martin manning wrote: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?
The chart represents the triode with the signal applied to its grid, but the effect of the other triode sharing the same Ra and Rk with its grid tied to ground is included (Note I did not show the Ra and Rk doubled originally, which is now fixed). 90k is where I had to set the following stage load (Rl) to get the AC load line to appear in about the right place. Since the software doesn't model this configuration, I had to fool it. I worked out a method to solve this graphically, but I haven't drawn it up yet.
tubeswell wrote: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).
As I said above, you can keep books whichever way you want, but the result won't change as far as the DC op point goes. When looking at AC the triodes are not parallel because the grids are not tied together.
martin manning wrote:90k is where I had to set the following stage load (Rl) to get the AC load line to appear in about the right place. Since the software doesn't model this configuration, I had to fool it. I worked out a method to solve this graphically, but I haven't drawn it up yet.
So do you mean that you adjust RL, until the output voltage/gain matches the expected value?
Thanks Martin, amazing, and above and beyond. Even more amazing I pretty much understand what I'm seeing on the graphs, still not much chance of figuring it out myself but I learned a lot. In terms of listening one must have a ton of patience, this stuff is subtle, but it's there. The amp has always had a nasal quality to the upper mids and a sharp treble - like turning the presence too high. The amp is working 100%, not much left, tweaking the input tube, which I had ignored until now, helped. Either it added some bass and shifted my perception of the tone, or it actually shifted the tone downward, but it's a bit richer and smoother. The El34s added low end and shifted the tone and it's why I preferred them, but w/ the 6SL7 biased better the 5881s sound fuller now.
What I'll never get is that with the pot I was going from 1k to 5K and it was all pretty much fine. You'd think it would sound ghastly at some point, but nope. If this was a truss rod, you're talking like 3 turns! I've never tweaked a truss rod more than a quarter turn! Tubes have too much latitude - they work fine even when totally whacked - hard to learn with that much latitude. Kind of like making bread - very hard to make exquisite bread, and very hard to make bad bread.
Doesn't seem likely with Ra and Rk to limit current. If you let the grid float the op point would move along the load line until the tube is fully turned on. If there is positive grid voltage applied I suppose the grid might overheat.
Btw, I've changed my thinking on this input stage again. Watch this space...
martin manning wrote:Btw, I've changed my thinking on this input stage again. Watch this space...
Oh no - my poor poor amp. I don't have the heart to tell it there was a mix-up and it needs an other operation. Was supposed to be my home thoroughbred piece but it's turned into my mule.
And Tubeswell, you bum, you and your raise the voltage idea. While it's ptp, I realized trying it it would be a 15 minute piece of cake, just lift the first dropping resistor and tack in a choke across the lugs, it would give me a 5F6a power string. Add some bigger stoppers on G2 and off I go. I should have a 5H/150mA choke on hand too. There's no end to the carnage.
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Vg for Triode B remains constant while tube current for B changes, and Triode B is sourcing changes in current through the cathode. So:
a) Ig-k for Triode B must therefore be changing. To the extent that Vk for triode B is kept in check by Ck, then this must have some (subtle) effect on the half boost point.
b) Ia for Triode B is changing, but the rate of change is different to the change in tube current for Triode A, because Vg-k for B is constant (to the extent that Vk is kept in check by Ck). Does this not affect the transconductance of Triode A (because the cathodes and the plates are tied)?
Thanks! It's very nice graph which clearly demostrates the rather heavy AC load seen by V1a when only one side is driven. Now the jumpered inputs make a lot of sense - higher gain and a lot more headroom.
Gone beyond my pay grade, but noting that in Fender's original plan the input tubes were grid-leak biased, not sure how that influenced their choice of using a shared resistor.
rp wrote:Gone beyond my pay grade, but noting that in Fender's original plan the input tubes were grid-leak biased, not sure how that influenced their choice of using a shared resistor.
You can see that your choice of Rk is spot-on, though!
In a simulation of the grid biased version the triode A load line is pretty much the same, and there is only a small difference in the gain and frequency response.
Like Jazbo8 says if you jump the two inputs (using a Y-cable, paralleling the triodes) you get more gain (~6dB) and perhaps more headroom, but the bias point will be off-center.
jazbo8 wrote:Thanks! It's very nice graph which clearly demostrates the rather heavy AC load seen by V1a when only one side is driven. Now the jumpered inputs make a lot of sense - higher gain and a lot more headroom.
I have a nice vintage British Y-cable that came with an old long gone Marshall - and it sounds really good in parallel - fat and punchy. I'll likely wire the inputs to single and parallel. They were like that originally, now I got to do it again, I hate doing inputs. I tried a choke too, didn't really effect the preamp voltage all that much so it's a no go there. I'll do the inputs and that will be end - in the immortal words of Roberto Durán, "No Más".
I would love to see your layout or a complete gutshot.
Not to copy... only to marvel. I have been doing a lot of p-t-p on some mic preamp prototyping and would love to incorporate a few of the fine points...
Tubeswell & Martin... Thanks for the breakdown! I thought my ears popped, but I think it was the info moving some stuff around to set up shop in my brain.
"Education is what you're left with after you have forgotten what you have learned" - Enzo
dorrisant, thanks! let me update my picassa images and I'll post a link. there's an old build post but the image links are likely down by now, and I've changed a few things around.
