Fender Cathode Cap Values

[plexi50]

This has been a really interesting experiment for me this past month. I have changed the preamp cathode cap values from 25uf to 2.2uf and listened carefully to determine which values sounded best. Im done. The 2.2uf values is killer

If you make a note and turn the bass pot up and down it you will notice that the bass travels up and down the full rotation of the bass pot

Thats nice because using the 25uf cap values all you get is BASS from CCW to CW

Using the 2.2 uf cap values allows the treble and mids to really blend much better and best of all the bass is actually functional and not over bearing

[HotBluePlates]

Sometime in about 1996, I had a '67 Princeton Reverb that I opened up and measured the value of every component.

I noticed that all of the "3-legged critter" caps (cathode bypass dual-caps, with two + leads and one - lead) had dried up somewhat and reduced in value. The typical range was something like 1-8uF for each bypass cap.

I thought the amp sounding amazing stock. Being new to amp tinkering, I tried replacing all the bypass caps with new 25uF caps (because that's basic maintenance, right?). I don't think that amp sounded any better, but did sound a little bassier.

[eleventeen]

VERY interesting, plexi! I might ask, what type of guitar are you playing for your tone test? I've never been able to turn a AB763 "bass" control much above 3-4 without excess mud...and that is with Strats.

[plexi50]

eleventeen i have found that using one 25uf & one 2.2uf on a single preamp tube  (12AX7) tightens up the bass response and sounds 100% better getting the bass under control

Lets face it: If you have a Bassman or any other Fender amp for that matter the bass has always been all or nothing on the dial

Im 90% finished tweaking my Bandmaster and i have to say it is frickin sweet

I set out for the Deluxe reverb tone and i have to say i think i have it nailed down

Of course i have a 40 watt OT in this amp but i believe it only inhances the headroom and depth of that creamy tone all the better without the restraint and faster breakup of a 20 watt OT

I am using a Telecaster & Stratocaster. I have not yet tried my LP because i am refretting it some time in the next 5 years

Pulled the frets 3 months ago and just dont have the time now to be in an uninterupted situation to finish it

In general an LP i believe will also have it's own wonderful Mojo

Now that i have been trying the 2.2uf caps in certain preamp positions i can see why they called the Bassman the Bassman

It does make for a real bassy amp

Most all vintage Fender amps have a bass that is just to much and as well sort of defeats the scooped mids from really standing out on there own without the bass being the leader

After using lower output coupling cap values (I settled on .033) and a .1 for the bass pot all you have left is the preamp cathodes cap values to fine tune that bass response

Im still thinking there may be one more thing to add and that would be a resonance pot. I have used them in the past with great results

But i am not going to be able to drill any holes in this amp. Thats a big No/No for me and any vintage amps i own today

Steve and PRR can tell you i was a swiss cheese hole drilling fool for many years. Never again unless it's between two slices of bread  :shocked:

One more really awesome thing i did was to remove the bright switches and there guts. In place of the guts i put in a SPST switch for both channels to the old carling switch frames

I paralled a 56K resistor to the slope and ran the wires to the SPST switch to give me an even more woody tone by switching between a 100 & 33K slope frequency

Before i installed the 2.2uf preamp cathode cap the slope change to 33K was too muddy using the 25/25 values

2.2uf is King. Now i have so many different combinations of true EQ response that the amp is a dream and fun to play

Blues Heaven

[Fresh_Start]

I found measurements similar to HBP's when measuring cathode caps in a 196? Blackface P.R. although not as drastic.  The first two stages had 19uf & 17uf respectively.  Reverb driver 12uf & reverb recovery 11uf.  The biggest drop was in the PI driver stage - only 6uf.  I didn't replace any of the cathode bypass caps.

My limited experience has been that each amp/guitar/speaker/music combination may call for different mixes of coupling caps.  So far, 2.2-6.8uf appeals to me for the first preamp stage.  Over 10uf goes below guitar frequencies when coupled with a 1.5K cathode resistor.

And don't forget new caps vary +/- 20% usually.  Not important with 25uf, but it could make a difference with something as small as 2.2uf or .68uf.

Cheers,

Chip

[jjasilli]

This has been a really interesting experiment for me this past month. I have changed the preamp cathode cap values from 25uf to 2.2uf and listened carefully to determine which values sounded best. Im done. The 2.2uf values is killer

I agree and like 1uF even better.  It seemed my Princeton & Champ really sprang to life with 1uF.  I think it yields clarity by reducing muddiness.  For regular guitar there's not much going on below say 100 Hz.  And the 2 bass strings have enough power to come across anyway.  With standard size caps the amp is waisting energy processing ambient noise. 

However if the amp will be used for bass, keyboard, or de-tuned guitars I guess the standard size caps make sense.  Leo had to please a wide market.

I have a feeling this could be a provacative topic.

[plexi50]

Quote: It seemed my Princeton & Champ really sprang to life with 1uF.  I think it yields clarity by reducing muddiness

Your explanation is right on what i experienced and better stated

Heck now you got me thinking of trying a 1uf here or there

All i can say is my amp is producing an Ampeg Gemini or Gibson Crestline series tone with nice breakup and a larger headroom of course

Ole Hickory!

[jojokeo]

Don't forget that if you bias the first stage w/ a 25uF you can minimize those lows w/ a much lower coupling cap of around .002uF instead of the standard .02uF. You supposedly get the added hum rejection benefit from a fully bypassed first stage according to a respected author. But, there's always several ways of skinning the same cat. Also, put your voltages and info into Merlin's load line plotter program and you can "see" the frequency curves you're discussing and listening to, and it can save you a lot of time in your tweaking change-outs and re-testing.

This subject is coinciding w/ an amp I just finished where it has three voicings similar to Plexi's mentioned except that mine's done w/ biasing instead of tone stack manipulations. It's quite interesting, fun, and eye-opening switching between Fender and Marshall modes on the fly. There's more than just bass response to be found as it affects so much more. Biasing towards cut-off and saturation has it's own yin & yang.

[plexi50]

Preamp tube biasing?

[jojokeo]

Preamp tube biasing?

Yessiree. 

[J Rindt]

Preamp tube biasing?

A "preamp" tube must be biased, same as a "power tube", or any other triode, pentode, tetrode. You may not think of it because they are usually cathode biased. But the bias is taking place none the less......

[plexi50]

I understand the peramp tube biasing. What i ment to ask is were you just using different cathode resistor values and switching between them instead of using different bypass cap values?

[jjasilli]

Yes the tube is biased by the resistor, not the cap.  More to the point, our delight with low bypass cap values can be measured.  The values of the plate resistor, cathode resisitor and bypass cap form a circuit with a predictable frequency repsonse.  E.g., my circuits used (if I remember ) 220K plate resisitor, 3.3k cathode resistor & 1uF bypass cap.

Someone posted a calculator or chart for this, but I can't find it.  If you're out there please point us to it.

No doubt a certain combination of those values has a resonance that works well with a particular amp.  ANYWAY, the point is that if we can calculate the frequency range we like, then it should work with different gain configurations.  I.e.:  I like a 1uF bypass cap with a 220K plate resisitor, 3.3k cathode resistor.  What cap value matches that frequency response if other resistor combinations are used, such as: 100K plate 1.5K cathode; 330K plate 2.2K cathode, etc?

[plexi50]

I want to get into more of the math in calulating different frequency data. Would be good to get it all on paper and into the computer library

My memory is already taxed

[Fresh_Start]

Someone posted a calculator or chart for this, but I can't find it.  If you're out there please point us to it.

When in doubt, check the "Useful References" sticky thread...

Oh heck, here is one:
http://ampbooks.com/home/amplifier-calculators/cathode-capacitor/

BTW once you figure out how the frequency-reactance chart works, it can be a dandy tool.

Cheers,

Chip

[topbrent]

Here is one chart.

[topbrent]

And another.

[topbrent]

one more

[J Rindt]

With Merlin's red line chart, you can see the stage is pretty much fully by-passed with a 2.2-4.7 mic capacitor. That is, as far as guitar and human ears are concerned. That is a lot of great info. Thanks for the post......

[plexi50]

Wow! Thanks all. This info and handy calculator with charts is like having a new amp to play with.  Great!

Now as far as the frequency goes, are the higher frequencies less bass and the lower frequencies more bass?

So from what i gather i am at 93HZ using a 1.5k resistor and 2.2uf bypass cap right?
 

[tubeswell]

Yeah there's another chart somewhere that spells out the frequency of different bass and geetar notes for you. If I can find it in my stash, I'll post it.

[jjasilli]

When in doubt, check the "Useful References" sticky thread...
Thanks Freshstart!  That calculator is what I was looking for. 

[Fresh_Start]

At the risk of sounding like a broken record, see the PDF attachment to the "References" sticky for frequency of guitar notes.  Reply #2.

Cheers,

Chip

[Pleximan]

I want to get into more of the math in calulating different frequency data. Would be good to get it all on paper and into the computer library

Hey plexi50,

F=1/2*pi*C*Zk

Whereas Zk is the Cathode impedance not the Cathode resistor alone.

Assuming a 12ax7 with a 100k load

Cathode Impedance is calculated by
Rk in parallel with Rp+PL/mu+1
= 1500//1609 = 776

Plug 2.2uf / 776Zk into the above formula  = 93 hz

This is the frequency where the the cathode transcends from being bypassed and frequencies below 93hz are attenuated.

As we are including the tube characteristics in the formula you can see by plugging in a 12AT7 in the same circuit for example the frequency would be 85hz

Hope that helps

[Fresh_Start]

Pleximan - that helps a LOT.  It explains why my experiments didn't match what I was getting with the forumula just using the cathode resistor value.

Cheers,

Chip

[plexi50]

Thanks Pleximan.

[Merlin]

Yeah there's another chart somewhere that spells out the frequency of different bass and geetar notes for you.

There's one here:
http://www.freewebs.com/valvewizard2/OtherStuff.html