PAS 2 - Normal Behavior?

So I have a Viking 88 reel to reel which has its own tube preamp. I had been running it directly into an amp and it sounded really good. However to get the vu meters moving the volume knobs had to be set to at least one o'clock which is way too loud. Normal listening levels is between 9 and 10 o'clock. I was told I needed to run it into another preamp with which I could control the volume and allow me to turn up the Viking to whatever I wanted so I just got a PAS 2. I am using the Tape Amp input. What I have found is that the Viking sound comes through regardless of the volume setting on the PAS. The PAS will increase the volume, the tone controls work, etc. but when I turn the volume all the way down, the Viking still comes through loud and clear running at the same volume level as I did without the PAS. Is this normal or should the PAS completely silence the Viking when the volume is turned all the way down?

rodneyk915 wrote:So I have a Viking 88 reel to reel which has its own tube preamp. I had been running it directly into an amp and it sounded really good. However to get the vu meters moving the volume knobs had to be set to at least one o'clock which is way too loud. Normal listening levels is between 9 and 10 o'clock. I was told I needed to run it into another preamp with which I could control the volume and allow me to turn up the Viking to whatever I wanted so I just got a PAS 2. I am using the Tape Amp input. What I have found is that the Viking sound comes through regardless of the volume setting on the PAS. The PAS will increase the volume, the tone controls work, etc. but when I turn the volume all the way down, the Viking still comes through loud and clear running at the same volume level as I did without the PAS. Is this normal or should the PAS completely silence the Viking when the volume is turned all the way down?

Which volume ( control) are you discussing ? The one on the tape or pas ?
Please clarify
And what connectors are used on the tape and the pas ( i guess that 4 should be
involved, 2 is "tape out" on pas, connected to "line in" in the tape, "line out" on the
tape should be connected to "tape in" on pas.

On the pas. If the volume on the pas is all the way down, instead of silence, the volume of the tape through the speakers is the same as on the tape player itself. Increasing the volume on the pas does increase the tape volume even more which it should it just will not provide silence when turned all the way down.

So before, I ran the tape straight to an amp with the tape volume at about 9 oclock. Now I run the tape through the pas with the tape volume still at 9 oclock and the pas volume at 0 and it is just as loud as when I ran the tape straight to the amp instead of silence.

Right now I'm not concerned with recording until I know everything is right so all I have is cable out from Amp Out on the tape player to the Tape Amp input on the pas and the monitor switch set to Tape. I am not returning anything back to the tape player just to the amp. I hope that helps, sometimes I can be clear as mud

rodneyk915 wrote:On the pas. If the volume on the pas is all the way down, instead of silence, the volume of the tape through the speakers is the same as on the tape player itself. Increasing the volume on the pas does increase the tape volume even more which it should it just will not provide silence when turned all the way down.

So before, I ran the tape straight to an amp with the tape volume at about 9 oclock. Now I run the tape through the pas with the tape volume still at 9 oclock and the pas volume at 0 and it is just as loud as when I ran the tape straight to the amp instead of silence.

Right now I'm not concerned with recording until I know everything is right so all I have is cable out from Amp Out on the tape player to the Tape Amp input on the pas and the monitor switch set to Tape. I am not returning anything back to the tape player just to the amp. I hope that helps, sometimes I can be clear as mud :lol:

My guess is that your tape deck has a low impedance output AND the pas has a
less then perfect volume pot.
If, by mistake, a speaker output is connected to the input of a PAS and the volume pot
is turned to zero, high current from the speaker output could actually destroy the pot.

rodneyk915 wrote:So I have a Viking 88 reel to reel which has its own tube preamp. I had been running it directly into an amp and it sounded really good. However to get the vu meters moving the volume knobs had to be set to at least one o'clock which is way too loud. Normal listening levels is between 9 and 10 o'clock. I was told I needed to run it into another preamp with which I could control the volume and allow me to turn up the Viking to whatever I wanted so I just got a PAS 2. I am using the Tape Amp input. What I have found is that the Viking sound comes through regardless of the volume setting on the PAS. The PAS will increase the volume, the tone controls work, etc. but when I turn the volume all the way down, the Viking still comes through loud and clear running at the same volume level as I did without the PAS. Is this normal or should the PAS completely silence the Viking when the volume is turned all the way down?

Does your PAS have the impedance modification? If not, do it:

To do the impedance mod - per the Dynaco literature:

1. Obtain two 100,000 ohm, 1/2-watt 10% tolerance resistors.
2. Unsolder and discard the two 510,000 ohm (green-brown-yellow)
resistors connected to the audio-output sockets on the back panel of
the preamplifier and replace them with the 100,000 ohm resistors.
Solder all connections.
3. Snip out and discard the two 62,000 ohm (blue-red-orange) resistors
on the PC-5 circuit board adjacent to eyelets 8 & 18. This completes
the modification.

Given the relative difference in cost, I would use 5% resistors and
screen them as well.

Do you have any other sources connected to the PAS? Do they mute entirely when the VC is down to 0?
Try the "spare" input and see if things change.
Does the 88 have a line-level output not tied to the VC? If so, use it into the PAS.

As you are not recording, does the action of the VU meters actually matter?

peterh wrote:
My guess is that your tape deck has a low impedance output AND the pas has a
less then perfect volume pot.
If, by mistake, a speaker output is connected to the input of a PAS and the volume pot
is turned to zero, high current from the speaker output could actually destroy the pot.

The impedance output of the tape deck is 20k ohms. The volume pot may be something I do need to look at. Probably a dumb question but is this something I can test with a meter or do I just have to swap out the pot with a new one and check the volume? Thanks

rodneyk915 wrote:

peterh wrote:
My guess is that your tape deck has a low impedance output AND the pas has a
less then perfect volume pot.
If, by mistake, a speaker output is connected to the input of a PAS and the volume pot
is turned to zero, high current from the speaker output could actually destroy the pot.

The impedance output of the tape deck is 20k ohms. The volume pot may be something I do need to look at. Probably a dumb question but is this something I can test with a meter or do I just have to swap out the pot with a new one and check the volume? Thanks

The pot's are quite cheap , new ones will most likley improve channel balance etc.
Note that you should use 250k pot's, lower resistance will load the riaa to much.

I managed ( a long time ago) to find alps 250k with loudness on ebay.
Tubes4hifi used to have good replacement pots, maybe the spares are transferred to the
new site - Bob, could you fill in here ?

Peter W. wrote:
Does your PAS have the impedance modification? If not, do it:

To do the impedance mod - per the Dynaco literature:

1. Obtain two 100,000 ohm, 1/2-watt 10% tolerance resistors.
2. Unsolder and discard the two 510,000 ohm (green-brown-yellow)
resistors connected to the audio-output sockets on the back panel of
the preamplifier and replace them with the 100,000 ohm resistors.
Solder all connections.
3. Snip out and discard the two 62,000 ohm (blue-red-orange) resistors
on the PC-5 circuit board adjacent to eyelets 8 & 18. This completes
the modification.

Given the relative difference in cost, I would use 5% resistors and
screen them as well.

Do you have any other sources connected to the PAS? Do they mute entirely when the VC is down to 0?
Try the "spare" input and see if things change.
Does the 88 have a line-level output not tied to the VC? If so, use it into the PAS.

As you are not recording, does the action of the VU meters actually matter?

Not sure if it has that mod, I will have to take a look but since it looks stock except for the selenium rectifier change out and the filter caps being replaced that was mentioned by the previous owner, I doubt it has been done. Just curious but what can be expected by doing this mod and thanks for explaining how to do it

Right now I do not have any other source connected to the pas. I have tried using the Spare input but it gave the same results. The 88 has two outputs: Amp Out and Monitor Out. I have tried both outputs into both Tape Amp and Spare inputs and the signal still passes through. The Amp Out has an impedance of 20K ohms and the Monitor Out has an impedance of 5K ohms. I even tried using the the two outputs going into the two other high impedance inputs of the pas but still no change. I was hoping there would be a lot of PAS 2 owners that would be saying "yes that's the way it's supposed to be" or "nope, you've got a problem" so I would know whether I need to be looking for a fix or not.

I will be recording but I wanted to take this one step at a time to make sure it worked right at each phase. At this point I don't really care about the VU meters, I am mainly trying to figure out if this is the way the PAS is supposed to work, if there is possibly a faulty component(s) that is causing it, the tape deck is overpowering the PAS, or if the original builder of this kit mis-wired something. I really appreciate your help!

If you use the "tape out" connectors and connect them to the tapedrive, then it is importent that
the loading ( input impedance of the tape drive) is more then 100k, preferrably > 250k)
If not you need a buffer , myself built a buffer in an external box for both the tape-out and
"audio-out" signals. The buffer itself is quite easy, a single ECC82 10 resistors and 4 caps
per stereo-channel.
Unfortunately there is no spare power in the PAS, the buffer has to be externally powered.

Oh ok, thank you for that info Peter, much appreciated!

You are not using the Viking to record, so have only one set of stereo interconnects between the Viking and the PAS.

The output impedance modification is not relevant at this time.  Do it later...and the resistor values need to be selected specifically for your intended power amp.  These instructions are for use with a Dynaco Stereo 120 or Stereo 80.  They aren't even sufficient a change for the Dynaco Stereo 150, which the PAS circuit can not optimally match.  The Stereo 400 is okay.

I'm thinking along the lines of a defective volume control or associated wiring.  If you have an ohmmeter, what is the DC resistance between the bottom side of the volume control and chassis ground?  It should be zero, or close to.

Thanks GP for clarifying that. Yeah, I am only running one connection from the tape deck to the pas right now to help limit the possibilities. I don't know what the resistance is so I will check that this evening when I get home. Thanks again!

[quote="rodneyk915"]
Not sure if it has that mod, I will have to take a look but since it looks stock except for the selenium rectifier change out and the filter caps being replaced that was mentioned by the previous owner, I doubt it has been done. Just curious but what can be expected by doing this mod and thanks for explaining how to do it

This mod makes the PAS compatible with about any modern SS power-amp, and does tighten up (to my ears, anyway) the bass for many contemporary tube amps. For the record, it will be fine with the 150, 400 and the 416 with this mod. Of course the 80 and 120.

Right now I do not have any other source connected to the pas. I have tried using the Spare input but it gave the same results.

Try another line-level source such as a tuner - the point is to determine if your Volume Control has crapped out, or if there is some issue specific to one input over another.

The 88 has two outputs: Amp Out and Monitor Out. I have tried both outputs into both Tape Amp and Spare inputs and the signal still passes through. The Amp Out has an impedance of 20K ohms and the Monitor Out has an impedance of 5K ohms.

Impedance, or resistance?

I even tried using the the two outputs going into the two other high impedance inputs of the pas but still no change. I was hoping there would be a lot of PAS 2 owners that would be saying "yes that's the way it's supposed to be" or "nope, you've got a problem" so I would know whether I need to be looking for a fix or not.

If you have a problem with the VC there are any number of after-market options to install. As NO PAS2 has the actual X-Mod (where the tone controls are out-of-circuit when in the neutral position) you will face considerable pressure to cut out the tone-controls. I am preaching heresy, here, but resist that pressure for now as you can always do it later.

I will be recording but I wanted to take this one step at a time to make sure it worked right at each phase. At this point I don't really care about the VU meters, I am mainly trying to figure out if this is the way the PAS is supposed to work, if there is possibly a faulty component(s) that is causing it, the tape deck is overpowering the PAS, or if the original builder of this kit mis-wired something. I really appreciate your help![/quote

One step at a time gives you the best results as you will be able to identify what did what. Multiple simultaneous mods always leave too many questions.

[quote="Peter W."]

This mod makes the PAS compatible with about any modern SS power-amp, and does tighten up (to my ears, anyway) the bass for many contemporary tube amps. For the record, it will be fine with the 150, 400 and the 416 with this mod. Of course the 80 and 120.


Sorry Peter, this is NOT correct. There is no specific "impedance mod" for the PAS. The PAS 2/3, in stock form, is designed to operate into a specific load impedance due to the way in which the Bass tone control functions. It is, perhaps, the most misunderstood aspect of PAS operation because, what the load is actually comprised of, is somewhat obscure. The PAS 2/3 wants to "see" a load of about 50K in order for the low end response to be flat when the bass control is centered. That load consists of two internal resistors, 62K and 510K, as well as the input impedance of the power amplifier it is driving. The "parallel" combination of these determines the total load. The intended "external" load for the PAS 2/3 is about 500K which, in parallel with the internal resistors, results in a total load near 50K. Any significant alteration of the total load will result in the low end response not being flat with the bass control centered. Lower impedance results in bass cut and higher in bass boost.

The internal resistor values can be altered to better match the PAS to different power amplifier input impedances, within limits,  such that the total parallel equivalent is near 50K. The modification you described would be valid ONLY for the case in which the power amplifier had an input impedance of 100K. In this case, the (now) 100K internal resistor in parallel with the 100K PA load would result in the desired 50K total load for flat bass response. Carried to the theoretical limit, the complete removal of the internal resistors would allow the PAS to drive a power amplifier with a 50K input impedance. Many SS power amplifiers are lower than this. Bypassing the tone controls, which the PAS 3X does when set to the center position, will allow the PAS the drive a much broader range of power amplifier impedances while maintaining flat response by virtue of the fact that the offending bass control is bypassed, resulting lower output impedance. But still, cannot drive any and all SS amplifiers. The only "universal" impedance modification would be an additional buffer stage following the PAS.

Besides the impedance issue, there is also the potential for problems due to the small DC voltage the appears at the output of the PAS 2/3 (not 3X). Altering the values of the internal load resistors also changes the level of the DC voltage that appears at the output. In stock form, this is about 100mV DC. With your modification it would be nearly double that. Although not a problem as long as the following power amplifier input is AC coupled, it certainly could be with certain DC coupled amplifiers, such as Dynaco's own ST-70, which may exhibit stability issues with a DC level of that magnitude.

I am only bringing this up since there is already so much misinformation out there in regards the PAS that I just couldn't let it pass. At this point the OP should not be concerned about modifications, valid or otherwise. The problem at hand is, first and foremost, one of basic troubleshooting. Unless I missed it in the comments above, I have not seen mentioned the specific power amplifier that is being driven, which may be helpful in diagnosing the problem.

[quote="audioregenesis"]

Peter W. wrote:

This mod makes the PAS compatible with about any modern SS power-amp, and does tighten up (to my ears, anyway) the bass for many contemporary tube amps. For the record, it will be fine with the 150, 400 and the 416 with this mod. Of course the 80 and 120.


Sorry Peter, this is NOT correct. There is no specific "impedance mod" for the PAS. The PAS 2/3, in stock form, is designed to operate into a specific load impedance due to the way in which the Bass tone control functions. It is, perhaps, the most misunderstood aspect of PAS operation because, what the load is actually comprised of, is somewhat obscure. The PAS 2/3 wants to "see" a load of about 50K in order for the low end response to be flat when the bass control is centered. That load consists of two internal resistors, 62K and 510K, as well as the input impedance of the power amplifier it is driving. The "parallel" combination of these determines the total load. The intended "external" load for the PAS 2/3 is about 500K which, in parallel with the internal resistors, results in a total load near 50K. Any significant alteration of the total load will result in the low end response not being flat with the bass control centered. Lower impedance results in bass cut and higher in bass boost.

Thanks, audioregenesis, for reaffirming this once again (I've done it how many times on this forum?).

It's true, folks. In fact when I was in the repair business I would insist that PAS owners BRING IN THEIR POWER AMPS if they required a modification to the PAS's output. I watched frequency response at the low end as a variable resistor, temporarily installed in place of the load resistors, was adjusted for flattest low end response WITH THE POWER AMP. Then I measured the variable resistor and made up a custom load resistance using selected parallel resistors.

audioregenesis wrote:
...I am only bringing this up since there is already so much misinformation out there in regards the PAS that I just couldn't let it pass. At this point the OP should not be concerned about modifications, valid or otherwise. The problem at hand is, first and foremost, one of basic troubleshooting. Unless I missed it in the comments above, I have not seen mentioned the specific power amplifier that is being driven, which may be helpful in diagnosing the problem.

Thanks for the responses, lots of good info! Right now the amp is a Carver AV 405 which has an input impedance (line) of 47K ohms. In the middle/end of October I plan on getting the ST 70 kit and using that for my amp. I find that right now there is very little bass with the tone controls neutral and the loudness switch off. However, by adjusting the tone controls and activating the loudness switch I can get a really nice sound. I agree with you about not doing any mods at this point. I want to keep everything simple and clean and solve one issue at a time before I change anything that might cause or complicate another problem.

I defer to the experts on the PAS2-3 non-X behavior. I have had, over time, two non-X PAS units, purchased for nearly nothing and had so many people lust after them that I sold them, retaining only my 3X.

However, over time, I fed the PAS (with the resistor change) into:

Dynaco 416, 150, 120, 80, 70, 35
Scott LK150
Revox A722
Citation 16, 19

And found the results uniformly reasonable. Not my favorite pre-amp for any number of reasons, or I would still have one.

Of the amps, I retain the 80, 120, LK150, 722, 16, 19, 70, 35. They have stood the test of time and with the various mods to the SS Dynaco devices are fairly bulletproof - which they were emphatically not so much in the past. The 35 is a little gem into my smaller AR speakers in a smaller room.

rodneyk915 wrote:
Thanks for the responses, lots of good info! Right now the amp is a Carver AV 405 which has an input impedance (line) of 47K ohms. In the middle/end of October I plan on getting the ST 70 kit and using that for my amp. I find that right now there is very little bass with the tone controls neutral and the loudness switch off. However, by adjusting the tone controls and activating the loudness switch I can get a really nice sound. I agree with you about not doing any mods at this point. I want to keep everything simple and clean and solve one issue at a time before I change anything that might cause or complicate another problem.

With a 47K load impedance the loss of bass response would be significant. I had a quick look online at the service manual for the AV 405 which indicated an input impedance of only 39K. The PAS would certainly not be happy with that, but still is not the root cause of your problem.

Let me see if I understand your current setup - you have a Viking 88 R-R with the Amp. Outputs connected to the Tape Amp Inputs of the PAS, and the Audio Outputs of the PAS connected to the Inputs of the AV 405. You are connecting to the Front L and Front R inputs of the AV 405 I assume?

As far as your volume level problem, one thing that comes to mind that could cause that is a bad ground connection on the volume pot. Do you have a multimeter to make some simple resistance measurements?

You are correct about the connections. I just tested the lower left leg of both pots on the volume and they show 0 resistance.

rodneyk915 wrote:You are correct about the connections. I just tested the lower left leg of both pots on the volume and they show 0 resistance.

So the pot grounds read 0 ohms relative to chassis ground, which is good. Now set the volume control fully counter clockwise and measure the resistance from the wipers to ground. Usually this will be a few ohms.

Fully counter clockwise I get about 0.16 on each pot. Turned fully clockwise I get 160 on the front pot and 145 in the back pot. Both measurements on the wiper.

rodneyk915 wrote:Fully counter clockwise I get about 0.16 on each pot. Turned fully clockwise I get 160 on the front pot and 145 in the back pot. Both measurements on the wiper.

Get ANOTHER SOURCE ..  See if your VC works.  REPORT BACK ...

I connected the Xbox into the Spare. Same situation where the sound continues to be heard when Volume set to 0.

rodneyk915 wrote:Fully counter clockwise I get about 0.16 on each pot. Turned fully clockwise I get 160 on the front pot and 145 in the back pot. Both measurements on the wiper.

The purpose of this quick test was, primarily, to determine if the pot wiper would approach ground potential with the volume control fully counter clockwise. If that were the case, then no signal should appear at the input to the PAS line stage, and thus, no sound from your power amp.

From your readings, I suspect you meter was set to the 200K scale, thus the wiper is at 160 ohms with respect to ground when fully counter clockwise. While a bit higher than a typical "clean" pot it is still very low with respect to the total value of the pot. As it is, the signal from your R-R would be attenuated by about 60dB (1000 times) by the time it got to your power amp. I looked up the specs. for your Viking 88 and found that it will produce 2VRMS output with the amp output set to max. Even in this case, with the volume on your PAS set to min, only 2mV would appear at the input to your power amp. According to the AV 405 specs. an input level of 100mV will produce an output of 1W, so with 2mV your power amp would only produce 20mW. That should not be very audible.

Something quite odd going on here and I would not be ready to declare the volume pot bad just yet. Your min/max wiper readings indicate a pot that is withing the realm of reasonable results.

OK. Then you need to get a schematic and follow the circuit from the source to the output, and make sure that everything is wired correctly. Either the VC is DOA, or there is a serious bit of miswiring. But, by the numbers: Source plugged in to "Spare". Selector to "Spare". Volume to 0. Stereo to full width. Tone to flat. Balance to center. Tape Monitor to "Input". Loudness "Off". Filter "Off". Same result?

If so, you need to follow the circuit and see where you are going off the rails.