Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on March 16, 2012, 05:06:22 pm
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What would be the best method to lower my plate voltage in a 2xEL34 JCM800 type circuit about 40-60VDC?
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One of the possible solutions
(http://www.el34world.com/Forum/index.php?action=dlattach;topic=13012.0;attach=27729;image)
Kagliostro
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Is there a way to reduce the power tube plates and not the entire B+ rail?
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Yes, split the B+ rail into 2.
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So you mean do what kagliostro posted but Y the B+ before that and use the other leg of the Y for the preamp?
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No, that's in the CT of the PT so it wont work if you want to split it.
But you can do it with a VVR, that has only the power tube scaled.
Tubenits got a bunch of schemos in the schemo section with the PSU like this.
Brad :icon_biggrin:
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For a voltage drop of 50V or less, i would use a dropping resistor. Or use a SW to short it for full voltage. A DPDT SW could be rigged to simultaneously swich fixed bias, if desired.
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For a voltage drop of 50V or less, i would use a dropping resistor.
Can you elaborate how you would implement this? Are you talking about a resistor in series with the OT center tap? And if so, what approximate value both wattage and resistance are we talking?
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As told one other way is to have separated branch
(http://www.el34world.com/Forum/index.php?action=dlattach;topic=13012.0;attach=27727;image)
you can duplicate the resistor from which exit the B+, with one other that is larger in resistance about value, if you have calculation difficulties as I have, you can start with a large value, measure the voltage, than lower the value and check a new time the obtained voltage till when you reach the desired voltage level
or, as suggested, you can use a circuit like a VVR with fixed voltage using a trimmer instead of a pot
Kagliostro
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I use 1 resistor after the rectifier. Determine the current draw of your power tube(s). This is essential knowledge, to set bias, etc. Then you know your desired voltage drop, and the current draw. Use Ohm's Law to calculate the resistance value needed to get your voltage drop. Then use the Wattage Formula to get its power rating. To work on amps you really have to do this math. Post for more help.
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ok, i'm trying to wrap my head around all this, but one question i have off the top of my head is to jjasilli, and that would be this. Doesn't that resistor cause the amp to have a lot of sag? I ask because i've tried that before to GET sag and it indeed did. And thats using a resistor much smaller than a typical dropping resistor. So i must be missing something but thats why i'm asking.
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Yes, a resistor will tend to cause sag. Imho, in response to the original question, that's the best way! :grin:
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Well, thanks 4 the replies. However, i decided to just swap the tranny. Let me explain why i was asking about this and see if you have an opinion as to whether it was indeed plate voltage or just B+ voltage period or even something else altogether i was experiencing. I have 2 PT's i've used in this amp, a hammond 250mA that measures 440VDC at the plates and a magnetic components thats 150mA and puts 480 to the plates. One other difference is the hammond's heater voltage is higher than normal at something like 7 VAC. I have been using the hammond for quite a while now but because the heater voltage is high i was thinking maybe the buzz i was experiencing might have to do with that. So i swapped the MC back in. I then noticed that playing the amp in a music mix i was noticing a annoying amount of very hgigh frequency, As tho you had a EQ hooked up and boosted the hell out of 8k. So i swapped the hammond back and not it's fine again.
So the question is, do you think that was due to the extra 40VDC on the plates, extra voltage in the preamp, or what? I'm asking because if it IS indeed the extra PA plate voltage (and yes, i did calculate the bias at the plate voltage and adjust with each tranny) that made it sound much better, i'm now wondering is i still should lower it to get and even better tone. I always hear about lower plate V being browner and even believe i read a post here or somewhere just recently about tonal benefits running higher current thru lower voltage. What do you think likely caused the extended nasty highs?
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Higher heater voltage does not cause buzz. Also, 7V heaters is not terrible loaded; and is normal UNloaded. One neat bypruduct of a bucking tranny is that it knocks down heater voltage. Also, a 40V difference in pleate voltage @ over 400V is less than a 10 percent variance -- still within spec
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Well, i just mentioned the heater to explain why i tried swapping in the first place. But the real reason for this thread was to try and lower plate voltage to get a better tone because when i swapped trannies for the heater issue (not an issue, but at the time i thought maybe) i found the tone issue i mentioned and thought if that was a product of higher plates then going lower would give even better tone. So now i started thinking even with my 440v PT back in, what if i went to 400...browner tone? But alas, i stuch a resistor on the OT center that brought the plates to 410 and i heard and felt nothing. So i'm just going to leave it. Sounds great anyways so it's not like i'm trying to rectify a issue, just that wacky never ending quest for magic. :icon_biggrin: Still wondering why that PT that gave me 480V at the PA plates had that nasty top end. But i figure it was probably happening in the preamp with the extra VDC.
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Did you mention which power tubes or current draw? Maybe neither PT is causing sag.