The two channel amp 5C2+6SJ7 optional grid leak/cathode biased circuit

Jerry garrcia

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On the finishing line of the plan for this jazz guitar amp. Main purpose is archtop, semi-hollow and tele (Biltoft blade neck pickup) Bebop playing. At this stage I’m holding back on the reverb tank but might put it in later with a mosfet driver.
Will be building in a 5F2 chassis, Hammond 290CAX PT, Hammond 1760D OT, SS rectifier, 6v6, 6SC7 (for the 5C2 channel) and a 6SJ7 for the optional bias channel. Have really loved the grid leak biased pentode in previous build but want to have a pentode that also will handle pedals well and want to try the sound of the 6SC7 in a preamp stage (previously only used it as a phase inverter).
Have a don’t know which speaker to use. Have a Jensen C8R, Jensen P8R and a WGS g8c.
Planning to use @printer2 bias switch for the 6SJ7 tube and the cathode biased circuit will be based on the instrument channel of the GA-50. Might change the plate resistor as @FenderLover suggested for better clean tone.

Was thinking of using the 275 VAC secondaries and the rectifier heater windings to a LED instead of the regular pilot light that has caused me troubles in the past.

Any tips and tricks?
About the filter caps, initial plan was 22uF x3 but maybe a 47uF as the first filter cap and 22uF for the next two. Inputs?
 

Jerry garrcia

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It seems like everything will fit. A good sign.
62CA933A-74A6-4322-A180-98E23F0814FC.jpeg

Will solder the grid stoppers on the sockets.
A 47K is the standard in a GA/50. Since I’m only using one input for each preamp I’m wondering if I should increased the resistor value?
Since it is a Cathode biased amp when the biased switch is in cathode mode I’m wondering the value of the cathode resistor? Seen everything from 47K to 1 M. Ideas which resistor value to chose?
 

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Jerry garrcia

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I just realised that when I’d soldered all components in place on the board that my solid metal ground bus “wire” had gotten legs. It really pi**ed me off since I hade a couple of hours of amp time before the family wakes up and start to give me shores to fix. “Intonate my guitar”, “can you drive me to a friend”, “we really need to shift tires on the car” etc.
what can I use as an alternative? A well tinned stripped wire?
 

wangdaning

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I mean you could technically connect each part of the bus by wires between the lugs. It does not have to be an external bus. It really depends on how you set it up, but normal 20-22 awg wire is fine.
 

Jerry garrcia

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Aha. Might have to do that if I want to work on it today.
Don’t really like all the ground wires going to a power and a preamp ground point in this build since everything is quite squeezed in. A common ground bus would make it a bit “cleaner”. Now I have two separate channels and a bias switch on a champ size board in a champ size chassis.
615D1C23-F0F4-48E4-A5E4-E0A1A4E02B4B.jpeg
 

printer2

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What values did you use for the cathode and plate? I don't want to trust my eyes and my monitor.
 

Jerry garrcia

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I’ll finish a schematic in a couple of hours. Was doing it now but the wife can’t get off my back. She wants me to cut down a tree…
 

Jerry garrcia

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What values did you use for the cathode and plate? I don't want to trust my eyes and my monitor.
AEB1C474-75A4-416D-BD3D-F60DEC070E35.jpeg

My first schematic. Uncertain of the resistor to 6v6. 470k or 470R? Probably a lot of errors…
 

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Jerry garrcia

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What values did you use for the cathode and plate? I don't want to trust my eyes and my monitor.
Since I can’t get the tone circuit work for a two channel amp and one volume knob, I will use a separate tone control for each preamp tube. It will fit since I’ll tried it with the last amp you helped me with.

For the 6sj7:
cathode 2.2K
Plate 100K
Took those values of the instrument channel of the GA-50. Hopefully it works 😬.

For the 6SC7:
Cathode 2.5K
Plate 270K

The 6SC7 is a funny tube due to the common cathode for both triodes. Initially I was planning to use the second half of the 6SC7 as an extra gain stage but couldn’t get it to work so I just copied the 5C2 circuit.
 

Jerry garrcia

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That grid leak resistor on the 6V6 can't be a 470R. 470k it is...
Thank you!
I had that initially but changed it since I realised that I have a 470R an a grid resistor on another amp…
Might need to change that one. What effect could it be to have a 10 times lower value on a 6v6 grid resistor? Need to read up on that. The amp is quieter than it should be.

Anything else that is totally crazy?
 

Jerry garrcia

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I am confused by your 6SC7 first triode configuration. What's going on there?

Don't you need to connect to the second triode grid?
I am confused as well.
Forgot to draw a line. A lot of multiple work was going on. I wanted to draw a schematic, the dog needed to go out, was cutting down a large pine tree and a builder came by our country house. Since I’m a man I only got one X and one tiny Y chromosome and having a hard time doing multiple shores at one time. Now hopefully correct.
It’s the last picture.

Used the original fender schematic and a layout already out there.
23AFA3B2-0666-4CD0-B08D-8D5F7C2462A9.jpeg
EB85B5B4-4535-4AA2-B546-40E3B6B62D14.jpeg

95685B0F-DBF2-4CA0-A22F-3F602D175C9C.jpeg
 

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tschwarz

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The pre-amp pentode is switchable from "normal" cathode bias to "antique" grid leak current biasing. Never seen that before! Did you pull that idea from some existing design? I'd love to do some research on the topic...
 

Jerry garrcia

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The pre-amp pentode is switchable from "normal" cathode bias to "antique" grid leak current biasing. Never seen that before! Did you pull that idea from some existing design? I'd love to do some research on the topic...
Yep. That’s the whole point. Love them octal preamp tubes. Only used grid leak biased 6sj7’s before. Did two builds with them. One 6SJ7+5B3 deluxe (grid leak pentode and a channel with regular fenderish 12ay7 for clean sound) and one GA-5+5F1 reverb (same grid leak pentode and same 12ax7 + a spring reverb, not complete yet due to a problem with the tone circuit, and realised today that the 6v6 had a 470R cathode resistor instead of a 250R). The problem with that grid leak biased circuits is that they don’t handles pedals very well. Wanted to use @printer2 idea with the bias switch. He’s the man when it comes to this. I just wanna learn and build amps that suits my purpose. Had a couple of spare 6SC7’s and read some good reviews of the sound of the 5C2 amp so I thought I would combine the switchable bias circuit and a 5C2 circuit. Due to lack of space in the spare 5F2a chassis I have, I skipped the 5y3 and will use UF4007 diodes instead.
@printer2 is the man you wanna talk to regarding the switch between different biases when it comes to octals. He has produced various different posts with this bias switch. And a REALLY nice guy.

If I get this to work I want to build a four channel amp with two different, switchable bias pentode circuits + one 6SC7 channel + one 12ay7. Upon that I want to be able to use 6v6’s and 6L6’s and have a switch that controls which power tube section to use. And maybe a power reducing circuit?
 
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printer2

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The 6SC7 input needs a grid leak resistor. The 100k on the pentode channel may reduce the signal and roll off the highs more than desired, then again maybe not. Depends on the guitar and the cable capacitance. Would I have done the triode part that way? No. But now looking at it I wonder how it will be? There is only one problem with the triode channel (other than the grid leak resistor). The cathode capacitor value of 22 uF is too small. There is a good chance of you getting a thing called motorboating. The problem is the second triode signal is inverted as compared to the first stage. So when the first stages is going up, the second is going down. Theoretically the AC signal across the cathode resistor should be shorted to ground and the tube would know any better. But at some low frequency the impedance of the capacitor will increase. Hey look at that, a capacitor calculator (yes I know the math, I also can multiply and divide without a calculator, but I have less useful things to use up my brain cells with).


(the 2.5k resistor will probably be a 2.2k or a 2.7k now days.) The calculator does not have a 6SC7 but a 12AX7 will show the issue well enough. Say we use the following numbers in the calculator, 22 uF, 250k, 100k, 100k. Hit the calc button and the bypass capacitor is starting to roll off at 20 Hz. This would be no issue as far as the frequency response of each individual stage, lots of space between 20 Hz and 82 Hz that there will be no concern of frequency rolloff.

But the curve going down at the frequencies below 20 Hz is a concern. The slope of a single stage of filtering (which is what the resistor and capacitor acts as) goes down at 6 dB per octave. The stage shows 34 dB gain at 20 Hz. At 10 Hz it is 28 dB, at 5 Hz - 22 dB, at 2.5 Hz - 16 dB, at 1.25 Hz - 10 dB, at 0.625 Hz 4 dB, at 0.3 Hz it will have no bypass capacitor action.

So about once every second to two seconds you might hear a thump, thump, thump...

Which sucks. (my first attempt building an amplifier in the mid 70's turned out to be a low frequency oscillator). Darn, wish we could lower the frequency way below where it can interfere with our sonic bliss. A good thing capacitance is cheap to get now days. A 12-15V capacitor with 220 uF to 470 uF is peanuts as compared to when the early Gibson's were made. Might make things tight on the board, or the capacitor hanging off the side of the board if need be. But something that is necessary in my opinion.

Just noticed the 470R, depends on the voltage. A BF Champ has a 470R with a voltage of 350V on the plate. In the datasheet a 250V plate has a bias of -12.5V with a 45 mA plate and 4.5 mA screen current. So 12.5 / (45 + 4.5) = 252R. Darn.
 
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Jerry garrcia

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The 6SC7 input needs a grid leak resistor.
I was planning of attach the 68K grid leak/grid stopping resistor directly on pin 4 on the 6SC7 socket and the 0.047uF + 68K on pin 4 on the 6SJ7 socket. Read that it might decrease hum (and don't have much space on the board). The original GA-50 schematic has a 47K grid leak resistor. When reading about it, it seems like it doesn't matter?
The 100k on the pentode channel may reduce the signal and roll off the highs more than desired, then again maybe not.
I was thinking about that. The original schematic does state a 100K in that position. But I have also read that there are some concerns with the amp producing a "dull" sound. Also I only have 100K 1W resistors and they are large and ugly. Have some nice looking 470K and 1M resistors. Should I change it?

I did also read some stuff that people have concerns with the plate current and its effect on the primary winding of the impedance of the OT in that circuit. Don't really understand what they mean with that.
the 2.5k resistor will probably be a 2.2k or a 2.7k now days.
A 2.2K it will be.
Would I have done the triode part that way? No.
Hey, I just copied what is already produced. As you know I don't really care if it is a true clone/exact copy. I just want to have a nice sounding amp. If anybody has a better idea I'm all ears. I might produce this and use a "better" triode design for the next build or change the components and design after a while? Time and money is all I got, not :)
There is a good chance of you getting a thing called motorboating
Thank you! Strange that they used this design if this problem might occur. I just ordered 10, 16 and 20V capacitors with 220 and 470uF on my phone during a board meeting. Quite similar in size. Will a 470uF be better and will a 10V work?

Thanks for all the inputs! Thinking of putting this thing in an old tube radio.
 




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