Converting an old Conn organ amp into a Pseudo-6G2

[Britt Rossman]

Hey folks,

A friend wants me to convert this old Conn amp (2x5881, 5U4GB, 2x12a_7) into a guitar amp, so we're starting with a brownface Fender 6G2 Princeton as a platform. Converting for use with 2x 5881 (6L6GB) that are OEM tubes (along with that recto ), and splicing in a modified James tone stack.

In checking the PT, I noticed the (center-tapped) heater windings give 12.6v, so I was curious if I could wire the 5881's in series, while making the 12a_7's parallel? According to Merlin's book, using 12v filament lines (2x voltage, 1/2 current) will reduce potential filament hum by 6dB compared to 6v lines...

So, is this series/parallel filament arrangement possible? Or should I simply tie the filament winding together and use the center-tap as the other side, then create an artificial ground in the standard fashion, thus creating a 6v filament line?

[Leo_Gnardo]

So, is this series/parallel filament arrangement possible? Or should I simply tie the filament winding together and use the center-tap as the other side, then create an artificial ground in the standard fashion, thus creating a 6v filament line?

No point in having a grounded center tap AND adding a pair of R's for "artificial" CT. DO ground your CT then run one output tube off one 6.3V side, and t'other off the other side. Only 2 pre tubes, similar, one off each side. No gain monster here, I doubt you'll have much problem with filament induced hum in your preamp stages.

I like your plan, a more powerful version of the wonderful old Brown Princeton, an favorite of mine.

[Pdavis68]

Watch the filament voltage in that. I had a Conn Caprice that I stripped. I ended up selling the amp, but because it was powering over 40 tubes (and their filaments) in the tone generator, I suspect without the load of all those tubes, the filament voltage will probably go up out of spec.

[olddawg]

Watch the filament voltage in that. I had a Conn Caprice that I stripped. I ended up selling the amp, but because it was powering over 40 tubes (and their filaments) in the tone generator, I suspect without the load of all those tubes, the filament voltage will probably go up out of spec.

A couple of diodes in parallel forward and backward in with the filament line will drop it .6v per pair.

[Pdavis68]

A couple of diodes in parallel forward and backward in with the filament line will drop it .6v per pair.

I doubt they're DC. But you can use resistors to drop it.

[Pdavis68]

Doh, nevermind. I see what you mean.

[eschertron]

So, is this series/parallel filament arrangement possible? Or should I simply tie the filament winding together and use the center-tap as the other side, then create an artificial ground in the standard fashion, thus creating a 6v filament line?

If I read this right, I'd have to say please don't short the filament leads together thinking that you'll double the current capacity.

edit: and yes, using the whole 12.6vac to help cancel the hum will be a good thing, as mentioned. Be sure to 'reverse phase' the legs for each 5881.

[Britt Rossman]

Did I do it right?

If I read this right, I'd have to say please don't short the filament leads together thinking that you'll double the current capacity.

I was planning on using the center tap AS an "outer" lead, and short the outer leads together for the other "outer" lead to make a 6vAC line. THEN I was thinking of putting everything parallel across that and having to THEN throw in a humdinger.... IDK what I was thinking... Current handling really isnt gonna be an issue, and was never a worry in my mind - the total draw I'll have is under 2A at 12v... wow!

Be sure to 'reverse phase' the legs for each 5881.

What exactly do you mean by this? Do you mean run the filament line like this..? :

12v upper lead-->5881-a pin 2-->pin 7-->5881-b, pin 7-->pin 2-->12v lower lead (see diagram)

Regardless, I think I have a simple way to execute this... Maybe this is what folks have said, but I still really wasn't *getting it*... Y/N? Thoughts? ...or concerns?


For reference, here's a 5881 datasheet:
http://frank.pocnet.net/sheets/127/5/5881.pdf

[Leo_Gnardo]

I was planning on using the center tap AS an "outer" lead, and short the outer leads together for the other "outer" lead to make a 6vAC line.

Those outer leads will have 6.3 VAC on them all right, but reverse polarity from each other. Short 'em together like you plan & you'll have a hunk of scrap metal fast fast fast.

THEN I was thinking of putting everything parallel across that and having to THEN throw in a humdinger... IDK what I was thinking...

Unnecessary complication.

Current handling really isnt gonna be an issue, and was never a worry in my mind - the total draw I'll have is under 2A at 12v... wow!

What exactly do you mean by this? Do you mean run the filament line like this..? :

12v upper lead-->5881-a pin 2-->pin 7-->5881-b, pin 7-->pin 2-->12v lower lead (see diagram)

Regardless, I think I have a simple way to execute this... Maybe this is what folks have said, but I still really wasn't *getting it*... Y/N? Thoughts? ...or concerns?
[ATTACH=CONFIG]37749[/ATTACH]

That would work fine. You could bring the point where 2 5881 filaments join to CT & ground too, then one tube would not depend on the others filament being continuous to light up.