Help with pre-amp out.

[marcoloco961]

I'm working on a small 40 to 45 watt bass amp design and would like to add a pre-amp output. It uses 3 - 12AX7's and 2 - 6l6gc's.

Present circuit goes :

Normal inverting gain stage, TS, inverting gain to CF, Inverting gain w/local feedback to LTP PI. At what point would be the best to pull the preamp signal to an output? Any schematics would help immensely. Thanks in advance for any suggestions.

[Cliff Schecht]

Preamp out would be after the preamp, before the PI. With something like this you could play around with where you tap off and see your results. It may be better if you add some sort of buffer to the output, but this isn't always reasonable. Can you pull signal off of the cathode of the stage before the LTP? If it has feedback from somewhere else going into the cathode, this might not be a good place. But if the cathode is just an RC setup, it's a nice low impedance source that has all of the preamp flavoring.

It would probably help if you drew a schematic for us .

[marcoloco961]

Preamp out would be after the preamp, before the PI. With something like this you could play around with where you tap off and see your results. It may be better if you add some sort of buffer to the output, but this isn't always reasonable. Can you pull signal off of the cathode of the stage before the LTP? If it has feedback from somewhere else going into the cathode, this might not be a good place. But if the cathode is just an RC setup, it's a nice low impedance source that has all of the preamp flavoring.

It would probably help if you drew a schematic for us .

OOh alright, I'll post my super duper top secret schemo....NOT.....LOL. Thanks for the help Cliff. This thing started life as a Wreck, then morphed to a Bassman 50 for about 10 minutes, now it seems to change on a daily basis. I am getting closer to the tone I was shooting for. I think this version of the schematic is current, although as I said, I have been goofing with it.

The cathode of the last stage prior to the PI is just a 2.7k resistor, no bypass cap, no Fb directly on the cathode, but that is the stage with the funky FB. Would that still be a good spot to tap? Or would you try several different spots? Also, would you suggest a pot to vary the signal strength at the pre-out?

[gearhead]

Hmm. Help a brother out here. What kinda bias you get for V1a (and b)? Two diodes with a 100n cap in parallel to ground or just a diode to ground?

[jaysg]

Pull it off the output of the cathode follower.

[marcoloco961]

Hmm. Help a brother out here. What kinda bias you get for V1a (and b)? Two diodes with a 100n cap in parallel to ground or just a diode to ground?

Each 1n4148 yields about .6V of resistance, so two puts you at about -1.2 VDC. The cap is merely a snubber to kill any noise the diodes might cause. The two diodes are in series, with the cap paralleled around both diodes. Each different color led diode will offer you a different voltage and IIRC somewhat serves as a CC source to stabilize the stage. Either way, it makes for a damn quiet amp when used in the V1 spot. The diodes also help to keep the gain of the stage high with, or without a bypass cap. If you want to know more you should buy Merlin's book, or just go to:

http://www.freewebs.com/valvewizard1/gainstage.html

Open the PDF and look at about page 35 of 37, you will see a chart of the different voltages associated with the different LED's.

............... Then buy Merlin's book from him so I don't feel so bad about all the good ideas I have gained from his work. The variable tube compression is also a spin off of one of his great examples in his book. Cheers.

[marcoloco961]

Pull it off the output of the cathode follower.

Thank you sir, I will begin the experiment at that point. Do you think I should use a pot on the output to make it variable?

[gearhead]

............... Then buy Merlin's book from him so I don't feel so bad about all the good ideas I have gained from his work. Cheers.

Thanks! When I saw the diodes (esp the V1B, no parallel cap) was wondering if there was some odd asymetrical bias going on as the input swung from positive to negative. Constant voltage above a minimal current is a cool application.

AFA Merlin, double thanks. A great source and finally got something to tell the Mrs. I want for Christmas!!

Edited to add: Does anyone know if he has a power amp book in the works? See he has the back end covered with his power supply book.

[marcoloco961]

............... Then buy Merlin's book from him so I don't feel so bad about all the good ideas I have gained from his work. Cheers.

Thanks! When I saw the diodes (esp the V1B, no parallel cap) was wondering if there was some odd asymetrical bias going on as the input swung from positive to negative. Constant voltage above a minimal current is a cool application.

AFA Merlin, double thanks. A great source and finally got something to tell the Mrs. I want for Christmas!!

Edited to add: Does anyone know if he has a power amp book in the works? See he has the back end covered with his power supply book.

Not sure on that one. There is some info on the power stages on his page.

[marcoloco961]

I have another question pertaining to the pre-amp output on a tube amp for the folks @ TAG. I'm feeling like this is a stupid question, but it has been bugging me while I have thought about the pre-amp out.

Knowing that it is not good to run a tube amp without a speaker load, how would you go about running the amp for the pre-amp out only. If the signal is broken at the pre-out, is it still bad to have no load on the OT. Or would I need to work in a dummy load on the OT for use as a pre-amp only?

I am wanting to pull the signal out before the power section and use another amp for the power, as 40 watts just isn't enough power for some jobs. I am not finding any designs for a tube amp with pre-amp out. Someone set me straight please.

[quayhog]

Scan through the Gibson service manual and look for Monitor jacks.
I've posted links to two different types. The first noted (GA-77RVT) taps the signal from the grid to kathode connection of the PI.

http://www.el34world.com/charts/Schemat ... -77RVT.pdf

The second one noted (GA-19RVT) taps the monitor out at the speaker jack connection.

http://www.el34world.com/charts/Schemat ... -19RVT.pdf

I tried this type of connection once in a home built and got a good shock.

[marcoloco961]

Scan through the Gibson service manual and look for Monitor jacks.
I've posted links to two different types. The first noted (GA-77RVT) taps the signal from the grid to kathode connection of the PI.

http://www.el34world.com/charts/Schemat ... -77RVT.pdf

The second one noted (GA-19RVT) taps the monitor out at the speaker jack connection.

http://www.el34world.com/charts/Schemat ... -19RVT.pdf

I tried this type of connection once in a home built and got a good shock.

The way the second design taps off of the OT secondary, I am assuming would be seen as a load and not put strain on the tranny. Correct? I'm not sure I want to get shocked though........ I really wanted to grab the signal before the PI and PS. Is there a way to do this properly?

[David Root]

Merlin's next book (#3) is apparently going to be the power amp book. I am awaiting delivery on his power supply book.