V2 Cathode Resistor Mod In A 5C8 Twin, Value?

[rp]

OK, I'll beat you all to it - Priceless! But seriously...

I'm going to make channel 2's 6SC7 cathode biased rather than grid-leak, any suggestions on the proper value for the cathode resistor? The 5D8 with novals has 470 ohms as the value but for V1 & V2 all strapped together, so 820 I'm thinking but not sure how it all translates to the 6SC7, and with it's single cathode for both plates, and it's gain, etc.

And, BTW can anyone hear a difference btwn 25mf and 250mf as a pre-amp tube's cathode bypass cap? I don't think I hear any diff after 4.7 but that might just be me. I'd just as soon use 25mf as space is tight and I have those on hand.

[tubeswell]

The 6SC7 is somewhere between a 5751 or a 12AX7 in tube characteristics, so run them like you would a 12AX7

[martin manning]

Looking at the data sheet it isn't too clear how the common cathode relates to the charts which are marked "each section." Since the Fender schematic shows a common 250k plate resistor, and the table suggests something around 500k, I think you could assume that the data sheet does consider the sections as being truly separate. With that the Rk should be 2k2 to 3k depending upon what your supply voltage is for those stages (180 to 300V). Maybe you can take a shot and see what cathode voltage you get, then tweak it to get ~2V.

[rp]

Maybe you can take a shot and see what cathode voltage you get, then tweak it to get ~2V.

That makes good sense. I just found this, which suggests that if Fender had used cathode biasing on the first stage of the 5C8 they'd have likely used 2500. I'll start there. Thanks.

[M Fowler]

Here is the schematic I used to build a Bassman 5b6.

[martin manning]

OK I divided by two instead of multiplied above; fixed now. I plotted the load line and I'm convinced that the plate curves are plotted for each section, as if the cathodes were separate, and the table Rp values are for one plate load ('cause you could), but the Rk values shown there are for a common cathode resistor ('cause you have to). So 2k7 would be a good bet, and look for about 2V on the cathode(s).

Edit: notice on the Princeton there are separate 250k plate loads and the two triodes are cascaded, whereas on the 5C8 there is a shared 250k plate load and the triodes are paralelled- but they have separate inputs. For practical purposes the plate load on the 5C8 is 250k, like the princeton. The Bassman circuit Mark posted has a similar 270k load, so yea 2k7 looks good.

[Colossal]

Here is the schematic I used to build a Bassman 5b6.

Mark,

Did you use 2M2 input grid leak resistors or the more typical 1M? How about grid and screen resistors on the power tubes?

Thanks,
Dave

[M Fowler]

Here is the schematic I used to build a Bassman 5b6.

Mark,

Did you use 2M2 input grid leak resistors or the more typical 1M? How about grid and screen resistors on the power tubes?

Thanks,
Dave

Dave,

I'm not really sure. I checked my photos but can't see the resistors but I am assuming that I used 2m2. The amp is at the music store so I can't check easily either.

Wired it just like the schematic shows no screen resistor and no grid resistors on the 6L6s.

Mark

[Colossal]

Dave,

I'm not really sure. I checked my photos but can't see the resistors but I am assuming that I used 2m2. The amp is at the music store so I can't check easily either.

Wired it just like the schematic shows no screen resistor and no grid resistors on the 6L6s.

Mark

Right on, thanks Mark. I was just wondering. Bet that is a nice sounding amp. Looks good too. Love that octal tone.

[M Fowler]

Dave,

The longer the amp is on the better the tone we swear we can hear the amp changing for the better not worse and not red plating of course.

I really should do a video of that amp or sound clip, I love the octal tone.
I prefer this 5b6 over my Gibson EH150.

Mark

[Colossal]

Dave,

The longer the amp is on the better the tone we swear we can hear the amp changing for the better not worse and not red plating of course.

I really should do a video of that amp or sound clip, I love the octal tone.
I prefer this 5b6 over my Gibson EH150.

Mark

That's very cool about the edge going to the 5B6 vs. the EH150!

[M Fowler]

I'm putting another one together in Princeton Chassis but I plan to use a board not PTP.

Also, plan to use screen resistors and grid resistors, 470k and 1k5 most likely. I bet I will notice a tone difference.

Mark

[rp]

notice on the Princeton there are separate 250k plate loads and the two triodes are cascaded...

Totally missed it. Interesting, a bit different from its contemporary 5C1, thanks for pointing this out even though it would have worked out the same.

Here is the schematic I used to build a Bassman 5b6.

I missed this one too, thought I had gone through them all, this one settles it but now I wonder if I should set up V2 5B6 style or later 68K/68K-1M style as I intended?

Did you use 2M2 input grid leak resistors or the more typical 1M? How about grid and screen resistors on the power tubes?

Good, good - this discussion went just where it needed to go, I was going to ask about this next. Lack of screen grid R's on the power tubes doesn't much bother me until I see the voltages. My Deluxe clone and tons of others, real and clones, kick it out reliably, though I have some special bottles for this I'd like to be conscientious about. If the mojo here turns out to be in the cranked zone, I might add them. Lack of stoppers is more troubling but these old low gain relics seem to be fine the way they were designed. Hoping for the best and I want to hear then au naturale before messing around. Will wait to see what happens when maxed.

I'm putting another one together in Princeton Chassis... plan to use screen resistors and grid resistors... I bet I will notice a tone difference. Mark

Please report back.

That's very cool about the edge going to the 5B6 vs. the EH150!

Hahaha, there's a reason Fender ate Gibson's lunch when it came to amps. Had Charlie Christian not died young bet you 10 1950 dollars you would have caught him on a Fender soon enough and few would have remembered the EH150. Not that I don't want to build one.