Dynaco Pas 2 impedance matching question

rshadley

Active Member
I know this has been discussed before, but I have a Dynaco Pas 2 preamp that I want to use with a Sargent Rayment SR-540 tube amp. The current input is via a 226K ohm variable resistor that sounds distorted at the max input level. I would like to replace these with a set of gold plated RCA jacks and want to match the impedance. Looking at the ST70 schematics, it appears that I just need to remove the potentiometers, connect the positive end of the input jack to pin 5 of the 7199 tube socket, a 470K ohm resistor from the positive end of the input jack to ground and a 10 ohm resistor to the negative end of the input jack to ground. Can anyone confirm this. I will leave the existing input jacks in place on the Sargent Rayment for possible reconversion with new variable resistors.
 
I know this has been discussed before, but I have a Dynaco Pas 2 preamp that I want to use with a Sargent Rayment SR-540 tube amp. The current input is via a 226K ohm variable resistor that sounds distorted at the max input level. I would like to replace these with a set of gold plated RCA jacks and want to match the impedance. Looking at the ST70 schematics, it appears that I just need to remove the potentiometers, connect the positive end of the input jack to pin 5 of the 7199 tube socket, a 470K ohm resistor from the positive end of the input jack to ground and a 10 ohm resistor to the negative end of the input jack to ground. Can anyone confirm this. I will leave the existing input jacks in place on the Sargent Rayment for possible reconversion with new variable resistors.
The PAS-2 is able to drive the ST-70 which is 470k Ohms. It also is easily adaptable to drive a 250k load but you must remove a resistor. Read the back of the manual, there is a simple procedure to do this.
 
Right-o. I would not dive into the power amp. Dynaco had this covered in 1960 in the PAS. Although, distortion may be another matter.

Are you saying I should not use the Sargent Rayment with the PAS?
 
I just re read your post, you are saying not to change out the input resistors in the Sargent Rayment as it may cause an issue?, I should just remove the resistor in the back of the PAS.
 
I know this has been discussed before, but I have a Dynaco Pas 2 preamp that I want to use with a Sargent Rayment SR-540 tube amp. The current input is via a 226K ohm variable resistor that sounds distorted at the max input level. I would like to replace these with a set of gold plated RCA jacks and want to match the impedance. Looking at the ST70 schematics, it appears that I just need to remove the potentiometers, connect the positive end of the input jack to pin 5 of the 7199 tube socket, a 470K ohm resistor from the positive end of the input jack to ground and a 10 ohm resistor to the negative end of the input jack to ground. Can anyone confirm this. I will leave the existing input jacks in place on the Sargent Rayment for possible reconversion with new variable resistors.
Any schematics of the Sargent ? Otherwize it is difficult to speculate in the effective input inpedance.
Distorsion at full level on the sargent pot seems to indicate that the actual input impedance is much lower then
226k

It _might_ also be another issue, PAS-2 has a habit of DC offset of the output, this
should be cured with a small cap ( could be mounted inside the PAS close to the output posts)
Depending on the 540 input it could actually cause distorsion. Do test with a cap ( 0.22 would do)
 
Last edited:
pretty hard to get a schematic for the Sargent Rayment, I believe the variable resistor is corroded at the high end, the distortion was there when I was driving the amp with my computer. I have recapped the Sargent Rayment, coupling caps and the two can capacitors. I replaced the silicon rectifier and two associated caps and tried the PAS. It would make an occasional POP that could be heard through the speakers. I than replaced the multi can cap with a 10, 30, 30, 30 uf capacitor and re orientated it so that the 10UF section was the first capacitor off the 12x4 tube. I also replaced the associated resistors with metal oxide at 2%. I disconnected the two 510K resistors and gave it a try with the Sargent Rayment. After warm up the popping was still there. I suppose its now time to start to replace the caps and the resistors in the PC5 and PC6 boards. Any idea where the popping sound maybe coming from. I am trying to keep the PAS as original as possible.
 
pretty hard to get a schematic for the Sargent Rayment, I believe the variable resistor is corroded at the high end, the distortion was there when I was driving the amp with my computer. I have recapped the Sargent Rayment, coupling caps and the two can capacitors. I replaced the silicon rectifier and two associated caps and tried the PAS. It would make an occasional POP that could be heard through the speakers. I than replaced the multi can cap with a 10, 30, 30, 30 uf capacitor and re orientated it so that the 10UF section was the first capacitor off the 12x4 tube. I also replaced the associated resistors with metal oxide at 2%. I disconnected the two 510K resistors and gave it a try with the Sargent Rayment. After warm up the popping was still there. I suppose its now time to start to replace the caps and the resistors in the PC5 and PC6 boards. Any idea where the popping sound maybe coming from. I am trying to keep the PAS as original as possible.

Now you are describing another problem "POP".

Back to the original problem "distorsion at max level of 226k". PAS delivers DC at it's output, and if no cap in the way
a 7199 ( or any tube) might get wrong working point if DC is applied. A cap in serial solved this.

As regards to POP, it seems to me more likley that it comes from external disturbances. Some fault isolation
strategy is needed here.
Start with disconnected PAS and shorted inputs. Turn on and wait long enough. Any pops ?
Next, connect PAS but don't turn it on, repeat listening.
third turn on pas, turn down PAS volume. Any POP's
forth, activate the source , apply "Normal volume" Any pop's ?
 
thanks for the link, I will check for the coupling capacitors. I bought the PAS 2 from Shop Goodwill online site years ago. Just know getting around to playing with it. The paperwork that came with it included the PAS 3X upgrade. The face plate and knobs have been changed out to the PAS 3. I was checking the pots at some point to insure that they were actually replaced but at the time I was thinking that they were the original PAS 2 pots as they did not have an indent in the center position. I also believed that the rotation was at 280 instead of 210 but will check them all again.
 
Ok, let me try and clear things up. I purchased the Dynaco Pass several years ago and thought it was a PAS 3. The back of the unit has the PAS 2 decal. I looked through the original paperwork and it included the PAS 3 upgrade (assuming this was for the faceplate and knobs) as well as the PAS 3X upgrade paperwork. The unit is not in front of me now but at the time I tried to determine if the complete PAS 3X upgrade was performed. I pretty sure that the capacitors were added but will check and add some photos. I than tried to determine if the treble and bass pots had been changed but I was thinking that they had a full 280 degree rotation. Not sure why somebody would buy the upgrade and not install the pots. I than stashed the unit away for several years. I pulled it out a couple months ago and replaced the selenium rectifier and associated capacitors. I than connected it to a rebuilt Magnavox 175-67 that I replaced the 470K ohm resistors with two 100K alps blue velvet mono pots. It played fine but had that occasional pop (did not adjust volume, bass or treble, it would just make a popping sound while listening) that I mentioned. I assumed that the can capacitor needed replacing. At the same time I was working on the Sargent Rayment SR 540, replacing the coupling caps and multi can caps. I than replaced the multi can cap on the Dynaco PAS, removed the 510K resistors from the output jacks and connected it to the SR 540 with a computer as the source and it sounded fine for at least an hour. Moved it upstairs along with the SR 540 and listened again and the popping sound started again. Hope this clears the muddy waters.
 
Ok, let me try and clear things up. I purchased the Dynaco Pass several years ago and thought it was a PAS 3. The back of the unit has the PAS 2 decal. I looked through the original paperwork and it included the PAS 3 upgrade (assuming this was for the faceplate and knobs) as well as the PAS 3X upgrade paperwork. The unit is not in front of me now but at the time I tried to determine if the complete PAS 3X upgrade was performed. I pretty sure that the capacitors were added but will check and add some photos. I than tried to determine if the treble and bass pots had been changed but I was thinking that they had a full 280 degree rotation. Not sure why somebody would buy the upgrade and not install the pots. I than stashed the unit away for several years. I pulled it out a couple months ago and replaced the selenium rectifier and associated capacitors. I than connected it to a rebuilt Magnavox 175-67 that I replaced the 470K ohm resistors with two 100K alps blue velvet mono pots. It played fine but had that occasional pop (did not adjust volume, bass or treble, it would just make a popping sound while listening) that I mentioned. I assumed that the can capacitor needed replacing. At the same time I was working on the Sargent Rayment SR 540, replacing the coupling caps and multi can caps. I than replaced the multi can cap on the Dynaco PAS, removed the 510K resistors from the output jacks and connected it to the SR 540 with a computer as the source and it sounded fine for at least an hour. Moved it upstairs along with the SR 540 and listened again and the popping sound started again. Hope this clears the muddy waters.
Well, not really.

As source for the POP's i would start with backtracking the Se and caps replaced, look for bad solders or loose wires. Poke with a woodstick and see if the POP can be provoked. I would NOT start replacing things as they most
likley is working as good as when the amp was built.
The pot's may pop WHEN TURNED but they dont pop when not turned.

As regards for the DC on output, a simple measurement on the output of the PAS will confirm is any DC is present. On a PAS-X it is not, a PAS will have some DC. I think we are talking about 0.1V, certainly enough to upset a amp input.
 
Photos
 

Attachments

  • 9ED73B77-B77C-4D4D-8FF3-058238A11670.jpeg
    9ED73B77-B77C-4D4D-8FF3-058238A11670.jpeg
    59.9 KB · Views: 18
  • E6A3EE93-5B13-4637-BF0D-19E081F276CC.jpeg
    E6A3EE93-5B13-4637-BF0D-19E081F276CC.jpeg
    74.3 KB · Views: 19
  • F3275D05-9CFE-4E96-998D-BF73431C4120.jpeg
    F3275D05-9CFE-4E96-998D-BF73431C4120.jpeg
    80.7 KB · Views: 17
  • 33CCCA5A-03C5-44EC-B857-4C8BE9F81BC7.jpeg
    33CCCA5A-03C5-44EC-B857-4C8BE9F81BC7.jpeg
    58.8 KB · Views: 19
  • 3B67E2E8-146E-40E7-A6D1-79013B285ACF.jpeg
    3B67E2E8-146E-40E7-A6D1-79013B285ACF.jpeg
    74 KB · Views: 16
  • 91E6A9EA-8DA4-4B59-BD4C-D0F8C079FF65.jpeg
    91E6A9EA-8DA4-4B59-BD4C-D0F8C079FF65.jpeg
    58.3 KB · Views: 18
  • 395E1AAA-5791-4148-B8C0-0842328EDC8B.jpeg
    395E1AAA-5791-4148-B8C0-0842328EDC8B.jpeg
    53.9 KB · Views: 19
  • F9C123E8-34A1-44BB-87B6-CFFE217D9765.jpeg
    F9C123E8-34A1-44BB-87B6-CFFE217D9765.jpeg
    65.9 KB · Views: 18
  • 46AD56C5-9CE9-494E-9DD9-03E69EAF0F54.jpeg
    46AD56C5-9CE9-494E-9DD9-03E69EAF0F54.jpeg
    71.2 KB · Views: 19
  • 361FBD02-DFCB-4D49-BA68-32E98A681F59.jpeg
    361FBD02-DFCB-4D49-BA68-32E98A681F59.jpeg
    61.3 KB · Views: 17
the photos contain shots of the front, back, pas 3x upgrade paperwork, the 1uf caps, the resistors that were changed on the blend switch as well as the replaced selenium rectifier and multi can cap that I replaced. The only other item of note is that I changed the light to an LED light. Looks like the treble and bass pots are original to the PAS 2 and not the upgrade. I had time to play with it last night. After power up I get a pop after about 6 minutes followed by several lighter pops over the next few minutes. I can replicate the pop by rotating through the blend switch. After five or so minutes, no more pops even while rotating through the blend switch. That same pop can also be heard when moving the input selector switch to phone. this pop happens even after listening for a few hours.
 
Last edited:
the photos contain shots of the front, back, pas 3x upgrade paperwork, the 1uf caps, the resistors that were changed on the blend switch as well as the replaced selenium rectifier and multi can cap that I replaced. The only other item of note is that I changed the light to an LED light. Looks like the treble and bass pots are original to the PAS 2 and not the upgrade. I had time to play with it last night. After power up I get a pop after about 6 minutes followed by several lighter pops over the next few minutes. I can replicate the pop by rotating through the blend switch. After five or so minutes, no more pops even while rotating through the blend switch. That same pop can also be heard when moving the input selector switch to phone. this pop happens even after listening for a few hours.
Was the pop during riaa / phono play ? Then the .1 caps on the PC-6 could be suspected.
If the pop was during to listening on high level inputs then PC-6 is not the culprit.
Was it both channels ?

As for the 1uF output caps, it will remove DC on output and is applicable for non-X too.
 
Both channels which would lead me to think power supply. I changed the selenium recitifier first and than gave it a test and the pop was there. Thinking the multi can is not the problem. Not sure why it pops when I move the blend switch and what the 33k and 68k resistors that were changed on the blend switch as part of the 3x upgrades may or may not have to do with the pops
 
Both channels which would lead me to think power supply. I changed the selenium recitifier first and than gave it a test and the pop was there. Thinking the multi can is not the problem. Not sure why it pops when I move the blend switch and what the 33k and 68k resistors that were changed on the blend switch as part of the 3x upgrades may or may not have to do with the pops
It might be mechanical vibrations that trigger the pop. As you say, both channels makes me believe a
bad solder/bad contact, and the points recently soldered are suspected.
 
Pops happen on all(tuner, spare and phono) typically with in the first 5 to 7 minutes as well as when I change the blend switch. After that it may play trouble free for a couple hours.
 
Any opinion on the resistor change on the blend switch as part of the PAS 3X upgrades without changing adding the PAS 3X pots. Original PAS 2 had 270k and 570k as aopposed to the 33k and 68k for the PAS 3X upgrades with the new pots. Should I change these back?
 
Any opinion on the resistor change on the blend switch as part of the PAS 3X upgrades without changing adding the PAS 3X pots. Original PAS 2 had 270k and 570k as aopposed to the 33k and 68k for the PAS 3X upgrades with the new pots. Should I change these back?
Not needed, they will affect the rate of blend. But do check the solderings!
 
The voltage at the first cap was about 425 at 120 volts input. Before I checked the other volts I backed the variac down to about 115 volts which put 405 volts at the first cap off the 12x4 I Than took readings on the other three section of the multi cap and they were all low from the schematics by 10 volts. It is currently running off 120 volts I did not check the other voltage test points. Both caps are 6800 at 50 volts and the diodes or 1n4007’s. Just in case you missed in one of my earlier post, the 10, 20, 20, 30 multi cap was replaced with a 10, 30, 30, 30 multi cap where I used the 10uf section as the first cap off the 12x4 tube because the rca tube manual specifies 10uf as the max for the 12x4. I believe the original Dynaco design had a 20uf as the first cap
 
Last edited:
Back
Top Bottom