Proper (stock value) volume pot for '69 Vibrolux and questions

Discussion in 'Amp Central Station' started by Krackle, Apr 30, 2021.

  1. Krackle

    Krackle Tele-Meister

    Posts:
    178
    Joined:
    May 10, 2013
    Location:
    Chicago
    Morn Peoples,

    The Volume pot on both channels of my Vibrolux yield not much volume
    between 1 and about almost 3..i mean..not much at all

    I've popped the chassis and all of the tone pots including Reverb and tremolo are clearly marked on the backs..

    However, both Volume pots in this amp have no markings on the back at all..
    There are markings on the sides of the pots but i can't read them..except for a few letters/numbers before they disappear around the side of the pot..facing the wrong away of course..

    Im thinking the pots were swapped out at some point for a different taper and/or resistance value..

    What is the stock, original resistance value and taper for the two volume pots in
    a 69 Vibrolux? (250k audio taper??)

    I never liked the taper of Fender volume pots..0 to Loud as hell-before hitting 3 on the dial..But, what's in there now seems way strange (for a fender amp)

    The overall volume on this amp feels low in the first place..sounds great and all but I have very very little in what should be the clean range just after 3 on the dial..doesn't feel right..even with the old Oxfords in it..
    my '70 Deluxe reverb is almost louder (has a much more efficient speaker tho)

    good morning
     
    Old Plank likes this.
  2. schmee

    schmee Doctor of Teleocity Ad Free Member

    Posts:
    15,767
    Joined:
    Jun 2, 2003
    Location:
    northwest
    That is normal on vintage Fender amps. Every one I have anyway. Leave it alone.
    It may not be the pot itself but just the amp architecture. But it could be I suppose.
    Normal:
    1-2 almost nothing.
    2.5 get's it going but it may not sound that good.
    3 is the lowest I will set to sound good.
     
    Krackle and corliss1 like this.
  3. Krackle

    Krackle Tele-Meister

    Posts:
    178
    Joined:
    May 10, 2013
    Location:
    Chicago
    Hi Schmee...thanks..

    Weird to hear because every dang Fender I've owned (all silverface around 1970 give or take) are off to the races volume-wise when you hit 2..

    And the overall loudness feels low for a Vibro up to 4-5 on the dial..
     
  4. BobbyZ

    BobbyZ Doctor of Teleocity

    Posts:
    13,693
    Joined:
    Jan 12, 2011
    Location:
    Snellman MN
    If I'm not mistaken the volume pots are 1 meg in pretty much every Fender and a whole lot of other amps as well.
    However if the amp isn't very loud on 10, me thinks you have other problems going on!
    I see a lot of amps only actually working on one power tube, for various reasons. Dead tube seems like the obvious reason but I've found its usually something else, probably more often.
    Of course low volume can be other things and if someone's been in there you'll need someone, like a tech, to figure it out.
    There's parts that if original that will cause issues too, electrolytic capacitors. If it's got OEM Oxford speakers it's definitely in the old enough for service category.
     
    Krackle and Old Plank like this.
  5. Krackle

    Krackle Tele-Meister

    Posts:
    178
    Joined:
    May 10, 2013
    Location:
    Chicago
    in my first post i had the question about the fact that both volume pots on the amp (dry and wet channel) were completely different than the tone and reverb pots..

    no markings on back of the volume pots

    ..all the other pots were clearly the same manufacturer..same structure of the spec markings on back of pot..that puzzled me and made me think they had been swapped out..
     
  6. Krackle

    Krackle Tele-Meister

    Posts:
    178
    Joined:
    May 10, 2013
    Location:
    Chicago

    Hi BobbyZ
    Amp's been recapped including filter caps..
    Power tubes checked and biased..
    Was also blackfaced..
    Tech didnt find anything out of line circuitwise
    But..we weren't really tking about the volume issue..

    As for 'loud on10' I mean..it's loud, it sounds good, although
    getting into that 'too much distortion is too much' zone

    but im not, like, running for cover volume wise..
     
    BobbyZ likes this.
  7. dan40

    dan40 Friend of Leo's

    Posts:
    2,234
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    It may just be the low sensitivity of the Oxfords that has you feeling that the amp's volume is a touch lower than it should be. Speakers with a higher sensitivity rating would definitely give the amp a volume boost.
     
    Krackle likes this.
  8. Krackle

    Krackle Tele-Meister

    Posts:
    178
    Joined:
    May 10, 2013
    Location:
    Chicago
    Hey Dan..thanks,

    Yes, for sure all considered..I've always shelved my original speakers on older amps..never had a pair in decent enough condition..These Oxfords sounded good right off so i decided to play them for a while and forego the usual modern speaker search..

    I actually like the restrained volume but i get basically no sound till i'm approaching 3 on the volume dial
    Thought it would be a good idea to ask for thoughts.. make sure things were right..
     
  9. Wally

    Wally Telefied Ad Free Member

    Posts:
    39,705
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Re: those 10” Oxfords. Those speakers did not get the wide voice coil gap revision in 1966 as did the 12” Oxfords. Those speakers are not your problem. The volume pots should be 1 Meg ohm audio tapers, and they should have the same date codes..within a range...as all of the other pots. It is not usual for BF/SF amps to have volume controls that come on all of a sudden, in my long experience....until you get into the J tapers in the MV era. Modern Blues/Hot Rod amps on the other hand do have that problem.
    Imho, your amp needs a tech. Fwiw, the changes over the life of the VR were minor...except for the mixed Biasing in the 1968 circuit, which lasted until the 1970 circuit.The 1970 and later VRs are very similar to the 1964 circuit but for some changes in the PI area.
     
    BobbyZ likes this.
  10. schmee

    schmee Doctor of Teleocity Ad Free Member

    Posts:
    15,767
    Joined:
    Jun 2, 2003
    Location:
    northwest
    A Vibrolux should be loud and full at 4-5 for sure. Yeah, my BF Fenders definitely get there by 3, some at 2.5 more or less. But at any rate, there is a big jump where the amp comes alive in that range.
    The other possibility is a bad pot, I have had at least one bad volume pot. It eventually became inoperable in certain areas of the sweep, stopping all sound entirely! It was making noise and I finally discovered the issue as it popped etc when turned. I hardly ever touch the volume control so it took some time to discover that.
     
    Krackle and BobbyZ like this.
  11. BobbyZ

    BobbyZ Doctor of Teleocity

    Posts:
    13,693
    Joined:
    Jan 12, 2011
    Location:
    Snellman MN
    You could try one simple test. Pull a power tube and play it just long enough to get a good listen. Then put the tube back in and pull the other one, don't change settings. Should sound pretty much the same. If it doesn't try the other 6L6 in the socket with lower volume or whatever.
    What that does is isolate the two circuits you've got between the phase inverter, where the signal splits and the output transformer where the signals comes back together to drive the speaker.
    The 6L6s are part of that circuit of course and you'll find out quickly if one of those is weaker.
    If it's not a 6L6 then everything from the phase inverter tube itself, (it's really two tubes in one bottle) through the circuits to the output transformer itself are suspects.
    If it sounds pretty much the same on each socket then you're fine there.
    I've never run into it personally but the output transformers in some if the "small OT" Fenders like this and the Pros and Bandmasters do fail
    @Wally would have more insight on this than me!

    And maybe the amp is fine. Hard to tell from where I'm sitting. :)
     
    Krackle likes this.
  12. Bob Arbogast

    Bob Arbogast Tele-Afflicted Silver Supporter

    Posts:
    1,782
    Joined:
    Mar 16, 2003
    Location:
    Lowell, Michigan USA
    I've not had the opportunity to measure pot tapers in black- and silver-panel Fenders. The schematics seem to indicate a 35% taper for the volume pots. That would account for how loud an original amp would be at 4-5. If the volume pot were replaced with a standard audio taper (10%), the equivalent setting would be around 7-8!

    In my builds, I have 10% taper. I like the more gradual rise in level. I'll admit, though, that at first I did scratch my head to hear people talk about a VR at 4 being really loud. Not with a 10% taper volume pot!
     
    Krackle, BobbyZ and schmee like this.
  13. Krackle

    Krackle Tele-Meister

    Posts:
    178
    Joined:
    May 10, 2013
    Location:
    Chicago
    Wally
    ..RE the volume pots with no markings on the back..as compared to the all the others..So this would/could indicate a change of pots by a previous owner?..or an oddball lot at Fender that day?
    ..And so, The proper Volume pot resistance for a 69 Vibrolux is 1000k?

    BobbyZ,
    ..Just did the test you suggested..everything seems close enough volume-wise in all formations of 6L6 swap outs..
    .I have done some reading on the OP transformer..it has
    a 'bit' of a reputation..it's the 022848 version..has a rep here and there for having issues..

    Bob Argo,

    ...Im thinking, based on Wally's comment that both Volume pots were swapped..cant read the markings as they are on the side of the pot (all other tone pot markings are on the back) and flipped the wrong way..thy are clearly different pots..

    ..The Volume pot in the wet channel of this Vibro has a tiny bit of play in the stem and if you push or jiggle you cen get a clicky response..Anyhow, they are mystery pots. no idea what the specs are on them..
     
    BobbyZ likes this.
  14. Wally

    Wally Telefied Ad Free Member

    Posts:
    39,705
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    I am thinking those pots are not original. They should be of the same make as the others, and the codes should be marked in the same way as the other pots.
    Re: That 022848 OT. I hav end ear heard of any problem with these failing. Many of them did get replaced in VRs, Pro Reverbs, Bandmasters, Ab763 Pros, and the AB763 Vibroverbs like SRV used because they are small OTs. They do not make as much power as do the OTs in say the Bassman amps of the same era...they do not punch at volume and they saturate early. That said, the VR is my favorite Bf/SF Fender guitar amp, and I like the way they act....especially in this day and age when huge volumes are not the desired endgame, right? I Have a 1964 OT of this type that is perfectly healthy. It is one of those that got pulled for a replacement with a larger OT.
    As for the sweep on the 1meg audio pots that were in the Fenders prior to the MV era, I just plugged into a 1972 Princeton Reverb with the original volume pot.....perfect reaction imho. The volume comes on just below 2 and has a sweep that increases the output as you turn it up...from 2 to 10 it has a proper reaction. I have never had a problem with those volume pots and their sweep. The 1Meg J sweep pots that were used in the preamp volume control of the MV amps is another thing, though.
     
    Krackle and BobbyZ like this.
  15. BobbyZ

    BobbyZ Doctor of Teleocity

    Posts:
    13,693
    Joined:
    Jan 12, 2011
    Location:
    Snellman MN
    Yeah I've never had an issue with one of the small Fender OTs just read where others have.

    I also like Fenders old volume pot taper, I'd never even noticed you have to turn them a ways till I read something about it, probably here. At the time I had a fair number of black and silver faced Fender and sure enough they all did it, make sound at 1 or 2. One of those who cares things for me, numbers on amp knobs are really meaningless. Useful for the one amp only, not useful from one amp to the next.
    My '74 1987 Marshall sports volume pots out a trashed 68 Bandmaster. The ones in the darn thing went from off to rip your head off loud, like switches. It made the amp completely unusable unless you were somewhere you could play that loud. Actually had some new one meg pots at the time but I know the old Fender taper. You can take a Twin Reverb and easily dial in whatever volume you want. That's how that 1987 is now great cleans at lower volumes and it rocks loud. With the old ones, (and they may not have been original) it went right past the clean tone zone. It completely transformed that into something I'm not scared to turn on anymore. Lol
     
    Krackle likes this.
  16. Dacious

    Dacious Poster Extraordinaire

    Posts:
    9,917
    Joined:
    Mar 16, 2003
    Location:
    Godzone
    People didn't like the Vibrolux, Bandmaster, Pro Reverb output because it saturated early. Past six with most guitars it gets more compressed. Not big and clean like a Twin or clean but fat like a Bassman.

    At six that amp is usually way too loud judging from my 78.

    When the tech did the filters did he also replace the small bypass caps on the fibre board? They also go and need replacing.

    The amp can definitely evidence low and poor quality volume if one or more is bad, ditto the bias cap.
     
    Krackle likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.