Tube radio conversion - Dorm amp

[Leviman]

Hey folks, I've been lurking around here and posted about my Gibson GA-15RVT I modded and now my text project is going to be the ever hated AA5 guitar amp conversion! I hear lots of people saying "don't do it, you need to add and isolation transformer!!" Yeah... so what? That is very easy to add. Alright so lets dive in!

YES IT WILL GET AN ISOLATION TRANSFORMER AND GROUNDED POWER CABLE

My radio I got off ebay for about $20 (shipped!) It's a Westinghouse model H821L5 aka, An AA5. pretty generic it's got a tube line up of 12BE6, 12BA6, 12AV6, 50C5, and 35W4. Respectively they're, RF Oscillator, 1st I.F., Pre amp, power amp, and rectifier. (schematic is attached) So the thing is, most people tap into the volume pot (aka, just before the pre amp) to run guitar in. But! The 1st IF is a pre amp stage too! It's just not really labled as such. So! My plan is to tap into that, more gain anyhow With probably a .047mfd coupling cap to the input. But if I disconnect the 1st I.F. Transformer, it looses the 18ohms to ground that the screen gets. Should I add a screen resistor here? Lr perhaps my guitar will behave as the screen resistor? especially if I use a shorting jack (which I probably will).

So that's the plan so far. Other than that, it's going to get new power supply caps (of course), and maybe a new electrolytic in the alarm clock (yes, it's a clock radio, so I can wake up to rock!!) . And the schematic lists this "voice coil 2.65 ohms". Do you think this would be a four ohm speaker then? Certainly there is a reactance along with that voice coil resistance. That would be pretty cool if I could provide a four ohm output for a cab! I can't imagine the tiny 6" speaker will sound to great.

Anyhow, NO THREAD is any good without pictures so. Pictures.
[IMG577]http://i966.photobucket.com/albums/ae14 ... 4Large.jpg[/img]
Wow! all original westinghouse tubes. I bet these are worn out
[IMG577]http://i966.photobucket.com/albums/ae14 ... 1Large.jpg[/img]
[IMG577]http://i966.photobucket.com/albums/ae14 ... 5Large.jpg[/img]
It'll be two pins and this much quiter than a single el84 amp!
[IMG577]http://i966.photobucket.com/albums/ae14 ... 3Large.jpg[/img]

The schematic is attached. Suggestions are what I'm looking for! Show off your knowledge! My room mate suggested I make the clock hands move with the guitar. I like that idea, maybe hook 'em up to the guts of a VU meter? Oh and what rating on the isolation transformer should I get? Keep in mind this is a cheap project so I'm thinking along the lines of this: http://www.mouser.com/ProductDetail/Tri ... F80A%3D%3D


EDIT: Heres the schematic
[IMG:739:1023]http://i966.photobucket.com/albums/ae14 ... L5_164.jpg[/img]

[rdjones]

I hear lots of people saying "don't do it,

Please, PLEASE seriously reconsider doing this.
There's a reason there's
a) no exposed metal parts, and
b) an AC power interlock that prevents powering up when the case is open
It's a HOT chassis with no ground. Dangerous.

And the schematic lists this "voice coil 2.65 ohms". Do you think this would be a four ohm speaker then? Certainly there is a reactance along with that voice coil resistance. That would be pretty cool if I could provide a four ohm output for a cab! I can't imagine the tiny 6" speaker will sound to great.

Speakers are rated by AC impedance, not DC resistance so it's a 2.65 ohm speaker.

Suggestions are what I'm looking for!

The safest suggestion here is to use it as it was designed, as a clock radio.

[Leviman]

Read again please. My plan is to make it safe to operate. If you have any suggestions to help with that I'd love to hear 'em. But I'd like to kindly suggest anyone who's going to tell me to wimp out on this project to go away. Also as for the speaker, the schematic reads "voice coil 2.65 ohms" which I take to mean DC resistance, which will be less than AC impedance, yes I understand this. I'm saying does a 2.65ohm DC impedance sound about right for a 4 ohm AC impedance? And I realize my schematic was screwy, I fixed it now. I'm just trying to build a low watt amp that's not just another champ clone. Anyone can do that, but who will have a guitar amp that they can set to wake them up in the morning?

[M Fowler]

Volume 1 building tube amps by Tino Zottola has this project in it complete page by page building instructions, schematic and BOM.

Mark

[FUCHSAUDIO]

Buy a table radio with a transformer power supply. I bought a few off the Bay for less than 30 bucks, refinished and made into nice little snot-watt amps for myself, friends, guitar techs etc. Jensen makes a 4 or 6" "guitar speaker" which, if you limit the low end, work well. A push pull 12AT7 or 6FQ7 or 6SN7 (if you h ave octal sockets) with the Mojo 1580 output iron work well. This is dangerous for sure. Play the guitar, touch a radiator, die. Not fun.

[M Fowler]

Tino uses a isolation transformer 117v 60 Hz with 120v out at 1/4 amp.

OT is 2k5 3w rated 8 ohm secondary

Mark

[Leviman]

This is dangerous for sure. Play the guitar, touch a radiator, die. Not fun.

If I add an isolation transformer and a grounded cord would it not be safe then? Because that's the plan...

[M Fowler]

Yes if you do that it would be safe at least your isolating your self from all that voltage where in the original circuit you would not be isolated.

Mark

[Leviman]

Well that's the plan... And I think I've determined that the isolation transformer I linked to in the original description will work. The rating on the bottom of the radio is 120 volts and 30 watts so that works out to .25 ampres which is within the specs on the transformer. Cool!

[FUCHSAUDIO]

Have fun be safe !

[Leviman]

Alright! So does everything I have sound good? Like it'll work? Haha

[Leviman]

Alright folks, I've re-done the schematic to include my modifications to most importantly, make the thing safe, and secondly, to make it an amp rather than a radio.

[IMG699]http://i966.photobucket.com/albums/ae14 ... atic-1.jpg[/img]

Alright, so as you can see I've tapped into the amp at the screen of the 12BA6 and coupled it with a .047mfd cap (this alone should help with safety, right?) and I've added a 1meg screen resistor for screen dissipation as well as a reduction in gain (I hypothesize that this thing will have plenty). I've also added a shorting speaker out jack, I'm assuming that the stock speaker has a four ohm impedance given that it has a 2.65ohm voice coil (I figure, fudge some reactance in there ). I've also re-wired the power's input, added a ground and moved the clock circuitry to be less integral to the amp. Because I will for sure have this clock still run. It'll be an alarm-amp .

Now for my questions. What is going on in terms of signal chain? Are those "diode" connections on the 12AV6 letting the signal into the screen on that tube? I've never seen "diode" connections so I'm no sure what's going on. Or should I hack that I.F. Transformer out all together and run a coupling cap to the screen of the 12AV6?

Alright, so please take a look, tell me what you think and give any suggestions! Thanks!

Oh and I tried to make my modifications to the schematic match in style, just for fun It's kind of funny to see a C13 connector on an AA5!

[Leviman]

Well you guys have not been tons of help but I figure I'll ask. So I got the parts and pieces I needed in the mail today and started wiring it up. Well it turns out that how I have my circuit setup it didn't work. So I placed my input lead on the up side of the volume pot (as most people do) and sure enough, that worked. So then I tried adding a .047uf coupling cap from the anode of the 12BA6 (IF amp) to the up side of the potentiometer. Now when I place my input on the grid of the 12BA6 it just buzzes a bit. No sound from the input (guitar). Any ideas why this could be? Could that tube be bad? Or am I just stupid for trying to turn an IF amp into another pre-amp stage?

I should note it also does not work as a radio.

[M Fowler]

Here is another schematic.

[Leviman]

This amp is based off a printed circuit board so that schematic is of no use to me. Do you have any idea why the 12BA6 doesnt want to work as a regular pentode? Is it just not designed to amplify audio frequencies? Or is it just a bad tube?