Choosing power tubes?

[Justin Thomas]

Hi all,

Embarking on my fourth build, a gutted old organ chassis of unknown circuit. I know it originally came with 2x6973 & 3 12A_7s, and an octal recto. I just hooked up the PT, no recto, and I got a 396-0-396 HVAC, = ~560VDC. This is completely unloaded. I plan to use a 5Y3 just for the voltage drop. Which would stick me at -~510VDC. I don't want to buy new tubes, and I had already put in octal sockets for intended 6V6s (JJs, for the 500V max). So, I have to pick some power tubes. And probably beef up my filter caps - a 4-section can @ 475V. The originals were 500, so I would guess the original ran pretty much under that, so I'm banking on lots of voltage dropping from loading.

I was wondering about my loaded B+ - would it be reasonable to say that I can figure 560V - 50V (recto drop) -30V (power tube load) = 480V? A little more drop would be nice; could I assume even more voltage drop? I could cram in a couple 10W resistors, up to 1K... I plan on 3 preamp tubes, but I know they don't count for peanuts. I also have a few pairs of UOS 7591As, which I know would handle the voltage, but draw more heater current. Other choices? I'm between 6V6 & 7591. I'd feel safer w. 7591 voltage-wise, but am concerned about heater current. The 6V6 would probably be a better match for the OT, power-wise. Note - the PT and OT are both Schumacher, so it may be a little unserspec'd anyway. I know how to do the stress tests. And there's not a whole lot of room for a series pair of filter caps to beef up...

I didn't pay for any of it, so although I don't <want> to blow it up, I'm not out anything (besides a really nice old PT)…
Thanks for any help!

Justin

[Justin Thomas]

Okay, on checking some of my schematic archive, looking at vintagey 2x6V6-powered amps (some guitar amps, a couple small PAs) with a 5Y3 and 2 preamp tubes, seems to drop between 40-60V from current draw, in addition to the 50V drop from the 5Y3.
Sure, it's inefficient, but that's part of my goal here...

Justin

Edit: Just found a schem of an old guitar amp (yay for Googling!) with 2x6973, a 390-0-390 PT, five pre tubes, and a 6CA4. The loaded B+ on that one wound up ~435, so I'm a bit more optimistic. May add a single lytic in series with my first can section to beef up that first cap; I can lower the screens a bit for some wiggle room there.

[DRH1958]

Have you got a variac? One thing I've noticed is generally, the full load voltage will be about what you get with 105-110 VAC into the primary depending on the % regulation of the tranny which is of course unknown. So this would put you somewhere around 350-0-350 VAC for full load voltage out of the secondary. So that's ~490 VDC full load. Then figuring 50 volt drop with the 5Y3, this gets you to 440 VDC.

Edit: simul-post. I guess this bodes well since we came within 5 volts of each other.

[Justin Thomas]

No, no variac or scope... Just my Fluke112 & Dim Bulb Tester! And a sense of adventure, and knowing how to not burn down the house or myself. The parts, on the other hand... (evil grin here)

The 396-0-396 was an actual measurement without the current limiter, so that's what I'm working with at real ~125VAC mains.

And I guess the filter caps might get stressed if the tubes are out for testing! >:O
I <could> buy a new can, though I don't really want to...

Justin

[big_teee]

You probably don't want to change the PT?
But, If you plan on ever making a bass amp, that would be the Berry's.
If it would handle enough current.

I too am starting a 6V6 build, and I bought CT Iron.
I bought this one.
Fender Power Transformer, Deluxe, Deluxe Reverb, Tweed Tremolux, 125P23B, 025130, 041316, 120V
Weber makes some Copper caps, with various amount of Voltage Drop.
T

[Justin Thomas]

No, not wanting to change any major parts. I have a sneaking suspicion that it powered more than just the 5 tubes it came with, being an organ chassis, and it was full of holes and leftover octal sockets for umbilicals. So I'm gonna prolly just go with the JJ 6V6s and let it cook; I think it'll live! I might should order another cap can, though - a 525V one, just for a little wiggle room.

A small bass head would be cool, though. And even with 6V6s, I think I can do it. I like an old-school gritty sound, anyway, and people always gripe that my Bassman is too loud... But my original intent is a Spitfire w. an extra channel, an EF86.

Justin

[Chuck H]

You might be able to get a tighter bead on what voltage drop to expect under load by measuring the PT secondary HV winding DCR. I had a recent, extreme experience with this which demonstrated it's importance. If you feel your OT is under rated for a pair of 7591's you could also consider any of the various methods for reducing voltage like a powered MOSFET, a big zener or a string of smaller zeners so that you can use any of the lower voltage small bottles. Dropping some volts would also allow you to use the existing 9 pin sockets and EL84 tubes, getting you closer to the Spitfire design, and therefor sound. Higher than spec voltages for a given design can yield unpredictable (and undesirable) results IMHE.

[mozz]

I usually tack a 4k power resistor on there to simulate what to expect. Download duncans power supply simulator.

[Chuck H]

I usually tack a 4k power resistor on there to simulate what to expect. Download duncans power supply simulator.

PSUD2 is very accurate. You do need to plug in all the correct parameters though. Including the HV winding resistance. I can usually get within 10V (or less) accuracy result in final builds. When it's not within that range it's usually because I failed to edit a default parameter. So, user error. It's free and it's good. Download it!

[Justin Thomas]

Sounds like a good first download for my new laptop! When I open it, that is... been waiting for a rainy day, and this is it.

Chuck, I'd already drilled the chassis for 8-pin sockets, figuring I can fit adapters if I found it was safe to go to EL84s. The 6973s have a max plate voltage of ~440, so I knew I was possibly treading dangerous territory. But not so big a deal for JJ 6V6s.
It seems like EL84s & 6V6s have similar drive requirements, so I'm willing to screw around a bit with stuff. I'm not looking at "cloning," so much as "inspired by." Build one was your Spitfire-w.-Cheese; Two was a Spitfire except without the MV and w. fixed bias, as a small recording bass amp; Three was an EF86 AC4=>Cathodyne=>EL34s.

I try to do something new with each one, and this is probably why I average one build a year...

Justin

Rather than start another post: in doing more research, seems a 5R4 also has a pretty hefty voltage drop, and lists a much higher AC Voltage rating per plate, and I have a few on hand. I'll have to pop them in the tube tester to make sure they don't short, and maybe give those a shot before ordering a new 5Y3...

[g1]

Rather than start another post: in doing more research, seems a 5R4 also has a pretty hefty voltage drop, and lists a much higher AC Voltage rating per plate, and I have a few on hand. I'll have to pop them in the tube tester to make sure they don't short, and maybe give those a shot before ordering a new 5Y3...

This may be handy for you, lists all the rectifier variants and voltage drops, currents, heater currents etc.

[DRH1958]

Rather than start another post: in doing more research, seems a 5R4 also has a pretty hefty voltage drop, and lists a much higher AC Voltage rating per plate, and I have a few on hand. I'll have to pop them in the tube tester to make sure they don't short, and maybe give those a shot before ordering a new 5Y3...

That's a great alternative that I use often that can deliver twice the current of a 5Y3 if needed and it only uses 2A for heater current just like the 5Y3, so it sounds like a win-win..

[Justin Thomas]

Got most of it wired up, should be finished in a week or two... No idea how it will sound, since I do true PTP... usually some noise issues. I will put all I've learned so far in it, with as much help from my first build as possible... But dang, copying the layout just isn't gonna happen! Will report on voltages, estimates, etc. Hoping for no flames!

Justin

[Justin Thomas]

Got it all wired up & fired it up tonight. No hum, no buzz, no noises to troubleshoot! And it's basically true PTP; there are some terminal strips. It's a little bright, and a little low in gain, but I haven't checked any voltages yet. The low volume may have to do with the screens; not knowing what to expect for voltages, I dropped the screens a bit more than usual. Otherwise, I've got my first build to compare to to make changes in the preamp. It sounds really good, and the wiring could use some tidying. Now to get a cabinet in the works...

16 years later, I'm <STILL> saying this is the best place out there! Thank you all!

Justin

[Chuck H]

Got it all wired up & fired it up tonight. No hum, no buzz, no noises to troubleshoot!

It's weird when that happens for the first time. You might even think it's a trick and look over your shoulder expecting Alan Funt. The first time I ever switched on an amp, "fived" all the knobs and heard near silence I thought the amp wasn't working! Then I turned up the guitar and found the amp was working perfectly. Since then it's morphed into other experiences. Like turning on a new build, fully expecting it to work as predicted and getting a loud hum. Spent two hours trying to figure out what I did wrong only to find it was a crappy preamp tube fresh out of the box. Full circle now I hope you get that experience on the next one (sans the two hours of trouble shooting). With greater knowledge of layout and grounding culminating in a nice, quiet build you'll come to expect it. Then you'll be surprised when it DOESN'T happen. Good times.