Can anyone tell me step by step instruction on how to convert an old phonograph tube amp to a guitar amp.

Can anyone tell me step by step instruction on how to convert an old phonograph tube amp to a guitar amp. It has a 6bq5 power tube 6X4 rectifier tube, and a 12aX7 pre tube. Can anybody please tell me how to do this. You would be so appreciated. I have soldering skills, but not too much electrical knowledge. I know what resisters are, I know what transformers are, I know what capacitors are, and I know the very basics of their functions, but I can't understand how to tear it down and make it a good guitar amp. I want to remove the feedback circuit so that it will distort naturally. I would also like to modify the equalization and tone to better suit guitar amp. Please help. You would make me so happy. I cant afford to buy a good tube amp, and this is my only hope of making a great sounding amp. Thanks so much. Please be as detailed as posible in these step by step directions. You are awesome if you can help me.? Thanks
The amp is from a 1959 Newcomb model R124V phonograph with an inboard 6"X9" speaker. It works perfectly with no crackling knobs or nothing like that. The amp is super clean and well cared for The tubes inside are 6BQ5 FROM AMPEREX BUGLE BOY. The rectifier tube is 6X4 from sylvania, made in USA. The 12AX7 is made in Britain, but I can't see a name on it. I have many spare 12ax7's lying around though.
Thanks so much in advance.

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Re-design5 years ago
Those were sold as classroom amps.

Since it's 50 years old, you should replace a few things before using it.  If you don't you stand a chance of the elec. capacitors shorting out and distroying other parts in the process.  Check out this site.  You should complete this step before altering the amp. since you need to make sure it's working perfectly before altering it any.

Note that if there is no power transformer then this amp is very dangerous to work on and extremely unsafe to use as a guitar amp.  any short will connect you directly to full mains power.  A power transformer would isolate you somewhat and limit the amount of power you would get in a shock.  Still powerful but much less dangerous.  Either way if you get enough to kill you, you are still dead.

Here is a great article on converting a garden variety of tube amp to a reasonable guitar amp.

Have fun!

If you get stuck and need help consider joining this forum.  It has a lot of members who really know old tube radio and amp stuff.  There is a forum dedicated to tube amps.  They will probably razz you a little for taking the amp out of a piece of vintage gear but not too bad.

meman-mark (author)  Re-design5 years ago
Thank you.
I am in the process of changing out all the resistors and capacitors even though it worked flawlessly already. Thanks for the links and tips. No one seems to know if these have a feedback circiut that needs to be taken out. The amp has a power transformer and a output transformer. So i think this amp is pretty safe to use. Thx.
Don't worry about the feedback loop right now. Lots of guitar amps have them. I've been unable to find a schematic for this amp and until you do you won't be able to figure out the feedback issue.
meman-mark (author)  Re-design5 years ago
Great, THX Would it help if I could get the schematic? Can I post it here?
I found one.  Here is a link to it.
rickharris5 years ago

There are dangerous voltages inside that amplifier so take all precautions and NEVER NEVER work on it switched on or even plugged in.

1. You may need to build a preamp for the guitar signal as it is very low.

2. 1 place to access the amplifier is by tapping an input onto the volume pot - Middle pin i.e.the wiper. One end will be ground the other end the audio input the middle is the output of the pot going to the amplifier.

3. If it has a gramophone pick up on it you may be able to use that input directly as the output from a crystal pick-up is similar to a guitar.
meman-mark (author)  rickharris5 years ago
Thanks for reply. Yes you are very right to warn us ameture electronics tinkerers. I have been playing with electrical components ever since I was a little boy, and I am pretty careful about that stuff, and I know that Capacitors and the filters can hold large amounts of current even after they are unplugged. Thanks for being cautious. I still think there is a feedback circiut that needs to be removed correct? I would definately need detailed instructions how to make a preamp section. I am hoping someone could give me step by step instructions.
You can find circuits for guitar pre amps around the web fairly easily.

Accessing the Amp via the phonograph input or the volume control gets round all of the RF stage of the radio part.

Typical radio block diagram below.
iceng5 years ago
if you describe a guitar output
And I can find the circuitry
then we can design input changes you need.

meman-mark (author)  iceng5 years ago
I am not certain what you mean by describe guitar output? I would like a amp that will distort through the speaker and amplify it. This is a low wattage amp so, I am probably looking at about 5 to 10 watts max. It sounds really clean and bassy without modifications. Thanks for the reply. I hope I answered your question.