# Tech question and amp tech in Cobourg/Peterborough area



## traynor_garnet (Feb 22, 2006)

My buddy just got back into playing again after a 20 year break. Anyway, I found him an great deal on a Garnet Rebel combo. It's a stencil (RAM) but looks identical to the Garnet (even has the Garnet badge and model number on the back!). It was such an amazing deal at $115 that it was hard not to take it for myself!

Anyway, it is a Rebel model R90B and my buddy says it barely breaks up, even at full blast with humbuckers. It does use a 12au7 in V2, so when I hear it/see it this weekend I may stick in a 12ax7 and see if that makes a difference. If it doesn't break up with the higher gain tube, then the circuit itself must be designed to give very low distortion.

If the circuit needs some minor modding to give up the dirt, I was wondering if there is a good tech in the Cobourg/Peterborough area of Ontario (where my buddy lives). I am guessing it would only take a small change/mod to get the amp to rip like most Garnets, but I don't want my inexperience friend, going to some hack, and coming back with a heavily modded amp full of suspect mods and needless changes (and probably sounding like crap).

Any amp tech recommendations or modification thoughts for this circuit would be very much appreciated.


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## Scottone (Feb 10, 2006)

He could check with "Hank to Hendrix". I beleive that a buddy of mine used their guy


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## dtsaudio (Apr 15, 2009)

You might want to try a 12AT7 in that position instead of a 12AX7. The current demands on the AU7 are such that the X7 is a bad match, and will run poorly. The AT7 is closer and can handle the current.


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## nonreverb (Sep 19, 2006)

The R90 schematic does not show the amp using a 12AU7 but rather a 12AX7. However, knowing how these amps varied in design, it's possible that your friends could be an exception. It would be best to check the bias resistors on the tube socket and compare them to the schematic. If they're the same, then my guess is someone's dropped the wrong tube in it.


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## traynor_garnet (Feb 22, 2006)

nonreverb said:


> The R90 schematic does not show the amp using a 12AU7 but rather a 12AX7. However, knowing how these amps varied in design, it's possible that your friends could be an exception. It would be best to check the bias resistors on the tube socket and compare them to the schematic. If they're the same, then my guess is someone's dropped the wrong tube in it.


Here is a pic of the chassis. As you can see, the middle tube is clearly labelled 12au7. I am hoping it comes to life with a higher gain tube so that he doesn't have to get the circuit modded to give up some gain.

I am taking a 12ax7 and a 12at7 to test in that position (thanks for the suggestion dtsaudio).


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## dtsaudio (Apr 15, 2009)

There must be a lot of variation in Garnet amps. I pulled out my schematics, and they show the R90 with a 6SN7 and 6V6 tubes. The schematic for the Rebel II/LB100 uses 12AU7 and 6L6 tubes. Interesting. I also found one other amp in the Rebel series that uses a 6SL7 and 6L6 tubes. The 6SL7 is very close to a 12AX7 in a bigger bottle. That one however is a PA amp.


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## jb welder (Sep 14, 2010)

If it barely breaks up at full tilt with humbuckers, I would think there is a problem that needs to be repaired, or it has been modified.


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## Wild Bill (May 3, 2006)

I never saw Gar use a 12AU7 as the preamp input tube. Occasionally he used them as a phase inverter. It's quite common for someone to try a lower gain input tube like the 12AU7 to try to get RID of early breakup!

So indeed the amp may have used a 12Ax7 as the original input tube. For a better check, using a 12AU7 for a sub is actually not a good idea anyway. There are a lot of tubes in this 12A*7 family that fit the same socket and pinout that have different gain characteristics. However, the 12AU7 uses different values of plate resistors for optimum distortion figures. Plugging one into a 12AX7 socket without changing those resistors might add some unwanted harsh dirt.

If you want to check this, look at the bottom of the socket inside the amp and check the resistors connected to pin 1 and pin 6. The pins are numbered clockwise, with the gap between pin 1 and 9 at the bottom.

A typical value for a 12AX7 would be 100K. It will probably have coloured bands on the body as a code for the resistance value. 100k would show a brown, a black and a yellow band, with a last band on the right bieng silver or gold, indicating 10% or 5% tolerance, respectively. Sometimes Gar used a higher value for more gain, like 220k, or red, red, yellow.

If the amp was intended to use a 12AU7 the resistor values would be much lower! Typically you would see 47k, or yellow, violet, orange or even 22k - red, red, orange.

Most of the different gain tubes really don't have enough difference in gain for anyone to notice. Guys who don't really know any electronics will see a chart showing that a 12AX7 has a voltage gain factor of 100, while another tube might show a value of 70. So they will pontificate on how their impressive hearing notes the differences. In actual fact, the gain factors of all the tubes in the preamp string multiply the signal as it passes through each tube stage, and there are two stages inside every such preamp tube. The end result of all that multiplication is so much excess gain that the difference from 100 to 70 works out to "mice nuts"! No one will actually hear it.

The exception is the 12AY7, with a gain factor of 40. This will indeed reduce the gain enough to be heard, pushing the point of breakup farther up the dial. This was actually the predecessor to the 12AX7 for Leo Fender. He used it in his first amps up and didn't start using the 12AX7 until the mid 50's. This one is truly just "plug in" as a sub and there are no worries about different resistor values.

Like old times, this is probably more info than you wanted but hopefully some folks will find it useful.

Wild Bill


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## WCGill (Mar 27, 2009)

Bill, welcome back. I had a white tolex Garnet here awhile back and the input was a 12AU7 if I remember correctly and I thought that was rather odd, but being a Garnet, not really.


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