Converting a phonograph amp to a guitar amp

[Malanga]

Hello all!

Well, I would like to build a vacuum tube guitar amp for my nephew. I have some electrical skills and have assembled kits and recapped several Hi-Fi receivers in the past. What I had hoped to do was to use the tube amp I pulled from a Motorola portable turntable that had a dead platter. The tube amp uses a 12AX7 and a 6V6GT for the tubes. I have read that others have used these type of amps to make some pretty decent low-wattage guitar amps. Anyone know where I could find a how-to for doing this? Thanks!

[6267]

Here are some online resources:

http://www.geofex.com/article_folders/old2new.htm
http://www.netads.com/~meo/Guitar/Am...a2g-notes.html

This amp should have two transformers, one a bit larger (or a lot sometimes) than the other. If there is only one, this will not be a safe guitar amp, and could pose immediate and lethal danger to any musician using it. Some old amps avoided using a power transformer by connecting the amp directly to the line. When these malfunction, or are even plugged in wrong, they will connect the line to the chassis, which is connected the the guitar jack, which is connected to you. Bad news!

Depending on the kind of phono cartridge, you may have not enough gain or too much gain already present. Further, if you imagine a graphic EQ, a phono stage will be boosting the bass and cutting the highs quite considerably, since the records are recorded with the opposite EQ (called RIAA eq).

You will want to remove this EQ filter network (probably resistors in series and caps to ground, maybe more complex) to recover an even response. It
's generally after the first amplification stage. If you post a schematic, I am sure someone will be able to point it out.

You will probably want to mess with the feedback resistor/cap combo, or remove feedback entirely to get a better guitar tone. Some amps wire this to a "presence control"

Personally, I wouldn't try to redesign the entire amp. Most of it will work fine already for guitar, but you will need to work on the input stage. Here, you can examine a couple guitar amp schematics. Try to find one with similar voltages present with all the tubes in.

Good luck!

[Malanga]

Thank you for the advice! I have been trying to find a schematic, but so far no luck. The only chassis number I found is 27D638505.

As you can see, it does have the power transformer as well as an audio.

Here are some pics of the amp:

[6267]

You could definitely trace out the schematic by hand, it doesn't look too complex in there. Good project chassis! If you're like me and can't always remember the resistor color codes or whatever, it's at least a good start to just draw in generic resistors & caps. Google jschem for a good multi-platform schematic drawing program...it has tube symbols, at least.

It's probably input voltage amp > equalization > voltage amp > 6v6. Some phono cartridges didn't really need as much equalization, have you tried just plugging in?

You aren't going to have too much gain available for a tone stack, unless you replace the tube rectifier (looks like a 6x4?) and replace it with diodes. You could then use a 6j6 or 6av6 for one or two more gain stages. As is though, it'll sound great...some of my favorite homebrew amps don't have much in the way of tone stacks, I just make 'em sound good, and build another one for a different tone .

[baddog]

Just plug in and give it a shot . I am working on a 50's phono amp that sound prety decent stock but of course i like to tinker with stuff.

[wrathfuldeity]

Found an old circa 50's Bogen record player with 12" jensen in the lid and a mic, just pluged in...sweetness, eventually had a cap/filter job & tweed mod; 6l6, 5Y3GT, 6AV6.

[baddog]

Cool find , check this out.
Bogen Phonograph with a tape out , radio in and mike input.
tubes
1 5y3gt
2 6V6gt
1 6sL7
1 6SJ7
1 6SC7

i Havent started it up yet but will after i order some parts

I have roughly 7 tube phono amps i recently scored from a guy cleaning out a storage building. I got them all for free just for helping.

[wrathfuldeity]

baddog, looks like you have a Bogen vp17, pm me if you want a copy of the photofacts

[baddog]

Wrathfuldeity

Yes it is a Bogen VP17 you have a good eye. I would be happy to just have the schematic but i will take whatever you got. I sent you an email already. I tried to fire it up with newer tubes but only got a hum that got encreasingly louder so i shut her down and ordered new caps. I did order new electrolytics also. I would like to see a bigger pic of the amp but it looks cool from what i can see. I recently ran into a guy who had a storage unit and helped him clean it out. I picked up 7 portable phohographs and other oddball stuff from the 50's and 60's. Kid in a candy store type of thing. I recently conlerted a voice of music PA amp into a 1x10 combo amp with a vintage jensen speaker. I will pist pictures later if anyones interested.

thanks
baddog

[Regis]

Originally Posted by Malanga

Thank you for the advice! I have been trying to find a schematic, but so far no luck. The only chassis number I found is 27D638505.

As you can see, it does have the power transformer as well as an audio.

Here are some pics of the amp:

This looks like a cool little amp. The rca input looks like it goes directly to the input of the preamp tube. You could wire up a typical Fender input but for starters I would just replace the rca jack with a quarter inch jack and see what happens when you fire it up. Be sure to insulate it from the chassis or you will get hum.

The cap can is probably dried up and you can install caps underneath to the terminals of the can to fix that.

I doubt that this amp has RIAA eq, It's probably a typical RCA tube manual type of circuit. A couple of cathode bypass caps and maybe dink with the tone stack and it will probably be a Champ like screamer.

Check the pot codes to determine the vintage of the amp.

[Dave Curtis, dB AudioTech]

Originally Posted by Malanga

Thank you for the advice! I have been trying to find a schematic, but so far no luck. The only chassis number I found is 27D638505.

As you can see, it does have the power transformer as well as an audio.

I recently rebuilt an old ('60) Voice of the Music phono amp. Wicked small, it fits right into the old tweed Victor projector speaker I have.

I basically rewired it as a 5F1:
http://www.schematicheaven.com/fende..._5f1_schem.pdf

[cavefish]

I have a +40yr Westinghouse G69ACS1 record-player/phonograph that I want to convert to a guitar amp. I read the discussions and info, and I'm glad to find so much enthusiasm.

The Amp is 90watt stereo. Two power stages two 6BQ5, 6CA4, two 12AX7. Panel has treble, bass, and volume control knobs, and a stereo control knob left-right balance (push stereo, pull mono). Speed knob and ON/OFF knob on the deck.

What I really want to figure out is what the cables are for. There's two grey wires that look like speaker out's and a set of yellow and white cables. I fired it up and it works, except the needle is broken (don't care), I don't a have speaker to test it with. What is a good speaker range to test it with? 12inch 30watt 4Ohm speaker or 50watt? The reason I ask this because it is old equipment... I had a Traynor 50watt combo which drove a 30watt speaker, and plugged it to 140watt Mesa cab.

Is each output 90watts or 45watts per channel?

The schematic file is
62_Westinghouse_H69ACS1_139.djvu
but it doesn't work. I've tried 3 diff pieces of software to open it.

I impatiently await your input.

[baddog]

As far as can see your amp probibly puts out about 5 watt per channel on a good day. 1 x 12ax7 and 1 x EL84 per channel and 6CA4 Rectifier. The gray wires are the signal wires and im gessing the other wires are for the speakers. To make sure the yellow and the white wires are speaker wires see if you can find where the go in the amp. See if they conect to the output transformers at any point. A 4ohm speaker should be fine for tesing the amp but you want to make sure you have a speaker hooked up to each output transformer during tesing or you can damage the output transformers or maybe more.
In my experiance anything that says highfidelity or stereophonic doesnt make a good guitar amps. What i look for is old monoblock amplifiers like organ amps,old stromber carlson lunch box PA amplifiers , Portable Phonograph with built inb speakers. But like anything you take what you can get and the one you have sounds like a good first build.

BD
Here is pic of my latest beast mad from an tone cabinet amplifier
http://viewmorepics.myspace.com/inde...ageID=15247852

Pics of some stuff i scored
http://viewmorepics.myspace.com/inde...ageID=14016667

[baddog]

Originally Posted by Dave Curtis, dB AudioTech

I recently rebuilt an old ('60) Voice of the Music phono amp. Wicked small, it fits right into the old tweed Victor projector speaker I have.

I basically rewired it as a 5F1:
http://www.schematicheaven.com/fende..._5f1_schem.pdf

Sweet little Victor speaker I ran into a victor speaker like that but it was all metal no tweed. I should have picked it up when i had the chance.

BD

[cavefish]

Thanx Baddog for the quick reply . The yellow and white wires hookup to the transformers (forgot to add that). The gray wires are connected to the 'Loudness' knob (volume). But what are they for? Input/output?

I'm going to test it out today and tyr to figure out more stuff.

[Dave Curtis, dB AudioTech]

"The schematic file is
62_Westinghouse_H69ACS1_139.djvu
but it doesn't work. I've tried 3 diff pieces of software to open it."

Try Irfanview; don't forget the plugins.

[cavefish]

Originally Posted by Dave Curtis, dB AudioTech

"The schematic file is
62_Westinghouse_H69ACS1_139.djvu
but it doesn't work. I've tried 3 diff pieces of software to open it."

Try Irfanview; don't forget the plugins.

LizardTech DjVu 3.1 opened the file but it was blank . I just tried Irfanview and it failed, decode error . If someone can open it for me that would be great, and convert it to pdf/jpeg.

[cavefish]

Simply works. I plugged in my guitar, bypassed the record deck, used a small 6Ohm 50Watt speaker for output. I definitely need a pedal for gain and eq control. Next, i'm going to build a single speaker enclosure and shop for a new speaker. I've built a open tube style mount like the Ampeg and Univibe amps.