Presence and bias wiring differences amongst Fender Bassmans

[dehughes]

So, I'm looking at both the 5F6-A and 6G6-B Presence pot wirings, and I'm wondering if there is an appreciable difference in terms of the impact on the circuit and on tone/functionality of it. That is, I'm trying to design an amp that covers as much of the different Bassman tones as possible, and am unsure how to wire the pot so as to allow it to do what it does both in the 6G6-B and 5F6-A schematics.

As well, I'm debating as to how best to wire in the fixed bias pot/resistors/caps, as gain, the amp in question is going to be a synthesis of tweed/brown/black Bassmans.

Any advice is greatly appreciated...thanks.

[jjman]

5F6-A runs the DC current for the PI thru it. I prefer the 6G6-B approach of separating the ac from the dc. But if you use good pots it’s no big deal.

I believe that using a ~5k resistor and a ~5k pot under the 6G6-B approach would yield the same tonal results as the 5F6-A approach. Either way the dc is seeing ~5k and the ac is seeing 0-5k.

[dehughes]

5F6-A runs the DC current for the PI thru it. I prefer the 6G6-B approach of separating the ac from the dc. But if you use good pots it’s no big deal.

I believe that using a ~5k resistor and a ~5k pot under the 6G6-B approach would yield the same tonal results as the 5F6-A approach. Either way the dc is seeing ~5k and the ac is seeing 0-5k.

Cool. Thanks. That makes sense. I was hoping to use PEC or Clarostat sealed pots for this build, but then I don't know if I can get them in all the proper values (5k, 50k, 100k, 250k, and 1M).

[Structo]

Yeah, a lot of presence controls sound scratchy due to DC being there.

I can't remember where I saw it but there is a way to decouple the DC so you don't get that.

But it usually isn't that bad because you generally don't sweep that pot that much anyway.

[dave g]

Yeah, a lot of presence controls sound scratchy due to DC being there.

I can't remember where I saw it but there is a way to decouple the DC so you don't get that.

But it usually isn't that bad because you generally don't sweep that pot that much anyway.

The Dumble ODS runs this way...all the DC bias current from the PI flows to ground through the 390 ohm resistor in parallel with the presence pot. The presence pot itself is not grounded, but goes to ground through the 1uF cap.

[Structo]

I have my D'Lite wired like that.
The pot has a tiny bit of scratchiness to it.

[dave g]

It could just be a scratchy pot

[David Root]

dehughes, PEC makes their carbon pots in all those values. Digikey sells them. I like them better than the Clarostat but they do cost slightly more.

[dartanion]

One note on PEC pots is that they are larger than your run of the mill CTS or Alphas. If your chassis is 2" tall, it's a tight fit and may give you headaches wiring and soldering them installed in the chassis. I would advise in this case to wire and solder them, then install the whole harness in the chassis if you have a tight fit.

[Sonny ReVerb]

The last chassis I built, I put the pots in their holes upside-down on the outside of the chassis and wired them up as much as I could. Then I took the whole assembly and put it inside the chassis. It worked great and I had much better access to the pots while soldering. Just make sure you put them in upside-down on the outside or they'll be backwards when you try to put them inside.

[dehughes]

Hey, great info everyone! Thanks so much!

As for the PEC, I'll totally check out Digikey. Thanks.

As for their size, I believe I have all the room in the world, as I have medium sized chassis with few controls needed, so I should be good. I'm thinking about doing Matchless-style PTP on this one, so we'll see how I go about wiring things up beforehand or not....

One other question: What's the consensus on the different bias setups for the various Bassman iterations? Is one preferable? I'm going to essentially be running a 6G6-B power section (proper PT from Heyboer, 5881 pair, fixed bias) so should I just copy the 6G6-B bias setup exactly, or are there better ways to setup this fixed bias? I ask mostly because I'm a cathode bias man at heart, and this will be the first fixed bias amp I've built.

Thanks!

[Wayne]

The 6G6-B bias scheme would be fine if tubes were as consistant as they were in "the good ol days". I would replace the 27k resistor with either 15k or 18k with a series 10k pot to ground, with the pot wired as a rheostat. That will give you a range of adjustment to accomodate tube variances.

It's important when including a pot in the bias supply that pots' usual failure mode is that the wiper loses contact. Wired as I described, pot failure would pull the bias more negative, thereby saving the tubes.

W

[Andy Le Blanc]

Peavey's have a 5k resistor where one would expect the pres. pot. to be,
but has a 25/25 cap, the same you use for a pre stage bias, with the + hooked
to where you would normally insert the FB. The FB and which ever control network
you choose is hooked to the - side of the cap.
It seems to be about where you would expect the amp to be tonally, and no scratch.
Fenders have always lacked some very basic considerations, to the point where
a scratchy presence pot. has become an expected traditional product feature,
that builders struggle with. Noise free operation is a recent and unrealistic
expectation unfairly placed upon old Fenders.

It can be a $1 solution, you don't have to go with pricey mil. spec. parts.
for traditions sake.

[dehughes]

The 6G6-B bias scheme would be fine if tubes were as consistant as they were in "the good ol days". I would replace the 27k resistor with either 15k or 18k with a series 10k pot to ground, with the pot wired as a rheostat. That will give you a range of adjustment to accomodate tube variances.

It's important when including a pot in the bias supply that pots' usual failure mode is that the wiper loses contact. Wired as I described, pot failure would pull the bias more negative, thereby saving the tubes.

W

Very cool. Thanks. This would look more like an AA864 Bassman bias setup, then?

[dehughes]

Peavey's have a 5k resistor where one would expect the pres. pot. to be,
but has a 25/25 cap, the same you use for a pre stage bias, with the + hooked
to where you would normally insert the FB. The FB and which ever control network
you choose is hooked to the - side of the cap.
It seems to be about where you would expect the amp to be tonally, and no scratch.
Fenders have always lacked some very basic considerations, to the point where
a scratchy presence pot. has become an expected traditional product feature,
that builders struggle with. Noise free operation is a recent and unrealistic
expectation unfairly placed upon old Fenders.

It can be a $1 solution, you don't have to go with pricey mil. spec. parts.
for traditions sake.

Cool man. Thanks. That makes sense...