Results 1 to 28 of 28

Thread: Vibro Champ Voltages

  1. #1
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0

    Vibro Champ Voltages

    Hi guys,

    I'm doing maintenance to a 75 Vibro Champ. As usual I got high dissipation for the 6V6. I'm trying to address that issue but the results are kind of weird. I replaced the 470 cathode resistor with a 1K 5W one and the 25/25 cap with a 47/50.

    The thing is that I get a voltage drop across the resistor of about 72.8 which is very high for the cap I used. And it's out of specs according to the schematic

    I took some readings with the stock components, before the change:

    PT red wires to rectifier 296.8 VAC
    PT green wires to lamp and heaters 5.43 VAC

    Filter section voltages 261.4, 278.5 289.2

    Pin 1 V1: 190.6
    Pin 3 V1: 1.2
    Pin 6 V1: 187.5
    Pin 8 V1: 1.12

    Pin 1 V2: 130-143 value oscillates.
    Pin 3 V2: 1.22
    Pin 6 V2: 279.5
    Pin 8 V2: 131-143 value oscillates.

    Pin 3 V3: 269.3 (plate)
    Pin 4 V3: 280 (screen)
    Pin 8 V3: 55.7

    Plate voltage on 6V6 is 55.7, already high.

    I know that rising the voltage drop I'll drop the current on the tube. But looking at the numbers I guess I should rise the plate current a little bit too. I don't get why the voltage drop on the cathode resistor is too high on this one.

    Any pointers on that?

    Thanks a lot guys.
    Carlos.

    1 Not allowed! Not allowed!

  2. #2
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    I took measurements with the original components for 6V6 dissipation.

    This is what I got:

    Cathode Resistor: 251.8 ohms (almost half the value that the schematic says).
    Voltage drop: 20.3V (within specs).

    Current would be around 80.6mA.

    Plate Voltage: 248.1V (really low).

    Tube Dissipation: 20 watts aprox.

    This makes me wonder, how can I get almost the correct BIAS voltage with cathode resistor value drifted almost to half the value. I need to increase plate voltage to get the correct tube dissipation.

    Probably what the amp needs is a recap job.

    1 Not allowed! Not allowed!

  3. #3
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,459
    Thumbs Up/Down
    Received: 119/0
    Given: 159/0
    What is the Vdc at the 6V6 control grid terminal 5?
    If not 0, then suspect the coupling cap of leaking dc or the grid leak being open.

    1 Not allowed! Not allowed!
    Last edited by pdf64; 08-06-2018 at 05:47 PM.

  4. #4
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    Quote Originally Posted by pdf64 View Post
    What is the Vdc at the 6V6 control grid terminal 5?
    If not 0, then suspect the coupling cap of leaking dc or the grid leak being open.
    It reads 4.10v

    1 Not allowed! Not allowed!

  5. #5
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    2,655
    Thumbs Up/Down
    Received: 199/1
    Given: 400/8
    Ouch. Check that coupling cap & grid leak!

    Justin

    1 Not allowed! Not allowed!
    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  6. #6
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    Hi Justin,

    Thanks for your kind answer. I'll check the coupling between the stages to ensure everything is working properly, I have a doubt, do this issue influence the voltage over the plate on the power tube? I know it changes the operation conditions for the tube but the voltages seem to be low after they go to the filter caps.

    Thanks.
    Carlos

    1 Not allowed! Not allowed!

  7. #7
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,459
    Thumbs Up/Down
    Received: 119/0
    Given: 159/0
    Do not doubt that coupling caps passing dc will cause tubes to draw way too much current and that HT voltages will sag significantly as a consequence.

    2 Not allowed! Not allowed!

  8. #8
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    Quote Originally Posted by pdf64 View Post
    Do not doubt that coupling caps passing dc will cause tubes to draw way too much current and that HT voltages will sag significantly as a consequence.
    Great, I'll order the parts and let you know how the thing goes.

    Thanks a lot.

    1 Not allowed! Not allowed!

  9. #9
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,459
    Thumbs Up/Down
    Received: 119/0
    Given: 159/0
    Bear in mind that the power tube cathode cap looks to have been subject to significant overvoltage, so consider replacement.

    1 Not allowed! Not allowed!

  10. #10
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    pdf64 I have a doubt, as I understand, to check a coupling cap for leakage, you need to lift the negative leg and measure voltage on it, isn't it?

    I understand that there shouldn't be any DC voltage after a coupling cap. But I just have that doubt. If you could clarify that to me that would be much appreciated.

    All the best mate.
    CE

    0 Not allowed! Not allowed!

  11. #11
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,459
    Thumbs Up/Down
    Received: 119/0
    Given: 159/0
    If you’ve got a regular digital multimeter it will have an input resistance of about 10M ohms, and then yes, it would be fine to do as you describe.
    You also need to check that the power tube control grid leak is good, eg its 220k path to 0V is good (amp de-energised).

    0 Not allowed! Not allowed!

  12. #12
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    Let me check if I understood the last bit of information you gave me. Right between 220K resistors, there should be -34volts (that is the BIAS voltage for the tubes), according to the schematic. You mean that the voltage on the grids after the 220K should be 0 as well?

    0 Not allowed! Not allowed!

  13. #13
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,459
    Thumbs Up/Down
    Received: 119/0
    Given: 159/0
    What amp are you looking at there?
    Doesn’t seem to be a vibrochamp. If it is, could a schematic link be provided.

    0 Not allowed! Not allowed!

  14. #14
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    It's a 75 Vibro Champ.

    There are a couple of shcematics on the internet. I'm asuming this one is the correct, since the 6V6 has the small cap between pin 5 and 8. The other one is the AA764 (earlier that same year).

    Click image for larger version. 

Name:	VChamp.jpg 
Views:	29 
Size:	437.2 KB 
ID:	50038

    1 Not allowed! Not allowed!

  15. #15
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,459
    Thumbs Up/Down
    Received: 119/0
    Given: 159/0
    So where’s the negative bias voltage? This is a cathode bias power amp.
    Are you getting mixed up with the Princeton?

    0 Not allowed! Not allowed!

  16. #16
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    Sorry totally my mistake. I have another thread about a 64 Princeton Amp and got mixed out with the schematics. You are right.

    0 Not allowed! Not allowed!
    Last edited by caesparza; 08-12-2018 at 12:39 PM.

  17. #17
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    Got the parts to replace just to realize that I missed that the ground leg of the 220K resistor was disconnected.

    Poor visual inspection from my part, sorry guys.

    Now the grid on the power tube is around 0V and the voltages on the rest of the amplifier got closer to the schematic.

    I checked again the BIAS values and got this:

    Plate Voltage: 370 VDC (went up almost 100V).
    Voltage Drop across cathode resistor: -13.58 VDC
    Cathode resistor value: 241.1

    Making the calculations I get Plate Current of 56.32mA and Dissipation of 20.48Watts (Very hot). Now the tube is not glowing blue, and the sound got cleaner without that abnormal distortion I was experimenting before.

    I guess that now I can proceed to set the Bias for this fellow, should I aim to 29mA for the power tube?. What do you think about the values so far?.

    Thanks for your kind help.

    1 Not allowed! Not allowed!

  18. #18
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,399
    Thumbs Up/Down
    Received: 372/1
    Given: 312/0
    I find this a good guide.

    http://www.tedweber.com/webervst/tubes1/calcbias.htm

    It recommends about 29mA for a 6V6 single ended with 370V plate. FWIW: A tube glowing blue is not an issue. Glowing red or red plating is another issue all together.

    Edit: Also, when calculating plate voltage, don't forget to subtract the cathode voltage if you measured to ground.

    1 Not allowed! Not allowed!
    Last edited by The Dude; 08-28-2018 at 12:57 AM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  19. #19
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    Replaced cathode resistor and cap for 1K5W and 47uf50V.

    Plate voltage: 418VDC
    Cathode resistance: 998 ohms
    Voltage Drop: 24.57VDC
    Plate Current: 24.61mA.
    Dissipation: 10.36W (86.33%).

    That look a lot better, I'll play with the amp and see how it sounds.

    0 Not allowed! Not allowed!

  20. #20
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,399
    Thumbs Up/Down
    Received: 372/1
    Given: 312/0
    Is that 418V plate to ground or to cathode?

    0 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  21. #21
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    Plate to ground The Dude.

    0 Not allowed! Not allowed!

  22. #22
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,399
    Thumbs Up/Down
    Received: 372/1
    Given: 312/0
    You'll need to subtract the cathode voltage for a cathode biased amp. The spec is plate to cathode. The tube doesn't know if its cathode is grounded or not.

    1 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  23. #23
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    You mean reading voltage from cathode (pin 8) to ground and then subtract that amount from Plate Voltage? That would be voltage from Plate to cathode? According to weber page, would that be equivalent to get the voltage from pin 3 and 8?

    Sorry, everything is explained on that page.

    Thanks a lot for the info!

    0 Not allowed! Not allowed!

  24. #24
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    I don't get something here.

    The cathode voltage would be the same Drop Voltage across the cathode resistor. According to that, the results would be like this:

    Plate Voltage: 418
    Cathode Voltage: 24.57
    Plate Voltage for BIAS calculation: 418-24.57 = 393.43
    Cathode Resistor: 998 ohms
    Voltage Drop on cathode resistor: 24.57
    Plate current: 24.57/998 = 24.62mA
    Power Dissipation: 0.02462*393.43 = 9.69Watts. (80%).

    I'll need to raise a little bit that dissipation.

    Does it make sense?

    If I measure the voltage between pin 3 and pin 8 I get 1.18 VDC I don't know exactly what to do with that value. That is supposed to be the plate to cathode voltage and is not similar to the subtraction of the cathode voltage from the plate voltage.

    0 Not allowed! Not allowed!
    Last edited by caesparza; 08-28-2018 at 02:56 PM.

  25. #25
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    5,429
    Thumbs Up/Down
    Received: 525/15
    Given: 551/0
    Quote Originally Posted by caesparza View Post
    I don't get something here.

    The cathode voltage would be the same Drop Voltage across the cathode resistor. According to that, the results would be like this:

    Plate Voltage: 418
    Cathode Voltage: 24.57
    Plate Voltage for BIAS calculation: 418-24.57 = 393.43
    Cathode Resistor: 998 ohms
    Voltage Drop on cathode resistor: 24.57
    Plate current: 24.57/998 = 24.62mA
    Power Dissipation: 0.02462*393.43 = 9.69Watts. (80%).
    This ^^^ all makes sense, no worries.

    I'll need to raise a little bit that dissipation.
    You can if you want to, by reducing the value of the cathode resistor. But if the amp sounds good why bother? As Enzo keeps reminding us, these are guitar amps, not precision lab equipment. And as Duke Ellington keeps reminding us "If it sounds good, it IS good."

    If I measure the voltage between pin 3 and pin 8 I get 1.18 VDC I don't know exactly what to do with that value. That is supposed to be the plate to cathode voltage and is not similar to the subtraction of the cathode voltage from the plate voltage.
    I don't understand how this can be. Something is way off about that measurement.

    0 Not allowed! Not allowed!

  26. #26
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    Ok, with the parts available I put a 2K2 resistor in parallel with the 1K getting 685 ohms. I lifted the 330pf cap and tested again. Put a Sprage Atom 50/50 instead of the cheap 47/50 cathode cap.

    This configuration gives me a Plate current of 31.73mA and dissipation of 12.19Watts (101%) with Plate voltage of 406 and voltage drop of 21.74. Now the reading plate to cathode is consistent (385). I think I prefer the sound of the colder BIAS setting and the 330pf cap in, its cleaner and breaks up almost at 10 which is something I like.

    I'll go back to that setup and finish the amp. Thanks a lot guys for the help.

    0 Not allowed! Not allowed!

  27. #27
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    5,429
    Thumbs Up/Down
    Received: 525/15
    Given: 551/0
    Quote Originally Posted by caesparza View Post
    Ok, with the parts available I put a 2K2 resistor in parallel with the 1K getting 685 ohms. I lifted the 330pf cap and tested again. Put a Sprage Atom 50/50 instead of the cheap 47/50 cathode cap.

    This configuration gives me a Plate current of 31.73mA and dissipation of 12.19Watts (101%) with Plate voltage of 406 and voltage drop of 21.74. Now the reading plate to cathode is consistent (385). I think I prefer the sound of the colder BIAS setting and the 330pf cap in, its cleaner and breaks up almost at 10 which is something I like.

    I'll go back to that setup and finish the amp. Thanks a lot guys for the help.
    Good report on your experiment. Cathode bypass cap whether "cheap" or Sprague Atoms, make sure there's some space between the cap and cathode resistor. Cooking the cap with heat from the resistor shortens its life. Factory Champ builds often ignore this factor. I've seen caps partially melted.

    0 Not allowed! Not allowed!
    NON ILLEGITIMI CARBORVNDVM

  28. #28
    Member
    Join Date
    Aug 2013
    Posts
    98
    Thumbs Up/Down
    Received: 11/0
    Given: 0/0
    I guess the last factor about the sound of this little amp is the speaker. I'm not very fond of the original Oxford, not a very efficient speaker, last year I had Fender Champ with a Warehouse speaker that was just amazing. I think a swap in the speaker makes a huge difference at this size and wattage.

    0 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. how to get more out of a Vibro Champ
    By Groover in forum Guitar Amps
    Replies: 14
    Last Post: 02-17-2013, 04:08 AM
  2. vibro champ
    By tonalvision in forum Maintenance, Troubleshooting & Repair
    Replies: 9
    Last Post: 04-20-2010, 08:39 PM
  3. 66 Vibro Champ needs some help
    By eurekaiv in forum Maintenance, Troubleshooting & Repair
    Replies: 13
    Last Post: 02-08-2010, 05:29 PM
  4. Bulletproofing a Vibro Champ
    By thumbs in forum Mods & Tweaks
    Replies: 5
    Last Post: 12-09-2009, 01:31 PM
  5. Vibro-Champ Help
    By Rick1114 in forum Maintenance, Troubleshooting & Repair
    Replies: 16
    Last Post: 06-19-2008, 02:56 PM

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
  •