Using a 6SL7 in a tube overdrive pedal instead of 12AX7?

G · 20th March 2005, 04:34 PM

Hi All,

I am planning to build a small guitar amp for use at home and I thought of building a Real McTube II distortion unit into the amplifier. I would like to use a 6SL7 instead of a 12AX7 since I will have the extra room and since I like octals better. I looked at the curves of the two tubes and they look almost identical. Does anyone have any opinions on whether this would be an acceptable substitution? The circuit can be found here:

http://www.dogstar.dantimax.dk/tubestuf/mctube.htm

Giaime · 20th March 2005, 04:39 PM

Yes, you can do this... you should get very different sound than the usual 12ax7 overdrive...

Trout · 20th March 2005, 10:51 PM

Quote:

Originally posted by G
Hi All,

I am planning to build a small guitar amp for use at home and I thought of building a Real McTube II distortion unit into the amplifier. I would like to use a 6SL7 instead of a 12AX7 since I will have the extra room and since I like octals better. I looked at the curves of the two tubes and they look almost identical. Does anyone have any opinions on whether this would be an acceptable substitution? The circuit can be found here:

http://www.dogstar.dantimax.dk/tubestuf/mctube.htm

Hey,
I have a nice working McTube I built about 6 months ago, This sounds like an interesting MOD, Please Keep me posted on progress. Maybe if possible, A sound clip

There are nearly as many variations of 6S_7's out there as 12A_7s. This could be a fun project!
Gene

Colt45 · 21st March 2005, 01:25 AM

Go for it.. they are brother tubes as far as i can remember.

PRR · 21st March 2005, 03:20 AM

> like to use a 6SL7 instead of a 12AX7

Very different tubes.

> I looked at the curves of the two tubes and they look almost identical.

??? Well, they are both triodes, High-Mu, with 1 watt heaters. How different could they be? In fact the zero-bias lines lay on top of each other. But the 12AX7 has 40% more Mu than the 6SL7. And at the low currents typically run on 12AX7, the plate impedance is significantly higher.

Note that the -2V line on 12AX7 lies close to the -2.8V line on 6SL7.

> Real McTube II distortion unit

Fuzzboxes are all about "too much gain", so the loss of gain may be significant. Also Fred's plan "mis-biases" the tubes to get them off their most linear swing; a different tube will need re-mis-biasing to get the same/similar effect.

There is no octal quite like the 12AX7.

The octal with Mu=100 like 12AX7 is the rare (but only $6-$7) 6F5 (double-ended) and 6SF5 (single-ended) single-triode. Because it is an old design, and uses more heater (per cathode) than 12AX7, it may be more linear. Like the difference between Tri-X film in 120 and 35mm sizes: big difference in the knee of the curve.

Giaime · 21st March 2005, 12:34 PM

He can just try that tube... and see (hear?) the results. Usually lower gains in Real McTubes (and other starved plate pedals) are the best solution to get a better overall sound... In fact Fred himself(RIP) suggested to use lower gains tubes for better sound... For me nothing beats the creamy overdrive of an ECC82 when compared to the noisy fuzzyness of the ECC83...

20th March 2005, 04:34 PM	#1
G diyAudio Member Join Date: Apr 2002 Location: Belleville, IL.	Using a 6SL7 in a tube overdrive pedal instead of 12AX7? Hi All, I am planning to build a small guitar amp for use at home and I thought of building a Real McTube II distortion unit into the amplifier. I would like to use a 6SL7 instead of a 12AX7 since I will have the extra room and since I like octals better. I looked at the curves of the two tubes and they look almost identical. Does anyone have any opinions on whether this would be an acceptable substitution? The circuit can be found here: http://www.dogstar.dantimax.dk/tubestuf/mctube.htm __________________ Gavin

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20th March 2005, 04:39 PM	#2
Giaime diyAudio Member Join Date: Oct 2004	Yes, you can do this... you should get very different sound than the usual 12ax7 overdrive...

21st March 2005, 01:25 AM	#4
Colt45 diyAudio Member Join Date: Feb 2002 Location: Canada	Go for it.. they are brother tubes as far as i can remember.

21st March 2005, 03:20 AM	#5
PRR diyAudio Member Join Date: Jun 2003 Location: USA	> like to use a 6SL7 instead of a 12AX7 Very different tubes. > I looked at the curves of the two tubes and they look almost identical. ??? Well, they are both triodes, High-Mu, with 1 watt heaters. How different could they be? In fact the zero-bias lines lay on top of each other. But the 12AX7 has 40% more Mu than the 6SL7. And at the low currents typically run on 12AX7, the plate impedance is significantly higher. Note that the -2V line on 12AX7 lies close to the -2.8V line on 6SL7. > Real McTube II distortion unit Fuzzboxes are all about "too much gain", so the loss of gain may be significant. Also Fred's plan "mis-biases" the tubes to get them off their most linear swing; a different tube will need re-mis-biasing to get the same/similar effect. There is no octal quite like the 12AX7. The octal with Mu=100 like 12AX7 is the rare (but only $6-$7) 6F5 (double-ended) and 6SF5 (single-ended) single-triode. Because it is an old design, and uses more heater (per cathode) than 12AX7, it may be more linear. Like the difference between Tri-X film in 120 and 35mm sizes: big difference in the knee of the curve.

21st March 2005, 12:34 PM	#6
Giaime diyAudio Member Join Date: Oct 2004	He can just try that tube... and see (hear?) the results. Usually lower gains in Real McTubes (and other starved plate pedals) are the best solution to get a better overall sound... In fact Fred himself(RIP) suggested to use lower gains tubes for better sound... For me nothing beats the creamy overdrive of an ECC82 when compared to the noisy fuzzyness of the ECC83...

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