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back again with the old tube amp!

Discussion in 'Pro Audio Equipment' started by drumist69, Oct 3, 2005.

  1. drumist69

    drumist69 Active Member

    Ok. I've posted about this before, but haven't done anything yet, other than buy this old tube amp. The unit was pulled from a Zenith mono record player cabinet (circa 1956!). The guy I got it from re-capped it, replaced the weak tubes, and converted the ins and outs to 1/4" jacks. The amp is 15 watts. There is a 6J5 tube in the preamp stage, a 12AX7 (it says AF Amp-Phase Inverter in the specs...is this for the tone controls?), a 5y3gt rectifier tube, and a pair of 6v6gt for output tubes. First thing to do is replace the old two prong cord for a grounded cord! I'm thinking this can be used as a nice small guitar amp pretty much as is. I'd have to move a few things around to get it in a cabinet, but no biggie. The one thing I was wondering about was the possibility of using this as a mic pre. I know I don't want 15 watts going into my PC, so I was thinking the way to go was to use an attenuator to reduce the output. Today, I was reading about removing an output tube from a guitar amp to reduce the power for lower volume playing. I got to thinking, would it be possible to pull both output tubes and just use the preamp stage? This is my first exploration into tube electronics, and I haven't found any info regarding this idea yet. I know from reading in this forum that Kev would be one guy to talk to about this, so c'mon Kev, tell me what a stupid idea this is! If I pull both output tubes, what will happen? I have a schematic, but I'd have to scan it at work tomorrow to post it. Peace!
     
  2. MilesAway

    MilesAway Guest

    You can remove 1 tube in Class A/B amps ONLY to cut the power in half... there are impedance issues involved with this that i can't remember so you'd be better off looking this up. Regardless though, i *really* doubt an amp that old (and that low-power) is running in Class-A/B operation so chances are, that's not going to be an option for you.

    As for "pulling both tubes" and running it as a preamp... DON'T! If you just want the preamp functions, find out where the preamp goes into the poweramp in the schematics and wire up a jack there, in the same way an "FX-Send" would be wired in a modern amp.
     
  3. MilesAway

    MilesAway Guest

    sorry - just clarifying the first part: You can remove one tube in EACH PAIR for Class A/B amps...
     
  4. CoyoteTrax

    CoyoteTrax Well-Known Member

    At 15 watts output you're probably talking 8 Ohms...mayyyyyyybe 16. As a guitar amp I'm sure it will sound very cool, you just need a speaker enclosure to connect to, which can be bought cheap on ebay usually. Impedence is the next question though. What's the output impedence? Personally I've purchased a few on ebay for different tones. The last one I bought was just a late 50's, early 60's Viking enclosure loaded with it's original 8 in. speaker. Ya gotta love the sound of those old magnets. Mic'ing an 8 in. enclosure works for guitar AND bass. Paid like $15 or something like that :lol:

    Miles has the right idea about adding an output jack where the preamp section ends. Then you can use it as a pre-amp or instrument amp, which is an advantage of using antique RTR's for the same purpose.

    An attenuator will work as well to convert this unit in a nice DI when you want.
     
  5. drumist69

    drumist69 Active Member

    thanks!

    Thanks for the input, guys! Lets see, the load is 8 ohms. I have a speaker I will use until I score a cooler one. I've got a lead on a pair of 1950-1960 era Jensens ( I think they're 12's)...the guy will sell me the pair for $50!!! I'm going to check those out soon. As for the pre-amp jack...duh! That's a great idea! I should have thought of that! It looks like this amp will be my project for this winter. So now I have to move the in and out jacks, add a jack in the pre-amp stage, build a cabinet for the whole thing, and one for the Jensens if I get them. I will just build a single enclosure, and probably hold on to the other speaker, or sell it for way more than he's selling them to me for! If and when I get this project off the ground I'll post some audio clips of what it sounds like, and maybe pics. Thanks again! ANDY
     

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