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How to get more punch?

12/9/2000 8:34 PM
PeterF	How to get more punch? I have an Ampeg Gemini VI which over the years has been modified a lot. I made it into a head driving a 2x12 with Weber p12q's. It's altered to take 6L6's (it has NOS Phillip's WGB's in it right now). And I changed it to Cathode biased. I like the way it sounds now except that it seems just a bit flubberish in the low end like when I play riffs on the six string or low power chords. I feel this way whether I use a dual humbucker git or my tele. My inexperienced guess is that it needs more powerful low-mids for better punch. It has a bandaxall style tone circuit which I don't get. Actually, I just don't get tone circuits in general. Any Ideas out there for what I might try?

12/9/2000 8:34 PM

PeterF

How to get more punch?

I have an Ampeg Gemini VI which over the years has been modified a lot. I made it into a head driving a 2x12 with Weber p12q's. It's altered to take 6L6's (it has NOS Phillip's WGB's in it right now). And I changed it to Cathode biased. I like the way it sounds now except that it seems just a bit flubberish in the low end like when I play riffs on the six string or low power chords. I feel this way whether I use a dual humbucker git or my tele. My inexperienced guess is that it needs more powerful low-mids for better punch. It has a bandaxall style tone circuit which I don't get. Actually, I just don't get tone circuits in general. Any Ideas out there for what I might try?

12/10/2000 2:32 PM

Trace

quote:
"I have an Ampeg Gemini VI which over the years has been modified a lot."

Off the top of my head I would say that a schematic of the way the amp is now would be pretty much a must to help you dial in the amp.

quote:
"And I changed it to Cathode biased. I like the way it sounds now except that it seems just a bit flubberish in the low end like when I play riffs on the six string or low power chords."

Was the amp "tighter" sounding originally?

quote:
"I feel this way whether I use a dual humbucker git or my tele. My inexperienced guess is that it needs more powerful low-mids for better punch."

That is certainly possible. A schematic would help out tremendously. Do you happen to have one?

quote:
"It has a bandaxall style tone circuit which I don't get. Actually, I just don't get tone circuits in general. Any Ideas out there for what I might try?"

Well tone stacks do look pretty wacky at first! (ha, ha) no arguement there. I'm not a fan of the tone stack you currently have but that's just my opinion. I've heard amps that sounded great with it but it's just not my cup of tea is all

I'm not really sure what to recommend because I'm sure what sort of changes you have made to the amp already. Could you give us an idea as to the changes that were made?

Trace

12/10/2000 7:44 PM
PeterF	I guess it really hasn't had "a lot" of mods, just a few big ones like chopping it from a combo to a head. The original schem is here: http://users.aol.com/portaflex/schems/gs-15-r.gif At a glance, it's pretty accurate except the first and second coupling caps are .1 uf. That's not a mod. The caps are original. To take 6L6's the wires were switched for the correct pinout configuration and 470 ohm screen resistors were added. The bias supply was disconnected and the 270k bias resistors were tied to ground. The 100 ohm balance pot was removed and the cathodes were tied together then to ground through ~375 ohms and bypassed by a 25 uf cap. The tubes are drawing ~40ma. Hope that helps. Thanks PeterF

12/10/2000 7:44 PM

PeterF

I guess it really hasn't had "a lot" of mods, just a few big ones like chopping it from a combo to a head.

The original schem is here:

http://users.aol.com/portaflex/schems/gs-15-r.gif

At a glance, it's pretty accurate except the first and second coupling caps are .1 uf. That's not a mod. The caps are original.

To take 6L6's the wires were switched for the correct pinout configuration and 470 ohm screen resistors were added.

The bias supply was disconnected and the 270k bias resistors were tied to ground.

The 100 ohm balance pot was removed and the cathodes were tied together then to ground through ~375 ohms and bypassed by a 25 uf cap. The tubes are drawing ~40ma.

Hope that helps. Thanks

PeterF

12/10/2000 9:10 PM
Ken Gilbert	since it has a concertina splitter, you may be limited by the drive available to the output grids. what's the DCV on the concertina grid? it should be about 1/3rd of total B+ available at node C. if it isn't, tweak the 10M and 1M5 voltage divider until it is. you could also use a bigger cathode bypass cap on that shared output Rk. is that 40mA current draw for each tube, or both? kg

12/10/2000 11:18 PM
Trace	Peter; 1.) I would reduce the .1uF coupling caps (going to the output tubes) to .022Uf's. .1uF's may be allowing a bit too much low-end to pass on through. 2.) What is the value of the cathode bypass cap that is across the 3.3K cathode resistor? 3.) I would change the tone stack to a Marshall or a Fender style tone stack. Either one would be a big improvement in my humble opinion. I don't much care for the stock PI myself but that's just me! (ha, ha) Let me know how you make out! Trace

12/10/2000 11:18 PM

Trace

Peter;

1.) I would reduce the .1uF coupling caps (going to the output tubes) to .022Uf's. .1uF's may be allowing a bit too much low-end to pass on through.

2.) What is the value of the cathode bypass cap that is across the 3.3K cathode resistor?

3.) I would change the tone stack to a Marshall or a Fender style tone stack. Either one would be a big improvement in my humble opinion. I don't much care for the stock PI myself but that's just me! (ha, ha)

Let me know how you make out!

Trace

12/11/2000 3:18 AM
PeterF	Thanks for the replies. It's about 40ma per tube. and that 2nd stage bypass cap is a mere .1uf. I won't get a chance open it up for a couple of days to check those voltages but I'll get back after I do. Why would they put positive voltage on the PI grid? The schem in the Pittman book says it should be about 18v but for voltage at C it say 320 which would make it 48 v? maybe it's a typo. I don't really want to do anything drastic like switch to a long-tail PI or put in a different tone stack since I'm building an amp with those things anyway. The sound isn't too far off what I want but a little more of that low, round punch would really make the amp kick. PeterF

12/11/2000 3:18 AM

PeterF

Thanks for the replies. It's about 40ma per tube. and that 2nd stage bypass cap is a mere .1uf. I won't get a chance open it up for a couple of days to check those voltages but I'll get back after I do.

Why would they put positive voltage on the PI grid? The schem in the Pittman book says it should be about 18v but for voltage at C it say 320 which would make it 48 v? maybe it's a typo.

I don't really want to do anything drastic like switch to a long-tail PI or put in a different tone stack since I'm building an amp with those things anyway. The sound isn't too far off what I want but a little more of that low, round punch would really make the amp kick.

PeterF

12/11/2000 2:20 PM
Ken Gilbert	tweaking concertinas for max Vout peter, you want the voltage on the concertina-type splitter's grid to be about 1/3rd of the B+. the reason is that the cathode will follow the grid voltage within a few volts. for max signal swing from a concertina you want to divide the supply voltage in thirds, with 1/3rd across the plate resistor, 1/3rd across the cathode resistor, and 1/3rd across the tube itself, from plate to cathode. the only voltage you can explicitly set is the grid voltage, and if you set that to be about 1/3rd B+ then, as i said, the cathode will follow. you can't make any amp kick unless you drive the output grids well enough. this is not a topological mod at all, more of a component swap. with 40 mills per tube, you've got a total of 80 mills across that shared Rk, which puts your bias voltage at about 30VDC. that means, for full output, you need about 60Vpkpk out of the concertina. not impossible, but it won't happen if the quiescent conditions are not set up just right. kg

12/11/2000 2:20 PM

Ken Gilbert

tweaking concertinas for max Vout

peter,

you want the voltage on the concertina-type splitter's grid to be about 1/3rd of the B+. the reason is that the cathode will follow the grid voltage within a few volts. for max signal swing from a concertina you want to divide the supply voltage in thirds, with 1/3rd across the plate resistor, 1/3rd across the cathode resistor, and 1/3rd across the tube itself, from plate to cathode. the only voltage you can explicitly set is the grid voltage, and if you set that to be about 1/3rd B+ then, as i said, the cathode will follow.

you can't make any amp kick unless you drive the output grids well enough. this is not a topological mod at all, more of a component swap.

with 40 mills per tube, you've got a total of 80 mills across that shared Rk, which puts your bias voltage at about 30VDC. that means, for full output, you need about 60Vpkpk out of the concertina. not impossible, but it won't happen if the quiescent conditions are not set up just right.

kg

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