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    Alright, I want my bass back!

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    tygr1

    Posts : 35
    Join date : 2014-12-08

    Alright, I want my bass back!

    Post by tygr1 on Tue Dec 16, 2014 5:19 pm

    Hi again kids. So, for the last two days I've been running my super clean but stock PAS 3X with my ST70. Without the preamp, my bass was loud and tight, just right. With the PAS, it's not as loud nor as tight.

    What will get the bass back under control? Which upgrades/mods should I concentrate on. Will the regulated power supply help with it? Should I do that first or replace the boards first?

    Thanks again for any help.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Alright, I want my bass back!

    Post by PeterCapo on Tue Dec 16, 2014 5:44 pm

    My first thought would be to question exactly how original it is - there might have been some changes that are not obvious in the images, I don't know.  But, if your PAS-3X is as all-original as it appears, then you should be able to get flat bass response with power amps having a Z-in of 100k-ohms or greater, and the 1uF cap plays a role in this (back to this in a moment).

    Maybe the bass pot isn’t quite centered?  The easiest solution I can think of is to simply nudge the bass control up until it sounds right to you.

    Before replacing a lot of parts, I suggest just starting with the two original 1uF electrolytics that are connected from the front panel assembly to PC-5.  They are old, electrolytic, directly in the audio signal path, and replacing them might result in an improvement in the bass as well as a general improvement in sound quality – I suggest two of Mouser Part # 667-ECW-F2105JA here: http://www.mouser.com/ProductDetail/Panasonic/ECW-F2105JA/?qs=GoZnPdyRfjZScBQE8XcRvw%3d%3d

    With the new 1uF caps and nudging the bass pot up, you might be able to get it where you want it.  You could also try a warmer-sounding set of 12AX7s.  I have original Teles in my rebuilt PAS and they sound kind of bright to me.


    Last edited by PeterCapo on Tue Dec 16, 2014 5:52 pm; edited 1 time in total

    tygr1

    Posts : 35
    Join date : 2014-12-08

    Re: Alright, I want my bass back!

    Post by tygr1 on Tue Dec 16, 2014 5:51 pm

    Thanks Peter, I will , of course, try your tips first. You've been a big help up to this point, thanks.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Alright, I want my bass back!

    Post by PeterCapo on Tue Dec 16, 2014 5:54 pm

    I hope to be of help.  Wouldn't want you to regret your purchase, but, of course the time may come when you might feel it isn't worth any more effort.  We'll try to keep your cost down in the meanwhile. If you're ever in Massachusetts, stop by to hear my PAS/St-70.

    tygr1

    Posts : 35
    Join date : 2014-12-08

    Re: Alright, I want my bass back!

    Post by tygr1 on Tue Dec 16, 2014 7:07 pm

    1uf film caps have been ordered.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Alright, I want my bass back!

    Post by PeterCapo on Tue Dec 16, 2014 7:36 pm

    A few suggestions to get your bases covered, while you are waiting for the 1uF…

    If you do not already have one, I suggest downloading a copy of the PAS-3X manual, and, if you feel like it, tracing out the circuit in your PAS-3X against the schematic to see if you can find any differences.  If you have a meter and can safely do so, it would be a good idea to check the internal voltages against the table in the manual.

    Also, if you do not have any other 12AX7s on hand, consider picking up a different pair, perhaps something warmer-sounding than the Teles, for the line stage.  Would be nice if you can test the 12X4 somehow.

    Your earlier thread talked about your St-70 being too bright and then you changed the output taps and it got better.  You might consider posting some images of your ST-70 in that thread to see if the folks here pick up on anything out of the ordinary.

    tygr1

    Posts : 35
    Join date : 2014-12-08

    Re: Alright, I want my bass back!

    Post by tygr1 on Tue Dec 16, 2014 7:50 pm

    Actually, I tweaked the bass pots ever so slightly and the bass sounds right to me now. When i go in to change the caps, I will make sure the knobs are oriented correctly.

    Thanks again Peter.

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Alright, I want my bass back!

    Post by GP49 on Thu Dec 18, 2014 2:07 am

    A couple of hints:

    While most on this forum won't have the equipment to do so, I used to set up PAS preamp tone controls in the repair shop for flat response using a frequency sweep signal and an oscilloscope, and marked the front panel accordingly (on a piece of clear tape).  

    Also, if you are running a tape recorder, computer audio input, remote headphone transmitter or most other solid state equipment off the TAPE OUTPUT of the PAS, you will probably experience bass rolloff on PHONO; the output of the phono stage sees whatever is plugged into the TAPE OUTPUT, and if the input impedance of the connected device is too low, the bass from the phono stage will roll off.

    tygr1

    Posts : 35
    Join date : 2014-12-08

    Re: Alright, I want my bass back!

    Post by tygr1 on Sat Dec 20, 2014 2:59 am

    Earlier tonight I replaced the 1uf electrolytic caps on the tone controls with Panasonic film caps and got a nice tone improvement from the PAS. Thanks Peter Capo!

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Alright, I want my bass back!

    Post by PeterCapo on Sat Dec 20, 2014 10:51 am

    Well, that’s good news.

    I should mention that there is a lot of inertia behind the idea that replacing the selenium diodes is mandatory.  Although I do not generally disagree, I’m not sure how urgent it is if the 12AX7 filament voltage is in-spec per the original PAS-3X manual.  If it is in-spec, the filaments should still be checked with some regularity, but this is not a guarantee you’ll catch it in time if the selenium stack fails.

    In order to accurately determine if the filament voltage for the 12AX7s is in-spec or not, you need to be able to establish the reference AC line voltage with a variac, so that you measure 117 VAC across the power transformer's primary winding with your meter (don’t rely on the scale on the variac itself).  But, it just might be possible that later versions of the PAS might have power transformers that were wound for 120 VAC, so a little experimentation might be called for.  Offhand, I think a good way to establish a reference AC line voltage is to set the variac so that you measure 335 VAC at pin 1 and also at pin 6 of the 12X4 (although this is for the B+, it might be the best way to know that the proper AC line reference is established).  You could also look for 10.5 VAC between pins 3 and 4 of the 12X4, though this might not be the most reliable method if there is an issue with the filament circuit or even with the pilot light.

    Wouldn’t it be easier to simply replace the selenium stack without having to go through this rigmarole?  Replacing the selenium stack is a little more complicated in the PAS compared with, say, a Stereo 70.  When you replace the selenium diodes in a PAS, the 12AX7 filament voltage might run up too high because the replacement diodes will most likely have a lower voltage drop than even new selenium diodes would.  If the filament voltage goes too high, it could affect the sound quality in ways you might possibly not like and tube life could be shortened, too.  What you would have to do is to experiment with installing one or two small value resistors in series with the filaments to get the voltage right, which can be a tedious process.

    So, replacing the selenium stack in a PAS is not that simple.  If you decide to delve into it, I suggest starting with a 5 ohm resistor in each leg of the voltage doubler and see where you stand, again, always referenced to the voltage table in the Dynaco manual.  A 2W power rating could be okay for the resistors, but measure the voltage it is dropping and calculate the power dissipation to make sure.

    If the selenium stack is presently failing, then it is something that should probably be attended-to ASAP.

    tygr1

    Posts : 35
    Join date : 2014-12-08

    Re: Alright, I want my bass back!

    Post by tygr1 on Sat Dec 20, 2014 3:22 pm

    Won't replacing the power supply caps with a VTA board get rid of the selenium stack entirely?

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Alright, I want my bass back!

    Post by peterh on Sat Dec 20, 2014 5:02 pm

    tygr1 wrote:Won't replacing the power supply caps with a VTA board get rid of the selenium stack entirely?

    Yes it will. It has to since it's mounted at it's place.


    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Alright, I want my bass back!

    Post by PeterCapo on Sat Dec 20, 2014 8:07 pm

    tygr1 wrote:Won't replacing the power supply caps with a VTA board get rid of the selenium stack entirely?

    Yes, but you might still have the issue of the filament voltage being too high, because the replacement diodes on the VTA board will likely have less of a voltage drop than a new selenium stack would (if you could even find one anywhere).  It will also depend on what your AC wall voltage is - if it's not too high, you could be alright, but I'd definitely want to check it.  12AX7 datasheets specify 12.6VDC as the maximum, so the shortcut to my previous post, n°10, would be to just measure your filaments at your line voltage and see where they fall.  Again, it would probably be a good idea to check the wall voltage periodically to see if it varies.

    PS - I am thinking of the unregulated VTA cap board that I have in my PAS.  I am unfamiliar with the details of the regulated VTA power supply board, but, of course, if it regulates the filaments, then I assume it will keep them to not more than 12.6 volts?

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