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Need Some Guidance For Testing Process On Vacuum Tube Amp

  1. Feb 22, 2016 #1
    Hi Guys,
    I am having issues trying to repair this amp.
    http://www.tangible-technology.com/schematics/fender/HR-Deville/Hot%20Rod%20DeVille.pdf [Broken]

    A little history is in order I guess. I have not been working on guitar amps for too long, several months now. I have repaired perhaps fifty or so. Most were tube but some were solid state. Some were easy to fix and some were not so easy but I got them fixed in the end.

    I am having real issues with Fender Hot Rod Deville and Fender Hot Rod Deluex amps for some reason. I assume this is true because I do not have a process in place that will get to the answer to the problem.

    Is there anyone here who can take the time to go through the process with me?
    Here is a little video to kinda give you an idea of what I have to work with.


    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Feb 22, 2016 #2


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    I suggest that you try injecting a tone of a few volts into the grid of the output stage and then gradually work back a stage at a time until the faulty stage is located.
    Having located the area involved with the fault, start checking voltages around this stage.
    The usual fault is going to be leaking coupling capacitors or burnt out resistors, both of which will give incorrect voltages. The amplifier has quite a lot of solid state circuitry, which may be vulnerable to mains spikes and overload, so is worth looking at. Generally, do not assume tube faults in the first instance because they are robust items and more often go down gradually.
  4. Feb 22, 2016 #3
    Hi tech99,
    So, if I understand you I should inject say a 1000 hz sign wave at say 500mV ( you said " a few volts", not sure what you really mean by a few volts) and look for clipping on the scope after the plate and also after the out put transformer.

    I have to run out for about 30 min. Be back in a few.


  5. Feb 22, 2016 #4


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    Well of course, I don't know what fault you are seeking. But assuming it is no output, the power amplifier is designed to have a few volts applied to its grids, so you should hear the loud tone in a speaker. Of course, it is a push pull amplifier, so don't expect a symmetrical waveform out, but test on both sides of the push pull amplifier.
  6. Feb 22, 2016 #5
    Give me a bit and I will set the amp up and describe the issues.
  7. Feb 22, 2016 #6
    All voltages from the filter caps are within range. Very small amount of AC at filter caps, ie..no ripple. Plate voltage for both 6l6 463V . 61mV at TP 30 which has both out put tubes running at 27mA + or _ .3 mA.
    No resistors on the main board are open but I have not checked the values of all of them.

    Distortion at all settings
    The Normal / Bright switch does not seem to work
    Low volume ie very little amplification on channel 1 more on channel 2
    Out put tubes are over heating when a signal is injected but heat range is more or less normal at idle

    One thing I do not understand from the schematic...what acts as a plate resistor for the two 6l6's?
  8. Feb 22, 2016 #7

    jim hardy

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    they don't really have one like you're used to
    the output transformer lets DC flow with no power loss
    it presents "resistance" only to ac - it's a transformer. For AC it looks like (speaker impedance X turns ratio)^2.
    That's why you see output transformers specified in kilo-ohms.
    It's important that secondary never be open circuited when signal is present.
    Back to topic...
    So you have B+ on output tube plates
    working backward from that end of amp
    what are voltages at both screen grids ? Junctions Pins 4 & screen resistors R61 , R62 ...
    expect 80% to 90 % of plate voltage

    and on control grids pin 5 - negative 40 or so?
    Last edited: Feb 22, 2016
  9. Feb 23, 2016 #8
    Hi Jim,

    I will make those measurements. It is a bit late here so I need to drink some coffee before I go poking my best right hand into some 400 V stuff...lol...plus my left hand is stuck in my back pocket at the moment...lol

    Actually...I think I will get into this tomorrow.


  10. Feb 23, 2016 #9
    Hi Jim,

    R61 and R62 are 470 ohms within 3%

    the 6l6's have the following voltages
    plate 479v
    grid2 pin 4 478v
    grid1 pin 5 -51v
    cathode pin 8 0v

    plates on the 3 12AX7 250v, 245v. and 280v

    110 ohms on output transformer primary and 58 ohms center tap (both sides)and .3 ohms on secondary

    Unless I am really forgetting something, only thing that could cause the 6l6 overheating and the low volume is the output transformer.

    Just as a side note I assume the two 100uf 350V filter caps in series must have been cheaper that one 600V 47uf which is what I would have assumed they would have used...

    If I have lost my mind...lol...yell!!



    BTW..I have a bottle of scotch with the cap rusted shut I think I need to open...lol...this amp is making me a little crazy..lol
  11. Feb 24, 2016 #10


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    tech99 gave you good advice about doing some signal injection to the input of each stage and find out where the signal dies

    Also, I notice that there is a preamp out socket ( labelled on the top right of the schematic)
    so do you get a good clean signal out of this point ?
    If not, then the problem(s) obviously lie either in the earlier stages or the power supply
    and you are wasting your time looking at the driver or final stages

  12. Feb 24, 2016 #11

    jim hardy

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    okay you have good screen and plate voltage on 6L6's
    so those stages have no obvious excuse to not work
    that much negative on control grid says both tubes should be at 10 to 20 ma ooops 20 to 30ma (lost my bifocals) which i think you verified,
    sounds to me okay for class AB

    dave and tech have the right idea , inject signal and listen

    The approach i use is
    check in this sequence
    power supply voltages
    output stage
    then work backward
    control grids of 6L6's is where i'd start, you'll need a few volts there, then to tp22 and see what it sounds like,
    but you can work other direction, starting at front and tracing with o'scope
    jumping to someplace near the middle like j4 might tell you which half isn't working

    i see from your video you have an oscilloscope
    i think i'd set the function generator for an audio signal , i like 400 hz it's pleasant
    and start injecting that through a capacitor to inputs of each stage, ~a .1 to 1uf ought to do
    Last edited: Feb 24, 2016
  13. Feb 24, 2016 #12

    jim hardy

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    each 12ax7 has two halves
    i assume you checked all 6 plates ?
  14. Feb 24, 2016 #13

    jim hardy

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    Aha ! Just found their input signal, 4mv 1khz at TP1
    those unsigned voltages at TP's are AC with that signal present, i presume...
    they were thoughtful enough to show us stage by stage gain for the 12ax7's

    what a nice drawing
  15. Feb 24, 2016 #14
    Hi guys,

    Thanks for all the help.

    The issue I don't know how to deal with is that the output tubes are really overheating when I inject any signal into the amp. They do not over heat at idle. The temp at idle is around 300F and moves up to past 400F with a signal at which point I turn the amp off to keep from red plateing the tubes. The max per the data sheet is 450F.

    I assume that because the idle current is normal, bias is normal, plate voltage is normal, and there is nothing wrong with the tubes themselves there are only two other things that could cause the output tubes to overheat. One, the output transformer is bad or two the screen voltage is somehow out of control. Check me if I a saying something stupid....but I assume the only cause of overheating in power tubes is too much current or a impedance mismatch between the tubes and the speaker.

    I measured every wire on the output transformer for shorts...Primary 110.92 ohms...red center tap to blue 53.92 ohms red center tap to brown 57.92. On the secondary the green/ yellow to black .30 ohms.. There no shorts to the case are between any wires. I have no idea if these ohms reading are normal.

    I have removed many of the caps to test them and found nothing.

    One question I do not understand is a measurement on the first filter cap. There are two 350V 100uf caps directly after the bridge diodes and the reading from case ground to the + side of the second cap is around 470V...or close to that as I did not write it down. The first cap reads about half of that.???

    I am putting the amp back together and starting over with the troubleshooting this morning to see if I have just missed something or done something really stupid!!!

    This amp is becoming a PITB..!!!!...lol


  16. Feb 24, 2016 #15
    Hey JIm,

    I am putting everything back together now and will post everything I am doing in the next few.

    I saw the 4 mV and I assume that is pp but I guess it could be RMS..?????
  17. Feb 24, 2016 #16

    jim hardy

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    here's a snip from schematic
    looks to me per 4.3 tp1 is rms ,

    read dc at control grids when you apply signal
    does it change much?
    is R60 known to be 1 ohm by measurement?
    6L6's run frighteningly hot. Still i laud your caution with them.
  18. Feb 24, 2016 #17

    jim hardy

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    Do you notice R70 and R71?
    Their purpose, in addition to bleed down, is to force DC to divide equally between C31 and C32. Without them DC would divide according to the caps's individual insulation resistances which ought to be in megohms but can vary a lot.
    So if you got about 235, 470 DC to comon they're doing their job.
    Read the AC across each cap for a similar sanity check on their capacitance ratio.
    Last edited: Feb 24, 2016
  19. Feb 24, 2016 #18

    jim hardy

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    A check of the transformer would be to put a small number of volts across speaker terminals (amp powered down of course) and measure voltages on plate side R-Blue and R-Brown. Hopefully they're well matched.
    We dont know turns ratio
    so let's assume it's similar to this Hammond 40 watt( God Bless Hammond for staying in the business!)

    sqrt(6600/8) = around 29 to 1, 15 at centertap

    shorted turns will show as low turns ratio

    hopefully you get close to that result and it's balanced

    when nothing will show itself wrong - make 'em show themselves right.

    If you're overdriving those tubes they'll get hot
    how much signal are you injecting? How much signal shows at tp24 ?
  20. Feb 24, 2016 #19

    jim hardy

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    Aha ! look at schematic i'm still learning my way around it

    they show signal voltages on transformer
    94primary to 7.8secondary = 12::1 per half
    24::1 X 8 = 4647 ohm centertap , maybe it's closer to 30 watt transformer
    look for that ballpark impedance and balance
  21. Feb 24, 2016 #20
    ok Jim...let me read all your last post...be back in a bit
  22. Feb 24, 2016 #21
    OK..I had it set to pp and will change to rms
    R60 reads 220K per the schematic..???
    Did you mean R60? that measures 1 ohm and is new as I changed it the other day..this is TP 30 used to measure the bias

    read dc at grid 2 pin 4 of the 6l6???
    I will read the grid now.

    ok 5 volt drop when signal is applied at pin4 6l6 475 down to 470


    tp7= 55.1
    tp8= 1.2v

    I am not sure the relay is working because I am getting low voltage on down the line...think I will change the relay as they area normal failure point in these amps.

    Also the is no real AC leakage on the filter caps..checked that
  23. Feb 24, 2016 #22
    Also Jim I remover the op trans and these are the reading.

    blue to brown 110.92 ohms
    blue to red 53.28
    brown to red 57.92

    green to black .60 ohms
    green to white .49
    green to green/yellow .53
    green/yellow to black .30

    no shorts to the trans case any where

    r70 and r71 are not open but I would have to remove one end to read them because of the cap
    Last edited: Feb 24, 2016
  24. Feb 24, 2016 #23
    I need to take a break for a bit...lol
  25. Feb 24, 2016 #24

    jim hardy

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    if you read equal DC across them they're not open
    equal AC across those two filter caps will tell you they're equal capacitance.. AC reading should be less than ~10% of the DC reading , that's a quick check on your power supply filter

    Transformer is probably okay
    Shorted turns does not change the ohm reading very much but it changes the inductance a LOT .
    you have to detect that by either inductance meter (and i have no idea how much to expect) or voltage measurement - shorted turns i think ought to change the apparent turns ratio by quite a bit.
    Industrial transformers we check by measuring milliamps versus applied volts.

    In my younger days i'd connect a Simpson 260 to a primary, set it for RX1 which puts tens of milliamps through the winding,
    then remove the meter lead. If the transformer's "inductive kick" did not "bite" my fingers still on the transformer leads i knew it had shorted turns.
    Ahh the wonders of analog meters...
    If your DMM has a "peak capture" function you could arrange a similar test with a low DC source - AA or 6V lantern battery

    As the guys said earlier we may be wasting time poking around output stage -- but on the other hand we're in this to learn and polish skills.

    I'll look at those TP voltages later today or tonight

    small change in grid voltage is consistent with more plate current and is expected

    thanks for the 1 ohm check, yeah we both meant R66 i think , the cathode current sampler resistor
  26. Feb 24, 2016 #25

    jim hardy

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    okay tp7 and tp8 show you're getting through V2A
    have you hung a scope there to see if it's a ~faithful reproduction of input? You complained of distortion...

    lots of old hands here on pf , i look forward to learning .
    In my power plant i had to go very slow and deliberate in troubleshooting because the machinery was big...
    it spilt over, now i'm way slower than i should be on small equipment
    if you guys get frustrated with me plow right ahead and i'll watch
    old jim
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