Garnet stencil (Granada) with no AC isolation transformer

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cancon

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2011-03-06 2:58 am
  • #1
    Hi everyone,
    I've got my first amp on the bench where I'm completely stumped about what to do as part of a general service job. This particular amp has no AC transformer.

    It is a Garnet stencil amp (Granada) from the early 70's, model 10P-T.
    (link to circuit removed)

    So...the amp works best as I can tell. I'm currently hunting for a schematic, this amp runs on 2x 50C5 tubes (both Garnet silk screens), a 12AX7 and another unknown dual triode.

    Any advice about how to proceed? Is it safe to run? How about adding a grounded AC cable, or is that an issue due to the lack of AC isolation?

    Thanks in advance

    adam
     
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    JonSnell Electronic

    JonSnell Electronic

    Member
    2012-12-28 12:03 am
    The Jurassic Coast, England. GB
    www.jonsnell.co.uk
  • #5
    cancon said:
    Hi everyone,
    I've got my first amp on the bench where I'm completely stumped about what to do as part of a general service job. This particular amp has no AC transformer.

    It is a Garnet stencil amp (Granada) from the early 70's, model 10P-T.
    Here are some shots.

    So...the amp works best as I can tell. I'm currently hunting for a schematic, this amp runs on 2x 50C5 tubes (both Garnet silk screens), a 12AX7 and another unknown dual triode.

    Any advice about how to proceed? Is it safe to run? How about adding a grounded AC cable, or is that an issue due to the lack of AC isolation?

    Thanks in advance

    adam
    Click to expand...
    The unknown triode is probably a 6SN7 or 6SL7.
     
    JonSnell Electronic

    JonSnell Electronic

    Member
    2012-12-28 12:03 am
    The Jurassic Coast, England. GB
    www.jonsnell.co.uk
  • #6
    Speedskater said:
    It might be similar to the old All American Five AM Radio. That radio had 5 tubes with the filaments all wired in series so that they totaled 115 Volts. Only tubes were chosen that could operate with a plate voltage of 115 Volts. Most dangerous common item that you ever saw.

    See:
    All American Five - Wikipedia, the free encyclopedia
    Click to expand...

    In the UK we had, up until 1971, radio sets and televisions wired with a "Live chassis". The valve heaters were wired in series and stabalized with a ballast resistor. If the mains was connected the correct way around, as the neutral wire was actually at Earth potential, it was relatively safe to use. I have called at a number of houses with the old type televisions that were wired the wrong way round and the aerial cables were damaged by full live mains being fed along them when the aerial isolators had gone faulty. Very dangerous!
     
    Last edited:
    C

    cancon

    Member
    2011-03-06 2:58 am
  • #7
    As hard as it is to believe, there was never an AC transformer in this amp's design, it was intentionally left out, most likely to save on manufacturing costs. Apparently the 50C5 tube was also very common and cheap, hence its usage. Apparently these amps are only 3W. I just checked and the heaters are definitely wired up in series from one end of the AC to the other, however, pin 9 on both dual triodes is disconnected.

    See here: Garnet Stencil Amps - Different Brand Names - garnetamps.com - Home of the Garnet™ Amplifier Company. The CSA code points to Garnet and I believe it (not to mention the silk-screened tubes). If I can get my hands on the schematic I wouldn't be surprised if it were a recent tech drawing and not original, these stencil amp schematics were probably never released. But more to the point, simply put an isolation transformer and that's that? how many VAs should it be rated for?
     
    Last edited:
    C

    cancon

    Member
    2011-03-06 2:58 am
  • #8
    cancon said:
    As hard as it is to believe, there was never an AC transformer in this amp's design, it was intentionally left out, most likely to save on manufacturing costs. Apparently the 50C5 tube was also very common and cheap, hence its usage. Apparently these amps are only 3W. I just checked and the heaters are definitely wired up in series, and also I didn't realize the 50C5 has a 50v filament, so it starts to make sense.


    See here: Garnet Stencil Amps - Different Brand Names - garnetamps.com - Home of the Garnet™ Amplifier Company. The CSA code points to Garnet and I believe it (not to mention the silk-screened tubes). If I can get my hands on the schematic I wouldn't be surprised if it were a recent tech drawing and not original, these stencil amp schematics were probably never released. But more to the point, simply put an isolation transformer and that's that? how many VAs should it be rated for?
    Click to expand...
     
    Tubelab_com

    Tubelab_com

    Member
    2005-07-14 4:18 pm
    West Virginia panhandle
    www.tubelab.com
  • #9
    Amps like these were relatively common in the 60's, and yes they can be lethal. Get a Triad N-68X isolation transformer and wire it between the line cord and the amp. Then add a 3 wire line cord with the green wire tied to the chassis. The Traid is under $15 at Mouser, but a similar Hammond may be cheaper in Canada.

    Live chassis guitar amps are not possible. A live chassis makes for a dead guitar player. These amps did have a paper and wax capacitor connected between the line and YOU! How many of these old caps are leaky....ALL OF THEM! Yes, people have been killed by these old amps. Add the transformer before playing with it.

    I have resurrected a few amps like this and the Triad will run them OK. The two 50C5's are probably in push pull and probably made 5 to 7 watts. The mystery triode is probably another 12AX7 or maybe a 12AU7.

    It's also possible to add a real power transformer, rewire a few things, swap the output tubes for 6AQ5's, possibly change the OPT, and get 10 to 12 watts, but the stock speaker will fall apart if it isn't dead already.
     
    C

    cancon

    Member
    2011-03-06 2:58 am
  • #10
    Well, I was wrong - this is definitely a Gar Gillies original drawing:


    [IMGDEAD]https://farm9.staticflickr.com/8242/8588590178_08b8265c5f_z.jpg[/IMGDEAD]

    What I got from the schematic donor is that Gar had suggested to make sure the 2-prong plug is at least polarized, OR add an isolation transformer, and when grounding the amp via 3-prong power plug, to only earth the transformer chassis and not the amp chassis.

    Can someone explain why its not enough to have the polarized configuration so that 'hot' is always fused? That should technically take care of any shorts. If I were to guess, one serious problem would have to be 'unknown' ac power sources, that you never know if the electrician wired up the socket appropriately.

    Thanks for the input thus far, learning a lot here!
     
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