smoothing "enhance" cap

[10thTx]

Thought I'd share something that worked on my D-style inspired amps.

The idea to try this started because ........

I built this amp using ECL84 tubes which is a triode/pentode tube. I used the triodes to do a LTPI push/pull amp. I had some challenges getting the tone to be as smooth as I wanted because it had some of that chimey high end "hash".

So I guessed the high end "hash/fizz" had to be in the ECL84 triode being used in the LTPI because smoothing caps anywhere else wasn't making that much difference?

So I thought about trying a smoothing cap across the plate resistor of the triode carrying the signal into the LTPI. It worked great.

Then I tried the same idea on my Tweed BluezMeister and Tweed Overdrive Special.

It has given similar results in all three amps.

1) clearer high end tone with no loss of harmonics, blooming,
sustain, or touch sensitivity. In other words, it seems like it left the
high end frequencies and removed only the high end hash/fizz

2) it added smoothness to the high end tone & gave it a sweeter tone

3) it seems to somewhat improve the attack on the notes playing fast
riffs similar to what I experienced when I had a compressor/sustain
pedal. The notes articulate more clearly playing fast riffs.

I tried different value Silver Mica caps and different value Ceramic caps.
Sometimes one would sound better than the other depending on the amp.

I have NO idea if this would benefit anyone else's tone? But I thought I'd share the results and describe the tone observations as best as I could.

I don't remember seeing a schematic where someone has done this specifically with the triode in an LTPI?

Anyhow, I thought I'd pass this on. IF anyone else has tried this or does try it ........... I'd sure be interested in the results (whether positive or negative)?

With respect, 10thtx

[groovtubin]

Thought I'd share something that worked on my D-style inspired amps.

The idea to try this started because ........

I built this amp using ECL84 tubes which is a triode/pentode tube. I used the triodes to do a LTPI push/pull amp. I had some challenges getting the tone to be as smooth as I wanted because it had some of that chimey high end "hash".

So I guessed the high end "hash/fizz" had to be in the ECL84 triode being used in the LTPI because smoothing caps anywhere else wasn't making that much difference?

So I thought about trying a smoothing cap across the plate resistor of the triode carrying the signal into the LTPI. It worked great.

Then I tried the same idea on my Tweed BluezMeister and Tweed Overdrive Special.

It has given similar results in all three amps.

1) clearer high end tone with no loss of harmonics, blooming,
sustain, or touch sensitivity. In other words, it seems like it left the
high end frequencies and removed only the high end hash/fizz

2) it added smoothness to the high end tone & gave it a sweeter tone

3) it seems to somewhat improve the attack on the notes playing fast
riffs similar to what I experienced when I had a compressor/sustain
pedal. The notes articulate more clearly playing fast riffs.

I tried different value Silver Mica caps and different value Ceramic caps.
Sometimes one would sound better than the other depending on the amp.

I have NO idea if this would benefit anyone else's tone? But I thought I'd share the results and describe the tone observations as best as I could.

I don't remember seeing a schematic where someone has done this specifically with the triode in an LTPI?

Anyhow, I thought I'd pass this on. IF anyone else has tried this or does try it ........... I'd sure be interested in the results (whether positive or negative)?

With respect, 10thtx

Koch amps use A one meg resistor! lol!! maybe try that too!! Nice goin!

jim@Omegaamps

[John_P_WI]

10thtx,

Curious as to why you only placed across 1 anode resistor? How does the tone compare / differ to the usual practice of placing a cap across / between both anodes of the PI (What the marshall guys call the fizz cap)?

Thanks for all of the schemes and ideas you post!

John

[martin manning]

10thtx, you put this cap on only one side? I think what you are doing is shunting HF to B+, which is AC ground.

[10thTx]

The decision to place it on one side was somewhat an arbitrary decision based on the unfounded idea that the signal path was going into "that" side. So I tried the cap on "that" side.


It worked so well for me that I didn't see a need to try anything else or add it across the other LTPI plate.

How does the tone compare / differ to the usual practice of placing a cap across / between both anodes of the PI

I have tried a 10p cap across the anodes of an LTPI on maybe 4 to 5 amps and never liked it & always removed the cap shortly after trying it. I thought it muted the tone and killed the high frequencies waaayy too much for my liking. Made the amps sound somewhat lifeless to me.

I think the typical smoothing cap of anode to cathode works well on the preamp. However, this seemed to get rid of the high end hash more effectively for me and gives the notes a slightly somewhat faster attack, IMO. Keep in mind that my amps are not Dumble clones.

What I liked is it made the higher frequencies clearer (removing hash) but did not seem to remove the higher frequencies that I could hear.

I hope someone else will try it and report what they hear. This may not work on other amps? I don't know? It might make a Dumble amp sound worse? However, I am very pleased with the result on my amps.

I see numerous posts about Dumble clones having some "fizz" on the OD that is not wanted and lacking smoothness the builder is wanting.

Maybe if someone has not found a solution, this very simple (easily reversible) mod would be worth trying out? Thought I'd share it with the hope it may be useful to someone else?

With respect, 10thtx

[topbrent]

Mesa uses that arrangement on the Lonestar amps, but it is on the opposite side of the PI as compared to your implementation.

*See the 120pF on the 100k PI plate.

[exocet]

Mesa uses that arrangement on the Lonestar amps, but it is on the opposite side of the PI as compared to your implementation.

*See the 120pF on the 100k PI plate.

I'm no expert but I think that this is all to do with stabilising the GNFB. There are a few locations where this can be applied - the Marshall 47pF cap was a very crude attempt, Mesa refined this a bit more to lower the roll off point still further.

Merlin Blencowes (Valve Wizard) book gives a good account of this.

[topbrent]

To expand the idea, Mesa has used caps in parallel with, and across both PI plates on many of their amps. This example is the Nomad, but it is used very regularly in their lineup.

[mlp-mx6]

Since a LTPI is a differential amp, it makes sense to me that it would not matter which plate resistor you put this on. Perhaps my thinking on this is wrong...

[Structo]

Another way to do it is to place the cap from the input grid to ground on the PI.

I did this on my D'lite and it really smoothed out the OD.

[sonicmojo]

Yet another place to potentially skin the cat, but adjustable and earlier in the signal chain. It is subtle, kind of like you are describing, at <400pF values. This came in handy for me on my Bluesmaster and especially when switching to a bright guitar. I'm not sure what the tonal difference would be pre/post PI. It would be an interesting A/B test. Thanks for sharing.

https://tubeamparchive.com/viewtopic.ph ... ht=#161826

[groovtubin]

Another way to do it is to place the cap from the input grid to ground on the PI.

I did this on my D'lite and it really smoothed out the OD.

SO you did insert another cap to grid to block grid voltage right?

jim

[Gaz]

Yet another place to potentially skin the cat, but adjustable and earlier in the signal chain. It is subtle, kind of like you are describing, at <400pF values. This came in handy for me on my Bluesmaster and especially when switching to a bright guitar. I'm not sure what the tonal difference would be pre/post PI. It would be an interesting A/B test. Thanks for sharing.

https://tubeamparchive.com/viewtopic.ph ... ht=#161826

Well, I think the idea you mentioned is good for cutting overall high-end and preamp buzz, but does nothing for PI buzz, which to me is where it can get really nasty.

[Structo]

After reading more of Merlin's book on preamp design, I was experimenting with different
smoothing caps on my ODS amp.

He touches on the anti-RFI cap or on page 40.
Between that and page 54 he discusses bright caps.
Then on page 54 he touches on using caps to trim harshness off of the top end.
Or to boost the lower frequencies as well.
I notice a bit of that on my ODS amp when in OD.
So with these caps we can just shunt that crap to ground.
Values to try would be 47pf on the low side to 330pf on the high side.
As you increase the values you will find that more and more treble is cut.

One of the places Merlin recommends a cap is across the input to the amp.
He recommends between the grid and ground using a 1.2n or .001uF cap.
This will help shunt any radio frequencies or other high end hash to ground.

I tried a 1.2n ceramic I had and felt that it was a bit much.
It took too much off of the top end.

The trouble with these non-HRM amps is that everything we do to the clean channel
affects the OD channel.
So there is a bit of compromise when tuning both channels.

I removed that cap and then tried a cap from OD2 to ground, much better.
I think I tried a 330pf cap there first.
But it lost a little too much treble so I backed it down to 230pf and that seems pretty good.
You don't want to trim too much high end off of the top or it can start to sound muddy.

I then revisited the input grid on CL1 and placed a 15pf cap from grid to ground.

The terminal strips holding the grid resistors provide a handy anchoring place for these caps.

I may try something on the PI tomorrow.

It's good to have your amp setup so you can hook it all so you can give it a listen before getting too far along.
Don't do more than one or two tweaks before you take a listen so you are sure which tweak is bad or good.