Champ with 12DW7 preamp

[Baguette]

Hello,

I'm wondering if someone ever designed a Champ (5F1) with a 12DW7 instead of the regular 12AX7?

I've seen posts on the interweb mentionning people who did swap the AX for a DW in their regular Champ, but it just does not make any sense to me since a regular Champ layout with a 12DW7 preamp would put the low mu triode first then the hi mu triode second.

I'm actually thinking about designing it like this:

Hi mu > volume > low mu > 6V6

On paper it would be great. Especially since I think 5F1 have too much gain to start with, which is not very musical when cranked (mostly that second 12AX7 triode getting smashed up big time). However, I really like them about halfway on the volume dial, + or - depending on pickups etc. Anything above 3 o'clock is just a harsh mess IMO.
Putting a low gain / high current triode as a 2nd gain stage may surely help the tone being higher headroom, bigger and less harsh. Gain could easily be tweaked with plate load and cathode resistors, bypass cap and NFB applied to the 2nd triode.

Thoughts? First hand experience?

Thanks for reading.

[sluckey]

There is a version of the 12DW7 that has the triodes reversed.

[vibrolax]

There is a version of the 12DW7 that has the triodes reversed.

That would be the JJ ECC 823.  The "normal" 12DW7 is the JJ ECC 832.

[jojokeo]

Hello,

I'm wondering if someone ever designed a Champ (5F1) with a 12DW7 instead of the regular 12AX7?

I've seen posts on the interweb mentionning people who did swap the AX for a DW in their regular Champ, but it just does not make any sense to me since a regular Champ layout with a 12DW7 preamp would put the low mu triode first then the hi mu triode second.

I'm actually thinking about designing it like this:

Hi mu > volume > low mu > 6V6

On paper it would be great. Especially since I think 5F1 have too much gain to start with, which is not very musical when cranked (mostly that second 12AX7 triode getting smashed up big time). However, I really like them about halfway on the volume dial, + or - depending on pickups etc. Anything above 3 o'clock is just a harsh mess IMO.
Putting a low gain / high current triode as a 2nd gain stage may surely help the tone being higher headroom, bigger and less harsh. Gain could easily be tweaked with plate load and cathode resistors, bypass cap and NFB applied to the 2nd triode.

Thoughts? First hand experience?

Thanks for reading.

Agreed that too much "bad distortion" doesn't equate to good tone. But you must think why and what kind/type or the cause of this is to either remedy or control it differently? Apart from your idea have you ever tried using voltage dividers to help cure your angst!? Try putting a 220K/220K limiting your plate output after your existing 2nd stage and see what this does? I've used this technique many times in various situations which works like a charm on many occasions. Those values gives you half signal voltage output. It helps keep blocking distortion away but doesn't harm the overdrive/distortion character in many situations. It will also help your volume control's range become much more useful as a result too. You can try different combinations too, 100K/220K for a little higher output or 470K/220K for less than 1/2. Just remember the signal has to come out of the middle where the two join. One end from your tube's coupling cap (dropping resistor), and the other (the load resistor) goes to ground.

[kagliostro]

What Jojokeo say

an alternative is to split load resistor and take signal at the junction of the two resistors

http://www.blueguitar.org/new/articles/blue_gtr/amps/bp010629.pdf

Franco

[drgonzonm]

Hello,

I'm wondering if someone ever designed a Champ (5F1) with a 12DW7 instead of the regular 12AX7?

I've seen posts on the interweb mentionning people who did swap the AX for a DW in their regular Champ, but it just does not make any sense to me since a regular Champ layout with a 12DW7 preamp would put the low mu triode first then the hi mu triode second.

Hi mu > volume > low mu > 6V6

Thanks for reading.

 

While I am as ugly as your neighborhood gargoyle, I believe you have correct concept.  Draw up your concept in JSCHEM and post.  I would provide the design for both the ecc823 and the ecc832.  and call it a PMAHC or cramp.   If you make it switchable, maybe you can patent it. (Bad idea, my gargoyle is showing,  patents are expensive to generate.)   

[Baguette]

Hello,

Thanks for the voltage divider / split plate load suggestions (I've used them numerous times - also tried NFB, all successfully).
But the whole point of the 12DW7 approach is to have that high current triode right before the power tube, which an AX is not.
I guess I'll have to try it myself. I usually like an AU later on in the circuit, so why not?

[drgonzonm]

Hello,

Thanks for the voltage divider / split plate load suggestions (I've used them numerous times - also tried NFB, all successfully).
But the whole point of the 12DW7 approach is to have that high current triode right before the power tube, which an AX is not.
I guess I'll have to try it myself. I usually like an AU later on in the circuit, so why not?

I have an M8e, rickenbacker amp, of of some what similar design to the the early champs, I replaced a 12ax7 with a 12au7, and found the amp darkened.  You might like the sound. 

[jojokeo]

If you make it switchable, maybe you can patent it. (Bad idea, my gargoyle is showing,  patents are expensive to generate.)

Do you really think this is a new and novel idea thought up before anyone else?

I replaced a 12ax7 with a 12au7, and found the amp darkened.  You might like the sound. 

Not likely, the 12au7 has a much greater bandwidth (more than 5 times) & frequency response range over a 12ax7.

[HotBluePlates]

I replaced a 12ax7 with a 12au7, and found the amp darkened.  You might like the sound. 

Not likely, the 12au7 has a much greater bandwidth (more than 5 times) & frequency response range over a 12ax7.

+1.

The 12AU7 has a lower internal impedance, and if you also use a lower-than-12AX7 plate load resistor (which stills nets almost all the gain), then the output impedance of the stage is likewise reduced (2 ways). Lower Zout generally equals extended treble range (though it won't make an amp "bright" all by itself).

[drgonzonm]

If you make it switchable, maybe you can patent it. (Bad idea, my gargoyle is showing,  patents are expensive to generate.)

Do you really think this is a new and novel idea thought up before anyone else?

I replaced a 12ax7 with a 12au7, and found the amp darkened.  You might like the sound. 

Not likely, the 12au7 has a much greater bandwidth (more than 5 times) & frequency response range over a 12ax7.

patentable: a simple swap on the triodes might be, all it takes is determining whether someone has patented the concept,and paying that $70,000 for the patent. 

The darkening of the amp when the 12ax7 was replaced  by the 12au7 on M8e was my observation.  Please note, the published schematics are wrong for the m8e   

[HotBluePlates]

... The darkening of the amp when the 12ax7 was replaced  by the 12au7 on M8e was my observation.  Please note, the published schematics are wrong for the m8e

Taken at face-value, you may have heard simply less-distortion.

People often claim triode mode operation of their output tubes sounds darker than pentode mode, after adding a switch to select between the two. Often, careful measurement shows what they really heard was less distortion, where fewer added harmonics also meant less added treble in the sound.

Where you heard "darker" many how swap a 12AU7 in place of a 12AX7 hear "brighter". It's application-dependent.

[drgonzonm]

The M8e amp is designed as a warm amp, to be coupled with a Hawaiian guitar.  regular guitars can sound tinny on te M8e.   IMO, with a 12au7, it is nowhere near as warm on an electric guitar.  (tested with a mini strat).