Converting a VOM 167A for guitar

[spiderjames]

I am a newbee to amp building but have successfully built some basic electronic projects and done some simple mods to my valve junior. I recently aquired a Voice of Music amplified speaker model 167A and feel it could be modded into a cool little guitar amp. Here's the unmodded schematic:

Before I talk about all the cool things I want to do to it. I want to make sure I'm properly identifying what I have. am I correct in thinking this is an EL84 7push pull amp with a 6AU6 preamp an a 12AX7 as a phase splitter? I know the basic symbols on the schematics but am quite a bit ignorant with regards to what all except the very basic circuits look like. Any education will be greatly appreciated.

[RWood]

You are correct.

You can indeed do a lot of very cool things with this amp. I'd be inclined to start it as an 18 Watt lite. One channel, proven design and well documented on the web. Oh, and they sound great too. It typically uses a 12AX7 dual triode input tube instead of the 6AU6 pentode, but you could make it work with either. Many people who like the sound of a pentode start it with an EF86 there. Lots of choices but at this stage of your apprenticeship I'd recommend working with something about which there is a LOT of info, and other builders' examples.

RWood

[spiderjames]

Thanks for the quick response. With this amp I was thinking of doing some maintenence, replacing some caps, clean everything up. Then make some basic mods. I am bypassing the volume and tone circuitry since it just plugs in on the front end. Later I will use them to add a fender style volume, treble and bass controls. I think I have Identified the feedback loop and the acoustic contour circuit which I intend to remove.

Attach a jack and I should have a basic amp with no controls:

Am I good so far? Will this function like this?

[olddawg]

As Mr. Wood suggested, an 18 Watt Lite Marshall variant would do you well. I would suggest that you gut it and start over and build up a new fresh turret board assembly rather than trying to modify the existing point to point construction. While you're at it, replace the tube sockets with new ceramics and use new pots and jacks. All of this is really very inexpensive and will save you many headaches. You have the expensive good parts. Another option is a Vox AC 15.

[Chuck H]

Yup. Gut it. Since guitar amps are often called upon to ask very high amplification of signal, layout will be an issue. Better to start in the right direction rather than try to make a silk purse from a sows ear. The 6au6 isn't a strong choice as a voltage amplifier even for a pentode. Better to change it for another 12ax7. A pair of 12ax7's and a pair of el84's with a 5y3 rectifier can certainly make a very nice guitar amp. Follow a known layout and grounding scheme to save A LOT of headaches. You'll learn plenty along the way and end up with a good amp.

The mistake most converters make is to follow the advice "get it running properly and then change it to your tastes." But this leaves you with an amp that's not made for guitar, with an incorrect layout, being made to serve a higher gain application. Nine out of ten times there are parasitic oscillations and extra noise from the ground scheme. Oh, and lots and lots of time spent trying to make a square peg fit into a round hole. Do yourself a favor and don't go this route.

[olddawg]

Also you can use a solid state bridge and an LED pilot lamp. I doubt if you will tell a lot of difference from the tube rectifier, it will raise the B+ a bit and free up extra heater current if you want to add another preamp tube.

[Chuck H]

Also you can use a solid state bridge and an LED pilot lamp. I doubt if you will tell a lot of difference from the tube rectifier, it will raise the B+ a bit and free up extra heater current if you want to add another preamp tube.

Provided the tiny OT can handle the extra watts from the higher B+. I've converted two VOM's. They both made great little guitar amps. But they both had small OT's.