How to remove phase inverter tube from design?

[BoomTexan]

I'm designing a parallel 6L6GC single-ended Fender Bandmaster clone, and I just realized that I really don't need the phase inverter at all. How should I go about removing it from the circuit? Should I just split the signal coming out of the preamp stage into 2 parts, get it to hit the grid stoppers, and then go straight to power tubes? Is there something else I need to do too?

My schematic so far is pretty cut and dry with some minor voltage and output transformer alterations, nothing that could be a problem really. I do have a potentiometer controlling the negative feedback loop and a switch that will drop one of the current signals to a tube for 1/2 power operation, but that's not influencing the design all that much.

This is mostly a learning project, something to do to really get to know the power stage and see if I can be a little oddball with my designs. I've got a friend lined up that wants me to build him one for $500, so I'd like to have a functioning design at least.

 

[peteb]

How should I go about removing it from the circuit? Should I just split the signal coming out of the preamp stage into 2 parts, get it to hit the grid stoppers, and then go straight to power tubes? Is there something else I need to do too?

basically.

I would find a particular single ended output stage and copy that. The 5F1 shows a coupling cap followed by a grid leak resistor and no grid stopper.

 

[Ten Over]

I'm designing a parallel 6L6GC single-ended Fender Bandmaster clone...

Which one?

 

[BoomTexan]

Which one?

6G7, not the A variation.

 

[BoomTexan]

basically.

I would find a particular single ended output stage and copy that. The 5F1 shows a coupling cap followed by a grid leak resistor and no grid stopper.

Alright, thanks, I was pretty sure of that, I just wanted to make absolutely sure. Also, thanks for the suggestion.

 

[Ten Over]

6G7, not the A variation.

You might consider replacing the PI with a single gain stage so that you have some place to inject the NFB.

 

[BoomTexan]

You might consider replacing the PI with a single gain stage so that you have some place to inject the NFB.

Honestly, the person I'm building it for is going for that balanced blonde overdrive sound at low volumes, so I'm not really sure if he even wants that. I'll ask, but I think he'd rather drop the gain stage and leave the feedback loop out. I was planning on having it on switch-on/off status anyways. Plus, I'm running pretty close to the mA that the power transformer can handle, so I don't want to risk it with another tube.

I do have one more question about the power stage. It seems that on the bigger blonde amps, the grid stopper resistor is missing. 6G2 Princeton has it, but the 6G7 doesn't. Is there some reason for this? Just planning on leaving it be, but I'm just seeing because I'm very curious as to why they would want that.

 

[Ten Over]

I do have one more question about the power stage. It seems that on the bigger blonde amps, the grid stopper resistor is missing. 6G2 Princeton has it, but the 6G7 doesn't. Is there some reason for this? Just planning on leaving it be, but I'm just seeing because I'm very curious as to why they would want that.

The 1.5k grid stopper attenuates signals that are way above audio frequencies. This lowers the probability that the tube will oscillate at those high frequencies. The grid stopper has no effect on audio frequencies.

 

[BoomTexan]

So just a safety/failsafe feature rather than a noticeable audio difference?

 

[Wally]

Curious I am… Are you not concerned that this drastic change in the output section will have an equally drastic change in what this amp does sonically compared to the 6G7? Imho, a low output 6G7 would run 2 x 6V6s with a LTP PI, fixed bias….maybe on a switch to go to cathode bias, and an adjustable NFB loop. A parallel, single-ended output is going to change things drastically it seems to me. ???? I could be wrong. Also, he must be a goooood friend to build at that price, imho.

 

[Ten Over]

So just a safety/failsafe feature rather than a noticeable audio difference?

Sure, as long as you consider the prevention of ultrasonic oscillation a safety/failsafe feature.

 

[lemme35]

I'm designing a parallel 6L6GC single-ended Fender Bandmaster clone, and I just realized that I really don't need the phase inverter at all. How should I go about removing it from the circuit? Should I just split the signal coming out of the preamp stage into 2 parts, get it to hit the grid stoppers, and then go straight to power tubes? Is there something else I need to do too?

My schematic so far is pretty cut and dry with some minor voltage and output transformer alterations, nothing that could be a problem really. I do have a potentiometer controlling the negative feedback loop and a switch that will drop one of the current signals to a tube for 1/2 power operation, but that's not influencing the design all that much.

This is mostly a learning project, something to do to really get to know the power stage and see if I can be a little oddball with my designs. I've got a friend lined up that wants me to build him one for $500, so I'd like to have a functioning design at least.

let see pictures

 

[Phrygian77]

So just a safety/failsafe feature rather than a noticeable audio difference?

They also serve the purpose of keeping the impedance from dropping to almost nothing as the grid approaches 0 volts.

 

[peteb]

Do you have a schematic to follow?

this is classic

The rest of the info:

(link removed)

 

[Wally]

Or something from a much earlier age….

https://schematicheaven.net/gibsonamps/gibsonette.pdf

 

[printer2]

The 12AX7 in the PI stage takes about 1mA each section, if your PT can't take two more mA then you are in trouble. Unless you are talking heater current. I would think changing to cathode bias, single ended and no NFB is going to change the sound away from being a Bandmaster. Not to say it will sound bad, but who knows until you fire it up. The loss of the gain from the PI should be rather noticeable. Stick in a 6AT6 maybe? Or a 12AU7 if you want 9-pin. Still a little low as far as gain but not that far off. They eventually changed to include grid stoppers in all the designs. Something to be said about an ounce of prevention...

 

[BoomTexan]

let see pictures

I'll be finishing the preamp stage early tomorrow, I'll send pictures then. I'm just making absolutely sure that I'm getting everything right, so its taking a while.

Do you have a schematic to follow?

this is classic

View attachment 971048

The rest of the info:

(link removed)

I'm taking inspiration from that, the Double Six, and the GA-8, just adjusting voltage accordingly.

Curious I am… Are you not concerned that this drastic change in the output section will have an equally drastic change in what this amp does sonically compared to the 6G7? Imho, a low output 6G7 would run 2 x 6V6s with a LTP PI, fixed bias….maybe on a switch to go to cathode bias, and an adjustable NFB loop. A parallel, single-ended output is going to change things drastically it seems to me. ???? I could be wrong. Also, he must be a goooood friend to build at that price, imho.

Well, this amp started out as a cut and dry 6G7 with parallel output tubes, but as time went on, the need for other modifications to run that setup became necessary. Now we're here, with the reverb tank from a Vibroverb in place of the vibrato channel, a parallel tube setup that I haven't been able to find anywhere else, a missing phase inverter, the preamp from a 6G7 Bandmaster, and no negative feedback loop. I think at this point the amp is pretty much entirely unique.
Well, I was able to source all the parts for 250, and I have a ton of tubes lying around from a panic buying spree, so I quoted 500. I estimate it'll take me 10-15 hours to build if I really really take my time, and 15-25 an hour is good pay for me.

The 12AX7 in the PI stage takes about 1mA each section, if your PT can't take two more mA then you are in trouble. Unless you are talking heater current. I would think changing to cathode bias, single ended and no NFB is going to change the sound away from being a Bandmaster. Not to say it will sound bad, but who knows until you fire it up. The loss of the gain from the PI should be rather noticeable. Stick in a 6AT6 maybe? Or a 12AU7 if you want 9-pin. Still a little low as far as gain but not that far off. They eventually changed to include grid stoppers in all the designs. Something to be said about an ounce of prevention...

Well, since I have a Vibroverb reverb tank, there was an extra 1/2 tube in that section that further amplifies the reverb signal and the normal channel signal together, so I'm using that as an extra gain stage in lieu of the phase inverter gain stage. It should perform the same function, after all, each signal only hits one half of the 12ax7, whereas both signals are hitting the same half of this 12ax7 in this idea.
I was talking about heater current as well, I'm about .3A off the max, but given that my current ideal power transformer has a 200mA output, and people recommend 90-100mA per 6l6 to be safe, I don't want to push it too far. Thats why I want to keep it at 3 preamp tubes. I could probably go to 4 with no real problems, but still...

 

[BoomTexan]

While posting this I noticed a loose end connecting the power and preamp and reverb stages together, I've gone back and fixed it.

 

[printer2]

Couldn't just take a picture of the whole circuit oriented so use that do not use our phones do not have to turn our heads and scroll back and forth trying to match up the pages? Not being critical, just a snarky suggestion.

 

[BoomTexan]

Couldn't just take a picture of the whole circuit oriented so use that do not use our phones do not have to turn our heads and scroll back and forth trying to match up the pages? Not being critical, just a snarky suggestion.

Sorry, haha, just took all those closeups so that if you couldn't see something in the circuit, you could get a closer look, but then I forgot to take the actual picture.