Cathode Bypass Effecting Tone Stack Bass Control

[Chuck H]

You are using a very low value (270R) NFB series resistor. Together with the 100R shunt resistor this means a lot of NFB and consequently low power amp gain. Try increasing to 820R (as used in TRs) or higher and take the feedback signal from the 4 Ohm output if available. This will increase power amp gain (and stability margin). As a result less PI drive is required for full power.

This is a good catch. I'll add that the schematic shows the NFB coming from the output jack rather than a specific secondary tap. Helmholtz implied this, I think, but I wanted to be clear. Since the NFB loop is a voltage only dependent circuit you should use a dedicated secondary tap. The output of the amp is a voltage @ current circuit. Which is different. Deriving NFB from the output jack means your NFB level will change with the impedance switch. With the most NFB happening when the 16 ohm tap is selected.

[ToneHenge]

Doh! Yeah thank you for catching this. I've actually built this amp twice, and the NFB was originally tied to the 4Ohm tap of the OT. Somehow I messed that up this second time around and that made it's way into the schematic.

[Chuck H]

Doh! Yeah thank you for catching this. I've actually built this amp twice, and the NFB was originally tied to the 4Ohm tap of the OT. Somehow I messed that up this second time around and that made it's way into the schematic.

You're not alone. In fact you're in good company. I've seen many examples where even major manufacturers have carried over prior design circuit values into a new model without making concession for the new operating conditions. I've done it too only to discover "Hey, This doesn't work here." And like your experience it has sometimes been this forum that steered me back in the right direction. Learning is important. But so is reaching. To some degree this genre of guitar amps is a black art. Usually art in the true sense, but sometimes with a dose of alchemy. Certainly there are parameters and known explanations for why what has gone before sounds good. But sometimes someone had to get out of the box OR experience a happy accident to discover the circuit that made it happen because if all we did was follow rules no one would ever try something different. The important thing is to keep building, tweaking and learning.

[loudthud]

And can you guys spell it out for me on how to leverage the gain pot and an additional resistor to create an interstage divider. Would this resistor be placed from the wiper to ground? I suppose that parallel resistance would pull down the resistance of the wiper-to-ground side of the pot.

Look for the Volume pot in this example of the Marshall 2203.

https://el34world.com/charts/Schemat...ad_100watt.gif

[HaroldBrooks]

Hi all,

I'm going down the endless tweaking wormhole on a design. I thought the amp was sound a little flat with all tone controls set at noon, and decided to boost treble a bit to make things sound more lively. A convenient approach is adding a bypass cap to the cathode of the triode gain stage that precedes the tone stack. That gave the desired effect but now the bass control seems to behave more like a gain knob than an EQ knob. The gain stage is V2a below:
[ATTACH=CONFIG]56460[/ATTACH]

I put a large .1uF orange drop in as the cathode bypass cap on V2a. Going off of the cathode bypass calculator on ampbooks.com, I wanted to place the gain boost more towards the treble frequencies at 1kHz+. My guess is that I've affected the way that the tone stack is being loaded by the preceding gain stage but I'm not entirely sure.

Any ideas on what's going on here?

Edit: Took out the cathode resistor bypass cap, and this bass control still sounds more like a volume knob to my ears. Maybe that's how Fender tone stacks are? I might try something like a baxandall next time around.

I have a Hi-Fi (Zenith amp) I converted to a guitar amp that has your two problems (cathode bypass cap on V2 kills theTone controls), and Bass control that sounds like a volume control just adding a boatload of very low bass and increasing overall gain a bit.



The first problem I believe on my amp has to do with the Presence circuit coming in right at the Cathode of the second gain stage. The addition of a cathode bypass cap (attempt to raise gain a bit) seemed to kill the tone control action, so I gave up on adding a cap in that position.

The Bass control brings in so much very low Bass, that it seems to cripple the amp while playing with high gain, Even after I added a smallish coupling cap (.01uf) just after the plate, and prior to the Bass control pot. The Treble control also does something weird, when you roll off all the treble, it seems to cause Bass transients to come up and peak at a high amplitude, and that almost wipes out the signal as well. Adding a small cap again after the plate of V2 but before the Treble pot helped a bit, but still has some issues when you roll off all the treble. The old tone pots in my amp measure a bit higher than the schematic, so that might be part of it.

Bear in mind in my case I was messing with a Hi-Fi amp to begin with, and driving the heck out of it, so you probably would expect some problems along the way as this was never the intended use, particularly with tone controls set up for Hi-Fi clean use. In any event, if I don't peg the Bass control, or totally back off the treble on this "converted" amp, it sounds superb !

Hope this helps in some way.