Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chaswahl on July 23, 2022, 05:24:05 pm
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Maybe I'm just dense, but I can't figure out why the plate and cathode resistors to the reverb driver shown (left side) are called out to be 5W and 2W respectively.
(heavy dashed line at bottom of the first file uploaded is ground bus, at 0 V)
I didn't make these values, ratings or voltages up -- straight from the original schematic and service manual. If I do the math, based on the resistor values and the voltage drops across them, I get
Plate - P = Vē/R = 40 x 40 / 2200 = 0.727 W (spec'd at 5, or nearly 7 x calculated wattage)
Cathode - P = Vē/R = 4.2 x 4.2 / 330 = 0.054 W (spec'd at 2, or 37 times calculated wattage)
Now, that's at quiescence, and I don't know how much swing, or change in voltages, there is in operation, but those factors still give me pause. I can see that the reverb triode on the left has about 2 x the plate voltage as its brother the recovery triode. And unlike the tremolo circuit, where voltages are given for both trem on and trem off (much higher when off than when on), there's only one voltage state given, and I don't even know which state (though the switch is drawn open -- reverb on -- on Ampeg's schematic too.
If anyone can explain why Ampeg chose those values, I'd appreciate it! Obviously I'm a reverb newbie.
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conservative, cautious engineering. I like it. Don't you? :icon_biggrin:
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Take the perhaps unlikely case of V3a going dead-short. I make that to be over 25 Watts. I guess they did not wish to cover that extreme smokelessly.
If I assume one section of 6SN7 can be like 6,000 Ohms, and overlook R39, we have 250V/(2200+6000+330) which is 29.3mA and 7.3W total, mostly in the 6SN7. So it does sound generous.
After you get past 1/2W parts, more watts is not a lot more money. Your 'savings' can be totally lost after a few 'accidents'. Also, not that Ampeg could afford to care, all resistors will fail eventually, and cooler lives longer. For a furnace-fan controller, I happily paid for 100W in 4 resistors for a 7W job; I don't like heat failures in sub-zero weather.
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Thanks a lot. I'm doing a layout, and the size of those resistors was a bit of a pain, but they worked out OK. The 0.47 uF 400 V cap (coupling cap between V3a plate and reverb tank) is no fun to shoehorn in either.
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OK. The 0.47 uF 400 V cap (coupling cap between V3a plate and reverb tank) is no fun to shoehorn in either.
where is the tube pin in relation to the reverb send jack? i usually run the capacitor from tube pin to rca jack on capacitor-coupled reverb stages because yes they're big
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where is the tube pin in relation to the reverb send jack? i usually run the capacitor from tube pin to rca jack on capacitor-coupled reverb stages because yes they're big
Now there's an idea! I've got it on the board now, but will consider what components I can elegantly hang off the tube pins (all the tubes are octals).
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Maybe I'm just dense, but I can't figure out why the plate and cathode resistors to the reverb driver shown (left side) are called out to be 5W and 2W respectively.
The v3A 2k2 plate resistor is seeing 40V which translates to 0.7W, so 5W rating is over-designed. The cathode resistor is even more-so. One possible practical advantage to oversizing resistors (besides being super cautious) is additional heat-handling capability would reduce resistor thermal noise. However, its a reverb driver and so all the signal noise goes into the reverb pan input transducer and becomes mechanical spring torsion anyway, so... its mainly over-caution.