Ampeg VT-40 Rebuild

[smacbride]

Hello all, I've used this board for some very good information on debugging and building amps so I thought I would share some pictures and notes on a recent "restoration" project I completed.

The amp is a 1976 Ampeg VT-40.

I’ve had this amp for about 20 years.  Over time it’s been in the shop on numerous occasions for various reasons.  The latest problem developed after having the amp sit in my basement for several years without getting turned on or played.  A local tech replaced all the power supply caps etc. but the amp still had very loud “crackle and pops” and was un-playable.  The mid-range controls didn’t seem to work and the distortion channel was “fizzy”. He suggested that the PCBs could be holding moisture from being in my basement all those years.
 
So I decided to go down the path of replacing the PCBs with a turret board…

First I had to develop a layout to see if this could even work.  I decided that I would try to stay as close to the original layout as possible with the tube sockets on the bottom of the board. I spent several hours with the actual amp and the schematic working out a viable layout.  I developed 6 versions over a period of several weeks before I came up with something that was plausible.

[smacbride]

Some pictures of drilling out the board and populating it with components...

[smacbride]

Installing the board in the amp. 

[smacbride]

First power up and setting the bias for the power tubes.  I decided to go with an old set of 6l6s I had laying around.  The amp originally called for 7027A tubes but tube data sheets show the 6L6 as being a good match.  The schematic also says you can run 6550 tubes as well.

With my layout I was not able to put the reverb pan in the chassis, so I decided to mount in in the bottom of the cabinet instead.  I put some rca jacks on the back of the amp.

Now the amp sounds really good!  The distortion channel actually sounds great as well.  You can dial up the distortion a bit and get a nice gain sound.

The mid-range controls now work again as well. 

[jjasilli]

A truly brave & bold project!   

[sluckey]

Yes it is!

[eleventeen]

You did spectacular! Very inspired.


One lesson to be learned is: It can take what seems to be a completely ridiculous amount of time to derive a layout.

[smacbride]

Thanks for the kind words.

Without resources like this site, a project like this would not be possible.

Much thanks goes out to the community here!

[shooter]

It can take what seems to be a completely ridiculous amount of time to derive a layout.

Without resources like this site, a project like this would not be possible.

great words to live by!  Nice work!

[eleventeen]

Aha. At first I thought the component side of the board was exposed but now I see the black metal plate that the tubes poke through.


I assume you got the drill pattern for the tube sockets from this metal plate?


Nice! 

[smacbride]

Actually the plate is just a thin sheet of circuit board material.  I drilled pilot holes for the tubes through both pieces clamped together so the bigger holes would line up.  I was planning on lining the inside of the plate with metallic tape but the amp seems to be pretty quiet so far...

[PRR]

> replacing the PCBs with a turret board...

That's incredibly bold and labor-intensive.

Boards will dry. I woulda put a heat-lamp on it for a month. Then store it dry and play it several hours every month.

Bravo.

> The distortion channel actually sounds great as well.

WTF did you do it it?? That was the worst sound I ever heard. Seems like a fuzz box but the "fuzz" was rusty steel wire abrasive. Maybe hip for 1976, and maybe not even then.

> the 6L6

The 7027 were, IMHO, "starter tubes" like that wee ink tank you get on a new printer. The factory heavy-duty option, 6550, was there for a reason. My school's starter-tubes actually ran for decades, and then one shorted and burned the PT.

7027 is 6L6GC with a funny pinout, so reliability will be similar.

I really think these amps, if not used by broke teenagers in a barn, should be fitted with 6550.

[smacbride]

RE the distortion channel:  The biggest difference is that the schematic shows 1N456A diodes in that circuit and the suppliers I used for parts didn't carry those so I used 1N4148A instead.

I'm not an expert by any means but they might have different signal clipping thresholds thereby giving less signal distortion??? Just a guess on my part.

I think I'll try the 6550 tubes at some point.

[eleventeen]

" different signal clipping thresholds thereby giving less signal distortion"


Right. 4148's are Si, three digit diodes are usually Ge. Breakover voltage in Si about .65, in Ge, about .2


(Not familiar with the circuit, just a general comment)

[PRR]

1N456 is surely Silicon. A high-price 4148.