Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on June 10, 2017, 11:23:33 am
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Is 7.3vac too high for tube filaments?
Pt has two primary voltage leads, 115 and 125, using the the 125v (my wall 120v) I get a B+ of 387dv, the 115 primary gives me 415dc, I like the tone better with the higher voltage but of course the filaments get higher too. With the 125 primary, I get 6.7 on the filaments, a little high but ok. On the 115v primary I get 7.3 on the filaments, too high...?
Second, I have the first filter before the stand/by and unloaded (S/B off) I get 430 vdc and the cap is rated at 450, too close. I was thinking if I put the first filter after the S/B, the B+ would be always be lower to that first filter. Playing the amp the B+, is 415, is that too close to the 450 rating?
Thanks as always,
al
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I would use the 125v primary tap on the PT. I wouldn't worry about the filter cap, but if you must, then use a 500V cap.
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You must state if your voltage reading is with the filament circuit loaded (tubes in) or unloaded (tubes out). If loaded, yes, too high.
Fix: a pair of power resistors, one in ea leg of the filament supply. Use Ohm's Law to figure resistance value value and W rating. Cut that in half, to get each R value.
Or, a diode will half rectify & drop about 1V.
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Or, a diode will half rectify & drop about 1V.
Won't a half wave on one filament wire change the voltage to dc?
Amp is loaded (tubes in), with Amp power on and S/B off the ac on the tubes filament is 7.7v, S/B on, ready to play, the voltage drops to 7.3, too high. The diode trick seems much easier than two big power R's. Maybe I'm not getting the diode trick right, was it a diode (1N4007) on one filament wire, cathode facing tubes to drop that volt, that would be a perfect 6.3v.
thanks,al
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Use the 125v primary tap and call it done.
Do not use a 1N4007. It is only rated for 1 amp! If you insist then use diodes rated for the total filament current. I bet it's more than 1 amp. Connect two diodes back to back and in series with the filament string. Diodes can be rated for only 50VAC but they must be rated for high current.
REPEAT... Use the 125v primary tap and call it done.
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I would use the 125v primary tap on the PT.
Yeah, makes sense, no "micking" involved, thanks.
Make any difference if I put that first filter after the S/B, (take rectifer to S/B first) would only take a few minutes? As the voltage after S/B on that cap is 415v rather than 430v. Don't mind me, I get a little anal, just want to learn all I can, not so much this amp but future stuff. You've forced me to read more rather than ask too many Q's here, now I'm getting a much better understanding, thanks, al
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Use the 125v primary tap and call it done.
Do not use a 1N4007. It is only rated for 1 amp! If you insist then use diodes rated for the total filament current. I bet it's more than 1 amp. Connect two diodes back to back and in series with the filament string. Diodes can be rated for only 50VAC but they must be rated for high current.
REPEAT... Use the 125v primary tap and call it done.
Missed your post, I didn't know ac and dc rating for diodes are different. Probably just physiological in my mine that 387v sounds better than 415. I'll just use the 125 primary
Thanks, al
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I didn't know ac and dc rating for diodes are different.
They're not different. Diodes don't know AC from DC. They just know PIV (Peak Inverse Voltage). If you exceed the PIV rating for only a few microseconds then the diode may fail.
But voltage rating is not the limiting factor in the suggested application. The diodes you choose must be able to safely pass the current required by the filament string. That may be 3 or 4 or 5 amps or even higher current. A 1N4007 handles the voltage just fine but will quickly fail if it needs to pass 5 amps!
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That Deluxe Lite plays very well, thanks for posting the schematic. Much better than the 18 watt, 6V6 Super Lite I had in the old Crate conversion.
I have an OT, unknown values, looks to be about 20 watts. Has two secondaries, yellow and green but unknown. I put a wall wart, 6.86 acv output into the primary, plate to plate got .21v on the yellow, .14 on the green. So, the ratio is 32:1 yellow, 49:1 green. Squared, 1066 Yel, 2401 green. Kind of lost my math, want to use it in that Deluxe lite.
I thinking: 8 ohms x 1066 is close enough to 8,000 impedance, using the yellow sec. with an 8 ohm speaker seems to be great...? But not sure.
The green sec., 49:1 windings ratio squared is about 2400, 2400 x 8 = 19,200 (no good for 8 ohm speaker) 2,400 X 4 = 9600. So I'm assuming the yellow is good for a pair of push/pull 6V6's with an 8 ohm speaker load and the green is good for a 4 ohm load.
An I doing this right? I think plate voltage comes into this too..?
al
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I think plate voltage comes into this too..?
yup, use your plate voltage and current to solve for ohms (primary), then use your speaker ohms to make it so, if you can.