AMP NERDS: Which channel should I... ?

JohnnyCrash · May 5th, 2006, 04:39 AM

As I usually do, I've been blabbering about my latest build, an old '50s Fender Twin clone head. The 5D8 to be exact, anyways...

My question is:
Which channel should I experiment with a 12AX7 tube on, the Normal or the Bright?

This thing is currently setup with 12AY7s (as it's supposed to be, in stock setup).

--

Also, I'm thinking of ways to do a simple channel switch between the two channels, this way I could have a footswitchable "normal channel" with the 12AY7 and "extra push channel" with the 12AX7... I could go really low tech and use my ABY switch into either channel input.

Perhaps I'd lose the 4 inputs (jumpering them was OK, but not that cool), and use the "unused" half of either channel's preamp tube as an extra gain stage or two to make a tweed Twin with a sort of more modern style Clean and Dirty channel (well, not exactly modern, but you get the picture).

This thing dimed has the coolest, BIGGEST rock tone. I was more than surprised at what came out of this '54 circuit! And it can do nice "warm" country with ease... like most OLD Fender amps, single coils sound AWESOME/best through it (holy crap P90s are badass), but my Les Pauls have been giving my Telecasters a run for their money in this amp.

JohnnyCrash · May 5th, 2006, 04:47 AM

Oh, and no Master Volume suggestions! HAHA

I like MV amps plenty, but this 25watter just has all of that fatness from the 6L6s cranked, I don't need a whole lot of preamp fizz, just looking to fool around with an "extra push" in the front end...

I bet somebody could make a fortune off of this circuit mod'ed like this - a "modern two channel" (you know, a Clean/Dirty, not it's old Normal/Bright setup) tweed with natural non-MV, cranked OD.

Especially in the boutique market... perhaps it's close (very close) cousin, the Bandmaster would be a better circuit for this though...

PhatTele · May 5th, 2006, 10:06 AM

Well, schematically, the only difference between the two channels is a cap going across the the Bright channel's volume pot. In fact, you could make the channels the same by either adding or subtracting that one cap.

The other thing that's going on is that both sides of both tubes are sharing one cathode resister and bypass cap. You could really do some experimenting by adding an additional cathode resister and bypass cap....giving you one set per tube. This would allow you to dial in each tube individually while keeping the "bright cap" on or off the volume pots. You'd have to change the value of the resister for each tube to start (probably start with 820 ohms since the both cathodes of each tube are connected). Then you could really maximize each tube's gain and tone by playing with the resister and cap values separately. You could go even further by putting in a separate cathode resister and bypass cap for each half of each of those two preamp tubes. You'd probably bump up the four resisters to 1.5K to start (which is a fairly normal value) and play around from there.

These are easy and reversable mods. You can connect the new cathode resister and bypass cap directly to the tube pin and run them to ground using a solder lug connected to one of the tube's mounting screws/bolts. I've done it in Tweed Bassman's and BF AB763 circuits. It works great and you learn a lot from fooling around with the values of these components.

JohnnyCrash · May 5th, 2006, 02:00 PM

My main thought was:

Which channel would sound best with a hotter preamp tube?

But you said something that I should've thought of...

I can put a push/pull switch to simply add the "bright" cap into the circuit when wanted...

In fact, I can forego all 4 inputs and just have one input, a "Bright switch" and use the extra tube halves for more gain stages...

IDK if I should be straying too far from the original... it sounds so good, I don't think I even actually want extra gain. IDK.

Thanks though, you got my brain working :)

PhatTele · May 5th, 2006, 02:21 PM

I've tried the 12AX7s in place of the 12AY7s in Tweed Deluxes, Tweed Supers, and Tweed Bassmans. Invariably, if I'm leaving the circuit stock and just play with the tubes, I always go back to the 12AY7. There's just too much gain with the 12AX7. The amps start breaking up at around 3 on the dial. Leo had it right w/the 12AY7.

Now, if you split up the cathode resisters and bypass caps like I mentioned, you can dial in the gain of a 12AX7 a little more precisely by making the tubes run a little cooler plus you can make them a little less agressive and boomy by lowering the bypass cap value...like Marshall did in the early days.

May 5th, 2006, 04:39 AM	#1 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	AMP NERDS: Which channel should I... ? As I usually do, I've been blabbering about my latest build, an old '50s Fender Twin clone head. The 5D8 to be exact, anyways... My question is: Which channel should I experiment with a 12AX7 tube on, the Normal or the Bright? This thing is currently setup with 12AY7s (as it's supposed to be, in stock setup). -- Also, I'm thinking of ways to do a simple channel switch between the two channels, this way I could have a footswitchable "normal channel" with the 12AY7 and "extra push channel" with the 12AX7... I could go really low tech and use my ABY switch into either channel input. Perhaps I'd lose the 4 inputs (jumpering them was OK, but not that cool), and use the "unused" half of either channel's preamp tube as an extra gain stage or two to make a tweed Twin with a sort of more modern style Clean and Dirty channel (well, not exactly modern, but you get the picture). This thing dimed has the coolest, BIGGEST rock tone. I was more than surprised at what came out of this '54 circuit! And it can do nice "warm" country with ease... like most OLD Fender amps, single coils sound AWESOME/best through it (holy crap P90s are badass), but my Les Pauls have been giving my Telecasters a run for their money in this amp.

May 5th, 2006, 04:47 AM	#2 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	Oh, and no Master Volume suggestions! HAHA I like MV amps plenty, but this 25watter just has all of that fatness from the 6L6s cranked, I don't need a whole lot of preamp fizz, just looking to fool around with an "extra push" in the front end... I bet somebody could make a fortune off of this circuit mod'ed like this - a "modern two channel" (you know, a Clean/Dirty, not it's old Normal/Bright setup) tweed with natural non-MV, cranked OD. Especially in the boutique market... perhaps it's close (very close) cousin, the Bandmaster would be a better circuit for this though...

May 5th, 2006, 10:06 AM	#3 (permalink)
PhatTele Tele-Holic Join Date: Jun 2003 Location: Richmond, VA Posts: 756	Well, schematically, the only difference between the two channels is a cap going across the the Bright channel's volume pot. In fact, you could make the channels the same by either adding or subtracting that one cap. The other thing that's going on is that both sides of both tubes are sharing one cathode resister and bypass cap. You could really do some experimenting by adding an additional cathode resister and bypass cap....giving you one set per tube. This would allow you to dial in each tube individually while keeping the "bright cap" on or off the volume pots. You'd have to change the value of the resister for each tube to start (probably start with 820 ohms since the both cathodes of each tube are connected). Then you could really maximize each tube's gain and tone by playing with the resister and cap values separately. You could go even further by putting in a separate cathode resister and bypass cap for each half of each of those two preamp tubes. You'd probably bump up the four resisters to 1.5K to start (which is a fairly normal value) and play around from there. These are easy and reversable mods. You can connect the new cathode resister and bypass cap directly to the tube pin and run them to ground using a solder lug connected to one of the tube's mounting screws/bolts. I've done it in Tweed Bassman's and BF AB763 circuits. It works great and you learn a lot from fooling around with the values of these components.

May 5th, 2006, 02:00 PM	#4 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	My main thought was: Which channel would sound best with a hotter preamp tube? But you said something that I should've thought of... I can put a push/pull switch to simply add the "bright" cap into the circuit when wanted... In fact, I can forego all 4 inputs and just have one input, a "Bright switch" and use the extra tube halves for more gain stages... IDK if I should be straying too far from the original... it sounds so good, I don't think I even actually want extra gain. IDK. Thanks though, you got my brain working :)

May 5th, 2006, 02:21 PM	#5 (permalink)
PhatTele Tele-Holic Join Date: Jun 2003 Location: Richmond, VA Posts: 756	I've tried the 12AX7s in place of the 12AY7s in Tweed Deluxes, Tweed Supers, and Tweed Bassmans. Invariably, if I'm leaving the circuit stock and just play with the tubes, I always go back to the 12AY7. There's just too much gain with the 12AX7. The amps start breaking up at around 3 on the dial. Leo had it right w/the 12AY7. Now, if you split up the cathode resisters and bypass caps like I mentioned, you can dial in the gain of a 12AX7 a little more precisely by making the tubes run a little cooler plus you can make them a little less agressive and boomy by lowering the bypass cap value...like Marshall did in the early days.