help please

Gorbayov · 2013-02-14 6:09 pm

hello, im new here..
i actually dont know much about building amps but i have here a diy preamp that i got 8 years ago, i didnt use it much, but recently i bought an electric guitar and i use amp simulator on pc, and i found out that if i put the guitar through the preamp before pc it has very nice tone.
it has 2 seperate volume pots, one for the right channel and one for the left channel, which i didnt like at first but since im using only one channel for the guitar its ok, but the pots sucks, very hard to control the volume, every centimeter of change is alot of volume change, and when its all the way down it still has lots of volume.
i know how to solder, so i thought i would open it and replace the pots, maybe with one stereo pot..
but after i freed the pots from the case and moved it a bit it got disconnected from the other side (both of them).
now i want to know how do i wire it back?? where it should go??
i remember that the ground went to the center of the tube slot, and the middle went to somewhere else in the tube slot, and the third one was connected to the input jack.

i cant find the one who built the preamp.
please help me rewire it so i can play my guitar again..
thanks

Update - Solved

Gorbayov · 2013-02-14 6:30 pm

i forgot to mention that it has 3 tubes..
one is 6X4/EZ90

but the pots were connected to two other tubes, one says ECF80 and im not sure about the other one because most of the marking faded, i can tell it said Amperex and after i searched amperex the web to find it, i found that looks exactly the same as 6GH8A, so i guess thats the tube.

these tubes are somehow connected to one another, and one pot was connected to one of them and the other pot was connected to the other.

M Gregg · 2013-02-14 7:11 pm

Hi,

First please be aware that tube amps have high voltage that can stay up after power off...shock hazard!

The first thing to do is post a picture of the pot positions (As clear and with as much detail as possible) Then someone may be able to help..

Also look here..scroll down to putting your pot to use...

Potentiometers (Beginners' Guide to Pots)

Regards
M. Gregg

M Gregg · 2013-02-14 7:18 pm

Also,

The pot type used is probably wrong it needs to be a log type pot to get the control.. so you need a log dual gang pot of correct value..

Regards
M. Gregg

Gorbayov · 2013-02-14 7:34 pm

thanks gregg.
ok i can put pictures but it looks like a mess, everything is connected so tight.
i dont know if you can understand something from these pics but i will try

first, all of it:
[IMGDEAD]https://s17.postimage.org/rl7kyxs2m/IMG_20130214_193807.jpg[/IMGDEAD]

now the tube slots
[IMGDEAD]https://s17.postimage.org/la6yw9g7y/IMG_20130214_193954.jpg[/IMGDEAD]
[IMGDEAD]https://s17.postimage.org/7f4301wf2/IMG_20130214_193924.jpg[/IMGDEAD]
[IMGDEAD]https://s17.postimage.org/b1ehqfs66/IMG_20130214_194059.jpg[/IMGDEAD]

about the pots type, well i know that, but right now i only want to know how to rewire the pot, or any other pot..

Mooly · 2013-02-14 7:42 pm

Why don't you attach the pictures to the forum

To add a photo, files or non standard files.

First click "go advanced" in the box below the "quick reply" message box. Doesn't matter if you decide half way through a message to do that, it carries it foward.

Then click "Manage attachements". Maximise the new Window so that you can see all the text.

Click browse in the first box at the top and find your picture. Repeat for any more pictures.
Click upload... a message appears "uploading"
When complete the files will show as being attached. Now click the small text that says "close this window"

The pictures should now be attached and when you submit your post they will appear.

Make sure your pics aren't too big, a couple of 100k is plenty, and many members object when they are massive and it alters the margins
It tells you in the attachments window what max sizes are allowed.

If you want to attach a file that has a non standard format for example excel, circuit simulation etc then try putting the files in a zipped folder and attaching that.

Gorbayov · 2013-02-14 7:58 pm

hey mooly thanks, i didnt know that.

M Gregg · 2013-02-14 8:44 pm

ECF80..

M Gregg · 2013-02-14 8:46 pm

6GH8..

M Gregg · 2013-02-14 8:48 pm

If its a stereo amp..

Then the two channels should be the same..? So the pot should go to ground at one end and to the input at the other end the middle (slider)goes to the Grid of the input tube maybe?

6X4 is a rectifier HT/B+<<< This power not signal...so its just finding the correct Grid connection (on the other tubes) one will come from a capacitor from the first tube Anode at a guess...that means the one that has a resistor to Ground and nothing else is the input..just a guess...

Regards
M. Gregg

Gorbayov · 2013-02-14 8:53 pm

i dont know what that means

is it possible it was connected on pin 9?? i think i see that something on this pin was disconnected from there but im not sure
can it harm the amp or my guitar if i try soldering it to pin 9?

M Gregg · 2013-02-14 9:04 pm

Pin 9 is the grid of the triode,

So it could be an input...however what is conected to it? does it have a capacitor connected to it and a resistor or just a resistor?

You must get this correct because if you connect to the wrong pin you could have high voltage on it and your guitar will become live!

You need to have more help from an engineer you cannot just take a guess!

Regards
M. Gregg

Gorbayov · 2013-02-14 9:14 pm

yeah i thought you guys may help me as engineers

pin 9 is connected only to the ground with a resistor.

M Gregg · 2013-02-14 9:21 pm

Gorbayov said:
yeah i thought you guys may help me as engineers
pin 9 is connected only to the ground with a resistor.

Are you absolutely sure? nothing else....

Regards
M. Gregg

Gorbayov · 2013-02-14 9:22 pm

well im sure its connected only to the ground and nothing else.. about the resistor, well it looks like a resistor but it has no colored rings, can it be something else?

M Gregg · 2013-02-14 9:30 pm

Pin 9 could be the input and if its only got a resistor to Ground then maybe OK..

However its your choice...I would test Pin 9 to ground with a meter set to high volts DC and make sure, however if you dont have experience with high voltage testing....then I would have to say you still need help...

Never take chances with tube circuits...so pin 9 would have only a resistor one end to pin 9 and the other end to ground and nothing else connected to pin 9 at all no other wires nothing..Then it would be an input.

Again I take no responsibility because I cannot see what you see! (I'm not there).

Regards
M. Gregg

Gilgy · 2013-02-14 9:48 pm

Gorbayov said:
i dont know what that means
is it possible it was connected on pin 9?? i think i see that something on this pin was disconnected from there but im not sure
can it harm the amp or my guitar if i try soldering it to pin 9?

Very possibly. Especially as you don't know what tube types you're dealing with.

With no disrespect intended, I think you should consider the possibility that you don't know enough about the amp you're dealing with to make a safe diagnosis and repair of the problem. Really I would recommend further reading, research and learning before you attempt to service the device, especially if it's going to be connected to a guitar (and therefore your fingers).

Gorbayov · 2013-02-14 10:07 pm

thanks for all the warnings..

but just so i know if pin 9 is only connected to the ground with a resistor, it must be input?

i now see that pin 8 also only connected to the ground with a resistor and nothing else..

Gorbayov · 2013-02-14 10:11 pm

i am pretty sure now it was connected on pin 9, cause i see that a flat wire was connected there and the only flat wires i see are coming from the inputs.

Gorbayov · 2013-02-14 10:15 pm

btw you can see pin 9 on the pictures, its connected with the brown resistor that says 7414

help please

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