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Old 15th October 2012, 12:22 AM   #1
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Default Need a second opinion

Hello!
I have a Bottlehead Stereomour configured for 45 tubes.
In stock form, it's running the 45 tubes at 300volts (360-60) and dissipating at 11.25watts (1.6K cathode resistor). I think this is pushing the tubes to its limit.
The power supply is 160 volts going thru a voltage doubler (pseudo-dual). Rectified DC is about 410 volts. Then it goes to a 1.2K/10W dropping resistor. I changed these resistors to 2k and it drops the voltage to 280 volts (p-k).
Is it okay to drop the voltage further to about 250-260 by increasing the the resistor value? Is there any disadvantage to using high-value resistor?
Right now, I've dropped the voltage to 270 by using a variac but I can only go down to the lowest voltage acceptable for the filaments (-5%) .
Thank you in advance.
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Old 15th October 2012, 12:31 AM   #2
AJT is offline AJT  Philippines
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yes, you can also reduce the B+ by adding a series resistor to the secondary ac leads, imho, this is also a good arrangement as it limits surge currents, your traffo will be happier...
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Old 15th October 2012, 12:58 AM   #3
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Thank you kabayan. You mean adding the resistor before the rectifier diode? As for the resistor value, do I just use ohm's law to get the desired drop?
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Old 15th October 2012, 01:29 AM   #4
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you're welcome, that's the idea...best to use a wire-wound pot and then adjust it under loaded conditions, then once you get the desired operating point, you can get the nearest standard value resistor....

you can chop the 2k resistor into 3 680ohm series resistors so that you have a CRCRCRC filter, that way you can get the ripple voltage really low....
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Old 15th October 2012, 11:56 AM   #5
DF96 is offline DF96  England
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Too much series resistance in a power supply can boost gain at subsonic frequencies. This may be OK, provided that your coupling caps are not too large so subsonics can't make their way through the amp. Unfortunately the modern fashion is to use large coupling caps.
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Old 15th October 2012, 12:47 PM   #6
AndrewT is online now AndrewT  Scotland
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DF,
can you explain further?
Is this anything to do with "motorboating"?
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Old 15th October 2012, 01:11 PM   #7
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Quote:
Originally Posted by DF96 View Post
Too much series resistance in a power supply can boost gain at subsonic frequencies. This may be OK, provided that your coupling caps are not too large so subsonics can't make their way through the amp. Unfortunately the modern fashion is to use large coupling caps.
i understand, the rule of thumb is to use just the right amount(minimal) of coupling capacitance to give you a roll-off just below 20hz...
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Old 15th October 2012, 03:22 PM   #8
DF96 is offline DF96  England
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At low frequencies decoupling caps/smoothers have less effect, so any series resistance in the supply is seen as extra anode load by the valves. This could
a) boost subsonic gain in a simple voltage amplifier
b) reduce subsonic gain in a transformer-based stage
c) introduce inter-stage feedback which may be positive or negative, and frequency-dependent

The point where decouplers stop working may or may not overlap with the point where couplers stop working. Modern practice uses much bigger caps in both places than 50 years ago, so it is possible that what would have been a near-subsonic issue now becomes a syllabic-rate/envelope issue as it all happens lower in frequency. In olden days PSU's used big chokes and coupling caps were 10nF; now we use big PSU caps and coupling caps could be 0.47uF.

If it gets as bad as actual instability then it is called motorboating, but it could muddy-up LF stuff even if it is not oscillating.
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Old 15th October 2012, 06:00 PM   #9
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Why not better to try a voltage regulator since you have sufficient voltage to drop. This way the output impedance seen by the valves from supply is greatly reduced, and THD also reduced?
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Old 15th October 2012, 08:11 PM   #10
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Quote:
Originally Posted by DF96 View Post
Too much series resistance in a power supply can boost gain at subsonic frequencies. This may be OK, provided that your coupling caps are not too large so subsonics can't make their way through the amp. Unfortunately the modern fashion is to use large coupling caps.
Thank you DF!
The coupling caps are 0.1uf.
How much is too much resistance? I think I need around 2.7K to get to the correct voltage. I think I read somewhere that too much resistance can also make voltage stability worse.
Is there other way to shed around 100 volts?
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