Dynaco PAS-3X Preamplifier. 3 stereo low level: RIAA phono, NAB tape head. An update of the PAS-2 series preamp to include the tone control and output stage modifications. Classic Valve Design PAS Line Mod Less is more! This single tube linestage allows you to select the overall gain for your PAS 2/3/3x preamp. Classic schematic Dynaco PAS PCB's.DYNACO PAS PREAMP (Models 2, 3, and 3X) PAS 2/3 Owners Manual (full) PAS Repair Guide. Replacing the PAS Volume Control. Removing the PAS Tone Controls. Replacing the PAS Selector Switch. Dynaco PAS Preamplifier Selector Switch Replacement Installation & Wiring Manual Page 1 PAS Preamplifier Selector Switch Rev 2 Introduction. It also can be adapted to make a killer tube mic preamp. Mk3/Mk2 boards require 2 12AU7/ECC82 and one 12AU7/ECC82,12BH7 or ECC99 for each amplifier. ST70 Troubleshooting & Triode Connecting. If you have a PAS-2 or PAS-3 preamp and also use the phono section you may be interested in upgrading the. Dynaco PAS-3 RIAA phono equalization upgrade - photo. Dynaco PAS-3 RIAA phono equalization upgrade. Dynaco Pas-3 preamplifier project Richard Philpot. Subscribe Subscribed Unsubscribe 9 9. Installing a PAT4PWR Updated Power Supply in a Dynaco PAT-4 Preamp - Duration: 10:01. Dynaco PAS- 3 (Vintage) Preamplifiers reviews. I rebuilt two Dynaco PAS preamps with AVA rebuild kits: first, a PAS- 2 that I further modified after getting it back from AVA’s free check up. The second time around I rebuilt a PAS- 3, with boards supplied by AVA and my own parts, the same that went in the finalized version of the PAS- 2. Stereophile reviewed the AVA- rebuild of the Dynaco PAS preamp back in 1. John Atkinson was mildly positive in his review and recommended the product with a C rating. From the photograph and description, it appears that Frank Van Alstine is still selling the same kit. Maybe a few caps and resistors have changed, but I’d bet that’s about all. It sounded at least as good as one might expect for the money invested – and here I’m not considering the value for my labor of some tens of hours. But still, it wasn’t quite up to the task of doing duty in a modern hi- end system. Here’s the thing: if one were to rebuild the preamp exactly to the specifications of AVA and stop there, they would be missing out on some seriously unrealized potential. Further mods can bring this little retro unit up to a level that will compete with the best of today’s preamps, making it worthy of at least an A- Stereophile rating. It’s fair to admit the caveats: as the gain is not large, and the output impedance is fairly high it should drive only a single amp with high input impedance (> =6. But these limitations will not be problematic to most. Simplicity is a virtue in itself in minimal corruption of the audio signal, and offers the secondary benefit in that one can afford to install exotic audiophile parts, as they are few in number. Another good thing is that compared to other components, the PAS preamp is really easy to work on. Van Alstine Super PAS 3 KIT preamp based on a Dynaco tube preamp. Van Alstine Super PAS 3 preamp KIT Rebuilding a vintage Dynaco preamp Part I PA211 Upgrade and Replacement transformer for Dynaco PAS2. You could build a nice buffer preamp or mic. Check out our reverb transformer and a twin reverb schematic for your tube reverb project. Once you cut out a hole under the power supply section (described below), everything is readily accessible. Below are the mods I did after first building the kit to AVA specs. Most of my parts came from the Parts Connexion in Canada; I also got some from Sonic Craft, e. Bay and Triode Electronics. Maybe 7. 0% of the sonic upgrade ultimately realized was from the simple act of replacing the Mylar (aka polyester) coupling capacitors. When you check out the AVA website, you will see that the company’s new products contain these horrid capacitors. That’s really sad, because I have every reason to believe that the circuits are well designed. Most high- end tube amp manufacturers of today use metalized polypropylene caps or better, and for good reason. Mylar caps suck the life out of the music, giving a slow, muddy, closed- in, spatially unfocused sound with weak bass, and dramatically rolled- off highs. I replaced the 6. F Mylar caps with Auricap metalized polypropylene; replaced the 0. F Mylar caps with Mundorf oil/silver; and the 0. F caps with Sonicap Platinum (Teflon film/ tin foil). If you are interested, there are plenty of online reviews of these products. I can say, conservatively, that the sonic upgrade was astounding. If you are disinclined to go all out with the many changes suggested below, at least dump your $0. Mylar caps in favor or something better, such as those listed above. The replacement caps will be bigger, but you can get them in there, in some cases by installing them upright. In order to tweak the power supply section, you’ll need to be able to get at it. Drill a hole in the chassis under the power supply board big enough for your nibbler, then nibble out a rectangular hole big enough to get to all the parts on the underside of the PC board. Before you start, loosen the screws that attach the circuit boards, so that the vibrations of drilling and nibbling won’t be transmitted to the boards and their parts. It will take you a while to do the nibbling, so it’s a good project for a rainy weekend. The kit provides mil- spec, general purpose Dale resistors. These are decent, but you can definitely do better. In the power supply section I installed Mills 2. W wirewound resistors. These are remarkably uncolored and are the first choice for higher wattage applications. Don’t bother replacing the resistor supporting the Power On LED; it won’t make any difference. In the signal path of the line stage and phono sections, I went with Vishay S1. Texas Component TX2. These bulk metal foil resistors are dear in price, but you’ll only need 7- 8 of them per channel, and they are transparent in a way that is almost unimaginable. In areas of lesser importance, I used PRP resistors that are still a big upgrade from the dry sounding Dales. If you don’t want to spring for the expensive Vishays, PRPs everywhere is still and excellent choice. Wire colors the sound, just like passive parts. Cutting out the underside of the PS board means your AVA kit wires are a lot longer than they need to be. So replace them with shorter lengths of high- end wire. I used 2. 0 and 2. AWG “silverfuse” hookup wire sourced from LAT International. It’s wonderful stuff and affordable at a less than $2 per foot. I used 2. 0 AWG for power supply wiring (and this is not too wimpy as the preamp only consumes . The kit- supplied wire is good enough for the filament supply - both phono and linestage - and the off/on LED; it’s pointless to replace this. Speaking of wiring, do you really need the ungrounded AC receptables on the back panel? These went out with bell- bottom pants. Yank them all out and this will enable you to shorten and simplify the AC wiring. Their absence will leave holes in the back that will improve ventilation, which I feel is otherwise inadequate. After- market power cords sound vastly better than the hardware store lamp cord provided with the kit. That’s because a good power cord acts as a low pass filter, rejecting the high frequency noise on the AC mains. So again, get out your nibbler and cut out a slot adjacent to the transformer for a male IEC connector. It shouldn’t be hard figuring out how to wire it. The middle ground tap of the connector is a bonus that can be used in a way I describe later. Then plug in a power cord worthy of this component that you are steadily improving. I like the LAT International AC- 2. Electrolytic capacitors. The caps provided with the kit are general- purpose industrial types that are okay, but again, you can do better. Don’t bother replacing the caps that support the tube filaments, because you won’t hear a difference. But the others that filter the B+ supply voltage (6 of them in all on the PS board) should be upgraded. If you never use the phono section the number drops to 4. I got very good results with Mundorf Mlytic types. Make sure to equal or exceed both voltage and capacitance ratings of the originals. Adding more capacitance will lower the noise floor and improve the dynamics and bass. And it’s not just about quantity; some caps filter better than others. Also, consider Jensen 4- pole electrolytics. The replacements will likely be bigger, but there is room for them in the box. I’ve replaced diodes in other components with high- speed FREDs, with excellent results, but in this component there is nothing to be gained in diode replacement. The silver mica capacitors in the phono section are just middling. Upgrade to Rel- cap polystyrene film/foil types for a sweeter more articulated sound on LP playback. The kit runs input signal wire from the tape loop switch to the balance control, back to the volume control, back in the opposite direction to the blend switch, then to the line stage inputs. This is needlessly complicated. Here’s how I simplified things. First, nix the blend switch. If you are like me, 9. In its place I installed a mono ladder stepped attenuator for the right channel, that I built myself with PRP resistors and a 2. TKD selector switch from e. Bay. Where the stereo volume control is, you will, of course, install an identical mono ladder stepped attenuator for the left channel. If you hold out for a good price for the switches on e. Bay, you can build the pair of attenuators for around $5. You will find that compared to the metalized plastic volume controls, they seriously open things up and are thus worth every bit of cost and assembly time. Two mono attenuators with a 2. B taper that I used, give reasonable control over balance, so you can disconnect the balance control without missing it. Now, the signal runs from the selector switch, to the tape loop switch, to an attenuator, to the line board. Your wire runs will be much shorter, which means better sound. You could even go one better and bypass the tape loop switch in the likelihood that you gave your tape deck to the Goodwill years ago. One more thing: the schematic shows a 4. Since you are using 1. So remove the 4. 7 k resistor and you’ll get a little more output and transparency, with no adverse effect. The original tranny in my PAS- 2 seemed to be cooked and the wiring insulation was so brittle it broke off when touched. So I took, through e. Bay, a replacement from Triode Electronics in Chicago, the same company that built the original trannies. But they claim the latest models user thinner enamel on the wire, allowing fatter diameter conductors, and thus lower DC resistance. I can’t verify by comparative listening, however, that the replacement sounds better than the original. I once saw a toroidal transformer on e. Bay, marketed for use in PAS preamps, but by the time I made up my mind to act the listing disappeared. It should be worth the try if it reappears. The cooked tranny and boards in my original units are evidence that the preamp hasn’t sufficient ventilation. About 2. 5W is not a lot of heat, but if left on for long periods in a box that can’t breath, things inside can still get pretty hot. So I took a pencil and ruler and drew a diamond shaped pattern of grid lines on the lid, and where they intersected I drilled holes with a drill press. I don’t think that the spacing and size of the holes or arrangement are very important. What counts is there are sufficient openings for cross- ventilation. Mapleshade makes thick- walled brass cylinders that clamp onto tubes.
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