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Thread: Stereo guitar amp trouble shooting

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    Stereo guitar amp trouble shooting

    Hello
    I've built a stereo guitar amp
    20 watts a side using 7189s in pushpull (actually using Russian 6P14-EVs) these can handle higher voltages.
    It's a cut and paste of fender ideas basically a couple of AB763 deluxes with a 6G16 bias vary tremelo and individual bias adjustments(off of one supply).
    It's built all in a line with the two power amps at one end and the preamps at another.
    The B+ runs down one side with the filter caps next to each section and B+ node they correspond to.
    The preamps has it's own ground near the input jacks and everything else goes to a single ground point by the PT
    This is a scheme I used on my previous builds successfully and I put up a picture from the hoffmans site which shows the method.
    It's pretty successful considering it's kind of an experiment.
    Everything works and it sounds really good
    It has a little more hiss than my other builds but not at all unusable.
    Because I'm using a Hammond organ AO-14 power transformer 375-375 I have quite high B+ up to 460 on the plates of the power tubes. Nothing has lit up yet but it is worrisome.

    One thing that is strange is that if I try lifting the ground at the AC plug (checking for ground loops) the amp makes a very ugly buzz, something non of other builds do. I don't plan to lift the ground at all in practical use but it still seems like it is a symptom of something amiss in the build
    I have checked all the solder joints and particularly the main grounds.
    Any ideas?
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    Last edited by Magic Potion; 04-18-2018 at 11:00 PM. Reason: 453 volts to brain stem

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    Maybe the PT insulation is failing.
    With the mains ground lifted, is there any Vac between the amp chassis ground and the ground of the wall outlet?
    Be very careful, one hand in pocket etc!

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    So I put a ground lift on the AC plug at the power bar the amp is plugged into (a properly grounded 3 prong power bar)
    and measured between the power bars earth ground and the amps chassis and I get 235 volts AC
    turn the amp off, voltage drops, turn it back voltage returns along with loud evil buzz.

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    Last edited by Magic Potion; 04-19-2018 at 09:38 PM.

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    How about if, rather than a complete lift, you put a 10k resistor between the amp chassis and mains ground; what is the chassis Vac then?
    Take extreme care!

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    Hi Before I do that i was just going to say that curiously i swapped the primaries on the PT around (they're grey and black) I put the black on the neutral and the grey on the line (usually it's just two blacks so there's not really an orientation) Strangely I now measure 127VAC on the chassis instead of 235VAC. regarding the 10K resistor will a 2 watt resistor suffice?

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    Last edited by Magic Potion; 04-19-2018 at 11:10 PM. Reason: brain malfunction!

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    OKay I tried what you said with a 10K resistor between chassis and mains ground. Meter between chassis and wall socket ground and i measured .57 VAC instead of 127VAC and the loud buzz seems to be gone.

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    I think it's reasonable to condemn the PT as faulty, unsafe to use, as there seems to be a problem on the insulation between the primary and metalwork.
    At the moment it's not actually dangerous, but insulation that's failed 'a bit' can later fail 'a bit more'.
    Anything less than perfect is unacceptable. I don't think that any competent tech / builder / manufacturer would allow an amp with that PT out of their shop.
    Sorry

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    Ah well
    It's good to know regardless and I'm not dead!
    Which so far is also good!
    Thanks!

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    Supporting Member loudthud's Avatar
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    Can you contact the manufacturer of the transformer? Ask them if they Hipot test their products and at what Voltage. The symptoms described could just be capacitance between primary and other windings.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
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    You know I don't even know what hipot testing is, how curious , Ill have to look it up

    I should've said its' a donor PT from a 50s Hammond M2 organ AO-14 Amplifier . It was bit of a gamble to try it I guess. I've had very good luck with other Hammond organ chassis and Transformers (actually the OTs on this build are from Hammond Amps I think both from Hammond AO-63s, Deluxe size 30-1 OTs used with EL84s and with an 8 ohm tap.)

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    Well Looking at transformers on the Hammond site I can get a bandmaster/concert/pro PT for $93 Can which is about $72 US dollars, Also as they're in Canada too shipping is relatively quick
    http://www.hammondmfg.com/pdf/EDB290EX.pdf
    it's 330 0 330 276 ma which I think should just do the job with my 4 6P14P-EV power tubes (65ma each and 7 preamps tubes)
    It will also give me a lower B+ which will be a little better for the Power tubes.

    MY amp is SS rectified as I want a clean machine so This PT is also a little cheaper as it doesn't have 5V rectifier heater taps.
    i thought about buying another Hammond AO-29 PT locally but though it would be only $50 it might be throwing good money after bad and also though they are quite substantial they're current rating might be a little low for my build.

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    I was looking at this site regarding testing PTs
    Power Transformer

    in testing the primaries it says :

    Using an ohmmeter, measure the resistance of the primary (usually Black - Black wires) and the resistance of both leads to the chassis. The primary should be under 1K ohms. If it is not, the transformer is dead.
    Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm.
    If it is shorted or less than 100K ohms, the transformer is bad. If it is between 100K and 1M ohm, unsolder the primary leads from the terminals they contact and measure again.
    If it is now less than 1M ohm, the transformer is failing, and should be replaced.
    If it is over 1M ohm, there is a component connected to the wiring leading to the power transformer which is leaking to the chassis that needs to be traced down.

    I get inifinite resistance to chassis from the primaries
    Could a leaky filter cap be giving me B+ to ground?

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    Supporting Member eschertron's Avatar
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    Under voltage conditions the insulation could be breaking down. That's why the "hi-pot" (high voltage potential) testing was suggested. The test above can cull out a definitely bad PT, but may not detect all failing PTs. Sorry I did not answer your question

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    Bent Member Chuck H's Avatar
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    First, kudos to Pete for his concise and fast troubleshooting and guidance

    Second, to our new member... Wow Magic Potion! That's a very ambitious build and you nailed it. Were it not for the bad PT you'd have been good to go. That's rare for an experimental build of this caliber. Welcome to the forum.

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Quote Originally Posted by loudthud View Post
    ...The symptoms described could just be capacitance between primary and other windings.
    Yes, that a possibility, but it's difficult to be sure whether the current is due to coupling or imperfect insulation, without access to a megger or hipot tester.
    As it's a vintage PT, perhaps best to play it safe and assume the worst.

    Quote Originally Posted by Magic Potion View Post
    ...I get inifinite resistance to chassis from the primaries
    Could a leaky filter cap be giving me B+ to ground?
    The big problem with a regular meter is that the test voltage is only 9Vdc.
    A bad bit of primary insulation might test fine at 9V, but when subjected to mains voltage it could allow mains current to leach past.
    That's why high voltage testers are needed https://en.wikipedia.org/wiki/Hipot

    The B+, being a secondary, doesn't have a current path back to mains ground.
    Just checking, has your build got a death cap, eg about 0.047uF between the incoming mains live and ground? As that would cause these symptoms.

    And yes, great work with your project, very impressive!
    The plan to bring the HT down a bit is good.

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    Last edited by pdf64; 04-20-2018 at 07:09 PM.

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    Thanks very much for the nice comments and all the trouble shooting advice
    as far as replacements go it seems the Hammond 290EX doesn't quite have the heater current capacity I need and I'd probably be better of with the 290FX which is what they are selling for twin reverbs. http://www.hammondmfg.com/pdf/EDB290FX.pdf

    It's good to have less worries about the transformers capacity, it will probably run cooler but it will add a couple lbs which is a big consideration in this build.
    I've built smaller amps(a couple vibrolux reverbs and deluxe reverbs) recently so I wouldn't have to carry my lovely sounding but 80 lbs super reverb around to smaller shows.

    The motivation(besides fun and curiousity)behind this amp was to have a stereo rig in one comparatively light amp instead of carrying two almost as heavy amps

    I was also happy to make use of a 2x12 combo cabinet I made about 8 years back that has been kicking around under-utilized for all that time. In it's current format the amp weighs about 44lbs.

    These pictures show it roughed into the cabinet before final modifications.

    I have since made a new back panel and completed some more of the reconfiguring of the cabinet.
    It had been used for one of my vibrolux's previously and a sherwood S-1000 conversion before that so the cut outs for the amp face plate and the back panel were wrong.
    It also now has a speaker extension plug for the right channel so I can use one of my mini leslies with it.


    It has 2 x 12" speakers one Eminence Little Texas (Clean, really efficient, and LIGHT! 4lbs!!) and a 60s approx. 20 watt Rola organ speaker which is nice sounding too.
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    Last edited by Magic Potion; 04-20-2018 at 11:24 PM. Reason: Tsunami

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    I was monkeying around and i discovered a couple of other unsual things which I was wondering if someone could tell me if they were related to my likely faulty PT

    When I removed my PT from it's original chassis I had made a wiring map, but then lost it (duh) so I had inadvertently shrink-wrapped the heater centertap and created an artificial one using 2 100 ohm resistors to ground which worked fine

    in light of recent exploration around the PT
    I was checking two wires that I had shrinkwrapped and set aside. One had no resistance to the heater wires. A centertap??. I measured the AC between this wire and each filament and it was half the heater voltage. BINGO!

    The other wire had 3.5 ohms to one primary and about .7 to the other. the primaries have about 3.4 ohms between them. I think this other 'primary' is the 110 volt tap which I have since checked by measuring between it and the black primary, yup 110 volts)

    I decided to hook up the newly discovered real heater centertap
    i disconnect my artificial one and hooked up the proper one then turned my amp on...my pilot light wouldn't come on
    I flipped the switch a few times, checked the power and tried it a few more times,no pilot light... then it worked..then it didn't. weird!

    I left the switch on to see if it was just the pilot light. the tubes heated up quite bright and the amp came on buzzing louder and ringing like it was a reverb tank starting to feedback
    Everything was turned down though.
    I tapped a tube and it was hugely microphonic and rang like a bell, I tried the next..and then all of them.They all rang like bells!! I then turned the amp off. I should've taken a heater reading but I felt like i was just meddling and something was very wrong, so I disconnected the heater centertap and hooked up the artificial one again. The amp worked as before and so did the pilot light!

    Is this part of the PT problem or symptomatic of something else

    I also think it's weird that my primaries seem to have polarity. I have a grey, a black and a blue one(110 tap)
    Is it because there's the extra primary tap?

    The main primaries are the black and grey ones

    When I reversed those ones while testing the AC between the chassis and wall ground before I was able to drop the AC voltage on the chassis by half??
    What was that indicating??

    I included the part of the hammond M2 organ schematic that has the power supply. I think the AO-14 amp this PT came out of was from an M2 organ
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    Quote Originally Posted by Magic Potion View Post
    ...I also think it's weird that my primaries seem to have polarity. I have a grey, a black and a blue one(110 tap)
    Is it because there's the extra primary tap?

    The main primaries are the black and grey ones

    When I reversed those ones while testing the AC between the chassis and wall ground before I was able to drop the AC voltage on the chassis by half??
    What was that indicating??...
    I think that may indicate that the part of the primary winding where the bad insulation lies is closer to one end of the winding than the other.
    To be expected really, it's more likely to be that way than if it was exactly in the middle of the winding.
    And it may be a further indication that the issue is bad insulation, rather than capacitive coupling etc.

    I can't think what might be going on with the apparent CT on the heater winding

    Really though, I would get that PT outta there, and sling it into the ecycling dumpster

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    Don't forget the joker g1's Avatar
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    I don't think that was a heater center tap, you said it had no resistance to the heater wires. (assume you meant 'open')
    Perhaps it was a ground shield or something. That might give the AC reading to the heaters like you got while the artificial tap was connected.

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    In the absence of an insulation tester you can check earth leakage by inserting a 1500 ohm resistor shunted with a 0.15uf capacitor in the mains earth connection. Measure the voltage across the resistor and using ohms law calculate the current. It should be less than 0.5mA with the mains checked with both polarities.

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    Old Timer J M Fahey's Avatar
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    1) what Country are you writing from?
    2) what´s mains voltage where you live? Show what outlet are you using.
    3) you have a gross wiring/grounding/both error which may be potentially deadly, I´ll suggest some measurements but only after you answer these questions.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Magic Potion View Post
    So I put a ground lift on the AC plug at the power bar the amp is plugged into (a properly grounded 3 prong power bar)
    and measured between the power bars earth ground and the amps chassis and I get 235 volts AC
    turn the amp off, voltage drops, turn it back voltage returns along with loud evil buzz.
    I find this very very worrying.
    You didn´t answer (I hope you are still alive) but now guess you live in a 120V mains Country; then how the heck can you have 235V on the chassis?.
    Even if you have 127V !!!!!!
    Voltage between chassis and mains Ground MUST be ZERO.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Voltage between chassis and mains Ground MUST be ZERO.
    Good catch on the verbiage here. Indeed, since the mains ground should be physically connected to the chassis, even with the actual ground wire lifted there should still be virtually zero volts with this measurement if there's a million volts on the chassis. Confusing.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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    Apologies I used the wrong terminology, see posts 2-7 . The reading I was talking about was taken between the chassis and the wall socket ground, with a ground lift on the power chord going to the amp. I live in Canada and yes 120 VAC and still alive. I've done this test as suggested above, post 2, several times and get the same result. I have a new twin reverb type PT on the way from Hammond.
    Thanks for the concern.

    Embarrassingly, it turns out my excitement with the heater centertap must have been something shorting, likely from me moving things about to hook up the centertap. Further testing between the heater tap and each heater wire showed it to actually be the heater centertap (1/2 heater voltage between the centertap and each heater) and when tested again and connected to ground it was functioning perfectly.

    The amp still works and sounds great, I was playing it today.

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