Page 3 of 3 FirstFirst 123
Results 71 to 104 of 104

Thread: Magnatone Custom M10

  1. #71
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    Quote Originally Posted by Helmholtz View Post
    The original amp works with hybrid bias (schematic see post#1): 100R cathode resistor(s)
    correct! and now I am going to change to full cathode bias and try the following R values: 250, 270, 330, 390, 470

    Quote Originally Posted by Helmholtz View Post
    Though the resulting B+ with the new PT isn't quite clear yet, I expect that something like 250R will give suitable biasing below 100% dissipation.
    with the power supply as is (with the new bridge rectifier) and isolated from the rest of the amp (I cannot have it connected as there will be a lot of current going through the power tubes without the negative bias voltage) I am getting about 338 VDC



    what do you guys think, is there still hope to use this power supply?

    0 Not allowed! Not allowed!

  2. #72
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,205
    Thumbs Up/Down
    Received: 2,342/1
    Given: 1,335/2
    Rep Power
    8
    I am getting about 338 VDC
    Now this seems very low. Is this with a reservoir ecap connected??

    I don't think it is possible to produce a negative voltage from a HT tap when using a bridge rectifier, so your proposal won't work.

    I just saw that there is a BJT stage that requires a -32V supply. So while full cathode bias would provide a solution for the power stage, I think you need a different PT that allows for the -50V supply.

    1 Not allowed! Not allowed!
    - Own Opinions Only -

  3. #73
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    Quote Originally Posted by Helmholtz View Post
    Now this seems very low. Is this with a reservoir ecap connected??
    no, the ecap is not connected yet as if I connect the 12-pin connector i would be engaging the power tubes as well. so this is purely what's coming out of the diodes without being smoothed by the ecap. this will add a bit of voltage, right?

    Quote Originally Posted by Helmholtz View Post
    I don't think it is possible to produce a negative voltage from a HT tap when using a bridge rectifier, so your proposal won't work.
    no, nooo... I wasn't suggested I could get negative voltage there, -50 V it's just the nominal according to the schematic. I am actually getting 0 V

    Quote Originally Posted by Helmholtz View Post
    I just saw that there is a BJT stage that requires a -32V supply. So while full cathode bias would provide a solution for the power stage, I think you need a different PT that allows for the -50V supply.
    then I am back to looking for a new Power Transformer?

    if that is the case, the HAMMOND 370BX seems to be the closest (601.4 VAC) being able to provide the 400 VDC with the original FULL WAVE RECTIFIER

    https://www.hammfg.com/files/parts/pdf/370BX.pdf

    however and for the BIAS VOLTAGE: all the Hammond transformers I've seen, they all deliver 50 VAC and I am failing to see how 50 VAC are going to become -50 VDC

    https://www.hammfg.com/electronics/t...rs/classic/300

    0 Not allowed! Not allowed!

  4. #74
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,205
    Thumbs Up/Down
    Received: 2,342/1
    Given: 1,335/2
    Rep Power
    8
    this will add a bit of voltage, right?
    Yes, considerably. My x1.4 formula only applies with smoothing cap and without load. Without smoothing cap many meters won't give a meaningful DC reading as the voltage is varying heavily.

    then I am back to looking for a new Power Transformer?
    I am afraid, yes.

    however and for the BIAS VOLTAGE: all the Hammond transformers I've seen, they all deliver 50 VAC and I am failing to see how 50 VAC are going to become -50 VDC
    Too high AC is no problem as DC level can be easily reduced by a voltage divider or (preferably) a 50V zener + series resistor.

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 02-11-2020 at 11:07 PM.
    - Own Opinions Only -

  5. #75
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    thank you, sir

    if you guys can take a look at the Hammond 370BX and agree with buying that one I will order it (because they range from 75 mA to 575 mA for HV and from 2.5A to 9A with the 6.3VAC)
    Of course I am open to other transformer suggestions.

    thx!

    PS: it doesn't matter I am full circle back to getting a new PT. I've learned a few things I did not know about the rectification process I am thankful for

    0 Not allowed! Not allowed!
    Last edited by TelRay; 02-11-2020 at 11:22 PM.

  6. #76
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    6,917
    Thumbs Up/Down
    Received: 1,684/2
    Given: 1,216/1
    Rep Power
    18
    Or, you could go with g1's suggestion in post #43 and build a cap coupled negative supply off the HV winding if you want to save the cost of a new transformer.

    Edit: Attaching the relevant section of an Ampeg V4. You'd need to adjust values for your particular application.

    Click image for larger version. 

Name:	Negative-Supply.jpg 
Views:	11 
Size:	180.3 KB 
ID:	56935

    1 Not allowed! Not allowed!
    Last edited by The Dude; 02-12-2020 at 12:12 AM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  7. #77
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    Quote Originally Posted by The Dude View Post
    ...build a cap coupled negative supply off the HV winding if you want to save the cost of a new transformer.
    Edit: Attaching the relevant section of an Ampeg V4. You'd need to adjust values for your particular application.
    Click image for larger version. 

Name:	Negative-Supply.jpg 
Views:	11 
Size:	180.3 KB 
ID:	56935
    yes! i was taking a look at that and trying to figure out how to calculate the CAP and R values in RED (I think I can still use the same 4007 Diode and the .047 uF cap is a standard)
    where is the "?" connected to, ground?



    thx!

    PS: those +400VDC will be achieved after going through the 40uF/450V cap (the value is there to represent the HV DC)

    0 Not allowed! Not allowed!
    Last edited by TelRay; 02-12-2020 at 12:59 AM.

  8. #78
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    6,917
    Thumbs Up/Down
    Received: 1,684/2
    Given: 1,216/1
    Rep Power
    18
    Yes. The "?" is ground.
    Yes. You can use 1N4007. Notice polarity for negative voltage.
    Cap values should be fine, although I might use higher values to provide more filtering for the BJT stage.
    You should need only to make resistor changes to R49 and R60.

    Also, you can simply pull the output tubes until you get voltages correct.

    2 Not allowed! Not allowed!
    Last edited by The Dude; 02-12-2020 at 02:04 AM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  9. #79
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,205
    Thumbs Up/Down
    Received: 2,342/1
    Given: 1,335/2
    Rep Power
    8
    Some good info contained here: http://www.valvewizard.co.uk/bias.html

    Might get a little tricky to get -50V from standard circuit values and lowish 400V HT. But with some resistor value adjustments to the amp -40V should suffice.

    1 Not allowed! Not allowed!
    - Own Opinions Only -

  10. #80
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    thank you so much guys, I like the spirit of "trying to use what we have and save some money"

    however, I believe I can get a new transformer for about $70, the specs are as follows:



    58 mA on the HV enough?

    0 Not allowed! Not allowed!

  11. #81
    Supporting Member
    Join Date
    Nov 2016
    Location
    Nova Scotia
    Posts
    226
    Thumbs Up/Down
    Received: 116/1
    Given: 298/1
    Rep Power
    4
    58 mA seems too little. This one https://www.hammfg.com/files/parts/pdf/370DX.pdf at 104 mA with the same voltages would be better, although maybe still a bit on the light side. 370EX might be a better choice at 144 mA https://www.hammfg.com/files/parts/pdf/370EX.pdf

    18 watt Marshall replacement from Hammond is rated at 140 mA as a point of reference.

    3 Not allowed! Not allowed!

  12. #82
    Professional Lurker eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    2,364
    Thumbs Up/Down
    Received: 522/0
    Given: 1,366/1
    Rep Power
    11
    this ^^^^^^

    I built an amp from scratch, 2x 6V6, with a 100ma PT. I learned the hard way that I needed more. Go with the 370EX if you're thinking about 2x 6V6 or equiv.

    1 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


  13. #83
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    thank you guys!

    FYI the PT we were trying to get to work (369BX) delivers 86 mA so I guess this just confirms that even if we got it to provide 400 VDC and -50 VDC it would have felt short on the current side, right?

    I am looking for Hammond because that's the one that came with amp and I know they have some good reputation. I am open to other manufacturers if you have a recommendation.

    Best,

    1 Not allowed! Not allowed!

  14. #84
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    new PT ordered, will report back next week

    0 Not allowed! Not allowed!

  15. #85
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    hello y'all, news from the front...

    - had the new power supply installed (after doing some mechanical engineering)
    - put the rectifier back to full wave
    - removed the 2x death capacitors from ANTI-HUM 1 and 2 (as the amp already has now a 3 prong mains connector properly grounded)



    - VOLTAGES as expected:
    * HV +395 VDC [NOMINAL 400 V]
    * BIAS -67 VDC [NOMINAL -50 V]
    - CURRENT DRAW (reading at variable isolation transformer): 0.4 A

    So... the amp is alive!



    There's not much indication of reference voltages in the schematics (and some of them are impossible to read), so I have measured the following ones:



    QUESTION #1: How can I achieve the required BIAS VOLTAGE?
    QUESTION #2: (for my understanding) what's the "damage" i am inflicting by running the amp at a 35% lower (or higher) BIAS VOLTAGE? Killing the power tubes life span? running them to cold? how safe is to keep testing the amp like this?

    At a certain point I connected the REVERB TANK and the overall sound (both WET and DRY) got distorted.

    I played like that for a while (5-10 minutes) until I've heard a loud "pop" coming from a blown capacitor (connected to PINs 1 and 6) on the 12AU7 tube driving the reverb.



    The blown capacitor was new and one of the electrolytics i have replaced. What I do not remember is if the original capacitor was polarized or not. On the schematic there's no indication of polarity, though. I would usually not replace a cap if i am not certain about polarity (but this was done a few weeks ago, so i don't remember).
    In any case, this is the polarity I have used:



    I have researched the Schematics for the Magnatone M10"A" and it shows a polarized cap with the opposite polarity I have used :/



    QUESTION #3: did I get the polarity wrong (is it not clear in the schematic)?

    0 Not allowed! Not allowed!
    Last edited by TelRay; 02-20-2020 at 08:24 PM.

  16. #86
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,205
    Thumbs Up/Down
    Received: 2,342/1
    Given: 1,335/2
    Rep Power
    8
    Yes, wrong polarity. Positive must go to the plates. (In case of doubt just varify voltage polarity with your meter with bad cap removed before connecting a new cap.)

    Bias voltage can be adjusted by lowering the 12k resistor to 8.2k. Increasing the value of 120k resistor will drop the BJT supply.

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 02-20-2020 at 08:18 PM.
    - Own Opinions Only -

  17. #87
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    thank you sir!

    0 Not allowed! Not allowed!

  18. #88
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    hello guys,

    mission successful! after playing with resistor values for a while I got the right VOLTAGES for the BIAS and the REVERB. Also put back the old (1uF 450V) CAPACITOR on the reverb tube and everything works



    now that all VOLTAGES are within spec I need to test this baby extensively to find all the other nuances I need to fix... but you knew that was coming

    thank you!!!

    PS1: I needed to go much higher than I expected in the resistor value for the BJT supply (120K to 450K)
    PS2: I do not remember who told me not to expect a dirty sound out of this amp... but at full volume it overdrives very graciously

    2 Not allowed! Not allowed!
    Last edited by TelRay; 02-21-2020 at 08:09 PM.

  19. #89
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    I am digging this 8" Speaker for recording
    Made this video for you guys
    (No external FX, no sound post processing apart from some EQ)


    3 Not allowed! Not allowed!

  20. #90
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,889
    Thumbs Up/Down
    Received: 1,785/21
    Given: 1,461/0
    Rep Power
    14
    Quote Originally Posted by TelRay View Post
    I am digging this 8" Speaker for recording
    Made this video for you guys
    Impressive chord tones, nice "growl" that seems to appear after the attack. So... what's the speaker?

    1 Not allowed! Not allowed!
    Enjoy. Every. Sandwich.

  21. #91
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    thank you, mister

    SPKR 8" OXFORD 80L5N-1 16 Ohm 14oz AlNiCo// 465-435 [WEEK 35 1964] // 25-1016

    however it’s missing the 3" McGREGOR Tweeter 25 1015 // 789434 9 [WEEK 34 1964] as it was blown

    1 Not allowed! Not allowed!

  22. #92
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,889
    Thumbs Up/Down
    Received: 1,785/21
    Given: 1,461/0
    Rep Power
    14
    Quote Originally Posted by TelRay View Post
    SPKR 8" OXFORD 80L5N-1 16 Ohm 14oz AlNiCo// 465-435 [WEEK 35 1964] // 25-1016

    however it’s missing the 3" McGREGOR Tweeter 25 1015 // 789434 9 [WEEK 34 1964] as it was blown
    Ah, an oldie-but-goodie. For some reason I thought you might have had a current production speaker there, something I could find "on the rack". No matter, and don't sweat the missing tweeter. It would just distract you with tinny racket from an overdriven amp. The Oxford delivers a gutsy sound, plenty impressive. With you playing thru it, of course!

    I have an M13 in the shop awaiting my attention. Way too many knobs!

    1 Not allowed! Not allowed!
    Enjoy. Every. Sandwich.

  23. #93
    Senior Member
    Join Date
    Dec 2007
    Posts
    968
    Thumbs Up/Down
    Received: 22/0
    Given: 2/0
    Rep Power
    13
    Quote Originally Posted by Leo_Gnardo View Post
    Ah, an oldie-but-goodie. For some reason I thought you might have had a current production speaker there, something I could find "on the rack". No matter, and don't sweat the missing tweeter. It would just distract you with tinny racket from an overdriven amp. The Oxford delivers a gutsy sound, plenty impressive. With you playing thru it, of course!

    I have an M13 in the shop awaiting my attention. Way too many knobs!
    On the M13, don’t be surprised if the schematic doesn’t match the amp exactly. On my M13A, I found significant changes, even though the amp appeared to be original from the factory. It had the schematic stapled inside the cabinet too.

    1 Not allowed! Not allowed!

  24. #94
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    Hi y'all,
    dropping a quick note to know (and wish) everyone is OK in these crazy times
    be safe!

    1 Not allowed! Not allowed!

  25. #95
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    Hi all,

    I have recently been watching some videos from Uncle Doug on youtube... and I like this guy, very instructive!

    I was specially interested in the detailed method description to calculate power dissipation on each output tube (measure resistance through the output transformer to the output tubes plate, measure voltage drop between the same points and plate voltage to ground to calculate the power dissipation)
    so I decided to try that on my amp.

    With the amp turned off and disconnected from the mains I measured the RESISTANCE from point “A” to each PIN 7 on the POWER TUBES (marked in RED in the schematic below)

    R1: 154.7 Ohm
    R2: 243.5 Ohm

    The issues appeared when I turned ON the amp and measured VOTAGE DROP across the same points, I could simply not achieve a STABLE VOLTAGE READING. Using the meter’s AVG MAX and MIN functions I was able to measure:

    Vdrop1: (1.9 +/- 1.0) V
    Vdrop2: (3.8 +/- 1.2) V

    The PLATE VOLTAGE (PIN 7 to GROUND is also changing by a range of 2 V)

    Vplate1: (384 +/- 2) V
    Vplate2: (382 +/- 2) V

    Trying to understand where the fluctuation was coming from I checked (measured 47 and 51 nF) and replaced by new ORANGE DROPS, the 2x 47nF Capacitors (marked in BLUE in the schematic below). NO CHANGE

    I discovered that the rate at which the VOLTAGE DROPS are fluctuating is related to the SPEED of the VIBRATO FUNCTION. However the variation exists with the EFFECT turned ON and OFF (I had to add an external FOOTSWITCH to turn it OFF).



    LINK to full SCHEMATIC: https://www.magnatoneamps.com/schema...natone_M10.pdf

    REMINDER: all electrolytic caps have been replaced with new ones

    Any ideas (sure you have some)?

    0 Not allowed! Not allowed!
    Last edited by TelRay; 04-17-2020 at 01:37 AM.

  26. #96
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,627
    Thumbs Up/Down
    Received: 2,179/24
    Given: 5,488/11
    Rep Power
    24
    I'd think the trem is not turning fully off. Try removing that last 12DW7 tube (phase inverter). See if you get a stable reading.

    2 Not allowed! Not allowed!
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  27. #97
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    good call, the readings are indeed stable with the PI 12DW7 pulled out of the circuit AND the tremolo OFF (unstable if it's turned ON)

    I get now:

    Vdrop1: 2.4 V
    Vdrop2: 2.9 V

    Vplate1: 378.5 V
    Vplate2: 377.2 V

    POWER 1 = 2.4 V / 154.7 Ohm * 378.5 V = 5.9 W
    POWER 2 = 2.9 v / 243.5 Ohm * 377.2 V = 4.5 W

    QUESTIONS:
    1) what do you think about the tubes being too cold for a maximum power dissipation of 13.2 W for these 7189As?
    2) what about the difference of 30% between the two tubes?
    3) is the "trem not turning fully off" a normal thing given the design of this circuit or something that needs to be fixed?

    thx!

    0 Not allowed! Not allowed!

  28. #98
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,627
    Thumbs Up/Down
    Received: 2,179/24
    Given: 5,488/11
    Rep Power
    24
    I'm not that familiar with the Magnatone's so not sure about the tremolo thing. Can you still hear it when it's turned off via footswitch while you are playing? If not, I don't think it's a problem.
    The difference between power tubes could create hum, if no hum issues then they should be fine. Same with the cold bias, no issue if it sounds good.
    If you wanted to tweak the idle current, you don't have adjustable bias so you would have to alter the bias circuit or decrease the cathode resistor.

    1 Not allowed! Not allowed!
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  29. #99
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    Quote Originally Posted by g1 View Post
    I'm not that familiar with the Magnatone's so not sure about the tremolo thing. Can you still hear it when it's turned off via footswitch while you are playing? If not, I don't think it's a problem.
    no sir, I cannot hear it at all. so I guess I'l leave it like that unless someone else jumps in


    Quote Originally Posted by g1 View Post
    The difference between power tubes could create hum, if no hum issues then they should be fine.
    yepp, that's exactly why I started looking into this matter, there is some 60 Hz humming with nothing connected and not affected by volume or tone controls

    Quote Originally Posted by g1 View Post
    Same with the cold bias, no issue if it sounds good.
    If you wanted to tweak the idle current, you don't have adjustable bias so you would have to alter the bias circuit or decrease the cathode resistor.
    in that case what makes sense to do first? I assume changing the cathode resistor value to increase POWER and then try balancing the TUBES
    Guidance on how to do both will be appreciated

    For what I've seen the 7189A are not widely available so I might not be really super interested in pushing them much. The question is that maybe running them at less than 50% is too low (unless the Magnatone guys had the power section over engineered on purpose)


    thx!

    0 Not allowed! Not allowed!
    Last edited by TelRay; 04-17-2020 at 03:10 AM.

  30. #100
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,205
    Thumbs Up/Down
    Received: 2,342/1
    Given: 1,335/2
    Rep Power
    8
    IIRC, the amp has both fixed and cathode bias. To increase idle current lower the absolute value of grid bias by increasing the 22k resistor that connects to the bias filter cap.

    there is some 60 Hz humming with nothing connected and not affected by volume or tone controls
    Unbalanced output tubes will introduce 120Hz ripple (not 60Hz) hum.

    To allow for balancing you may use 2 separate cathode resistors of around 200R, but one lower than the other as required.
    Or you may use a 50R resistor to ground + a 200R/2W balancing pot on top, wiper connected to 50R, outer pot lugs to the cathodes.

    2 Not allowed! Not allowed!
    - Own Opinions Only -

  31. #101
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0

    Thumbs up

    thank you very much!

    Quote Originally Posted by Helmholtz View Post
    IIRC, the amp has both fixed and cathode bias. To increase idle current lower the absolute value of grid bias by increasing the 22k resistor that connects to the bias filter cap.
    to re-cap, it's been written that the amp has a "partial cathode bias" (post#51) or "hybrid bias" (post #69), I understand that "both fixed and cathode bias"... they all mean the same, right?
    the bias circuit is currently as the original schematic

    as a reminder, the POWER TRANSFORMER was supplying a -67 V bias voltage that we had to bring down to the nominal -17V by adjusting the 12K R and to -32 V by tweaking the 120 K R.



    now we are going to modify the 22K resistor to increase the BIAS idle current (this one inside the red rectangle)



    (pardon the maybe basic question) is this change (22K R) going to only modify the CURRENT (mA) or also change the VOLTAGE at point "F". If so, should then I play again with the value of the (old value) 12K R to maintain -17V at point "F"?


    Quote Originally Posted by Helmholtz View Post
    Unbalanced output tubes will introduce 120Hz ripple (not 60Hz) hum.
    To allow for balancing you may use 2 separate cathode resistors of around 200R, but one lower than the other as required.
    Or you may use a 50R resistor to ground + a 200R/2W balancing pot on top, wiper connected to 50R, outer pot lugs to the cathodes.
    like this?




    thx!!!

    0 Not allowed! Not allowed!

  32. #102
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,205
    Thumbs Up/Down
    Received: 2,342/1
    Given: 1,335/2
    Rep Power
    8
    it's been written that the amp has a "partial cathode bias" (post#51) or "hybrid bias" (post #69), I understand that "both fixed and cathode bias"... they all mean the same, right?
    Yes, it's a combination of fixed and cathode bias.

    is this change (22K R) going to only modify the CURRENT (mA) or also change the VOLTAGE at point "F". If so, should then I play again with the value of the (old value) 12K R to maintain -17V at point "F"?
    To increase idle current (and thus plate dissipation), you need a smaller negative voltage (e.g. -15V instead -17V) at point "F". (Actually -15V would be a higher = more positive voltage than -17V, that's why I wrote "lower absolute value of grid bias" above). This can be achieved by increasing the value of the 22k resistor.

    The picture with the 2 separate cathode resistors shows what I meant. The tube with the lower current will require a lower cathode resistor for balance. This can be achieved by paralleling an additional resistor. But the balancing pot would be a more elegant and versatile solution.

    Do you have 60Hz or 120Hz hum?

    2 Not allowed! Not allowed!
    - Own Opinions Only -

  33. #103
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    Quote Originally Posted by Helmholtz View Post
    Yes, it's a combination of fixed and cathode bias.
    verstandeden!

    Quote Originally Posted by Helmholtz View Post
    To increase idle current (and thus plate dissipation), you need a smaller negative voltage (e.g. -15V instead -17V) at point "F". (Actually -15V would be a higher = more positive voltage than -17V, that's why I wrote "lower absolute value of grid bias" above). This can be achieved by increasing the value of the 22k resistor.
    very clear

    Quote Originally Posted by Helmholtz View Post
    The picture with the 2 separate cathode resistors shows what I meant. The tube with the lower current will require a lower cathode resistor for balance. This can be achieved by paralleling an additional resistor. But the balancing pot would be a more elegant and versatile solution.
    I like elegant

    Quote Originally Posted by Helmholtz View Post
    you have 60Hz or 120Hz hum?
    good question... I think I have both



    my reasoning is that it is 60 Hz, yes. Because I see 60, 120, 180, 240, etc
    But... if it was purely 60 Hz then the 120 should be lower in intensity, right? additionally the harmonics from the 120 Hz Hum re inforce the ones of the 60 Hz and I see re-inforced peaks at 240 Hz, etc
    so i believe there's also some 120 Hz which should be coming from the unbalanced tubes.
    do you agree?

    0 Not allowed! Not allowed!

  34. #104
    Member
    Join Date
    Feb 2019
    Location
    San Francisco, CA
    Posts
    268
    Thumbs Up/Down
    Received: 75/0
    Given: 341/0
    Rep Power
    0
    hi all, hope you guys are still healthy and safe

    i've been investigating the issue a bit more and found out through a suggestion from a friend that i have not checked the 270 K resistors. oh, surprise, one of them was not great at 347 K (+29%) and the other was marginally OK at 297 K (+10%). So, both replaced by nice 1% tolerance resistors.

    Also double checked the 1 K and 1 Ohm resistors, all OK

    Re-checked POWER DISSIPATION and got:

    POWER 1 = 6.4 W
    POWER 2 = 4.9 W


    still too cold and 30% unbalanced

    changed the 22 K to a 34 K R following previous suggestions and got a bias of -13.0 V

    POWER 1 = 9.4 W
    POWER 2 = 7.7 W


    I want to be a bit more conservative than the 9.4 W with this tubes as I've seen they are not easy (or cheap) to find and. the unbalance was better at 22%

    at that point I decided to swap the tubes (7189A tube 1 went to socket #2 and viceversa)

    POWER 1 = 7.5 W
    POWER 2 = 8.5 W


    still marginally hot (for my conservative approach) but the balance was at 13%

    changed to a 30.7 K resistor and -13.8 bias

    POWER 1 = 6.9 W
    POWER 2 = 7.7 W


    Now the difference is at around 12% and I think running the tubes at about 60% of max power will give them many years of happiness

    After that I connected a decal box and tried increasing resistance and power to hear how much "punch" i was missing because of this conservative approach and have not really noticed a difference.

    2 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. Replies: 4
    Last Post: 02-16-2017, 06:18 PM
  2. Diy Magnatone
    By lowell in forum Build Your Amp
    Replies: 5
    Last Post: 08-08-2015, 07:15 AM
  3. Magnatone M-30
    By caverat in forum Schematic Requests
    Replies: 0
    Last Post: 10-04-2013, 04:12 PM
  4. Magnatone MP1 help
    By davohilts in forum Maintenance, Troubleshooting & Repair
    Replies: 11
    Last Post: 10-28-2010, 05:09 AM
  5. Magnatone Maesrtro 150
    By unclebubba in forum Schematic Requests
    Replies: 0
    Last Post: 03-23-2010, 08:08 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
  •