So I've acquired a 93 series Magnavox stereo tube amp chassis with original transformers and new tubes and having some thoughts before I start to repair/restore it. Would this be a good candidate to convert to a guitar amp? I know it doesn't use the same types of tubes traditionally found in a guitar amp. I know it is has two push/pull circuits while a traditional guitar amp has one but I was curious. What if I introduced a fender style tone stack to help shape the frequency curve? I could use it with a stereo speaker cabinet to utilize the stereo amplifiers. Or am I just way off base and this compliment of transformers and tubes are not well suited for the application?
Nor sure if this will work , Photobucket image.
The output tubes and transformers will give you a lot of amp options. For the preamp tubes you can stick with the 6EU7's or go 12AX7's instead. The power transformer is built for two amps so if you were to build only one channel as your amp you will be carrying some extra weight.
The output tubes and transformers will give you a lot of amp options. For the preamp tubes you can stick with the 6EU7's or go 12AX7's instead. The power transformer is built for two amps so if you were to build only one channel as your amp you will be carrying some extra weight.
Last edited:
Sounds like a chimey "jazz chorus" waiting to be made.
I'd be loathe to feck with the amp too much (for a variety of reasons) but you could add a stand alone preamp and put the lot in a box with two 10" or 12" speakers. Throw a stereo chorus or reverb* between pre-and-amp and you would get a nice result. Maybe add a switch to remove the feedback.
You could probably light up another 12AX7 or small signal pentode without the power transformer sulking, it's possible that the amp has a "preamp" connector with B+, GND and heater already. Worst case you'll need another heater tranformer as another 5mA of B+ won't be noticed.
* either throw in two springs or add the reverb to one channel and subtract from the other. You've already got -ve inputs on each PI. PS - a TL074 will recover your reverb as well as a tube - there's no tone shaping going on here. Or tear the guts from a chorus pedal - feed it from the output tube bias which'll be near enough 10V
Last edited:
?? 6BQ5 is the US number for EL84. 6EU7 is a 12AX7 with a different pin-out.
Amps is amps. The topology is fine as far as it goes. It is expects an old-style high-output needle. For guitar you need at least one more gain stage in front.
I have one of these and they actually have a hole punched out in the chassis for another tube socket, as if it's there for a preamp tube. I believe it's also stamped '6EU7' on the chassis next to the empty hole.
93-04 Ampl/Mixer Magnavox Co., Commercial Wireless and Devel
I had had the same sort of idea, build a case for it with stereo speakers and run each channel for the output of my Rockman with stereo chorus. Bought parts for the job and then other things came up and I never got back to it... one day.. one day...
Not going for stereo you could double up the output stage on one phase inverter and have the other two triodes for a preamp. Dump the tube rectifier and go SS you should have 350V on the EL84's 😀 and with the preamp triodes freed up you could have a 30W Marshall 18 Watt. 😉 Or go Vox, lots of options.
Thanks for all the replies! I wasn't aware those tubes are essentially the same as the EL84 and 12AX7. I thought I might need another pre-amp stage, especially if there was a tone stack added in, but I wasn't too sure. I may attempt adding that extra stage and going with a mono amp set up, eliminating one of the channels. That does leave the extra headroom with the power transformer but it's what I have on hand at the moment. Thanks again for all the advice and ideas everyone!
What type of amp are you going for, clean or overdriven? If you want a clean sound I would consider using both channels and driving both inputs them from the same source.
Also note that the 100ohm cathode resistor for the output tubes is common to both channels, so pulling one set of tubes will change the bias, whatever effect that might have. The resistor will only see half the current it normally would so the bias voltage would be half what it normally is.
Also the 10K resistor in the power supply is common to both channels, so if you pull out one 6EU7 the voltages on the other will go up a bit.
Also note that the 100ohm cathode resistor for the output tubes is common to both channels, so pulling one set of tubes will change the bias, whatever effect that might have. The resistor will only see half the current it normally would so the bias voltage would be half what it normally is.
Also the 10K resistor in the power supply is common to both channels, so if you pull out one 6EU7 the voltages on the other will go up a bit.
I would be going for a cleaner tone. So what I'm gathering from all of this is that it will be quite an undertaking, which I'm ok with. I'm fine with spending a lot of time with my hobbies. It looks like the best plan forward is using the pre-amp tubes from both channels combined to a single or sending a single source through both at the same time, and running through a single channel of power tubes. The other option, as I see it, is adding an additional pre-amp tube to each circuit and running it in true stereo mode. The first option will have much more on the power transformer than needed and I will have to be careful with, well everything, but taking special note for the power supplied to the tubes and the output bias(which I was planning to make adjustable anyhow). Again, Thanks for all the help everybody! I really appreciate the different approaches to this idea.
For a clean tone both channels should be better, that way each set of tubes is doing half the work a single set would have to do for the same volume level, the sound should be cleaner and the tubes should last longer, I would think. The disadvantage is needing two speakers.
You could throw a socket in that spare hole, put in a 12AX7 with one half as the preamp, follow that with volume and tone stack, then use the other half as a buffer to drive the inputs to the 6EU7s.
I would do as little as possible to the original amp circuit, from what I understand it's a pretty good little design, meant to be used in stereos and televisions, so they're designed to be run at a reasonable volume for long periods.
Good luck with whatever coarse you choose.
You could throw a socket in that spare hole, put in a 12AX7 with one half as the preamp, follow that with volume and tone stack, then use the other half as a buffer to drive the inputs to the 6EU7s.
I would do as little as possible to the original amp circuit, from what I understand it's a pretty good little design, meant to be used in stereos and televisions, so they're designed to be run at a reasonable volume for long periods.
Good luck with whatever coarse you choose.
Oh, ok, I think I see what you're getting at. I was intending to build and use a 2x10 or 2x12 with this so running a single channel to each speaker is completely reasonable in my book. Adding an extra 12AX7 to drive the volume/tone stack and buffer the preamps is a very interesting idea. I suppose it would help to leave the circuit close to stock if I ever wanted to resell, which I probably won't but still. Good thing is that when I picked this up it was non working in the first place so I won't be ruining/changing a stock working amp. Thanks again for the advice! I'll post pictures of it completed if I get it finished and working in a reasonable time.
It is a Paraphase phase splitter. It is an old obsolete design, the second phase takes the output of the one channel after the triode and then scales the signal down and inverts it. So you have added distortion going to the second phase, especially if the previous triode clips. Not the best for high fidelity if you push the amp. But being for guitar, a little extra distortion is not always a hardship. I played around with the Paraphase and found that it added a little more fullness to the sound but when overdriven by a large amount the sound is not too pleasing. In your case how you want to use the amp it might be a good choice adding a little more 'tube' flavor rather than the Cathodyne or Long Tail Pair.
Any amps you like the sound of?
Any amps you like the sound of?
Good to know. I play a lot of blues, so a little breakup at full volume is fine(not that I play at full volume very often). I'm a fan of old fender and vox style amps so I was going to make the tone stack reminiscent of a fender when it comes to targeting the frequency response/modeling.
- Status
- Not open for further replies.