Wolfeman: got a tip for me? (Amp Tech)

[KHE]

he means the first two resistors after the hi-input-jack.
1MegaOhm goes to ground, the 68kOhm to the grid of V1a.

how did you check the input-jack? maybe there is a lot of oxidation on the contacts or bad solderjoints.
can you post better pictures of the input-jack/pcb/tube-socket?
do you have a voltage-meter?

do i see this right: is there a shielded cable from the input-jack to the tube-socket (grid of V1)? maybe this cable is bad solderd and there are some wires from the shield making contact with the signal?

 

[greyskull]

I resoldered the shielded connection between V1 and the input Jack just to be sure, and its a new valve base and input socket, so oxidation is pretty minimal! Yes I have a voltage meter...

 

[KHE]

how much experience do you have with high voltages and electronic? (for "advanced" measurements)

meanwhile, try this:
dissconnect the powercable, wait 30mins, just to be sure that the supplycaps are discharged
plug you guitar-cable in the input-jack, but on the other end no guitar.
measure the resistance on the free end of you guitar cable, between signal and ground (to make sure there is no short-cut)

after that, make a resistor-measurment with your voltage-meter between the free end of you guitar-cable and the grind-pin of the tube-socket (once on the solder-lug and once direct on the contacts for the tube)
do you measure something between 0kOhm and 70kOhm? then this connection should be ok (it`s a common modification to remove/lower the 68k-resistor to lower the noise of the amp)

what do you measure?

 

[greyskull]

how much experience do you have with high voltages and electronic? (for "advanced" measurements)

meanwhile, try this:
dissconnect the powercable, wait 30mins, just to be sure that the supplycaps are discharged
plug you guitar-cable in the input-jack, but on the other end no guitar.
measure the resistance on the free end of you guitar cable, between signal and ground (to make sure there is no short-cut)

after that, make a resistor-measurment with your voltage-meter between the free end of you guitar-cable and the grind-pin of the tube-socket (once on the solder-lug and once direct on the contacts for the tube)
do you measure something between 0kOhm and 70kOhm? then this connection should be ok (it`s a common modification to remove/lower the 68k-resistor to lower the noise of the amp)

what do you measure?

I've managed to fix amps before without killing myself

Okay I'll grab the Meter and have to go get my amp.


Apologies Lasse

 

[KHE]

just wanted to ask, i hope you understand

check also the "ground-switch-contact" of the hi-input-jack. if this contact doesen`t open or there is a shortcut, then the signal is bleeded to ground after C3.
to check this, plug in you guitarcable in hi-input with no guitar, make a resistor-measurment betwenn the ground of your guitar-cable and switching-contact of the low-input (no cable plugged in, measure on the solder-lugs)

 

[greyskull]

just wanted to ask, i hope you understand

check also the "ground-switch-contact" of the hi-input-jack. if this contact doesen`t open or there is a shortcut, then the signal is bleeded to ground after C3.
to check this, plug in you guitarcable in hi-input with no guitar, make a resistor-measurment betwenn the ground of your guitar-cable and switching-contact of the low-input (no cable plugged in, measure on the solder-lugs)

I did check the leaf spring before and tried also forcing it shut to see if it had been bent, but no joy; but I'll check again.

Forgive my Ignorance... how does removing the 1 meg and 68 k resistor help?

 

[KHE]

no, not removing just measure to make sure theire values/wiring is ok (but i think they are ok).
if you would remove the 1meg, the amp would still work (if its not broken ), but if you would remove the 68k and don`t replace it, then you will get no sound, because this is a serial-resistor with your input-signal.

damn language-barrieres ...

 

[greyskull]

That's what it thought!
Thanks for your help! much appreciated, I'll report back!

 

[greyskull]

Okay finally got my meter out..

There is One meg resistance between the cable and ground when plugged into both inputs..

I get 2.02 meg on pin 2 Of V1
I get 67.9 k on pin 7 of V1 (shielded cable)
The 68 k and 1 meg check out ok... 67.8 k and 1.012 meg respectively

However!!!!! I think one of the 100 k resistors i think has bitten the dust!! do you think that would be a decent reason for the hi input to be dead? or am i talking arse?

 

[KHE]

yes, if R4 is dead, then the hi-input does not work. what do you measure across this resistor (Ohm, not voltage)

if you think you can do it, you can also measure the voltage from the anode of V1a to GND. but please!! be very carefull, leathal voltages!! (do it only if you know very excatly what you are doing, i will not be the one who gave you instructions to kill yourself )

 

[greyskull]

Don't worry buddy, I made a cap drainer with a huge cement resistor and measured all the caps before sticking my hand in! There's literally NOTHiNG on the meter with this resistor! completely fucked! A few other resistors also 100 k, so I'll pick some up tomorrow! What wattage/voltage /kind should
The resistors
Be?

 

[KHE]

i would play safe and use a 2Watt / 500V MetallOxideFilm-Resistor, not the carbon-comp-crap.
http://www.tube-town.net/ttstore/pr...oxidschicht-Widerstand--2-Watt--100-kOhm.html

 

[greyskull]

metal film yeah?? sweet.
I'm gonna have to just grab it from maplin for now to keep it going, so It won't be one of those cork sniffing sort of resistors.
Marshall used 1/2 watt throughout the 2203 I think; although in the re-issue they use 1/4 watt

Any benefit to 2 watt ? or just longer life?

 

[greyskull]

Christ this 2 watt is HUGE.
also picked uop a load of .6 watts

bugger. these .6 watts are only rated to 250 v.

Okay this is wierd.. maybe im measuring like a spazz.. BUT

when I measure the 100 k Resistor between the prescence pot and the impedence selector IN the circuit it gives me 4.3 k.


when i Remove said resistor it comes up as 96 k (ie within tolerance) WTF!?!?

Edit... snipped brite cap whilst I was at it to see what would happen.

Will report back!

 

[KHE]

this "wrong" measurings are because you measure them "in-circuit". depending on the circuit / what you want to measure, the current from the volt-meter flows throu other components and give you a wrong value.

 

[greyskull]

Thought as much!
Did a little maths based on what was around it, and it made sense!
Finally got it plugged in...
HOLY SHIT!!
Between replacing that resistor and snipping the bright cap, this thing sounds KILLER so much less fizzy, and more balls at gain settings other than max!!

Thank you so much Guy's for all your help!