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Thread: Changing Plate resistor question.

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    Changing Plate resistor question.

    Hi,
    I've picked up a chinese amp as a project. Looks like it may be losely based on a Fender Bassman.

    I wish to give it a bit more drive and I understand that I can change the plate resistor on the first 12AX7. It's currently at 100K, I'm looking to push it to 220K in the first instance.

    Just to confirm, I am supposed to be changing the resistor that is connectoed to Pin 1 on the tube and not pin 6. Can't seem to get clarification on this with any of the tube documentation I've have read.

    Thanks.

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    Probably but it depends on how the amp has been laid out. If the input jacks were in pin 2's circuit it would be a good indicator.
    Pete

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    Supporting Member Jazz P Bass's Avatar
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    Pin 1 & pin 6 are both plate pins (12AX_ type tube).

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    Ah, I see. Yes the input is via pin 2. Many thanks.

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    Thanks Jazz.

    So would I change the value of resistors on both plates? ie a value of 220K for both 1 & 6?

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    Last edited by Shawnobi; 11-28-2013 at 03:49 PM.

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    Supporting Member Jazz P Bass's Avatar
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    Without some kind of documentation on the circuit involved, I wouldn't recommend anything.

    How about drawing the first tube circuit.
    It doesn't have to be fancy.
    Plate resistors, cathode resistors, coupling caps for a start.

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    Good idea! I believe you've seen my other post noting the lack of schematics. I'll get on it. Many thanks.

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    First draft. Its PCB so a bit of a bitch!
    Click image for larger version. 

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    Changing the plate resistor from 100k to 220k will only increase the gain a little (about 20%) but it also changes the bias which may result in the bottom peaks of the signal being clipped. You should also change the cathode resistor to correct the bias.

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    Iíve just seen your schematic and itís biased way cold (probably to clip off the tops of the peaks for distortion. Do you want that?) As thatís the case R8 could be changed to 220k for a bit more gain and a more normal bias without changing the cathode resistor (assuming the cathode RC is grounded).

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    Don't forget the joker g1's Avatar
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    Are R1,R2, R67 going to input jacks? What is the value of R4?
    I suppose it is also possible that this is not the "first" 12AX7. The reference to "first" is in terms of the circuitry, not the physical location.
    So what you need to determine is which is the first grid (pin 2 or 7) that the input jacks are routed to. Then you need to draw the circuit around that particular grid.

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    Quote Originally Posted by Dave H View Post
    I’ve just seen your schematic and it’s biased way cold (probably to clip off the tops of the peaks for distortion. Do you want that?) As that’s the case R8 could be changed to 220k for a bit more gain and a more normal bias without changing the cathode resistor (assuming the cathode RC is grounded).
    Yes I'm basically looking to give it a bit more gain and distortion.

    I'm a little confused after some early morning checks. It seems the High & Low inputs come in via R2 & R67. I'd kind of presumed that half of the triode dealt with the high channel and the other half dealt with the Low. So affecting the plate resistor on Pin one would only affect 1 channel.

    Looking at it again. The cathode resistors (R4 & R10) are 1.5K. (Very fenderlike). It seems more common practice to change the these from what I am reading. So I think I will start by changing these to 820 Ohm values (More Marshall like).

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    Last edited by Shawnobi; 11-29-2013 at 09:48 AM.

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    Does your chinese amp have a name? Maybe there's already a schematic out there we can find for you.
    Why do you say it's loosely based on a Bassman? Which Bassman? The first Marshalls were themselves based on the late 50's Bassman, with 820Ω cathode resistors, so 820Ω cathode resistors are not brand specific, in my experience.

    If you do try the higher value plate resistor, you may, in addition to the biasing issue, find that you'll increase the noise substantially, and the higher impedance may or may not upset the operation of the next stage.

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    Quote Originally Posted by dwmorrin View Post
    Does your chinese amp have a name? Maybe there's already a schematic out there we can find for you.
    Why do you say it's loosely based on a Bassman? Which Bassman? The first Marshalls were themselves based on the late 50's Bassman, with 820Ω cathode resistors, so 820Ω cathode resistors are not brand specific, in my experience.

    If you do try the higher value plate resistor, you may, in addition to the biasing issue, find that you'll increase the noise substantially, and the higher impedance may or may not upset the operation of the next stage.
    The amp is a Subzero H50R. Also known as the Belcat H50R or the Strauss SVT-H50R. The reason I believe it is developed from the Bassman is that it is a 50W 3x12AX7 & 2x6L6 configuration. Also the use of Cathode resistor value of 1.5K with a 0.47uF bypass seems very Fender like.

    I've changed the cathode resistor value (coming in on Pin 8 of the 12ax7 - Input feed) to 820 ohms from the 1.5K. It's better but still too clean when driven. So 2 questions.
    1. Should I also change the cathode resistor on the other side of the 12ax7 (Pin 3) which goes off to the gain knob to 820 ohm also to achieve more 'crunch'?
    2. Should I change the cathode bypass capacitor on the input side to something larger (250uF for example) for more 'crunch'.

    Crunchy thanks!

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    Quote Originally Posted by Shawnobi View Post
    I've changed the cathode resistor value (coming in on Pin 8 of the 12ax7 - Input feed) to 820 ohms from the 1.5K. It's better but still too clean when driven. So 2 questions.
    1. Should I also change the cathode resistor on the other side of the 12ax7 (Pin 3) which goes off to the gain knob to 820 ohm also to achieve more 'crunch'?
    2. Should I change the cathode bypass capacitor on the input side to something larger (250uF for example) for more 'crunch'.
    1. You could end up with less crunch by doing that. The low frequency gain will be reduced because of the 820R and 0.47u bypass capacitor.

    2. No, 47u is already more than enough with a either a 1k5 or 820R resistor.

    I think the 5F6A and early Marshal first cathode resistor is 820 ohms because it is common to both 12AX7 triodes so the effective value is 1.64k for each triode and I think they used 820 ohms for the next triode to make sure the plate voltage was below 180V so that the heater to cathode voltage of the directly coupled cathode follower was below its 180V max rating. I’m just saying that 820 ohms isn’t a ’magic’ value for crunch.

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    Thanks Dave.
    So I guess I'm back to wondering how I make this thing overdrive a bit more when I wind the gain knob up....

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    Supporting Member Jazz P Bass's Avatar
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    How about a pedal out in front?

    A simple 'boost' pedal should do it.

    Or a Tube Screamer.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Shawnobi View Post
    Thanks Dave.
    So I guess I'm back to wondering how I make this thing overdrive a bit more when I wind the gain knob up....
    With a complement of 3 each 12AX7s, I'm guessing that your amp uses 1 for the PI, and the other two are used for gain functions - but that may not be the case. Without knowing exactly what the signal flow through the amp looks like, we're all blind men poking at an elephant. Changing a resistor can make drastic (that is to say, noticeably audible) differences in an amp's gain, but it has to be the *right* resistor in the *right* location. As has been posted above, the resistors at the plate or cathode of stage 1 or 2 can have an effect on the sound, but it won't take an amp from clean to crunchy, and may not even be the effect you're looking for.

    A drawing that shows the signal flow through the preamp tubes, with the plate and cathode component values, AND any interstage divider component values, AND the tone stack, will go a long way to bringing the issue you propose here into the light.

    Many amps will have a "boost" function that is nothing more than a component that bypasses some or all of the tone stack, allowing a substantial increase in gain. The best part of that is it can be put on a switch!

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    I'm tempted to build a tubecaster. My problem is that I'm a compulsive tinkerer. I like the idea of tuning the amp rather than adding additional hardware. Also modifying will help my understanding of how amps work.

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    ToneOholic! big_teee's Avatar
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    Quote Originally Posted by Shawnobi View Post
    I'm tempted to build a tubecaster. My problem is that I'm a compulsive tinkerer. I like the idea of tuning the amp rather than adding additional hardware. Also modifying will help my understanding of how amps work.
    I have a prototype head amp that is not in a case, that I experiment with.
    If you are going to change the preamp components often, I would use a Turret board.
    They are the easiest to change resistors and caps on.
    I'm changing and adding Cathode bypass Caps all the time when I'm looking for a particular sound.
    Good Luck,
    T

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    Quote Originally Posted by eschertron View Post
    With a complement of 3 each 12AX7s, I'm guessing that your amp uses 1 for the PI, and the other two are used for gain functions - but that may not be the case. Without knowing exactly what the signal flow through the amp looks like, we're all blind men poking at an elephant. Changing a resistor can make drastic (that is to say, noticeably audible) differences in an amp's gain, but it has to be the *right* resistor in the *right* location. As has been posted above, the resistors at the plate or cathode of stage 1 or 2 can have an effect on the sound, but it won't take an amp from clean to crunchy, and may not even be the effect you're looking for.

    A drawing that shows the signal flow through the preamp tubes, with the plate and cathode component values, AND any interstage divider component values, AND the tone stack, will go a long way to bringing the issue you propose here into the light.

    Many amps will have a "boost" function that is nothing more than a component that bypasses some or all of the tone stack, allowing a substantial increase in gain. The best part of that is it can be put on a switch!
    I'd love a scheme for this thing! It's quite hard to draw my own from the PCB, things get quite confusing. I understand what you are saying though, I'm just a typical Mod-Newbie wanting the moon on a stick It does have a boost switch. I should look into the circuit that is on.

    Big-Teee - Not a bad idea. I have a fear of track lifting on the PCB if I start pulling components in and out quite often.

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    For reference - here she is with a few 'beautification' mods!
    Click image for larger version. 

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    I've taken the liberty of redrawing your partial schematic here.

    I'd love a scheme for this thing! It's quite hard to draw my own from the PCB, things get quite confusing.
    When you get back in there, there should be a capacitor connected to the first plate (pin 6) and R3. I've labelled where that should go "A" on my schematic.

    Try to get as far as connecting point "A" to point "B" and we should have much better info for you.

    While you're at it, jot down the voltages at the tube terminals too.
    Attached Files Attached Files

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    That's really kind of you, thank you. I should have chance to open it up tomorrow evening with a bit of luck.

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    Quote Originally Posted by Shawnobi View Post
    For reference - here she is with a few 'beautification' mods!
    Click image for larger version. 

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    regardless, she looks like a beauty!

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    Here we go. FYI C3 is a 100pF Ceramic disc capacitor.


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    Kingsway Guitar - Thank you

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    Last edited by Shawnobi; 12-03-2013 at 08:57 AM.

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    Here's the schematic for the Strauss SVT-H50R. See if it matches your subzero.
    Attached Files Attached Files

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    Quote Originally Posted by dwmorrin View Post
    Here's the schematic for the Strauss SVT-H50R. See if it matches your subzero.
    Amazing! I tried Strauss but never got a reply. That certainly looks the same. Its even stamped Belcat which is the other brand its sold under. Thank you.

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    There's an unused 12AX7 gain stage! Tinkerer's heaven!

    'I have a fear of track lifting on the PCB if I start pulling components in and out quite often'

    Consider fitting small turrets/solder pins into the pcb holes, once components are removed.
    Then the trial replacement parts can be easily fitted and swapped out, all done on the component side of the board, with minimal stress to the pad / track.
    Pete

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    Quote Originally Posted by pdf64 View Post
    There's an unused 12AX7 gain stage! Tinkerer's heaven!

    'I have a fear of track lifting on the PCB if I start pulling components in and out quite often'

    Consider fitting small turrets/solder pins into the pcb holes, once components are removed.
    Then the trial replacement parts can be easily fitted and swapped out, all done on the component side of the board, with minimal stress to the pad / track.
    Pete
    I've got some breadboard type turrets at work.

    I noticed that V2B seemed to be empty. So how do I best rope that gain stage in to my preamp circuit? That could be exactly what I'm looking for!

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    Quote Originally Posted by dwmorrin View Post
    Here's the schematic for the Strauss SVT-H50R. See if it matches your subzero.
    Hey everyone, just a quick note. The Subzero branded H50R has an 8R output and, from the schematic, the Strauss has 16R output. Resistor 27 (R27) is actually 56k rather than 75k as shown.
    I suppose there may be other differences, but I'm yet to really dig into this amp.

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    Well, you may find minor differences, but main ones are visual ones (Tolex, grill cloth, etc.) and of course the silkscreened label.

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