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Post subject: 58 Harvard Bias to hot ?
Posted: Fri Jan 13, 2012 1:55 pm
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Just checked my now favorite amp and got 348 plate voltage @ 34mA , what do you guys think of that acceptable or not. By the way I think it sounds great right now.


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Post subject: Re: 58 Harvard Bias to hot ?
Posted: Fri Jan 13, 2012 7:31 pm
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Race, IMHO that seems a little warm. Art

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Post subject: Re: 58 Harvard Bias to hot ?
Posted: Fri Jan 13, 2012 7:40 pm
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Hi Race66,

If that is Cathode current for one tube, that's close to 11 watts idle (about 80%), fairly hot for a 14 watt tube in the 5F10 amp, but not too unusual. If your current reading is the actual Plate current, the idle is closer to 12 watts (85%), too high IMO. You could easily change a resistor and lower the idle current if you want, or use lower current draw tubes. Some output tubes will run cooler, and some will run hotter in the same circuit. You will typically get longer tube life with a cooler bias, but if it's too cool, you can lose tone.

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Post subject: Re: 58 Harvard Bias to hot ?
Posted: Sat Jan 14, 2012 12:03 am
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I think if you have a good output tube (like RCA or Sylvania), you'll be ok at 11 watts. One thing you may want to check... if the bypass cap has been changed to another value. Like 50 mfd, instead of OEM 25 mfd. And also, the value of that grid-to-ground 220k-ohm resistor hanging off the 6V6GT's grid. Optimally, should be within 5% of that value. This resistor also helps set the idle bias, as well as the cathode tied 470-ohm resistor.

As long as you aren't red-plating the 6V6GT. Or the amp is OD too soon in volume range, I think you'll be ok.

FWIW--- I like putting a screen resistor between the power supply and the 6V6GT screen grid (470-ohm, 1/2 watt is fine). This helps keep the screen at a lower voltage than the plate and prevent thermal runaway. And I like wire-wound, low inductance cathode-tied 5 watt resistor from cathode to ground. Tube seems to like these better. Like I said FWIW... but it's ok to let sleeping dogs lie, if all is well, as it is. :)

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: 58 Harvard Bias to hot ?
Posted: Sat Jan 14, 2012 12:10 am
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Your amp has a dropping resistor between B+ and the screen grid power supply. So, no worries there.

Sorry, page wouldn't take edit function. :cry:


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Post subject: Re: 58 Harvard Bias to hot ?
Posted: Sat Jan 14, 2012 7:25 am
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I think that the OPs amp is the 5F10 ('58) which is a fixed bias, with two 6V6 and the Cathodes are connected to ground (no bypass cap), not the 6G10 which is a Cathode biased single 6V6 output tube. :idea:

http://www.prowessamplifiers.com/schematics/fender/Harvard_5f10-Schematic.html

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Post subject: Re: 58 Harvard Bias to hot ?
Posted: Sat Jan 14, 2012 9:05 am
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Any suggestions on what resistor value to use and would it be the 6800 before the rectifier that I swap out ?


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Post subject: Re: 58 Harvard Bias to hot ?
Posted: Sat Jan 14, 2012 9:12 am
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Just a note it is a 5F10


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Post subject: Re: 58 Harvard Bias to hot ?
Posted: Sat Jan 14, 2012 10:30 am
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The 6800 and 56K resistors, and the diode form a simple voltage circuit to supply a negative DC voltage to the control grids of the output tubes. The more negative the grid voltage, the lower the idle current through the output tubes. The closer to zero that the grid voltage gets, the higher the idle current through the output tubes (red-plating at some point).

Note the schematic shows -27 volts where the two 220K resistors connect (control grid circuit). So, more negative voltage here will decrease the idle current. Typically, leave the 6800 as is, and increase the 56K (make sure that it actually is the value that the schematic shows) to a value that will give you to 24 to 31 mA Cathode current for one tube (at approx 348 Plate volts), adjusting for the change in Plate voltage to have between 7 and 10 watts at idle (50% to 70%). Maybe something close to 65K to 75K as a start and check the idle current.

You could also put a 25K pot in series with the 56K for an adjustable bias. If you know the range of current draw for your amp, you could even change the 56K to a 25K and use a 50K pot in series, making sure that the tubes won't get too hot over the range of adjustment. :idea:

Keep in mind that this can all be different when using another type/brand of output tubes. :!:

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Post subject: Re: 58 Harvard Bias to hot ?
Posted: Sat Jan 14, 2012 11:50 am
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Shimmy, thanks again. For the correct schematic. In that case, the push-pull amp should prolly be biased in the Class AB1 range. 80% idle bias wattage is be bit much. Closer to Class "A." This is the Princeton Reverb issue, all over again.

If you don't mind altering the circuit to a more useful one, esp in light of the different power output tubes you'll be inserting into it--- I'd put a 10 or 25k-ohm pot into the circuit, like Shimmy said.

This will give you the flexibility to use various output tube brands & vintages (they all pass different amounts of current-- and therefore have different bias points). And not have to play with fixed resistors, like I do with a couple of my PR--- which I've kept stock circuitry.

Maybe best to use one of those pots with a short shaft and drill a hole into the chassis, so you can adjust the pot without removing the chassis from the cab. Adjustment is from the side where the trannies sit. Then you can adjust the pot, with the amp on and in the cab (connected to the stock speakers). Just a thought.


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