Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: yorgle on March 24, 2020, 11:00:19 am
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I'd like to use a vintage PT I have lying about as part of a Les Paul Jr. build using a 6x5GT rectifier tube instead of a 5y3 (my PT lacks 5v filament wires). My wall voltage at 122.5 and I'm getting 350-0350 on the secondaries/ 7.2 on the 6.3 filament leads. The max plate voltage according to the 6x5 datasheet is 325 so it seems like I should add a bucking transformer on the primary side to pull things down a bit, unless there is room for extra voltage like with power tubes?
As an experiment, I stuck a 270 ohm 10W power resistor on one of the primary leads (about a 19v drop) which reduced the secondary high voltage to 300-0-300 and filament leads to 6.29 vac. So I think a 12v bucking transformer would be just about right. Any thoughts or suggestions are very much appreciated.
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With load, (as you see ) Power Transformer will do the job, no need bucking transformer.
PT with no load read alway higher voltages.
Why don't you use solid state rectifier diodes ?
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Thanks for the reminder about no-load voltages. I guess I'll wait until I get some tubes wired in before getting too worried about the PT.
Why don't you use solid state rectifier diodes ?
I like the glow of tubes.
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I suspect that 350-0-350 PT will give you a lot more B+ than that little amp wants. You'll probably need to increase the size of that 470Ω dropping resistor considerably.
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b
I like the glow of tubes.
Why not :laugh:
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I suspect that 350-0-350 PT will give you a lot more B+ than that little amp wants. You'll probably need to increase the size of that 470Ω dropping resistor considerably.
Yikes! Just tested it with a SS rectifier and measured 640vdc! Remember that scene in Jaws when the sheriff first sees the shark, stumbles into the cabin and says, “you’re gunna need a bigger boat.” I may have to rethink this idea.
Edit: It occurred to me that those square SS rectifiers are full wave bridge. So that’s twice the voltage the 6x5 will output, right?
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> tested it with a SS rectifier and measured 640vdc!
Without a capacitor??
350-0-350 (700VAC) rectified onto a cap is 1.414X or 980 Volts DC.
700VAC rectified withOUT a cap is 0.9X or 630VDC (all ripple).
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> tested it with a SS rectifier and measured 640vdc!
Without a capacitor??
350-0-350 (700VAC) rectified onto a cap is 1.414X or 980 Volts DC.
700VAC rectified withOUT a cap is 0.9X or 630VDC (all ripple).
Correct, no cap.
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Edit: It occurred to me that those square SS rectifiers are full wave bridge. So that’s twice the voltage the 6x5 will output, right?
Correct. All the numbers PRR posted are 2X the voltages you would have with a conventional two diode rectifier.
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Plugged in my shiny new 6x5 tube tonight and thankfully nothing smokes, but I do indeed have 490vdc at the first filter cap and only a volt or so less at the second cap. Of course, without the other tubes installed, these are still no-load values. Tomorrow I’ll plug in a 6v6 and the other tubes and see where things stand.
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....I do indeed have 490vdc ... still no-load values. ....
Oddly, 490V is just what the 6X5 sheet predicts no-load. Even with one or two 6V6 to load it, these are high voltages. Maybe this vintage PT is from something big?
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Tubes are in and the amp sounds great up to about 70% volume, then it starts to whistle like a steam engine, so I’ve got some troubleshooting to do. Hopefully it’s just a bad preamp tube. I made two deviations from the Gibson schematic (added a trem o nator tube and used only 2 (not 3) filter caps, but I don’t think either of these changes are causing the whistle. Here are some voltage readings I took after 30 minutes of playing:
V1 (6sj7): plate 150, screen 20, cathode grounded
V2 (6v6): plate 318, screen 273, cathode 16
At 6x5 output: 372
At 1st filter cap (after 470 resistor): 355
At 2nd filter cap (after 22K resistor): 273
None of the schematics I’ve found here or elsewhere on the web include voltages, but these numbers seem reasonable for this circuit. Anyone see anything out of the ordinary?
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> None of the schematics I’ve found here or elsewhere on the web include voltages
https://www.ampbooks.com/mobile/classic-circuits/gibson-ga-5/ ?
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Wow, thanks for that link, PRR. That’s exactly what I need.