Passive attenuator for guitar amp

I've been playing a Bassman lately. I'd like to build an attenuator to let the tubes saturate without serious hearing damage!

This project seems perfect.

https://www.tube-town.net/diy/tt-pos/pos50-01.JPG

But amp output impedance is 2 Ohm. The schematic shows no option for 2 Ohm impedance.

How should I calculate the resistors for 2 Ohm impedance?

Thank you

I don't like this scheme very much...

For line out I'd use 500Ω and 4k7 resistor.

For speaker out you should go for a L-pad attenuator and use any tool online for vlues calcs. Here is one http://www.sengpielaudio.com/calculator-Lpad.htm

Pay special attention to power in each resistor because it WILL get HOT and it have to get a lot of power to heat. If your amp is 100W depending on the attenuation wanted you should go for more than 100W resistors. If you want 20dB atten (1/4 percived level, I think it's nice for a bassman full up for confortable listening) resistor will be handling 81W when your bassman puts 100W, if you want to saturate output stage it will be putting more than 100W and mos't in R1 of the scheme in the web, so it should be 200W at least... Could be 4x50W in series-parallel. (I don't know if it could be 100 2W resistors but it will have some inductance that may affect if layout is not well worked)

Any case you should be specting a lot of heat and a massive heat sink and probably a fan will be necesary.

JS

EDIT: I often recomend lower sensitivity speakers and less speakers but hard to get a guitar speaker that handle 200W... (I'd love to build a SE amp, something like AC4, with an 10' alnico eminence to recording at home really hot with low levels)

L-Pad Calculator
From this cool diy build.

Jaz

I have a THD hot plate, it is just a L pad but it have a bass and highs controls. I have found that the bass buton is a good extra when you are atenuating more than 8 db, I have noticed no loss of treble.

Thanks for the L-Pad calculators, they are very useful.

I liked the THD Hot Plate and it's a good design to get some inspiration.

How do you think the bright and deep switches are wired?

the bass boost is a couple of caps "bypassing" the atenuator and the high boost is an inductor also "bypassing" the atenuator.

I use a THD Hotplate, it's very helpful, but there's another approach I just learned about from this forum:

Making a minor modification to the actual amp to give you a knob that will lower the B+ to your power tubes thus lowering the output volume while still allowing them to provide distortion to the output.  It's called power scaling, or variable voltage regulator VVR.  Anyway, I'm looking into building one (there's like 6 components total in the thing) to put into my tube preamp but it's mostly used for guitar amps, and I wouldn't mind putting one into my guitar amps when I'm done.

There are some tricks to the VVR. (1) You have to worry about scaling the bias if you have a fixed bias amp and (2) Do you scale the preamp B+ or not? Experimentation is in order, based on the amp you're modding.

--mark

Just curious to the implementation on the new Fender Bassbreaker 45:

"Output level knob takes power from 45 watts to a single watt and anywhere in between, tailoring full-powered Bassman overdrive capabilities for studio, stage and arena – players can dial up many “sweet spots” with various combinations of level and channel settings"

As it seems it does sound good on this amp.

There are also other models which simply use a PPIMV for 'power-reduction', but those don't get all thumbs up.

maybe a built in attenuator ?

best way ist o get a smaller amp,

beatnik said:

maybe a built in attenuator ?

Click to expand...

Sure possible. These usually don't seem to get the praise as this Bassbreaker got though.

I expect some B+ variation. Not too difficult, but should be done right.
If this amp was using power-scaling (see London Power), they might have made more PR for it - unless Fender came up with a smart alternative.  Curious when the schematic appears.

CJ said:

best way ist o get a smaller amp,

Click to expand...

Agreed, but being able to use one amp for different tasks sounds attractive. Based on reviews they got it right here.

Having both a smaller & larger amp sounds good as well though ;-)