.1 caps between PI and output tubes:Fender BF

[bsaul]

How important, in terms of tone, are the .1 capacitors that are situated between the phase inverter tube and the power tubes, in blackface fender amps?

[Ken Moon]

If you remove them, you will kill the tone

But seriously, are you asking if the value of 0.1uF is important to the tone (yes, in that it plays a part in controlling how much bass gets through to the power amp - you can lower the value to get a tighter, less flubby bass, as is done in most hi-gain amps), or are you asking about different styles/brands of caps in this position, and the effect on tone (yes, there's some effect, but not as pronounced IMO as the coupling caps before the PI).

I used to use Vitamin T's in that position, mainly because of a recommendation from someone with golden ears, but switched back to my old standby Mallory 150s when I had a Vitamin T cap short out.

In a BF circuit, I think you'd be hard pressed to hear much difference between brands/styles in this position, but the only BF amp I've personally experimented with these caps in, is my Deluxe Reverb, so I can't testify to the effect in other BF amps.

And, experimenting is fun and relatively cheap, so try a couple of fancy caps there and judge for yourself

[labb]

Fender's Tweed Deluxe amps before the 5E3 used .05 caps in that postiton. Fender went to the 0.1 cap when they brought out the 5E3. I just built a 5B3 that has gotten a few ataboys from some players,in particular one Jazz player. It has .05 caps in all of the coupling positions..I would say give it a try and see what you think.

[Structo]

Labb,

Did you use NOS or vintage 6SC7's in your build?

I've been wanting to build an amp with octal inputs for a long time.
Can describe your amp say compared to a more common 5E3 that people are more familiar with?

I see on the 5B3 they don't even bypass the first tube!
So I assume it doesn't have very much gain? Perhaps a great harp amp?

Thanks, Tom

[labb]

I used vintage tested 6SC7's. The 6SC7 is not in production at this time. The 6SL7 tube data wise is practically the same and it is in production. Go over to the AX84 site and take a look at the thread"5D3 vrs. 5E3 Distortion." Fred ran a model of the out put of the 5B3 and posted it in that thread for me (Mac Dillard). You can see the curves. Pretty interesting..The cleans are sparkling and the break up to distortion is very smooth.

[orrong65]

A little bit off topic, but I am experimenting with the coupling caps in a single-ended champ style amp (see schematic). The issue is the same - 0.1uf coupling caps and their effect on tone and gain.

The amp tone is a bit muddy, especially at high volumes. I have changed the 0.1 coupling cap in the second stage to a .022 (similar to the Champ 600), which reduces the mud a bit, but it drops the overall volume level as well.

I presume these caps are set at 0.1 to pass the whole signal, just as they are in the blackface PI?

[Alexo]

Hey guys, here's a very handy link: http://www.opamplabs.com/rfc.htm

As you can see, a .1 cap with a 220K grid-to-bias R gives you a high-pass filter that starts at 7 hertz! A .022 only brings you up to 32hz, which even a 15" woofer will struggle to reproduce.

FYI, the low E is somewhere around 75 hertz, give or take.

I think this is more of a philosophical design choice in the old Fenders than a tonal one. Their approach seemed to be to do all of the tone shaping in the preamp, and to leave the pi/power stage as clean and hi-fi as possible (at least by the bf era).

Having said that, the lower that high-pass filter starts, the less phase shift you get in the upper frequencies, but since when do we care about phase shift in guitar amps?

It is a common mod for 5E3's to swap out those big ol' caps for something that will tame the low end a little more, and honestly, I think BF deluxes could probably use some help in that department too. 6V6's can get a little flubby in the low range, especially without nfb, as in the early deluxes.

You aren't going to hear too much of a noticeable effect though until you get up to maybe .01uf caps or so.

[David Root]

Since you are playing with the time constant, you can alternatively reduce the traditional 220K bias grid leaks to 100K. This may reduce the output a little when cranked but it will cut the farting out (blocking distortion) that may rear its head with 0.1s and 220 Ks cranked up.

[tubeswell]

Another way of reducing the fartiness is to put small grid stoppers at pin 5 of each of the output tubes. 1k5 to 4k7 ought to do it. try 1k5 first.

[Roe]

how can the grid resistors reduce bass flab?

personally I like .1/68k and .022/220k

[Ken Moon]

The grid stopper and the input capacitance of the tube form a low-pass filter.

To hear any roll-off effect from power-tube grid stoppers on typical BF amps, I've found you have to go up to about 10k - but my ears are shot, so what do I know

edit: this obviously doesn't address bass rolloff, but high-end rolloff (duh) - I think the grid stoppers can reduce blocking distortion, but that's not the same as reducing flub.

[Roe]

The grid stopper and the input capacitance of the tube form a low-pass filter.

To hear any roll-off effect from power-tube grid stoppers on typical BF amps, I've found you have to go up to about 10k -...

agreed!

[tubeswell]

What I meant by 'fartiness' was blocking distortion, not bass flab. The grid stoppers inhibit grid current, that may otherwise occur at the grid of the output tubes as a resulting of the longer discharge time of the (bigger/.1uF) caps affecting the bias of the output tubes, and thus the grid stoppers can help in avoiding blocking distortion. JM2CW

[Ken Moon]

flub, flab, fart...

Are we talking guitar amps or too much beer and mexican food?

As my Grandpa used to say, "If we had some pineapple with this, we could make Hawaiian music"

I'm not sure why I find that so funny...

[tubeswell]

flub, flab, fart...

Are we talking guitar amps or too much beer and mexican food?

As my Grandpa used to say, "If we had some pineapple with this, we could make Hawaiian music"

I'm not sure why I find that so funny...

Food for thought.

(or music for digestion)