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Old 1st November 2012, 10:12 AM   #1
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Default Acceptable variance in plate voltage?

I've recently built a 6G2 Brownface style Princeton. It's a great sounding little amp and seemingly runs perfectly fine, but the plate voltage is about 10% higher than it should be.

Factory schematic indicates I should be measuring about 315V at the 6V6 plates, but with an RCA 5Y3 i'm reading 350V, and with a Sovtek 5Y3 it's 364V.

Now pretty much every component in the build is over spec'ed (1 watt resistors, 630V capacitors) for a hard life, but even i'm still a little uncertain as to why my readings are so high.

Oh yeah, i'm a newbie at this amp building game, so if this is a perfectly obvious and dumb question please forgive me
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Old 1st November 2012, 10:29 AM   #2
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Bias the 6V6 a bit hotter. More current thru the 6V6 and the voltage will drop, due to high impedance PSU.
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Old 1st November 2012, 10:33 AM   #3
SY is offline SY  United States
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Check your line voltage- that's often higher than nominal, and the voltages in the amp will scale accordingly. If the amp is rated for 220V and your line is 240V, you'll see everything roughly 10% high.
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Old 1st November 2012, 10:45 AM   #4
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Thanks guys. The transformer is a Classictone 40-18027 - with 120 and 240V taps. Being an Aussie i'm obviously using the 240V.

I will try to run the bias a little hotter, but i'm running my bias pot as hot as the trim will let me and it's still a too cold (measuring 12 & 16mA respectively, should be ~21mA). It's running in series with a 15K resistor, so I might flip this resistor out for a 7 - 10K resistor so a hotter bias is possible.

Does that sound like a good plan or should I opt for a different resistor value?

Last edited by Ravenhaller; 1st November 2012 at 11:08 AM.
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Old 1st November 2012, 11:57 AM   #5
DF96 is offline DF96  England
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Remember that in the days of old, when valves ruled, a normal resistor had a tolerance of 20% and a few high stability resistors were 5%. Provided you are not running too hot, 20% variation should be fine for most circuits. Exceptions may be where DC coupling is used.
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Old 1st November 2012, 12:27 PM   #6
AJT is offline AJT  Philippines
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a resistor in series with your power traffo secondary is a sure way to lose some voltage....cost of 2 47ohm 5 watter resistor....you can adjust this value till your confident that you have the correct voltage...

btw, are you using cathode resistor biasing of fixed biasing?
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Old 1st November 2012, 02:13 PM   #7
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I used the original fixed bias design, for some reason my playing style gels better with fixed bias amps as opposed to those of cathode bias.

Here's my bias circuit, mounted on the main turret board:

Click the image to open in full size.


I have to ask - the original schematic calls for a 30K resistor in the bias circuit, which i'm guessing would come close to achieving the ideal 21mA bias point for 6V6's. My 25K trim pot is virtually bypassed and i'm still cold with only a 15K resistor biasing the power tubes. How could this be?

Again - forgive me if this is an obvious query, but i'm still a greenwood builder

Last edited by Ravenhaller; 1st November 2012 at 02:15 PM.
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Old 1st November 2012, 10:08 PM   #8
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Quote:
Originally Posted by Ravenhaller View Post
I've recently built a 6G2 Brownface style Princeton. It's a great sounding little amp and seemingly runs perfectly fine, but the plate voltage is about 10% higher than it should be.

Factory schematic indicates I should be measuring about 315V at the 6V6 plates, but with an RCA 5Y3 i'm reading 350V, and with a Sovtek 5Y3 it's 364V.

Now pretty much every component in the build is over spec'ed (1 watt resistors, 630V capacitors) for a hard life, but even i'm still a little uncertain as to why my readings are so high.

Oh yeah, i'm a newbie at this amp building game, so if this is a perfectly obvious and dumb question please forgive me
If it sound great don't touch it.
so what you have an extra 10% in voltage increase that means your output amp has a couple of db of extra headroom.

Just make sure your heaters are not over voltaging as that will wear down your tubes quicker. stay 5.5 to 6.5V on the heater.
Fenders Voltage readings are taken with 110V applied to the circuit. a 10% margin is ok provided the heater voltage is never above the design maximum 6.5V. As your rectifier diode wears, this voltage will drop too.
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