Newbie needs help w/conversion.

[gwenzel]

I'm converting a Recordio Reel to reel to a guitar amp. I believe that I can remove the BIAS OSC section by just cutting it out and bringing the mic input directly into pin 2 of the pre-amp. I also plan to remove the tape head section. My question, and remember that I'm new to this so I may be asking a dumb question but when I remove the tape head section what do I do with the connections into C10, C17 and C18. (see schematic).

I'm also planning to remove the internal speaker portion, leaving only the ext. connector. I'll be adding a 3 wire plug, fuse, removing the motor and cleaning-up the power switch. Also can anyone tell me from the schematic what the wattage of this amp will be?

[RWood]

I can remove the BIAS OSC section by just cutting it out and bringing the mic input directly into pin 2 of the pre-amp.

Yes but what you've got there is a grid-leak biased input, not the best for guitar. I recommend you use a standard Fender-style input triode. The 68k grid blocker is not real critical but the 1M from grid to ground is. Then from the cathode, 1.5k to ground bypassed with 25uF/25V capacitor. The plate resistor should be changed from 150k to 100k. Check out most any classic Fender schematic to see the details.

when I remove the tape head section what do I do with the connections into C10, C17 and C18. (see schematic).

Here's what I whould do:
Leave C10 but remove the connection there to the AM tuner input
C17 to ground, rather than to the function control.
C18 (variable capacitor?) Remove it and the conneciton to the head

I'm also planning to remove the internal speaker portion, leaving only the ext. connector. I'll be adding a 3 wire plug, fuse, removing the motor and cleaning-up the power switch. Also can anyone tell me from the schematic what the wattage of this amp will be?

2 watts from the 6AQ5, according to the data sheet
NJ7P Tube Database Search

You can wire the whole thing a la tweed Champ or Princeton, if you want to cut out all the confusing and unnecessary stuff. Pick any classic single-ended one channel circuit (although youve got an unused triode if you want to make a different sounding channel, for variety)

I'd also re-do the filament wiring to be a daisy-chanied parallel pair rather than one side grounded.

RWood

[gwenzel]

RWood, Thanks for all the Help.

I think I understand all of your suggestions with the exception of the last one. I'm not sure how to go about re-wiring in a daisy chained parallel pair. Again I'm new to all this, could you give me an example or point me to somewhere on the schematic or to another schematic that would show it.

Thanks again for your help, I'm trying to learn as much as I can from researching this stuff but sometimes you have to get someone who knows what they're doing involved.

Greg

[RWood]

Sure. On your schematic it shows the two-wire filament secondary from the PT with one "hot lead" going from tube to tube while the other lead is connected to ground. Then each of the tubes' other filament pins is also connected to ground, using the chassis as a means of conduction. This works - indeed my '59 Princeton had its heaters wired this way, a '66 Kalamazoo 12 that I had in here recently was wired like that, and lots of others.

It's a noisy way to go, though, and it can fairly easily be improved upon. You'll need to tightly twist together your filament pair of wires. I like solid core wire for this because it stays together well, and I use 20 gauge in low power amps like this. Lots of varying opinions here.

From the PT filament secondary you'll go to a pair of terminals that are isolated from ground. Then from there to the pilot light if yours is a 6.3V lamp, and also to the rectifier's two filament pins (3 and 4 for yours). Keep the pair twisted together as close to the socket as you can, and keep it routed away from the grid wires. Some people like to press them against the chassis, some like them flying up in the air - the point is it keep them away from any signal carrying wires.

From the rectifier's pins now your next length of paired wires goes to your 6AQ5, also 3 and 4. Then from the 6AQ5 to the nearer of the 6AV6 (3 & 4) and the 12AX7 (4+5 & 9). When you do the 12AX7 lead your wire in through the gap between pins 1 & 9, pick up pin 9 there and continue with your other wire straight across the of the socket to get pins 4 +5 as one. Don't encircle the socket with filament wire, in other words. And if the 12AX7 is the final tube, which i suspect, then you only need to lead a pair to it, and not one from it.

So far so good? All you need now is a reference to ground. There are many techniques to do this, so feel free to read up on it. The method I like is easy and it elevates the ground a bit, also helping to reduce potential hum. I use the artificial center tap-to-cathode technique. To do this, connect one end of a 100Ω resistor from heater pin 3 on your power tube, and connect the other end to it's cathode, pin 2. Now take a second 100Ω and connect it from heater pin 4 to cathode pin 2. Pin 2 will have a lot on it - a big 330Ω resistor to ground and a 30uF cap to ground - so you'll have to get creative about where you actually attach it.

Another way to do this is with a humdinger, a pot that helps you account for unequal 100Ω resistors or an unequal filament supply. Merlin's got some great stuff about this here.

Good luck; let us know how it goes.

RWood

[gwenzel]

Wanted to give you an update, Phase one is complete. I removed all of the external connections including the motor and AM radio tuner connections. I swapped out the grid resistors, added the cathode resistor and cap. Replaced the power cord with a three prong and added a fuse. Fired it up and played through it. It sounds very cool even thru the small 4"x6" and 4" speaker that came in the reel-to-reel. I removed them from the cabinet and added a 1/4" plug so I could use them thru the ext. speaker jack.

Phase two will be to cont. to remove the unused components, terminating some of the connections I talked about earlier and maybe adding a tone stack. I'm still planning to make the heater wiring changes at some point.

RWood Thanks again for all your help. I'm learning alot.