Tone Stack Q

[Fred G.]

Ok, here comes a question from somebody who really should know better :-thnk

I have modded a Bandmaster into a '59 Bassman/Marshall JCM 800 single channel configuration. For the most part, I went with the JCM800 component values, with some variation in the tone stack. The tone stack has a 56k slope resistor, 500 pF treble cap, and .022 caps for mid and bass resistors. I have stayed with 6L6 output tubes.

I need to shed some "apparent" bass. The tone stack pots are 250K treb, 25k mid, and 1 Meg bass. If I reduce the bass pot value to 500k or 250k, will I accomplish this - what will be my end result? What effect will this have on the circuit?

OR, what about dropping mid pot down to 10K? or perhaps dropping both mid and bass pots to Blackface values? I've always had a hard time grasping this, even with using the Duncan tone stack sim.

Thanks in advance!

[Albert Kreuzer]

Yes, a 250k bass pot will give you less bass.
If that isn't enough, you can change the "mid" cap (C3 in Duncan's TSC) to .047µF. This will give you less mids.

HTH
Albert

[pdf64]

I always prefer a 500k log bass control for 5F6A type tone stacks, they give far better resolution. And wire it to the wiper of the mid control, rather than the hot track end.
If still too much low end, how about reducing a coupling cap? Try halving the one at the input to the phase splitter, ie if it's 1nF, take it down to 500pF. Peter.

[MWJB]

You don't say which Bandmaster, tolex or tweed? The most relevant factor is whether you have also incorporated the cathode follower driven tone stack, with the volume control at the preceding stage (tweed/Marshall), or whether you have just changed tone stack values to tweed spec, whilst retaining an input stage feeding a tone stack from its plate, with the volume control following the treble (tolex Fender style).

In the tolex style, 2 triode preamp circuit you don't need such a large bass pot as you do in the tweed/Marshall cathode follower circuit, 250K will be plenty.

[TheElectricMoron]

Maybe this

I would just change the slope resistor to 100k and the treble to 250-270pf. And give a thot to changing the connector caps. Way easier than swapping the pots and you have lots of options.

Charlie

[Fred G.]

GREAT GREAT GREAT Answers!!!

These answered many things I had "thought" might work, but I just wasn't sure, and I'd rather ask first before doing it trial-and-error.

MWJB - it's a 5F6A/JCM 800 clone - All the way. cathode follower, lowered preamp B+, extra gain stage in front with post-PI master. Only diff is 6L6's and some minor circuit tweaks I used out of "practicality".

Just too much dang BASS! Even with bass pot on "1".

Thanks much, fellas, these are great answers!

[Fred G.]

Interesting...

I've been fooling around with the Duncan sim again - from what I'm seeing, the best way to "clobber" available bass is to take the mid pot down to 10k.

I admit I need to study more about tone stacks. Does this dropping the mid pot to 10 k make sense?

Thanks for your patience!

[TheElectricMoron]

Well, if you turn the 25k mid-pot to "0" and you still have too much bass then changing the pot gives you nothing. And if it does reduce the bass then just leave it turned down. More k's on the mid just give you more mid. Zero is still zero as far as anything audible goes.
The smaller value connector caps and bypass caps will yank out the bass. I use a .003mf connector (3000pf) to the PI in almost everything. And .02's on on the PI. Still have LOTS of bass.

JMO of course YMMV --- but it is real easy to do and undo

Charlie

[MWJB]

"I admit I need to study more about tone stacks. Does this dropping the mid pot to 10 k make sense?" No it doesn't, turning the pot to 10K will do the SAME thing.

5F6A & JCM800 are sustantially different, do you have a schem of exactly what you have done?

Subbing 100K slope resistor will sound smoother & bassier, doesn't sound like the way you want to go, JCM800 has a 33K slope.

5F6A had a 12AY in V1, this will reduce bass & gain.

[Fred G.]

Look at a schem of a JCM 800 single channel MV amp, change slope resistor to 56K. Now substitute 6L6's for EL-34's. That's my amp, except it's wearing Fender clothing.

When I set bass control at "0" on Duncan sim, then reduce Mid pot value to 10K, it shows quite a decrease in lower freq's.

Is the sim hosed up?

[pdf64]

No, it's just showing that the bass control is wired to the hot track end of the mid pot, rather than the wiper (as is the BF arrangement). Therefore the the bass can never be turned lower than the value of the mid pot.
Which is why you could try wiring the bass control to the mid pot wiper. Peter.

[Fred G.]

Peter,

I've brought the amp home, I'm going to rewire according to what you spec'ed. I get what you're saying, I might have a Q or two for ya after I take out the chassis.

I remember reading about this somewhere before years ago, but I'd forgotten it.

Are you saying Wire like the BF stack or wire like the Marshall stack (in the Duncan sim)?

Thanks!

[pdf64]

What works best for me is to wire it up like the BF arrangement, use the classic 5F6A/Marshall component values except sub a 500k log for the bass pot. Peter.

[Fred G.]

Peter,

Got it. I was a little confused by the wording in the previous post.

I'm hoping to get to it this weekend, but there's a lot going on with kids and homecoming and all that jazz...

Thanks! I appreciate it.