Was: Re: PAS Transformer Replacement
166.250.0.62 |
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| Posted on March 20, 2011 at 17:49:25 | |
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In a post on Feb. 1, 2009 titled "PAS Transformer Replacement" (I'm not going to revive a 2-year old thread), it was suggested that the Hammond 290AX transformer might be a usable high-quality replacement/upgrade for the Dynaco PAS2/3 PA211 power transformer. The specs on the 290AX (the real one, from Hammond, not the knock-offs) are: Primary: 120VAC. Secondaries:325-0-325@81mA for B+, 6.3V@2A for filaments, 5V@2A for rectifier tube size: 3.0in.Lx2.5in.Wx2.8in.H Weigh: 2.85 pounds. It's a drop-thru, meant to sit in a hole in the chassis, and be wired from underneath. It can be made to fit in the unmodified chassis with a custom pop-up bracket with 4 bolt holes that screws into the original mounting holes, but you'll have to ditch the selenium rectifier for a pair of diodes to make room. That said, the physical mounting isn't the main issue, contrary to the original post. If you're replacing the tranny because the old one is dead, one of the drop in replacements from specialty Dynaco parts houses like Triode will probably serve you, though there's some mixed rap on the Triodes. If you are looking for a beefier tranny because the filament current is sagging under load (a common observation) you may want to consider the following first: - replace the pilot lamp (240ma - 400ma draw) with an LED (30ma-50ma) - replace the selenium rectifier (it drops 1.0-1.5V, consuming 100ma under load) with a pair of fast recovery 3A rectifier diodes - definitely replace those old 2000uf filament filter capacitors (may have ESR of several ohms or more) with new low-ESR 3300uf 35V electrolytic caps - replace the power supply filter can with a new 20/20/20/20uf or 30/20/20/10uf can. This isn't in the filament current path, but if the preamp is sagging under load, it may be drawing excess current in the B+ path, and sucking it out of the 12V path due to ESR in the big caps. Even better for a little more money, there's a 80/40/30/20uf@525V can for $75. - bypass the PS filter caps with 0.02uf 630V MKP film caps (because of the wound construction of electrolytics, they have increasing R at higher frequencies). - check all the capacitors (don't bother with the pF caps) and replace any that are out of spec by more than 50% or have ESR more than a few tenths of an ohm (I know, more work than you want to do). Use PP film caps rated at least 450V. If your tranny is actually alive, 80% of you won't need a new one if you do these. If you are among the 20% who are still having a problem, the alternatives are problemmatical. Sellers on Ebay sell Chinese-made toroid transformers as "drop-in" replacements for PA211s. Not so fast - you'll need soft-start circuitry to protect against inrush current 40 times greater than that for an upright transformer. They also run just on the edge of saturation. When they do saturate, they generate all kinds of nasty high amplitude high frequency artifacts that can destroy your tubes. You need a filter to block these, and a clamping circuit to avoid current saturation that'll overheat the transformer and cause a meltdown. In short, to use these, you need a regulated power supply. The sellers don't tell you that. That brings us back to the Hammond 290AX. The filament supply on the PA211 is 12V@800ma, very wimpy. The filament supply on the Hammond 290AX is only 6.3V@2A - 12V@1.05A if we can step up the voltage easily. It already looks dubious, because that's only an extra 250ma over the PA211 800ma, and we haven't considered the I^2*R losses in the voltage step-up circuit. You'll need a pair of big caps and a pair of diodes, just like what's already there, and a small .4ohm (or .39ohm) 2W resistor to drop the resulting 12.6V to 11V or so, the level output by the original selenium rectifier. The current passes through the additional series cap with ESR probably in the .1ohm range, so the loss is 2A*2A*.1ohm=.4W. The resistor is in series with the secondary, and the loss there is .8V* 2A=1.6W, total 2.0W. At 12V output, that's a parasitic loss of 167ma from the circuit to use the 290AX's 6.3V filament winding. That means we're only getting 250ma - 167ma = 83ma of additional current for the circuit out of this tranny. The 290AX B+ current at 81ma is >8 times that of the PA211, but B+ current wasn't the problem with the design of the PA211. The tranny runs 4 12ax7 @1.2mA plate current = 4.8mA plus rectifier tube and parasitic losses, so we're probably covered with 10ma of the PA211, but a little more headroom might be needed under load. I note that the replacement PA211 drop-ins from Triode are rated 660V@15ma, quite good enough. The 2009 thread left the impression that the 290AX was a "better", i.e. upgrade transformer. It IS better quality, but it won't solve the problem of insufficient current rating for the filament winding, particularly if you've modded the box with cathode followers or anything else that consumes more filament current. Most other alternative trannys will have the same issue with filament winding at 6.3V, because triodes and most other tubes have 6.3V heaters. Dynaco for some strange reason, wired the 12ax7 triode pairs in series, then the tubes in series, so requiring a total voltage of 25.2, though they dropped the winding voltage a bit to give the tubes longer life. The problem that the further from source a series triode is, the more it'll get staved for current under load is an issue for a re-design. Bzzzzt.
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| RE: Was: Re: PAS Transformer Replacement, posted on March 23, 2011 at 07:53:22 | |
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Posts: 43
Location: Central New Jersey Joined: September 8, 2007 |
Thanks for the info guys. Sal
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