5879 smoothing caps

[tubenit]

Any reason I couldn't use a smoothing cap from the plate to the cathode pin on the 5879?  Thought I might experiment with this to see what it sounds like?

I've discovered I like the smoothing effect of using a smoothing cap across the plate resistor AND from plate to cathode on 12A_7 tubes.


Thanks, Tubenit

[Tone Junkie]

Cool T

[sluckey]

No problem. You can do that. Another idea is to connect the cap between plate and grid. Look at V3A in this amp...

http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

[phsyconoodler]

Dumble used those.they do indeed work.I use either 270pf or 500pf on my builds.I'm sure the 390pf will be a great inbetween value.

[jojokeo]

They all will work but one I haven't seen is btwn Grid & cathode. Have you tried that also?

[tubenit]

Thanks Sluckey and others for the feedback!

With respect, Tubenit

[DummyLoad]

thanks for sharing TN.

--DL

[tubenit]

OK, I tried the 390p from the 5879 plate to cathode pins.

I like the results ............ it did add smoothness. I have heard Geezer use the term getting rid of the high end "hash" & it does tame that reasonably well. It wasn't bad to begin with but this is a creamier/smoother OD now. 

I can not tell any loss in touch sensitivity. There may be a tiny loss of high end chimeyness? I think the harmonics are about the same? Perhaps they may be a little less?

However, for some reason, it seems to increase the "blooming" effect just a little bit also where the sustained notes swell.

I still have the .001 across the plate resistor but simply added the 390p smoothing cap also.

With respect, Tubenit

[Geezer]

Cool! More stuff for me to try!  

Thanks for the update, T!

G

[38Super]

Thanks Sluckey and others for the feedback!

Speaking of feedback - that's what you've got workin' there with a cathode to plate cap.  Your gain stage has NF effective at higher frequencies.  You could also add a resistor in series with this small cap to adjust the magnitude and frequency of the smoothing effect.


rob

[Fresh_Start]

Thanks Sluckey and others for the feedback!

Speaking of feedback - that's what you've got workin' there with a cathode to plate cap.  Your gain stage has NF effective at higher frequencies.  You could also add a resistor in series with this small cap to adjust the magnitude and frequency of the smoothing effect.


rob

Trying to clarify for myself - a small cap across the plate resistor is a high-pass filter to (AC) ground.  I guess you'd say it's a "low pass" filter in terms of the signal really.  The cap between the plate and the cathode also is a high-pass filter to (AC) ground (assuming a cathode bypass cap).  However, the plate to cathode cap also introduces negative feedback because the signal on the plate is out of phase with the signal at the cathode.  The only resistance it is in parallel with is the internal anode-to-cathode resistance.

Have I got that right?

Another thought - isn't the signal on the plate normally much greater in terms of voltage swing than the signal on the cathode?  That must have some implications in terms of how the negative feedback affects the  signal.

Chip

[Tone Junkie]

Tubnit are you trying this on the 5879 in the TOS or the bluesmiester hybred you made with the 5879 tube.
Thanks Bill

[tubenit]

I only have two amps now.  The Tweed Overdrive Special with 5879 in the OD. The TOS now has that one knob TBM tone stack instead of the 5E3 tone stack.  So it's in that amp.   And the Tweed BluezMeister which only has 12A_7 tubes and no 5879.

With respect, Tubenit

[tonewood]

" However, the plate to cathode cap also introduces negative feedback because the signal on the plate is out of phase with the signal at the cathode.  The only resistance it is in parallel with is the internal anode-to-cathode resistance.

Have I got that right?

Chip "

I'm thinking that you are correct when you say:

"The cap between the plate and the cathode also is a high-pass filter to (AC) ground (assuming a cathode bypass cap)"

Since the high is passed to ground, it won't introduce neg feedback. If you lifted the cathode bypass cap then it would introduce nfb.- I think??

[JayB]

" However, the plate to cathode cap also introduces negative feedback because the signal on the plate is out of phase with the signal at the cathode.  The only resistance it is in parallel with is the internal anode-to-cathode resistance.

Have I got that right?

Chip "

I'm thinking that you are correct when you say:

"The cap between the plate and the cathode also is a high-pass filter to (AC) ground (assuming a cathode bypass cap)"

Since the high is passed to ground, it won't introduce neg feedback. If you lifted the cathode bypass cap then it would introduce nfb.- I think??

Correct. There might be some feedback based on the ESR of the cap, but it's ever so slight.

[tubeswell]

Chapter 2 of Merlin's 1st book has a summary of treble-cut and anode bypass cap options.  He notes that the option with the parallel cap and plate resistor does allow for a path for HT noise to be coupled to the plate, but that in a pre-amp stage this aspect might not amount to much. Something to think about if you find that option too 'noisy' I guess. JM2CW

[jojokeo]

Depending on playing style ofcourse losing too much high end frequencies can quickly help make an amp sound dull and suck the real tone, life, and soul out of the guitar, so one must be very careful here. It'd be a shame when so much care, thought, & money is made while selecting tone woods, pickups, strings, speakers, tubes, etc. and it's all being colored or muffled and can hardly be distinguished or heard clearly the way it should be. When an amp is properly fine tuned and the mark has been hit, it plays equally well and clear w/ single coils of Strats and Teles to the humbuckers of a Les Paul or semi-hollow body. And the same with whatever speaker & cab is being used meaning it sounds the way that instrument or speaker is supposed to sound like and you know it when you feel it resonate in your hands and sustain in your finger tips. This is just MY definition of "perfect".

A similar analogy of sound waves through air is the sun's energy, intensity and light spectrum as it penetrates the ocean. The first parts of the light spectrum to go is the the red, yellow, and orange. This represents audio highs to mids. Within the first five meters the red begins to be filtered away, w/in 15 the yellow, and so on until only the blue is left which can go much further, up to 150 meters the same way the lowest bass frequencies can travel and be heard so far away. Sound waves are the same. The uppermost, sunlit layer of the ocean is where 70 percent of the entire amount of photosynthesis in the world takes place and is called the euphotic zone. This upper region so abundant with life is the place to strive to explore and appreciate the true character and fine nuances of the tonal spectrum, careful not to get too close and fall into the muddy, dark, and undefined lower depths of the aphotic zone. :)

[Geezer]

This upper region so abundant with life is the place to strive to explore and appreciate the true character and fine nuances of the tonal spectrum, careful not to get too close and fall into the muddy, dark, and undefined lower depths of the aphotic zone. :)

Hmmmm, are you making some backhanded statement about Bass players? 

[jojokeo]

  Yeah, just sort of kiddin around a little to break up the monotony but there is a bit of truth too. It is a fine line when a treble bleed cap can give such an improvement in clarity but be too much and w/out it it's too dark and unclear say on a simple 1M Vol control. Then I'll try a 47k or 100k in series w/ the cap and that seems to be ticket. It's very sensitive at time.
But I get what tubenit is going after in getting the tone just right in a particular gain stage and keep it from being brittle and smoothing things out. Big thumbs up for the continued quest! 

[tubenit]

Jojokeo expressed some concerns about smoothing caps lessening the amps articulation and highs. So this is how I've resolved that
concern.

losing too much high end frequencies can quickly help make an amp sound dull and suck the real tone

My Tweed Overdrive Special now has overdrive that is as smooth as the Tweed BluezMeister Special.  And the TOS has better harmonics, sustain, and blooming with a vowel tone. I still like the clean better on the TBM, however.

I am continuing to learn about tweaking amps and thought I'd share this idea. I find the smoothing caps to be pretty essential to get a super smooth overdrive in this TOS amp. However, as mentioned, it does take away a little of the high end frequency and perhaps lessens the high end harmonics also.

So here is a before and after schematic of the mods. Note on "before" the coupling cap after the second gain stage was .02 and the coupling cap after the 5879 was .01.

I added a smoothing cap to the second gain stage and then experimented only to find that the smoothing cap with changing the .02 to an .01 came very close to the original clean tone BUT smoother.

Then I added the second smoothing cap on the 5879 from plate to cathode. And so I lowered the .01 coupling cap to .0047.  It should be noted that the Dumblish clones used a .0047 there anyway instead of the .01 that I had been using.  AND I added a 250p cap across the drive pot.

That allowed the OD to be super smooth BUT still retain very nice highs & harmonics. Absolutely no loss of good tone. This is the best tone that I've gotten.

And this is the best overdrive tone that I've ever gotten!
With respect, Tubenit

[Fresh_Start]

tubenit - thanks so much for sharing your process with us!  Sometimes we are lucky enough to see Version 1.0, 1.4, 2.0, and 3.7 of a circuit in development but don't get the details on the "Why?" of each progressive set of changes.  Seeing the Before & After schematics along with an explanation of what was lacking in the earlier version and why you think the After works is fantastic.

Thank you! 

Chip

[jojokeo]

Thanks for the explanation and info tubenit it really helps. Is there a V2a and does it seem that using a an ax7 is too much gain for that stage? Have you tried paralleling V2 or cascading that too? I'm also wondering how it would sound w/ V2 set-up as a CF there?

[tubenit]

Thanks for the explanation and info tubenit it really helps. Is there a V2a and does it seem that using a an ax7 is too much gain for that stage? Have you tried paralleling V2 or cascading that too? I'm also wondering how it would sound w/ V2 set-up as a CF there?

A 12AX7 works fine in the V2a stage. But I prefer either a 12AV7 or 5751 which I think is more musically sweet to my ears.  I have not tried a CF there. Geezer tried a parallel gain stage there at the front of the OD and said it didn't sound very good. That was quite a while ago though. 

I've thought a CF with a tone stack there could possibly be a winner?

With respect, Tubenit

[tubeswell]

Its difficult to get a great clean and a great O/D within the same channel because of the highs and lows you 'have to' lose to get a great O/D, or because of the mids you 'have to' lose to get a great clean. Been down that road (and it was paved with good intentions).

[terminalgs]

No problem. You can do that. Another idea is to connect the cap between plate and grid. Look at V3A in this amp...

is that 20M R Sunn's way of preventing a "pop" when the switch is engaged?

the (B) part of that switch is interesting,,  toggling the bypass cap of either v1b or v3b...

thanks for posting the schematic!