Old Accordion Tube Amp

[drumslinger2000]

Hey Guys,


     i ran across this old tube amp that was made in Mexico for the Accordion.  it was given to me by a friend who said it no longer works and it's just collecting dust at his place.  The face plate has the markings C.V.S.A  10-N.  Tried searching for this but no real hits on it over the web.  He said some work had been done on it and it fried the 6AU6 in V1.  Most of it is still original, still has the two conductor lamp cord, the Volume acts as the on/off switch, no markings on the PT or OT so can't identify, it's all PTP work and couldn't find a schematic similar to it which is why i made a layout of the amp to understand it better.  The power section has two 7408/6V6, a 5Y3 rectifier, a 6AU6 in V1 and a 12 series (12AX7, 12AY7, 12AT7..etc) in V2.  A few of the coupling caps are ceramic disc (don't understand this or maybe it's all the builder had at the time or maybe it's part of the Accordion sound) but i would like to get this amp back in working order but with some upgrades that are needed.  Some of the electrolytics have some swelling and most of the carbon comps look brittle.  I have attached a PDF of the layout and the Visio file as some fresh eyes would be welcomed in finding errors.  I'm taking pics of the amp today and will upload when done.  All of you have a Wonderful and Happy New with family and thanks for your input/advice/help in advance.


Regards,


C




**layout and visio updated.  had 100k from V2 pin 1 going to ground. 

[drumslinger2000]

Here are some of the pictures.

[drumslinger2000]

a few more..

[drumslinger2000]

just a bit more..

[drumslinger2000]

still a bit more....

[drumslinger2000]

and the last 2.

[PRR]

> made in Mexico for the Accordion.

My colloquial Mexican is super-rusty, but isn't that a picture of a guitar next to "Instrumentos"?

And the circuit is 90% one of the Fender Deluxes, isn't it?

The 6AU6 stage is super odd. 6AU6 should have more pins wired. Ah, 'au6 can be used as low-gain Triode, either G2 to Plate or just using G2 "as" plate in low power work like this. But even then, when I compare pinouts, it's just wired wrong.

As it is originally a Guitar amp, you expect a hi-Mu triode in front. Because this amp omits the two "channels" and mix-stage, it does not have to be the real high-gain dual 12AX7. And Millones of single hi-Mu triodes were used in AM radios. 6AT6 is super typical. Again the K is just wired wrong, but the other pins are not right either.

Agree the power electrolytics look perished, and I wonder if it is time to build a proper doghouse on top to keep these out of the way and safe. This is clearly "studied from" Fender, and Fender used doghouses.

Wow, 20% tolerance resistors! Actually I would leave any of these which are not clearly toasted. "Dry" may be the way they came from the factory. (It is possible these are the 1940s resistors the USA threw-out in the 60s to make stock-room for 10% resistors, or they may be local production.) Mostly tube amps work all the same with 20% parts. They may not have aged well. But I'd leave as much "mojo" in this as possible. Echos of now-gone musicians.

But obviously it is a nest of serpents. Inspect as possible. Remove tubes. Outside on a dry board with a long cord. Lamp-limiter a very good idea. Stand back, plug in, wait an hour or more until the fireworks stops. (Actual fire, stop now.) That won't stress the DC side, so install rectifier and repeat the show. When done, carefully bench-test for PT AC, then inventory the toast and order replacements. I bet *everything* can be sourced from Fender Deluxe shopping list. (Except the single triode 6AT6, but it can use DL AX7 R and C.) 

[drumslinger2000]

Hey PRR,


     Yes indeed it's a guitar on the front but from what my friend told me it was built for an accordion and totally agree that it has similarities to a Deluxe, most likely the 5E3 circuit.  I was a little stumped when he told about the 6AU6 and had to look it up.  Never seen an amp with one.  Perhaps a hybrid radio/guitar amp?  A proper doghouse is on the list along with other upgrades for safety and ease of tweaking.  I'll be sure to check all the resistors and keep what i can.  A proper board will be made to keep from all the crisscrossing and accidental shorting when checking things.  Perhaps removing the 6AU6 completely and replacing with another tube type would be less of a hassle.  Oh yeah, in case you haven't noticed the tube sockets are riveted to the chassis!  LOL


C

[sluckey]

I'd fix it just as it is. Then evaluate it for a while before 'improving' it.

[PRR]

> told me it was built for an accordion

It could be they had the guitar faceplates printed up and used them when they branched into other markets.

But the way you have drawn the 6AU6 just won't work. Do you have any other evidence it was always a 6AU6? If not, I would suspect someone stuck it in there for no good reason. I also suspect whoever wired what is probably a K pin to a B+ resistor was confused. (I now see this is your Visio, not the as-found connection.)

And yes, I noted the rivets so I didn't go off on a 9-pin trip. Though there is a 9-pin punching next to it.

_I_ would steal a known-good DeLuxe layout, only omit half the first 12AX7's parts and follow 6AT6 pinout. 6AT6 is a super-common type (inexpensive, $6) and all you will find are Golden-Era NOS.

[sluckey]

Many of the old "blue diamond" Ampeg amps had accordion and guitar inputs. Hey, Lawrence Welk had just as many accordion pickers as guitar pickers. It's purely marketing scheme. Absolutely no difference in the electronics.

[drumslinger2000]

PRR,


    The only info i have is what my friend told me but he DID say that the 6AU6 had fried some time back (several years) when he last turned it on.  Perhaps he was mistaken by the type of tube to use in V1.  HOLD UP!  It seems there was another tube rolling around the bottom of the amp cabinet and this one is a 6AV6. 


C

[drumslinger2000]

Hey Sluckey,


    Thanks for clearing that up.  I thought maybe there was some kind of hidden mojo with amps made specifically for accordions.  I'll search Doug's inventory of Ampeg schematics and see what similarities there are.

[sluckey]

Your mystery tube is a 6AT6 or 6AV6, not a 6AU6. Fender Harvard 5F10 used one also See link...

     http://sluckeyamps.com/harvard/harvard_5f10_schem.pdf

[drumslinger2000]

ChAChING!!!  Thanks Sluckey!  Man, talk about an almost exact layout except for the missing two inputs.  No wonder he fried the 6AU6.  Thanks again Sluckey and PRR.  Time to fire up the soldering iron.  Happy New Years guys!!!


Regards,


C

[drumslinger2000]

Your mystery tube is a 6AT6 or 6AV6, not a 6AU6. Fender Harvard 5F10 used one also See link...

     http://sluckeyamps.com/harvard/harvard_5f10_schem.pdf

Hey Sluckey,


     I was looking over the layout and schematic from the link and something caught my eye.  The bypass cap (25uF @ 25V) coming off the cathode V1 pin 2.  Is that oriented right?


C

[sluckey]

Nope. It's backwards!

[drumslinger2000]

by the way......NICE schematics and layouts!

[sluckey]

I just copied the old Fender layout. I'll fix that cap. I'll probably use your indicator lamp too. I've already added it to my shapes stencils.   

BTW, I actually own that old 5F10 Harvard amp.

     http://sluckeyamps.com/harvard/harvard.htm

[drumslinger2000]

By all means, have at it.  I have a big file of parts somewhere on these hard drives but can't seem to find it.  Might be on my backups.  I'll try to locate and post it.


DUDE!!!  Excellent restoration!! 


C

[PRR]

> another tube rolling around ... a 6AV6.

Yes, yes, yes.

There's a dozen similar tubes of different Mu. I can't remember them all. A 6AT6 (one I remembered) will work fine. A 6AV6 will work fine, AND the U/V similarity explains why there was a pentode in the mix-up.

6AV6 is half a 12AX7 plus two diodes you will ignore.

The diode-diode-triode tubes were VERY common. When you design a radio, you plop a rectifier and an audio power tube, then the RF and IF stages. To join them you need a diode (or two) and a voltage amplifier. Almost every post-WWII radio had a d-d-t tube. Different bases and different Mu to suit the build. Going a bit up or down on the Mu is a small difference in Vol knob setting, so you could tweak the showroom "loudness". Incidentally they get used where you need "a half a twin triode".

[drumslinger2000]

Hey PRR,


     Yup a 6AV6.  It was actually wedged/stuck in at the base of the cabinet.  Couldn't see it because of the back cover.