Announcement

Collapse
No announcement yet.

Having a hard time debugging this one...

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • nickb
    replied
    Ground one end only, preferably at the driving end.

    What are the blue/brown wires that go across the board? They look like they might come from the output tubes and go to the output transformer. Also they go right past the screened leads presumably with sensitive signals. If this is the case they need to be re-routed.

    Diagnose your hum/buzz. Turn down the volume controls and see what happens. Is this 60Hz heater hum or 120hz power supply buzz?
    Last edited by nickb; 04-25-2016, 05:49 PM.

    Leave a comment:


  • mort
    replied
    Originally posted by Jazz P Bass View Post
    Did you ground both ends of the shield?

    Most times, you do not.

    Sometimes, you do.

    Sometimes, neither.
    I tried neither and one. Did not try both. Will though.

    Leave a comment:


  • Jazz P Bass
    replied
    Originally posted by mort View Post
    I've tried several different combinations of grounding and nothing seems to change the hum. The hum was not present at all before the shielded lead got put in. If I change it back to a regular wire then it will go back to being unstable again on the presence mode. ]
    Did you ground both ends of the shield?

    Most times, you do not.

    Sometimes, you do.

    Sometimes, neither.

    Leave a comment:


  • mort
    replied
    I've tried several different combinations of grounding and nothing seems to change the hum. The hum was not present at all before the shielded lead got put in. If I change it back to a regular wire then it will go back to being unstable again on the presence mode. Hmmmm....


    Here's a photo to show the grounding. I split the grounding buss to get the phase inverter and bias grounds off of the preamp grounds. The power section grounds, bias and PI all ground together toward the left. And the preamp section all gets grounded back to the chassis on the right. That's how I generally do grounding schemes and don't run into to many problems. Should I ground the shields somewhere else? I tried putting a 500pf cap from the shielded lead to ground and it reduced the hum but also dampened some of the glassiness of the top end so I removed it.

    Any other ideas?

    Leave a comment:


  • mort
    replied
    With a little more prodding around I found that the wire from V3 P2 to the diodes still did in fact have a strong effect on the stability of the amp, so it got removed and replaced with a shielded one, grounding the shield, and this seems to have stabilized the amp while in presence mode, but now there is a general hum regardless of where the volume is set or if anything is plugged in or not. Did I introduce a ground loop via the shield? Doesn't seem like it should have... I'm gonna put a shielded cable on the input to V1 just for precaution and maybe work the ground scheme a little different and see what happens.

    If there are any more thoughts please share. I'll check back frequently.

    Leave a comment:


  • nickb
    replied
    When someone says 'noise' I take it to mean hiss and that is why I said what I did in #17. What you just described sounds like it's borderline unstable.

    Could be the feedback phase. Remove the feedback and see if it's any better. You can try swapping the phase but my guess it is correct as is.

    Instability is due to output signal getting back to an earlier stage. It could be poor location of wires, keep big swing signal wires like output transformer primaries away from anything. Keep wires going from plate to grids close to he chassis to help reduce capacitive coupling of unwanted signals to them. Unfortunately there are many way for this to happen and most of them are to do with layout.

    Leave a comment:


  • mort
    replied
    ok so I did find a minor wiring error and corrected it. With the presence switch open it's got good full sound but there's still a small amount of unwanted noise riding on it. And when the presence switch is closed there are noises that sound kinda like oscillations and also the guitar sound distorts in a sudden and unpleasant way. Those leads associated with V3 P2 are MUCH less sensitive than they were earlier though.

    Leave a comment:


  • mort
    replied
    Ah... I never installed the 100k & F.B.

    doing that now...

    edit: scratch that. I DID install the F.B. but might have found a mistake on my layout.. will report back in a few.
    Last edited by mort; 04-24-2016, 09:14 PM.

    Leave a comment:


  • mort
    replied
    ok, done with dinner and back at it. It is making some varying high frequency noise that seems to be associated with V3 and V4. When I get my small needle nose pliers near the lead that goes to V2 P2, it starts making ALOT of noise and moving that lead around has ALOT of effect over the noise(just realized this is with the presence switch closed). With the presence switch open the guitar sound is much better, seemingly full volume, but still has some unwanted distortion riding on it.

    Here are some fresh readings after the above circuit corrections. I unplugged the tank and left out the 3 triodes associated with the reverb.




    V1
    p1 122.5
    p2 .00
    p3 1.02
    p6 124.7
    p7 .00
    p8 .97

    V2
    p1
    p2
    p3
    p6
    p7
    p8

    V3
    p1 190.3
    p2 .35
    p3 2.13
    p6
    p7
    p8

    V4
    p1 185.1
    p2 -.93
    p3 1.9
    p6 276.9
    p7 63.89
    p8 76.1

    POWER TUBES
    p3 458.4 & 456.5
    p4 379.5 & 379.5
    p5 -57.4 & -57.6
    p8 0 & 0

    Leave a comment:


  • nickb
    replied
    Originally posted by mort View Post
    ok I tacked in a 5M and a 1.1M for temporary and I now have guitar sound coming through the speaker!
    Well, so long as it's your guitar then that's good

    Originally posted by mort View Post
    Also hooked up the tank and the reverb does work.

    The output is low and has a little noise in it but I have to break for dinner.

    You guys kick ass. Be back in a while...

    Oh and I'm accustomed to having to swap the OT leads on a fresh build sometimes. Usually because it goes BBBBBRRRRRRMMMMMMPPPPPP on start up. Could that phase reversal(?) problem also just cause low output like I'm experiencing now?
    If the phase were wrong it would tend to oscillate or at least be louder. Noisy suggests to me that you are having to crank the volume up to hear anything and that suggests low gain early on around V1a - check that area. Do the hi & lo inputs work as they should? Also turn down the reverb just in case the noise is coming from there.

    Leave a comment:


  • mort
    replied
    ok I tacked in a 5M and a 1.1M for temporary and I now have guitar sound coming through the speaker! Also hooked up the tank and the reverb does work.

    The output is low and has a little noise in it but I have to break for dinner.

    You guys kick ass. Be back in a while...

    Oh and I'm accustomed to having to swap the OT leads on a fresh build sometimes. Usually because it goes BBBBBRRRRRRMMMMMMPPPPPP on start up. Could that phase reversal(?) problem also just cause low output like I'm experiencing now?

    Leave a comment:


  • nickb
    replied
    Originally posted by mort View Post
    another good catch. I had read that as 5.6K and that's what's installed. I have a 4.7M I can sub in. do I also need that 1.5M to ground for the circuit to work?
    You'll need the 1.5Meg to OV. The aim to to get about 1/4 to 1/3 of the supply voltage at the grid.

    Leave a comment:


  • mort
    replied
    Originally posted by nickb View Post
    V4b voltages are wrong and once again the reason is the schematic is wrong - that 56K(?) should not be going the supply.

    On the prowessamp link it shows 5.6M to supply and 1.5M to OV
    another good catch. I had read that as 5.6K and that's what's installed. I have a 4.7M I can sub in. do I also need that 1.5M to ground for the circuit to work?

    Leave a comment:


  • nickb
    replied
    V4b voltages are wrong and once again the reason is the schematic is wrong - that 56K(?) should not be going the supply.

    On the prowessamp link it shows 5.6M to supply and 1.5M to OV
    Last edited by nickb; 04-24-2016, 07:45 PM.

    Leave a comment:


  • mort
    replied
    ok I tacked in a .01 cap and the DC is off of pin 7 and my plate voltages on V1 are now 101 and 106. Still no output though

    Leave a comment:

Working...
X