Another Bassman Build - Tweed 5F6A : New Digs

[martin manning]

Looks great TM, and I bet it will sound fantastic! It appears that you have an extra 6.3VAC winding dedicated to the pilot lamp? Custom wind on ghe PT? Also I see a number of similarities to the Modernized 5F6-A layout I posted last fall- I recognize the adjustable bias arrangement, the standby switch downstream of the reservoir, and also the relocated grounds. No elevated filament string, though?

[ToneMerc]

Looks great TM, and I bet it will sound fantastic! It appears that you have an extra 6.3VAC winding dedicated to the pilot lamp? Custom wind on ghe PT? Also I see a number of similarities to the Modernized 5F6-A layout I posted last fall- I recognize the adjustable bias arrangement, the standby switch downstream of the reservoir, and also the relocated grounds. No elevated filament string, though?

Thanks Martin, yes it's a custom wind, my take on a Triad 8087 with a twist. Yes, there are some similarities; on Fender builds I see no reason to do it exactly as Leo did decades ago and I will typically relocate the standby switch and use balancing resistors with series caps at the first node. Also, I normally relocate grounds while using a buss bar. I did mount the bias pot in the same location. I really wanted to mount a bias pot and test point on the chassis, but there's not much real estate on this chassis type.

On one of the 6G6B's I built last year I used the ground switch a bias test point toggle, on this chassis style that wasn't so practical, so I did copy the way that the ground switch is used on your modern layout as a PT termination point, it was a great reference.

My original intent was to wire up all four outputs using RCA jacks, then I got to thinking there's absolutely no reason to do it that way. Then once I got the board mounted and mocking up final fitment, the bottom most or forward facing 1/4" phone jacks were really in the way, so I only wired it with two jacks. Had I known I was going to use only two phone output jacks, I would have mounted a CTS bias pot in the bottom left output jack. However, considering one would most likely be removing the back for access anyway, I don't think really matters much where, as long as it funtional some how.


TM

[ToneMerc]

Installed tubes and fired it.

Pri: 124.5 VAC
Unloaded DC at 5AR4: 479.8 VDC, 698 VAC at pins 4 & 6
Filament voltage at V1: 6.87 VAC

V1 RCA 12AY7: 157.4, 162.1, 2.35
V2 RCA 12AX7: 191.3, 1.15
V3 GE 12AX7: 251.5, 263.2, 32.8
V4 & V5 Tung-Sol 5881 : 436 VDC plates, 434 VDC screens
Voltage at 220K junction: -48.4, at cathodes, V4: -30.4, V5: -34.8

V6: GE copper post 5AR4

Bias circuit: 50V tap>15K>22K>30K bias pot, plenty of range with 50's era 5881's and plate voltage.

TM

[martin manning]

With those voltages it looks like you are ready to go live...

I came to the same conclusion re bias test jacks and trimmer. Removing the upper panel gets you full access, so it's no big deal. Similarly, for a single output impedance two Switchcraft wired as usual is a much better route than the RCA's, unless you are going for an accurate reproduction. I still like the idea of elevating the filament string on circuits like this.

On a back burner, I've been doodling a 5F6-A layout that puts the filters inside the chassis. Maybe I'll get around to building it soon.

[M Fowler]

Nice build Mike.

I put my filter caps between the PT and tone board using tag strips.

[martin manning]

I put my filter caps between the PT and tone board using tag strips.

It was your build that got me started in that direction, Mark.

[Blackburn]

ironically, I wired the pre with some cotton jacketed wire from Jupiter and later I realized that it's that high purity crystal stuff that tends to be faster and inherently brighter sounding and I felt the need to rebuild because of the harshness. Even at the most clean setting it could be very brittle sounding, but some days it was quite nice especially with my R8's neck pup! Super jazz tone!

DB, when you say high purity crystal stuff, was that wire silver plating over ultra high purity copper or just straight cotton cloth over non-plated high purity copper. Just trying to get a feel for where that brightness might have come from.

Thanks man.

Dave

That particular wire was non plated and was pretty small with a cotton jacket. I only realized that it was special when I compared it with a magnifying glass to some pvc jacketed mono crystal wire I bought later on. Crystal wire has a VERY different appearance to regular off the shelf copper most use. Much darker bronze and magnified you can see tiny specs of crystal. Very rich looking compared to the other. Looking back at the 5B6, I'd say the brightness was a combination of things like that wire, higher plates and playing that amp through the Elegante, which tends to not care for 6L6s, as it bumps the highs, in my experience. They were RCA 6L6GC Blackplates too, but that wire really did seem to have quite the effect on the tone. Can't wait to try this newer stuff I have in an amp that will be complimented by it.

[Blackburn]

Installed tubes and fired it.

Pri: 124.5 VAC
Unloaded DC at 5AR4: 479.8 VDC, 698 VAC at pins 4 & 6
Filament voltage at V1: 6.87 VAC

V1 RCA 12AY7: 157.4, 162.1, 2.35
V2 RCA 12AX7: 191.3, 1.15
V3 GE 12AX7: 251.5, 263.2, 32.8
V4 & V5 Tung-Sol 5881 : 436 VDC plates, 434 VDC screens
Voltage at 220K junction: -48.4, at cathodes, V4: -30.4, V5: -34.8

V6: GE copper post 5AR4

Bias circuit: 50V tap>15K>22K>30K bias pot, plenty of range with 50's era 5881's and plate voltage.

TM

Sweet build, dude! Love those old tubes! I dig my brown base 5881s as well. Think we may get a clip or two?

Where's a good place to get those resistors?

[statorvane]

Yes, very nice build. Got a cab? What speakers are you going to use? My first build was a 5F6-A driving two 10" Emi Legends in a narrow panel style cab. Very loud and Fendery.

[ToneMerc]

Yes, very nice build. Got a cab? What speakers are you going to use? My first build was a 5F6-A driving two 10" Emi Legends in a narrow panel style cab. Very loud and Fendery.

Thanks
No cabinet just yet, I will be ordering a pine cabinet from Mojo and most likely will be filling it with a combination of alnico and ceramic Weber 125's

TM

[jelle]

Hi TM, any progress? The chassis looks so nice, that deserves a happy ending!

[ToneMerc]

Hi TM, any progress? The chassis looks so nice, that deserves a happy ending!

No Jelle, I have too may soldering irons that are hot to finish this up, been prepping "ground up" Princeton Reverb and Twin Reverb head builds.

TM

[jelle]

Nice, sounds like you are playing with the fender side of things. Have fun!

[Blackburn]

Hey TM, have you settled on choices for V1, 2 and 3 yet? If so, which have you decided on? I'm thinking I'll gets some 50s GE 12AX7s and maybe a 12AY7 for mine.

[ToneMerc]

Hey TM, have you settled on choices for V1, 2 and 3 yet? If so, which have you decided on? I'm thinking I'll gets some 50s GE 12AX7s and maybe a 12AY7 for mine.

Haven't deviated from my initial choices

"NOS and ANOS Tubes:
V1: 11/56 GE rebranded RCA 12AY7 D getter
V2: 12/60 RCA 12AX7A short gray plate
V3: 5/59 GE 12AX7 long plate horseshoe getter
V4 & V5: 1/59 Tung-Sol 5881 matched pair
Rectifier: GE copper post 5AR4"

TM