Overdrive section in Dumble124 compared to Geezer BluezMeister

[acheld]

Greetings!  It's Michigan cold here in Maryland (not a good thing) so I'm hunkered down dreaming up new plans.

I've built several iterations of Dumble's #124 over the past few years, and really love the sound.   However, there are a few things I want to change/simplify with the 124:

1.  I never ever use the FET input.
2.  I never use the bright switch, very rarely use the "Mid-Boost."
3.  OD channel is always in use.
4.  Pre-Amp boost is used sometimes, but is noiser than desired.

So, I've been thinking about Geezer's "Tweed BluesMeister" as an alternative, though with modifications.

Comparing the OverDrive sections between the two circuits shows that they are nearly the same, with some variation  in component values.

I can't seem to figure out the purpose of some of the caps.   For instance, both circuits have plate to cathode caps (circled in red)-- what is their purpose and how would they effect tone and playability?   Do these provide some high frequency rolloff?  Why include Rk and Ck in the path?

And in Geeezer's circuit, there are plate capacitors in parallel with the plate resistor (circled in blue) -- what do they do, and how would they affect tone and playability?

[triode]

>> I can't seem to figure out the purpose of some of the caps.

The plate  caps are there to do a few things, remove some (very very) high frequencies, not sure if that has an impact here, really, but more importantly, to try to prevent parasitic
oscillations.

The plate to cathode caps are there to introduce positive feedback and change the sound of the distortion. You are picking the cutoff frequency that you are feeding back with the
size of the capacitor.

[Willabe]

.....  really, but more importantly, to try to prevent parasitic oscillations.

I doubt Geezer was having trouble with parasitic oscillations. Tubenit might remember.

But Geezer and tubenit both used those caps as they called them 'smoothing caps'. They smoothed out the amps tone.

[Willabe]

There's pictures of David Lindley, lap steel, plugged into the FET channel on his Dumble. 

[tubenit]

DaGeezer did not have problems with parasitic oscillations leaving him to add those caps to the best of my memory. 


He liked to experiment and often sort of blazed his own trail to achieve the tone he liked.  I think he has been one of the more innovative builders on this forum. My sense is he never felt confined to stay inside the lines of "tradition" and trusted his own tone values more than those of others.  I found DaGeezer inspirational and he is a great guitarist!!

To my ears, not all the original Dumble amps and Dumble clone that I've heard on YouTube (or some sound clip) sound great with their overdrive. Some sound harsh to me. My viewpoint is just because it says Dumble doesn't mean it's a good sounding amp.

My memory is those caps across the plate resistor there were to smooth the OD tone. Same reason for his "psuedo tone stack" in the OD section which dialed down some higher frequencies.  If you look at DaGeezer's  HoSo56, he used a smoothing cap across the plate resistor of the 5879.

Regarding the "enhance cap" which is mostly a 120p to 220p value across the plate resistor of the LTPI entrance, there is a thread one can find using search that explains that. It smooths out the higher frequencies to my ears and I've been using it on all my LTPI amp designs for yrs now.

I think of the mid-boost more as a single coil or humbucker switch meaning with the mid-boost on for single coils. Most of my Dumble inspired amps have a modified version of both the mid-boost AND the PAB especially. Take a look at this tone stack and switching options. This has become my preferred after trying many variations. The original Dumble PAB has a big jump and lots of hiss to it. I never liked it and didn't use it.


I think of the Tweed BluezMeister, the Tweed Overdrive Special, the D'Mars and the Carolina Overdrive Special amps from a perspective of build them close to the schematic posted and then spend alot of hrs tweaking things to get the tone you want. 


I'd build something and spend many many hours experimenting making changes one at a time. There were probably more experiment changes that I reversed than changes I kept over the first few years of experimenting. I'd build an amp. Get past the honeymoon period and then spend a month or two off & on tweaking it to my tastes in tone. As long as the experimenting is within established guidelines for safety .......... I have found it a useful mindset to have.


With respect, Tubenit

[acheld]

There's pictures of David Lindley, lap steel, plugged into the FET channel on his Dumble.

   Yeah, but that's David Linley playing!

I've had no luck whatsoever with the FET channel.  It works, but seems harsh to me.  Honestly, I love the soft overdrive tone using the normal input, so I don't really have the motivation to make it work well.

Lots to unpack in Tubenit's reply, much appreciated.   
 

[pdf64]

... both circuits have plate to cathode caps (circled in red)-- what is their purpose and how would they effect tone and playability?   Do these provide some high frequency rolloff?  Why include Rk and Ck in the path?

And in Geeezer's circuit, there are plate capacitors in parallel with the plate resistor (circled in blue) -- what do they do, and how would they affect tone and playability?

The "plate to cathode caps" and "plate capacitors in parallel with the plate resistor" are the same, if there's both they're in parallel.
They're a HF rolloff.
The cathode is bypassed so = AC 0V.

Anode resistance is in parallel with the anode load resistor is in parallel with the AC load.
So with regard to the #124 first stage about 60k // 100k // 350k = R of the RC LPF.