alphaking
Joined: Jan 02, 2013
Posts: 214
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Posted on May 21 2015 06:47 AM
TomH wrote:
I tried searching for this topic to no avail. Yet, I'm pretty sure that this has been discussed. So, forgive me if this is old hat.
I just bought an early 70s Vibrosonic amp with a JBL D130-F 15" speaker. It hasn't arrived, yet, but I'm really excited about getting it. It's 100 watts. I've read that you can remove either the outer two 6L6 tubes or the two inner 6L6 tubes in Fender amps to cut the wattage in half (50 watts). That's appealing to me, because I won't need 100 watts for 90% of what I do with the amp. Is doing this harmful in any way to the amp? Thanks!
The main thing here is that it will change the impedance of the amp. I've been running my Surfer with 2 tubes vs 4, but that's because it's a 4 ohm amp and I'm running it with an 8 ohm JBL D130. Pulling 2 tubes changes the impedance from 4 to 8, so it works in this application. If you have a combo, the speaker is probably matched with the transformer's impedance, and you may not want to do that. The only reason I did was so I could run the 8ohm D130. But it's still pretty loud, and I don't know that the pros would outweigh any cons in your case.
Chris
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JObeast
Joined: Jul 24, 2012
Posts: 2762
Finknabad, Squinkistan
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Posted on May 21 2015 10:16 PM
Do others with technical insight corroborate what Alpha is saying, that halving the number of power amp tubes doubles the output impedance? What are the downsides to this?
alphaking wrote:
TomH wrote:
I tried searching for this topic to no avail. Yet, I'm pretty sure that this has been discussed. So, forgive me if this is old hat.
I just bought an early 70s Vibrosonic amp with a JBL D130-F 15" speaker. It hasn't arrived, yet, but I'm really excited about getting it. It's 100 watts. I've read that you can remove either the outer two 6L6 tubes or the two inner 6L6 tubes in Fender amps to cut the wattage in half (50 watts). That's appealing to me, because I won't need 100 watts for 90% of what I do with the amp. Is doing this harmful in any way to the amp? Thanks!
The main thing here is that it will change the impedance of the amp. I've been running my Surfer with 2 tubes vs 4, but that's because it's a 4 ohm amp and I'm running it with an 8 ohm JBL D130. Pulling 2 tubes changes the impedance from 4 to 8, so it works in this application. If you have a combo, the speaker is probably matched with the transformer's impedance, and you may not want to do that. The only reason I did was so I could run the 8ohm D130. But it's still pretty loud, and I don't know that the pros would outweigh any cons in your case.
Chris
— Squink Out!
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JObeast
Joined: Jul 24, 2012
Posts: 2762
Finknabad, Squinkistan
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Posted on Mar 31 2018 12:58 AM
I hope I didn’t miss something important about Weber Mass Attenuator capacity as I purchased a 100 w version and hope to be able to use it on my BF’d Twin. The 100w attenuator is mostly for a 20w RMS Ceriatone Lead & Bass soni can crank that amp, but I was hoping I could safely use it with my larger amps like Blond Bandmaster and the Twin. Does the device need to be, like a speaker, rated about 200% or more of the amp’s RMS to safely crank it?
ncgalt1984 wrote:
I had this same issue. I'm running a 100 watt Gomez Surfer head and I wanted to be able to push it to get better tone out of it. I decided against pulling the tubes simply because it's only a 3db difference. I ended up getting a Weber Mass 200 attenuator and couldn't be happier. Of course, it doesn't sound as good as it would if it were completely un-attenuated and cranked up. But, it sounds much better than it did when I was playing it on low volume and not pushing the amp into its "sweet spot".
Also, if you were to go with an attenuator, everything would just stay at 8 ohms impedance so there's no need to unplug a speaker or swap them out.
— Squink Out!
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tubeswell
Joined: Sep 24, 2011
Posts: 1424
Wellington, NZ
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Posted on Mar 31 2018 01:46 AM
Yes, pulling 1 pair of output tubes in a 2 x pair amp (like a twin reverb or a showman) will mean that the load resistance through the output transformer needs to be doubled in order to keep the remaining pair of tubes optimally loaded. So if the amp normally runs a 4 Ohm speaker, then plugging in an 8 ohm speaker instead, will achieve this.
Of course it won't harm the remaining pair of output tubes to run them into half the required load in most situations where you don't dime the amp. However, the tube life expectancy will tend to be shorter, especially where you are running the tubes hard. And because they're working into 1/2 the normal load, they won't work as efficiently.
Lastly, all other things being equal (i.e. operating voltages being the same, bias being the same, and the load being doubled), you will only achieve about a 3dB reduction in maximum volume by running 1/2 the number of output tubes.
— He who dies with the most tubes... wins
Surf Daddies
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stratdancer
Joined: Dec 11, 2013
Posts: 2537
Akron, Ohio
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Posted on Mar 31 2018 06:06 AM
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Squid
Joined: Aug 22, 2010
Posts: 1019
Portland, Oregon with Insanitizers
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Posted on Mar 31 2018 12:21 PM
Tom, a speaker power attenuator (power soak) may be the best solution to an overpowered amp. Several product reviewers were surprised and delighted at how much it improved their tone, and wrote that every guitarist with a tube amp should use one. I saw many versions and different makers, varying widely in price. The particular device I saw the most enthusiastic praise for costs about $200 including shipping and comes in 4 ohm and 8 ohm versions.
https://www.ebay.com/itm/TubeCube-Pro1004-Attenuator-100-Watt-RMS-200-Watt-Peak-2output-4ohm-LineOut-/251469499176?hash=item3a8cc00b28
— Insanitizers! http://www.insanitizers.com
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TomH
Joined: Mar 15, 2006
Posts: 766
Oberlin, Ohio, USA
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Posted on Apr 01 2018 03:11 PM
Thanks Squid. Attenuators are a good option. That said, I have a 65 Reissue Twin Reverb that I use this way as a 42/85 watt amp. First, I removed one speaker, the speaker on the opposite side from the transformer. This makes it a balanced 53 lb amp. Then, I put the speaker in a lightweight pine open-back 1x12 cab. So, as a 42 watt amp, I remove the two inner 6L6 tubes and leave the cab at home. I didn't notice any decrease in tone. Then, for the occasional un-miced outdoor gig during the summer months, I put the two 6L6 tubes back in and bring the cab, plugging it in the back.
It's a really nice sounding amp with lots of head room both ways.
Last edited: Apr 01, 2018 15:13:36
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Surfadelphia
Joined: Sep 04, 2017
Posts: 435
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Posted on Apr 01 2018 07:40 PM
skeeter wrote:
There's also THD Yellow Jackets, tube converters.
I have no experience with them whatsoever, so I can't say if they're good or bad.
http://www.thdelectronics.com/product_page_yellowjacket.html
I have some experience using the THD Yellow Jacket tube converters. What they basically do is convert 6L6, 6V6, or EL34 power tubes to EL84's typically halving the wattage. They also convert the amp to Class A and require no re-biasing. I used them in a Fender Hot Rod Deluxe that was way too loud for my purposes. Basically, I was able to achieve "the sweet spot" tonally at a lower volume level. However, there could be some changes to the tonal characteristics of the amp as you could be going from 6V6's or 6L6's to EL84's which break up differently, etc. I thought it was an improvement in the Hot Rod Blues Deluxe. The nice thing is that it is a completely reversible modification, doesn't require a tech and won't hurt the amp. That being said, ultimately a good lower power amp is the best way to go.
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Badger
Joined: Nov 16, 2013
Posts: 4537
Wisconsin
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Posted on Apr 02 2018 06:11 AM
One thing I did with a Vibrolux Reverb was to simply swap out the power tubes for something from Doug's rated "early distortion." Doesn't make it fuzzy, just saturates (aka gets to its sweet-spot) a little sooner without changing the tone of the amp. This way I can keep the overall volume of the amp down a bit more & still get that sound that says the amp is working for its dinner.
The Yellow Jacket thing mentioned above would also be a cool option, albeit with tonal changes naturally because of the tube used.
— Wes
SoCal ex-pat with a snow shovel
DISCLAIMER: The above is opinion/suggestion only & should not be used for mission planning/navigation, tweaking of instruments, beverage selection, or wardrobe choices.
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