5B6 Modded Layout

[reddead83]

Been thinking about building a modded 5B6. Can someone check my layout please?

Thanks!

[reddead83]

Schematic.

[labb]

Word of caution for what it is worth...I built a couple of the 5C3 Deluxe amps that use the 6SC7 tubes...Had a lot of trouble finding one that was not microphonic.

[reddead83]

I appreciate the heads up. Did you try the new Sovteks? I heard they’re pretty good.

[sluckey]

The horizontal yellow wire that connects from the 220K/500pF just below the treble pot should connect to the BOTTOM side of the 250K board resistor, ie, the plate of V1. As drawn, it is connected to B+.

The yellow grid wires are connected directly to pin 5 on the output tubes. That's just like the schematic. But you have 1.5K grid stoppers mounted on the socket (a good thing IMO). You need to move the yellow wires from pin 5 to pin 6 in order to use the grid stoppers.

The layout is missing the red wire on the OT primary.

[labb]

Did try the Sovteks. They worked pretty good. I would have to go back and check but seems that on the last amp I rewired it and used Tung-Sol 6SL7's. Minor change to do that.

[reddead83]

Might have to do that labb, thanks.

Thank you sluckey. Attached is the revised layout.

[reddead83]

Here is the layout with a 6SL7 for V1.

[reddead83]

Realized I should probably have a coupling cap on the plate before the tone control right?

[uki]

Very interesting, would you please share the schematic as well ?! Thank you !

[sluckey]

Realized I should probably have a coupling cap on the plate before the tone control right?

yes

[66Strat]

The tone stack will eat up quite a bit of signal. You may want to consider cascade configuration of the V1a and V1b triodes rather than parallel. The tone stack could be placed either between the two gain stages or after the second.

[reddead83]

The tone stack will eat up quite a bit of signal. You may want to consider cascade configuration of the V1a and V1b triodes rather than parallel. The tone stack could be placed either between the two gain stages or after the second.

Complete amateur here. That would look something like this?

[66Strat]

Close. There needs to be a plate resistor, coupling capacitor, and grid leak resistor added between the second gain stage and the PI. I have added suggestions for consideration to your schematic. The cathode resistors were increased to 5K to maintain the same bias voltage as when the two triodes shared a common cathode resistor. I also recommend reducing the input grid resistor to 1M and adding an input grid stopper to reduce noise. You will need to play around with coupling capacitor and by-pass capacitor values to see what you like. I provided starting point ranges for each.  See attached.

[PRR]

That's a LOT of gain.

[shooter]

LOT of gain.

square waves sound pretty good in a death metal sorta way 

[66Strat]

Yes there is too much gain. The V1A gain stage is about 57. The tone stack loses about 10db, or .68 of the signal. A distributed plate load in the V1B gain stage using a 10K resistor in series with a 240K resistor should bring the overall preamp gain back pretty close to the gain in the original circuit.

[reddead83]

Already have a 5150III for death metal so I'm good on that front. 

Modified the input to later Fender two input. Any comments?

[PRR]

> Any comments?

Why the 240K:10K divide on the 2nd stage?

Counting on thumbs, I'd say not enough gain.

[66Strat]

PRR, my suggestion of the distributed load may have led the OP astray. I did not go through the voltage gain calculations that you performed and may have erred in my approach and calculations. I focused my attention to the triode immediately following the tone stack. My objective was to tune the gain of the triode section to just make up for the signal loss in the tone stack. I estimated the signal loss in the tone stack to be 10db or a voltage loss ratio of 0.32. To offset the loss, I was looking for a voltage gain of approximately 3. I calculated a voltage gain of 57 for the triode using the formula VG = u x RL / (RL + rp). The RCA RC19 manual shows the voltage gain as being closer to 44.  I can’t explain the discrepancy. Using a VG of 44, as reflected the manual, the distributed load needed for a voltage gain of 3 would be closer with a 20K resistor in series with a 230K resistor, 44 x 20K/(20K + 230K) = 3.52. That is if my estimate of a 10db tone stack signal loss is correct. If your estimate of a 0.10 voltage loss ratio for the tone stack is correct, then a voltage gain of 10 would be needed to offset the loss. A 56K resistor in series with a 200K resistor for a distributed load would approximate this gain, 44 x 56K/(56K + 200K) = 9.6. All of this is assuming that I did not err in my calculation. The distributed load is just one approach in solving the problem. Another approach may be a better.

[sluckey]

Your schematic is beginning to look a lot like Ampeg. I'd just build this...

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b12n_b15n.pdf

[reddead83]

Your schematic is beginning to look a lot like Ampeg. I'd just build this...

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b12n_b15n.pdf

That’s what I was thinking. Might just build it stock or move onto something different.

[PRR]

> I calculated a voltage gain of 57 for the triode using the formula VG = u x RL / (RL + rp). The RCA RC19 manual shows the voltage gain as being closer to 44.  I can’t explain the discrepancy.

Too hot to beat my brains (or thumbs). "rp" from the datasheet show-off numbers is often far from where we work a tube in audio amplifier service. You could figure it out. Some long-ago junior engineer compiled that data for us. Take the table data.

But while looking at 6SL7 data, skip-past the first page and go to the bottom of page 2. Yes, at 2.3mA the Mu is 70 and rp is 44k. But the proposed circuit shows a 250K plate resistor. To fit this to the show-off data we would need an 825V supply! (Sheet cites 250V 2.3mA in tube. 2.3mA times 250K is 572V drop in plate resistor.) No, more likely we have 250-300V total supply, half of that on the 250K, so we are near 0.5mA. On the Mu and rp  graph near 0.5mA, rp is up near 77k and Mu has drooped to ~~65. (Does that work out?)

> If your estimate of a 0.10 voltage loss ratio for the tone stack is correct,

I recognized it (the "James") but you can rough-analyze by eye. Note that top-and-bottom parts are 10:1 ratios. 220K and 22K. 0.001 and 0.01. Not shown explicitly: this stack can only be "flat" if the pots are 10% taper (mechanical center gives 90:10 resistance). In the "flat" centered-knob setting the loss is 10:1. (Actually 11:1... nobody ever makes that distinction.)

I wasn't saying "No!" I would say the proposed 240K+10K should be only lightly-tacked in initial build and then adjusted on test.

[MFowler]

I build the 5B6 amp heads stock circuit but wired PTP so I use a small Hammond chassis with cage top.
Love the stock tone circuit and still use the .1 coupling caps but others like to change those to .047 or even .02 they say more guitar friendly.


The 6SC7 by Sovtek is good as well as the 6SL7 tubes.


I don't feel the 5B6 needs a tone stack.


Mark

[66Strat]

PRR, Thank you for your insight regarding plate current, plate resistance, mu. That really helps me to fill in the blanks. I found a GE 6SL7GT datasheet that has the chart that you mentioned. Referencing this chart at 0.5 ma plate current, plate resistance is about 75K and mu is about 64. I recalculated the voltage gain using these numbers with a 200K load and came up with a voltage gain of 46. This is very close to the VG of 44 reflected in the RCA table for similar but not exact operating points. Thanks again for the insight.

"datasheet show-off numbers", priceless!

[66Strat]

Your schematic is beginning to look a lot like Ampeg. I'd just build this...

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b12n_b15n.pdf

That’s what I was thinking. Might just build it stock or move onto something different.

Build the stock circuit as a starting point. This really is a cool circuit. You can always tweak coupling caps and tone control caps later. The only change that I would make is to use a 6SL7GT with the cathodes tied together. This would be the equivalent of a 6SC7. The advantage of using 6SL7's IMO is greater selection of tubes. I would build the amp in a head format and shock mount the tube sockets with silicon washers to further help in preventing microphonics.

[reddead83]

A lot of great replys. Appreciate all the insight and comments. Seems like the consensus is to build it stock since it’s a great circuit as is so I’ll do that.