Dissecting The D Schematic

[Goldhedge]

I haven't seen a thread that explains what goes on inside the amp circuits, so I thought I would start one with the input/preamp section. I want to understand what each component does instead of throwing it all together and hope 'it works'. It might even help with troubleshooting? I'd like to understand what it is that makes a Dumble 'different'. Please feel free to embellish and correct my understanding.

I found Merlin Blencowe's "Designing Valve Preamps for Guitar and Bass" in my ramblings across the net and have studied the common cathode triode gain stage. He certainly makes subject come alive. I recommend getting the book if serious about designing amps!

I also suggest checking out 'vacuum tube' on wikipedia to get a better understanding of what a tube consists of. http://en.wikipedia.org/wiki/Vacuum_tube

I've been reading and attempting to analyze the pre amp and tone stack section.

In laymen's terms, how I've assimilated its operation is this: The valve has to be DC 'biased' so that AC current can be amplified through it. 'Biasing' is kind of like a 'fixed' rheostat in that, at one end of the spectrum there's not enough power to light up the room, whereas at the other end the bulb is so bright that it burns out. What fixed biasing does is to put the valve at an operational mid-point where it will run efficiently and stay there without varying. Valves come with spec sheets that tell us what the best set up is, so there's no need to reinvent the wheel.

Biasing a Triode - This is so universally used in nearly all amplifiers that it's considered to be 'common'. Dumble used it in the attached schematic, so it's nothing special. Remember, the goal here is to set the valve up for an optimum operational capacity. It's a lot like tuning a guitar. It works best when everything is in agreement.

The guitar pickup sends a small signal that varies between 100 mV and 1 V RMS to the grid of the 'B' triode (TRD B). This in turn (through the magic of biasing) gets amplified and sent to the 'tone control' section. The tone control section changes the AC voltage that the TRD 'A' triode grid sees and amplifies. Flipping the Rock/Jazz, and Deep switches changes the resistance of their respective control pots, which in turn changes what the 'A' triode grid sees (and amplifies) and sends to either 'Distortion' or the phase inverter/power valves.

The capacitance keeps noisy DC out of the pots and alters the Hz seen by the 'A' TRD grid which in turn determines treble, mid, bass. The cap before each pot is what passes the desired Hz. Adjusting the pot changes the voltage the 'A' grid sees.

I attempted to follow each of the Treble/Mid/Bass controls to see what they 'see', but it's really intense!

Mid: Has no 'switch' so it's always in the circuit unless on 'rock' and one side of 'deep'. It sees 100K resistor (R5) and a 250K pot (P1) so it's variable from 100K to 350K ohms. There's a 0.01uF cap (C2) which keeps DC out and allows the midrange Hz to pass at all times through C3 where it takes a variety of paths.

Treble: (C4) 510pF cap (very small - so probably used to keep any stray DC out of the tone stack and cuts only the very high Hz out?) then a 250K pot (P3) and a switch that when on 'Rock' sees…

There's "Bass" and "Deep" and I don't know what the difference between the two are.

I haven't built one yet, so I don't know what components were 'gooped', but I'd guess it's this tone stack that he hid?

I'm guessing that Dumble's contribution to this is the tone stack as the amp design is a fairly simple layout in that it's a 'standard' design found all over the place. Am I close? What did he do that makes his 'design' special?

Comments?

[vibratoking]

Most of what you are after is contained somewhere on this site. The organization leaves a little to be desired, but it is here. First, I would say read, read, and re-read the threads. It is time consuming, but worth it. There is no shortcut that I am aware of. Second, Merlin's books are good. If you really want to understand the gory details, you will have to have a basic level of electronics knowledge. Richard Kuehnel has some very good books that go into the gory details of how each block works, how to make approximations, and how to derive the exact functionality. He then goes on to compare the approximations to the exact analysis. This will give you a good idea of how to estimate and how to do a complete analysis. I am guessing that you want a thorough understanding. This is one good way to get it. Good luck.

[ToneMerc]

Most of what you are after is contained somewhere on this site. The organization leaves a little to be desired, but it is here. First, I would say read, read, and re-read the threads. It is time consuming, but worth it. There is no shortcut that I am aware of.

Amen, you got to put in the work. Gotta search, read, stare and compare x 10.

So makes a Dumble, a Dumble?.....well which one?

Off the top of my head.........four to five core models; 5 ODS generations as we refer to them, 6 different tone stacks, 4-5 different preamp schemes, 3-4 power amp and PI schemes and at least 4 different DC power schemes.


TM

[Zippy]

So makes a Dumble, a Dumble?.....well which one?

It's not "What?" but "Who?"

Howard makes Dumble amps... Or is it Alexander?

Hmmm, what is the difference between Howard- and Alexander-era amps?

Inquiring minds...





Nah.

[ToneMerc]

So makes a Dumble, a Dumble?.....well which one?

It's not "What?" but "Who?"

Howard makes Dumble amps... Or is it Alexander?

Hmmm, what is the difference between Howard- and Alexander-era amps?

Inquiring minds...





Nah.

Exactly..........lot's to digest

TM

[Goldhedge]

Most of what you are after is contained somewhere on this site. The organization leaves a little to be desired, but it is here. First, I would say read, read, and re-read the threads. It is time consuming, but worth it. There is no shortcut that I am aware of.

Amen, you got to put in the work. Gotta search, read, stare and compare x 10.

So makes a Dumble, a Dumble?.....well which one?

Off the top of my head.........four to five core models; 5 ODS generations as we refer to them, 6 different tone stacks, 4-5 different preamp schemes, 3-4 power amp and PI schemes and at least 4 different DC power schemes.


TM

I get the "it's here somewhere", I was hoping everyone could pitch in and give their idea (link), so the 'next guy' might get it.

I don't need a Fourier transform series in-depth study, just basics. I'm still looking. I'll check out the other book too, but it sounds deeper than I need to go.

Just knowing there are various Dumble tone stacks is helpful guidance. I knew of 3 OD's and an HRM. I'll look for the others...thanks!

I know of #124, #183 and #102 are their other designations you guys have named like that?

Feel free to point the way.

[turbo5speed]

I know of #124, #183 and #102 are their other designations you guys have named like that?

Feel free to point the way.

Those are the amps serial numbers.

[Structo]

Goldhedge,

Most of what you posted is general tube theory.
That is good you laid it out but most here are pretty advanced in knowledge.
We kind of expect people to do their homework, then ask questions if they don't understand something.

If you go to the Technical section there is a Sticky there called Reading Material on Steroids.

It lists a lot of books and other information about tube amps.

[Goldhedge]

Goldhedge,

Most of what you posted is general tube theory.
That is good you laid it out but most here are pretty advanced in knowledge.
We kind of expect people to do their homework, then ask questions if they don't understand something.

If you go to the Technical section there is a Sticky there called Reading Material on Steroids.

It lists a lot of books and other information about tube amps.

Thank you.