Saturday, August 30, 2014
Here's a quick test of the newest version of Bodie. I'll update the schematic soon. Basically just cascaded the first two stages instead of running them in parallel. A couple more tweaks to come too.
Wednesday, August 6, 2014
Saturday, July 5, 2014
So, I got the Rocket finished a while ago. I decided to add a Vox "normal channel" by using the spare gain stage through a gain pot into the other side of the phase inverter. It took a little tweaking, namely an additional grid leak, stopper, and coupling cap, but it's worth it.
Drive is reduced slightly on the TB channel, which is fine, and the single gain stage normal channel can drive the amp into overdrive just fine. The "cut" control still works for both channels, of course, since it's after the phase inverter.
I used the other side of the PI (the second triode's grid, which is usually grounded with a large cap) because the two channels are out of phase. I kept the two channels permanently bridged because I didn't feel like adding three input jacks and the ability to blend the two channels is great. You can dial in some mids to get a thicker sound than a typical Vox, or use either channel independently.
It's not a revolutionary idea, I mean, the AC30 uses the PI to mix the normal and overdrive channels with the reverb/trem signals on the second grid. Typically the normal and tb channels can't be bridged, so this is novel, at least, while bringing the pi closer to the original.
I'm tempted to add channel switching by using an on/off/on SPDT to shunt either grid to ground with a large cap. Heck, why not do it with a relay and footswitch? Ah, but how to power a relay?
Unfortunately, the rocket's going to be out of commission for a while due to a shorted output tube, but if anyone is looking for ideas about the Trainwreck "unused triode" I just have two words: normal channel.
Oh yeah, and here's a nasty gut shot as I still haven't drawn an updated schematic!
I also moved the hv fuse to the front panel because it just made way more sense there from a wiring perspective.
I'm so amazingly lazy, I'm just going to copy/paste what I posted at MEF:
Finally took some measurements.
I pulled two tubes because my dummy load is only 100W.
Va = 487V
Ia = 29.9 mA (49% idle dissipation; a little cold, sure)
at full clean output: 24.1V into 10.1 ohm resistor for 57.5W
So for the screens we have, at full output:
27.3Vac across 1.991k screen resistor = It = .0137 A
DC ammeter in series with screen resistor = .00892 A
And the equation for screen dissipation in distributed load connection is:
P = EI - ei
where e = (1/2)(Ns/Np)sqrt(RlPo)
and i = sqrt(It^2 - I^2)
e = (1/2)(.125)sqrt(10.1R * 57.5W) = 1.506 V
i = sqrt(.0137^2 - .00892^2) = .0104 A
P = (487 V)(.00912 A) - (1.506 V)(.0104 A) = 4.43 W
...which is a whole pants load different from (487V)(.00912A) = 4.44 W
But considering the maximum dissipation for a 6L6GC screen is 5W, I'd say with 2k screen stoppers we're in the clear. Unless, of course, I'm doing things horrendously wrong. It would be interesting to repeat this with different values of screen stopper; I kind of doubt that I lucked into such a good value of screen stopper.
In lieu of a thermocouple ammeter, I measured the AC voltage across the screen stopper. This is probably a significant source of error. The result does seem reasonable, when comparing how the screen voltage swings at a 12.5% tap instead of a 40% tap. I was expecting to see a minor reduction in screen dissipation due to power being dissipated in the load, and this is what I found, but I guess I was hoping it would be a little more.
On top of that, my plate voltage is lower than what I've measured previously, and the bias is on the cold side.
Now, what dawned on me after I posted this is that these calculations are independent of the value of the screen stopper. That is, without a screen stopper at all, the maximum screen dissipation at full output is roughly 4.5W. At least a hundred ohms should be used as a screen stopper anyway, to prevent oscillation.
I'm not sure what'd going to happen during overdrive, so a hefty screen stopper is probably still a good idea. Even with 2k stoppers, this bad boy'll put out 115W of clean power before clipping. Even if it did put out a clean 135W (mfg ratings are always dubious), it would be virtually indistinguishable from the 85W an earlier BF/SF Twin would put out.
Tl;dr - don't worry about it. 6L6s are just fine. That's not to say a pair of KT88s wouldn't be fun too, but there's no need to worry.
I'd like to get measurements under overdrive, but that'll have to wait until I get a true RMS meter.
Here's a link to the thread where I originally posted: Music Electronics Forum
Thursday, May 15, 2014
Tuesday, April 1, 2014
So! Finished the power tube wiring. Used one bias resistor for the inner pair and one for the outer, so one set can be pulled if need be. The tubes are biasing at 13W and 13.5W, which should be fine.
I also got the PI wired - I promise it's in there somewhere. Pretty much as far away from the power tubes as possible. Next time, I'm totally drilling the chassis; I'm lucky it's quiet.
Well, that's not entirely true. I did my best with the grounding and layout that I could. And the preamp isn't wired in yet, but compared to the "are you sure it's on" Dynaco gold standard for quiet power amps, this guy's not quite there. Compared to my Twin with the master on 0, though, the Rockette is an order of magnitude quieter. And it's not even connected to the chassis yet!
Okay, enough. I thought that the massive leads to the output tube grids would be a death sentence, and I'm glad they're not. Also, those chicklets are totally 630v film caps. I vaguely remember Mouser being out of the particular caps I wanted and apparently I never followed through on ordering them. Same reason there are some different 470k metal films in there.
Oh well. I'm also running out of teflon wire, so the color scheme is probably going to get even wackier in the preamp.
I really need to give this thing a different name.
I have some cascode wackiness planned for the unused triode in V1 so I can get both the AC30 top boost channel and something like the AC30/4 normal channel at the same time. Bridged, even. I'll decide if that's worth it after I get the stock schematic working.
Or I could still try the "jumpered Plexi" thing. I should at least try the amp as Ken Fisher might've designed it first.
Maybe that's some variant of "blind urge to mod" syndrome. "Blind urge to make a design more 'original'" syndrome. Eh. It's my basement; I make the rules!
Saturday, March 22, 2014
Finally got a little more work done on the Rockette. I need to think of a better name.
Got the power amp finished, except for the grid leaks. Maybe I will put that PPIMV in after all. The PI tube is all the way across the chassis so I'll probably need shielded cable anyway.
Also finished wiring in the filter caps. I don't like turrets; tag strips have ruined me forever.
It's crazy how time-consuming this is when you're not painting by the numbers. If I had a board of the preamp components that I could just plop in, I'd be done by now.
Hopefully I can avoid noise problems, because fixing them is going to be a bear. The "distributed star" grounding is going to help, and I should be able to attach the components to the preamp sockets in a "bicycle spoke" pattern also, which is pretty much ideal.
Time will tell, but I'm optimistic.