1960 Motorola Stereo RP to Guitar amp conv.

[thomasks]

I just aquired an 1960 Motorola stereo record player and would like to convert it to a guitar amp. It has 12AX7 preamps and ECL82 pwr amp tubes. With a EZ81 tube rectifier. I would like to know if anyone could help me to bridge the left and right channels into a mono amp. (or maybe I could make a clean and distortion channel) Any suggestions? What type of modification would I need to do to make this sound good as a guitar amp? I have uploaded the schematic, and would appreciate you looking at it. Since I am a newbie

[Rayme]

I hope you get an answer as I posted the same question awhile back and still no response. I have the same setup only Admiral and would like to know if I can somehow run both channels to one speaker without damage. I have converted a few mono systems with no problems and would like to utilize both output trannies in this setup. Tried searching for answer and still no luck. Again, I hope you receive the answer for both of us and good luck on your project! Rayme

[thomasks]

I have run both channels to one speaker, and no damages so far. In the amp the "stereo" switch just tied both channels together basically (although each channel had it's own pots, they are double stacked pots using the same knobs) there was an internal speaker in the cabinet seperate from the outboard speaker jacks. I took the rca jacks that went to the turntable and tied them together going to a 1/4" guitar jack and plugged in a guitar to it, and it worked. I removed the armature from the turntable motor and just used the coil as a load resistor. I need to figure out how to modify the circuit so as to use one channel for clean, and the other for distortion and add channel switching to it (hopefully utilizing a footswitch). I know that the tone stacks need to be modified, and the tubes can be driven hotter then they are now on the distortion channel. Let me know if you hear anything ok? I will keep you posted if I get any responses.

KT

[Regis]

Seems like what you have there is three single ended Champ like amplifiers. I would start by rewiring the inputs to Fender style inputs aka 33k/1meg, and put the volume control on the other side of the tube after the tone control.

Change the cathode resistor on the preamp tubes to something more guitar-like, either fender or marshall, add bypass caps to taste.

I personally wouldn't go with the clean/distortion way, I would wire it in stereo so I could use a stereo chorus and have a wet on the left, dry in the middle and inverted wet on the right.

[thomasks]

Thanks Regis!
I checked out your web page, and the article on modifying preamp circuits was very helpful. Since I am a newbie though... Any suggestions for values on the cathode resistors? I would like a Marshall type circuit if I could. Do I have to put in beefier OT's? If I increase the drive on the tubes? Any other helpful pages you could suggest?
Thanks again for your time and patience.

[Regis]

The traditional Marshall values are 820ohm/320uf and 2.7k/68uf. Marshall used one in one channel 1 and one in channel 2 in the 1987 and superleads, the JTM-45 used a shared cathode with the 820ohm/320uf setup. Fender is traditionally 1.5kohm/25uf.

You could to the marshall in one channel and the fender in the other.

No you don't need different transformers. Have a look at the Fender and Marshall tonestacks, you could put one in one channel and the other in the second channel. Fix up the front end, then see how it sounds. Then maybe you could mess with the power section, but there isn't much you can do other than make it resemble a tweed Champ, you can't go wrong with that, it's the icon of single ended guitar amps. Maybe someone else could chime in here about that, the power section of this amp is a little confusing.

It looks like the ELC82 is similar to an EL-84, data here.

[thomasks]

Thanks!
One more quick question?
How can I go about removing the motor coil from the power circuit? I took the armature out of the motor, and I have been using the coil for a load resistor for now. But the coil is 33 ohms at 120 Volts that would be 450 watts! Could you look at the schematic and let me know what you think I should do about the turntable motor?
Thanks,

KT

[Regis]

It looks like the motor is attached via a plug of some sort? And the AC cord goes through it to the amp?

You need to install a grounded plug anyway so just pull all that shit out and put a three pronged grounded cord in, attach the ground wire to a transformer bolt or something. The white and black of the cord attach to the power transformer primary before the fuse. If you need to, drill a new hole and install a grommet for the cord. Go here for a diagram.

Dunno what you are talking about using the motor as a load??? You mean in lieu of using a speaker? Don't do that, the amp needs to see an 8 ohm load.

[Rayme]

I think I know what you are talking about and the coil is not needed if you are doing away with the turntable. They usually have a plug-in connector and I just unplug it and pitch the coil. If there is no plug I would just follow the leads to their origination point and clip them. Generally this is a unnecessary circuit for a tube amp. Hope this helps. Rayme

[thomasks]

I was thinking the same thing. Although I will have to look closely at it. I have found on some old record players that they used to use the coil of the motor to drop the voltage in the power circuit that went to the heaters on the tubes. And it was an integral part of the power circuit.
I once removed the motor coil and just bypassed the motor circuit and fried the tubes in a previous project! That is why I was asking for another opinion based on the schematic I attached.

KT

[Rayme]

KT, your right, I did have one awhile back that used that circuit for the field coil speaker and I connected the two wires with a 10watt resistor which was too small and fried and smoked, so I replaced it with a 20watt resistor and it is still running today. So if you have to have the circuit, I would try a 30watt.
Rayme