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Old 5th September 2013, 04:07 AM   #1
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Default 5F1 BREADBOARD READY TO BUILD?

I think I've got my prototype nailed down, and want to start filling up an eyelet board, drill a chassis, and stuff it all into a Squire cabinet.... Have a subchassis with a weber WRVBPT OT and a ss bridge already built, will sit on the bottom of the cab (will put weight on the bottom, leave room for small chassis up top). Finally tweaked the B+ resistors (are 1200 & 10K) to give me decent preamp plate voltages (203/185), and 326v on the 6V6 plate (and 320 on the screen). Sounds good with guitar, breaks up nice at 7+ on the vol pot. Plate dissipation calcs out to 12.8 watts (used cathode - screen current method).
Question: running a SigGen instead of guitar to look at the signal/distortion, I see that watts go up dangerously with hotter input signal (like 150mV). Get 14+ watts.... will using an effect pedal that has any gain at all burn this thing up? Do all tube amps bear this potential risk?
Pics are of my "pegboard" breadboard, my "Squirt" (Squire/tube) cab & chassis, and scope shots of the AX7 pin 6 and 6V6 plate at vol=10. Comments? Suggestions? Thanks for all your help - I have learned tons from this forum!
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File Type: jpg SQUIRT CAB.jpg (55.1 KB, 65 views)
File Type: jpg AX7 PIN 6s.jpg (281.2 KB, 64 views)
File Type: jpg 6V6 PLATEs.jpg (270.5 KB, 60 views)
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Old 6th September 2013, 03:28 PM   #2
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Hey guys....appreciate all the views, but I had a question buried in my post. How does one keep tubes from being driven out of safe range by effects/gain pedals? Thanks!
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Old 7th September 2013, 05:44 PM   #3
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Howdy fellow Californian and fellow champ builder!

Insufficient information. Is your custom power supply capable of doing damage to the output tube or output transformer if the speaker load impedance is too low and you also overdrive the whole thing? With a proper speaker load it's going to run out of voltage supply and just put out square waves regardless of the input level, naturally limited before any trouble. You probably saw the limit at that 14 watts, but a real 5F1 with a 5Y3 won't even do that.

It's not a 5F1 anymore with a SS bridge; do you have a schematic and parts list including the exact power transformer? What's the input and output power rating of that Weber output transformer? With the B+ voltage you have, fortunately it will sound like a slightly hot 5F1.

Usually the class-A amp consumes nearly constant current and has nearly constant B+, but the output is ultimately limited by the power supply even if you try to drive it way too hard; it won't put out any more than full-rail into the load. You can overdrive the input of a real 5F1, and perhaps make the input distort regardless of the volume control, but if you turn the volume up the power protection is ultimately limited by the B+ voltage into the output transformer load. If the output transformer has too low an impedance because the speaker load is too low an impedance, the amp is still protected by the very limited current of the dual-halfwave 5Y3. So a real 5F1 is only capable of 3 to 5 watts output and can't really get into much trouble. A 5F1 power supply with a Hammond 270X only puts out a few watts and can't even really hurt a real 6V6.

The 2A line fuse doesn't protect against overload; blowing at 220 watts it just prevents fire in the event of a power transformer failure or tube short, not really even protecting the power transformer very well.

The 5Y3 is like two diodes for a center-tapped transformer output. If you use a SS bridge there instead with the same transformer you will have significantly higher B+ voltage and WAY more current capability. So I assume you chose a completely different power transformer. Did you fuse the B+?

Exactly how did you measure those power figures? They sound high even with the slightly high B+, but I usually measure at the speaker; I guess your figures include the DC that the output transformer eliminates. Generally I drive the output tube to only about half its plate dissipation value.

What exactly are you afraid of? Hurting the output tube or the output transformer? Is your custom power supply capable of doing damage or not? Your overbuilt power supply may not have protection if the speaker load is too low, but otherwise I don't see a problem.

Since you made a power supply that can handle it, see what it sounds like with an EL34 or 6L6.
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Old 7th September 2013, 05:51 PM   #4
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BTW if you put the power transformer and rectifier at the bottom of the cabinet that sounds great except the power lines are long; why not put the entire power supply down there. Put at least an initial filter cap down there so your power supply wires are not radiating switching noise. Then another cap at the power input to the amp to filter out any noise picked up by the long power lines. Make sure the high-voltage wires are well-protected for the sake of any human hands in the area. Twist them I guess, then put them in a sheath. Getting the power transformer farther away might be a wonderful improvement, so maximize that hum reduction by running filtered DC in the long lines.

Make sure the tubes are sufficient distance from the speaker magnet. Make sure the amp gets sufficient cool air.

Last edited by cyclecamper; 7th September 2013 at 05:57 PM.
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Old 7th September 2013, 06:04 PM   #5
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If time ever allows, we should get together. I cold bring various 6V6s to try, an EL34, a 6L6, a KT120. Could be fun.

Right now I have a bunch of projects on hold until I replace my oscilloscope.
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Old 8th September 2013, 05:19 PM   #6
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Wow! Glad to meet an (obviously) more knowledgable Champ builder, and local as well! All your points are well taken and make sense. I goofed calling the Weber WRVBPT an OT... its an inexpensive unit, with 260V/100mA (no CT) and 6.3V/3A secondary windings, no 5V winding, so won't support a rectifier tube. But with a 1000V bridge I get plenty of B+. I measure power sorta indirectly (but came from Tone Lizard): cathode current minus screen (pin 4) = plate current. Read voltages across the cathode 470 ohm and a screen 15 ohm (I just inserted it to do the test). I just got precision 1 ohm'ers from Hoffman, will put one on pin 3 and get plate current direct. I have a test data spreadsheet, to record about 10 voltages everytime I make a change (B+ resistors, cathode bias, tubes, OTs etc) so I know what my oddball PTs are supplying. I do have a std Champ PT as well. My current breadboard is a bit oscillytic and noisy - want to reorient things and add shielding so I can get clean scope shots (my pics are at full vol, lower volumes show lots of hi freq that obliterates the sigGen signal I use to adjust for desired clipping).
I'm proceeding with my little "Squire - Tube" build, hoping it will sound decent. Am putting an Ext Spkr jack on it. Want to keep it simple, but there are so many things to play with I just have to commit to one setup.... Thanks for posting! Jeff
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Old 8th September 2013, 05:35 PM   #7
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Default BUILT MY EYELET BOARD!

From some 1/16" phenolic laying around, and 1/8" eyelets from Hoffman.
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