More 'life' to high plate cleans

[Charlie Wilson]

Marcus, I found some good stuff here:
https://tubeamparchive.com/viewtopic.ph ... b&start=15

I hope this is the right way to link to a past topic.
CW

[martin manning]

Couldn't I just make it like the Bluesmaster boost? I noticed that there's no resistor on the bass pot at all in that, it is a 500k pot and a 50k mid pot. The PAB is fuller on the BM which got me wondering about the #102 PAB.

For PAB you could lift the whole stack like in the BM, which could be done using only one set of relay contacts. Then you'd have the other set available to bring on some mid boost at the same time. I say some because you could use the relay to parallel a cap of any size across the treble cap instead of shorting it like the mid boost switch does.

[ayan]

Martin,
I am not sure about a couple of the claims you’ve made thus far in this thread. I’ll go through them one by one, hopefully I can get some clarification:

1. Cathode bypass Frequency Break

You said: “You can ball-park where the middle of the bass roll-off will be by using the 1/(2 Pi RC) formula where R is Rk and C is Ck. For 5uF and 1k5 that's 21Hz, but it will be 1-2dB down an octave higher than that at 42Hz. You can see that doubling the value of the cap will move it down by a factor of two, or one octave. It'd be easy to test by tacking another 5uF across the existing one. Worth a try.”

Ballpark, kind of yes… The formula actually needs to consider the equivalent cathode impedance which is the parallel of the cathode resistor and (Ra + ra)/(mu + 1)

Ra = the total load resistance, which is Rp in parallel with the input resistance of the next stage, Rl. If there is no Rl, Ra = Rp

ra = the internal plate resistance of the tube

mu = the mu of the tube

If you do the math for a typical 12AX7, for 1.5K and 10uF, the frequency break point at the cathode is 22 Hz. For your values, 1.5K and 5 uF, it is 43 Hz. So the numbers you suggest are actually off by quite a bit with the approximation you used.

2. Boosts

You said: “Comparing a full lift to the Skyliner PAB, I see about 2dB increase across the board, i.e. no significant change in the frequency response (looks like a typical first-order high-pass filter).”

Did you actually lift the ground off the bass and middle pots of a Skyliner tone stack and it sounded to you similar to the PAB normally implemented in amps with that tone stack? I find that strange, because lifting the ground on that should basically turn the circuit into a “mega boost” like in the Blues Master amps with the Fender/Marshall type tone stack. For clarification, I am referring to just lifting the gorunds and not separating the treble and bass pots by a 22 Meg resistor, etc.

Then, you said: “For PAB you could lift the whole stack like in the BM, which could be done using only one set of relay contacts. Then you'd have the other set available to bring on some mid boost at the same time. I say some because you could use the relay to parallel a cap of any size across the treble cap instead of shorting it like the mid boost switch does.”

This further supports the reasoning of your first claim, implying that lifting the grounds alone would not give you a mega boost. Again, did you actually try this in a Skyliner amp?

Thanks,

Gil

Couldn't I just make it like the Bluesmaster boost? I noticed that there's no resistor on the bass pot at all in that, it is a 500k pot and a 50k mid pot. The PAB is fuller on the BM which got me wondering about the #102 PAB.

For PAB you could lift the whole stack like in the BM, which could be done using only one set of relay contacts. Then you'd have the other set available to bring on some mid boost at the same time. I say some because you could use the relay to parallel a cap of any size across the treble cap instead of shorting it like the mid boost switch does.

[martin manning]

Hi Gil, for cathode bypassing, if you use Rk and Ck, what you get is the frequency close to the mid-point of the treble boost ramp. The upper break point will be about one octave higher than that as I mentioned above. For 1k5 and 10uF, the quick-and-dirty method gives 10.6Hz, and doubling that 21.2, as compared to your more rigorously calculated 22Hz. For 5uF and 1k5, I get 21.2 and 42.4, as compared to your 43Hz.

On the boost, some time ago I put together SPICE simulations of the Skyliner tone stack in Rock, Jazz, and PAB modes, and that is what I used to get the Dumble PAB vs full lift comparison. The full lift was modeled by lifting the mid and bass pot grounds in PAB mode, so that would require a DPDT. Otherwise, you would get mega boost. Instead, one could just disconnect the bottom of the treble pot, which gives a result almost identical to PAB, and then use the other set of contacts to do a mid boost.

[ayan]

Hi Gil, for cathode bypassing, if you use Rk and Ck, what you get is the frequency close to the mid-point of the treble boost ramp. The upper break point will be about one octave higher than that as I mentioned above. For 1k5 and 10uF, the quick-and-dirty method gives 10.6Hz, and doubling that 21.2, as compared to your more rigorously calculated 22Hz. For 5uF and 1k5, I get 21.2 and 42.4, as compared to your 43Hz.

Ah, I do understand you now.

On the boost, some time ago I put together SPICE simulations of the Skyliner tone stack in Rock, Jazz, and PAB modes, and that is what I used to get the Dumble PAB vs full lift comparison. The full lift was modeled by lifting the mid and bass pot grounds in PAB mode, so that would require a DPDT. Otherwise, you would get mega boost. Instead, one could just disconnect the bottom of the treble pot, which gives a result almost identical to PAB, and then use the other set of contacts to do a mid boost.

Now I understand as well.

Thanks,

Gil

[martin manning]

...and thank you for pointing out the difference in the boosts!

[talbany]

I've got a 50W high plate, non-HRM that is lacking a bit on the cleans, single coils and buckers. Sounds a little too 'flat' dark and a bit too much compression. I like the somewhat 'singing' violin like quality, but the cleans dont sound '3D' and lively enough, especially with more mids dialed in (which I like for the OD)
Had a new production tung sol in V1 Plate voltages were roughly 10V low, swapped it out for a brand new one, and the plates went back up to about 190V. Sounded better, but still lacking. bright switch helps a bit, but causes ear fatigue and can get a bit harsh, so I often flip it back off within a few minutes
What's the best course of action to liven the cleans up? Should I add the NFB back to V1? I'm considering just dropping the plates to 180K/150K because I'd like some more dynamics from the clean channel, too. I dont think it needs to go all the way down to 100k, though. Thoughts or suggestions? Should I attack this one triode at a time? I attached an image of what I believe to be a fairly accurate representation of the preamp in my amp.

IMO..This is a common problem that has plagued the high plates? (Non-HRM)
The main issue being how do we balance the Clean tone with the OD side.. If we get the clean side lively with plenty of sparkle as well as a kind of softer low end then the OD becomes too loose and too bright(I personally think the V1B loop was put there for the OD side)..This is one of the reasons why I've said I have to get these amps up in volume to get any real life from the clean channel..I've always liked the Clean side of the low plates better and the OD side of the high plates..IMO..these are the fundamental problems that are in the amps design and when you start using things like feedback loops and bright caps snubbers, treble bleeders these are more like band aids over the problems and you always have other side effects that can cause other issues!!..Where does it end..It doesn't!!
If we start to compare the Dumble design to other amps we are used to like say I want my cleans to sound more like like a Blackface Fender or VOX or a Marshall??.Your talking about a totally different design?
With the dumble where do you start?..Voltages/Tubes/Component choice/lead dress/cables/guitar/loops/speakers/effects/etc..
All these things can effect these things you mention?

Welcome to Dumbleland

Tony

[bluesfendermanblues]

Or build an amp with dumble skyliner tone stack and OD PLUS a fender clean channel. It only takes one extra tube.

[Charlie Wilson]

Tony, those are expensive band aids .
CW

[norburybrook]

well I took my newly tweaked amp out on a studio session yesterday and I must say the clean sound was very nice with my Telecaster , didn't use the overdrive at all as there was another guitarist with me on the session and we split things to compliment the tracks. He ended up doing most of the OD stuff with an 80's Lukather vibe, I took clean and Bill Frisell vibe. I did switch the OD in for a few in between takes jams and it sounded great too.


So I'm happy with low plate clean....high plate OD,classic tone stack. All 4.7uf Cathode caps ,0.05uF mid cap, 47pf master cap. Makes for a very versatile amp.

Marcus

[Charlie Wilson]

Ok Marcus with all that recording going on no excuse for not posting some clips of your amp.
CW

[norburybrook]

Ok Marcus with all that recording going on no excuse for not posting some clips of your amp.
CW

. I'm on acoustic duties for the next few days but will try and sort something out


Marcus

[rogb]

well I took my newly tweaked amp out on a studio session yesterday and I must say the clean sound was very nice with my Telecaster , didn't use the overdrive at all as there was another guitarist with me on the session and we split things to compliment the tracks. He ended up doing most of the OD stuff with an 80's Lukather vibe, I took clean and Bill Frisell vibe. I did switch the OD in for a few in between takes jams and it sounded great too.


So I'm happy with low plate clean....high plate OD,classic tone stack. All 4.7uf Cathode caps ,0.05uF mid cap, 47pf master cap. Makes for a very versatile amp.

Marcus

[waves hands over crystal ball] I think I can see a 100w 2nd Gen. hybrid in your future Marcus. The OD is sweet but the cleans have a [/cliche alert] 3-D quality!

[norburybrook]

Roger, what would that be? I have a 100w Blues master that has great cleans and a good overdrive. In fact I played it yesterday after coming back to my studio and I realise I've made the #102 now quite similar to the BM.


I've been enjoying my Train wreck though recently and wonder if that's more my thing.




Marcus

[Charlie Wilson]

I believe this is what Rodger is talking about and is going to be my next build. https://www.youtube.com/watch?v=7NuDnwgnTA4
CW