2204 with 1 tube?

[Zben3129]

Well heres the thing,

Your low input is working fine. That means the problem lies where the high and low inputs differ. The only place they differ (judging from the layout, a schematic would help) is the first half of that 12ax7. It looks to me like the high input uses both halfs of that 12ax7, while the low input bypasses the first half and uses the second. So that leaves us with the 12ax7 itself (which you already checked), and the components that the first half of that 12ax7 use. These components are : 1m resistor, 68k resistor, 2k7 resistor, .022 cap, and .68 cap. So I believe the problem is somewhere in those components, or their endpoint connections (input jack, ground, pins 6 7 and


Atleast that is what I am thinking, people with more tube amp experience correct me if I am wrong (happens occasionally sometimes often)   



Zach

[wavenator]

you are completely correct
the high input uses both stages of the 12ax7 while the low only 1

well ill check those componnets
anyway it can be one of the caps, cause i checked all the resistors

zach, thx a lot for your help, if there is anybody else who can add something, please help

ill update of there will be any change

[morcey2]

I know this is a little late since you said that you already ordered a pair of EL34s, but get a pair of JJ KT77's.   They're drop-in replacements for EL34's (make sure you bias them correctly, but that's necessary for any new set of power tubes).  They are the best sounding new-production EL34/6CA4 type tubes out there.  I can't keep them in stock because every time someone tries them in an amp they've brought me, they leave with them. 

Matt

[wavenator]

oh thx bro for that
ill order some after the amp will work properly ^^

[teletroy]

how about a photo?  Seems it would be much easier if  we could it.   I've never build one of these,  I dont have a schematic to look at, but I'd guess either something is wired wrong or you have a bad component.  Check the parts that aren't shared since the one input works fine.

good luck - you might try a tube amp board, you might be able to get a little more assistance too.

Long ago  I told someone - 'well it's wired correctly I checked several times.'   To which they replied " If it was wired correctly, then it would work correctly."   They were right, I had it wired wrong.

[BrianJ]

This happened on my first build.  I had the whole thing apart twice until I measured DC on the grid of the second gain stage.  It had like 20 Vdc when it should be zero.  The coupling cap was leaky out of the box and was passing some dc.  Just a thought, the symptom sounds like a bias problem on a preamp tube and this would certainly follow the "isolate the first stage" thinking everyone else is coming to.

[Zben3129]

Be careful reading those voltages...use one hand etc etc 

http://www.ax84.com/safety.html

Zach

[wavenator]

http://mail.google.com/mail/?ui=2&ik=9e3c8375f4&view=att&th=11d8c83e81b0fc1f&attid=0.1&disp=inline&realattid=f_fm8guqii0&zw
http://mail.google.com/mail/?ui=2&ik=9e3c8375f4&view=att&th=11d8c83e81b0fc1f&attid=0.2&disp=inline&realattid=f_fm8gv4xq1&zw

photos from inside

btw, there still has a problem
the same distortion
dunno why, checked all the grids, seems fine
0 volts

[wavenator]

guys, maybe we've found the problem
v3 reads 12-13 volts on pin2 and 7 (grids)
on the voltage table i got from ceriatone, those both grids should have 0 volts
where is the problem?

[slacker]

I think the grids of V3 should have some voltage on them. The 1 meg resistors from the grids are connected via the 470ohm resistor to the cathodes so there will be some DC there. I don't know if it should be 12 volts though, what does ceriatone say the voltage should be on the cathodes (pins 3 and 8 ).

[wavenator]

41 volts on both

[Zben3129]

few things,

Sorry if you've already specified, but are you using a kit, or did you get your own components.
If you bought your own, check the voltage rating on your .68uf cap. If its not sufficient you may have an open/shorted cap.
Also, put your multimeter on beep mode (needs audible beep) and test for continuity across the cap. Listen for a very short beep right when you put the leads across the cap.

Try removing the tubes and taking a voltage at those points where you got 12v before. V1, V2, and V3 aren't noted on the layout, I am assuming V3 is the phase splitter?


Zach

[slacker]

41 volts on both

There's no way you should have 0 on the grids then, assuming that V3 is the phase splitter. According to this site the grids of V3 should have about 30 volts on them. http://mhuss.com/MyJCM/vchart.html

[wavenator]

well i got there only 12
anyway zach, i dont hear any beep with the beep mode thru any cap
maybe its impossible to check with this DMM

yes, i bought a kit
and the cap seems ok, just @#$%in dunno whats the problem

yes v3 is the phase inverter
btw, did you see the pics?

[wavenator]

btw, i've just check the beep with the amp "on" and not off

22nf the right one from pin1 on v1 havent send any beep

is this ok?

[Zben3129]

Sorry, should have told you to try it with the amp off.

If it beeps right when you connect the leads across it I think it is good, as the tiny beep is the cap charging and then once it charges it blocks DC (no continuity) and only passes AC. If the amp is on, then the cap has already charged.

Zach

[wavenator]

zach i dont get any beep at any cap in the circuit

anyway
dont you think the problem is at the v3 pins 2 and 7- which reads 12 volts both
it should be more - about 30, isnt it?(grids)

[Zben3129]

I would say it looks to run the grids at about 22-24 volts actually

Zach

[wavenator]

can you plz help me to find the problem
i am tired of this amp

please man, i really need you

thx a lot btw

[Zben3129]

maybe try reflowing the solder joints of those components i mentioned (be careful the underboard wires don't fall out when you do this)

Also, take a DMM, again in beep mode, and connect the - lead to ground, and then probe every point that should be connected to ground in that area to make sure you have the correct connections.

Zach