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Thread: 25 watt valve pa bass cut question

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    25 watt valve pa bass cut question

    Hey there this is my first post. I'm in the process of restoring a valve pa and then altering it for guitar. I'm trying to determine the value of a blown resistor. It is part of a circuit that connects to a bass cut switch at the back of the head. From the second stage cathode of a 12ax7 the circuit goes through a 470k resistor then when the switch is connected in parallel through the blown resistor and a .047 uf capacitor. This then connects to a 20uf cap and then to the heater (pin2) of the two 6l6 valves. I'm replacing the 20uf electro but I'm wondering what the resistor value may be and two if it would be possible to replace the switch with a pot. Any insight to the circuit valued. Thanks

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    Lifetime Member Enzo's Avatar
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    And the brand and model of PA is... ?

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    It's an Australian brand called awa.

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    The model is pa1003z. I can't find a schematic anywhere

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    Noodle of Reality Steve Conner's Avatar
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    Is the resistor burnt so bad that you can't read the value? What physical size and shape is it? Can you post a picture of the burnt item and surrounding guts? I think it's most likely to be a power supply dropper resistor that was taken out by an electrolytic cap going bad.

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    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    picture

    the first stripe on the resistor is blue.
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    Noodle of Reality Steve Conner's Avatar
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    Have you got a bigger version of that picture? I can't really see much of anything.

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    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Sorry about that. This one should be a better size.
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    Noodle of Reality Steve Conner's Avatar
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    Hmm, OK, it looks like the bass cut switch is part of the negative feedback around the output stage, kind of like a presence control, and that resistor is part of the bass cut thing. Still no idea what value it is though, sorry but maybe someone else will.

    It looks like a Williamson type design though, which is more of a hi-fi amp. You're probably going to want to rewire the whole thing to a Marshall or Fender circuit, or whatever, so I wouldn't worry too much.

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    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Thanks. Since this part of the circuit is connected to a switch I've left it off for now. I've replaced the electro caps and a few plate resistors and we are beginning to rock.
    I have a 68 k resistor on the input. It's plenty loud enough for now. I've noticed that when I have an over drive on, if I tap the metal telecaster knobs I get earthing buzz. Do I have an earthing problem?
    Also if I was going to overdrive the second stage could I replace the 220k resistor that feeds into its grid with a 1 meg pot? (I'm new to this, I've made pedals for a while, restored some valve radios and I'm trying to get an angle on valve amps etc.) Thanks again

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    Quote Originally Posted by Steve Conner View Post
    It looks like a Williamson type design though, which is more of a hi-fi amp. You're probably going to want to rewire the whole thing to a Marshall or Fender circuit, or whatever, so I wouldn't worry too much.
    I thought the Marshall Major as well as some Gibson amps wrere based on the Williamson circuit?

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    I've fixed the first problem (bad connection on the input. Can someone point me in the right direction for the second one (how to overdrive the second stage) Much appreciated. Thanks

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    Noodle of Reality Steve Conner's Avatar
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    Did you try it with a Tube Screamer or similar drive pedal? That will overdrive the second stage sure enough

    If you want the answer that I think you wanted, then you probably need to add an extra gain stage. It usually takes about three tube gain stages to get things really grinding, when you consider losses in tone stacks and such like. If you want to wail like Van Halen, you probably need four stages.

    If it's a multi-channel amp, you may be able to steal a triode from one of the other channels. We can't really help with that unless you trace the schematic and post it.

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    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Thanks, I've built quite a few overdrives and fuzz's and luv them all. I've experimented with the different ways to saturate a transistor and was trying to apply it to a valve. I'm going to trace the schematic. I suppose I was thinking about a pedal I built which was a three stage jfet based on the marshall 18. Three jfets and a subtle overdrive. My amp has two 12ax7 tubes that feed into two 6l6 tubes. There is a volume inbetween the first side of the first 12ax7 and the second side. I thought maybe adding a higher gain in there may have broken up the sound a little (not like van halen, just a simple blues thing). At the moment it has a very clean sound which is cool also, I'm just trying to get a feel for this stuff. Thanks again.
    ps Hey one of my favourite guitarists the late great Robert Quine used a Tube screamer.

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    awa valve pa schematic

    This has been a steep learning curve for me. After some reading I set about drawing up a schematic for my 25 watt valve pa. I documented it without the added phono/radio input.( these give me 4 extra holes for pots and a place for a standby). I like the sound of the amp as it is but want to enhance it. I figure I'll add a fender style treble, middle and bass tone stack and a post volume pot. (maybe a brite capacitor also). As for placement, from what I've read the best place for the tonestack would be on the pink line between the second and third 12ax7 stage and the best place for the post volume would be either after the tone pot or a dual pot on the green lines between the fourth 12ax7 and the 7025 grid. Any advice on this is greatly appreciated. (eg is this going to create too much gain?)
    Also if anyone has advice on the blown resistor value I'd love to hear, I know the first line was blue so it's a 6 something. I figured a 600k but more likely a 6k.
    Finally I have an extra space for another valve (eg 12ax7) which I may put in later down the line (good idea?) I've drawn the schematic to the best of my abilty, I think it's pretty much error free
    (*third version seems to be getting closer!). Thanks again
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    Last edited by 9 volts; 02-28-2007 at 06:41 AM. Reason: updated schematic. Third version Thanks for the tips!

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Right off the bat I really need a better picture of that resistor that is blown/broken in half and it looks like there could be some omssions or errors on your schem.
    It is possible to have B+ on the grid of the second triode but how is the cathode grounded for bias reference?
    Is the second triode using another 220K anode resistor, which is written in but shown as a cap?
    If there is just a tiny amount of NFB delivered to the cathode of triode #2 I guess a very high value is OK but I am not so sure that 470K is really a 470K on the cathode of the second 12AX7 triode.
    I don't see any way to bias the triode unless it is fixed bias brought out by the plate supply, to the grid, forming a voltage divider on the grid... which again looks to be the voume control.... which would be painfully noisy and not very effect as it would change the bias point and the volume!! Not right.
    It looks like a simple voltage divider for the negative feeback where the cap bypasses the NFB voltage at specific frequencies to ground, around the cap through the switch.
    This is kind of what a presence control would do when bypassing NFB to ground around the 4k7 or 5K pot in that control.

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    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

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    fixed schematic

    Sorry the 220k resistor is fixed. Two separate resistors to plates of the first two triodes.
    I've added a coupling cap before the volume in the schematic...whoops. (the amp actually sounds quite good so the errors will be with the schem). I'm currently rechecking again. Awa were based on phillips I think.

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    Last edited by 9 volts; 02-28-2007 at 05:46 AM.

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    I'm sure there a lot of folks who know better than me but I'll offer my two cents here. Resistors commonly come in decimal multiples of 6.2 and 6.8. If you get that portion of the circuit straightened out it might be possible to deduce what that resistor should be, but there are quite a few things with the schematic that don't look right to me.

    First, the 2.2M input and 1.8k cathode resistor are certainly grounded. Looks like probably via the ring of the in jack. That junction of the to 8uF cap -'s and the two 1k cathode resistors are probably just all grounds. That would make the 1.5k and 2ohm the Cathode bias resistors for the power tubes and the 20uF the cathode bypass cap. Speaking of which, having two resistors in series like that seems highly unusual to me.

    The junction of the two 220ks at the grids of the power tubes is probably a ground and perhaps the junction of the 2.2k and 2.2M between stage 2&3 is too? It's hard to say because Stages 3&4 look to be the phase inverter but it's not right the way it is. I don't get what that 56k and 10uf in series is (is the 10uf a cathode bypass in the P.I.?) and the feedback, which the bass cut looks to be a part of, often goes to the phase inverter. I don't think that junction between the .1uF and the 470k right at the end of your turquoise line should be there. I would look for where you might be missing points of the circuit that are grounded to the chasis.

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    awa schematic

    Thanks for the tips.
    Ptron, you were pretty much right in your observations.I've corrected the mistakes in the schematic and have also included photos of the 1.5k to 2ohm resistors (second photo). I've also uploaded a detailed photo of the bass cut circuit. It's covered in two photos. The first shows where the 470k connects to the cathode (first photo) and the second (third photo) shows where actual bass switch etc. ). Anyway... thanks again.
    Ps- The 10 uf cap is non polar.
    I'm working on a 'version four' schematic which is a little easier to read. Will post when completed.
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    Last edited by 9 volts; 02-28-2007 at 01:15 PM.

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    im looking at mine and that resisitor is blue red brown gold, so i guess that makes it 6.2K? with 5% tolerance, im following your project, today i put a new power cord on some sylvania 6l6's and some nos 12ax7s, ran my cd player into the phono and i have sound yippee! i removed all teh radio garbage from teh circuit. keep going with this man its been great.

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    extra info

    This site is excellent, it has a little bit of info on these awa/phillips/stc PA amps
    and lots of information in general on austrlian guitar amps
    http://www.ozvalveamps.elands.com/awa.htm

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