Tubes mounted on the PCB good or bad thing?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by nemp, Mar 9, 2006.


  1. nemp

    nemp Member

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    I've heard this is not the best way but is there an advantage to mounting tubes on the PCB board? Should i be more careful with playing too long or with out a fan?
     
  2. Guinness Lad

    Guinness Lad Silver Supporting Member

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    Lets see, Guyton, Custom Audio Electronics, VHT, Tone King and bunch more do, it all depends on how well the amp is made nothing more. This has been argued to death along with PTP verses circuit boards. If PTP was superior as said before the avionics industry would still be doing it. My Guytron has supports within 1/2 inch of each tube which keep the circuit board from flexing when installing and removing tubes. I'm sure others have thought this out as well.
     
  3. mbratch

    mbratch Member

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    I wouldn't confuse this question as a debate between PTP and PCB. I think you have to distinguish HOW the tubes are PCB mounted.

    My opinion is that if tube sockets are directly mounted to the same PCB with all the other circuitry, then that's a bad thing, especially if they're EL84's. Crate and Ashdown do this on some guitar amps, and both have reliability issues.

    Fender and many other manufacturers mount their tubes on a separate PCB that is then buss-wired to the main board. This is a much better design IMHO. At the very least, heat sensitive components and circuit board areas should be kept away from where the tubes go.

    BTW, how much tube-based circuitry does the avionics industry still use?
     
  4. VacuumVoodoo

    VacuumVoodoo Member

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    There are electrical safety factors that disqualify some methods of mounting tube sockets on pcbs. IEC norm 60065 (IEC norms are in force worldwide, same number but with different prefix in different countries, prefix "EN" in Europe):

    -max allowed pcb temperature is 90 degC under worst case conditions i.e. +10% overvoltage on mains supply and max power output from the amp. This disqualifies low sockets with short pins directly.

    - creepage clearances between pcb tracks and solderpads. Anode supplies up to and over 500V require clearances of ca 6mm (ca 0.23"). Standard pcb sockets with short pins don't pass this criterion.

    From reliability and safety point of view best solutions are:

    - high sockets with widely spread pins that conform to temperature and clearance requirements, they may still have some mechanical issues though.
    - pcb designed as if the socket was to be mounted on it but sockets mounted on the chassis and connected with wires to the pcb. IMO this is a prefered method.
     
  5. mbratch

    mbratch Member

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    There are tube sockets made for PCBs. My previous amp (that I sold) had them. But, the PCB was a nice toasty brown around the EL84 sockets. And some of the manufacturing RTV-like stuff used to hold some parts on the PCB melted and oozed around from the heat. ;)
     
  6. nemp

    nemp Member

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    Do Peavey C30s and H&K do this?
     
  7. claptonisgood

    claptonisgood Member

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    If one were never to change a tube, then I don't think PCB mounted sockets are a bad thing, but, in the real world--especially the world of tweakers--every time a tube is pulled and put back in, it puts a strain on the PCB...My last experience with this was just yesterday. I had a Peavey Delta Blues in for repair--the preamp tubes lit up but the power tubes were dead. I got lucky...the way Peavey puts this together it is with three PCBs connected with ribbon cables and wire leads that are bent at 90 degree angles. One of those wire leads must have been cold soldered at the time of manufacture and the subsequent changing of tubes over the years had worked it electrically loose. I was doing a "pliers yank" test and happened to find it. Off the top of my head, I have seen about a half-dozen amps this past year with problems related to PCB mounted tube sockets.
     

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