Bass boost switch on brown deluxe?

[Quatro]

Hey,

 I have a brown deluxe (6G3) type circuit up and running and was wondering if anyone had an idea for a simple bass boost or deep switch that can be added to the circuit?

http://music-electronics-forum.com/attachments/2952d1219372350-deluxe_6g3_schem.gif

[HotBluePlates]

... a simple bass boost or deep switch that can be added to the circuit? ...

Swap the 25uF bypass cap on the first gain stage for a 0.1-1uF cap. This is now your new "Normal" setting. Adjust coupling caps to be "bright but not too-bright" with the new bypass cap value.

Place the 25uF cap in series with a ~100kΩ resistor, with this series pair from the 1st gain stage cathode to ground; put a SPST switch across the resistor so that it shorts the resistor when set for "Deep".

It seems like all deep switches/bass boosts restore near-normal bass, while the non-deep setting destroys low end. That's how this recommended approach works as well. The key to making it work is the re-jiggering cap values in the circuit for the brighter normal setting so that the added bass is obvious in the Deep setting.

[Quatro]

Thanks HBP,

Alas, I had a feeling that might be the case.

[Willabe]

You could try making the PI input .01 cap larger, but I don't know about putting it on a switch.

You could also try making the .01 tone cap on the normal channel smaller.

And what speaker do you have in it right now? Changing the speaker can make a very large or enough of a change in the overall sound.


                Brad    

[HotBluePlates]

You could try making the PI input .01 cap larger...

Due to the bootstrapping effect, the input 1MΩ resistors on the phase inverter look like at least 2MΩ and maybe more like 5MΩ. -3dB for 2MΩ and 0.01uF is ~8Hz. So there's nothing to be gained by making that phase inverter input cap bigger.

I'm still thinking cathode bypass caps give the most guaranteed bang for the buck.

[terminalgs]

Another option is to add an Ampeg style ultra-lo on one of your channels.

example:
http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_B25.pdf

See C3/.02 and R6/39K in series to ground following C2/.1 coupling cap.  the ultra-lo switch places a 1M in series to ground  to turn ultra-lo off..

this works well.

try a .02 in series with a pot to ground, tune to suit.  If .02 is too much, try a .047 or a .001...

[Willabe]

So there's nothing to be gained by making that phase inverter input cap bigger.

So that's another Gerald Weber mix up?


           Brad       

[HotBluePlates]

So that's another Gerald Weber mix up?

I'll just say I had to un-learn a lot of things I read in Weber's books. A lot of hype and a few instances of just bad info.

Dave Funk had some very not-nice things to say about Weber in Funk's book, which were thinly veiled (though they didn't call Weber out by name).

But regarding the phase inverter cap, it seems like it should be right, unless you know about bootstrapping.

For instance, look at a Deluxe Reverb schematic. The long-tail has voltages indicated at the cathode and at the non-cathode end of the cathode bias resistor (470Ω). You know there is no d.c. through the 1MΩ grid resistors (in a typical gain stage, this results in 0vdc at the gids), so the tube bias is the voltage across the 470Ω, or 77v - 75.5v = 1.5vdc. This is a bias voltage consistent with what you're used to seeing in other preamp stages.

Now if you probe with your meter at these 2 spots to measure voltage, all is good. If you try to measure that 75.5v which should appear at the phase inverter grids (because no d.c. flows, the voltage is the same at both ends of the 1MΩ), you won't get 75.5v. You'll get something like 40-some volts instead.

Why? Your meter most likely has a 10MΩ input impedance (if you're unlucky it may be more like 1MΩ), and the bootstrapped grid looks not like 1MΩ+ to ground, but several-MΩ. When you place 10MΩ as a parallel path to ground, and it's not 10x or more resistance as the circuit impedance, the parallel path presented by the meter drags the circuit voltage down.

Try it for yourself... Probe cathode and grid of a long-tail, and ask yourself could a 12A_7 possibly function with 20-30vdc difference between the grid and cathode?

Anyway, this is how an amp like a Deluxe Reverb can have a 1000pF coupling cap going into the phase inverter and not sound like all the bass is gone (0.001uF = 1nF = 1000pF).

All this is one of the reasons I'm kinda excited about having a HP 412A DC meter, which has an input impedance of 200MΩ on most ranges.

[sluckey]

Several of the Bassman amps even use a 500pF at the input to the LTP PI.