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Dirty-South Blues Harp forum: wail on! >
Removing power tubes and impedance mismatch?
Removing power tubes and impedance mismatch?
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Joe's_harpin
8 posts
Jun 25, 2014
7:52 AM
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Has anyone pulled the inside power tubes on a fender twin reverb-100 watts to reduce it to 50? Did it reduce the life of the remaining tubes because of impedance mismatch?
Also a work around is to disconnect one of the speakers reducing the load on the power tubes, has anyone tried this and how was he sound?
Recently pulled out the twin to use gigging, using a bullet >a tech21 blonde>CC analog delay>clean channel on the twin. Running at 6 for volume but would like to lower the wattage and see if I can increase the volume on the amp. I was originally going to use the pedal lineup for the pa but the combination into the twin....its fat, its sweet, its an analog dream! -thanks
Last Edited by Joe's_harpin on Jun 25, 2014 7:55 AM
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5F6H
1796 posts
Jun 25, 2014
8:26 AM
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The classic (BF/SF/RI) Twin reverbs are designed with some mismatch in mind, e.g. if you were to employ the "Ext Spkr" jack with another 4ohm cab, with the original compliment of 4 tubes, you would see the same degree of mismatch.
I very much doubt that you would notice a significant increase in wear, unless something else was adrift (badly biased, very worn tubes already).
I didn't like the sound of my Twin with 1x12" in comparisons between 2x12" & 1x12" YMMV.
"Running at 6 for volume but would like to lower the wattage and see if I can increase the volume on the amp." In a technical sense, this is illogical (lower power & more dB), I'm assuming that you want to turn the knob up more for a higher apparent setting at lower/equivalent level of sound out?
Even running 2x power tubes (one either side of the OT winding) the amp will be fairly loud (I run my twin with just 2x power tubes & 2x10" via converter rings, or a 3x10" cab), you might try pulling one (leaving 3 installed), or two tubes on one side rather than pulling them evenly with respect to OT secondary winding, this will unbalance the OT and reduce headroom noticeably. If it's a SF Twin with bias balance, you might also try deliberately unbalancing the 2 pairs, ensuring the hotter pair was still in spec (under 35mA - Confirm with probes, don't do this blind & hope for the best). I'd even look at running the cooler end of bias, at least 7mA on the lower side.
Oh yes, you can always blow softer too! ;-)
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www.myspace.com/markburness
Last Edited by 5F6H on Jun 25, 2014 8:27 AM
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Joe's_harpin
9 posts
Jun 25, 2014
10:55 AM
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5F6h - 69 SF, Thanks for the input, I don’t have any tech experience with amps, but had read about the tube reduction at http://fenderguru.com/amps/twin-reverb specifically the removal of the inner two power tubes for a reduction to 40-50W.
Yes “ turn the knob up more for a higher apparent setting at lower/equivalent level of sound out”- thinking there might be benefit of open circuit for sound quality- less power, less volume-closer to maximum capacity? Not looking for breakup but the same idea of pushing a smaller amp to get the best sound quality.
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5F6H
1797 posts
Jun 25, 2014
11:56 AM
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Well, if you're not looking for more breakup, then the unbalancing of the tubes/bias might not be ideal for you.
Swings & roundabouts regarding open circuit & sound quality, if you turn the amp full up you'll have some series resistance somewhere earlier (mic vol control, pedal turned somewhat down), if you turn the mic/pedal full up, you'll have some series resistance in the amp's volume control (turned somewhat down)...you want some series resistance somewhere to curb brightness & feedback.
Have you tried using the #2 input on the amp, from the pedal out & adjusting from there?
Ultimately, there'll likely be a fairly narrow range of W out at the speakers before you hit a ceiling of feedack (not saying that there won't be any advantage, certainly try it) but depending on what you have set hi/low will largely affect the smoothness/softer edge vs aggressiveness/brighter attack of the sound & sweep of mic control vs amp control (whichever you prefer up/down is valid).
Actually pushing the amp harder, means more volts at the speaker (be they dirty, or clean) & a louder amp.
I guess you could also try a 5751 or 12AY7 in the preamp of whichever channel you are using (V1 farthest preamp tube to the right, looking at the back of the amp, is the preamp tube for the normal channel, V2 next to it is the preamp tube for the vibrato/reverb channel). Try the preamp tube subs with both the stock 12AT7 in the phase inverter & with a 12AX7 in the phase inverter (preamp tube at farthest left, next to the power tubes).
Be sure to let the amp cool before subbing tubes, even 12AT7 get pretty hot.
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www.myspace.com/markburness
Last Edited by 5F6H on Jun 25, 2014 1:53 PM
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5F6H
1798 posts
Jun 25, 2014
3:50 PM
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Another option is to use 2x 6L6/5881 power tubes and rewire the 2x12" at 16ohms in series, this will mismatch the load in the opposite direction, but still be tolerable.
This could possibly open up the option of JJ6V6 power tubes (I'm not recommending any other brand in this instance without specific details of the amp in question), but you would *need* to get the amp checked over by a tech to ensure a useable range of bias (17-20mA per tube?) & compatible plate voltage (SF twins vary by ~100vdc) - again, don't just whack in the parts in & hope for the best.
FWIW I have tried 4x6V6 in my twin, but mine runs very high voltages (>510vdc) & sounds harsh like this, a lower voltage amp might be more receptive? Just thinking out loud here...
Don't open the amp up, or try to measure anything unless you know *exactly* what you are doing, the amp runs lethal voltages & retains them even when unplugged.
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www.myspace.com/markburness
Last Edited by 5F6H on Jun 25, 2014 3:53 PM
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