[Joeztech]

I recently bought a Dynaco PAS-2 from ebay and began restoration. This unit had been modified to just be a phonograph preamp. I ordered a 100k ohm volume control with loudness tap, new polypropylene capacitors and metal film resistors plus a new modern function switch that is much smaller than the OEM switch which had been removed. I had to rewire the front controls to the circuit boards and then changed parts. I added two 12AX7A tubes from some used spares I had on hand. I used the small OD shielded audio cable material between the jackfield and the function switch. This helps eliminate cross-talk between high level signals and adjacent cables. Some old resistors are still present but will be changed later. A new power supply board and electrolytics will be installed where the filament filter capacitors are presently under the old selenium rectifier (silicon rectifiers had already replaced that but the old part was left in place as handy connection lugs).

I did run into a few problems after first try - I had left one wire off of the blend switch and corrected that, restoring audio on the right channel. On phonograph function the left channel was intermittent and touching anything on the PC-6 board would cause the audio to come and go. It turned out that one very fine strand of shield wire had gone into the hole in the PC board where the signal lead was soldered. I removed the strand from the hole and all was well.

I listened several hours and found that overall the sound is quite pleasing but just a little more forward in the midrange than my PAS-3X. The tubes I installed consisted of a 1960s vintage Telefunken and a late 70s RCA from my used tube stock. These two tested with the best emission of the ones I had on hand.

I also added a 1/2 amp fuse in a clip-in holder under the chassis. The unit will need a new face plate to look its best. Additional cleaning is still needed and the volume control shaft will be shortened about 1/2 inch to allow the knob to be closer to the front panel.

You can see some pictures at: http://s1213.photobucket.com/user/rubal1...sort=6&o=0

Joe

[Steve D]

Nice restoration. 

 I have the exact same PAS2 and an ST70 in original condition that I am planning to restore and upgrade this winter. I am considering going with the Tubes 4 hifi upgrades and was wondering if you have an opinion about them vs. Curcio, Van Alstine, etc.

Thanks,

Steve D 

[Joeztech]

Steve;

I did use some parts from Tubes4HiFi and have bought a number of parts from them. I used their PC-6 and PC-5 replacement circuit boards with parts to rebuild my PAS-3X preamp. I like the quality of the circuit boards as the traces adhere far better than the OEM phenolic boards. I went with their re-layout of the OEM design which makes parts locations more logical and has lead spacing more appropriate for modern poly types of capacitors. Curcio, Van Alstine, DynakitParts, Tubes4HiFi and others all have some great offerings of modified circuits for improved audio and from reviews I believe they all provide some improvement over the OEM design. I have yet to try those. I am considering installing a VTA board for the ST-70 for improved sonics, but have not done that yet. The ST-70 has very pleasing sound that I have found no fault with after new parts were installed on the original PC board which was also modified to use 6GH8a tubes. I used a modified version of Van Alstine's input band-limiting filter on the ST-70. I did modify the ST-70 power supply to have more high voltage rating (around 600VDC) to give more ability for the circuit to survive overvoltage and also to increase the filtering capacity. That was done under the chassis using radial lead type electrolytics and equalizing resistors. The 4 sections of the existing can capacitor was changed such that two sections each were bridged together to filter the two lowest B+ voltages that feed the PC board.

The PAS-2 restoration could have been done neater regarding the input audio cables. I originally had it so, but it turned out the function switch I had versus the install information from another source did not jive completely and I wound up moving wires at the jackfield to correct the problems encountered. In retrospect I should have used my ohm meter to verify each connection point from the A and B signal lugs at the center to each peripheral lug of the new type switch and I could have avoided the mess I wound up with. However, the end result worked well, so I don't feel like self flagellation at this point. It works as originally designed except for more rapid volume advancement as the volume control is turned. The new switch does not provide for tape head function so that position becomes another high level input for say a CD player or another AM/SW or FM tuner. The volume control I bought from Tubes4HiFi had the loudness tap so that feature was restored to the preamp. I often listen to music at low levels to avoid interfering with TV in an adjacent room and use the loudness feature.

Joe

[Steve D]

Joe,

Thanks for the info. I'll probably do similar mods to yours.  The ST70 sounds a bit thin and flabby in the bass and a bit dulled in the highs to me.  The PAS2 phono section sounds good to me but I would like to upgrade the line amp and power supply like you have done.  Did you keep the tone controls?  Yeah, the loudness compensation at low listening levels is very useful.

Do you plan any other upgrades?  What speakers are you using?

Steve D

[Joeztech]

Steve;

Yes the ST-70 is known for somewhat loose bass due to power supply limitations and too much gain on the low end. The power supply upgrade (similar to one offered by Joe Curcio) plus the lower limit of around 14 to 16Hz and high end limit of 20kHz while the amplifier itself has a wider frequency range helps. The highs are quite good if the preamp is and the tubes are in good condition. I did notice that the restored PAS-2 has a better high end frequency response than the PAS-3X for whatever reasons. Yes I did keep the tone controls. I am old enough that my hearing at high frequencies is poor and I compensate by advancing the treble as needed. I have a Hafler multi-frequency equalizer that I could use and need to try soon.

I am using my AR-2Ax speakers that I bought in 1964. I have had to repair them by re-gluing the woofer surrounds and replacing the tweeters with ones suggested by the Classic Speaker Pages site. This brought them back as good as new. I also have a set of home constructed acoustic suspension speakers using some 10in. woofers from some RS Optimus-6 speakers, new mid ranges and some AR silk dome tweeters. I used 3-way cross-over networks from Parts express with L-pads to tweak the midrange and tweeter levels. They actually sound quite good - far better than they did with the original RS tweeter only plus woofer design. The home constructed speakers require more drive level than the AR speakers do. Right now I have them both in my study where I can switch between A and B speakers on a given recording and even change to 4 ohm impedance and use A and B at the same time. I just started the evaluation of the two speaker types and need more listening time. I have an audio signal generator that I can use to run some frequency sweeps to see if either speakers have noticeable peaks. Of course the room gets involved too as does speaker placement and listening position.

It is possible you might like the VTA board offered by Tubes4HiFi, which is a long-tailed pair design. Some others offer similar designs but not necessarily with the individual grid bias like the VTA unit. I like the individual bias setting for each output tube because you can compensate for slightly different tube conduction to balance the output better and not have to use matched pair tubes. Periodic bias checks can correct for changes in tube current as the tubes age.

I plan to install four bias adjustment controls, one for each tube and install 10 ohm resistors at each tube's cathode allowing direct reading of each tube's current. Those cathode voltages can be brought out to pins on the octal sockets at the front of the amplifier for easy access with a meter. I removed the B+ feed and heater feed to those two front sockets as I will never be using them with PAM preamps. These new bias controls will be installed and used with the existing OEM driver board equipped with 6GH8As and some minor resistance changes in the bias control network to keep the adjustment range right after adding the four controls instead of the original two controls.

I am having fun along the way. Hope that you achieve your goals and have fun with your own restoration and/or modifications. Check out the DIY Tube forum too.

Joe

[Steve D]

Seems like you have a real nice sounding system there.  I also want to keep the tone controls and will probably go the same route you did with the PAS2.  Luckily my front panel is in mint condition and just needs some cleaning.  I am missing a knob though I believe knob kits are available.

With regards to the ST70 I am not sure whether to go the VTA route or with another upgrade and design my own constant current sources for the cathodes of the 6CA7 tubes. This will require running class A which gives lower distortion but less output power.  A capacitor storage bypass could be added to allow more peak power without clipping.  I'm not sure the extra effort will produce measurable listening improvements especially with my 65 year old ears!  But it would be a fun project.

Thanks for the tip on the DIY Tube forum, I have just registered there as SteveBigD.

Steve D