Needing info on modding an old VM speaker amp.

[IHeartRics]

I was at a rummage sale not long ago and picked up a VM (Voice of Music) model 103 powered speaker enclosure that was made back around 1959 for $3.00. It has what looks to be an 8" speaker and about a 2" speaker with cap for highs. On the side it has two stacked pots, like an old Dano or Silvertone, one set for power and volume and the other for treble and bass. The wood finished speaker cab looks like it belongs to an audio set up and has small screw-on table legs - very 50's / 60's.

I'm thinking of making it into a little low watt practice combo amp. It's currently about 8 to 10 watts. I plugged a guitar into it using an RCA to 1/4" adapter. It's clean and cool but a little too quiet. Put a distortion pedal in front and it gets pretty mean and louder, which inspired me more. It uses an EZ81 rectifier, two EL84's in the power section, and a 12AU7 and 6AU6 in the preamp section. I swapped in an 12AX7 and got a little more volume.

So after that long intro, here is my question - how hard would it be, and what would have to be done to mod the amp to use another 12AX7 instead of the 6AU6 to get more power/volume? Can it be done? Is it something an amateur like me could do? Any help would be appreciated.

Note that I did have the tubes tested and they are all OK and fairly strong, and I also understand that if I use a higher efficiency speaker I should get a bit more volume too.

[kiramdear]

Chip, do you have any pictures? I have a 1953 VM with the amp that slides in and out on a little shelf. It used to belong to The Jefferson Airplane and lived backstage at The Family Dog in its heyday. I think it has an eight watt rating with one 10" speaker. Great little amp, and I'm curious what yours looks like.

Pictures of one just like mine are here: viewtopic.php?f=19&t=383117&p=457655&hi ... c+#p457655

[soundmasterg]

The 6AU6 is a 7 pin tube and the 12AX7 is a 9 pin tube. In order to swap them, the socket would have to be removed and a 9 pin socket would have to be added. You would also have to rewire the socket and possibly add more wires and other components like resistors and capacitors too. This may involved drilling, and will probably require a hole punch too. The 6AU6 is a pentode while the 12AX7 is a dual triode. The 6AU6 probably has the capability to have more gain than you need if the circuit is reconfigured. It also will have an interesting sound that is closer to an EF86 like what was used in the early AC30 and the AC15 Voxes. If it was mine, I would leave the 6AU6 in there and reconfigure the circuits in the amp to get the tone and gain that you would like.

Greg

[IHeartRics]

Kira, That's a cool amp you have there. Here are some pics of the powered speaker:

[IHeartRics]

The 6AU6 is a 7 pin tube and the 12AX7 is a 9 pin tube. In order to swap them, the socket would have to be removed and a 9 pin socket would have to be added. You would also have to rewire the socket and possibly add more wires and other components like resistors and capacitors too. This may involved drilling, and will probably require a hole punch too. The 6AU6 is a pentode while the 12AX7 is a dual triode. The 6AU6 probably has the capability to have more gain than you need if the circuit is reconfigured. It also will have an interesting sound that is closer to an EF86 like what was used in the early AC30 and the AC15 Voxes. If it was mine, I would leave the 6AU6 in there and reconfigure the circuits in the amp to get the tone and gain that you would like.

Greg

Thanks Greg. So if I understand you correctly, I would be better off keeping the 6AU6 and reconfigure the circuit. But I have no idea how and what to do. I can do simple soldering, thats it, and that is what I was hoping for in my original question, which is obviously not the case. It sounds like if I want this to get louder and a desired tone, like a Vox, I'll need to take it to a competent tech, right?

[jps]

Wow, original tubes, no less!

OTL?

[IHeartRics]

Wow, original tubes, no less!

OTL?

Yep, and tested fine.

Forgive me for my ignorance, but was does "OTL" stand for?

[jps]

Output TransformerLess. I only see one transformer, and I assume it is the power transformer.

[IHeartRics]

Output TransformerLess. I only see one transformer, and I assume it is the power transformer.

Oh, OK. Yes, only one. The schematic is a little tough to read as it is glued inside that cabinet, but reads that it is the power tranny. I think some of the original Fender Champs only had one, didn't they?

[jps]

I don't think Fender made any OTL amps, I'll look at my Weber or Pittman book, as they both have lots of schematics in them. Julius Futterman was famous in hi-fi circles for his OTL tube amps.

[IHeartRics]

I don't think Fender made any OTL amps, I'll look at my Weber or Pittman book, as they both have lots of schematics in them. Julius Futterman was famous in hi-fi circles for his OTL tube amps.

Thanks Jeff. Let me know if you find out anything. I saw a picture of the inside of an early Champ and it looked like there was only one tranny.

[jps]

Every schematic I have of Fender amps have both power and output transformers, perhaps some of the older OTFers are small enough to have been hidden behind a tube or something else that blocked it's view from you.

[soundmasterg]

The 6AU6 is a 7 pin tube and the 12AX7 is a 9 pin tube. In order to swap them, the socket would have to be removed and a 9 pin socket would have to be added. You would also have to rewire the socket and possibly add more wires and other components like resistors and capacitors too. This may involved drilling, and will probably require a hole punch too. The 6AU6 is a pentode while the 12AX7 is a dual triode. The 6AU6 probably has the capability to have more gain than you need if the circuit is reconfigured. It also will have an interesting sound that is closer to an EF86 like what was used in the early AC30 and the AC15 Voxes. If it was mine, I would leave the 6AU6 in there and reconfigure the circuits in the amp to get the tone and gain that you would like.

Greg

Thanks Greg. So if I understand you correctly, I would be better off keeping the 6AU6 and reconfigure the circuit. But I have no idea how and what to do. I can do simple soldering, thats it, and that is what I was hoping for in my original question, which is obviously not the case. It sounds like if I want this to get louder and a desired tone, like a Vox, I'll need to take it to a competent tech, right?

Yes, the 6AU6 should have plenty of gain for you, but it is probably setup right now for lower gain. To change it over to a higher gain circuit, the parts around it, mainly the resistors would need to be changed to different values. In order to do this though. I would need a copy of the schematic to suggest changes, and would also need to know the current voltages in the amp at various points. Any competent amp tech would need the same info. That schematic on the side of the speaker box is very helpful, but won't have the voltages on it....those you would have to measure with a DMM. The 6AU6 in this amp is the preamp. The 12AU7 is the phase inverter, though it could be a cathodyne inverter (basically the same thing, just a different type of circuit), in which case half of the 12AU7 would be the driver tube and the other half would be the phase splitter. The EL84's are the power tubes, and this is mono from all appearences, so it is probably a 10-15 watt amp. The big transformer there is the pwoer transformer, and is quite large for a small amp such as this. It could have been setup to run some other device through those octal plugs on the side of the cabinet. Your schematic may tell you...but you have to be able to read it and understand it. The output transformer may be inside the chassis, but you would have to pull the chassis to see. If you do so, be careful not to touch any of the circuitry inside until you safely discharge the caps. They may still have high voltages on them.

A competent tech is probably your best bet. You probably aren't in my area (Portland, Oregon) but if you were I'd be happy to take a look at it.

Greg

[IHeartRics]

Every schematic I have of Fender amps have both power and output transformers, perhaps some of the older OTFers are small enough to have been hidden behind a tube or something else that blocked it's view from you.

Thanks Jeff. Or it could be under the chassis too.

[IHeartRics]

Yes, the 6AU6 should have plenty of gain for you, but it is probably setup right now for lower gain. To change it over to a higher gain circuit, the parts around it, mainly the resistors would need to be changed to different values. In order to do this though. I would need a copy of the schematic to suggest changes, and would also need to know the current voltages in the amp at various points. Any competent amp tech would need the same info. That schematic on the side of the speaker box is very helpful, but won't have the voltages on it....those you would have to measure with a DMM. The 6AU6 in this amp is the preamp. The 12AU7 is the phase inverter, though it could be a cathodyne inverter (basically the same thing, just a different type of circuit), in which case half of the 12AU7 would be the driver tube and the other half would be the phase splitter. The EL84's are the power tubes, and this is mono from all appearences, so it is probably a 10-15 watt amp. The big transformer there is the pwoer transformer, and is quite large for a small amp such as this. It could have been setup to run some other device through those octal plugs on the side of the cabinet. Your schematic may tell you...but you have to be able to read it and understand it. The output transformer may be inside the chassis, but you would have to pull the chassis to see. If you do so, be careful not to touch any of the circuitry inside until you safely discharge the caps. They may still have high voltages on them.

A competent tech is probably your best bet. You probably aren't in my area (Portland, Oregon) but if you were I'd be happy to take a look at it.

Greg

Thanks for all the info Greg. There is a web site dedicated to VM products where I could probably get the schematic. The one inside the cab will be "fun" to access. I'd try to have to take a photo of it and hope it comes out.

If this helps, the input for the amp is a mono RCA cord to the volume pot, which is connected to another RCA cord, which plugs into an RCA jack on the amp (grey cord on the far right in the 3rd photo). The smaller octal plug runs to the on/off switch. The larger octal plug connects to the pilot lamp, speaker, and the treble/bass pots. Tracing these wires was gross, as their covering is getting sticky and gooey (plasticizers migrating out of the vinyl).

Oregon is a bit of a drive. But I appreciate the offer!