2204 with 1 tube?

[wavenator]

zben
you havent understand
i know this amp

it cant be distorted like it is atm
i resoldered all the joints, same problem
checked all values
can i be bad preamp tubes?

[Zben3129]

Hmm...

I guess it could be a tube

sounds like something is either biased hot or getting fed to much signal, I'm not farmiliar with the amp however so I can't think of where to tell you to check

[wavenator]

i think also it feeds more voltage then need to
does any one have any voltage reads for this amp?

[DougH]

Again, check with Ceriatone. They should have all the target voltages for the amp.

[drewl]

How about when you use the low input?
are you grounding any of the preamp cathodes (pins 3 & 8 of the first 3 tubes)?

you can remove the 470k/470pf treble peaking circuits and it still won't over saturate.
do you proper preamp tube grid loads (pins 2 &7)?

are your filaments wired properly? 6v ac between pins 4/5 and 9 of the 12AX7's.....and between 2&7 of the EL34's.

is the negative feedback tap connected?

[wavenator]

thx for trying to help man

no, in the low input the sound is great

yes i'm grounding all the preamp cathods

what do you mean about the treable peaking, the 2 in parallel from the high input jack?

my filament are ok

what is the negative feedback tap?

[d95err]

no, in the low input the sound is great

Sounds like you've isolated the problem. That is good news! Check the first stage again, and again, and...

[drewl]

the 470k resistor and 470pf cap in paralell is known as the treble peaking circuit.
you can replace them with a 68k to 100k resistor to beef up the sound.

Most fixed bias amps use negative feeback to clean up the sound.
that is part of the output signal from the output tranny is "fed back" through either a 47k or 100k resistor to the phase inverter.
Without the neg. feedback ther will be ALOT more gain in the amp.

[wavenator]

http://www.ceriatone.com/images/layoutPic/marshallLayout/JCM800_2204Ceriatone.jpg

can you please tell me which wire/part is the negative feedback tap i should check

ill try today to change the treble picking circuit and ill update

[Zben3129]

The (green? colorblind) wire coming off of the terminal labeled '4 Ohm' on the "impedance selector switch" going to the 100k resistor is your negative feedback tap

Zach

[wavenator]

oh as i thought
well thx a lot man
ill check all and ill back
btw

if ill take out the 470/470 resistor and cap , would that be a good indication for the problem? or ill get a worst sound?

[d95err]

If the amp sounds fine on the low input then negative feedback has nothing to do with the problem.

EDIT: Have you tried replacing the first preamp tube? If you don't have a spare, try switching the preamp tubes around and see if it makes any difference.

[drewl]

The problem is he's getting way too much gain in the high input.
Lack of feedback may not be noticeable when using the low input since it's gain is alot lower.
Even so, when using the high input he should be able to dial down the gain with the preamp gain pot.

[wavenator]

ok
i've tried both, change the 470/470 with a 100k resistor, i checked the feedback tap
nada
dunno what to do
i've also tried another tube, same problem

the main problem that really disturbs me is that in the high input i dont have any sustain
the sound is cut, when i pick the treble strings , after 1-2 secs the sound getting weak and bad

do you have any other suggestions?

[Zben3129]

check the 2k7 and .68uf cap on the far right of the board. Make sure you put the underboard wire that connects this to ground.
check the 100k resistors go between pin 1 and pin 6
check the .022 cap from pin 6
check the 68k resistor from high input to pin 7 on the outside ecc83
check the 1m resistor from high input to ground (on the jack)

I would check in that order, as I ordered them what I thought was most likely


I suspect one of these will resolve your problem. Please, do these one at a time. That way when you fix it, you will know what went wrong for future builds. Also, if you happened to mess something up, you would know exactly what part you messed up.

Zach

[wavenator]

zach
thx a lot for the help

i blew a fuse so it should wait for tommorow
but a suspect thing
when i check resistance from high input to ground i get a read for few ohms
can i be possible?
maybe the 1 m resistor isnt connected good ?

[Zben3129]

Are you using switched jacks for your input?

If so then you need to have a cord plugged in to the high input to take these measurements accurately.

You can still check all those things without a fuse, just won't be able to test any solutions if you change anything.


Check them visually, and also check them with a DMM

No tube in the socket.

See if you can give me this info...write what the resistance you get across the parts are (put one DMM lead on one leg of resistor/tube pin and one lead on the other)
2k7 resistor -
68k resistor -
1m resistor -
pin 1 and pin 6 -



Zach

[drewl]

With nothing plugged in there will be 0ohms to gnd since the jacks shorts the input to reduce noise.
If the cord is plugged in and it's still zero ohms that's your problem.
The first stage is not conducting.

[Zben3129]

If he gets a splatty high-gain sound I'd think theres almost no way he can have a short to ground. More likely is that 1m resistor is a 100k or 10k or 1k.

Probably just shorting jacks with no cord.

Zach

[wavenator]

yea yea i had a mistake
the resistance is 1m

anyway
i checked all wires- all connected
checked all resistors values = all correct

the only things i can guess are trannies, can caps, lead dress(possible?), not good caps(dont have how to check)

any other suggestion? just dunno what to do