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Post subject: Preamp tube replacements
Posted: Tue Apr 20, 2010 12:06 pm
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I just replaced the power tubes in my 1978 pro reverb (70w). I am looking at replacing the preamp's now. What do I need to know when replacing these. Should I do all of them together, is there anything I should keep in mind. I have never replaced preamp tubes before. Is it only nessesary to replace some? The tone isn't what it used to be on the amp and I am trying to get it back to sounding like it did. The Power tubes helped quite a bit also.

Thanks.


Pre amp: Normal: 7025 Vibrato: 7025, ½ 7025, Phase Inverter: 12AT7 (long tailed), Other: Reverb Driver: 12AT7 Reverb Recovery: ½ 7025 Tremolo: 12AX7 (photoresistor)


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Posted: Tue Apr 20, 2010 1:16 pm
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You will do well to locate some NOS 7025 tubes and use those everywhere they're specified as per the tube chart inside your amp's cabinet. The 7025 is a military/industrial equivalent to the garden variety 12AX7 and has a significantly enhanced S/N ratio for quieter operation in audio gear. As well, a good quality 12AT7 at the V6 position (the phase inverter) will definitely yield audible differences which should keep a smile on your face.

HTH

Arjay


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Posted: Tue Apr 20, 2010 2:59 pm
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Thank you very much for your response. That's exactly what I was looking for.


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Posted: Tue Apr 20, 2010 5:38 pm
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Happy to help out, S&D.

Arjay


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Posted: Wed Apr 21, 2010 5:15 am
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Pay close attention to the pins when replacing the preamp tubes,they're easy to bend.You'll see a space between the pins that's wider than the other spaces and you need to look at the tube socket to line up the "key",that's what it is without the actual "key" that's on the power and rectifier tubes.


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Posted: Wed Apr 21, 2010 7:37 pm
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S&D, you don't have to replace all the pre amp tubes at one time. I would prioritize. Arjay's suggestions are spot on. But if you rarely use the normal channel, then I'd leave V1 alone. If the Vib channel is your main choice, then V2 is very important, it's the basic signature of your sound. A high quality, tested and matched 12AT7(V6), would be my other investment. If the amp is sounding good and functioning in all other respects, I'd go this route. These two tubes could easily cost over $100, from a reputable dealer. But like Arjay said, you'll be smilin'. Art

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Posted: Wed Apr 21, 2010 8:26 pm
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Right you are, Art.

If the effects channel is used primarily then those tubes in the 'normal' channel could conceivably be left "as is".

Personally, I've grown fond of using both channels in all my vintage blackface amps via a Radial Tonebone A/B/Y which gives these geriatric rigs a quasi channel-switching capability not originally envisioned by Leo.

Rawk on!

Arjay


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Posted: Wed Apr 21, 2010 10:14 pm
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Yeah Arjay, actually I probably use the normal channel in my amps the most. But I have them voiced with .02 Alessandro caps to get that piano -like bandwidth and natural guitar sound and feel. Never tried a switching pedal, but you're sure right about the convienence and versatility. Don't play out much any more, except church or music fests, and that's mostly acoustic. But I do have an old Dead Head bandmate, that won't let me rest too long. Let's see if I remember "Deal" ? Art

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Posted: Wed Apr 21, 2010 10:34 pm
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LOLOL

We've only got one Dead tune on our current play list.

♫ "Ridin' that train, high on cocaine......" ♫

On a Twin or Showman I like to configure the 'normal' channel a skosh on the bright-and-chimey side then dial in some additional "fattitude" on the vibrato channel for a smoother creamier vibe. When slaving both channels together it delivers a "Marshall-esque" tone, especially when I punch up the Fulltone OCD.

Arjay


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Posted: Thu Apr 22, 2010 5:04 am
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While on the subject of the BF normal and vibrato channels,Gerald Weber has a mod that converts the normal channel to a tweed tone,I'm not planning on doing it but it is interesting.
Has anyone tried something like that?


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Posted: Thu Apr 22, 2010 8:03 am
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Such a mod would require a modest amount of re-wiring and re-configuring since the tweed amp's tone stacks are cathode-follower based and located further "downstream" in the preamp circuit. As well, some decrease in screen and plate voltages in both the first and second stage of that channel would be required since the tweed circuit does not push the preamp tubes quite as hard (which helps give them their characteristic compression when driven hard to the point that power-supply "sag" occurs). Such a conversion, if engineered and performed correctly, might be very interesting (and useful) indeed.

Arjay


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