Littoral
230 posts
Jan 04, 2011
6:04 AM
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Got this years ago from Piazza and 4/10's, pull a power tube and run 2 speakers.
Any experience with this?
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5F6H
460 posts
Jan 04, 2011
6:28 AM
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Pulling a tube will increase distortion...massively. The amp is now running single-ended, like a champ and will have very little, or no headroom before distortion. That said, the amp won't necessarily sound "half as loud", it won't keep up in harder conditions but may be fine in quieter/moderate surroundings. Victoria for one do this on request as a factory harp mod, via a switch to disconnect one tube. I once did a jam with a Bandmaster, halfway through the night the amp really seemed to get into it's stride & hit a hot, raspy tone...at the end of the night I saw that one of the power tubes was just rolling around in the cab!
There is a theoretical risk that the Output Transformer can become magnetised if running only one power tube in a Push-Pull set up. If you are going to do this for extended periods then it is probably a good idea to pull the other power tube from time to time, rather than constantly run with the same tube pulled.
Some amps, like Kendrick's Texas Crude, have controls to vary how hard each tube works in push-pull, making the amp more like a single-ended amp. You can also unbalance the "phase-inverter" for a similar effect, by subbing the preamp tube next to the power tubes with a 12DW7, the amp will be unbalanced but will make more power than with one tube only.
You don't necessarily need to unplug 2 speakers.
For maximum power you will need both power tubes in there, there are other avenues that you can explore to increase distortion & fatten tone, if required.
Don't ever pull a power tube in a cathode biased amp, this will kill the remaining power tube. This suggestion only applies to fixed bias amps like the Fender 4x10"s.
Last Edited by on Jan 04, 2011 9:29 AM
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Greg Heumann
965 posts
Jan 04, 2011
8:38 AM
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5F6H - what a wonderful response. Great information, well written. Thank you.
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/Greg
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5F6H
461 posts
Jan 04, 2011
9:30 AM
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Thanks Greg.
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Jim Rumbaugh
360 posts
Jan 04, 2011
10:21 AM
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@5F6H said,"There is a theoretical risk that the Output Transformer can become magnetised"
That's new information to me, but I'm not surprised to read it. My question, wouldn't the same "risk" be present for any single output tube amp?
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intermediate level (+) player per the Adam Gussow Scale, Started playing 2001
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5F6H
462 posts
Jan 04, 2011
11:41 AM
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Good question Jim, the answer is that Single-Ended (SE) transformers are designed with an airgap to eliminate dc core saturation, the cause of the magnetisation.
Push-Pull (PP) transformers do not need or have this airgap. Most transformer designers would shudder at the thought of running a PP OT in SE mode, but then most transformer designers don't build amps, or play harp. Fender's factory fix for SE Tweed princetons with blown OTs was to replace them with tolex PP Princeton transformers wired SE for an 8ohm load...these amps work fine & sound great.
Last Edited by on Jan 04, 2011 11:43 AM
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Littoral
231 posts
Jan 04, 2011
12:25 PM
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5F6H, thanks, I was hoping the post would find you. I've researched this issue numerous times with techs over the years becuase I've been concerned about damaging my amp -an early tan concert (Jan 60) w/the reverse order volume bass controls. I've never had any trouble with 1 less tube. Changing sockets, I hadn't considered that and it sounds like a good idea. I know of the de-mag possibility but haven't heard of it actually happening. I would prefer to run the amp with both tubes because the bottom end is unbelievable but I lose the punch. I was considering a harp attack pedal to allow for more parameters. Don't know. Any suggestions would be appreciated. I'm skeptical because the only thing I've ever liked between the mic and the amp is my brown reverb unit.
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7LimitJI
268 posts
Jan 04, 2011
12:44 PM
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I would ask what it is you are looking for ?
Earlier break up ?
Fatter sound ?
Lower volume ?
Gigging amp ?
Recording amp?
If you can describe what you're after there might be a solution.
Dropping pre amp voltage, running 6v6's,changing the bias, caps etc.
But the concert was designed as a 40-45watt gigging amp.Pulling power tubes severely compromises some of the components.
If you only need half the power, why not sell it and buy a 15 to 30 watt amp. Or keep it and buy another??
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The Pentatonics Myspace
Youtube
Why don't you leave some holes when you play, and maybe some music will fall out.
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Littoral
232 posts
Jan 04, 2011
1:02 PM
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7limit, it gigs more than fine with 1 tube so "severely compromises some of the components" isn't an issue. The tone and volume is righteous. I am a long time amp junkie with 25+ 8-30 watt amps already (all 1947-1963).
Again, I'm experimenting in general and adding the harp attack pedal was a thought. A lot of credible sources seem to like it -and going straight to the PA with a pedal sounds like a lot lighter load.
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harpwrench
410 posts
Jan 04, 2011
1:23 PM
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The Harp Octave pedal was the best solution I found with my Super Sonny.
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Joe
www.spiersharmonicas.com
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