Twin vs. Deluxe

Chat about Equipment Info

Postby jonarobb » Fri Oct 24, 2008 11:07 am

Pete B. wrote:I haven't tried it myself, but anybody try yankin' two tubes from their Twin and maybe even replacing the remaiing two with 6V6's, in order to open up the preamp volume to the 7-10 range?... (the idea being to hit the preamp tube cream without peeling the paint off). Just curious.

I've played (and heard others play), some '65 Reissue Deluxes that sounded really good to me.
I thought the tone and reverb were great for the lower volume playing I am becoming more fond of.
I played Steel through a '65 Reissue Twin recently and it sounded pretty good to me also.
I've heard some quality control horror stories on both amps, though.


Do not pull any power amp tubes on a Twin and do not put any 6V6's in a Twin. The one thing to do to a Twin to give it some warmth is to score a good quad of older PHilips/Sylvania 6L6GC's that can take the abuse, and then jack the current draw(bias) way up. If your're into revoicing the preamp from a power supply standpoint then you'd go right for the plate load and cathode resitors. I can make a Twin sound like a Marshall or anything in between. But all that pulling or swapping tubes in any positions. It's pure BS.
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Postby Jon S. » Sat Oct 25, 2008 5:56 am

Never put 6V6s in an amp that wants to see 6L6s. The caveat on that statement is that I have heard secondhand report of people safely replacing 6L6s with - and ONLY with - new stock JJ 6V6s and a rebias. That's because, construction-wise, the JJ 6V6 is in a class by itself, robustness-wise.

However, in fact, it is perfectly acceptable to pull half the power tubes in a Twin or Twin Reverb. You just have to take care to do it right in two ways:

(1) pull them in the correct pair (in a Twin, that's the two middle tubes -or- the two outside tubes). This is because the tubes function in a push-pull paired capacity. A rebias shouldn't be necessary for this (amp tech John Phillips explains why below) but it never hurts to at least check your bias when pulling or changing any power tubes in a Class AB amp.

The only reason it [a rebias] would be [necessary in these situations] is on an amp with a lot of sag in the power supply (tube rectified), where the B+ voltage may rise slightly with two tubes not drawing their current - and even then probably not - but the Twin is SS rectified and has a pretty stiff power supply, so it should make no difference.


(2) accomodate the change in ohmage through a speaker change (what follows are John Phillips words):

When you pull two tubes you get an amp output impedance of 8 ohms - double the normal impedance, not half. To keep things in match, you'd have to double the speaker impedance too, to 8 ohms, but you can't with the same speakers. So, another way would be to set the amp to 2 ohms, if it had such a setting, still with the 4-ohm speakers. Halving the amp impedance is like doubling the speaker impedance.

Now... since you can't change the amp's output impedance (assuming it is a 100W Twin), you have an 8-ohm amp connected to 4-ohm speakers. This is a bit hard on the tubes, although it won't do any harm to the amp [Jon's note: this is because Fender amps generally are engineered to be able to handle this level of mismatch safely]. So instead, if you reconnect the speakers in series (should be easy, assuming the original push-connectors are still there), you have a 16-ohm load on an 8-ohm amp. Equally mismatched, but less bad for the tubes. It won't cause any other problems either, since the power developed is lower and the flyback voltages won't be any higher than for normal operation.

Alternatively... if you're still concerned with volume, just disconnect one speaker as well! Instant 8-ohm load.


Last thing I'll say now is that someone - like jonarobb - may not personally prefer the tone of a Twin running on one pair of tubes but never confuse this with the ability to do so safely. The content of this post addresses the latter. Thousands of guitarists have, and do, run their Twins safely this way and prefer the attendant volume and tone changes (though the actual volume change may be less than you'd anticipate!)
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Postby Pete B. » Sat Oct 25, 2008 8:42 am

That's pretty much what I have heard Jon S.... But as I said, I haven't had a reason to try it.
Which part is the Pure BS part exactly?

Steve Kimmock claims to regularly use Fender tube amps at Twice and/or Half their reccomended speaker Ohmage rating (and says this is the general no-harm-done rule), for tonal purposes (he writes a fair amount about this on his gear page on his site).
http://www.online-discussion.com/SteveK ... bb3675edfb

I'm using an Alembic F2B into a SS head for my high-power rig these days (Lexicon LXP-1 for 'verb).
I have a Fender Vibrosonic that I am now considering selling or trading for a good sounding Deluxe Reverb.
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Postby jonarobb » Sat Oct 25, 2008 10:42 am

The fact is, in a Twin Reverb the drop from say 100 watts to 50 watts is 3-4db. It's nothing, completely un-noticable. What you're really hearing is the narrowing of the frequency bandwidth. That, in and of itself negates any pulling of power tubes. Especially if The Grateful Dead's Hi-Fi approach to sound appeals to you....

Can you do it? Yes, sure. Should you do it? NO, definately not.

The BS part is addressing the amount of mis-information on the web about this. Without adjusting the impedance any change you hear is strictly the tubes and OT struggling to deal with the inefficiency on the OT's primary and secondary. What you end up with is a farty inefficient power section.

The Schumacher OT's were hardy units when they were new. The 2:1 mismatch philosophy is not something I subscribe to. I have a box of burnt up OT's here from various Fenders where users experimented with impedance mismatches, tube substitutions, etc...

Some sort of attenuator will address power amp saturation or revoicing the preamp will address some preamp break-up.

Sorry I was so blunt about that. I apologize. But it's not just personal preference, it is what it is.
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Postby Jon S. » Sat Oct 25, 2008 11:03 am

jonarobb wrote:The fact is, in a Twin Reverb the drop from say 100 watts to 50 watts is 3-4db. It's nothing, completely un-noticable. ...

ooo

... it's not just personal preference, it is what it is.

I disagree. There are differences in volume and tone that slap you across the face and other that are more subtle and feel-oriented. I would never attempt to argue with your position that it's "completely un-noticeable" (your words) for you. You hear what you hear. We need to be careful, however, when projecting our senses/sensibilities onto others. There is more happening with the change we're discussing now than is measured in average sound pressure levels (dbs), including frequency and harmonics adjustments and our psychoacoustical interpretations of them (i.e., two sounds can have the exact same measured db level but be interpreted by folks as noticeably louder or softer due to other factors).

EDITS: Amp feel/responsiveness, too (the latter is particularly noticeable when cutting from 4 to 2 power tubes in my Tone King Continental using the internally pre-wired 40/20 switch, though, as we all agree, the associated impact on volume is nowhere near half). I would also say that I think it should be easy to hear a 3-4 db volume increase or cut, e.g., when replacing a speaker rated at 96-97db with a 100db replacement.

I would strongly encourage anyone who is curious about this mod to try it for yourself and form your own judgments.
Last edited by Jon S. on Sat Oct 25, 2008 11:20 am, edited 4 times in total.
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Postby jonarobb » Sat Oct 25, 2008 11:16 am

Jon S. wrote:
jonarobb wrote:The fact is, in a Twin Reverb the drop from say 100 watts to 50 watts is 3-4db. It's nothing, completely un-noticable. ...

ooo

... it's not just personal preference, it is what it is.

I disagree. There are differences in volume and tone that slap you across the face and other that are more subtle and feel-oriented. I would never attempt to argue with your position that it's "completely un-noticeable" (your words) for you. You hear what you hear. We need to be careful, however, when projecting our senses/sensibilities onto others. There is more happening with the change we're discussing now than is measured in average sound pressure levels (dbs), including frequency and harmonics adjustments and our psychoacoustical interpretations of them (i.e., two sounds can have the exact same measured db level but be interpreted by folks as noticeably louder or softer due to other factors).

I would strongly encourage anyone who is curious about this mod to try it for yourself and form your own judgments.



I can agree with some of that jon. I'm from the camp of clean available power. Again, schooled in The Dead's philosophy of maximum frequency bandwidth, and efficient power supply. Again, you said it as well, the perceived sound difference is not pressure related, it's the change in the way the output transformer does it's job. That change takes shape with a negative connotation from my viewpoint. So, yes, that's a personal preference based on work experience and taste in tone.

I also encourage everyone to experiment. I mean, hell, one time I smoked angel dust while on qualudes and the outcome was amazing. I don't remember any of it but I was told I was in fine form. ;)
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Postby Jon S. » Sat Oct 25, 2008 11:21 am

Thanks, jonarobb. These conversations are great ways for all of us to discuss and think about tone from all angles. :)
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Postby Pete B. » Sat Oct 25, 2008 4:31 pm

Cool.
I just pulled the center two tubes on my Vibrosonic.
Sounds a little clearer/thinner in the bottom end, in a pleasing way.
One of the tubes I pulled had something clanking around in it, it was that little round thing (looks like a washer) that appears to be hooked to a stem in the other tubes.
I have the 1st channel jumpered to the second channel (through a phase inverter), both volumes about 3.5... treble cranked.
Sounds good for just playing at basement volume.
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Postby Jon S. » Sat Oct 25, 2008 8:53 pm

FWIW, I have no familiarity with that amp and couldn't vouch for whether its tubes are arranged analogously to a Twin (i.e., make sure you know what you're doing with it!).
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Postby Pete B. » Sun Oct 26, 2008 3:11 pm

I checked it out... Identicle to a Twin across the board, but with an 8ohm output transfromer for a single 15" JBL-D130f.
Pull either outer two or inner two tubes.
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