5F6A Settings

63 vibroverb

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Interesting. Never heard of this. It would be difficult to mount this "mod" on a switch though, so a subtle change would be hard to notice. Do you have any idea what actually happened electronically? I have the impression that the 5f6a Bassman does not like too much preamp gain.

Don't think I'm educated enough to explain it well scientifically (perhaps Wally can), but the 12AU7 has a lot less gain than a 12AX7 but a lot more current than a 12AX7. This provides a stronger and smoother push of the signal thru the EQ section before hitting the phase inverter and power section. I really like it.
 

Antoon

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Don't think I'm educated enough to explain it well scientifically (perhaps Wally can), but the 12AU7 has a lot less gain than a 12AX7 but a lot more current than a 12AX7. This provides a stronger and smoother push of the signal thru the EQ section before hitting the phase inverter and power section. I really like it.

Thanks. I did some quick reading and found some older posts by Wally on this subject. I am probably going to try this but do not have any 12au7s currently. Did you notice any reduction in gain? My 5f6a can be harsh in some situations. I always thought that parasitic oscillation was probably causing this.
 

63 vibroverb

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Thanks. I did some quick reading and found some older posts by Wally on this subject. I am probably going to try this but do not have any 12au7s currently. Did you notice any reduction in gain? My 5f6a can be harsh in some situations. I always thought that parasitic oscillation was probably causing this.

Thankfully, a lot of old NOS 12AU7s can still be had pretty cheap. They last a long time in the cathode follower. They’re good for microphone tube preamps as well. I’ve read they do well in the tube overdrive pedals too. No reduction in gain since the CF isn’t a gain stage, just sweeter and more usable EQ. I also like a 12AT7 in the phase inverter, could help tame the harshness in your bassman. Since the originals used a 12AY7 (step below a 5751 in gain) in V1, that could also be a spot to try a lower gain tube if you have a 12AX7 in there. I like the chain of 12AX7 —> 12AU7 —> 12AT7 personally.
 

Gris

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Good stuff. I’ve been running an AY in V1 but never thought to swap V2 or V3. I have a few AU as i use them sometimes in blackface reverb circuit.
 

mad dog

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Pretty much as JRapp says. I always jumpered mine also, adjusting channel volumes based on the guitar plugged in. As in, a humbucker guitar, more brite channel. A rather bright single coil guitar, more normal.
 

ocduff

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My home brew Bassman has the ‘59 style tone stack, phase inverter values, and Presence control wiring. I also run US Tung Sol 5881 which don’t have a lot of bass. I don’t think those values sound good with 6L6GC which have less mids and more bass and with them I’d likely prefer the schematic tone stack values.

My settings are Normal Channel, Bass 3, Midrange 8, Treble 6 and Presence 6.

I have tried every combination in my amp - even have wired the Presence like the true 5F6 w the mercury vapor tube (didn’t work well) - and it’s what I like best.

I like a slope resistor that reads a bit high. Like 115K.

It’s fun playing with these values. With the schematic drawn values it always sounded like the sound was coming out of the side of the amp rather than the front if that makes sense.

I do think wiring the Presence like the blonde Bassman is an upgrade for either tone stack and in fact I think only the Mercury vapor tube versions had the Presence control wired per the schematic.
 

tomward

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...geez I love this thread.
Swapping tubes sounds good but is there a hidden down side?
I'll have to check these options out.
What is the series resistor on the treble pot about?
My amp has a high e string that sounds "snappy" not chimey or "bell" like.
It does not ring ... it snaps.
AND the low E sounds ...maybe muffled.Not that piano low note sound so often spoken of.
Could the tone stack be governing these tones?
 

tomward

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...geez I love this thread.
Swapping tubes sounds good but is there a hidden down side?
I'll have to check these options out.
What is the series resistor on the treble pot about?
My amp has a high e string that sounds "snappy" not chimey or "bell" like.
It does not ring ... it snaps.
AND the low E sounds ...maybe muffled.Not that piano low note sound so often spoken of.
Could the tone stack be governing these tones?

Sorry for the double post.Proof I'm a digital dinosaur.
 
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Antoon

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For the time being I use the free spst switch on my amp for a 100k series resistor between the treble and the mid pot. This has approximately the same effect as increasing the treble cap value, and gives a vocal like mid high boost (nice).

Another free switch (in the ext speaker socket) i used for a switchable nfb, between the jtm45 and 5f6a value. This is very useful too, except that it is hard to see which setting i am using, because it is behind the back panel.

I am surprised to learn that so many players set the mid control so high. I always have it almost on zero. Especially with overdrive.
 
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Axis29

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For the time being I use the free spst switch on my amp for a 100k series resistor between the treble and the mid pot. This has approximately the same effect as increasing the treble cap value, and gives a vocal like mid high boost (nice).

Another free switch (in the ext speaker socket) i used for a switchable nfb, between the jtm45 and 5f6a value. This is very useful too, except that it is hard to see which setting i am using, because it is behind the back panel.

I am surprised to learn that so many players set the mid control so high. I always have it almost on zero. Especially with overdrive.

I do like the idea of the switchable NFB. That sounds like an interesting mod.

As for the Mids.... I find scooping them out gets you closer to the brown and blackface tones. I like warmer, fatter and thicker... more congested, almost.

As my sainted mother used to say, 'That's why they make chocolate AND vanilla....'
 

Antoon

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I do like the idea of the switchable NFB. That sounds like an interesting mod.

As for the Mids.... I find scooping them out gets you closer to the brown and blackface tones. I like warmer, fatter and thicker... more congested, almost.

As my sainted mother used to say, 'That's why they make chocolate AND vanilla....'

If you have a free space for a switch then two NFB settings is nice to have. It reminded me how much the massive NFB on a JTM45 is part of the Marshall sound. I still prefer the moderate NFB of the 5f6a for most of my playing but for certain sounds such as single note picking the JTM45 setting sounds better to me. I have found that I do not care for low or no NFB on a 5f6a. Too much mushy low mid distortion. I do find that after an hour of loud playing I have trouble being able to hear what setting I am in. It does not help that I cannot see the switch setting while playing.
 

Antoon

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I do like the idea of the switchable NFB. That sounds like an interesting mod.

As for the Mids.... I find scooping them out gets you closer to the brown and blackface tones. I like warmer, fatter and thicker... more congested, almost.

As my sainted mother used to say, 'That's why they make chocolate AND vanilla....'

True regarding the mid settings that I use. Together with the 100k series resistor that I mentioned my settings result in almost exactly a blackface response, as verified with the tsc. Yet the overdrive sound is so different compared to my (stock) DR.

Another thing I still have to try is a 12au7 or 12dw7 in V2. I will report back when I have.
 

63 vibroverb

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Another thing to consider is a larger value potentiometer for the Mid control. The stock pot is 25K (which is already really nice), but going to a 50K or even 100K pot would let you get some really gnarly fat overdrive on the higher end of the taper while still retaining the normal range on the lower end of the taper.

...geez I love this thread.
Swapping tubes sounds good but is there a hidden down side?
I'll have to check these options out.

My amp has a high e string that sounds "snappy" not chimey or "bell" like.
It does not ring ... it snaps.
AND the low E sounds ...maybe muffled.Not that piano low note sound so often spoken of.
Could the tone stack be governing these tones?

No downsides to preamp tube swapping. As long as the amp is off and the tubes are cool, no risk of damaging anything.

As far as your "snappy" instead of "bell" and muffled bass - it could be a matter of just your settings. Or it could be your strings or pickups. Tubes and speakers also play a big part. Hard to say without hearing it and playing it.
 

tomward

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Appreciate this response 63 Vibroverb.
Gonna try the larger pot.
Also see that my choke is too big at 10Hgoing to swap that for a 4H.
And change output transformer to 4200 CT

Thank you for the information
 

Antoon

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Lol, I am just about to decrease the value of the mid pot to make it easier to set ;-).

One other "mod" that I want to mention is linking both channels (internally) with one channel having a standard large 0.02 uF coupling cap and the other a 0.005 uF CC cap. Mixing those thickens the mid-high frequencies without adding bass. Perhaps there is also a small phase difference between both channels because of the different caps. Don't know, but the effect was (unexpectedly) positive.
 

Gris

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I’ve been running the amp jumpered with bass at 0 or close to it, the treble up hi like 8+ and mids like 4… presence 1/2 way up. I have AY in V1, AX thereafter. I’ll ty AU in V2.

Haven’t messed with any caps or resistors yet.

Still experimenting with different spkrs. I like the 50 yr old Jensen alnicos best so far.

I love how fat this amp sounds
 

Antoon

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I’ve been running the amp jumpered with bass at 0 or close to it, the treble up hi like 8+ and mids like 4… presence 1/2 way up. I have AY in V1, AX thereafter. I’ll ty AU in V2.

Haven’t messed with any caps or resistors yet.

Still experimenting with different spkrs. I like the 50 yr old Jensen alnicos best so far.

I love how fat this amp sounds

You might consider rewiring the middle pot as a variable resistor instead of a potentiometer, like the blackface TMB tonestack. That way you can entirely eliminate bass if you want to. Other settings will still sound the same. I like it more than the standard 5f6a wiring, which was always too bassy for me. If you like a higher treble setting you may also consider a 100k slope resistor instead of 56k.
 

loopfinding

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any 5F6A/JTM45 for me is always mid dimed (or 12'd, haha), treble adjusted at idle til hiss is reduced (and then futzing with brightness a hair), and then bass to taste depending on gain (usually just sub halfway). presence, eh, idk...baseline is around 3/4 of the way always, and then i just futz with it marginally as necessary after TMB is dialed in.
 
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