A blog of my tube amp design and modification work. Primarily my own builds, but occasionally I feature work I've done on others' amps (with their permission.)

Sunday, September 14, 2014

Silverface reverb

Okay, so during the silverface era, CBS decided to beat the living hell out of the reverb driver, lowering the cathode bias resistor to 470R. Easy fix here, just replace the cathode bias resistor with the blackface value of 2k2 bypassed with a 25uF cap.

Another nifty trick is to replace the reverb driver's grid leak with a pot, aka a dwell control. That's how it was labeled on the original reverb units, though it's a little disingenuous as it doesn't control how long it takes for the reverb to decay (I mean, how could it, right?) but instead it controls the level of signal sent to the driver, with the existing reverb pot controlling the level of signal coming off the recovery gain stage. With a dwell pot, you can dial in a much more subtle reverb sound that's more dynamic.

I just put the dwell pot in the place of the humdinger which had fried ages ago due to a heater-to-plate short. While I'd like a proper humdinger, a pair of 100R resistors off the pilot light is better than a blown pot!

Saturday, August 30, 2014

Bodie Test Recording


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

Final-ish Rockette Schematic

The previous .sch files appear to be lost, so I drew up a new one showing the current schematic of the Rockette.

Some of the parts values might be off, but dig that normal channel. Super useful.

Saturday, July 5, 2014

Rocket in the pocket

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. 

The last word on "ultralinear" Fenders

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.

Idle conditions:
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

NGD - 2013 Gibson SGJ

Okay, so not particularly amp related, but I need somewhere to park these images.

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!