Noise in Dumble circuits....

[eddieod]

Hi everyone,

I built several amps including some dumble-based circuits (especially the '70 s version which can be found on schematicheaven.com) now. The problem I encounter with the dumble-circuits is that the're so noisy...I never have such problems with other amps and preamps that I've built. I keep the signal paths as short as possible, all shielded as well, use carbonfilm resistors in the signal path...maybe this has something to do with it? Furthermore heavy power supply filtering, the usual lead dress-guides etc. etc. I'm no newbie when it comes to this, but...does anyone else encounter similar problems? Especially in the overdrive-circuit...

Like to hear from you all. Thanks alot in advance.

Ed's Custom Shop
Edwin Thoen
Maarssen
The Netherlands

www.edscustomshop.com

[pamaz67]

One of the most common cause is the use of filament supply (6.3v) for deriving a Dc current for the relays.
No chances when you use the same 6.3 v supply both for filament than for relays, to get a quiet amplifier.
Always use for relays a dedicated secondary or any other suitable transformer different from the filament one.
Ciao

Paolo

[bluesfendermanblues]

What kind of noise are we talking about?

Hum
or Hiss

Hum should not be an issue if you ground it properly. Check the layouts on the forum addressing this matter

From you post I tend to asume that you mean hiss, since you find it most apparent when the OD is engaged.

Well, in the the od you get a lot of gain in the circuit, which will provide hiss. IMHO it not worse than boogies og soldanos, in fact I find it is less.

You mention carbon resistors...They provide a lot of selfnoise and should only be used strategically in the signal path. Don't use them for plate resistors, which should be Dale RN65 and they don't make the same amount of noise.

Have you tried other preamp tubes, maybee you got a bad tube in there somewhere. V1 and V2 are critical for both sound and noise.

Personally I don't worry about hiss - just turn down you guitar volume in breaks and you'll be fine, but V1 has a major impact on the total sound of the amp.

[Funkalicousgroove]

Has anyone tried DC heaters? that may take care of some of this, I personally don't worry about it- tube amps are noisy

[glasman]

You can also lift the 100/100 ohm psuedo center tap reisitors a few volts off of ground and get rid of hum, Accoridng to Andy Fuchs it is almost as good as DC filaments.

Gary

[markr14850]

For me, a 10-turn 200ohm precision wirewound pot for balance, with about ~20v of elevation on the wiper does the trick. It's a *huge* difference. For people who have never tried this before, add this to an amp tonight!

Even with the 10 turn pot, I'm amazed at how small a window of movement it is for optimum hum silencing. And the best position is nowhere near centered.

The 20 volts just requires 2 resistors (for me, 470k and 24.7k) as a voltage divider and a cap (for me 47uf), fed from the plate supply. This is all easily mounted on a center-grounded 3 terminal strip.

I found I needed at least 20 volts. For those of you with scopes, you might find it interesting to look at the output of V1A with no input, and varying amounts of voltage on the pot wiper. The heater is like another input to the tube.

[markr14850]

duplicate post deleted.

[heisthl]

For me, a 10-turn 200ohm precision wirewound pot for balance, with about ~20v of elevation on the wiper does the trick. It's a *huge* difference. For people who have never tried this before, add this to an amp tonight!

Even with the 10 turn pot, I'm amazed at how small a window of movement it is for optimum hum silencing. And the best position is nowhere near centered.

The 20 volts just requires 2 resistors (for me, 470k and 24.7k) as a voltage divider and a cap (for me 47uf), fed from the plate supply. This is all easily mounted on a center-grounded 3 terminal strip.

I found I needed at least 20 volts. For those of you with scopes, you might find it interesting to look at the output of V1A with no input, and varying amounts of voltage on the pot wiper. The heater is like another input to the tube.

That's a great tip - I can't wait to try it (next time I order parts for the 200 OHM 10 turn pot). Thanks

[bluesfendermanblues]

Or you can do what Boogie did on the Mark III....

- take the 6.3 VA heater off V1 and
- put rectifier diodes, a small capacitor and a small resistor in line with the Heating supply string
- and vupti you got 6.3 V DC to put on V1

its pretty simple and works like a charm. Check a Boogie Mark III schematic to see what they did. You can solder the components directly to the V1 socket or put the stuff on a terminal strip.

[odourboy]

For me, a 10-turn 200ohm precision wirewound pot for balance, with about ~20v of elevation on the wiper does the trick. It's a *huge* difference. For people who have never tried this before, add this to an amp tonight!

Even with the 10 turn pot, I'm amazed at how small a window of movement it is for optimum hum silencing. And the best position is nowhere near centered.

The 20 volts just requires 2 resistors (for me, 470k and 24.7k) as a voltage divider and a cap (for me 47uf), fed from the plate supply. This is all easily mounted on a center-grounded 3 terminal strip.

I found I needed at least 20 volts. For those of you with scopes, you might find it interesting to look at the output of V1A with no input, and varying amounts of voltage on the pot wiper. The heater is like another input to the tube.

I'm not 100% sure I have this little circuit straight... could you post a schematic or layout of this this bit and how it's connected?

[markr14850]

I'm not 100% sure I have this little circuit straight... could you post a schematic or layout of this this bit and how it's connected?

Sure. Here's how I did it...

[odourboy]

Perfect! Thanks dude!

[greiswig]

I'm not 100% sure I have this little circuit straight... could you post a schematic or layout of this this bit and how it's connected?

Sure. Here's how I did it...

I don't understand what this circuit does. It would probably help to see it within the context of the overall circuit, but that's a lot to ask. So...

It's not clear whether this is replacing the filament current that's normally supplied by a filament transformer with something that taps off the bias supply?? Color me dense, but I'm not getting it...

[heisthl]

filaments need a ground reference for noise (hum) reduction, typically handled by a center tap or in the case of no center tap 2 100 ohm resistors one from each side to ground. This new circuit would take the place of the 2 100 OHM resistors and would require disconnection of the center tap if you have one. If everything was manufactured perfectly(not likely) you wouldn't need the adjustment.

[greiswig]

filaments need a ground reference for noise (hum) reduction, typically handled by a center tap or in the case of no center tap 2 100 ohm resistors one from each side to ground. This new circuit would take the place of the 2 100 OHM resistors and would require disconnection of the center tap if you have one. If everything was manufactured perfectly(not likely) you wouldn't need the adjustment.

Ah! Now I know where this circuit fits. Got it...thanks!