Motorola Phono Amp To Guitar Amp Conversion

[ThomasE]

I have an old Moto console I want to gut and use the internals for a guitar amp. Has 2 channels and a bass. Cool reverb tank. 2-8s and 1-12. Could some of you guys provide me with some info on what modifications will need to be performed in order to make this work? Thanks

[sluckey]

The stereo power amp chassis shown in moto1.jpg and moto2.jpg is the only part that's likely useful for guitar amp. You will need to add a guitar preamp with tone controls.

I'd start by getting a schematic of the power amp even if I had to draw it myself (probably quicker and cheaper) Then I'd chose a guitar preamp to add. Once you have this info you can begin to get more specific.

I would probably lean toward removing one channel (3 tubes and one output transformer). This frees up sockets and real estate for the guitar preamp. I'd probably chose a simple one tube preamp, ie, Fender AB763, Marshall 1974 normal channel, or Vox AC-15 normal channel.

[ThomasE]

Thank you for the reply! I'm in the process of tracking down a schematic.

[ThomasE]

After complete disassembly here is the preamp unit.

[eleventeen]

You have some interesting and probably useful elements there. One thing (and there usually twenty things) that might be a problem requiring some pondering time is that it would appear that the power amp chassis is only about an inch deep. If it's your intention to achieve a one-piece chassis that is your whole finished amp, it could be a problem mounting tone control pots within that chassis....because they might not physically fit There are small-body pots you could use.
 
Alternatively, it may suit your intended physical configuration to rework the preamp into your guitar-preamp and keep it in the separate chassis. That would seem like a way to go, you have 6 pots there. One goes away for the guitar input jack, which might leave you with vol-treb-bass-reverb-master......or vol-treb-middle-bass-reverb. The remote power supply plug arrangement is already there. That would give you more room to mess with the electronics and quite possibly you could leave the power amp almost unmolested. 


What kind of cabinet are you envisioning placing this into? Combo? (one piece amp/speaker) Or head-and-speaker cab? A few times, I have found dead solid state amps that are all set up with tolex and speaker (which you might want to replace, but the baffle board is all set to go. Sometimes you can find a dead old Peavey amp for $25 with a handle and corners and grill cloth----all those things take time and money and skill to execute, way more than $25, believe me.


But then, such an amp carcass might not accomodate the depth of the power amp chassis you have.


In general, there are the electronic considerations and I agree with Sluckey that you want to get a schematic of the power amp and to rework the preamp into a Fender or Vox type of thing. If you can get the Motorola model number, I bet you can find the schematic somewhere on the net. Then there are the physical considerations. Unfortunately, you kind of have to think about all these things all at once and it's very common that one thing you want to do makes another thing impossible. It's also possible that the tube sockets (typically wafer-type) and not very good for something you want to move around and which will thus take some amount of physical shock now and then. Are you planning on having reverb in the amp? Got room for the reverb can?


You certainly have a good candidate and some promising 12AX7s and transformers.

[HotBluePlates]

I'd use the power amp chassis 100% as-is. There are 2 single-ended EL84 output stages, each with their own output transformer (the Left & Right channels), plus a push-pull EL84 output stage (the Bass channel).

I would just assume you need a preamp for each on a different chassis. The original preamp chassis might be close enough to make a full-featured guitar preamp for at least one of the power chassis channels (and possibly more than one, depending on what the schematic shows & what features you want).

[ThomasE]

Thanks for all the info guys! I've searched the interweb for hours and no "free" schematics. Looks like I can purchase one for $5.99 from THESCHEMATICMAN. I think I'm going to keep and amp's current chassis and try and use/modify the separate preamp. As far as the enclosure, I'm planning on using the wood from the old console and build a cabinet. I want to use as much of the original items as possible. It's a sentimental thing. This huge console belonged to my grandmother and was left to me when she passed away. I would never get rid of it but have no room nor use for it in it's current condition. That's what got me thinking about the guitar amp.

[ThomasE]

Here are a few pics of the pre and tank. I would assume that since the tank was originally mounted vertical that it should remain that way in the new cab.

[ThomasE]

Got all the guts out, hooked up on the bench and lit the fire. Guitar plays fine through it as is (1/4 to RCA) but definitely is lacking in MOJO. Super clean, you can't get any distortion at all out of it. Would probably make a great acoustic amp as it sets. One correction, I stated in an earlier post the console had 2-8s and 1-12. Turns out to be 2-6s and 1-10. Will definitely upgrade those.

[HotBluePlates]

The 2 lightbulbs make this look like a fold-back reverb unit, which originally took its signal from a speaker.

[ThomasE]

Yes, the tank grounds to the chassis and connects to + on the left and right channels.

[ThomasE]

Here's the schematic........

[ThomasE]

Thanks again for all the info. So far I have got rid of the 4 RCA connections. Bypassed the stereo/mono part of R4. Installed one 1/4" plug input and tied into both left and right pre amps. Also removed circuit breaker and replaced with fuse. Removed "death cap" and installed 3 wire plug.

[PRR]

Gain from Radio/Phono says it needs 0.2V to make full output.

Guitar inputs are usually scaled for 0.02V input for full output.

Fast hack: use a Booster Pedal in front.

Next Hack: put 22uFd bypas caps at R8, R26, and R31, and the same points in the other channel; snip one end of R47. This looks like about 0.024V input sensitivity, which should be near-enough for guitar.

At this point you may learn why we do not put the Volume pot at the input of a high gain guitar amp. It will hiss, even when volume is turned down.

Snip and re-wire the Volume pot to between the Treb pot wiper and V3A grid. Take guitar straight into V1A using any standard g-amp input network.

The 3-channel scheme is awkward if you are not retaining the original speaker cabinet. It may not quite have the power-supply grunt to do a 36 Watt (four EL84).

Anyway, with reverb and all, you might just reconfigure the whole thing. Remove the treble-cut caps in the "Bass/Rev" power amp and use that as your Speaker Amp. Keep one of the Single-ended side-amps as a reverb driver. Retain the reverb recovery. Retain one Bass/Treb tone control set. Re-rig V1A/B as your Guitar preamp with Vol control between stages.

Keep the Loudness parts "K1" but put a switch to the Volume pot. I suspect we "need" such a network for satisfying playing at the very-low levels needed around family or neighbors.

I wudda kept the circuit breaker (unless busted). It is a much better plan than a fuse.

[ThomasE]

Gain from Radio/Phono says it needs 0.2V to make full output.

Guitar inputs are usually scaled for 0.02V input for full output.

Fast hack: use a Booster Pedal in front.

Next Hack: put 22uFd bypas caps at R8, R26, and R31, and the same points in the other channel; snip one end of R47. This looks like about 0.024V input sensitivity, which should be near-enough for guitar.

At this point you may learn why we do not put the Volume pot at the input of a high gain guitar amp. It will hiss, even when volume is turned down.

Snip and re-wire the Volume pot to between the Treb pot wiper and V3A grid. Take guitar straight into V1A using any standard g-amp input network.

The 3-channel scheme is awkward if you are not retaining the original speaker cabinet. It may not quite have the power-supply grunt to do a 36 Watt (four EL84).

Anyway, with reverb and all, you might just reconfigure the whole thing. Remove the treble-cut caps in the "Bass/Rev" power amp and use that as your Speaker Amp. Keep one of the Single-ended side-amps as a reverb driver. Retain the reverb recovery. Retain one Bass/Treb tone control set. Re-rig V1A/B as your Guitar preamp with Vol control between stages.

Keep the Loudness parts "K1" but put a switch to the Volume pot. I suspect we "need" such a network for satisfying playing at the very-low levels needed around family or neighbors.

I wudda kept the circuit breaker (unless busted). It is a much better plan than a fuse.

Thanks PRR!

Lot of good info. I have removed R47 from the circuit. The circuit breaker was bad so that's the reason I pulled it. You've provided a lot of info that I can apply in "stages". I've got the caps ordered so that will be the next step. This is my first overhaul like this so please excuse some of these questions. My first one is in regards to this The 3-channel scheme is awkward if you are not retaining the original speaker cabinet. It may not quite have the power-supply grunt to do a 36 Watt (four EL84). Please explain. My next question is in regards to K1. What is the purpose of this? and please explain a little more about a switch to the volume pot.  Thank you again for your time.

[ThomasE]

Here are a few pics of the new cabinet. As I stated earlier I wanted to use as much of the original console as possible. This is what I ended up with. I even managed to reconfigure one of the panels that held the original model and serial number of the console and used it as part of the back of the cabinet. Even though I'm transforming this into a totally different piece of equipment I want it to look as though it's something that Motorola would have / could have built back in that time. I'm going to be installing two 8"s along with the 10". Decided to keep the 3 channel configuration. I'll be keeping the preamp section separately from the power amp as from the factory. I still need to relocate the power switch and bulb to the preamp section for a little easier accessibility. I will be using the original speaker grill cloth along with the original Motorola badging. Still have to fabricate a panel for my controls though. I'm going to be covering the cabinet with faux leather or alligator material and I have a few questions. I have thought about coating the cabinet with a satin polyurethane before covering. After thoroughly drying would this adversely effect the contact cement from bonding properly? Or would it be best to just apply the cement directly to the bare wood?

[ThomasE]

Here are a few more pics.........

[ThomasE]

Here are a few pics of the cab after covering and speakers installed.

[TIMBO]

That's AWSOME, I'm a big believer in recycling and making the most of someone from the past's hard work and craftsmanship.

GREAT WORK

[sluckey]

Looks good. I like doing stuff like that too.

[smackoj]

love that cabinet! reminds me of the vintage Marshall 1974x head with the off-center chassis mount. cool stuff amigo. Question; what did you do about the preamp section? I have a similar Airline console amp that uses a power section with an EZ81 recto and two EL84 power tubes running single ended thru two separate output trans. Curious what you changed, modified, or built?

 

[ThomasE]

Thanks TIMBO and sluckey for the comments! This has been quite an undertaking since I'm fairly new at this. The members of el34 have been tremendously helpful! I couldn't have got this far without them. I have a few more hurdles to jump but if all goes well this baby will be done before the weekend.

[ThomasE]

love that cabinet! reminds me of the vintage Marshall 1974x head with the off-center chassis mount. cool stuff amigo. Question; what did you do about the preamp section? I have a similar Airline console amp that uses a power section with an EZ81 recto and two EL84 power tubes running single ended thru two separate output trans. Curious what you changed, modified, or built?

 

Thanks for the comments smackoj! As far as the pre....... that's what I'm in the process of finishing up. I'll be posting pics shortly. I have a few more questions that I'm hoping some of the guys can help me on. Stay Tuned!

[ThomasE]

 The mods I'm performing are based on el34 member "PRR" recommendations. Got a deal on some cool ODs. I've installed the .22uF bypass caps across R8,R26,R31,R15,R48 and R53. It already sounds considerably better. At this point I'm in need of a little additional assistance in order to finish. Here are the questions/concerns -

Attached are two images, one original one with possible mods. I'll be referencing below.

1. Moving volume pot - PRR said to install between Treble Pot wiper and V3A Grid. I'm assuming it literately means install in location#1 and not location#2. Is that correct? If so, why do we not want to install after bass and treble pots combined?

2. Guitar input directly into V1A - Just a simple 69k, 1meg (champ) input. Will this be sufficient?

3. Loudness "K1" - It is suggested to retain this but to put a switch to the volume pot. What is the purpose of K1 and a switch to the volume pot?

Thanks guys for the help.

[sluckey]

1. Moving volume pot - PRR said to install between Treble Pot wiper and V3A Grid. I'm assuming it literately means install in location#1 and not location#2. Is that correct? If so, why do we not want to install after bass and treble pots combined?

That's correct. It is after the bass and treble pot. Still gotta have the connection between treble pot and R12. See pic below.

2. Guitar input directly into V1A - Just a simple 69k, 1meg (champ) input. Will this be sufficient?

Yes

3. Loudness "K1" - It is suggested to retain this but to put a switch to the volume pot. What is the purpose of K1 and a switch to the volume pot?

K1 is a RC module that connects between the fixed tap on the volume pot and ground. Basically just a 68K resistor and a .002 cap. K1 basically makes the sound more bassy when the loudness (volume) pot is turned down low. This may or may not be useful for guitar, so put it on a switch and you can have it or not.

You may also find it useful to put a cap across R8 and R26 of your modified drawing. These are called cathode bypass caps and will increase the gain of each of those tubes. Typical guitar values range from .68µF up to 25µF. Bigger values allow more bass and may make the sound kinda muddy. Experiment to find your personal preference. Negative side of caps will connect to ground.

[ThomasE]

Thanks for clearing those items up sluckey. That's kind of what I was thinking but just wanted confirmation from someone that knows.  As far as the bypass caps, i've already added those to R8,R26,R31,R15,R48 and R53. Here are a few pics. Back to soldering...............

[ThomasE]

Well, it's been a while since I posted any updates on this amp project. It's complete now, looks and sounds great! These last posts are pics of the final assembly.

Preamp complete. Ever heard the phrase 20lbs of s**t in a 10lb bag. Here ya go.

[ThomasE]

Here are some images of the faceplate. Made from .025" polished brass. Image of waterslides are prior to 23 coats of clear. I used the original Motorola font but gave it my own twist. Motorola with some MOJO.

[ThomasE]

Additional Pics

[ThomasE]

Additional Pics.

[ThomasE]

Well that's the end of the show guys. I would like to thank sluckey, PRR & eleventeen. I couldn't have done it without you guys. Thanks to HotBluePlates, TIMBO & smackoi also.
One last shout out to one cool dude - Uncle Doug! If it wasn't for you, this cabinet covering would not have turned out as good as it did.

[eleventeen]

Nice job! Rather unique.

[AZJimC]

Slick Job! That looks great, and it's certainly one of a kind. I like the faceplate. I need to do that for a couple of amps. How thick is the brass?

[ThomasE]

Slick Job! That looks great, and it's certainly one of a kind. I like the faceplate. I need to do that for a couple of amps. How thick is the brass?

Brass is .025". Most trophy/plaque shops have it. If not, a brass kickplate for a door will work.

[ThomasE]

Found a couple more images of the amp installed in cab. Reverb is installed on the back to the left of amp.