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Thread: Deluxe Reverb problem

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    Deluxe Reverb problem

    Hi, I've built a deluxe reverb clone and have hit a snag. My screen and plate voltages on the 6v6s are both only 153v. I get a zero reading on pin 8 (through 1 ohm resistors), both mA and mV. I measured and set the negative bias voltage at -35. It's not a tube problem- already swapped another pair with same results.

    B+ is 440, then the second node drops all the way down to the 153v. The remaining power caps measure about as they should.

    Obviously there's no output, unless I dime channel 1 and get faint crackling notes. Vibrato channel is silent all the way up.

    Does this seem like a power cap or maybe transformer problem? I've checked the wiring and solder joints thoroughly.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by tyler8611 View Post
    Does this seem like a power cap or maybe transformer problem?
    Neither. There's a choke between the main B+ and the second power supply node that feeds the 6V6's screen grids. There would have to be something awful wrong with that choke for the screen grid voltage to drop all the way to 153V. Let's measure the resistance between those choke leads. Power OFF of course.

    Also possible the hi voltage nodes are wired in an out-of-order way.

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    Supporting Member The Dude's Avatar
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    I'm confused. You say, "B+ is 440". Then you say, "My screen and plate voltages on the 6v6s are both only 153v" The output tube plate node (B+) is before the choke and goes through the OT directly to the plates. You almost have to have something wired wrong.

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    Quote Originally Posted by Leo_Gnardo View Post
    Neither. There's a choke between the main B+ and the second power supply node that feeds the 6V6's screen grids. There would have to be something awful wrong with that choke for the screen grid voltage to drop all the way to 153V. Let's measure the resistance between those choke leads. Power OFF of course.

    Also possible the hi voltage nodes are wired in an out-of-order way.
    Ok I'll do that and report back. Also will check the doghouse again.

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    Quote Originally Posted by The Dude View Post
    I'm confused. You say, "B+ is 440". Then you say, "My screen and plate voltages on the 6v6s are both only 153v" The output tube plate node (B+) is before the choke and goes through the OT directly to the plates. You almost have to have something wired wrong.

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    I meant the voltage from pin 8 of the rectifier to the first power cap node. That is 440 and the second node feeding the screens is 153. The OT plate voltage is also 153.

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    Quote Originally Posted by Leo_Gnardo View Post
    Neither. There's a choke between the main B+ and the second power supply node that feeds the 6V6's screen grids. There would have to be something awful wrong with that choke for the screen grid voltage to drop all the way to 153V. Let's measure the resistance between those choke leads. Power OFF of course.

    Also possible the hi voltage nodes are wired in an out-of-order way.
    Between the leads is 154 ohms.

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    Supporting Member The Dude's Avatar
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    Yes. I understand. If you look at "pin 8 of the rectifier to the first power cap node", follow it up through the OT. It connects directly to the tube plates. If you measure 440VDC there, you should also measure it on the plates, unless you have a horribly bad, hot, melted standby switch. Measure DC on both sides of the standby switch. Is it the same on both with the switch closed?

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    Quote Originally Posted by tyler8611 View Post
    I meant the voltage from pin 8 of the rectifier to the first power cap node. That is 440 and the second node feeding the screens is 153. The OT plate voltage is also 153.
    Voltages to ground of course.

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    Quote Originally Posted by The Dude View Post
    Yes. I understand. If you look at "pin 8 of the rectifier to the first power cap node", follow it up through the OT. It connects directly to the tube plates. If you measure 440VDC there, you should also measure it on the plates, unless you have a horribly bad, hot, melted standby switch. Measure DC on both sides of the standby switch. Is it the same on both with the switch closed?
    True and everything is wired correctly. Weird problem.

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    Quote Originally Posted by tyler8611 View Post
    True and everything is wired correctly. Weird problem.
    True that. It's very weird to have 440V at one end of that choke and 153V at the other. If that's really the case it should be shooting flames. I'm still holding out for a mis-wiring. If you have an electronically knowledgeable friend have them give it a look-see. Sometimes a different set of eyes can pick out what's amiss.

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    Supporting Member The Dude's Avatar
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    IMO, the problem is before the choke. According to post #1 the plate voltage is also low. The plate voltage is derived "pre-choke". I'd like to know if the same DCV exists on both sides of the standby switch. Then, if we're good there, we see what the voltage is on the OT primary CT. It should be easy to follow the voltage and see where it's not. The B+ caps should be the same electrical point as the plates as far as DC is concerned, yet we have 2 different voltage readings, so I agree- something is not connected properly.

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    Don't know if this is your problem, but I replaced the filter caps in the dog house on my 78 SFDR.

    When I powered up, something was not right, I never measured any voltages and nothing smoked... but I knew something was "off".

    Powered down and checked my work... one of the cap leads I soldered into the eyelet was a bit long, and it missed the insulating card and shorted to the chassis.

    Maybe have a look in that area.

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    Quote Originally Posted by The Dude View Post
    IMO, the problem is before the choke. According to post #1 the plate voltage is also low. The plate voltage is derived "pre-choke". I'd like to know if the same DCV exists on both sides of the standby switch. Then, if we're good there, we see what the voltage is on the OT primary CT. It should be easy to follow the voltage and see where it's not. The B+ caps should be the same electrical point as the plates as far as DC is concerned, yet we have 2 different voltage readings, so I agree- something is not connected properly.
    I forgot to mention that I discovered earlier that on standby, the voltage readings were actually normal. Drop happens when I take it off standby. The switch is wired correctly with the leads going exactly where they should. Very strange.

    I also measured the resistance of the transformer and choke leads and they were fine. I drained the filter caps and measured those and the readings began around 90k and kept creeping upwards past 150k, so I'm not sure whether that's normal.

    I ran a flashlight under the filter cap board- no shorts. I'll have to keep searching.

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    Quote Originally Posted by galaxiex View Post
    Don't know if this is your problem, but I replaced the filter caps in the dog house on my 78 SFDR.

    When I powered up, something was not right, I never measured any voltages and nothing smoked... but I knew something was "off".

    Powered down and checked my work... one of the cap leads I soldered into the eyelet was a bit long, and it missed the insulating card and shorted to the chassis.

    Maybe have a look in that area.
    I thought that could be a possibility also so I looked under with a flashlight and saw nothing grounding out. It's a garolite board on standoffs so I was able to see.

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    Quote Originally Posted by tyler8611 View Post
    I forgot to mention that I discovered earlier that on standby, the voltage readings were actually normal. Drop happens when I take it off standby. The switch is wired correctly with the leads going exactly where they should. Very strange.
    Are you saying that, when you have the standby switch closed, your 440V B+ drops at the filter caps?

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    Quote Originally Posted by The Dude View Post
    Are you saying that, when you have the standby switch closed, your 440V B+ drops at the filter caps?
    The 440 B+ doesn't drop, but the second power cap node, 6v6 screens and plates drop down from normal range, while on standby, to abnormally low when switched from standby to play.

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    Again, please measure the voltage on both sides of the standby switch (switch closed) and tell us what you have. Then, measure the voltage on the center tap of the primary of the output transformer and report.

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    Quote Originally Posted by The Dude View Post
    Again, please measure the voltage on both sides of the standby switch (switch closed) and tell us what you have. Then, measure the voltage on the center tap of the primary of the output transformer and report.
    Ok the voltage to ground from the rectifier side is now 480. No idea why or how it jumped. Voltage on the other side, connected to the choke lead and OT center tap is 156. The remaining filter cap nodes are 127v for the PI and 111 preamp.

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    Hi

    When you check something, please don't tell us it is "fine", please tell us what you measured. We don't know what you consider fine, and we want to know specific values.

    You have 440 at the standby switch? OK. What do you have at the OT CT?

    To me "second node" is the downhill side of the choke, ie the screens. It bothers me you have the SAME voltage at both plates and screens, even though ther is a choke and transformer between those two places.

    What voltages do you get there with power tubes removed?

    The standby switch should be on at all times here. ON means closed, the operate position. You should have your 440v on both sides of that switch.

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    It may be helpful to note that in a parallel thread on TAG, the OP has revealed a 'trickle bypass' mod to the standby arrangement Deluxe Reverb voltages - Page 2 - The Amp Garage
    Hence standby mode is not as we would normally know it.
    Most amps described as clones aren't; it's way better to provide an accurate schematic, rather than provide text descriptions of deviations to an existing design.

    Quote Originally Posted by tyler8611 View Post
    Ok the voltage to ground from the rectifier side is now 480. No idea why or how it jumped...
    Maybe the mains voltage increased a few V? It's a good idea to monitor the mains Vac whilst undertaking such testing, as all voltages from the PT secondary will be a ratio of the primary voltage.

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    Last edited by pdf64; 05-30-2018 at 11:50 AM.

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    Quote Originally Posted by The Dude View Post
    Yes. I understand. If you look at "pin 8 of the rectifier to the first power cap node", follow it up through the OT. It connects directly to the tube plates. If you measure 440VDC there, you should also measure it on the plates, unless you have a horribly bad, hot, melted standby switch. Measure DC on both sides of the standby switch. Is it the same on both with the switch closed?
    Quote Originally Posted by tyler8611 View Post
    True and everything is wired correctly. Weird problem.
    PLEASE-ANSWER-WHAT-YOU-ARE-ASKED
    "True" is not an answer.
    "everything is wired correctly" is obviously not the case.

    EDIT:
    PDF64 wrote:
    It may be helpful to note that in a parallel thread on TAG, the OP has revealed a 'trickle bypass' mod to the standby arrangement Deluxe Reverb voltages - Page 2 - The Amp Garage
    Is that so?
    If so, why didn´t you mention it?
    Please post the schematic of said "trickle bypass".

    And remember:
    "Standby ON means switch closed, passing current to the OT and power tubes"

    "Standby OFF means switch open, NOT passing current to the OT and power tubes"

    Nobody knows what trickle Bypass actually means but just from its name it looks like a BAD idea.

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    Last edited by J M Fahey; 05-30-2018 at 11:48 AM.
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    Quote Originally Posted by pdf64 View Post
    It may be helpful to note that in a parallel thread on TAG, the OP has revealed a 'trickle bypass' mod to the standby arrangement Deluxe Reverb voltages - Page 2 - The Amp Garage
    Hence standby mode is not as we would normally know it.
    Most amps described as clones aren't; it's way better to provide an accurate schematic, rather than provide text descriptions of deviations to an existing design.


    Maybe the mains voltage increased a few V? It's a good idea to monitor the mains Vac whilst undertaking such testing, as all voltages from the PT secondary will be a ratio of the primary voltage.
    I've used the standby switch trickle bypass mod on a tweed deluxe build, tweed bassman build and princeton reverb build. Never had a single problem with it.

    I will check and monitor the mains Vac and provide readings.

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    Quote Originally Posted by J M Fahey View Post
    PLEASE-ANSWER-WHAT-YOU-ARE-ASKED
    "True" is not an answer.
    "everything is wired correctly" is obviously not the case.

    EDIT:


    Is that so?
    If so, why didn´t you mention it?
    Please post the schematic of said "trickle bypass".

    And remember:
    "Standby ON means switch closed, passing current to the OT and power tubes"

    "Standby OFF means switch open, NOT passing current to the OT and power tubes"

    Nobody knows what trickle Bypass actually means but just from its name it looks like a BAD idea.
    This reminds me of that other forum. Jesus.

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    I'll be providing exact detailed measurements all at once asap. I appreciate you guys.

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    Quote Originally Posted by tyler8611 View Post
    Ok the voltage to ground from the rectifier side is now 480. No idea why or how it jumped. Voltage on the other side, connected to the choke lead and OT center tap is 156. The remaining filter cap nodes are 127v for the PI and 111 preamp.
    I think several of you guys may have missed this response. This is both sides of standby, switch closed. 480 on one side, 156 other.

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    And I can certainly remove the trickle bypass if anyone thinks it's making this more difficult. But like I said, I've always used it. I think it's a sensible, but obviously not mandatory mod.

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    Quote Originally Posted by tyler8611 View Post
    I've used the standby switch trickle bypass mod...
    Have you made any other deviations from the DR AB763 schematic and layout? http://schems.com/schematicheaven.ne...b763_schem.pdf
    Please take a bit of time and review it carefully against your build.
    We'll assume that a grounded mains cable and chassis have been used, and the death cap and switch omitted, unless you say otherwise.

    Quote Originally Posted by tyler8611 View Post
    I think several of you guys may have missed this response. This is both sides of standby, switch closed. 480 on one side, 156 other.
    There may be something wrong with the switch.
    I suggest that it is removed from the circuit (at least that will remove terminology confusion), and that you then power up via a light bulb limiter.

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    Quote Originally Posted by pdf64 View Post
    Have you made any other deviations from the DR AB763 schematic and layout? http://schems.com/schematicheaven.ne...b763_schem.pdf
    Please take a bit of time and review it carefully against your build.
    We'll assume that a grounded mains cable and chassis have been used, and the death cap and switch omitted, unless you say otherwise.


    There may be something wrong with the switch.
    I suggest that it is removed from the circuit (at least that will remove terminology confusion), and that you then power up via a light bulb limiter.
    Deviations: 1 ohm resistors from pin 8 to ground on the 6v6s, a zener diode string to pull down an overvoltaged PT (also have zeners in my tweed deluxe), 100 ohm resistors on the pilot since my PT lacks a center tap, and copper bus preamp grounding.

    I don't have a light bulb limiter but I probably do have another new switch that I can substitute. I will check my parts bin.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by tyler8611 View Post
    Deviations: a zener diode string to pull down an overvoltaged PT
    Are those zeners between hi voltage winding center tap and ground? We had a recent thread where that arrangement caused the bias supply to overcharge = excessively high bias voltage. Why? because the current path through the bias supply presented a lower impedance than the current path through the zeners, causing the bias supply to act in an unexpected way. (If there was an entirely separate winding for the bias supply, not just a tap on a hi voltage winding, it wouldn't be a problem.) This isn't a problem with a tweed Deluxe because it is self-biased, iow has no negative voltage supply for fixed bias operation.

    This may explain one reason for some of your Deluxe's misbehavior.

    Another, if the meaning of "standby" is inverted, and you have a trickle resistor to partially charge the power supply, it would be expected to see 440V one side of the standby switch and some lower voltage say 153 on the other, when the switch is open. When the switch contacts are closed, the same voltage will exist on both switch terminals.

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    Quote Originally Posted by Leo_Gnardo View Post
    Are those zeners between hi voltage winding center tap and ground? We had a recent thread where that arrangement caused the bias supply to overcharge = excessively high bias voltage. Why? because the current path through the bias supply presented a lower impedance than the current path through the zeners, causing the bias supply to act in an unexpected way. (If there was an entirely separate winding for the bias supply, not just a tap on a hi voltage winding, it wouldn't be a problem.) This isn't a problem with a tweed Deluxe because it is self-biased, iow has no negative voltage supply for fixed bias operation.

    This may explain one reason for some of your Deluxe's misbehavior.

    Another, if the meaning of "standby" is inverted, and you have a trickle resistor to partially charge the power supply, it would be expected to see 440V one side of the standby switch and some lower voltage say 153 on the other, when the switch is open. When the switch contacts are closed, the same voltage will exist on both switch terminals.
    Yes, they're between center tap and ground. There is a separate blue 50v winding for the bias supply.

    I hear what you're saying on the standby, but somehow the reverse is happening. Doesn't make sense because the standby wires are going to their proper lugs on the switch. I will have to try a new one.

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    Quote Originally Posted by tyler8611 View Post
    Yes, they're between center tap and ground. There is a separate blue 50v winding for the bias supply.
    Is that an entirely separate winding OR a tap on the hi voltage winding? Most transformers intended for Deluxe have the tap. I say most because I haven't seen 'em all, maybe somebody makes a Deluxe power transformer with an entirely separate bias winding.

    I hear what you're saying on the standby, but somehow the reverse is happening. Doesn't make sense because the standby wires are going to their proper lugs on the switch. I will have to try a new one.
    If the "sense" of the standby function is inverted then a new switch won't help. Easy enough to apply an ohmmeter & find out which direction to point the bat so that the switch is closed; when the meter reads zero ohms, that's the "operate" position and should allow current to pass thru the power supply to run the amp. Yes it's a bit confusing: Fender and lots of other companies print "standby" on the chassis so that when the bat is pointed to the lettering, that's the operate position, and bat away from the lettering is amp on standby, iow not passing signal. Might be all you need to do is invert the switch position and that's that.

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    I think Leo's hunch is right, the zeners may be screwing up the bias supply.
    I suggest to take them out of circuit and review.
    I think that a light bulb limiter is second only to a multimeter as being most important resource on a tech's bench.
    I suggest to build one before proceeding further.

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    Quote Originally Posted by tyler8611 View Post
    Doesn't make sense because the standby wires are going to their proper lugs on the switch. I will have to try a new one.
    Don't go by the looks of the lugs on the standby switch, measure resistance. Zero ohms is the closed position.

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    Quote Originally Posted by Leo_Gnardo View Post
    Is that an entirely separate winding OR a tap on the hi voltage winding? Most transformers intended for Deluxe have the tap. I say most because I haven't seen 'em all, maybe somebody makes a Deluxe power transformer with an entirely separate bias winding.



    If the "sense" of the standby function is inverted then a new switch won't help. Easy enough to apply an ohmmeter & find out which direction to point the bat so that the switch is closed; when the meter reads zero ohms, that's the "operate" position and should allow current to pass thru the power supply to run the amp. Yes it's a bit confusing: Fender and lots of other companies print "standby" on the chassis so that when the bat is pointed to the lettering, that's the operate position, and bat away from the lettering is amp on standby, iow not passing signal. Might be all you need to do is invert the switch position and that's that.
    It's the one here: Fender Power Transformer, Deluxe, Deluxe Reverb, Tweed Tremolux, 125P23B, 025130, 041316, 120V

    And you were right about me stupidly forgetting the direction of the standby switch. Unbelievable. Haven't dealt with blackface amps in a long time. Totally slipped my mind!

    Now, the problem is the bias is only 4 mA max. So the zeners may indeed be causing this to happen. Currently, the bias tail resistor is 10k. The range resistor, which I may try changing next, is the standard 470 ohm. I was thinking of upping it to 1.5k, which was the next highest metal oxide resistor I could find.

    Then again, maybe I should go ahead and remove the zeners. I'll let you guys chime in.

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    Quote Originally Posted by pdf64 View Post
    I think Leo's hunch is right, the zeners may be screwing up the bias supply.
    I suggest to take them out of circuit and review.
    I think that a light bulb limiter is second only to a multimeter as being most important resource on a tech's bench.
    I suggest to build one before proceeding further.
    Thanks I will look into that.

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