you just don't have sound verify the vdc in blue

[fossilshark]

The resistor isn't blowing anymore, but there is 440v on the plate and 3.7v on the cathode.

[shooter]

Measure up the whole tube, also the "top" of the 100k, you stated B+ at plate was 440, so that will show what the PS is giving the tap

[fossilshark]

Pin 6: 357v
Pin 7: 0v
Pin 8: 3.7v

Top of plate resistors on V1: 390v

There is 490v coming right off the rectifier if that helps.

[sluckey]

Pin 6: 357v
Pin 7: 0v
Pin 8: 3.7v

These numbers look reasonable. But so far you have reported 0V, 440V, and 357V for pin 6. All without doing anything to the amp. So, something is changing. Is it the amp, or your meter, or you?

[fossilshark]

Yes sorry I forgot to clarify. The last values I gave were what my meter settled on, my previous measurements were taken right when I turned on the amp and I did not double check them I guess.