Need Shamanistic Guidence From The Chart Guys

[tubeswell]

hmm...

... I could parallel something or other but that will look klugey ...

You mean 'k-ugly'?

[tubeswell]

deleted

[tubeswell]

If you run a single triode off a 218V HT with a 100k plate resistor, then you want a 2k4 cathode resistor to get it roughly centre biased. (Thats with the grid sitting at -1.7V, plate at about 146V. For the same sort of thing with two parallel triodes (with shared plate and cathode resistors) you want to halve the values of the plate and cathode resistors (because there will be 2 x the current through each resistor) - edited

[tubeswell]

If you want cleaner sounds, get the HT supply up to 350V and use a 150k plate resistor with a 2k8 cathode resistor. Plate will sit at about 217V. Cathode about about -2.5V

Halve the plate and cathode resistances for || triodes with shared RP,Rk

[martin manning]

If you run a single triode off a 218V HT with a 100k plate resistor, then you want a 780R cathode resistor to get it roughly centre biased. (Thats with the grid sitting at -1.7V, plate at about 146V.

I think ~2k2.

[tubeswell]

If you run a single triode off a 218V HT with a 100k plate resistor, then you want a 780R cathode resistor to get it roughly centre biased. (Thats with the grid sitting at -1.7V, plate at about 146V.

I think ~2k2.

Ah yes sorry, that should have been 1.7V/0.0007A = 2k4, which when halved would be 1k2 (I was dividing the grid voltage by the maximum tube current Duh!). I'll just duck out and bury my head in a bucket now LoL - edited

[tubeswell]

Probably a more correct method would be to derive the load line with the intended plate load for the parallel stage but doubling the current values on the Y-axis. This would give a more accurate picture of the load line w.r.t. the grid curves, seeing as how 2 x matching parallel stages is the same as a single stage with the same grid curves but 2 x as much tube current. This will result in the load line being shallower and the bias voltage point and plate idle voltage shifting slightly.

[martin manning]

Both techniques are used, and you get exactly the same operating point and DC load line either way.

[rp]

I'm still trying things out - 1500R sounds good. I've updated that schematic - though it's still for 5881/5U4G voltages.

Tubeswell, the f'ing low voltages are Fender's doing not mine - on the orig layout the grid biased input tubes are at 60Vp, check out the later "new improved for '55" 5D8, same odd low voltages but with novals. Why they wanted to run these circuits in the non linear part of the curves beats me? It's not like you were gonna play it in your car! Limits of metal 6L6 and save $5 on a choke? I could put in a choke, larger screen resistors, and play w/ the power string, prolly a sure improvement but then it's not a 1953 Twin is it? Not that it's much of that anymore.

Here's what got me playing around, sounds killer, and why I put the 6J5 back from the paralleled 6SC7. Slowly getting there, and it's very quiet with the changes. The Edcor's good, but isn't the right OT for exact Fender sound, if I had some dough I'd try a Heyboer tweed Bassman type. But the cathode biased EL34s, 6.6K Edcor like each other well.

https://www.youtube.com/watch?v=AL-JKM4Hn-4

Meantime here's a grateful feathered head bonnet for you and Martin, there's only one, quetzal feathers are rare, you'll have to share it.

[tubeswell]

I like the Heyboer OTs and have used a few of them in my builds. For a 30-40W tweed twin, I'd go for the larger version.

http://www.tedweber.com/hy018343

I have one of these in a simple 2 x 6L6 cathode biased amp with a paraphase inverter and 2 x 12" speakers (so its kind of like a one channel tweed twin) and it comfortably handles the power of full-on tube current of two maximum-biased (21W) Russian '5881s' (running at about 400V on the plates). Schematic attached.

The soundbyte (attached) when dimed shows how well this OT handles power (apologies for the surf theme - this was a grab from one of the tracks on a recent demo, single coil guitar with P90s. No other effects except a reverb tank - on a low wet mix setting - in front of the amp)

[martin manning]

RP, so you now have 6SL7 with a separate input going to each grid, a shared 220k plate load and a shared 1k5 cathode resistor? What voltages are you getting (Va, Vk) without anything plugged in, and what is the supply node voltage? Finding a DC op point for this arrangement is straightforward, but the AC load line is not, and it is unlike a modern input stage. I'd like to see where the point you found by ear is and how it fits with the analysis. Also, when you play and listen, I assume you are plugging into one input only and leaving the other one unused and therefore grounded?

[rp]

Martin, I went with 1500R and soldered it in so I could get the amp right side up w/o the pot hanging in there. One channel at a time no jumpering. I updated the schematic once again, I'm back to the 5881s for testing but with a 5AR4, the schematic is for a 5U4 but the difference isn't much on the preamp stage.

Been playing it for last 3 hours at home turned up to about 1 o'clock but playing soft, digging in when I want to test it. Haven't played electric that long in a while (that's a good sign).

BTW I kept raising the cathode resistor trying to get the voltage up higher as 80V just seemed whacked, but then thought - 80V can be a lot, like if you stick your tongue on the plate, so I stopped worrying. Looking at the 5D8 w/ 88V I just figured go with it - Leo Fender never made a bad amp. BTW on that amp there's 100K Rp per triode, 470R across four triodes, or 820R per channel, or 1500R per triode. Granted it's for a 12AY7.

Tubeswell's amp sounds better, even if he's cheating with all that reverb but this is getting there. Edcors besides having a lot of clang have a lot of grind, not at all bad but not very tweed. If I run it at 16R into an 8R speaker and 5881s it sounds a bit more Fendery so maybe the 6.6K primary might also not be perfect - but it's all right if you aren't trying to clone a tone. It's very versatile, I can drop-in EL34s, 6L6s, 6V6s. I've never been able to hear such a clear difference mismatching the secondaries before, the Edcor is very revealing. Found a post on Hoffman w/ PRR recommending 6.6K / 250R Rk / 400V for EL34, maybe that's why the amp sounds so nice with the Pentodes.

http://el34world.com/Forum/index.php?topic=15615.0

Anyway, if 1500R isn't right then I've gone A/B'ing loopy. That can happen.

[martin manning]

Anyway, if 1500R isn't right then I've gone A/B'ing loopy. That can happen.

Not likely. Using your ears is probably the best approach here.

The two triodes share a common plate load, and if they are well matched they will settle with each one conducting similar current, and this point is located on the plate curves by doubling the plate and cathode resistances. The funky part is when an AC voltage is applied to triode A's grid while triode B's grid is held at ground potential. When triode A's plate current increases, triode B's plate current decreases because triode B's plate voltage must follow triode A's, but its grid and cathode voltages remain fixed. That makes the load resistance seen by triode A appear to go down, making the AC load line much steeper.

As a result, "center biased" in this arrangement requires a DC operating point that is decidedly off-center as shown in the plot below. I think I agree with your selection.

[jazbo8]

Triode A's plate current increases, but triode B's plate current decreases because triode B's plate voltage must follow triode A's, but its grid and cathode voltages remain fixed. That makes the load resistance seen by triode A appear to go down, making the AC load line much steeper.

The chart is for both tubes, right? Also how did you derive the 90k AC load?

[tubeswell]

... Also how did you derive the 90k AC load?

AC load for an inverting stage = plate resistor||grid resistor (of the following stage)

The bias point is in the same location (on the DC load line) as for the DC load line's bias point, but the gradient is steeper. (So the intersection of the AC load line with the x-axis will be at a lower plate voltage than the DC load line, and the intersection of the AC load line with the y-axis will be at a higher plate current than the DC load line). If the DC load line for 2 x identical parallel stages is drawn so that the plate current values on the y-axis are doubled (giving a shallower DC load line), then the resulting AC load line will be more accurately positioned w.r.t. the grid curves (methinks).