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Old 4th August 2005, 03:47 PM   #1
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Default fender clone, 6SJ7 vs. 6SN7

hey guys, first post for me!

i recently bought a tube amp chasis assemble pulled from an organ.

i want to turn it into a fender amp clone. heres the schematic im hoping to go with...

now, present are two of the three listed tubes. the third one needed is a 6SJ7, but I have a 6SN7. What is the difference in those two tubes? I'm going to do some research, this being not my first amp, but my first tube amp. also, do you suppose that since this is a working amp to begin with, the existing transformers should be compatible in the above schematic? am i assuming too much in that statement? Thanks!
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Old 4th August 2005, 05:33 PM   #2
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A nice-looking chassis, though may be some work before it sings again.

> a 6SJ7, but I have a 6SN7. What is the difference in those two tubes?

SJ is a single pentode. SN is a twin triode.

First: trace the existing schematic. After all, it IS a musical instrument amplifier already. It seems likely that 6SN7 plus 6V6 could be a good guitar-amp. In fact most of the classic Fenders used a twin-triode input stage, not a pentode. It takes $0.05 more parts to do a two-triode preamp than a single-pentode preamp. But the triode has less noise, and the added stage allows a better gain structure (you don't have to put the volume control on the 6V6 grid, ugh). Also gain-per-stage and impedances are lower, it is less likely to be unstable.

Before you get a lot further, verify that the organ-amp output transformer has a secondary impedance that suits available gitar speakers. It probably does, but there were some odd things made. If not, you will be buying a Champ OPT.

Assume the can-caps, and several of the wax/paper caps, are sick. Don't bet on the tubes being good: they may be fine, or may be ill.
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Old 4th August 2005, 06:41 PM   #3
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the OPT is rated for a 8ohm speaker, so that should be ok. So what you are saying is, it is probably better to leave it how it is, keeping the 6SN7? How will that effect the schematic? Sorry I'm new to this tube stuff. I planned on new tubes and caps/resistors as well. thanks.
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Old 4th August 2005, 07:05 PM   #4
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> it is probably better to leave it how it is, keeping the 6SN7?

It will probably work fine, with minor mods, less than a complete re-wire. My advice is: get it working before doing anything major. (I rarely follow my own advice....)

> I'm new to this tube stuff.

Then you need to know Frank's:

6SN7 pin-out:
Click the image to open in full size.

This is bottom-side of the socket, where the wires go.

The 6SN7 has two units. One of the Plates connects through a capacitor to the 6V6 Grid. The other unit's Grid goes to the input jack. Both Plates have ~100K resistors to a power filter capacitor. The Kathodes go through a ~1K resistor to ground. There is more, and there may be more complications like a gain-pot, but it is pretty easy to sketch a tube amp from inspection.

When you smoke-test: the 6SN7 Kathodes will be at about 1 or 2 volts. Their Plates will be very roughly 100 volts. 6V6 Cathode will be +15V to +25V, Plate and G2 at +200V to +350V. All grids MUST sit within a half-volt of zero: most likely fault after a no-voltage in the power lines is a high positive grid voltage due to a failed coupling capacitor.

I am very glad to see that fuseholder. Don't over-fuse it. 1A may be enough. With 5Y3 out of socket, it should power-up without blowing the fuse, and the 6SN7 and 6V6 should glow soft red in the center (proving you have 6V to the heater). (Some 6V6 have a black coating inside to hide the heater glow: it will get warm in a minute.) Try again with the 5Y3. Until the 5Y3 warms up, it should work the same as before. When the 5Y3 warms up, you will find out how sick your high-voltage system is. If a cap is dead-short, it may pop the line fuse. If it is only sorta-shorted, it will heat up and burst after a few minutes: do this outside and keep your face away. If the HV is good but the 6V6 grid or cathode capacitor is shorted, the amp will run but the 6V6 will run far too hot. It is normal for a 6V6 to burn your skin, so it can be hard for a beginning toob dood to know what is too hot. Check the cathode voltage. If it is low and the tube is just-warm, the tube is not sucking current. If the cathode voltage is low but the tube is very hot, the cathode resistor-capacitor bias scheme is shorted-out.
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