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Old 20th March 2014, 01:48 AM   #1
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Default Turning old tube stuff into guitar amps

I've learned so much playing with non-guitar tube stuff, which is great for me because I've only been doing this a couple years now. I would first notice the similarities to standard Fender/Marshall circuits, but it's been the differences that have taught me things. Like a tube intercom that (to optimize voice) had a cap across the OT (bass-cut), and a 0.005u coupling cap in the preamp (hi-cut). Then there was a Webcor phonograph (coolest little P-P 6v6 amp!), with the weirdest tone stack....turned out to be the RIAA restoration circuitry. I've also got a Manatt-King p-p 6L6 PA that I looked at a year ago, but shelved it because it was too wierd.... it works, but why/how? Plan on digging it back out and trying to figure out all the idiosynchracies it it....
Does anybody else have something like this to share, to help all us newbies? I've learned some much from you guys! Thanks
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Old 21st March 2014, 02:36 AM   #2
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Old tubed 6V or 12V car radios are cool. Many have 6V6 or 12V6 output tubes. Check before you tear one apart, some are worth big bucks if you can get it working and find a buyer. There is a guy that I see at guitar shows that takes whatever he can find and converts it to a guitar amp.
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Old 21st March 2014, 03:09 AM   #3
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Yeah, I bet someone restoring an old car would love to have a working authentic radio in it! I've heard about low-voltage tube designs using a "space charge" principle to allow them to work with very low plate voltages (somehow using the first grid to accelerate electrons to the plate). Amazing innovations from the past! I tore into the old intercom because I just couldn't believe anyone would use one, although I did document the original schematic....
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Old 21st March 2014, 06:23 AM   #4
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I hope you will find the following articles useful:

Converting Integrated/PA Tube Amps into Guitar Amps
Notes on Converting PA Amps to Guitar Amps
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Old 21st March 2014, 03:16 PM   #5
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Thanks jazbo! I've seen the first link (have it in my 3-ring "library"), but the 2nd is great! Especially "mu.com", on tube specs and comparisons. Thanks! I feel like a 2yo kid...every time I see something I don't understand, I ask "why"...then "why" again...! I don't mind reinventing the wheel, it's the journey. At my age, being still able to learn is precious!
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Old 31st March 2014, 11:26 PM   #6
seta1 is offline seta1  United States
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I've been doing this for years. PA amps are easy. They are pretty much already guitar amps. Most have those funky screw on input connectors which you need to replace with standard 1/4" phone jacks. The tone controls are generally perfect for guitar work. Usually Baxandall type and not the types of tone controls referred to as 'tone stacks' or Marshall Vox Fender type tone control circuits. Hifi amps are of the type where they either have only a line level type gain stage ( units out of consoles with Xtal phono inputs) or the type with a true RIAA phono stage ( more expensive amps like the Scott 99D). A cheap quick trick to enable the use of the RIAA phono input is to solder a 150 pf capacitor between the phono input jack and it's circuitry in the amp. This effectively negates the RIAA equalization yet retains the gain of the phono stage, which you will need to properly amplify a guitar signal. You can experiment with the value of this capacitor, but 150 pf has worked for me in the past. RIAA networks are designed to heavily boost bass while cutting treble. This capacitor heavily cuts bass while retaining the high frequencies. Like I said, it is a quick and dirty fix for the RIAA eq in a vintage tube hifi amp's phono stage, which is the only stage with enough gain for guitar use.
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Old 1st April 2014, 06:49 PM   #7
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seta1-thanks for the tips! I usually adapt those old "UHF" button connectors with a cut-down male connector - use them on my old radio RF signal generators. My PA has 3 input channels, all octal 12SJ7s. Want to find a dual triode 8-pin and rewire them to get another gain stage for guitar use. Need to look at Baxandall tone circuits, how they differ from the usual tone stacks.....been playing with tremelo circuits, both tube and SS. I hear the Magnatone actually had a true "vibrato" - wonder how they did that.
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Old 7th April 2014, 12:20 AM   #8
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The 12SJ7's actually have a lot of gain on tap and can sound very good if you choose the right circuit values, but the octal 12ax7 "equivalent" would be a 6sc7 but they have no lower gain subs with the same pinout and they have common cathodes I believe so limited circuits are possible.
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Old 7th April 2014, 03:59 AM   #9
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Thanks - I will play with the 12SJ7's, learn how pentode preamps work. Definitely more complicated that a triodes! I've added extra stages to tube amps, only to then have to attenuate the heck out of them to keep the distortion under control!
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Old 9th April 2014, 03:45 AM   #10
billc99 is offline billc99  United States
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Quote:
Originally Posted by bparnell57 View Post
The 12SJ7's actually have a lot of gain on tap and can sound very good if you choose the right circuit values, but the octal 12ax7 "equivalent" would be a 6sc7 but they have no lower gain subs with the same pinout and they have common cathodes I believe so limited circuits are possible.
I'm a big fan of 6/12 SJ7's too, have used them in several amps, most recently a Gibson GA-9 clone built with a 12SJ7 and two 12V6's in parallel SE, best sound of anything I've built yet and about 9 watts FWIW...sort of a 5C1/Les Paul Jr. with an extra tube.
The 6SL7 is sometimes used as an octal equivalent of a 12AX7 and does have seperate cathodes...but hum can be a problem from what I've experienced and have read (though it can be sorted out) The 6SC7 and 6SJ7 usually have metal cans and this can help with hum control...but the 6SC7 has common cathodes as was said so that limits its usefulness.
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