Dumb and Dumbler

[martin manning]

Usually pins 1 and 8 are tied together for either 6L6 or EL34 operation. The 1-ohm resistor goes from the pin 1-8 junction to ground, and the bias test point is the pin 1-8 junction. Just use the resistor's lead to join the 1 and 8 lugs.

Pin 4 is the screen grid. Pin 6 is not connected internally, and serves as a tie point for the screen supply wire and the screen resistor.

You must get the phase of the OT primary correct whenever you have global negative feedback, otherwise you will have positive feedback and a loud oscillation. In the Fender color coding the blue primary lead is in phase with the secondary, and is on the same side as the input (the inverting) side of the phase inverter. The "upper" coil as shown in the transformer schematic MIGHT be the in-phase winding, but I wouldn't bet on it. If there are dots to show the phasing, then you can be pretty sure they are correct. I would test it myself, or leave the leads long enough to swap them if it's wrong.

[dreric]

Usually pins 1 and 8 are tied together for either 6L6 or EL34 operation. The 1-ohm resistor goes from the pin 1-8 junction to ground, and the bias test point is the pin 1-8 junction. Just use the resistor's lead to join the 1 and 8 lugs.

Thanks but now I'm more confused.

For 6L6 I thought that pin 1 was not connected internally and therefore is used as a tie point for the 5.1k resistor which then attaches to pin 5. Pin 8 goes directly, no 1 ohm resistor, to ground and the bias test point. This seems to be how it's done on the 102 and 124 layouts.

For EL34 pins 1 and 8 connected, tap to bias test point and then 1 ohm 5 watt to ground. This seem to be how it's done on the 183 layout.

Am I mistaken??

Thanks

Eric

[martin manning]

If you tie pins 1 and 8, and you don't use pin 1 for a tie point for the grid resistor (stand it on end), then you can run either EL34 or 6L6. In other words, wire both amps this way and you are free to swap tube types if you want to without rewiring. Here's a pic:

Edit: Drawing revised to show internal wiring of 6L6

[CHIP]

Does it matter if the screen resistors are different for el 34 and 6l6's? (1k5 and 470 respectively)

[martin manning]

I don't know how sensitive EL34's are to lower screen resistor values, and it would depend upon the supply voltages, but both types can certainly be run with 5k-ish grid resistors and 1k-ish screens.

[talbany]

Does it matter if the screen resistors are different for el 34 and 6l6's? (1k5 and 470 respectively)

Chip
EL34's in general have a wimpy screen supply. if you plan on using both go with at least 1K like Martin mentioned to be on the safe side..

Tony

[CHIP]

Thanks Tony and Martin,
I plan on using El34's, (1.5k screen ), so it's good to know I could experiment with 6L6's in the future. (of course I realize I'll have to re-bias).

[dreric]

If you tie pins 1 and 8, and you don't use pin 1 for a tie point for the grid resistor (stand it on end), then you can run either EL34 or 6L6. In other words, wire both amps this way and you are free to swap tube types if you want to without rewiring. Here's a pic:

Edit: Drawing revised to show internal wiring of 6L6

Martin

I really appreciate you taking the time to help me. However I'm still confused on the basic anatomical differences between 6L6 and EL34 tube.

Every reference I have found demonstrates the 6L6 like this

http://www.drtube.com/datasheets/6l6gc-sed1996.pdf

without a connection to pin 1 either internally or externally.

While EL34 all seem to look like this:

http://www.drtube.com/datasheets/el34-sed1997.pdf

where pin 1 is described as grid 3.

Neither drawing matches to one you provided with the 6L6 internal connections in red.

Not trying to be a pain just trying to understand the differences between the two tubes and how they are wired. Am I missing something in my interpretation of the data sheets?

Thanks

Eric

[martin manning]

Eric, no problem. 6L6's do not have an internal connection to pin 1 as you say. The beam forming plate is connected to the cathode internally, though. 6L6 and 6V6 are beam pentodes, and do not have a suppressor grid as is used in the EL34. Notice the Svetlana 6L6 diagram in the first link you posted does not show this element at all, but it is there, and it is connected to the cathode internally as I drew it. My red X means that the internal connection from grid 3 to pin 1 as shown (the diagram is an EL34) is not made in the 6L6. In the 6L6 diagram below, which is from an old GE data sheet, the beam forming plate is shown as a third grid. 6V6's are the same.

Edit: corrected terminology

[dreric]

Martin, thanks for the patience and education, I think I have a better grasp on the tube differences at this point.

Here's the pictures of "dumb" 6L6's wired in the tradition 102-124 style, which I now understand is not a drop in for EL34s.

Feedback is always welcome!

E

[dreric]

Here's the power tube wiring for "dumbler" which is a EL34 based amp, which if I am correct in my understanding can take 6L6s with a re bias.

[martin manning]

The only caution to add viz. swapping between 6L6 and EL34 is that the EL34's use ~0.6A more filament current per bottle, so replacing EL34 with 6L6 is no problem, going the other way may challenge your PT's filament winding.

Wow, wiring two dissimilar amps at the same time... I think I'd get confused!

BTW, on the 6L6 amp I don't see any current sensing resistors between the cathodes and ground, yet there are bias test points connected...

[dreric]

This project has been on the back burner but it's still kicking along.

I'm stumped by something.

The 102 amp uses a voltage doubler circuit to feed the relays. I made the board and I'm using a 6v transformer which is giving me 17 volts into the voltage regulator. Then at the opposite pole of the regulator where it connect to the relays I'm getting 5.04v out?

The tandem build (bluemaster) uses the JB boards and a 12v transformer. It works fine.

I've tried replacing the regulator twice with the same result

Any insight? Thanks

Eric

[guitardude57]

Are you floating the CT on the tranny?

[dreric]

Are you floating the CT on the tranny?

yes

e