trouble installing Silvertone 1483 preamp

[EETStudent]

Im trying to integrate a Silvertone preamp into my amp and Im having some troubles. Basically, I cannot get sound through. The voltages seem correct, about 110-120VDC on the plates of the two stages, and about 140VDC into the phase inverter stage. I basically disconnected the existing preamp from two triode stages to accomodate for the stages required for the silvertone. I feel like im on the right track, because when I fire it up I can tap the pickup and get that slight popping sound. any thoughts on what might be happening?

[52 Bill]

Where does the "out" go to? You don't show a coupling cap, just a 220K resistor. Depending upon what the next stage is, you may be sending too much DC into the next stage.

[EETStudent]

This is the schematic for the power amp. The out is going to phase inverter grid, coupled by .001 cap. Now that i check, I totally overlooked that capacitor. I'll put it in and let you know the result. thanks a lot

[EETStudent]

capacitor wasnt it. i cant figure it out. Since Im changing the amps original preamp do I have to modify the power amp as well? seems like its just piecing the new preamp with the old power amp and it should work.

[EETStudent]

Ok so a further update. Finally got some sound, but something is still clearly wrong. The volume is very, very low. At 10, the volume level is probably equal to a 1 on a fully functional amp. The volume and master volume do bring the volume up and down, but not very far. I do have a 1uf 35VDC cap. connected to the B+, so maybe since its far exceeding its maximum voltage its creating a problem? Also, the tone controls arent responding. Clearly there are many issues to resolve, but three long nights of no sound and finally sound now is a huge relief.

[ricach]

Work backwards starting from the PI stage. Inject a guitar signal into this stage after the coupling cap - you should get nice sound, but not tons of volume. If not, continue towards the output for troubleshooting. If decent volume, next inject at the Volume pot wiper, now you should be getting good volume, if not, your problem is inbetween. If good volume, continue towards the input until you get a severe volume drop. This should identify exactly where the problem is.

Ok, just realized my thinking here is backwards, but you get the idea. Look where you find an unexpected change and work in the 'logical' direction.

[EETStudent]

hey man thanks a lot for that tip! thats something i would never think to do at this point, but it sure seems effective. ill be up all night working on that. Just out of curiosity, do you think the interaction of the preamp and the EL34s and an unconventional phase inverter could be causing a drop of some degree? maybe not, but i would hate to be focusing on something on one thing when another thing is creating the problem. thanks a lot

[ricach]

I would doubt it. I'm doing exactly the same thing you're doing (see couple posts down*) where I completely trashed everything from in front of the PI (just before coupling cap) and installed the exact fender tone stack you did. As long as the amp worked fine before your tone mod, I wouldn't expect any problems caused by design.

For what its worth - I had to do exactly the same thing (inject my signal in various places) because it didn't work. Turns out I forgot to wire my jack to the circuit - the wire was there, but just laying there. It appeared connected. duh.

(*my post is actually in the mods and tweaks section)

[Paul P]

The out is going to phase inverter grid, coupled by .001 cap. Now that i check,
I totally overlooked that capacitor. I'll put it in and let you know the result. thanks a lot

I don't know (but I wouldn't mind hearing from someone who does) what
applying 120v to your inverter grid did but it may not have been good for it.
Do you have another tube you can replace it with ?

Paul P

[EETStudent]

yes I have another 12AX7 i can put in there. are you saying that too much DC could have done some damage? If so, what sort of voltage level would be appropriate? im always interested in new ideas and such

[EETStudent]

so far ive injected my signal at the phase inverter, and also at the input. both places produce pretty much produce the same signal, although very slight difference in eq, and volume. I guess this is a sign that my preamp isnt functioning. On the phase inverter, i put my signal in at the grid, cathode, and plate, and all three produced sound. I then put my signal in at the last stage of the preamp just before the coupling cap to the phase inverter, at the same three places, and got no sound. im kinda stuck troubleshooting now. should those places have produced sound?

[Enzo]

If the PI makes output from the speaker, move back one step and inject there. That is signal tracing in reverse.

If the grid of the PI works, what connects to that grid and from where? Go there with your signal.

[EETStudent]

well, not sure exactly how much volume i should be getting from the phase inverter, but i am getting some volume. pretty low volume, but still some. i also did this. i basically wired a one stage circuit to see what would happen. i did a first gain stage, basically cathode resistor/cap to ground, input to grid, plate to B+, and there into the phase inverter. Although a little extra gain, but no volume increase. Should I have gotten volume there? Where does volume come from necessarily?

[ricach]

When I put my guitar signal into the grid of the PI, it was decent - loud volume. That told me everything from there to the speakers was working fine. I backed up to the grid of the 'b' side grid of my preamp tube, then the 'a' side. The output volume increased at each point, with TB and Volume controls working correctly as well. It was at this point when I discoverd the unconnected input wire. Since your output (at the speakers) is weak injecting into your PI, your problem is in the PI or power stage.

I should have stressed this earlier, but if using your guitar as a sound source, make sure you inject 'after' a coupling cap to shield you from high voltage DC coming back through your guitar - damaging the guitar and you. Basically stick with the grid inputs.