Champ question

[worth]

Why is a 2uf used on V1 pin 8 instead of the usual 25uf ?

[kagliostro]

A low value cap offers a low bypass to the AC signal and lower the gain

also a lower value give a different frequency response to the stage

Kagliostro

[stingray_65]

Cathode bypass caps are a favorite place of mine to start when shaping the voicing of the amp.

As K suggests, you can dramatically change the gain to begin with.

And again you can change the frequency roll-off.

It is just a guess,(as to why Fender used this value) but I suspect that with a humbucker equipped guitar the 2 uF would raise the low end roll off making it less "muddy". So again  humbuckers have a high output and the 2 uf  lowers the gain a tad maybe not hitting  V2 as hard.

I'm not sure when the AA764 was designed, but wasn't it around '72 that Seth Lover's WRHB was getting put into Fender guitars?

The history is all conjecture, and you did ask why.

If you had asked how the change affects the amp, K is correct in his reply

[sluckey]

I suspect it's related to the fact that that tube is also the injection point for the NFB loop. Both the other tubes still use Fender's typical 25uF.

[worth]

I wonder if this would be a good spot to add gain to a Champ.

[LooseChange]

I never noticed that before. I've always put a 20uf in there.

[worth]

I never noticed that before. I've always put a 20uf in there.

THAT is VERY funny!

[sluckey]

I just noticed that some of the later champs went back to a big cap.

[Boots Deville]

Yeah, I never noticed that either.  I even checked a few sources of that schematic to make sure the one posted wasn't an altered one, because based on the spacing of the label, it looks like the method of editing the schematic was applying some white-out over the "5" in "25".  (maybe that schematic pre-dates white out.)

[worth]

CBS Champ.. clearly 2uf

[worth]

Loosechange... I know you've been building desktop Champs. I guess you've unknowingly been making "high-gain" Champs ?

[Fresh_Start]

Don't forget that Champs had 8" speakers in little cabs plus small output transformers.  That's not a recipe for great bass response.  Given that Fender used 25uf for virtually all cathode bypass caps, there clearly was a good reason to go with 2uf here. 

Also note that the tone stack has the Fender standard .047uf mid cap and .1uf bass cap.  That's fine if you're feeding four 10" speakers in a big cab like the Super Reverb and want fat, thick tone.

I'll bet that LooseChange is plugging his Desktop Champs into bigger cabs with 10" or 12" speakers, and I know that he uses much "better" quality output transformers that can handle bass frequencies with less muddiness.  I'd bet that his tone stack cap values are lower too.

Cheers,

Chip

[PRR]

I have a suspicion that the stage-2 cathode cap makes nearly-no difference.

If you have one open, hay-wire it with 220u, 2u, and no cap at all. Have a friend listen. Can a blind listener reliably tell one from the other?

The only real tweak is the 2700 NFB resistor. With sturdy speaker, 10K or 27K is some more gain and significantly more "raw".

[jjasilli]

I think the cap is needed to get NFB signal up and around the 1500R cathode resistor.  With no cap, move the NFB connection to the top of the 1500R cathode resistor.

[PRR]

> to get NFB signal up and around the 1500R cathode resistor.

1.5K isn't huge; you don't think the NFB will get through?

> With no cap, move the NFB connection to the top of the 1500R cathode resistor.

Major change of NFB ratio; some change of DC bias.

I think Fender hit it pretty good, except a bit tame to suit the ~~1970 fashion and the small/inexpensive speaker Champs got. Keep the cap, loosen-up the 2700.

[Fresh_Start]

Drifting off topic, the NFB ratio is 1.7% correct?
47/(2700+47)
That NFB ratio is much less than the tweed Champ NFB ratio of 6.4% (1,500 / (1,500 + 22,000).

In the Blackface/Silverface circuit, does the NFB ratio apply equally to all frequencies or only those above 53 Hz? (which I think is the cutoff frequency for 1.5K || 2uf)

Cheers,

Chip

[HotBluePlates]

Yes, but the cathode resistor is bypass for frequencies above the turnover frequency for 2uF and 1.5k, which doubles the open loop gain. You could probably say that the 6.4% would have to be halved (to 3.2%) to account for the increased gain, so the actual feedback amount is only reduced by about half compared to the feedback in the tweed Champ.