voice of music 160

[dub136]

Hello

 Could someone suggest tweaks for a tweed  tone guitar amp  from this 1952 voice of music  tv front .
 I need to change the cap can for sure but dont know what  I need for the mic input.

Thanks in advance
Dub

[Willabe]

That'll proble make a real nice guitar amp.    

The coupling caps at c3, c7, c8 could be changed from .047 to .02, to lessen the bass, if it's too much bottom end and muddy sounding now.

You could add a cathode bypass across R20, (output tubes cathode bias R@300R) say 25uF@50v?
 
You could also try to change the input preamp grid and cathode R's to Fender values, + add the cathode bypass cap, but it might sound nice as it is now.

I think that's a para-phase PI, could be changed to a split load PI or a LTPPI.  

If you wanted more drive/distortion you could use the phono triode either in parallel or in series with the mic triode or use that triode as a tone stack recovery stage, if you what/need better tone shaping.

What are you hoping to get sound wise?


                               Brad      
  

[dub136]

Thanks brad
I was hoping for tweed deluxe 
I have another one of these  that was modded with 2 guitar inputs. one is darker than the other .
I asked the tech to write down his changes but he didnt.
Its close now I just  wanted some advice on possibilities.................

Dub
 

[Willabe]

Forgot to ask if it works? Have you played through it yet?

I don't know if you know about using a lamp limiter as a testing tool when 1'st powering up an old, not working amp, new build or when you've worked on the power supply of an amp? I wouldn't fire up that amp without it.

It can save you a lot of $$ and heartache by saving the PT, filter caps, etc.

Here's a link that shows you how to build one, it's a must for tube amp work.


                            Brad      

  

[Willabe]

I was hoping for tweed deluxe

Well you have the right tube count/complament already in that amp. So you can probly assume that the PT can handle it, altho it was not ment to be run at full blast. If the PT is about the same size as a Tweed Deluxe you'll proble be fine.

You could turn it into a 5E3 or even a 5D3. You could go with 1 of Doug's turret boards or make your own, Doug has what you need for this. Or you could use tag strips, might be what it was built with?

Can you post a few good pics of the amp your thinking about modding?

                        
                                     Brad      

 

[dub136]

Brad

Yes ive used it (got it off ebay)and checked plate voltages against the schematic .  they are okay.
Changed to a 3 prong plug and added a fuse. took the pots apart and cleaned them. theChanged the filter supply cap can and changed C3 C4 C7 C8 (.047)
 Its got a strong whine that is present with and without any input.
substituted other  tubes and cant find the whine  my Hoffman bias checker show one 6v6 much higher mA than the other.

Dub

[Willabe]

my Hoffman bias checker show one 6v6 much higher mA than the other.

Swap the 6V6's with each other and see if it follows the tube or if it stays with the same socket.

changed C3 C4 C7 C8 (.047)

To .02?

Its got a strong whine that is present with and without any input. substituted other  tubes and cant find the whine

Did you change c2 on the PI? That might cause strang things to happen if it's bad.

Could be lead dress? + there's no grid stopper on the input tube grid. If the wire from the jack to the grid is long, you might need to use shielded wire there. You might be able to find it by using a wooden chop stick or wooden dowel rod to move the grid wires around/away from other wires like plate wires while the amp is on, be very carefull.

I'd also add a 1K5 R to each of the output tubes grid and a 470R@3w flame proof type to each of the screen grids if you haven't yet? All standard stuff.


                              Brad    

[jjasilli]

Power tube imbalance:  could be the tubes are seriously mismatched.  But if swapping them doesn't resolve that issue, you might check to see hat there's -0- DC on ea power tube grid. 

PI:  I'm intrigued.  Guess it's a paraphase, with the lower half stealing signal form the upper half.

Preamp: I find a bit mystifying. I'm guessing the phono section has an interesting feedback & a presence-like control.  Was there some outside phono circuitry originally feeding this input?

If the amp is too dark, maybe consider converting the the mic channel to tweed deluxe specs.  Then see if it interplay's with the phono channel controls.

Just some ideas.

[dub136]

Willabe:
 the high mA is on one tube socket.

You guys are way past my tube amp knowledge. Ive read alot of hoffman articles :built a bias checker : can sorta read a schematic

replaced blocking caps C3 C4 C7 C8  .047 just like the schematic
I need to change C2 I guess
 Thanks to all who replied
If I can get rid of the whine Ill try to jumper the inputs and see about gain.
Dub

[Willabe]

the high mA is on one tube socket.

What are the mA readings your getting?

I think jjasilli is right, check to make sure that there's no dc on pin 5 (to ground) of each 6V6. Measure across R18, R19 and R21, with the amp off and see if they've drifted up a lot?

My next thought is some kind of imbalance on/from the PI, could be that 10uF e-cap,        but it should be changed anyway.

Last thought is bad OT, but....           I think that's a long shot.

Is the plate and screen voltage the same or very close (measured at the scoket pins) on both output tube scokets? I'd also measure the voltage at/on pin 3, cathode, to see what's going on there.  

                                  
                                                Brad    

[DummyLoad]

- change grid leaks (R4 & R7) to 1M.
- change RCA phono jack to 1/4" phone jack.
- add grid stopper (22k-68k to taste) to V1a and V1b. larger grid leak values will shave off high audio frequencies.
- phono amp (V1b) is a cathode follower - add 100K plate R, change cathode R5 to 1k-5k; value to suite taste; move C4 from cathode (pin8) to plate (pin6). clip R9 out of circuit remove C6 as well, if it's convenient.
- add bypass cap to R5 and R8 - 1uF-10uF to taste - i usually like 2.2 or 4.7uF there.
- add bypass cap to R20 - 47uf-220uF at 50V cap; + side of cap goes to pin8 - of side of cap goes to ground; again, value is to taste.
- add grid stoppers to 6V6s (V3 & V4); a value from 1K-10K will work.
- fiddle R23 value for NFB - higher R (brighter) or less R (darker, less gain).

whine could be RF; installing grid stoppers @ grids of V1 should take care of that.

pull the 6V6 tubes and power up with a lamp limiter - check pin 5 of 6V6 sockets for DC - should be be reading DCmV range to ideal of zero volts. you may have burned the body of a cap during replacement.

i attached a modified schematic of what i would do if the amp were on my bench.

good luck,

--DL

[dub136]

Ive got all the stuff to do the changes but have to wait for work to slow down a bit.
Question: can I put the 1 meg resistor right on the 12a shunt lug and then jumper the shunt and ground?

Thanks for the help
Dub

[sluckey]

Question: can I put the 1 meg resistor right on the 12a shunt lug and then jumper the shunt and ground?

Yes, if the other end of the 1 meg is on the tip.