Harmony 525 Rebuild and transformer question

Discussion in 'Amps and Cabs [BG]' started by JSC, Mar 25, 2016.

  1. JSC


    Mar 25, 2016
    Hello all-

    First post, I am a beginning bg student who has rebuilt a couple of bass guitars.

    I acquired a Harmony 525 in pretty bad shape. This is my first amp project (besides building 2 Little Smoky's and things like that). I have some electronics experience, enough not to get killed by a tube amp, but some things like impedance matching are a great mystery to me.

    The question I have about it is the output transformer. According to the schematics I've found, originally it was a Supro T3788 with a 5000:8 impedance ratio.

    At some point in this amp's life it was swapped out for a Merit A-3128, which has a cnter-tapped 8000 ohm primary and an 8 ohm secondary (among several other secondary options, which are bundled out of the way.)


    So, is this transformer acceptable? If I leave it in there instead of buying the currently-manufactured equivalent, is it going to fry the 6L6 tubes, the transformer itself, or the speaker? Or will it just sound like crap?

    I'm trying to find out before next weekend, which is a big HamFest over in Raleigh, where I will be able to buy new tubes if I need them (so I need to power up the existing tubes before then to check.) Thanks for any help. At this point I could theoretically put the tubes back in and power it up, but...


    More to read about this project if you want, but TL/DR:

    My goal isn't to restore this to "original" but make a functional amp out of it. I got it cheaply enough that if I have to replace it component by component, I don't mind, especially considering the education involved.

    Tolex: the original has this cool pimply sort of ostrich pattern on it, so I opted to leave it, and reglue it where it needed it and use leather repair to fill in some gouges.

    Cabinet: removed dead bugs, disassembled it for cleaning, replaced the finishing washers, cleaned/buffed the oval headed original screws. Flipped the name plate over and refinished the back and stenciled on a new name. The main panel is really badly pitted and I have to replace it. Hope to cut and bend that this weekend. The speaker is fine- coil intact, reads 8 ohms, cone intact. Grille material was filthy so I cleaned it carefully, but I'd like to keep it because of the racing stripe.

    Power: replaced the terrifying nonpolarized plug with a grounded 3 prong socket, replaced the rusty/gritty power switch and neon lamp, removed the power inverter switch, covered the power transformer wires with shrinkwrap where the insulation looked iffy, checked for continuity and resistance on the windings, removed the ancient 10/10/20 uf can filtering cap and replaced with new electrolytics.

    Checked other wiring, have new jacks and pots to install once I get a new panel cut. Patched up the shielding foil here and there.

    There's a whole other question about preamp 12AX7's versus AT7's, but I'm not worrying about that now. I can swap more caps, resistors, tubes, whatever later.

    I hope to do a little post on this on my hackerspace's blog once I'm done, with before/after pics, so I hope to be posting that soon.
  2. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    that transformer is only rated at 14 watts and the original is 35.

    I'd only assume the frequency response is rather poor and a if you check the rca 6L6gc data sheets a impedance of 4 to 6 k is common.

    8k being a little high.
    but understand the impedance of a transformer isn't set. the ratio is.
    so using a different rated speaker impedance will reflect a different impedance at the primary.

    maybe someone was trying to make a Princeton style amp and is using the 8k transformer for a 6v6 pair of tubes to be used with common 8 ohm speakers.
  3. B-string

    B-string Supporting Member

    Concerns also you mentioned a CT primary on the replacement, was this not so on the original?
  4. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    yeah depending on the amount of power the power transformer provides I'd assume the original amp is a 25 to 35 watt 6L6 pair.

    you can still power up the amp
    sounds like you did a lot of work already. but a 14 watt transformer would get pretty saturated. low volume test or use would okish

    if the transformer provides 8k on the primary using the 8ohm tap.

    running a 4ohm load on the 8ohm tap will provide a 4k load on the primary. which is a better match for a 6L6 pair.

    most fender amps had 4.6k primary
    for 6L6 pairs
  5. B-string

    B-string Supporting Member

    The spec sheet even suggests a 6V6 pair not 6L6(GC).
  6. JSC


    Mar 25, 2016
    Thanks so much for all of these!

    I mentioned the center tap primary because I thought the 4K option from that might be a better fit, but given the greater than 2X difference in wattage between the two transformers, I think I might just go ahead and order the proper replacement.

  7. JSC


    Mar 25, 2016
    Forgot to ask: given that this has two 6L6 tubes, one wired to each side of the output primary, is this what is called a "push-pull" amp?
  8. beans-on-toast


    Aug 7, 2008
    Yes. The amp will have a phase inverter before the power tubes which splits the signal into two, one inverted that goes to a power tube, the other non inverted which goes to the other tube. One push, the other pull.

    Note that there is a difference between a 6L6 and a 6L6GC. The GC can take higher plate voltages, they perform better. Most often, when people say 6L6, they mean a 6L6GC. Just wanted to ensure that if you buy any NOS tubes at the hamfest, you get the right ones.

    You might want to consider a 12AY7 or the quieter 6072A in V1. Sone tweed Fender amps used this tube for a smooth tone.
    Last edited: Mar 28, 2016
  9. JSC


    Mar 25, 2016
    Well, I got my new transformer and wired it in. I should be ready to plug in the tubes tonight and throw the switch, so I'll know if I need new tubes. they are in fact not 6L6's, but 6L6GC's.

    One more Q that I haven't been able to track down: why do some tubes, like the 12ax7s in this preamp, have metal sleeves over them? Is it a noise-reduction/shielding feature?
  10. beans-on-toast


    Aug 7, 2008
    Yes, they are radio frequency (RF) and (electromagnetic interference) shields.

    The small signal tubes are amplifying relatively low level signals and they compete with noise. The amp doesn't distinguish between the two, they both get amplified. In radios for example, the noise can be more pronounced than the signal which makes separating them difficult.

    Anything that you can do to lower the noise level makes the amp quieter.
  11. JSC


    Mar 25, 2016
    Hello again- first of all, thanks again for all the help. Here's a link to my makerspace's blogpost about the rebuild. It's long but if anyone cares to read the details about my first amp rebuild, there it is.
    Still sounding great, BTW.

    1967 Tube Bass Amplifier Rebuild - Splat Space

    PS. I recently scored an old Variac which I cleaned and fixed up. It would have been nice to have that; would have reduced some stressful moments when I powered up the amp.
  12. beans-on-toast


    Aug 7, 2008
    Congratulations on a job well done.

    A variac is also invaluable when servicing the amp. Line voltage varies throughout the day in many areas. This affects the voltages inside the amp. Use the variac to set the line voltage at 120 VAC, or whatever you choose, and take a set of voltage readings. Typical readings include: AC line voltage, AC heater voltages, DC plate and cathode voltages on all tubes, DC screen voltages on the power tubes, and DC voltages at each power supply node. If the amp is fixed bias, note the bias voltage for the optimal cathode current and plate voltage. When servicing the amp in the future, use the variac to set the same reference line voltage, take readings, and use the recorded reference voltages for comparison.
  13. JSC


    Mar 25, 2016
    Thanks for the Variac test info; so far I've mostly been using it to avoid the "loud noises and Magic Smoke" moments.
    beans-on-toast likes this.
  14. BassmanPaul

    BassmanPaul Inactive

    Make your self a lamp box. it consists of an insulated box with an AC receptacle and a lamp base mounted to it. A three core power cable connects into the box. Ground in (green) connects to receptacle ground. Neutral in (white) to receptacle neutral, larger pin - identified. Line connects to lamp base centre. The other lamp connection goes to the receptacle line contact. Power reaches the receptacle via the lamp base. By screwing different sized lamps into the base you can control the inrush current. If the lamp lights brightly the amp has a short.

    The lamp base should be the fully insulated variety as it does connect directly to live voltage.
  15. Primary

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