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Posted: Wed Sep 08, 2010 3:45 pm
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Arjay, I think that the 7025 has spiral wired heaters. This supposedly cancels out hum; kinda like the idea behind twisting AC lines within the chassis. The Sovtek 12AX7LPS has a similar wound heater wire. I found if you buy some of these Sovteks, selected for quietness and strength---they are pretty good tubes. Lots of hi-fi guys used this tube in sensitive phono gain stages, with success.

I have a stash of RCA 7025 black plates. They are great tubes. Getting a bit hard to find good NOS ones these days.


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Posted: Wed Sep 08, 2010 4:01 pm
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I think you may be right. I have a small stock of NOS Amperex 7025/12AX7A types and their heaters are braided.

Thankfully I "liberated" a large number of brand new mil-spec 7025s before I retired from the military (various mfrs, all US-made). I can keep my vintage Fenders happy for decades to come.

These -- along with many other vacuum tube types -- were actually headed for the base's dump before I intervened. Over sixty 6L6GCs alone (half of them RCA black-plates)!

:mrgreen:

Arjay

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Posted: Wed Sep 08, 2010 8:28 pm
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Wow... good call saving those!

I just pulled the tube out of V1 to take a look. It's the original for sure. Fender label 7025. Interestingly enough, in faint gray lettering is "12AX7" underneath. I seriously doubt any of the smaller tubes have ever been touched on this amp.

I think a retube will really spice up the tone!

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Posted: Wed Sep 08, 2010 8:48 pm
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My '78 ultra-linear Twin Reverb came with a full set of Fender-branded bottles when I bought it new. Several of them are still residing in the pre-amp (and working fine). These tubes were generally made by Westinghouse and GE though there may have been other vendors as well.

If/when you need to replace your 6L6's I'd recommend JJ's, Sov-Tek's, or Tung-Sol re-issues. The 75 pushes its glass substantially harder than say a Bandmaster or a Vibrolux Reverb.

Best of luck, HTH

Arjay

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Posted: Thu Sep 09, 2010 7:35 am
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Retroverbial wrote:
My '78 ultra-linear Twin Reverb came with a full set of Fender-branded bottles when I bought it new. Several of them are still residing in the pre-amp (and working fine). These tubes were generally made by Westinghouse and GE though there may have been other vendors as well.

If/when you need to replace your 6L6's I'd recommend JJ's, Sov-Tek's, or Tung-Sol re-issues. The 75 pushes its glass substantially harder than say a Bandmaster or a Vibrolux Reverb.

Best of luck, HTH

Arjay


I'll probably go with JJ's. I'm going to contact Eurotubes and see what they recommend for a full retube kit. And yeah, I've read that about how the 75 somehow jams 75 watts through a pair of 6L6 power tubes really pushes them to their limit. Right now it's got a pair of Mesa 6L6's in there.

Not sure if you'd know, but are these amps biasable? On the back all it has is the "hum balance" and "output tube matching" pots.

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Posted: Thu Sep 09, 2010 7:45 am
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As you've noted, the 75 has a quasi-biasing pot labeled as an output-tube matching control. It's not a traditional adjustable bias circuit but it's adequate for the task. I do not know how complex a conversion to a bonafide bias-adjust circuit would be. Despite the allegations, I wouldn't expect to see 75 watts at the output jacks -- a true sixty watts is a more logical assumption.

HTH

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Posted: Thu Sep 09, 2010 9:56 am
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Retroverbial wrote:
As you've noted, the 75 has a quasi-biasing pot labeled as an output-tube matching control. It's not a traditional adjustable bias circuit but it's adequate for the task. I do not know how complex a conversion to a bonafide bias-adjust circuit would be. Despite the allegations, I wouldn't expect to see 75 watts at the output jacks -- a true sixty watts is a more logical assumption.

HTH

Arjay


I was playing with the tube balance pot the other day. The amp is a bit noisy at idle - not so much that it's offensive - but I just wanted to see how it worked. From what I can tell, you just use your ears and dial it in until you hear the least amount of hum.

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Posted: Thu Sep 09, 2010 7:32 pm
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PTM, the 75 is biased by means of changing the 33K resistor( if that hasn't been changed) on the balance pot. IMO this is a very stable design, some what inconvenient. But once set-up with quality power tubes, it works fine in conjunction with the balance pot. You know for that idle hum, I would have the tube balance pot in the center of it's rotation. Then adjust the heater hum control for the lowest noise. Then the tube balance control. Actually, the power tube's current draw should be monitored while balancing the tubes, inorder to get the most out of that control. Just a suggestion Art

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Posted: Mon Sep 13, 2010 9:18 pm
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Just ordered myself a full compliment of new JJ's from Eurotubes today. Should be interesting to see what this amp sounds like following a retube!

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Post subject: Update:
Posted: Tue Sep 21, 2010 8:44 pm
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Just got my box of tubes from Eurotubes in the mail today. WOW! Did they ever wake that 75 up! It sounds like an entirely different amp! The high end sparkle is there now - and does this amp ever do a good blackface impression. And the gain channel... it sounds nothing like it did before. With the old tubes, anything above 2 on the pre-gain dial just turned to fizzy buzz. With the JJ's the whole sound and character of the lead channel is different. It sounds much more like a classic tube amp overdrive. With a sweet grind instead of a harsh buzz. Plus, it seems like I have a lot more play in the lead settings to get usable sounds.

Talk about a diamond in the rough. These amps really are the best kept secret in the Fender line. I swear, if they'd have slapped the name, "Deluxe" or something like that on them, they wouldn't be so rare and undervalued.

It's still not a Super Reverb, but after the retube I would rate the sound to be above that of an HRD amp. (And keep in mind, that's just with the new tubes slapped in, I still haven't even spent time to re-dial in the sound.)

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Post subject: Re: Update:
Posted: Tue Sep 21, 2010 10:39 pm
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Powdered Toast Man wrote:
It sounds much more like a classic tube amp overdrive. With a sweet grind instead of a harsh buzz. Plus, it seems like I have a lot more play in the lead settings to get usable sounds.


Glad to hear the new glass put a smile on your face, PTM. I had a feeling they would.

It isn't any secret that the 75 (and the companion 30 and the 140) were imspired by the amps that Mesa-Boogie were building. In some respects the Fenders were in fact superior (primarily due to legendary designer Ed Jahns who insisted on over-spec components in all critical areas) and even today the saturated gain channel sounds much more organic that the "fizz/more-fizz" tones of the HRD series. It was a tragic blunder that Paul Rivera did not allow this series of amps to fully gestate and develop a loyal following of customers, much like FMIC should've done with the equally-innovative Studio Bass combo.

Arjay

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Post subject: Re: Update:
Posted: Wed Sep 22, 2010 9:42 am
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Retroverbial wrote:
In some respects the Fenders were in fact superior (primarily due to legendary designer Ed Jahns who insisted on over-spec components in all critical areas) and even today the saturated gain channel sounds much more organic that the "fizz/more-fizz" tones of the HRD series. It was a tragic blunder that Paul Rivera did not allow this series of amps to fully gestate and develop a loyal following of customers, much like FMIC should've done with the equally-innovative Studio Bass combo.

Arjay


That, I totally believe, given that the amp pushes 75 watts through only two 6L6's and the use of 7025 tubes for the preamp instead of regular 12AX7's.

So $250 for the amp, $100 for tubes - only $350 into this thing and it's killer. I'm reasonably sure that if I ever get tired of it, I could flip it for at least $500.

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Posted: Wed Sep 22, 2010 9:46 am
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Most vintage Fenders used 7025s in the front end because they have a lower noise floor than the more common 12AX7.

Arjay

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