Sunday, December 28, 2008

Fender Champion 600 12DW7 / ECC832 Mod Part 2

Here's a look at the 12DW7 / ECC832 tube I used in the Champion 600 12DW7 Mod Part 1 .

Starting with the pin out diagram:

You can see from the diagram that, like other 12A*7 tubes, the 12DW7 has two triodes. Each triode has three connections - pins 1,2,3 and 6,7,8 respectively.

On most 12A*7 tubes (12AX7, 12AT7, 12AY7, 12AV7, 12AU7) the two triodes
in a single tube have identical characteristics.

In a 12DW7 the two triodes are different.

The section listed as Section 1 on the data sheet uses pins 6,7 and 8 and has characteristics identical to one half of a 12AX7.

The one listed as Section 2 uses pins 1,2 and 3. This section is identical to one half of the lower gain 12AU7 .

As an illustration, here's a Traynor YB1A with a 12DW7 in the first valve position:

Input 1 feeds the 12AU7 half of triode and input 2 feeds the 12AX7 half. The input is about 120 mV RMS and the meters are reading the output voltage from each of the two triodes.

The left hand meter reads 6.3 V for the 12AX7 stage output - that's a gain of 53 (6.3 V output/120 mV input).

The right hand meter reads 1.6 V for the 12AU7 stage output - that's a gain of 13 (1.6 V output /120 mV input).

So you can see the most obvious difference when swapping a 12DW7 for a 12AX7 is going to be a lower overall stage gain with the 12DW7.

This is also apparent if you look at the Amplification Factor in the data sheet (highlighted in yellow).

Amplification Factor
is related to the gain of the stage the triode is used in. It should not be thought of as the actual gain of a stage - it is actually the maximum possible gain in a stage designed to use that triode. As you can see in the measurements in the YB1A above, the actual stage gain for the 12AX7 was just above 50, even though the Amplification Factor for a 12AX7 is 100. The reasons for this are too complicated to get into here, but I'll post them independently at a later date. Suffice it to show that the 12AU7 stage has a significantly lower gain than the 12AX7 one (the combined gain for the two stages will be roughly the same as if you substituted a 12AY7 for the 12AX7).

The second and slightly more subtle difference is evidenced in what is listed as the Grid Voltage (highlighted in blue). Doubling this number will give a very rough sense of the possible input headroom available to the stage. It a very inexact number but good enough for the sake of illustration.

For Section 1 then the input headroom would be 4 volts. For Section 2 it would be 17 volts. A typical 12AX7 stage can output over 4 volts with an input of only 70 millivolts. At 120 millivolts it easily clears 6 volts:

Many guitars can output over 500 millivolts. And those voltages are in RMS - the peak to peak voltage is 2.828 times the RMS voltage (peak to peak voltage is what you really have to look at if you're concerned with the headroom of a stage).

So you can easily see that if the second stage is also a 12AX7 triode and there is no attenuation between stages, the second stage is going to be clipped. A 12AU7 has 4 times the input headroom of a 12AX7 (even more if it's run into the nonlinear part of it's curves). So using a 12AU7 as a second stage has some advantage if you're looking to keep things clean.

Plugging a 12DW7 into a Champion 600 actually places the 12AU7 section before the 12AX7 section, which is why the first stage is the one I rebiased in Part 1 of this posting.


Andorephus said...

Concerning the last paragraph; with the lower gain stage coming first in the Champ circuit, how will that differ from your earlier explanation which involves the higher gain stage first? Will there be any difference after the 2 stages?

akavalve said...

Hey Androphus,

I was wondering who might catch that! I'd been meaning to make a post of this - probably still will eventually.

I'd originally thought that the "section 1" of a 12DW7 started, logically it seemed, with pin 1(the plate). Looking more closely at the V4 schematic I realized that "section 2" plate is actually on pin 1 and that the section 1 has it's plate on pin 6. A bit backward and a reversal of the order I orginally thought they were in.

The European number for the 12DW7 is ECC832. JJ makes an ECC832 and also offers an ECC823 (the same tube with the sections reversed). Unfortunately it's a special run with a minimum order of $1000.

I did find one place (Radical Music) which carries a very broad selection of JJ's and actually had a stock of the ECC823's. I bought a few of them to try out.

I've had a chance to compare the ECC832 and the ECC823 briefly in the Champion 600 and the Valve Junior. I didn't rebias the stages - just popped them in.
The tests were quick but the initial report is that the ECC832 and the ECC823 sound pretty much the same in both those circuits. I'm getting ready to do some work on a pair of Valve Juniors and hope to make a more thorough investigation of the 832/823 a part of it. I'll let you know what I find.

Best, Andy

Ed Storer said...

It seems to me that the ECC832 or 823 would be suited for use in a cathodyne circuit as phase splitter & final gain. One side of the tube works at unity - so something with the gain and current capacity of a 12AU7 would work great and the other side is a gain stage where a 12AX7 would be desirable.

I have 2 amps based on a Princeton Reverb circuit and was thinking of trying it. On the other hand, it is probably a waste of money since 12AX7's are pretty reasonably priced.

I have a circuit diagram for one of the amps (Allen Sweet Spot) and the 12DW7 has the pinout backwards from what I'd need - 1, 2 & 3 are the phase inverter and 6, 7, & 8 are the final gain stage. I'd need the ECC823.

lookah said...

No one sells Ecc823's because JJ will only produce them in batches of 1000. So rewiring the tube is necessary. Just rotate it so you can keep the same leads. Takes 5 minutes. I'm messing around with a 12dw7(ecc832) in my princeton reverb. sounds good. but you need to change resistor values though to accommodate for the 12au7 side.

akavalve said...

I got mine from Radical Music:

Radical Music Inc
10031 N 31st Street
Phoenix, AZ 85028-4408
Phone:(602) 788-6499

They had a very broad stock of the JJ line a while back at least.

I actually have a few new ECC823 left from that order. If they don't stock them anymore I'd be glad to sell you one if you'd like to try it out. Just send me an email and we'll work it out.


Anonymous said...

Anyone tried using them in a circuit where one side is a gain stage, and the other a cathode-follower buffer (e.g., Marshall)? Seems to me the 12AU7 side would work well in the buffer part, leaving the gain stage more or less unchanged.