PA conversion...bass tone

[bluehevy75]

Hello,


I recently bought an old tube PA amp. It has been converted with 1/4" input jacks and 1/4" output jacks. It is about 30 watts and uses a pair of 6L6G's.

My goal is to use it for bass. I know this is a little unusual I'm going to run it as one half of a bi-amp rig.

I will run my lows through an Ampeg B-15 and run my highs through this old tube PA head and a 2x12 cab I've got.

I had some questions about the controls. There is a single tone knob that switches to four different settings. I was wondering if I could make some mods and change the sound of those settings.

I've included some pics. The tone control is at the bottom of the page.

My main goal I guess is to cut out some bass without it getting crazy bright. I know that tone preferences are pretty subjective but with an amp this old and a switch like that I don't know where to start.

Reading schematics and making them is completely new to me so thanks for being patient.

Anyway the switch has four settings I'll describe the resistors the best I can. The capacitors I'll have to do when I get home from work.

1st Pin=1st Postion=Deep Bass
2nd Pin=2nd Position=Mellow
3rd Pin=3rd Position=Crisp (So far I like this setting best)
4th Pin=4th Position=Brilliant



1. There is a 22K resistor connecting the 1st and 4th position. That is, going from the "deep bass" to "brilliant" settings. It is curious to me that they are linked this way as the "brilliant" setting actually has a boat load of bass. Which is counter-intuitive to me as I assumed "brilliant" settings would have less bass than "crisp" seetings but that is not the case with this amp--or at least the way it is configured.

2. The first postion seems to have what is a .003 capaciter that goes to a ground.

3. The second position has a 65k resistor running to a gound.

4. The second position also has a capaciter that runs to a ground but I need to check it out when I get home.

5. The third position has a 68k resister running to a ground.

6. The fourth position has that big gnarly cap running to a tube.

I'd love any ideas but will try to do something more helpful when I get home tonight.

thanks,
Kevin

[bluehevy75]

Here is the Baxandall circuit I'd like to make and add to my amp. I think the values for caps and resistors are good as I stole them off one of Piazza's schematics for the b-15n (1962).

I'm not positive that the values going in and leaving the circuit are correct I'll need to check them against the amp. I'm going to try to make a schematic for the amp here in the next couple of days. I'm not sure whether I should use the computer to do it or not. I used the Eagle free-ware. I know that I don't the software that well but it still took me longer than I would have liked to draw that simple circuit. I also couldn't find the ground function.

Anyway, look for more before the weekend is out.

[bluehevy75]

Well here we go. I drew up a schematic as best as I could for the head. I'm not sure that anyone will actually be able to read it but I tried. I attached the values for caps/res separately because I thought they would be unclear if I included them in the schematic.

Thanks everyone for the help on the tone circuit. I have changed the values accordingly.

So I guess my questions are:
1. Can I make mods to the existing tone circuit to cut out more bass?
2. Can I drop in the Baxandall circuit for more tone control in general?
3. Do I/can I/should I mod that phono circuit to make the amp more "useful"? Because I am never going to use that channel, jack, or volume control the way it is now.

I'd be interested in hearing what other kinds of questions or comments you have. See what you think. Thanks for your patience and thanks in advance for your time.

Kevin