I am in the process of building a Hiwatt variant based on the Triwatt preamp and a MusicMan style high voltage PS/Output section. I have built the PS board using an example high voltage schematic that has been posted previously on TAG. The only changes I made made are to increase the filtering to be more consistent with Hiwatt designs, the substitution of a 100R resistor for the choke, and a 220k vs 150k bleeder resistor.
Below are the unloaded voltages I am getting. The B1 and B2 look OK for ~700v plates and ~350v screens, but the voltages for B3-B5 are way high. I started with 2x22K/10w resistors as per the sample schematic.
Do I need to go even higher than 22k x 2 to drop preamp voltages to reasonable levels or is measuring unloaded voltages in this case so misleading that the voltages I am seeing should not be trusted? Or is there something else I am missing?
I would prefer to avoid a performance of the exploding 12ax7 trick.
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With no tubes in the amp or attached to those points you won't get any drop. I would advise series 47 UF 350 in place of the 450 volt caps or they will certainly blow up. You could add a 450 Volt 5-W zener string at the output of the dual 22-K resistors to limit the voltage and keep those current caps as well. That's a good safety measure and limits the B+ rail, thus protecting 12AX7's from 700 volts on the plates while the amp is in cold start mode.
lord preset wrote:I am in the process of building a Hiwatt variant based on the Triwatt preamp and a MusicMan style high voltage PS/Output section. I have built the PS board using an example high voltage schematic that has been posted previously on TAG. The only changes I made made are to increase the filtering to be more consistent with Hiwatt designs, the substitution of a 100R resistor for the choke, and a 220k vs 150k bleeder resistor.
Below are the unloaded voltages I am getting. The B1 and B2 look OK for ~700v plates and ~350v screens, but the voltages for B3-B5 are way high. I started with 2x22K/10w resistors as per the sample schematic.
Do I need to go even higher than 22k x 2 to drop preamp voltages to reasonable levels or is measuring unloaded voltages in this case so misleading that the voltages I am seeing should not be trusted? Or is there something else I am missing?
I would prefer to avoid a performance of the exploding 12ax7 trick.
Yes, B1 & B2 are exactly right!, my unloaded voltages are usually around 750-760V. With no bleeder resistor I have used a 47-50K 25W. I copy HAD, an 820K bleeder with a 40K 25W, 43K 25W with a 750K bleeder. Install the bigger resistor and once there is a load on the supply, voltages will be right in range.
TM
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Last edited by ToneMerc on Wed Apr 02, 2014 7:32 pm, edited 2 times in total.
FUCHSAUDIO wrote:With no tubes in the amp or attached to those points you won't get any drop. I would advise series 47 UF 350 in place of the 450 volt caps or they will certainly blow up. You could add a 450 Volt 5-W zener string at the output of the dual 22-K resistors to limit the voltage and keep those current caps as well. That's a good safety measure and limits the B+ rail, thus protecting 12AX7's from 700 volts on the plates while the amp is in cold start mode.
Thanks, but I am confused. Which 450v caps are you referring to? The 47uf cap on B+2 is seeing 379v. I was thinking that the 200uf/450v caps are in series with respect to the high B+ rail and would be good to 900v. Again, I was basing this off the "HIGH_VOLTAGE_PS_V2.jpg" schematic which calls for 100uf/450v caps in the same configuration.
As for the Zeners I might try that. But not having used them before I have some reading to do.
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I am referring to the three nodes on the right hand side of your diagram after the dual 22-K dropping resistors. I use a 47-K 25-W chassis mounted aluminum housed resistor in this location. D*mble (and Tony Bruno) used a 15 or 20-W WW on the power supply board. The Zener string can be two 200-V and one 50-V 5-W Zener diode in series to limit the cold voltage to the lower voltage supply for the preamp tubes. The main B+ caps in the doubler and your screen connection are fine.
Here you go.............diodes to ground will limit B+ to 450 V and when tubes warm up the voltage will dip below 450 because of the preamp tubes drawing current. The diode limit the B+ to the tube while they are cold. A good idea. I do this in my music man mods. Protects tubes and 450-V caps....
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FUCHSAUDIO wrote:Here you go.............diodes to ground will limit B+ to 450 V and when tubes warm up the voltage will dip below 450 because of the preamp tubes drawing current. The diode limit the B+ to the tube while they are cold. A good idea. I do this in my music man mods. Protects tubes and 450-V caps....
I think it's a tad low for this application, and the PI may want a little more filtering at that node Aleks. I built this a few times and found that to be the case. Thanks for dropping-in though !
