Some Weirdo Amps of The 80s

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• my hunch is that you saw a tusc at music guild in danbury around '94. and my follow up hunch is that you could see that very same tusc, at music guild in danbury today.

  i think you owned a 4x12 spectra half stack that a college classmate of mine had bought (at music guild) and then traded to you.

  *i have no official affiliation with above-mentioned store, other than spending way too many hours wheeling and dealing there as a broke high schooler with a need for gear in the mid-90's...

• Your comment about the AC30 variations and few "being what George and John (and Hank Marvin) had is correct. No Vox amplifiers made in the UK were exported to the US until the Beatles became famous, at least not enough to be common at all. There were tariffs in those days, and shipping was a bugger too. When the Beatles created a demand the US distributor licensed the name and had a line of solid state amps made by an organ company. Almost all the "real" tube vintage JMI Vox amps here now were privately imported later.

  In the UK there were several revival or reissue AC30 runs and while most had similar circuitry most were physically radically different. They are also usually sonically different as well.

  And if you do get "the real thing" keep in mind that they are a half century old, that they were inherently very hot running, and that extensive rebuilding is almost always needed.

  If you are going to buy a tube Vox amp and not get screwed you have a lot of study to do.

• It's pretty easy to build a correct homebrew AC30 clone because repro chassis are available from WeberVST, if you want the slide in chassis. I would make a back panel with some fans if you do.

  It's a complicated design with a lot of parts. It also runs those EL84s pretty hard. But it is a unique sounding amp.

  Since it runs open loop, no feedback, an interesting design tweak is to use an interstage transformer to drive the 84s-or use a pair of any-common-octal power tubes-and have the pre drive a single ended driver tube. If you use any of the dollar or two vertical deflection tubes out there you get some beautiful harmonic complexity. If you use a EL84 or even a 6AQ5 you get cleaner with some nice overdrive depending on your choice of interstage transformer.

  Don't expect this to be a Strat friendly amp. It does well with Ricks and humbucker type guitars-in my opinion. YMMV.

  DO use a power transformer with a lower B+ secondary and a solid state rectifier, a tube rectifier is wasted on Class A designs. 84s like 300 volts, no more.

  • you know what, i have so many old fkkn interstage transformers lying around, they just pile up, i think this is a good idea, i am gonna build a gtr amp with one... so are you suggesting like (input) - triode stage, tone stack, 2nd triode stage, gain control, single cathode follower triode stage to drive interstage transfo, and then to PP grids?

    • Take the existing pre section you have and see what the level is to the phase splitter. If it is high enough, drive the single ended driver tube with it, if not add a gain stage. The driver tube is now the single ended small power tube. The interstage must have a center tapped secondary, it acts as the phase splitter to drive the outputs. This was what they did before WWII. You may need to add some load resistors to the output section so as the driver tube sees the right load.

      When the amp goes into overdrive, the idea is you are overdriving both the interstage section and the finals. This takes a little thinking in terms of the gain structure and the interstage has to have the right characteristics. You get some of the single ended quality of overdrive as in a Champ, and the push pull overdrive.

      The example I heard sounded great, but that was a matter of happenstance, the builder just had an old UTC LS interstage that happened to work just right. The finals present a bridging load so all they care about is that at full drive they get a certain voltage. You do not want to drive the grids very far if at all into conduction.

      Any size power tube can be used as a driver given the right load and turns ratio, but the smallest one you can find that sounds good is going to be least wasteful of supply current and generate less heat. In a Class B amp the power the drivers generate is needed, but here it just goes into the load resistors. Hence the 84 will work fine, but, smaller ones will do with less power draw and heat. A 12AT7 bridged with both sections will work, it's good as a power tube for about a watt, but sonically won't do much. (This is what the reverb driver of a Fender does.) The reason the one I saw used a beam pentode vertical deflection amp was it distorts a lot (they weren't too linear) and the builder had them laying around doing nothing. No one wants these tubes. 6AQ5s are a little 6V6 and are cheap, but you need a seven pin socket, which is why the

      Interstage transformers, which both provided voltage gain and gave a symmetrical balanced output to drive the push pull finals were common until 1) using a phase splitter tube was cheaper and 2) the use of negative feedback made it infeasible. It works in this amp because there is no NFB. They stayed in use for Class B applications like the 400PS Fender, and AM transmitter modulators until the seventies. Fender also built a little bass amp using one, the Musicmaster.

      You should not need a cathode follower.The driver tube is a high impedance load. You COULD use a cathode follower config for the driver, but that defeats the purpose.

• Cool post. i've got two JMF Spectra H-160s . It's a single channel head with pretty great onboard distortion, passable reverb, and interesting but very noisy chorus. It's labelled as 250 watts into two 8 speakers or a single 4 ohm cabinet. It's hard to believe lots of wattage ratings on solid state gear, but this one is very very loud. The cleans are great. Tight and crisp, but not boxy. I'd buy another one.

  • hi brett. As luck would have it, i found a spectra 120 (i think...) combo the other day for $30. The overdrive channel is dead but otherwise it works great. excellent sounding solid state amps.

    • I have one with reverb and I love it! Paid $40 for it. It smokes all other small amps.

• great price chris. i paid 100 for one with no footswitch and 150 for the other with a footswitch. had a custom one button switch made by switchdoctor that only activates the distortion for 60 bucks. these jmf spectra amps are surprisingly rare and also relatively inexpensive for the quality of build and sound they generate. there's been a h-162 two channel head with footswitch and 2x12 cab on ebay for months for $299. the seller is in seattle and it's a local-pickup-only deal. sounds like there are some issues with the amp and the speaker cabinet, but the seller hasn't budged on the price. i sent him a message offering to buy the head and pay for shipping, but he declined. if somebody reading this post has a h-162, h-85, or other spectra heads, i'm probably interested. brettadamsblog@gmaildotcom.

• hey guys,
  i just got one of them vt 4x10 combos about an hour ago.
  i had cleaned/repaired it for a guy, and a month later he offered me the amp for 200. i jumped on it.
  this thing sounds freeking great. loud clear and growly like a good marshall crossed with a traynor. totally solid state with push-button vegematic switches. sick.
  it is louder than f*ck, too... the reverb and phase sound great, i am looking forward to using it outdoors at a couple shows this summer.
  may seem like they're lame, but if you can get a hold of one, check it out. very warm, kinda reminiscent almost of the old tube works guitar combos on steroids.
  they take pedals great, are crazy loud and apparently fairly rare.
  i think ya'll are based in new england, right? if so, and you'd like to see for yourselves, let me know, i'm in hartford ct area.
  peace!