Dumble SSS

[ThisLifeILead]

howdy folks, im nearing completion of a Dumble SSS clone. I got the board made and chassis nearly populated. BUT im working on the bias supply. im using the Fender Studio bass phase inverter for this project.
its a LTP going into a cathode follower to drive a pair of 6550's in AB fixed bias.

Here is my question, I am not using the ouput tube balance control I just want to incorporate a bias pot. looking at this unique design im having a hard time determining a pot value. it looks like I would replace the 2.7k 2W resistor with a pot to ground.....BUT what about the 2W current rating?

[Enzo]

So instead of stealing a circuit you don't want and trying to bend it into something you do, why not just steal a circuit like you want in the first place:

http://bmamps.com/Schematics/fender/..._Schematic.pdf

[nosaj]

So instead of stealing a circuit you don't want and trying to bend it into something you do, why not just steal a circuit like you want in the first place:

http://bmamps.com/Schematics/fender/..._Schematic.pdf

You think the problem is that one has Leo's name on it instead of Howard's?

nosaj

[Enzo]

I think the problem is looking for a cut and paste instead of understanding the circuits.

[nosaj]

I think the problem is looking for a cut and paste instead of understanding the circuits.

Kinda like trying to mate a chevy engine and a ford transmission? A lot of extra work to actually make them work well together.
Lot easier just to do as you suggest and use a tried an true circuit.

nosaj

[ThisLifeILead]

And it is cut and paste in this situation because this is an unorthadox circuit that im playing with. I would like to have it built and functioning so i can get a better understanding. Ive built hundreds of amps but im trying to wrap my head around this interesting design. I dont want a handout. I would like a more knowledgeable person chim in to help me understand where this design is coming from.
Why would the bias circuit in this amp use such low resistor values? The bias feed resistors are only 33k from the normal 220k or 100k. From what i understand the sound doesnt exactly change but the feel is greatly improved and the idea is to provide sufficient current to the output grids to starve off breakup. Since im only running 2 tubes ,not 6 like the original studio bass or svt it might not be as big of a deal. But we wont know until its fired up and tested.

[nosaj]

And it is cut and paste in this situation because this is an unorthadox circuit that im playing with. I would like to have it built and functioning so i can get a better understanding. Ive built hundreds of amps but im trying to wrap my head around this interesting design. I dont want a handout. I would like a more knowledgeable person chim in to help me understand where this design is coming from.
Why would the bias circuit in this amp use such low resistor values? The bias feed resistors are only 33k from the normal 220k or 100k. From what i understand the sound doesnt exactly change but the feel is greatly improved and the idea is to provide sufficient current to the output grids to starve off breakup. Since im only running 2 tubes ,not 6 like the original studio bass or svt it might not be as big of a deal. But we wont know until its fired up and tested.

Where is the rest of the schematic for the dumble?

Isn't the 2.7k resistor you want to remove connected to a pot already . Theres a 33k on both sides of the 400 ohm pot. Looks like the pot has 4 connections (seen them in fenders before).

nosaj

[loudthud]

The Studio Bass is a conservative design. The low grid impedance is intended to prevent thermal runaway of the output tubes. Note it uses six 6L6GC for the output stage even though only two are shown on the schematic. Another way to get a low impedance on the grids of the output stage is to direct couple to the 6L6 grids from the 12AT7 cathode followers because the 12AT7 won't require such a low impedance on their grids, but, and it's a big butt, you'll need a strong negative supply of perhaps -100V or -150V (something like the SVT). So this design is a compromise with a low impedance bias supply and capacitor coupling from the 12AT7 cathode followers.

[J M Fahey]

I would like a more knowledgeable person chim in to help me understand where this design is coming from.

You asked for it , you got it

Ainīt MEF wonderful?

[Chuck H]

Another consideration is that a bass amp is more prone to command a little grid current on occasional peaks. Not at all the case with guitar amps. Especially significant for a Dumble design is that it works best with a little tube overdrive. That character will change with a bias supply and grid loads that stifle power amp reaction to signal peaks and allow for more dynamics. It's my own humble experience (with a Dumble owner) that you actually DON'T want to stiffen the amps dynamics. That bias supply goes the wrong direction UNLESS this is a bass player that wants to play through a Dumble.?.

[ThisLifeILead]

Amps finished. Works great. I used a standard bias circuit and cap coupled the CF to the output grids. Next time I will direct couple the CF to the output grids and use a large negative bias voltage. The amp has local feedback circuits in the preamp and the dumble pre + studio bass P.I combination I was hoping to blend turned out great for my friend.

[J M Fahey]

Glad you finished it and are happy with results.