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

[sluckey]

Let me know if you think it is possible to put on a switch.

Here's a simple switch...

[bcmatt]

Let me know if you think it is possible to put on a switch.

Here's a simple switch...

Oh ya, cool! That would do it. Thanks!
I'll give it a try. Stay tuned....

[bcmatt]

Garnet 45B Cathode Switch Voltages.jpg

OK, I got the switch installed and did some more measurements. The regular Split Bias Resister method does run the tubes a little hotter than the Garter method but it's comparable I guess. I think I'm running them hot all around. I have one of my 4 470 ohm resisters that is a good deal higher at 482, but I wonder if I need to try 500ohm resisters instead since I'm running these 6V6s at about 115% plate dissipation with these 470s using the Garter method and the split cathode bias is up around 123%.

I'm thinking maybe I try 500 ohm resisters. What do you guys think?
I know what we are all really wondering about...
"How does the Split Cathode Bias compare to the Garter Method when it comes to the mismatched tubes?"

I'm scared to try it with such high dissipation, so maybe I'll try at half voltage. Unfortunately, I also have that one resister throwing the balance off.

[maxkracht]

Here's a simple switch...

When I was experimenting with this I just jumpered the ground side of the grid leak resistors together. Am I missing something regarding why this would be a bad idea? I assumed keeping the extra resistors in circuit but eliminating the balancing action would be a more fair comparison. I guess that also depends on what you want to compare.

I'm thinking maybe I try 500 ohm resisters. What do you guys think?
I know what we are all really wondering about...
"How does the Split Cathode Bias compare to the Garter Method when it comes to the mismatched tubes?"

I'm scared to try it with such high dissipation, so maybe I'll try at half voltage. Unfortunately, I also have that one resister throwing the balance off.

120% isn't too crazy but probably not good to leave it there. I wonder why the original resistor was so small? You could trim the value with added resistors in series until you find something that works, they don't need to be high wattage types.

[bcmatt]

When I was experimenting with this I just jumpered the ground side of the grid leak resistors together. Am I missing something regarding why this would be a bad idea? I assumed keeping the extra resistors in circuit but eliminating the balancing action would be a more fair comparison. I guess that also depends on what you want to compare.

I would think that would be the same, but still floating about 235 ohms above ground. I imagine it would give the same results. What did you find in your comparison? Was there significantly less balancing happening?
Because one of my cathode resisters is like 13 ohms off of the others, but eliminated when the garter is removed, I imagine my results are skewed.

120% isn't too crazy but probably not good to leave it there. I wonder why the original resistor was so small? You could trim the value with added resistors in series until you find something that works, they don't need to be high wattage types.

It was small because I chose rather arbitrarily based on what fairly large wattage resistors I had enough of. I ordered some 3 watt resistors in 1k (for paralleling 2 together to hit 500), 510r and 560r to try later this week. For now, I'll keep the VVR turned down while I make other changes to the circuit.

I'm trying to decide whether to start with the preamp, or if really I should upgrade the PI to a LTP right off the bat. I think I will add some grid stoppers as well.

[maxkracht]

Not the most scientific, i didn't measure the plate voltage each time, but maybe enough to give a bit of a trend. This is voltage across each cathode resistor, garter circuit on the left and grid leak resistors connected on the right. Tube in the left column stayed in the amp, tube in the right column was changed and allowed to heat up long enough for voltages to stabilize. Last measurements are the original tube back in the amp.

IMG_3413.jpg

Edit: Took some better measurements with a couple of the more mismatched tubes. Got 4 meters going at the same time... All 4 bias resistors are within 1 ohm of 427 ohms. Bias is cooler than I remember.

[bcmatt]

Cool, I'll take a look at those.
I'm just posting this now so I can look at it from another computer:

Garnet 45B Cathode Garter Mismatched both switch.jpg

[bcmatt]

IMG_3420.jpg

Edit: Took some better measurements with a couple of the more mismatched tubes. Got 4 meters going at the same time... All 4 bias resistors are within 1 ohm of 427 ohms. Bias is cooler than I remember.

This is really great testing! Thanks for this!
It sure looks like the garter circuit was keeping the mismatch in tubes down to 1/3rd of the difference that the split cathode circuit was doing. That makes it seem totally worth it to me to make more mismatched tubes useable.

I'll have to do some tests like that when I get my better matched cathode resistors swapped in.
My current mark up are rather a mess and my current resistors are skewing the garter method unfavourably because that is when my most wildy different resister is used.

Garnet 45B Cathode Garter Mismatched both switch2.jpg

[bcmatt]

I'm also continuing to mod this amp into something I might like more.
This morning I reconfigured the output tube sockets to add some 470r screen resistors, some 1.5K grid stoppers, and bumped the PI coupling caps up to more common .047uF values to anticipate this amp's transformation:
This is the current circuit now:

Garnet 45B Current Representation of Schematic.jpg

45B Oct23.jpg

I'm still trying to decide what to change next.... start at the input, or convert the PI...

[maxkracht]

I would probably mess with the preamp first, but thats just me. I'm guessing the tiny OT doesn't have the best specs so adding bass might not help.

[bcmatt]

I would probably mess with the preamp first, but thats just me. I'm guessing the tiny OT doesn't have the best specs so adding bass might not help.

That was what I was thinking last night, but for some reason this morning I had changed my mind and switched the PI to a Long Tailed Pair before reading your suggestion. I may want to reduce the coupling cap size again to keep the low end more controlled. I was just using the Rocket schematic as a guide.

It sure did enable this amp to become an unruly fire-breathing monster.
It really accentuated the preamp controls and you become very aware how much the bass control affects the gain level this circuit. The controls all become super interactive. There is way more than enough gain on tap now, but the question would be to tame it and make it a bit more controllable... Maybe confine it to more usable parameters. It can get into the super-compressed maxed-out fuzz tones.

I will have to mess with the preamp a bit because there is a fair amount of hum on the volume control and the volume pot is super noisy as you turn it. A little of that may be due to the cap that still needs to be added to for the VVR to keep DC leaking onto the pot from the grid at low voltages, but the pot is just bad too and cuts in and out.
Here is the current amp:

Garnet 45B Oct 23 Schematic.jpg

[bcmatt]

I think this layout may lend itself well to the songwriter preamp and controls:

[bcmatt]

Songbird 30 - Schematic.jpg

I find it interesting that the bright switch choices are so close to the same at 200pF and 250pF

[bcmatt]

20241024_103135.jpg

I went with some parallel caps for the bright switch since my collection is dwindling. My choices I settled on ended up being about 210pF and 420pF. They seem to be pretty useful choices. For the Bass pot, I only had a 4M and I guess it works pretty well... I'm not sure.
Since the cut needed to act intuitively as a "Treble" control, I had to wire it accordingly backwards from most cuts, and the only reverse log pot I had was not really giving me anything useful, I ended up using a 100KB (Linear pot) and that is a very useful range. I'll have to remember that for my other amps that use a cut and I want it to get brighter clockwise.
The other thing that is probably obvious is that the Bass and Volume pots actually do need to be grounded on the bottom leg. (Well, I'm still not sure about the bass for certain). I tried it according to the schematic first because I was wondering if it had some unique thing going on that counters all reason. This songwriter circuit is definitely cleaner than the stock 45B preamp, but I think also much more useful. It's mostly clean until the last quarter of the volume knob. I can get aggressive tones (almost Liverpool-like) with full bright switch and cranked volume. Then the cut and bass tweak it to taste. Then dialing back the guitar volume does clean it up nice. I wouldn't say it keeps quite the same volume clean when doing that like an Express would, but it's maybe almost like the Liverpool that way too. It's not a substitute for the full aggression that the Liverpool is capable of, but it gets to most of the gain that I will often use.

The other think I haven't tried yet that the Songwriter has is any NFB... since this is almost exactly a 6V6 Songwriter at this point. I'll probably try that at some point, but Next I want to dial in my cathode resister values.

[bcmatt]

So, this is the current Status of this Amp:

Garnet 45B Oct 24 Schematic.jpg

Some 3W 1K and 560r resistors showed up. I may test the 560R Resisters tonight for the 6V6 cathodes.