No Name Amp - Isolating the Power Amp section

[rattleandhum]

Hi there,

I have been mapping out the schematic on an amp I got at the junkyard (I think it may be an old Organ amp). Anyhow, I am in the process of figuring out where I should cut off the pre-amp in this thing and insert the line input from my Vox valvetronix for fun and thus isolate the power amp.

I am thinking I can just connect the line-input directly to the grid of the phase inverter (as that is where the volume of the amp connects to). Though I am sure I will need a coupling capacitor.

I realize I dont have the schematic finished yet. After working on it for about 4 hours I got tired.

Thoughts anyone? Am I approaching this right?



Thanks,
Kurt

[darrellcheng]

I'm not too familiar, but the vib. channel looks right because i use the same tube.

I want to know, however, how did u get somethin from a junkyard??? I would love to go treasure hunting

[rattleandhum]

I have a friend at the junkyard who in the past has called me when he knew some tube gear was given to it. At the same time I got a tube tester - though it doesnt work and i have no clue how to fix it. But it was fun to mess with.

Well, it's good that I have the vib channel right. Any thoughts on isolating the power section?

Something I forgot to mention - the power section is two 6L6GB's so...there you are.

The input is an rca jack which is interesting. I don't know enough about the eq curve for a phono input to know whether this has it or not though.

Thanks for looking!

[Enzo]

Don't confuse phono jack with phono channel. Phono jacks - RCA jacks - may have been developed for use with phono signals, they are used for all sorts of things. Certainly any stereo receiver has a ton of them for channels other than the phono. And closer to home, reverb pans use them. SOme Fender amps used them as speaker outputs.

A phono preamp - RIAA preamp - usually has an RCA jack for input, but might use something else.

If this really is an organ amp, I'd have doubts about it having a phono preamp stage. Phono preamps have the RIAA response curve, which is SERIOUSLY bass heavy and treble light. It is also a lot higher gain than a line input. SO the phono response eq curve you mention is not relevant here.

[rattleandhum]

I'm sure you're right. I have gotten it to accept a line level signal through the RCA, and it even came through the speakers, which is more than i expected. It sounded treble light most certainly, but it did not have the bass heaviness that you spoke of.

However, why use a 5879 as a gain stage if it already accepts a line level signal? I thought you would need to amplify the signal only if it was quiet like a microphone... Why not just build the amp to inject the signal right before the phase inverter? Is it so that the vibrato channel works?

(btw I did try to inject a line-level signal through the center pin of the volume control right before the phase inverter and I got no sound at all...)

Thanks for reading..

Kurt