Magnavox tube amp convertion to harp/guitar amp

[RitzRed]

Hey I have some old organ tube amps that I want to convert to harp/guitar amps. Has any of you all done this? The Magnavox amp I have has the tubes That make a good harp amp or that is what I read. The tubes are 2 6V6 tubes 1 12AX7 and 1 5Y3 tube the amp I have has the control knobs and a new 3 wire power plug the problem I am having now is where do I wire in the 1/4" inch input jacks and where do I wire in the output jack to the speaker? I see there is 2 different jacks that appear to be input jacks it also has wires that look like speaker wires the problem with the 2 input jacks if that is what they are?is one has 6 pin holes 5 in a circle one in the middle,that jack only has 3 wires hooked to it. the other jack is bigger with 6 pin holes all in a circle and there looks to be 6 wires hooked to it or 1 wire to each pin.I am not sure what model Magnavox it came from it said in the add it came from a working Magnavox Sterio/record player well no I just read it again and it says from a working consol it also says 164AB mono tube amp does anyone know where to find a schematic for this amp? I just bought 2 books that a amp tec reccomended one is all about tone the other goes through just about everything in a amp it looks to be a very good book with allot of information I have not read too much of it yet but it seems to tell what each part does it also has allot of schematics of amps in the back If I had the right jacks for this amp it is supposed to work. I have been told that you have to change some of the resistors and caps to make it sound right for guitar I will get to that once I make it work,and I have read through the one book I just got on tube amps and tone I would appreciate any advice hear on this amp as to where the jacks go thanks

[nashvillebill]

Welcome to the mysterious world of tube guitar amps.

While it's true that those tubes are common enough in many guitar amps, it takes more than just those tubes alone. An amp also has a power supply (using a power transformer), an output transformer, tone and volume controls, and miscellaneous parts such as resitors capacitors, sockets, a chassis, etc. All of these components are combined into the circutiry that's desired for a guitar amp. Fortunately, the two biggest-expense items--the power transformer (PT) and output transformer (OT)--that you have on hand will likely work for a guitar amp. So you're kinda well on your way....except...

It's not going to be quite as simple as merely soldering in an input jack. What you have is a power amp, without a preamp. You will need to add at least one tube to make a preamp gain stage, then have a volume and tone control circuit suitable for guitar use. But even before you get that far: you've got a 50 year old amp. Darn near every capacitor in that amp should be replaced. Oh, and those 50 year old carbon resistors? Probably out of spec as well. Oh and one more thing: Magnavox made that amp to interface with its (separate) tuner/preamp so some wires go out of your power amp section, up into the missing tuner/preamp, then back into the amp. Probably does the same thing with the speaker outputs, on my Maggie console there is a separate speaker selector switch.

So to really get this into a viable guitar amp, you'll need to save the two transformers (and set the tubes aside) and gut everything else in the amp. I'd even toss the tube sockets, they are too much trouble to desolder (buy new ones from Parts Express or Antique Electronic Supply for a few bucks each). You might save the steel chassis but it'll need a faceplate when you're done to make it look presentable.

Then rewire the amp using the PT and OT to match the guitar amp circuit of your choosing. You can get some ideas at www.ax84.com, I believe under the Building Blocks section there is a power amp using a pair of 6V6's. Or duplicate a Fender amp circuit.

Now a fair bit of technical knowledge will indeed be necessary. So read through some of the books downloadable free at http://www.tubebooks.org/technical_books_online.htm

[RitzRed]

Well thanks for the info I bought this amp to make it into a harp amp, I have a schematic here for a fender princeton and as far as I have gone through this schematic it is all the same as this amp so far the fender just has a 5Y3GT and 2 6v6GT,s and a 12AX7 this amp has all the same tubes so far the only thing I can find that the Fender schematics has that the amp does not is a 2 amp fuse. If I did,nt know this amp came out of a Magnivox consol and by looking at the Princeton schematics I would think it was just that I don,t see a preamp unless it is the 12AX7 tube but I am new to electronics so I don,t know whats what yea I have 7 guitar amps I need a good harp amp oh well I will figure it out thanks again

[JHow]

Ritz Red: Which Fender Princeton schematic are you looking at?

Also, if you want some free tube learning, take a look at these places:

How to design valve guitar amplifiers

If you can grok valve wizard's first chapter on how a triode works, you will be well on your way to achieving amp-fixing satisfaction.


New Page 1

This site is great and in his "Tube guitar amp tech pages" he has a section entitled "converting old mono and PA hi-fi amps for guitar use", which is almost what you are trying to do (I know you play harp, so its almost the same).

I would link to the exact document, but somehow geofex and my mac don't get along, so I can never link directly.

Tube Amp Schematics, Tube Amp Information, Tube Amp Projects

The above site I like for handy reference stuff, like for instance, how to wire hi-lo inputs. It should be simple, and it is, but how embarrassing when all that stands between an amp builder and harp-amp glory is a mis-wired input. "Hmm...mis-wired his input has Obi-wan...How embarrassing!"

[nashvillebill]

The 5D2, the 5E2, and 5F2 Princetons had one 12AX7--but they only had one 6V6 running in single-ended. The AA964 and CBS Princeton had two 6V6's in p-p like the Maggie--but they have two 12AX7's--one is needed for the phase splitter. Well technically the schematic has one 7025 and one 12AX7, but a 7025 is the low-noise version (in those days) of the 12AX7.

With the OT of the Maggie, you need to run a push-pull configuration with the pair of 6V6's. So you need a phase splitter, that can be done with one half of a 12AX7like on the 5E3, but you still need another tube somewhere.

[RitzRed]

there is a spot for a 6 pin tube on the side of the chasis maybe that is it but in this schematic of the princeton it has all the same tubes as the Magnavox amp I have I am not sure that it is a place for a tube or it is a jack all I know is I have to study into this more so I understand how all this goes together

[nashvillebill]

Some of the Maggies did have an empty spot for another tube, that is hardly surprising.

Hwoever, there isn't a single Magnavox comsole amp in existence that was pre-designed with an option to use as a guitar amp. The console amp section that you have was made to take a line-level signal out of the preamp/tuner (which you do not have) and amplify that. It was not made to amplify a lower-level guitar signal; that's the part you need to add.

Again, what Princeton are you looking at? There's a whole lot of them, some of them have designations such as 5D2, others were the AA964 for example. To my knowledge there isn't a single Princeton out there that has only three tubes (in addition to the fourth rectifier tube) with two of them being a pair of 6V6's in push-pull and the third tube a 12AX7. (However, I don't claim to be the ultimate Fender expert)

There's Princetons with one 12AX7 and ONE 6V6. Emphasis on the ONE. That's a single-ended amp...but your Maggie output transformer is a push-pull.

I do encourage you to please study some more as this isn't a simple matter of just soldering in an input jack and "hey presto! The guitar tone of the gods!" . I would suggest, in addition to the books jhow and I linked to, the "Electric Guitar Amplifier Handbook" by Jack Darr. This explains many of the basic circuits involved. With a Google search, you may find it somewhere downloadable free.

[RitzRed]

that is one of the books I have the other one is the guitar amp handbook by Dave Hunter, the way it is discribe in the book with the schematic says that the 12AX7 is the preamp but yea I continue to study and the tubes in the Magnivox are 6V6GT,s and the 5Y3GT I had to look at the tubes again the schematic in the book says it is a Fender Princeton5F2 I don,t see a preamp in that schematic but I will take your word for it and study on

[RitzRed]

After looking closer at these tubes there is a 12AX7,2 6V6GT,s and a 5Y3 the schematic of the Fender Princeton 5F2 has a 12AX7,2 6V6GT,s and a 5Y3 everything in the schematic is spot on the same as the Magnavox maybe I am missing something so I am going over it all again

[g1]

The 5F2 has only ONE 6V6. It is a single-ended output rather than push pull. Your amp is push-pull with TWO 6V6's, so it requires a second 12AX7 for a phase splitter circuit. Your push-pull power amp uses a different type of output transformer so it can't be wired up with a single 6V6 like the 5F2 is.
Look at your 5F2 schematic, you will see there is only a single 6V6.
Now please re-read what nashvillebill said in post #5 and #7.

[RitzRed]

ok well the schematic has 2 6V6,s it says it is a 5F2 here is a pic of the amp I am working with in a few I will attempt to take a picture of the schematic to send I don,t get it

[RitzRed]

you are right as I look at the schematic closer it has only one 6V6 tube no problem adding another 12AX7 I will just find a schematic, you know the quote on the bottom of your post is offencive if it bothers you to answer these questions why do you even answer them we are all ignorant just on different subjects

[g1]

you know the quote on the bottom of your post is offencive if it bothers you to answer these questions why do you even answer them we are all ignorant just on different subjects

Sorry you found the quote offensive, no offense was intended, just a poor attempt at humour. I have removed the one quote I suspect you were referring to as it was kind of obscure and has probably outlived it's usefulness.
And it never bothers me to answer questions. There are many things I am ignorant about and sometimes I have to ask about them too .

[Bruce / Mission Amps]

you are right as I look at the schematic closer it has only one 6V6 tube no problem adding another 12AX7 I will just find a schematic, you know the quote on the bottom of your post is offencive if it bothers you to answer these questions why do you even answer them we are all ignorant just on different subjects

If I might make a suggestion... many of those old amps do not run the power tubes with very high B+ from the rectifier filter cap section.
So to kick it in the tail.....
What I have done in the past with amps like this is to not use a tube rectifier at all.
Instead, use a solid state set of diodes on a terminal strip to the main filter cap for a little higher useable B+ rail.
Then strip all the wires off one of the old octal 6V6 and the rectifier sockets.
Use the old rectifier tube socket as the second 6V6 socket with the two 6V6s wired in push pull and cathode biased.
Here is the awesome part... use the old empty 6V6 octal socet rewired as a 6SL7GT dual triode, driver for the push pull circuit.
For harp I would suggest a paraphase driver.... but any two triode push pull circuit is fine.
This way you have way more gain then you will need and both triodes of the 12AX7 socket for mic gain, tone controls, volume etc., and one more triode for recovery.... to drive the first part of the push pull 6SL7GT.
Killer harp amp sound.

[nashvillebill]

Very clever, Bruce!

My Magie 175 s probably similar and it has 355 V on the B+, so going to SS rectification likely wouldn't get the B+ up too terribly high for those old 6V6's.