Preamp 101 Fender Champ

[captntasty]

I was looking up 12ax7 substitutes and came across a tube that was 12ax7 on one side, 12ay7 (I think) on the other - I think it is a 12dw7/ecc832. JJ and a couple other manufacturers are now making them again and there is even a ecc823 from jj where the sections are reversed.

Now the champ has only one preamp tube / 2 sections (obviously) - I am curious - what functions each section plays and how the different gains in each section effect the sound? - it seems harp players are mostly interested in these puppies.

Mostly, I just want to learn the inner workings of my amps...
Many Thanks
Patrick

[MWJB]

12DW7/ECC823 is typically a 12AX7 at the 2nd triode (pins 6, 7, 8) and a 12AU7 at pins 1, 2, 3. As you say JJ can make them the other way round.

They used be used in Ampeg PIs.

As a harp player myself, I sometimes like them in V1 of a Tweed bassman (no bright cap)/2 channel tweed amp, which gives the 2 channels different gain structures & characters, the lo-mu triode can be handy for very hi output Hi-Z dynamic mics/Shure CM & CR. The AX side can be good for moderate output crystal/ceramic, lo-Z dynamics with a lo to hi-Z transformer and lower volume situatuions where a thicker, more compressed tone might be desired.

Some harp players like them as PIs, for a lo fi, unbalanced LTPI sound.

In a 5E3 they could be used in V2 to drop some gain right before the PI.

For harp in champs, maybe worth a try, but that's a big gain drop on a circuit that only has 2 triodes to play with. Perhaps handy for plug in/plug out changes in non-mod friendly amps/the odd mic with a huge output (I'm talking tens of volts at the first tube grid). The lowest gain tube I can get on with in SOME small SE amps is a 5751, mostly I prefer a 12AX. I know that others like 12AY. I generally prefer the less severe gain drop you get with removing cathode bypass caps & using a higher mu tube.

Think of the overall mu of the 2 triodes in a champ/typical tolex Fender preamp tube as being multipliers, e.g. 19x100 = 1900 (it's not quite that simple but you get the idea), compared to 10,000 for 12AX, 4900 for 5751, 3600 for 12AT7 & 1936 for 12AY7. I find a 12DW looser sounding than a 12AY in an A/B comparison.

In a 2 channel tweed, where each half V1 is the input stage for each channel, the gain drop is less severe.

I suspect (though have never tried) that 12DW in a champ for guitar would be far too loose & lacking in dynamics with 19mu at the first triode, the reversed version (100mu at the input) MIGHT be feasible with a low loss preamp like 5F1 & big output humbuckers, but I'm speculating. Again, circuit mods to tailor gain & frequency response maybe more practical.

Be aware that for guitar, that the 12AU triode draws a lot more current & has a much bigger voltage drop accross the plate resistor (a 12AU will usually show around 35% of the plate voltage of a 12AX in a typical Fender triode), in some cases hard use could burn out the stock resistor. So in a Super Reverb (assuming you could ever get a usable sound), if you had say 260v on a 12AX7, that would be around 90v-ish on a 12AU and 292v dropped accross the 100K plate resistor = .852W.

[Enzo]

I am sure you already saw the 12AT7 as a lower gain alternative for the 12AX7. You might also consider the 5751 - which falls between those two. It is a popular alternative for mellowing out a stage.

Your 12DW7/ECC832 is also called a 7247.

[captntasty]

Ahh, so I get why it's a harp player thing now - mic impedance and sound for 2 different channels/inputs.

I am sure you already saw the 12AT7 as a lower gain alternative, etc...

Yes. I've used 12at7's, 6021's (industrial 12at7), 5751's... it's a sickness, I can't stop My collection is steadily growing. It's good to know that if I'm looking for a certain sound, say, a little chunkier or round I go to the 5751, for clean-clean and pretty flat I go to a 12at7... then there is my favorite: Amperex 12ax7 - a little edge, airy, sort of chimey.... I think I've settled on the fact that a 12ax7 is necessary in the Champ unless I'm looking for something very specific.

Anyhow, I do love trying different tubes - it's amazing how different they can be! I am really interested in what each half of the tube is doing - are they both gain stages? From what I gather, there would not be a phase inverter in an SE amp - nothing to invert, right? So are the two halves a cascade - first stage into the second then on to power tube? Just curious about the more technical aspect of an amplifier. (If you can't tell, I'm not even close to being any sort of EE)

Thanks
P

[Alex R]

Essentially harp players need to drop a lot of gain from a typical guitar amp, because their mics are (a) much higher in output than a guitar p/u, and (b) much more prone to sonic feedback. Which is why more 'traditional', vintage-type amps are popular with them. Well, ok, us, I play harp too.

Also there's a need to restore the mids, usually scooped out by a guitar tone stack. And cut some of the highs (but I like the treble-down-presence-up sound).

Actually though, all such efforts really just aspire to the condition of an old tube PA with high-Z mic inputs, which is what your typical 40s/50s blues harp player used every night, because it was right there on the stage with a taxi despatcher's bullet mic, or a high-z Shure stick as the 60s approached, plugged in ready and waiting. Those were the days.

I guess many harp players end up as techs because if you want good amplification and can't afford custom prices you just have to learn to build your own.