Convert an old Ampro tape recorder to guitar amp

[neckwizard1979]

Hello all! I recently inherited this old Ampro tape recorder from my grandfather's estate. It currenty has a bad switch and will not work. I would really like to convert this over to a very simple guitar amp using a 12AX7 preamp to a 6AQ5 or 6BQ5 (el84) power section with just a power switch, fuse, and a volume pot. I would still be willing to use the 6X4 as a rectifier, but I'm kind of concerned because I'm not sure if the tube is any good or not. I know the 12AX7 is good, and it sounded fantastic in the V1 position of my Valveking head. If I could use the existing parts from the tape recorder
(minus filter caps) that would be great. I'll be off work for the next couple of months for knee surgery and I need something to occupy my time. Any help would be greatly appreciated! I've include the original schematic.

[RWood]

You have everything there to build a good sounding, low power guitar amp. The difficulties in working with tape recorders are the chassis layout and also figuring out which 'tape recorder stuff' to remove.

The chassis layout for most of these tape recorders is not the typical, linear layout that Fender spoiled us with; it's often three-dimensional with tubes and components mounted up, down and sideways. So you need to stare at yours for long enough to decide whether you can work with it as is, or if you need to strip as many parts as you can and put them on a new 'normal' chassis. Most importantly, this will be the transformers, but may also include tube sockets, jacks, pots, and other hardware. Start fresh with resistors and capacitors.

Then simply pick a circuit to copy. There are dozens of three tube circuits which will get you where you want to be. The 6AQ5 can be used in most 6V6 or 6BQ5 circuits, so feel free to choose from amps such as Silvertone 1471, 1481, Vox AC-4, Champ, among many others. Another great example is the P1 from AX84.com, which uses two diodes instead of a tube rectifier but you can build it either way. If you use your 6X4 you'll get a greater voltage drop on your B+ supply

If you do want to keep the amp in the tape recorder, you can do it but you have to systematically remove or at least disconnect the tape recorder electronics. This includes the 6AU6 tube (bias oscillator for the tape head). One way to do that would be to remove most of the connections to the tube sockets, leaving the filament wires and the plate connection on the 6AQ5. Then rebuild per whatever schem you want, going point-to-point from the tube sockets to terminal strips to volume pots, etc. It is like a puzzle, and it's a great learning experience. Single channel, single-ended amps are perfect for learning about circuits.

Rwood

[neckwizard1979]

Thanks for the reply. I was planning on using a cake pan as a chassis to mount everything on. I'm going to check out those circuits you suggested. Thanks again.

[olddawg]

Gut it and start over. Use a diode rectifier. Check out the schematics for an Epiphone Valve Jr, A Fender Champion 600, or a Peavey Valve King Royal 8. Before you spend a lot of time and cash keep in mind that you can buy the Valve King combo for $99 right now sans tax and shipping. It is basically what you are trying to make complete with a proper chassis, cab, 8" speaker, and brand new with a warranty.

[neckwizard1979]

Really I need another amp like I need a hole in the head, so I'm really doing it more for the enjoyment of it. I think I've settled on a Fender Champ 5F1 schematic that I found online, it looks like it may possibly work. And yeah, I've already taken the tape recoder apart. I've removed the tubes, sockets, the eyelet boards, and the transformers.