Separate Cathode Bias Resisters for Dual 6V6 Amp -> Garter Biased 6V6 Songwriter

[Colossal]

Nice work. How do you like the voicing now with the Songwriter preamp?

[bcmatt]

Nice work. How do you like the voicing now with the Songwriter preamp?

It's turning into something I like alot more. I'd say it's up my alley but I need to try with a decent speaker. I definitely like the simplicity and usefulness of the controls.

[Colossal]

I've heard and played a SW30 clone and it was a great amp and yes, I liked the low-load, simple tone controls. Great amp! Excellent work on all of your experimentation with the cathode bias. It's been a good read. You may have mentioned it, but did you end up finding or having a tonal preference for one over the other?

[bcmatt]

I've heard and played a SW30 clone and it was a great amp and yes, I liked the low-load, simple tone controls. Great amp! Excellent work on all of your experimentation with the cathode bias. It's been a good read. You may have mentioned it, but did you end up finding or having a tonal preference for one over the other?

I'm still tweaking this thing. I'd say the sound difference is pretty negligible/unnoticeable while I was comparing the garter vs the split cathode modes and playing through some well-matched 6V6s even though now with the 560r cathode resisters, the Garter circuit gives about 96% dissipation and the split Cathode about 105% dissipation. But really this is with matched tubes and my goal is to be able to use tubes that were never matched, or have drifted too far apart over time to be used in other amps.

That's where I would assume I like the sound of some mismatch. So I would hypothesize that if they were a little off from each other, I would maybe prefer the sound of just the Split-Cathode allowing them to be a bit off. But if the 6V6s are further apart, I may prefer the sound of the Garter pulling them closer together. Who knows. I still have some experimenting to do.

In the meantime, I'm playing with my dropping resisters. I'm not sure what ideal voltages should be for the songwriter, but right now my V1 plates are reading 139V and my Phase Inverter plates are at about 250V. I was trying to pull them a bit closer by having nodes C and D both feed through their resisters directly from B as well as dropping the D resister to 6.8K. It didn't really make much difference at the actual plates, it just brought up the voltage at the D node a bit.

Does anyone know what the actual Songwriter voltages should expect with the parallel triodes in V1?

I also realize that the volume control doesn't really start to give much sound until 12 noon. I'm wondering if I'm screwing up my ground reference because I am adding another 1M resister on each of the parallel grids. (I have to move them off the input and I'm supposed to put it right before the grid after a cap for the purposes of the VVR).

Garnet 45B Oct 25 Schematic.jpg

[bcmatt]

Weird thing, when I switch between Split Cathode and Garter settings on mismatched tubes, the dissipation is going down slightly for the garter, but staying about the same distance apart.

I tried my first 2 mismatched tubes but one of them had it's cathode voltage continually climbing, so I turned it off and pulled that one out for the 3rd tube.

At full voltage, I got them dissipating at 98% and 87% respectively with the Garter circuit. I wasn't brave enough to try Split-cathode at full voltage since they were already 11% apart.

I pulled down the plate voltage to the low 160s and tried both circuit types again:

Garter: 19% and 16% dissipation
Split Cathode: 20% and 17% dissipation

Garnet 45B Oct 25 Mismatched Voltages.jpg

[maxkracht]

Weird thing, when I switch between Split Cathode and Garter settings on mismatched tubes, the dissipation is going down slightly for the garter, but staying about the same distance apart.

The way you have that switch gives you extra plate voltage with split cathodes, are you taking this into account?

In the 10 tubes I tried, there was one outlier where the split cathode was a slightly closer match, but I wasn't measuring changes in plate voltage each time. I'm sure there are some other variables.

[bcmatt]

The way you have that switch gives you extra plate voltage with split cathodes, are you taking this into account?

In the 10 tubes I tried, there was one outlier where the split cathode was a slightly closer match, but I wasn't measuring changes in plate voltage each time. I'm sure there are some other variables.

Yes, I measure the plate to cathode voltage again each time. It makes me wonder if I did something wrong. I don't have a tonnes of tubes to experiment with since the 3rd tube seems to be extra problematic.

Ooh, I guess I could use one of my matched pair too. It's buttoned for now (for the first time). I really love how weird this thing looks:

20241025_143201.jpg

It actually sounds pretty awesome too. It is a big experiment on how to use unmatched tubes, but I'm really falling in love with this thing.

[bcmatt]

I dropped a WGS ET-65 in this thing too, and it really sounds quite a bit better.

20241025_141619.jpg

[maxkracht]

Yes, I measure the plate to cathode voltage again each time. It makes me wonder if I did something wrong. I don't have a tonnes of tubes to experiment with since the 3rd tube seems to be extra problematic.

I think the beauty of this thing is there isn't too much to get wrong. I'm guessing you are doing everything right and there is some other variable we're not thinking of. Regardless, i'm glad the amp is looking and sounding good!

[bcmatt]

OK, I gotta say it again, I am really loving this thing.

I REALLY like the voicing of the songwriter preamp. It works so great with VVR and it is so easy to dial in tones without the volume pot being too touchy.
I also think this cabinet design is really awesome too. As far as convenience to getting into the guts of the amp it is horrible because you have to pretty much remove the speaker baffle to get the amp out. This would be the opposite end of the spectrum with 70s Traynor's being the easiest because you don't even have to take the chassis out; you just take the lid off.
But with this amp, the control panel is facing up at you at an angle right at hip height.

I am considering changing over my 5e3 head to be this songwriter preamp as well...

Can someone explain to me what's happening with the songwriter preamp V1 with the parallel input grids being separated by their own grid stoppers verses say V1 in the DC30 that ties the grids directly together after a shared grid resister? What affect does that difference have?

[martin manning]

Can someone explain to me what's happening with the songwriter preamp V1 with the parallel input grids being separated by their own grid stoppers verses say V1 in the DC30 that ties the grids directly together after a shared grid resister? What affect does that difference have?

The difference is in the value of the grid stoppers and the associated Miller capacitance effect. The DC30 (high input) has 34k into the paralleled Cg-a, where yours has 27k into each triode. If the DC30 grid stopper were reduced to 27k//27k they would be exactly the same on the high end. There is also the cathode cap difference, 1u vs 0.1u, which is giving you more mid-range gain.