63′ Gibson/Maestro GA-1RVT repair

I dig a good challenge // reverb and tremolo were DOA, more surprises waiting within … here’s a quick rundown of what I found …

(i) the PSU and overall wiring … everything looked and measured pretty ok considering age and corroded state of the chassis // that is, except for a filament voltage that laid at 5.8vac instead of the customary 6.3vac (right on the edge of no-go) … (ii) turns out the reverb pan guts were destroyed // three of the four ferrite beads that drive the coupling transformers inside the can were totally stretched out and beyond repair (weird) … I wired in a fresh pair of wires with RCA plug ends and tried a few pans I have handy here and none were giving a strong enough response … I then tried hooking up a Zoom G1 effects box and got great verb/delays and an interesting signal boost option … the owner could try his luck at finding a suitable reverb pan that will couple (quite unlikely considering the low-gain natuere of the drive and recovery circuits) … the external effector approach is very cool IMO and opens up a host of potential applications that goes well beyond what the basic design could do // keeping the original signal path and Tremolo circuit intact … to make this work I loaded down the reverb transformer output with a resistor to keep the signal from getting too massive there (the Zoom G1 has a high Zin so we can’t rely on that to keep the signal tame) … with a properly chosen load value this would not interfere with a reverb pan if we decided to use one, that option remains open // best of both worlds … (iii) the Trem, turns out there was a cold solder in the circuit, and in addition the shorting switch on the back of the Speed pot was no longer working // doubly dead … what I decided to do was to use the Reverb kill pedal and wire it to the Trem pot switch terminals … we now have a perfectly functioning and foot defeat-able tremolo … and with the reverb, now potentially an external unit, can be foot defeated as well // this made most sense … (iV) other things … I found the 620ohm Cathode bias resistor on the power tube to measure around 880ohms … I inserted a 5watt 600ohm resistor that actually measures 612ohms cold // Cathode bias voltage went form 30vdc to 18.5vdc, schematic shows 20vdc … the gain of the amp has gone up as a result, reaching max (stock) power … I did not tweak or alter any other part of the signal path // it could be modded for more gain, but I didn’t touch it … also, to try to remove some hum form the circuit I lifted the grounded side of the filament circuit and applied two 100 ohm resistors to balance the filament circuit w.r.t. ground … didn’t notice a huge change, but I left it balanced/modded

I tested the amp using the speaker in my Champ; that is, a Celestion Super8 // indeed, I was getting great sounds using the Zoom G1 in the reverb’s Send/Return circuit … killer actually

update:

after a first round of mods I was still annoyed about the amount of hum, even with the volume controls set to zero, the amplifier had // of course, this is very common with single-ended (SE) amps … after taking a closer look at the grounding scheme I noticed that the whole amp was grounded to the chassis via the input jacks … the only other time I’ve seen this was in an 90’s re-issue AC30, which was also noisy (in that case beyond repair) …

all I did in this case was re-route the two main current absorbers in the PSU circuit by giving them their own seperate attachment points in the chassis // as far as possible from the input jacks at the same time … this, in tandem with the balancing of the filament circuit, gave the amp a virtually hum-free performance … as it lies now, this is definitely an impeccable “recording ready” guitar/harp tube amplifier, with a natural tone that compresses naturally … amazing little amp // apparently Gibson shipped 232 of these in total between 1962 and 1963

For a full resolution scan of my art-work and more design details on this fine tube amplifier please consider joining my work forum and perusing through the Thermionics section:

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5 comments on “63′ Gibson/Maestro GA-1RVT repair
  1. Jim McConnell says:

    Hey, do you have that amp? If so would you like to sell it. Nice work! Thanks, Jim

  2. Rodney Kroetsch says:

    I have one of these amps. Dead reverb and trem. I am hoping to get it working again. I enjoyed this blog.

  3. Agu says:

    Hi,
    I would like to built small less than 2 watt with Reverb and Vibrato. I see than GA1RVT is a good choice for me. So I would like to join this forum.

    My backgroud experience on building Fender CHamp 5F1, Marshall 18 Watt, Spitfire Bassman 5F6 and Repair Tremolo (Photocell) Gibson GA20RVT, Guyatone GA330D, etc…

    OK
    thanks and BR
    Agu

  4. jcm says:

    THANKS FOR THE NOTE AGU … YES, WE CAN CERTAINLY TURN THIS INTO A DIY PROJECT AT THE FORUM … I WILL CONTACT YOU AND PROVIDE A NOTE (ABOVE) ONCE ALL THE MAIN BITS HAVE BEEN SOURCED … IN THE MEAN-TIME I WOULD RECOMMEND YOU PURCHASE MY “UNLEASHED CHAMP” PDF AND STUDY THE CHASSIS GROUNDING PRESENTED THERE … CHEERS

  5. jcm says:

    HEY ROD, HOPE YOU GET THE FX’S WORKING … WICKED LITTLE AMPS!!

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