I am building this amp http://www.woundcomponents.co.uk/300b-docs/SE001CHANNELA.pdf I only have a 240v/.1 amp transformer for the 6sn7/5687 supply. Their schematic specifies a 270 volt transformer. How can I figure the correct resistors to the tubes to get the recommended voltage? I have the power supply app but do not know how to make it work on this design. Any help would be appreciated
There's not enough information to calculate the correct (smaller) value of the 1k resistor.
Start with the 1k value, and parallel resistors with it until the 350V output is correct.
Once you have 350V, calculate the resistor's total power dissipation, from the voltage drop
and the total resistance, to make sure the power rating of the replacement resistor is enough.
I think PRR is just saying you can use the the single HV supply and add another stage of filter drop to power the input and driver stages (though the increased current through the chain will lower the 300B's plate voltage a little as well.)
Another take : If the output VDC of your "B" supply is 333 I hardly think it matters. It's a 17 volt difference from the design specified voltage which is ≈5% and on an unregulated tube amp circuit is no big deal.
Using Ohms law, the schematic 5687's plate load resistor drops .008 x 22,000 = 176VDC.
Subtract your 17VDC and you get 159VDC. 159/.008 = 19,875 Ω. 20KΩ is plenty close enough.
There are other ways to shift it, you can leave the plate load R the same and lower the cathode resistor value to keep the same current.
As for the 6SN7 , using Ohms law you can figure the current through the 12K resistor which is the 6SN7's current draw. (350-300=50 . . . 50/12,000 =.00417 A) Using that you can figure all the other voltages with Ohm's law.
( A higher voltage and current across the 'N7 might be better if your supply can provide it.)
My personal opinion - You're close enough. If you've already got the power supply, why not build the signal circuit as given (as a temporary build) and try different operating points by changing R values, looking to see what sounds best with the components you've got? If you do a Google search on "5687 operating point " you'll quickly see that you're not limited to the exact voltages in the schematic.
HTH
Another take : If the output VDC of your "B" supply is 333 I hardly think it matters. It's a 17 volt difference from the design specified voltage which is ≈5% and on an unregulated tube amp circuit is no big deal.
Using Ohms law, the schematic 5687's plate load resistor drops .008 x 22,000 = 176VDC.
Subtract your 17VDC and you get 159VDC. 159/.008 = 19,875 Ω. 20KΩ is plenty close enough.
There are other ways to shift it, you can leave the plate load R the same and lower the cathode resistor value to keep the same current.
As for the 6SN7 , using Ohms law you can figure the current through the 12K resistor which is the 6SN7's current draw. (350-300=50 . . . 50/12,000 =.00417 A) Using that you can figure all the other voltages with Ohm's law.
( A higher voltage and current across the 'N7 might be better if your supply can provide it.)
My personal opinion - You're close enough. If you've already got the power supply, why not build the signal circuit as given (as a temporary build) and try different operating points by changing R values, looking to see what sounds best with the components you've got? If you do a Google search on "5687 operating point " you'll quickly see that you're not limited to the exact voltages in the schematic.
HTH
I have built the amp using one transformer for everything. I want to use a second transformer for the signal tubes as I have found it usually sounds a little better this way. Also using a single supply means that I have to use either two rectifier tubes or solid state. I would rather power the 300b's with tube rectification. Sounds like the easiest way to implement this is to order a different transformer. Thank you for your help.
It would sound a lot better to shunt regulate the driver and not introduce the second power supply at all.
Why?
There used to be a lot of arguing for this in the 90's, not so much anymore!
There is a serious flaw in the power supply.
The first smoothing capacitors after the rectifier tube are series connected 2 x 470 µF, forming 235 µF as total capacitance. This is far too much and ruins the rectifier.
Maximum for GZ34 is 60 µF.
That schematic seems to "designed" by incompetent designer.
From the linked pdf:
"............The output stage supply is unusual as it uses a valve rectifier in parallel with solid state diodes to combine the low impedance and high current capability of solid state with the good sounds of valves. This allows a relatively large input capacitor to be used, something which is not possible with a valve rectifier alone and is good for regulation and therefore bass performance. If the arrangement looks unusual it is, and there are no doubt sceptics that will doubt it’s efficacy, but it does sound like a valve rectifier with good bass.........."
I wonder how much current the tube is supplying vs the diodes?
"............The output stage supply is unusual as it uses a valve rectifier in parallel with solid state diodes to combine the low impedance and high current capability of solid state with the good sounds of valves. This allows a relatively large input capacitor to be used, something which is not possible with a valve rectifier alone and is good for regulation and therefore bass performance. If the arrangement looks unusual it is, and there are no doubt sceptics that will doubt it’s efficacy, but it does sound like a valve rectifier with good bass.........."
I wonder how much current the tube is supplying vs the diodes?
"It would sound a lot better to shunt regulate the driver and not introduce the second power supply at all."
I do not want the sound of a regulated supply. I am using two sets of 6sn7/5687 tubes and the current draw is too much for one rectifier tube.
I have built the original design and you can hear the tube even with the diodes. I switched from 274b, 5u4, GZ34 and they all sounded different
I do not want the sound of a regulated supply. I am using two sets of 6sn7/5687 tubes and the current draw is too much for one rectifier tube.
I have built the original design and you can hear the tube even with the diodes. I switched from 274b, 5u4, GZ34 and they all sounded different
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