Gibson GA-50 tone stack

[yaanno]

Hey,

I'm struggling a bit to piece together the mechanics of this tone stack. To be honest I'm lost. Could you guys elaborate on what is going on here? Attached the "cleaned up" schematics of the circuit too (might contain errors). The Gibson style schematics are a bit hard to follow, at least for me. Thanks a million!

[Phil_S]

This is from a GA-50T not GA-50. There is a big difference. Be certain of which amp. Lower pot is not grounded. Some things are not placed correctly. The 500K instrument pot gets input to wiper from J2-J4. The long wire going down doesn't belong there. Maybe this helps?

[yaanno]

Oh my...

You are totally right, I was looking at the T version :D

The long wire goes to the other volume pot I omitted from the tone stack circuit as shown here on the correct schem, (thanks again!): https://www.prowessamplifiers.com/schem ... n/ga50.pdf

I'm planning to build a 2 channel version with one "mike" and one instrument channel, separate volume controls. Will update my schema with this tone circuit. I have a lot of russian 6SJ7s so the preamp is not a problem. On the other hand the phase inverter will be updated slightly either with 6N7s or 6SN7s. Luckily I have several NOS ones. It is very tempting to use a different pre pentode as I have 2 full crates of 6AC7 too. These might be finicky though. Will report back on the progress!

BTW your Gibson build (GA-20) is very inspirational, gotta love them old Gs :)

[yaanno]

I hope this looks better now.

[Phil_S]

Thank you for the kind words...I generally figure no one is paying attention to what I do.

I'm not so sure about using a 6N7 in place of a 6SN7. The 6N7 has a higher plate resistance and twice the gain. I don't believe this is a good thing for a paraphase inverter. My primitive understanding and limited experience with the paraphase suggests to me that this particular inverter is more about current (Amps) than voltage. You will be better served with a 6SN7.

If I were doing this build, I'd skip the mic channel. I don't think it offers anything useful. You can simplify the build a great deal with a single input the way I did the GA-20. Basically 6SJ7 > 6SN7 > pair of 6L6. Reducing the number of tubes will reduce the demand for heater current and HT current supplies. I think I'd skip the tube rectifier in favor of 1N4007's or something similar, but be sure to consider how that will raise B+ voltage when spec-ing your PT.

Are you going to build the tremolo? That looks like a good thing to include. For the tremolo to trigger oscillation, it needs a strong-testing dual triode. (OK, you can use two old-timey tubes that have a single triode.) I wouldn't hesitate to use a 12AX7 for that, though a 6SL7 should be fine.

If you have 6SJ7's why even think about 6AC7's? If you have the time and want to experiment, there's no harm in it I can see.

Of course, this is your build, not mine, so you should do as you wish. Just my 2¢ and thanks for the opportunity to comment. Good luck with it.

[didit]

Hello --

Perhaps you will find helpful insights via these two threads:

• GA50 aspirations
• Gibson GA50 Clone Noise Issue

Pay close attention to the struggles and ultimate success in that second one. Some lessons certainly.

Observations to share:

• In my view (and that of others who've played through my clone) the mike channel is cool. Try both and decide.
• Making the amp single channel and adding some switching to allow shifting the pentode preamp gain on might be interesting. So far though I've not bothered.
• Using 6SN7/6SL7 base and rolling tubes is interesting, and the additional gain of the SL does shift the amp from clean toward more warm distortion.
• Recommendation for silicon rectification is wise. Cheaper and more reliable, and with the "PI" choke primary filtering and level of power tube bias "sag" is off topic.

Good luck and have fun.

Best .. Ian

[Phil_S]

In my view (and that of others who've played through my clone) the mike channel is cool. Try both and decide.

I was unaware and asserted a view not supported by experience or evidence. I should not have said to skip the mic channel. Sorry and thanks for mentioning.

[Phil_S]

I hope this looks better now.

You still have the bass pot grounded. That's a mistake. The bottom lug goes to the 2.2K and then the PI.

[sluckey]

You still have the bass pot grounded. That's a mistake. The bottom lug goes to the 2.2K and then the PI.

That's not what my schematics show. That 2.2K is the PI cathode resistor and it connects to ground. Same for the ga50 and ga50T. Has nothing to do with the bass pot.

[Phil_S]

You still have the bass pot grounded. That's a mistake. The bottom lug goes to the 2.2K and then the PI.

That's not what my schematics show. That 2.2K is the PI cathode resistor and it connects to ground. Same for the ga50 and ga50T. Has nothing to do with the bass pot.

Gibson is well known for multiple schematics for one amp model. Here is what I'm looking at. Did I read it wrong? Is the schematic wrong? I am not invested in being right, so let's get to the bottom of it. I see a direct path the ground through the 500K pot.

[sluckey]

That's the same schematic I have.

All four pots in this snip are connected to ground. The 2.2K cathode resistor is also directly connected to ground. The 2.2K does not have to go through the bass pot to get to ground. Could it have been drawn more clearly? Absolutely. But that's not the Gibson way!

[Stevem]

It might interest you to see the schematic for my 54 GA77.

This amp sounds amazing and especially when you jump both channels.

Anyone of the 30 or so that have played it want to buy it.

This model amp was made in true Gibson fashion with like 3 different tube complements this version is the most rare and in fact took me two years to find a schematic for it.

A very big part of this amps feel and tone when cranked Is the 6J5 pi tube and the 15” Jensen Alnico magnet speaker.

You could sub the 9 pin preamp tubes for octal’s with a pt having more heater current.

[yaanno]

Thank you for the kind words...I generally figure no one is paying attention to what I do.

I think folks do here. I've been lurking around for a while, especially looking for the Gibson discussions.

I'm not so sure about using a 6N7 in place of a 6SN7. The 6N7 has a higher plate resistance and twice the gain. I don't believe this is a good thing for a paraphase inverter. My primitive understanding and limited experience with the paraphase suggests to me that this particular inverter is more about current (Amps) than voltage. You will be better served with a 6SN7.

I admit I stole this from the Fender Model 26, that circuit has a para PI too. But you are absolutely right, and while I checked the 6J5 characteristics, the 6SN7 very well resembled that tube.

Are you going to build the tremolo? That looks like a good thing to include. For the tremolo to trigger oscillation, it needs a strong-testing dual triode. (OK, you can use two old-timey tubes that have a single triode.) I wouldn't hesitate to use a 12AX7 for that, though a 6SL7 should be fine.

I really want a tremolo there :) I've been thinking inserting Sluckey's clever tremolo idea in the circuit once I get everything figured out, at least on paper.

If you have 6SJ7's why even think about 6AC7's? If you have the time and want to experiment, there's no harm in it I can see.

The reason is basically that I have plenty of them but I have a lot of 6SJ7s too. So yeah I will stick to that first. 6AC7 is not that great for AF purposes according to the RCA manual. Will figure that out :)

[yaanno]

• GA50 aspirations
• Gibson GA50 Clone Noise Issue

Pay close attention to the struggles and ultimate success in that second one. Some lessons certainly.

Thanks Ian, I missed the second one somehow. I basically used your annotated schem for figuring out the component values and voltages, so thanks for that!

Using 6SN7/6SL7 base and rolling tubes is interesting, and the additional gain of the SL does shift the amp from clean toward more warm distortion.

I'm sure everyone who attempted to reconstruct the 50 wanted to have that mellow jazz tone, well me too :) On that note I used the 6SL7 only in the pre section but the extra drive here is tempting!

Recommendation for silicon rectification is wise. Cheaper and more reliable, and with the "PI" choke primary filtering and level of power tube bias "sag" is off topic.

Thanks Ian and Phil, will definitely go this way. That 5V4 doesn't worth it at the moment.

[yaanno]

Could it have been drawn more clearly? Absolutely. But that's not the Gibson way! :mrgreen:

Interestingly the later schems been drawn much nicely, at least to my eyes. So I think my tone stack understanding is right per the original circuit?

The GA-Custom designated amp schemo (attached) is almost 100% GA-50 without the tremolo. It is a bit cleaner than the GA-50 one. What do you think?