ORANGE OR 120M - SCREEN GRID RESISTOR FAIL

[jujudubluz]

Hi guys !

first of all, i hope all of you are safe during this global crisis.

i took time to change power tubes in my vintage OR120M. ( EL34M - str - TAD )
So, i removed the old ones, put my probes to check bias, put new tubes in, turn it on, and within a minute, one of the 1K/5W fails.
Changed the faulty resistor for a brand new one ( same type ... ), same thing happen ... the same resistor start to smoke and fail.

Trying to keep the fauly tube / socket out of the equation,
I tried to run the amp with only 2 tubes / only one in each side ( turning my OR120 into a 80 ... ) keeping the impedance matched amp set @ 4 ohm for a 16 ohm load.
Everything seems fine, voltages and bias both look good.

Do you guys have an idea or procedure, where to start ?

JUJU

[pdf64]

A light bulb limiter is an invaluable tool for activities such as this; why risk burning parts up unnecessarily?
Powering unproven stuff up without one seems akin to a drop test.
I find it perplexing why folks are so resistant to them

[jujudubluz]

hey !

thank you for your advice pdf64 !
i do have a bulb limiter but i am not starting a new build or running it for the first time -
unproven stuff ???... i was changing power tubes in an amp that worked ...
i do have a light bulb limiter, i am not resistant to using it ... i just don't see your point !

thank you a lot anyhow -

[martin manning]

So it's ok with the tube that smoked the screen resistor removed? Sounds like a defective tube.

[jujudubluz]

yes Martin,

only using 2 tubes out of 4 works ( leaving the faulty socket / resistor unused ) works.
i will try to swap power tubes to make sure one of the 2 other " brand new TAD" is faulty.

thank you.

[pdf64]

My thinking is that the power tubes, being new, should be seen to be unproven. As they are components whose failure modes can result in damaging fault current being drawn, the use of a light bulb limiter is called for when first putting them into use.
I suggest that the light bulb limiter be used if re-trying the tube that previously smoked its screen grid resistor.

[jujudubluz]

ok -
thank you

[pompeiisneaks]

as an aside, I've actually been told that using a light bulb limiter on a brand new build is rarely super needed, as ever part is known brand new. So long as you've triple checked things, it's less important than use on an older amp that's showing issues already.

i.e. a whole lot of knowns means less need to safely check
a whole lot of uknowns means use the safety check of the limiter.

I can't be sure who told me that, but I kinda wanna say it was sluckey... he'll tell me I'm crazy probably

~Phil

[martin manning]

I'm not buying it, whomever it was. New parts can be defective, and triple checking is no guarantee that there isn't an error somewhere.

[pompeiisneaks]

and now that I'm thinking about it, before he corrects me, I think he was telling me about a variac, not a limiter

d'oh!

~Phil

[jujudubluz]

Ok !

you guys were right ! bad power tube !!!

swapped the two "good" power tubes with the other two "remaining" tubes.
after a few seconds, i heard a crackling noise / hiss ...
burnt another screen grid resistor.

I replaced the dead screen grid resistor, put back the good pair and everything went back to normal... bad EL34 !

Do you guys know of a tough EL34 ... the plate voltage are very high in these Old Orange amps.

thank you again !

JU

[wpaulvogel]

SED winged “C”
Ruby has some for high plate voltage too

[martin manning]

SED =C= are out of production, and very pricy as NOS. I recommend Electro-Harmonix.

[thetragichero]

as an aside, I've actually been told that using a light bulb limiter on a brand new build is rarely super needed, as ever part is known brand new. So long as you've triple checked things, it's less important than use on an older amp that's showing issues already.

i.e. a whole lot of knowns means less need to safely check
a whole lot of uknowns means use the safety check of the limiter.

I can't be sure who told me that, but I kinda wanna say it was sluckey... he'll tell me I'm crazy probably

~Phil

for somebody who keeps discovering new "stupid human tricks," the couple extra minutes powering up on a lightbulb limiter has helped me troubleshoot a number of issues before smoking components... after building a number of octave fuzzes i made a maestro-like diode ring out of 1n4007 instead of a proper bridge rectifier..... eventually i figured out why the lightbulb kept lighting up, thankfully with only my price being damaged

i won't even go into the old kustom solid state amp i kept smoking output transistors (and driver transistors made of unobtanium)....

[pompeiisneaks]

as an aside, I've actually been told that using a light bulb limiter on a brand new build is rarely super needed, as ever part is known brand new. So long as you've triple checked things, it's less important than use on an older amp that's showing issues already.

i.e. a whole lot of knowns means less need to safely check
a whole lot of uknowns means use the safety check of the limiter.

I can't be sure who told me that, but I kinda wanna say it was sluckey... he'll tell me I'm crazy probably

~Phil

for somebody who keeps discovering new "stupid human tricks," the couple extra minutes powering up on a lightbulb limiter has helped me troubleshoot a number of issues before smoking components... after building a number of octave fuzzes i made a maestro-like diode ring out of 1n4007 instead of a proper bridge rectifier..... eventually i figured out why the lightbulb kept lighting up, thankfully with only my price being damaged

i won't even go into the old kustom solid state amp i kept smoking output transistors (and driver transistors made of unobtanium)....

Yowza! Yeah sorry I got confused between the Variac and the Light bulb limiter, it truly is a great tool to avoid cooking things

~Phil