Univox Help Needed

[cdmarion]

Hello all,

I've just finished recapping a Univox 1221 amp head, I went throughout the entire chassis replacing any component that was out of spec, doing extensive work to the reverb circuit to eliminate excessive noise. When I fired it up I noticed that channel one, with reverb and tremolo had became somewhat muddy, it has lost its high and mid range sparkle. Channel two on the other hand has not changed at all, still has the sparkle that I like in this amp. My theory is that in bringing the reverb channel back to spec it in turn sucked some of my tone. These effects circuits can be tone killers anyway. Both channels go through the inverter phase and exit the power tubes and channel two sounds great, the only difference is that channel one goes through reverb and tremolo circuit so this leads me to believe the problem is in this circuit. So my gut tells me to start playing with coupler caps and maybe some resistor values in the reverb circuit to try and bring my tone back. Has anyone had a similar experience and does anyone have any suggestions on how I should proceed? Here is the link to the schematic. http://www.el34world.com/charts/Schematics/files/univox/univox1221.pdf

Thanks
Chris

[Willabe]

When I fired it up I noticed that channel one, with reverb and tremolo had became somewhat muddy, it has lost its high and mid range sparkle. Channel two on the other hand has not changed at all, still has the sparkle that I like in this amp. My theory is that in bringing the reverb channel back to spec it in turn sucked some of my tone.

I think you miswired somthing or put a wrong value part in. It has happened to the best of them.

You might have put in/changed an R with the wrong value. Like 10K for 100K or 100K for 1M, etc. Wrong value cap? Cold solder joint?

Go back over every thing you changed in the reverb/trem channel till you find it. Print out a copy of the schemo and cross off every part and wire 1 by 1 with a marker as you check it.


                     Brad     

[Ritchie200]

Hey Chris welcome aboard!  

Help me out here because maybe I am not understanding your post.  You mentioned that after you re-capped it and fired it up, Channel 1 had become somewhat muddy.  I am assuming that by this comment you mean that it changed AFTER you re-capped it?  If so, you may want to go back in this area and re-check the values of the parts you replaced and your solder work to make sure what you did has not caused the problem.  Good luck!

Jim

[Ritchie200]

Well, Brad beat me to it.

[Willabe]

Well, Brad beat me to it.

That's cuz I play 1 of these through a Marshall Major 8x15 stack!     


            Brad       

[plexi50]

Definatley go back over every single part. You will find the problem. The tone would not be that drastically different if all values were as they should be. Ive done it myself on a few ocasions.

And as far as Marshall Major with a 8x15 stack. That would end your hearing career. I lost some hearing thanks to Judas Priest in 2001 at Janus Landing

[Willabe]

Sorry to hear that Plexi, no pun intended.


                Brad     

[PRR]

Take out the 12AU7. Is that better?

[cbass]

I agree with others Probaly a wrong value part or a bad connection.

If your amp the old grey PIO caps with the rubber seals.Chances are they are all bad.Even if they measyre ok they could still be leaky.

[cdmarion]

Hey Chris welcome aboard!  

Help me out here because maybe I am not understanding your post.  You mentioned that after you re-capped it and fired it up, Channel 1 had become somewhat muddy.  I am assuming that by this comment you mean that it changed AFTER you re-capped it?  If so, you may want to go back in this area and re-check the values of the parts you replaced and your solder work to make sure what you did has not caused the problem.  Good luck!

Jim

Thanks guys for all the replies, Jim, yes the tone changed after my work. I'll go through this morning and recheck everything and post what I've found.

[cdmarion]

Take out the 12AU7. Is that better?

I've taken out the 12au7 and the 6an8 with no change in tone.

[PRR]

Aside from the 470K going into 6AN8, when 6AN8 and 12AU7 are pulled there's really no difference Ch-1 or Ch-2.

Yet you say there is.

So something is different. Probably hiding right under your nose.

I like to mix-up red-stripe and orange-stripe resistors.

[smackoj]

I'm not familiar with Univox. Does anyone have a picture and some info on the hx (history) of the amp?

[darryl]

I'm not familiar with Univox. Does anyone have a picture and some info on the hx (history) of the amp?

There is a website dedicated to all things Univox. It hasn't been updated for a while, but then Univox haven't been very active recently . . .

http://www.univox.org/

[smackoj]

thanks much. cool stuff and lots of it too!

[cdmarion]

Aside from the 470K going into 6AN8, when 6AN8 and 12AU7 are pulled there's really no difference Ch-1 or Ch-2.

Yet you say there is.

So something is different. Probably hiding right under your nose.

I like to mix-up red-stripe and orange-stripe resistors.

I hate to ask this but Im confused as to the signal path without these two tubes, without these tubes does the signal just go into the phase inverter? Bypassing the effects circuit all together? One tube I've haven't pulled is the 12ax7 in the reverb, I did find that i had replaced the 30uf 6v cap on the 12ax7 cathode with a .47u 6v. Originally it had a 47u6v so I've corrected that, will fire it up this afternoon and see if there is a difference.

[sluckey]

without these tubes does the signal just go into the phase inverter?

yes

[Ritchie200]

Well, Brad beat me to it.

That's cuz I play 1 of these through a Marshall Major 8x15 stack!     


            Brad       

Brad,

I really think you have a BAD case of tube envy.   But since you admitted you have a Tele, I would imagine you will be on a FOREVER quest to make it sound good - like Tubenit!

cdmarion, I think you are on the right track.  Now you need to look back at it and determine why you got the result you did.  Boy do we learn from our mistakes!

Jim

[cdmarion]

Ok, now it starting to make sense, if without the effects circuit tubes the signal goes straight to the phase inverter then the problem has to be in that first preamp stage. Correct? While I was doing the initial work I tried to tighten and clean the first 12ax7 socket pins, I thought the tube was a bit loose, when I got done it was worse than when I had started. So now the first preamp tube is excessively loose, could this be my problem, while the amp is on I can touch this tube and get a pretty good crackle. I'm going to go back and check all my component values one more time in this stage but I'm starting to suspect the tube socket. Any thoughts?

[sluckey]

I don't suspect the tube socket, but since it now crackles I would replace it.

The two preamp channels are identical. Just compare wiring and component values between the two. I suspect you used an incorrect value, made a wiring error, or used a part that is out of spec.

[cdmarion]

IT'S FIXED!!!! So i went through this thing one last time and I found that I had installed two of the 500pf caps wrong. One of them being the 500pf in channel ones preamp, The other is the 500pf cap between B3 and B4 on the schematic, these two sit  together on the board. It's hard to explain exactly what I did but as most of you suspected it was an error in my work.
I appreciate all the input that everyone gave me. Not only did I fix the problem, I've now got a better understanding of how this thing works. Again THANKS!!

[Willabe]

Hey alright, you found it.      

I really think you have a BAD case of tube envy.  But since you admitted you have a Tele

Yes, you've found me out. Hi, my names Brad and I have tube envy.      

When did I ever say I have a Tele? My best gals been a Strat for 20+ years now. Tore up 3 sets of frets on her.

I need to get back to practicing.          

            
                    
                                           Brad                

[Ritchie200]

Sorry pal!!  That was JoJo...

We good!

Jim