Inside the Blue Robbie tube preamp / Power supply for audio tubes

Discussion in 'Analog & Mixed-Signal Design' started by phantombox, Sep 24, 2016.

  1. phantombox

    Thread Starter New Member

    Dec 15, 2006
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    Hi.
    I have a Blue Robbie tube preamp that has the heater supply voltage somehow not working.
    This preamp uses a 6N23P tube, a Russian version of a European ECC88 or American 6DJ8.
    On both pin 4 and pin 5 (heater supply of the audio tube) there are -48Vdc present, while on one pin there should be -54V, in order to get a difference of 6V for the heater.
    After pulling every component that is somehow connected to the any of the rails I could not find any shorted or open cap or diode or a drifted resistor.
    Now I'm seriously considering to build a separate 6V psu for the heater circuit. Any idea on how to do using the existing -48VDC?
     
  2. #12

    Expert

    Nov 30, 2010
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    9 google searches and I can't find a schematic.
    Provide one or I'll have to use my crystal ball.
     
    Last edited: Sep 25, 2016
  3. phantombox

    Thread Starter New Member

    Dec 15, 2006
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    Here is the only one that I got. Unfortunately, the part of the PSU is missing...
     
  4. #12

    Expert

    Nov 30, 2010
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    That's unfortunate because that's probably where the problem is.
    I started out thinking the filament in the tube might be burned out, but there are so many parts that depend on the -48V line that the -48 couldn't possibly be derived from the -54V and the tube filament.

    I have used a 120VAC to 5V brick to power a 5Y3 rectifier floating on +315 VDC so I'm sure you can bolt on a 6.3V DC supply.
    This can't be a simple transformer. It has to be DC or it will pee sine waves all over your output amplifier.

    Still, you must look into the power supply to be sure the -54 isn't shorted to the -48 or that short will take out your new DC supply.
    As long as there is no knowledge of the power supply, this is risky business.

    Do you have the skills to sketch out the power supply from just looking at it?
     
  5. #12

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  6. phantombox

    Thread Starter New Member

    Dec 15, 2006
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    The external power supply works fine.
    It was originally bought in Germany and set for 230 VAC input, but since in my country we have 115-125 VAC (Venezuela is known for having a not-so-stable line voltage), I modified the wiring at the input of the toroidal transformer so that I don't have to put a step-up transformer in front of it. Although on the back of the Robbie it says "AC 10V/54V" just above the power input jack, the actual transformer output voltages are 12V/61V at both 230 VAC (before) and 115-125 VAC (after modification). Since the DC voltages inside the Robbie are regulated, that doesn't really matter that much. At least that is what I thought.

    And about "sketching out the power supply from just looking at it": the guts of the Robbie are somehow simple but at the same time incredibly complex. Component IDs (R1, R2, ..., IC1, IC2, ..., C1, C2, ...) are absent.
    It says MADE IN USA but I have the impression that it is actually assembled somewhere esle... (!)
     
  7. #12

    Expert

    Nov 30, 2010
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    That's good to know. Does it have the three power lines marked on the schematic and a ground?
    Does it work just fine until you plug it into the amplifier, then the -54V collapses to -48V?
    Have you tried a new tube?
     
  8. phantombox

    Thread Starter New Member

    Dec 15, 2006
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    I really don't know if the 3 power supplies are present at this point, since I destroyed a 78M06 regulator trying to pull it from the PCB. Tomorrow I'll get a new one. Before that, -54V collapsed to -48V. The 12VAC and 61VAC from the transformer convert to +48V and -48V, respectively. Ground is present
    I've tried with another known good ECC88 (same as 6N23P) but it doesn't work, either.
     
  9. phantombox

    Thread Starter New Member

    Dec 15, 2006
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    Yesterday I bought a new 7806. I installed it, accommodated the connector between tube socket board and main circuit board, and then checked the voltage on the heater rails. -48.5V and -54.5VDC. Hallelujah I thought, finally...
    Turned off the Robbie, inserted tube, turned it on and checked the rails again. Now it measures 47.5V and 48.5VDC.
    I really don't know what I am missing...
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    If its a European double triode, make sure you've read the number correctly - some in the ECC8x family have centre-tapped 12.6V heater.
     
  11. phantombox

    Thread Starter New Member

    Dec 15, 2006
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    Its an ECC88, with 6.3V heater, no center-tap
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    A lot of old scopes use that one - sometimes you find E88CC types which are "special quality".

    Lower spec scopes might contain; 12AT, 12AU or 12AX6, which are the American equivalents of the ECC81 - 83 range. Very occasionally, these can also be top grade selected parts.
     
  13. phantombox

    Thread Starter New Member

    Dec 15, 2006
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    Hi Ian.
    I know that. But the 12xx7 (not 12AX6...) tubes don't work on the Robbie. Only the 6DJ8 / 6922 / ECC88 E88CC) or specifically the Russian 6N23P which originally comes with the Robbie.
     
  14. ian field

    Distinguished Member

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    The design purpose of the ECC88 was for casc-ode amplifiers in TV tuners - but it is at least a low noise type.
     
  15. phantombox

    Thread Starter New Member

    Dec 15, 2006
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    Finally, I resurrected the Blue Robbie.
    First I cut the leads of the filament voltage supply pins 4 + 5. Then I added a small board with a 2nd LM7806 and heatsink, rectifier, 2 'lytic caps and 2 polyester caps.
    I took power from the 12VAC supply, and connected the 6VDC output directly to pin 4 and ground to pin 5 on the tube.
    After checking that everything was correctly assembled and that the tube actually does glow, I put everything together.
    Voalá, there it was working again.
     
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