Driving a single 6L6GC with 12AX7

[mrhotmark]

I am updating a Fender Champ. It used a 12ax7 to drive a 6V6 tube. Will the 12ax7 drive a 6L6GC tube adequately? Or a 6550? Or is a buffer (cathode follower) required?

Thanx all!

[tubelab.com]

My first guitar amp was a Champ. It was in the mid 1960's. In one of my early quests for more power I put a 6L6 in place of the 6V6. To the best of my memory, there was plenty of drive. I was using a homemade clone of a Vox Tone Bender fuzz box. It sounded bad to the bone..... but The unfortunate result of my early experiments was a slow meltdown. The power transformer couldn't hack the extra demands of the 6L6 and eventually fried. Why am I explaining this? Because I am pretty certain that if you leave the amp on for long periods of time you may have the same problem. So plan on a bigger transformer in your update.

A 12AX7 can drive a 6L6 without much of a problem, although the extended high frequency response may roll off. A 6550 requires a lower value grid resistor 220K is too high. A 6550 wants 100K or less. This makes the driver work harder. A 12AX7 may not cut it.

[Miles Prower]

"Will the 12ax7 drive a 6L6GC tube adequately?"

I doubt it. Here are the basic stats:

Cgk= 11.5pF
Cgp= 0.9pF

Vgk= 12.7Vp
Rl= 4.5K
Po= 6.5W

Given the Rl and Po, the output voltage is: 241.87Vp, for a stage gain of: 19. The resulting input capacitance will be: Cin= 29.5pF (Cgk + Cmiller). Add in some stray capacitance, and you can figure on Cin= 59.5pF (~60pF). For a high frequency of 25KHz, that capacitance will need 0.12mA, over and above the current for the grid resistor. A 12AX7A would be hard pressed to supply that. If it doesn't have the current drive capability, then you're looking at slew rate limited distortion. You'd be better off using a cathode follower for 6L6 grid drive.

You could implement that with a 12DW7, which incorporates a high u triode (u= 100) (comparable to one section of a 12AX7A) and a medium u triode (u= 20) (comparable to a 6C4). Use the high u for a voltage amp, and the medium u for the cathode follower.

[SY]

Miles, do you need 25kHz response for a guitar amp? And might slewing distortion (if indeed that is what happens) be a desirable?

It's hard to break the habits of hifi design when doing musical instrument amps. I know this the hard way; I've recently been diddling around with a guitar amp design and all my instincts keep leading me to an amp that sounds... neutral and boring.

[Trout]

Originally posted by mrhotmark
I am updating a Fender Champ. It used a 12ax7 to drive a 6V6 tube. Will the 12ax7 drive a 6L6GC tube adequately? Or a 6550? Or is a buffer (cathode follower) required?

Thanx all!

Hi, I have been doing alot with Fender Champ builds lately(snicker)
No seriously,
I used the standard circuit, Easily ran a 6L6 & 5881.( No Mod Drop In)
I felt there was no real performance increase, BUT! There were some desirable Tonal differences. If you wish to get rid of some of the traditional tangyness of the 6V6, Thats a really good way to go. I felt it seemed to have a bit less distoration on the upper volume range, But I looked at it as a small recording amp and liked the idea of yet another type of sound.

Gene

[kevinkr]

Yeah most current fender, marshall, kendrick, mesa and a plethora of boutique amplifiers use 12AX7 to drive 6L6, pretty much the standard approach these days. They're readily available as is the 6L6 at most music equipment stores - a major consideration.

An old favorite for driving pp output stages in guitar amplifiers was the 12AT7. Somewhat lower rp, lower mu and much higher current and dissipation ratings.


Kevin

[tubelab.com]

The good thing about the 12AX7 is that there are dozens of twin triodes with the same pinout that can be plugged into the same socket to give a different sound. Not necessarily a technically correct HiFi sound, but something that sounds cool with a guitar plugged into it.

12AX7, 12AU7, 12AT7, 12DW7, 5963, 5965, 12AY7, 12AV7, 12BH7, 12FV7, and the list goes on. Most of these have lower gain than the 12AX7 and may not drive the 6L6 without using a stomp box with gain in front of the amp.

None of these tubes will cause damage to this circuit, although some may not give the desired sound character. DO NOT try this on any stage where the triode is DC coupled to the output tubes without considering the effects on the bias.

[amperex]

No more power output that one could hear using a 6L6 in place of a 6V6 at same plate voltage & audio transformer. Just stay with the 6V6 as they are great sounding tubes.

How about updating the speaker?

[tubelab.com]

You might pick up a little bit of extra power using a 6L6 if you used a transformer with a slightly lower impedance. Granted the power increase will not be audible if operated at or below the point of clipping. I know from experience that when you overdrive a Champ type design by about 10 db (well into serious distortion) a 6L6 will sound different than a 6V6. To some it sounds louder.

The last "ultimate Champ" type amp that I built (sold long ago) used a Hammond 125 type transformer (i don't remember which one) with every secondary tap on the transformer wired to an impedance selector switch, to present a variable load to the output tube. The screen grid has a 3 position switch, triode, pentode, and pentode with a 1K resistor in series ( gives a good bluesy distortion with the 6V6).

I upgraded the power transformer to an Allied 6K7VG. To allow for different B+ voltages, I added a filter choke, and another switch to switch in one of two caps to make a cap input filter (gives B+ of 400 volts for 6L6) or no cap, which makes a choke input filter which results in a B+ of 345 volts for 6V6, 6K6 and 6Y6.

I also added yet another switch to select a solid state or tube rectifier (like the Mesa dual rectifier switch)

I also brought the cathode bias out to a front panel control (500 ohm 2 watt) with a 270 ohm resistor in series.

I am sure that there were a few other mods that I can't remember now.

Why all of this? Well this was the most versatile Champ on the planet. You could dial up just about any sound that you wanted from clean to "stack cranked to eleven", BUT you could dial up some dead power tubes if you didn't think about what you were doing. I sold this amp to my daughter's guitar teacher about 3 years ago and I believe he still has it. He used it for small club gigs on weekends and it was reliable. He carried several different tubes (output tubes and rectifiers) and a cheat sheet to remember which combination resulted in the proper sound for the job.

Somewhere I have a paper drawn schematic for this amp, and I will post it if I can find it. This amp was the last (and most outrageous) Champ type amp that I made.

[mrhotmark]

Yes I am updating my old Champ amp. The output transformer is now from a deluxe reverb reissue, the output transformer is now a hammond 125ESE. I plan to try 6V6, 6L6, 6550, or KT 90 in the output stage, no cathode bias. The bias tap created a -50 VDC bias voltage which feeds a 25K pot to either a 220K or 100K resistor. That's why I asked is a 12AX7 could drive all tubes, perhaps a second 12AX7 stage in parallel...I have the room. Thanx all!