Simple obvious effective Bassman tweak

[sliberty]

i built a Bassman head based upon AA864 bass channel only. I include the bass channel's deep switch and the normal channel's bright switch, as well as adding a mid control and a master volume. I use the amp for guitar with a closed back 2x12.

I really didn't like the deep switch. So I experimented with a few other ideas for the switch and finally hit on one that works for me. Part of the problem is that the amp has enormous bass on tap, so anything that enhanced bass was overkill. So I went the other way. Now, with the "deep switch" off, the first stage has a 1uF cathode bypass cap (I didn't have a .68uF on hand). Then when the deep switch is engaged, the usual 25uF cap is added in parallel to the 1uF. So off, the amp is a little more Marshally, and on, the big bass is back.

Don't know why it took me so long to arrive at this solution, but I am really happy with it.

[Stevem]

Swapping the amp over to the normal PI section from a BF twin or SR and such is really the way to go when you play gtr thru these non tweed Bassmans, but back the plate caps back to .022!

[sliberty]

Oh, well, I don't really agree. I love the way the Blonde and Blackface Bassman amps work for guitar, and especially the bass channel (although I know that is not really related to your PI comment). These amps are great for guitar, and very popular with guitarists.

Now, having said that, what impact does changing the PI as you've suggested have on the tone of the amp?

[Stevem]

Not much in the tone department, but a good amount in the touch and feel department.
I like to disconnect the feedback circuit, but then you also need to have both of the PI sections plate load resistors be 100k, not 82k and 100k as is the norm!
If you have strong testing preamp tubes you can have your first gain stage tube be a 12AY7 with the feedback disconnected.
With the feedback disconnected the amp transitions from the output stage being clean to dirty with more control so it doesn't jump right into clipping as it does now.
You will get smother highs by a bit and looser low end when the amp goes into clipping.
You may just want to start off by doubling the value of the amps current feedback resistor and see if that's going in a better direction for you or not.

[robrob]

This is my 5F6A tweed Bassman "Lead" channel mod. It uses the Marshall 1987 first preamp cap and cathode resistor values. With this mod there's a real difference between the standard Normal and Bright channels.

[img:400:752]https://robrobinette.com/images/Guitar/ ... hannel.png[/img]

https://robrobinette.com/5F6A_Modificat ... ad_Channel

It's possible to voice one of the 5F6A's channels with a more modern, tighter, Marshall "Lead" style tone. Remember, the only difference between the 5F6A Normal and Bright channels is the bright cap on the bright volume pot which bypasses high freqs around the volume pot at low volume levels. At max volume there is no difference between the Normal and Bright channels so voicing one differently is a useful mod. This mod will make your 5F6A much more versatile and will blow away your Marshall buddies.

I used the Marshall 1987 as my roadmap for this "Lead/Normal" channel mod. We have to separate V1's cathodes so we can use different value bypass capacitors on each channel. We must also remove the cathode jumper from pins 3 to 8 on the V1 tube socket. The "Lead" channel gets a much smaller, Marshall size 0.68uF 25v bypass capacitor to attenuate lower frequencies which will clean up and tighten the overdrive tone. The "Bass/Bright" channel uses the original, huge 250uF 25v cap so its tone will not change. The 250uF cap is large enough to boost the gain of all frequencies, including very low bass guitar frequencies. You can modify either the Normal or Bright channel but I modified the Normal channel because you need less high freqs in a sweet, modern overdrive (the opposite of the standard Fender mid scoop). I left the other channel as Leo Fender designed so I can still get the magical 5F6A tone when I need it.

Note that when we separate the cathode resistors we have to double their value to retain the same bias so V1B (unmodified Bass/Bright channel) needs a 1.5K 1/2 watt cathode resistor (original is 820 ohms) to keep the same bias and tone. For the "Lead" channel (V1A) we move up to a 2.7k cathode resistor to cool the bias for Marshall style asymmetric clipping distortion. V1A's bias point is shifted toward cutoff so the guitar signal's negative lobe will get clipped at cutoff with a lower level input signal. The negative lobe cutoff clipping will also occur much earlier than the positive lobe (which gets clipped much later by saturation). With this asymmetric clipping the positive lobe carries the undistorted musical content while the negative lobe carries the distortion. Asymmetric distortion often sounds "creamier" with less fizz than symmetric distortion.

We also want to reduce the value of the "Lead" channel coupling cap to 0.0022uF (from 0.022uF) to again suppress unwanted bass frequencies but more importantly reduce bias excursion recovery time and therefore reduce preamp blocking distortion in the second preamp stage which will also smooth and sweeten the overdrive tone. If you don't want to deal with splitting the V1 cathodes just installing the 0.0022uF coupling cap will go a long ways toward tightening up the tone of just about any tweed amp channel.

V1's extra caps and resistors can be a little crowded on the circuit board so it's a good idea to raise the hot cathode resistors higher off the circuit board than the coupling caps to aid cooling and minimize heat transfer from the resistors to the caps.

This mod works great with a higher gain 12AX7 in V1 (like the Marshall JTM45) and/or with the Gain Boost Switch mod.

[martin manning]

Rob, what is the source for the quote above?

[robrob]

Rob, what is the source for the quote above?

My 5F6A mod web page.

[davidjon_99]

Rob, what is the source for the quote above?

It's from the link he put up there. [EDIT: He beat me to it ]

I was just looking at that mod yesterday and I'm planning to do that to the 5e3 I just finished.

[sliberty]

Interesting. So, the way I implemented my "deep" switch is that the standard configuration is 1.5K and 1uF (not "deep"). With the switch flipped, I parallel a 25uF (effectively giving me 26uF - close enough).

But as you've reminded me, the std for a Plexi would be 2.7K on the cathode. So, if I change the std resistor to 2.7K, and add a 3.3K resistor in parallel with the 25uF capacitor, when I flip the switch, the 2.7K in parallel with the 3.3K would approximate the 1.5K Fender std resistor, while the capacitors approximate the 25uF Fender std capacitor.

I'll have to try this out...it might make that "not deep" position even more Marshally.

[robrob]

sliberty, I did something similar in this modded version of the Deluxe Micro:

[img1377]https://robrobinette.com/images/Guitar/ ... ematic.png[/img]