Results 1 to 23 of 23

Thread: Carvin Belair 2x12 Gain Stage

  1. #1
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3

    Carvin Belair 2x12 Gain Stage

    Hey guys, I could use a bit of advice on a problem I've encountered with a Carvin Belair combo amp (schematic below). On channel 2 there is significant hiss when the "Soak" (drive/gain) control is turned up past a specific point (in the neighborhood of between 6 and 7 on the dial). I'm scoping a test signal at pin 7 of V2:B (with the channel volume all the way down) and as I increase the drive pot I can see the amplitude increasing on a relatively linear curve until it jumps up significantly at the point where the hiss starts and then continues from that point in a typical fashion. The hiss is not present on the other side of C13. According to the schematic I should see about 300V at the plates on V2. In fact, the actuals are 147V on pin1 and 246V on pin 6. Based on that information can anyone point me in the right direction as to what the issue might be?

    Edit: I see now that there is a couple of threads on here dealing with similar problems with this amp. I'll start by looking through those.


    Carvin-Belair-Schematic.pdf

    0 Not allowed! Not allowed!
    Last edited by bobloblaws; 01-01-2019 at 12:43 AM.

  2. #2
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    11,753
    Thumbs Up/Down
    Received: 1,884/23
    Given: 1,463/35
    Rep Power
    27
    What happens to hiss when you remove your test probe from V2b-7?

    Hiss might be "what you hear" from an oscillation and probe could be an interference injecting antenna.

    What do you see when scoping V2b-6 and you rise/lower volume pot?

    At least, anything you see on pin 7 will be much more visible on pin 6 .

    Also, what do you hear?

    It should match what you see on screen.

    I´m not much worried at schematic indicated plate volotages, if anything I find *them* high, go figure.

    What you measured sounds more usual to me.

    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  3. #3
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    Also, how does signal react to the pot, the same 'jump' or different?
    As far as the DC voltages, agree with JM, what you measured sounds right, what's shown on the schematic looks wrong. The 2 halves of the tube have the same plate R's but different cathode R's, so they can't be the same. Is voltage at 'D' 385V? V2 pin3 around 3V and pin8 around 1V ?

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  4. #4
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Quote Originally Posted by J M Fahey View Post
    What happens to hiss when you remove your test probe from V2b-7?
    In this scenario I have the output connected to a dummy load and I'm using an audio probe to listen for the hiss at various points in the circuit. So if I remove the probe I don't hear anything. I probably should have clarified that I'm probing with the scope at times and with the audio probe at other times.

    Quote Originally Posted by J M Fahey View Post
    Hiss might be "what you hear" from an oscillation and probe could be an interference injecting antenna.
    But I noticed the hiss present via the output before I ever started probing so we can rule that out.


    Quote Originally Posted by J M Fahey View Post
    What do you see when scoping V2b-6 and you rise/lower volume pot?
    Same as pin 6, just louder, as you would expect.

    0 Not allowed! Not allowed!

  5. #5
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Quote Originally Posted by g1 View Post
    Also, how does signal react to the pot, the same 'jump' or different?
    Yes, same.


    Quote Originally Posted by g1 View Post
    Is voltage at 'D' 385V?
    Actual is 350V.

    Quote Originally Posted by g1 View Post
    V2 pin3 around 3V and pin8 around 1V ?
    pin 3 -> 0.7V pin 8 -> 2V

    0 Not allowed! Not allowed!

  6. #6
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    So could it just be a glitchy pot then? Or something about the taper that makes it act that way? If the signal is reacting the same way, then I would think the pot is the cause.
    As for the V2 cathode voltages, I got my pins 3 & 8 voltages reversed. For 350V at point D, your V2 pin3 at .7V and pin8 at 2V look correct. The schematic voltages shown for V2 plates are incorrect.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  7. #7
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Quote Originally Posted by g1 View Post
    So could it just be a glitchy pot then? Or something about the taper that makes it act that way? If the signal is reacting the same way, then I would think the pot is the cause.
    As for the V2 cathode voltages, I got my pins 3 & 8 voltages reversed. For 350V at point D, your V2 pin3 at .7V and pin8 at 2V look correct. The schematic voltages shown for V2 plates are incorrect.
    Just to make sure I understand what you mean, are you suggesting the pot is the source of the hiss? Typically with a master volume type amp you should be able to turn the gain control all the way up to get desired amount of saturation and control overall volume with the volume pot. I'm not sure I understand what you mean by "the signal is reacting the same way". The same way as what? To reiterate, there is a specific point on the turn of the pot where the hiss kicks in. If I probe a test signal with my scope at V2 pins 6 or 7 I can see the sine wave increase significantly in amplitude at the same point. If I recall correctly the jump in amplitude is apparent at the output as well (I would expect that in any case). Anyway, I guess my point is I don't see how the pot taper is relevant since there should not be that amount of noise regardless of where the pot is.

    Out of curiosity, how do you determine what voltages to expect on the cathodes? Also, based on what you said about expecting different plate voltages based on different cathode resistor values, are the plate resistor and cathode resistor the only factors determining what the plate voltage will be (given the same B+ and same tube type?).

    0 Not allowed! Not allowed!

  8. #8
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    11,753
    Thumbs Up/Down
    Received: 1,884/23
    Given: 1,463/35
    Rep Power
    27
    Please replace pot with a new one and post results.

    A pot with worn/dirty track leaving momentarily next tube grid "open" may account for all of your symptoms and then some, so let´s remove that out of the way first.

    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  9. #9
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Quote Originally Posted by J M Fahey View Post
    Please replace pot with a new one and post results.

    A pot with worn/dirty track leaving momentarily next tube grid "open" may account for all of your symptoms and then some, so let´s remove that out of the way first.
    OK, I'll try that, fingers crossed.

    0 Not allowed! Not allowed!

  10. #10
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Quote Originally Posted by J M Fahey View Post
    Please replace pot with a new one and post results.

    A pot with worn/dirty track leaving momentarily next tube grid "open" may account for all of your symptoms and then some, so let´s remove that out of the way first.
    On second thought, wouldn't it be feasible to first test that theory by simply bypassing the pot with a jumper, in effect having the pot wide open, but without the pot? Replacing the pot requires first disconnecting a slew of ribbon cables and wires with slide on connectors, unfastening all the pots, channel switch and input jack, removing the preamp tubes and then lifting the circuit board out. Seems like a lot of trouble if we are only guessing and there might be another way to possibly rule out the pot being bad.

    0 Not allowed! Not allowed!

  11. #11
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    13,003
    Thumbs Up/Down
    Received: 568/3
    Given: 298/0
    Rep Power
    28
    I agree.
    Jumper it out.

    1 Not allowed! Not allowed!

  12. #12
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    You were not clear on my question about 'does signal act the same way'.
    What I meant was, is there a big jump in level when you are turning the pot with a signal applied.
    Lets forget about the noise level with nothing plugged in for now. What I was getting at was the noise level relative to the signal. If there is an abnormal change in the signal level when adjusting the control, then there must be something wrong with the control, so that must be fixed first before being concerned with noise levels. (cleaning or replacement of the pot)
    If the pot is good, then you will get no abnormal jump turning it while you have signal applied.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  13. #13
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    Quote Originally Posted by Jazz P Bass View Post
    I agree.
    Jumper it out.
    The pot serves as the grid leak for the V2. I'd suggest resistance checks from wiper to ground through it's rotation. You could jumper the pot and still lose the grid reference to ground.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  14. #14
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Quote Originally Posted by g1 View Post
    You were not clear on my question about 'does signal act the same way'.
    What I meant was, is there a big jump in level when you are turning the pot with a signal applied.
    Lets forget about the noise level with nothing plugged in for now. What I was getting at was the noise level relative to the signal. If there is an abnormal change in the signal level when adjusting the control, then there must be something wrong with the control, so that must be fixed first before being concerned with noise levels. (cleaning or replacement of the pot)
    If the pot is good, then you will get no abnormal jump turning it while you have signal applied.
    Maybe I can shed a little light from a slightly different perspective. While I did see an abnormal jump in sine wave amplitude scoping at V2, when I instead listened to the same test signal through the speaker it was more noticeable as a change in frequency response rather than simply a bump in volume. For that reason I kind of leaned toward it being something other than a wonky pot. Anyway, I'm gonna try jumpering the pot here shortly.

    1 Not allowed! Not allowed!

  15. #15
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    Ok, but make sure the grid (pin7) has 500K or less to ground at all pot settings when measured with your meter.
    Even with your jumper in place.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  16. #16
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Quote Originally Posted by g1 View Post
    The pot serves as the grid leak for the V2. I'd suggest resistance checks from wiper to ground through it's rotation. You could jumper the pot and still lose the grid reference to ground.
    In the message you just posted you imply I can do these checks with the pot in circuit, do I have that straight? I just want to make sure.

    Quote Originally Posted by g1 View Post
    Ok, but make sure the grid (pin7) has 500K or less to ground at all pot settings when measured with your meter.
    So in other words make sure it doesn't show an open line, correct?

    0 Not allowed! Not allowed!

  17. #17
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    Correct.

    1 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  18. #18
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    Quote Originally Posted by bobloblaws View Post
    Out of curiosity, how do you determine what voltages to expect on the cathodes? Also, based on what you said about expecting different plate voltages based on different cathode resistor values, are the plate resistor and cathode resistor the only factors determining what the plate voltage will be (given the same B+ and same tube type?).
    I wanted to answer this eventually, you don't have to read it now but I'll get it in here anyway.
    I used your plate voltages to determine the cathode voltages. The current is the same at both ends of the tube. So the supply voltage less the plate voltage gives you the voltage drop across the plate resistor. Voltage divided by resistance gives the plate current. Same current through the cathode times the cathode resistance gives the cathode voltage.

    This is very handy for verifying schematic voltage listings, I've found several errors this way. Comparing to known 'standard' circuit values verifies any gross faults. (Fender blackface tend to be the most typical well known 'standard' values).

    Yes, with same B+, tube type, and circuit configuration, cathode and plate resistors are the only thing that affects the plate voltage (aside from slight variance between tubes of the same type).
    So right off the bat, I knew V2a and V2b plate voltages could not be the same due to different cathode resistors. Your numbers seemed correct (compared to standard values) and the schematic values could not both be. Further calculations proved your readings to be correct.

    By the same methods, if you were only shown the cathode voltage, but knew the value of cathode and plate resistors, and the voltage at the supply node, you could calculate the plate voltage.

    3 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  19. #19
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Well this is kind of strange. I went ahead and did some measurements as suggested. I didn't see anywhere where the grid lost the ground reference. Checking resistance from the wiper to ground perplexed me somewhat. Each end of the pot has virtual continuity with ground. As you can see on the schematic, the lug connected to C12 is connected to ground through the L1 relay. The max resistance I measured toward the middle of the travel of the pot was 135K which was a bit of a surprise since the pot is supposed to be 500K and I would have expected closer to 250K at the highest point. I don't know if that in itself is reason enough to change the pot. Anyway, then I tried jumping the top lug to the wiper/V2.B grid. The result also surprised me as the pot still worked, albeit with more signal passing to the grid. I'm assuming this is because in effect I'm connecting the grid directly to ground via the relay? It's all a little above my level of understanding. In any case, I discovered that when I took out the jumper and returned to the baseline test condition the hiss is virtually gone! Without having changed anything. I did give the pot a blast of Deoxit at some point yesterday, can't remember when exactly, maybe it was after I did the scoping and probing or maybe it took awhile to do it's thing, but it's seems OK now. There is still some noise, but much less than before and it comes on more gradually. So thanks a lot G1 and JM, the advice is always appreciated, and if you don't mind explaining why the design has both of the outside lugs on the drive pot connected to ground I'd appreciate that as well. Also, please let me know if you think the pot needs to be changed anyway based on the mas 135K measurement. Thanks!

    0 Not allowed! Not allowed!

  20. #20
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    11,753
    Thumbs Up/Down
    Received: 1,884/23
    Given: 1,463/35
    Rep Power
    27
    Maybe you measured with Drive channel set OFF.
    In that case *both* pot ends are grounded, and wiper set at exact center will measure upper half (250k) in parallel with lower half (250k) for a combined wiper to ground measurement of 125k .

    135k is well within tolerance for a nominal 500k pot.

    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  21. #21
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Quote Originally Posted by J M Fahey View Post
    Maybe you measured with Drive channel set OFF.
    In that case *both* pot ends are grounded, and wiper set at exact center will measure upper half (250k) in parallel with lower half (250k) for a combined wiper to ground measurement of 125k .

    135k is well within tolerance for a nominal 500k pot.


    Right, I didn't think of it in terms of parrallel resistors, it makes sense now. Also right re: the grounded lugs, 'cept the issue was I measured with the amp off instead of on, in which case both lugs were grounded in both channel switch positions

    0 Not allowed! Not allowed!

  22. #22
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,027
    Thumbs Up/Down
    Received: 1,795/24
    Given: 4,607/11
    Rep Power
    23
    Quote Originally Posted by bobloblaws View Post
    the issue was I measured with the amp off instead of on, in which case both lugs were grounded in both channel switch positions
    Yes, sorry I missed that, the relay is closed when the amp is turned off so the pot is grounded at both ends.
    It's purpose is to kill the signal coming from V1A while Ch.1 is selected.
    I hope the pot cleaning is what cured it. Otherwise, if the problem recurs, there is probably an iffy solder connection that got temporarily cured from being jiggled while you were working in there.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  23. #23
    Supporting Member
    Join Date
    Jun 2017
    Location
    Ontario, Canada
    Posts
    405
    Thumbs Up/Down
    Received: 34/0
    Given: 43/0
    Rep Power
    3
    Hey guys, I have another problem with this amp. I'm not sure if the preferred thing to do is start a brand new thread or carry on in this one. Maybe someone can let me know?

    Anyway, the "Acoustic Presence" control doesn't seem to be affecting the tone of the amp at all, or if it is it is imperceptible to me. I've done some troubleshooting but kind of at a dead end. The control is meant to work on the clean channel only. I verified that the relay is grounding the one pot lug when switching to the dirty channel. So the default would be the clean channel with presence/negative feedback. I cleaned the pot and it seems OK but it doesn't go quite down to zero ohms, the range is something like 40 ohm to 28 Kohm. C70, R70, and R71 are in spec and I can scope a test signal on either side of C70. On the pot side of C70 the amplitude of the signal is affected by turning the presence control, however, on the R70 side the signal barely changes.

    Can anyone suggest next steps?

    Edit: For the record I created a new thread and this was answered thusly:


    Quote Originally Posted by The Dude View Post
    From the manual:

    "The rear ACOUSTIC PRESENCE control adds a sibilance to the high frequencies of your guitar.
    Most presence controls work in the 3k to 4k range. However, the VINTAGE TUBE’s presence starts
    at a very high 8k Hz delivering 10 dB at 12k Hz and continues to 20k Hz which extends all the upper
    harmonics of your guitar. The amount of sibilance will depend on the speakers used. To keep both
    channels totally independent, the ACOUSTIC PRESENCE is switched by relay only into clean channel
    1. The effect of the ACOUSTIC PRESENCE will seem ever so slight, however, the result is added
    sibilance only to the ultra-high frequencies."


    I suspect it's working and, as the description says, ever so slight. If you're running a typical guitar speaker, frequency response may not even include 8K or higher and you would hardly notice it at all since we're in tweeter range.

    0 Not allowed! Not allowed!
    Last edited by bobloblaws; 01-11-2019 at 03:19 PM.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Carvin Belair - moding one channel
    By überfuzz in forum Mods & Tweaks
    Replies: 3
    Last Post: 03-16-2015, 04:01 PM
  2. Carvin belair 50W
    By überfuzz in forum Schematic Requests
    Replies: 2
    Last Post: 03-03-2015, 06:24 PM
  3. Channel Switching problem on Carvin Belair amp
    By jnsfoto in forum Maintenance, Troubleshooting & Repair
    Replies: 1
    Last Post: 03-05-2013, 11:34 PM
  4. Carvin BelAir troubles
    By Zouto in forum Maintenance, Troubleshooting & Repair
    Replies: 8
    Last Post: 04-06-2010, 02:10 PM
  5. Carvin BelAir - Low Voltage, Output, lot's of problems
    By wcrankshaw in forum Maintenance, Troubleshooting & Repair
    Replies: 9
    Last Post: 03-26-2010, 12:37 AM

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •