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Thread: 5F6A low volume

  1. #71
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Pedro Vecino View Post
    The stolen volts (it's funny) could come from having confused the center tap (CT) with the bias tap?
    I thought that too, but I see that while the two halves would not match, they would still need to add up to the total 694vac.

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    I hate to say it, but if we can trust the measurements, the most probable explanation is an internal transformer short between part of the HV secondary and the core.
    Needs to be verified, though.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by vinceg View Post
    I hade removed all secondary wires, exluded filaments, before make measuraments. Anyway, I'll remove also filaments. My transformer is Hammond 290DEX and these is datasheet. All wires colour match. Can you Tell me what to do, step by step, please?
    Yes:
    Please remove all secondary wires, including filaments, strip ends , tape them to a piece of wood so you can measure without need to hold them and remeasure.
    Draw a transformer winding schematic and back it up with a picture, as a double check.

    Weīll ask you for specific colour/tap pairs resistance and voltage measurements.

    If itīs a "known brand" transformer also upload its winding data plan or schematic, BUT, do not copy it and say "this is what I have", rather draw what you see at the transformer, both drawings may not match at all.
    You did less than half of this, so we canīt check for inconsistencies.

    And when I ask you to draw or measure something without looking at the transformer datasheet or trusting memory or written notes, please do that.

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    Also I wish to ask if you have been getting any other strange readings when using your meter? Many other threads ask that the battery in the meter be replaced in such situations. Also, you could try a different meter if you have one. These suggestions are not a guaranteed solution. Just an idea.

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  5. #75
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    Quote Originally Posted by J M Fahey View Post
    Yes:

    You did less than half of this, so we canīt check for inconsistencies.

    And when I ask you to draw or measure something without looking at the transformer datasheet or trusting memory or written notes, please do that.
    Ok, I hope I've done well until now. I removed all secondary wires and strip ends. I isolated the wires and I left Orange wires connect to chassis.
    Voltage at wall of my home is 220 VAC

    -I measured resistance between the two Red wires and I get 74 Ohm
    Voltage between the two Red wires is 690 VAC

    -I measured resistance between the two Yellow wires and I get 0.1 Ohm
    Voltage between the two Yellow wires is 5.09 VAC


    - I measured resistance between the two Green wires and I get 0.05 Ohm
    Voltage between the two Green wires is 6.4 VAC

    I have not measured between Bias and center tap because last night while I was measuring the 500mA fusein bias wire is burned, then I'm scared. I suspect the problem is at 50 VAC out winding.

    Now I'm waiting for instructions
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    Last edited by vinceg; 12-12-2018 at 09:41 AM.

  6. #76
    Senior Member Pedro Vecino's Avatar
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    With all secondaries disconnected (HT and bias), measure the resistance between the high voltage windings and the center tap to see if they are identical or very close (37 and 37 Ohms).

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    Old Timer J M Fahey's Avatar
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    Thanks, almost there, letīs complete the measurements.

    Yes, some are redundant, repetitive, "not necessary" .... and thatīs the point, we are trying to catch where/which the error is.

    So please measure and post AC voltage across:

    1) Red_T>Red_B (I call Red_T the top one, Red_B the bottom one, please attach a little piece of tape to wires to label them, "Red" alone does not tell me which is which and this is important)

    2) Red_T>Red/Yellow

    3) Red_B>Red/Yellow

    4) Red/Green>Red/Yellow

    5) Red_T>Red/Green

    6) Red_B>Red/Green

    Unless I forgot some, these are all possible High Voltage winding combinations, our main problem today; in theory they "should" add up and yield consistent results, but your earlier measurments did not match, at all, so we are testing the transformer "clean" to verify.

    After we are certain of what we have there, we can go on with other tests.

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    Quote Originally Posted by vinceg View Post
    - - -I have not measured between Bias and center tap because last night while I was measuring the 500mA fusein bias wire is burned, then I'm scared. I suspect the problem is at 50 VAC out winding.- - -
    Since you were able to measure the voltage at all the other secondary windings and the transformer doesn't seem to be in distress (Such as heating up excessively or blowing the main primary fuse) it should be OK to measure the remaining voltages.

    Specifically, measure the voltage between the Red/Yellow wire and each of the red wires and measure the voltage between the Red/Yellow wire and the Red/Green wire.

    Note that the 500mA fuse in the bias circuit doesn't blow because of a transformer problem. It blow because of excess current draw due to something in the downstream circuit. Could also blow because of a slip with a test probe causing a short.

    Edit: Oh - I see that Juan Fahey has posted while I was writing. Follow his instructions of course.

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    Quote Originally Posted by J M Fahey View Post
    Thanks, almost there, letīs complete the measurements.

    Yes, some are redundant, repetitive, "not necessary" .... and thatīs the point, we are trying to catch where/which the error is.

    So please measure and post AC voltage across:

    1) Red_T>Red_B (I call Red_T the top one, Red_B the bottom one, please attach a little piece of tape to wires to label them, "Red" alone does not tell me which is which and this is important) 690

    2) Red_T>Red/Yellow 340

    3) Red_B>Red/Yellow338

    4) Red/Green>Red/Yellow50.5

    5) Red_T>Red/Green290

    6) Red_B>Red/Green396

    Unless I forgot some, these are all possible High Voltage winding combinations, our main problem today; in theory they "should" add up and yield consistent results, but your earlier measurments did not match, at all, so we are testing the transformer "clean" to verify.

    After we are certain of what we have there, we can go on with other tests.
    Here I am. It is I who thank you and everyone else.

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    Quote Originally Posted by Pedro Vecino View Post
    With all secondaries disconnected (HT and bias), measure the resistance between the high voltage windings and the center tap to see if they are identical or very close (37 and 37 Ohms).
    Yes, it is identical.

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  11. #81
    Supporting Member eschertron's Avatar
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    340 + 338 = 678 pretty close to the measured 690 (little more than 1% off)
    290 + 396 = 686 even better.
    I'll let Juan do the final assessment, but I think you will not need to buy a new PT

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    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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    Quote Originally Posted by eschertron View Post
    340 + 338 = 678 pretty close to the measured 690 (little more than 1% off)
    290 + 396 = 686 even better.
    I'll let Juan do the final assessment, but I think you will not need to buy a new PT
    Depends on what happens, if RED/YEL is connected to chassis ground/ORG. I suspect voltage between RED/YEL and ORG/transformer core.

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    Quote Originally Posted by Helmholtz View Post
    Depends on what happens, if RED/YEL is connected to chassis ground/ORG. I suspect voltage between RED/YEL and ORG/transformer core.
    Can I check with multimeter if there is voltage between RED/YEL and ORG?

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    Quote Originally Posted by Helmholtz View Post
    Depends on what happens, if RED/YEL is connected to chassis ground/ORG. I suspect voltage between RED/YEL and ORG/transformer core.
    Unfortunately, your suspicion is founded. I measured the voltage between RED / YEL and ORG and it is 94VAC. So the transformer is gone?

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    Quote Originally Posted by vinceg View Post
    Can I check with multimeter if there is voltage between RED/YEL and ORG?
    You might, but the reading will not be very reliable, because of capacitive leakage in the transformer. But you can measure resistance between RED/YEL and ORG (after disconnecting from mains!), should read infinite/open.
    But I recommend to wait for Juans reply, because I assume that he is going to guide you through a more systematic and instructional analysis.

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    Quote Originally Posted by Helmholtz View Post
    You might, but the reading will not be very reliable, because of capacitive leakage in the transformer. But you can measure resistance between RED/YEL and ORG (after disconnecting from mains!), should read infinite/open.
    But I recommend to wait for Juans reply, because I assume that he is going to guide you through a more systematic and instructional analysis.
    Oops, I measured before reading your comment. Ok, I measured resistance and it's open (multimeter display show 0L)

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  17. #87
    Old Timer J M Fahey's Avatar
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    So far I think transformer is fine, all voltages and specially their combinations match perfectly; remember normal meters guarantee 2% precision, expensive ones 1% ... of full scale value, (just check datasheet) so less than full scale measurements have less than "official" precision ... yet these match incredibly well.

    Letīs go through this first: *maybe* a turn is touching the core laminations, we canīt discard that at least from safety considerations, letīs check it one way or another before going on:

    a) measuring voltage between a winding and core is a way, but sadly very high meter impedance makes usually very weak losses/undesired_coupling look larger than they are.
    Iīd connect a neon bulb if you have one from Red/Yel>Org , with, say, 100k in series, and check whether it lights bright (for a Neon)/weak/none.

    If unavailable, replace neon with a bright or extrabright LED (best if crystal clear) in anti-parallel with any 1N400x type diode you have: Led Cathode to diode anode, Led anode to diode cathode; the couple in series with a 100kresistor.
    I have found this to show visible light even with tiny current through it.

    or: Plan B, the (safe) brute force approach, and which I trust better, because it mimics actual use:

    b) connect Red/Yel to Orange , plug amp into mains , always through lamp bulb limiter and turn it on:

    * if lamp lights full brightness, transformer is shorted unusable

    * if it blinks and then becomes dark red or orange, the transformer is probably fine, weīll continue with other tests.


    Note: Iīm suggesting nothing new or unusual, just applying my OCD to a noble cause
    The old: "leave no stone unturned" or Enzoīs: "never search for reasons NOT to check something"

    Including:"but I already measured that"

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    Quote Originally Posted by J M Fahey View Post
    or: Plan B, the (safe) brute force approach, and which I trust better, because it mimics actual use:

    b) connect Red/Yel to Orange , plug amp into mains , always through lamp bulb limiter and turn it on:

    * if lamp lights full brightness, transformer is shorted unusable

    * if it blinks and then becomes dark red or orange, the transformer is probably fine, weīll continue with other tests.
    Remembering that at the moment the transformer is disconnected from the amp circuit, do you mean that i have to connect Transformer to circuit or test only the transformer?
    For 220VAC voltage at wall what wattage for the bulb? Incandescent bulb are hard to find, I have one of 60W.

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    Unfortunately the time zone does not allow us to be awake at the same time, but in Italy it is daytime and I can not stay still.
    So I connected Red/Yel to Orange and together to chassis, then plug transformer into mains through lamp bulb limiter (I used a 60W incandescent lamp) and turn on. The lamp does not light up. While transformer is turn on I redid the same measurements and I get:

    Red_T > Red_B 644 V
    Red_T > Red/Yel 329 V
    Red_B > Red/Yel 329 V
    Red/Yel > Red/Green 48 V
    Red_T > Red/Green 280 V
    Red_B > Red/Green 375 V

    I'm waiting for your response

    p.s. Before all above operations I test for continuity between Red/Yel and Orange and there is not continuity.

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  20. #90
    Old Timer J M Fahey's Avatar
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    Hey, we are only 4 hours away!!!
    Italy +1 GMT ; Argentina -3 GMT

    Back to the transformer, it seems to be perfect


    So now weīll reconnect it step by step.

    Start by reconnecting filaments .... *ONLY*
    Do not "save time" connecting anything else.

    1) Test first with bulb limiter that we have filament voltage across proper tube pins at every tube. No tubes plugged in.

    2) Now that each tube pin has about half that relative to ground.

    3) Now repeat tests with tubes in their sockets, *still* through builb limiter.
    I donīt care you will now have way less than 6.3V at each filament, we are just testing winding does not collapse/short under real load.

    Post results.

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    Quote Originally Posted by J M Fahey View Post
    Hey, we are only 4 hours away!!!
    Italy +1 GMT ; Argentina -3 GMT

    Back to the transformer, it seems to be perfect


    So now weīll reconnect it step by step.

    Start by reconnecting filaments .... *ONLY*
    Do not "save time" connecting anything else.

    1) Test first with bulb limiter that we have filament voltage across proper tube pins at every tube. No tubes plugged in. 6.20 V

    2) Now that each tube pin has about half that relative to ground.3.10 V

    3) Now repeat tests with tubes in their sockets, *still* through builb limiter.
    I donīt care you will now have way less than 6.3V at each filament, we are just testing winding does not collapse/short under real load.

    Post results.
    Now I accompany my daughter to school. Later, within an hour, I will do the test with the tubes in

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  22. #92
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    Quote Originally Posted by J M Fahey View Post
    Hey, we are only 4 hours away!!!
    Italy +1 GMT ; Argentina -3 GMT

    Back to the transformer, it seems to be perfect


    So now weīll reconnect it step by step.

    Start by reconnecting filaments .... *ONLY*
    Do not "save time" connecting anything else.

    1) Test first with bulb limiter that we have filament voltage across proper tube pins at every tube. No tubes plugged in.6.20

    2) Now that each tube pin has about half that relative to ground.3.10

    3) Now repeat tests with tubes in their sockets, *still* through builb limiter.~5V between filaments wires, ~2.5V between each filament wire and ground. On V1 the voltage between each filament wire and ground is ~2.2V and ~2.5V
    I donīt care you will now have way less than 6.3V at each filament, we are just testing winding does not collapse/short under real load.

    Post results.
    I presume that above values was less than 6.2V because of the lamp. By the way, this time the lamp lit up normally and immediately the brightness went down, you could see the orange tungsten filament, like tubes filaments.

    Here I am

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  23. #93
    Old Timer J M Fahey's Avatar
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    Ok, now connect bias winding *only* and check that you have bias voltage and that it reaches power tube grids.

    You will have less than expected values, say, -20/25V instead of around -50V, thatīs because of the bulb limiter, no big deal.

    I want to be certain that you have negative bias before we apply high voltage to plates and screens ... which would be deadly if unbiased.

    Like the other thread says: letīs check the obvious.
    We canīt assume "everything is fine" without proof.

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  24. #94
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Ok, now connect bias winding *only* and check that you have bias voltage and that it reaches power tube grids.

    You will have less than expected values, say, -20/25V instead of around -50V, thatīs because of the bulb limiter, no big deal.

    I want to be certain that you have negative bias before we apply high voltage to plates and screens ... which would be deadly if unbiased.

    Like the other thread says: letīs check the obvious.
    We canīt assume "everything is fine" without proof.
    I'm assuming 'bias winding' indicates 'bias tap' and 'center tap'. Let's be sure Vince understands before we spin out of control

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    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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    Quote Originally Posted by J M Fahey View Post
    Ok, now connect bias winding *only* and check that you have bias voltage and that it reaches power tube grids.

    You will have less than expected values, say, -20/25V instead of around -50V, thatīs because of the bulb limiter, no big deal.

    I want to be certain that you have negative bias before we apply high voltage to plates and screens ... which would be deadly if unbiased.

    Like the other thread says: letīs check the obvious.
    We canīt assume "everything is fine" without proof.
    Quote Originally Posted by eschertron View Post
    I'm assuming 'bias winding' indicates 'bias tap' and 'center tap'. Let's be sure Vince understands before we spin out of control
    In fact I'm not sure I understood. Juan talks about negative bias and this suggests that I need to connect the Red / Green wire (50V output) to the diode of the baseboard.

    Also, he says he want to check if I have bias voltage at power tubes grid.

    I have already measured the voltage between "bias tap" (Red/Green) and "center tap" (red/yellow).

    Anyway, I want to be sure I'm doing the right thing. Also, now the sockets are disconnect from all wires, then I have connect the 220K grid resistors to power tubes for I be able to read negative bias voltage.

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  26. #96
    Supporting Member eschertron's Avatar
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    Let's see if I understand. Do you have any connections other than the wires from the power transformer disconnected? If so, I don't think that was necessary. At this point, the red high voltage wires should be disconnected and taped off to make them safe. Other than that? Nothing that I can think of.

    With the amp (except the red wires) wired up properly, the bias circuit will take that 50vac from the red/green and red/yellow pair and convert that to a negative DC bias voltage that can be read between ground and the grid pin connections on the output tubes sockets. If you don't get the negative bias at the power tube pins, then any high voltage will kill the tubes PRESTO!

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    Old Timer J M Fahey's Avatar
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    To make it clear, we are going step by step.

    I never said "disconnect everything and restart from 0", whatīs implied is than any new suggestion *adds* to what was already there. Unless specifically told otherwise.

    So Red/Yel which is the high voltage winding is already connected to supply/chassis ground and stays there .

    Filament windings also stay there.

    Only "news" is that NOW we connect the bias tap where it belongs, the small bias supply, so to anything it includes: its own diode, resistors, capacitors, trimmer, etc. , I shouldnīt need to name all besides saying "connect bias tap where it belongs".

    connect bias winding *only*
    means "add that only and nothing else", since vinceg is very impatient, and when told to go one step forward, he walks 10

    Juan talks about negative bias and this suggests that I need to connect the Red / Green wire (50V output) to the diode of the baseboard.
    Yes.
    Also, he says he want to check if I have bias voltage at power tubes grid.

    I have already measured the voltage between "bias tap" (Red/Green) and "center tap" (red/yellow).
    You measured AC voltage at a winding, now I want you to measure Bias Voltage which is DC.

    Anyway, I want to be sure I'm doing the right thing. Also, now the sockets are disconnect from all wires, then I have connect the 220K grid resistors to power tubes for I be able to read negative bias voltage.
    Did you disconnect everything from power tube sockets?
    Why?
    I only asked you to disconnect *transformer* wires fron anything so they can be measured without interference, but nothing else.

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    I have all wires disconnected except those that go to pots. The red high voltage wires are disconnected and taped.

    Red/yelllow is connected to ground. If I connect Red/Green to diode and the grid resistors to power tubes I'll be able to read negative bias at pin socket. Right?

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    I had disconnected everything for safety before you gave the instructions step by step.

    Anyway I understood correctly and I assure you that I do not want to run.

    Ok, tomorrow morning, at 8.00, when you are steel asleep, I will proceed with this step and update you.
    It is inplied bulb current limiter use.

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  30. #100
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    I have not measured between Bias and center tap because last night while I was measuring the 500mA fusein bias wire is burned, then I'm scared. I suspect the problem is at 50 VAC out winding.
    From this I assume that there is a 500mA HT fuse wired between red/yel and ground, like in a JTM45 circuit. It is not shown in the layout.

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    Hi Vincenzo,
    I think it would be good if you will post an updated photo of the amp. The photos in your posts #2 and #3 are very good quality but each photo shows only a portion of the circuitry. Just one new photo is requested. A photo that shows the entire view of the inside of the chassis. This will help the members see first hand the current state of the amp.
    Sincerely,
    Bill

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  32. #102
    Stray Cap DrGonz78's Avatar
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    Quote Originally Posted by Reader View Post
    I think it would be good if you will post an updated photo of the amp.
    Just looking over the original pictures it appears that turret board is not mounted to the chassis. My thought here is that the underside of the board might have the ability to short something out to the chassis. This might not be the case but since it does not appear to be mounted I felt it worth mentioning.

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  33. #103
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by vinceg View Post
    Red/yelllow is connected to ground. If I connect Red/Green to diode and the grid resistors to power tubes I'll be able to read negative bias at pin socket. Right?
    Yes ...if wires leading to the socket are also connected

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    Goodmorning people!

    I connected the bias winding and check for negative bias voltage at power tubes grid. The voltage is -44VDC each power tubes.

    Before proceeding with the next step I want to point out that I replaced the rectifier valve with diode like picture below, in case this could be a problem for our work
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    Old Timer J M Fahey's Avatar
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    No problem.
    Now you connect Red_T and Red_B to the rectifier, and turn amp pn.
    Still with lamp limiter and remember to connect some speaker to the output.
    All controls set to 0.
    Do you get high voltage?
    Check main filter caps, plates, screens, and follow the line which feeds the preamp tubes.

    You will have less than normal, probably around 300V DC.

    Post results.

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