A blog of my tube amp design and modification work. Primarily my own builds, but occasionally I feature work I've done on amps for customers (with their permission.) Feel free to contact me at paulpampspgh@gmail.com if you're looking for a budget-conscious amp tech.

Tuesday, April 1, 2014

PI

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

The five percent nation of filter caps

Finally got a little more work done on the Rockette. I need to think of a better name.

Anyway.

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.

Thursday, March 13, 2014

Incremental progress

Not much time spent today, but I got the grid stoppers in place and the screens connected.

Really wish I had a chassis-mount resistor for biasing the power tubes, but I think I'm just going to plow ahead with the cement blocks I have on hand.

I did spend a little line on the load line for this 3k OT. Probably the only thing I ever thought "when am I ever going to use this" about in college was graphical analysis, rulers and pencils and such. "We've got computers now!"

Har, har.

Little did I know I'd be trying to extrapolate anode curves for a 6V6 at 350V, for fun.

...I gotta learn Spice.

Tuesday, March 11, 2014

I've got the power (supply) (almost)

So, pretty wild that I've had another productive day. Didn't have a ton of time, but still managed to get the power supply laid out and the power tubes halfway wired.

Oh, and I finally measured the Crate OT: 3k plate-to-plate (to 8, 16 ohms) which probably seems low for a guitar amp, but I'm thinking it'll be pretty close to ideal. I'll have to plot some loadlines, just to be sure.

Monday, March 10, 2014

Bridge to B+

Ugly, sure. But it works. For the first test fire, I forgot the bleeder. Yay, 375V! Um... Where's that cap drainer?!

I did leave the B+ fuse on the back panel. That whole corner of the chassis is completely off limits for any signal wiring. AC radiating everywhere.

Next donor chassis... I've gotta drill it. This is just silly.

Sunday, March 9, 2014

Rockette - more wiring

So, I got the PT primary wired up. The safety is wired to its own chassis connection, and the purple is just a static shield for the PT.

No rectifier yet, but we're getting 273V AC, both unloaded and with the tubes in (heater load). Forgot to check what the heater sags to, but unloaded it's at 6.8V AC, which is fine.

Admirable performance from a PT sold as 250V. Added bonus: it's rated for 100VA at 50 Hz. Long story short, it should be able to supply much more current than the amp can draw. Which is great, because how guitar amp PTs have been sized in the past irks me.

To that end, I don't think I'm going to install a sag resistor. At least not initially. I might abandon cathode bias later on too. We'll see how punchy it is, and if it's not enough I'll go from there.

I think I'm going to mount the B+ fuse internally. As much as I like the pretty fuse holder on the back, it's just going to be inconvenient for wiring. Same reason I installed a Carling instead of using the front panel power switch it came with.

Slow and steady...

Sunday, March 2, 2014

Heaters

...I hate wiring heaters.

I'm hoping having the heaters run perpendicular to most of the signal wiring + the correct approach to the sockets + an elevated humdinger should add up to no hum, but I guess we'll find out.

I probably should make a genuine elevation node for the heaters instead of just referencing the power tubes' cathodes, what with the DC cathode follower raising Vhk on v2b into dangerous territory.

Kind of moot until I can figure out where to put the bridge rectifier and the reservoir & screen supply caps.

The rest just falls into place though.