janka wrote:I'm going to order a 550vdc cap for my st-70. However, I've been warned going above 50uf on the first section can put my 5AR4 at risk. What is the benefit of the 80/40/30/20 @ 550vdc versus the 30/20/20/20 @550vdc ? Does the 80uf in the first section really pose a risk to the 5ar4?
Good question concerning connecting the 80 uf as first section. There are really two good reasons for keeping the first cap section relatively small, say 30uf and not much more. First it is a matter of rectifier tube reliability. Second it is a matter of performance. But there is a great performance advantage in putting a 80uf, or even larger, cap in the rear lug position. I'll start with the first idea.
The first section capacity reactance and the resistance of the power transformer high voltage windings are what primarily determine (and limit) the rectifier peak inrush current during an inrush event. Ordinarily, the relatively slow rectifier filament warm up prevents inrush at turn on, but if the amp gets hot switched or if your utility company should briefly interrupt your power, you will get the inrush event, guaranteed. During a brief power interruption, lets say 1 second or so, the first section cap voltage will fall to something less than 100 volts but the filaments of the GZ34 (5AR4) will still be plenty hot. When the brief power interruption is restored the inrush currents involved in trying to peak up the first section from a voltage less than 100 volts vs the usual 400 volts that ordinarily would have been there will easily exceed the 3.76 amp limit set by GE (for example)and you will be at risk of finding the coating of the rectifier cathode spread about the interior of your now useless GZ34. The color of that now powderized coating is white, BTW. Now for the performance advantage of keeping the first section in the 30 to 40 uf range.
There is an optimal value range for the first section if the highest possible operating B+ is to be had. Oversizing the first section will lengthen the time constant of the first section, composed of secondary winding resistance and the capacitance of the first section, and result in a lower peak voltage on the first section and a consequent reduction in B+ value. I have tried dubbing in various capacitance values in the first section and found 30 to 40 uf yields the best results.
There is, however, a really great advantage to upgrading the rear lug, or "C" section capacitance. The original 20 uf cap in this position allows some very substantial voltage swings in the plate B+ for the power tubes. I have measured voltage swings as high as 40 volts on this point when the amp is just driven into AB1 mode. To say this causes compression would be an understatement. Even at low listening levels I can hear noticeable improvement when this section is upgraded. 80 uf will reduce music induced power tube B+ voltage swings, all other things being equal, by about 75%. Personally, I prefer and use a "helper" cap of 210 uf.
IMHO, upgrading the rear lug capacitance is the most bang for the buck as upgrades go and you are sure to be pleased with the sound of your "new" ST-70. Hope this helps the ST-70 community. Rock on!
Last edited by dynacojoe on Thu Nov 22, 2012 12:29 am; edited 1 time in total