# Modifying Wattage Output?

Discussion in 'Amps and Cabs [BG]' started by baddarryl, Jul 6, 2019.

Oct 26, 2008
Cape Fear!
I know in guitar amps it is common to find a watt selection switch. My question is if I get an older amp can they be modified to do this and if so what should be a reasonable charge? I am looking for a tube guitar amp and the older ones obviously don't have this. Is it do able? Thanks.

Jul 26, 2008
Jax FL USA

3. ### BassmanPaulInactive

If you pull tubes the matching to the load will be way out of whack.

4. ### Wasnex

Dec 25, 2011

If the amp has six output tubes there is no practical way to manage the resulting impedance change unless you have speakers with non-standard impedance. But if the amp has 4 tubes, you can pull half of the tubes and the expected load will double. For example, a 4 ohm output will become an 8 ohm output.

You need to know which tubes to pull. The tubes are setup in two circuits that are connected to opposite sides of the output transformer (OT). In order to keep the current balanced in the OT you need to pull half the tubes from each circuit. The way the tubes are physically aligned varies from one amp to the next, so you need to actually understand enough about how tubes work to inspect to circuit, and decide which tubes should be pulled.

Another issue that needs to be managed is bias. When you pull half the tubes, the plate voltage will rise slightly. This will cause the remaining tubes to run a bit hot, so the bias should checked and reset if necessary to get the current back in the proper range.

Some big bass amps have 8 output tubes. So the process can be done twice. If half of the tubes are removed from the circuit, the 2 ohm output would become a 4 ohm output. Then, if you pulled half the tubes again, leaving only 2 active tubes, the 2 ohm output would become an 8 ohm output.

I personally don't think it's worth the effort. From what I have read, the power is not actually cut in half due to the increase in plate voltage. So the difference in volume is <3dB. Also because the power supply is not being taxed as heavily, there is a good chance the feel of the amp will change a bit.

BassmanPaul likes this.
5. ### Downunderwonder

Aug 26, 2009
New Zealand
But it is somewhat predictable, no?

6. ### BassmanPaulInactive

Yes but it depends on the particular amplifiers topology.

Last edited: Jul 8, 2019
7. ### agedhorseSupporting MemberCommercial User

Feb 12, 2006
Davis, CA (USA)
Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
It MAY be possible without increasing the risk for damage, but often the benefits don't justify the risk.

Ampslut and Nevada Pete like this.
8. ### Foz

Jul 26, 2008
Jax FL USA
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9. ### RedbrangusSupporting Member

Nov 19, 2018
Under The X In Texas
This was my first thought. Guitar players do this (I'm told) in order to get distortion from over-driven components at a manageable volume; OP, is that also your intent?

10. ### ScoopsWhy do we use base 10 when we only have 8 fingersSupporting Member

Oct 22, 2013
Sugar Creek, Wisc
If you are looking for your amp to be a bit quieter, I suggest using the volume knob to turn it down

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11. ### BogeyBassInactive

Sep 14, 2010
Is a few ways. Lower voltage, run in triode mode. Pull tubes and match the load to the impedance change.

Can be as simple as a switch and having preset bias for 2 different voltages.

Problem is adding the switches and tapping into pre established traces.
Likewise how do you drop the voltage. Different transformer? Big resistors?

All been done. Still a waste of time.

If you want low volume tube saturation.
Run pair of 6v6 at 10 watts. Run into a good but low sensitivity speaker.

You have to realize even a simple 95 to 98 dB single 12" speaker at 5 watts is still roaring loud at home.

10 watt 6V6 pair into a sealed 91 dB 10" speaker could be feasible design. Cheap to build. Not much plate voltage needed, and realitively small output transformer. Iron is kinda cheap...kinda

Just a simple already made used 5 watt Epiphone Valve junior is also feasible. Go used for about 100 to 150 bucks.
https://www.guitarcenter.com/Used/E...MIqJbkndOh4wIVlIzICh3sJwEhEAQYASABEgK8KvD_BwE

Turd class A mini TV tube.
But it's tube and 5 watts.
No need fart around with switches or plate voltage. Probably quick cathode bypass capacitor change in the pre to make it more usable for bass.

Same idea run it into a low sensitivity 10" speaker. The lower sensitivity also reduces volume.

1 watt sensitivity rating for speaker is just that. You can fiddle fart with wattage all you want. But 98 dB at 1 watt is still 98 dB.
Hence 91 dB or lower significantly lowers volume than piddle winking with wattage

Dual cone, large magnet Eminence B102
Sensitivity is horrible for single driver live application. 92 dB. Otherwise at low volume dam good flat response speaker with very low 45 hz resonate frequency. Good for recording and low sensitivity application.

LEGEND B102 - Loudspeakers | Eminence Speaker

Last edited: Jul 6, 2019

Nov 22, 2016
This is just my opinion. Take it for what it may be worth to you, if anything. I have been around the block a few times in the music business. But that doesn't mean what I say is right, or wrong. I may be making assumptions here I shouldn't be making. But here it goes:

For my money, instead of fussing with modifying an amp, I suggest acquiring a smallish tube guitar amp in around the twenty to twenty five watt range. Fender and Mesa are top tier manufactures of amps like this, and there are many others. Amps like that will actually play astonishingly loud depending on the kind of speaker cab you use (hint - hint). This should be ideal for club dates and studio sessions. Find one with the tonal and touch sensitivity characteristics you desire, and if more volume is required to carry the venue your playing at, mike up the amp with a Shure SM57 and run it through the PA. Nothing is more obnoxious to your fellow band mates (including your bassist) than a guitarist who over plays the stage. It just ruins everything for both your band mates, and for the audience too.

For example, I saw my first Marshal stack at the Fillmore Auditorium in San Francisco, California, when Cream made their original appearance there. Both Eric Clapton and Jack Bruce had twin Marshal stacks (2 heads / four cabs), and the effect was stunning. Never heard anything like it! Remember, back then, PA was used almost exclusively for vocals. But those Marshals were really something, and, of course, went on to become the sound of Rock.

At a later date I saw a group called Moby Grape at the Fillmore with guitarist Jerry Miller, who was eventually listed as one of the top 100 rock guitarists of all time by Rolling Stone Magazine. Jerry used two 59' Fender Bassman open back amplifiers on folding chairs, which was his signature set up. The Fillmore Auditorium was a big venue for its day and Jerry's amps did the job, but just barely. Then, at the end of their set, Jerry did something I had never seen done before (and I'd been to and played at a lot of shows). He took one of the mic stands off the front line, lowered it down, and placed it in front of one of his Fender Bassman's. I was horrified! He can't do that, I thought! Then, the Grape went into a jam and Jerry demonstrated just exactly why he became so frequently noted as a guitarist in Rolling Stone. His brilliant guitar work totally filled the venue with sound that had astonishing volume, depth and clarity. A harbinger of things to come, certainly, and something that taught me a lesson. The effect was easily equal to what Clapton's twin Marshall's did, covered the room better and was much clearer.

You'll rarely, if ever have anything on stage that will cover the audience as well, and as loud as even a modest modern sound system. And you don't have to over play the stage and annihilate the first ten or twenty rows of fans to do it. Remember, when you see a wall of Marshal stacks on a big stage (60 foot stage), in all likelihood only one of the stacks (or even half a stack or half a cab) is actually being used. A lot of those cabs don't even have speakers in them, and several of the heads will have nothing inside them other than a little battery to power a sham pilot light.

Anyway, I say if your regularly playing the big stages then you can afford to just purchase whatever amp with whatever power level you require for whatever gig your playing. You don't need to fuss with modifying anything. Otherwise, a 20 to 25 Watt amp should be ideal for you in any situation. Use a power soak for playing in your bedroom. You can always rent something if you absolutely, positively have to blow everybody off the stage. Good luck.

Last edited: Jul 6, 2019
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Oct 26, 2008
Cape Fear!
Yes

14. ### beans-on-toastSupporting Member

Aug 7, 2008
The Ampeg B-15R is a bass amp with a half power switch.

https://ampeg.com/pdf/B-15R.PDF

Note that changing the impedance tap is required to maintain a matched impedance when two of the four tubes are disabled.

How useful switching between 100W and 50W is debatable. 50W is still loud enough to shake a room.

15. ### J Wilson

For what you'd spend on this 'roll the dice' experiment, less money could be spent with the sure result of a second, smaller amp to do what you want.

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16. ### SactoBassA retired civil engineer who likes all-tube amps!Supporting Member

Jul 8, 2009
Lake Havasu City, AZ
I don't know if this would interest you, but the Fender Bassman 100T all-tube amp is 100 watts, and with the simple flick of a switch, it's 25 watts.

17. ### Moose22

Apr 14, 2019

Just, no. It's doable, but... don't.

Modifying an amp for something like this is more work than just building one. At least if you want it to sound decent. If you asked me to do this I would advise you that there are better solutions. If you insisted I would politely, but firmly, ask you to leave, like Hank Hill if you ask for a well done burger.

There are so many really great amplifiers out there to choose from I can't tell you. Just buy the appropriate amp for your situation. Maybe get one designed like this, or a decent master volume amp and live with the compromise. Or, if you really aren't willing to do that and need something lower wattage, get another amp. It'll be cheaper than trying to modify something and a whole lot better when playing.

agedhorse likes this.
18. ### Wasnex

Dec 25, 2011
You make it sound like a big deal, but it's not. It's just a matter of pulling a few tubes, resetting the bias, and compensating for the new impedance of the output taps. I don't think it's worth the effort, but not much effort is actually required if you know what you are doing and have an appropriate amp.

There several amps available that already have variable output power as a feature. Mesa Prodigy and Strategy come to mind. Someone mentioned the Fender Bassman 100T. Go for a Unicorn and track down a Trace Elliot V8 or Matchless Thunderchief. There are probably others that I have left out.

FYI, The Prodigy, Strategy, and V8 vary output power by changing the number of tubes in the circuit. The 100T and Thunderchief vary output power by varying voltages in the power supply.

There are also quite a few relatively low powered bass amps available. Ampeg PF20T comes to mind. Ashdown has made LB30 and CTM30 which are basically the same amp with different cosmetics. Ashdown also recently introduced the CTM30 Little Stubby which has a few new features.

Many low powered guitar amps sound great with bass. Just connect a bass speaker of the proper impedance if you want to push the amp any. Playing bass through a guitar speaker can sound fine at low volume, but many of them will not tolerate very much SPL.

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19. ### Moose22

Apr 14, 2019
But it is a big deal. You're assuming a lot here.

First, you're assuming this is a big boy amplifier with more than two power tubes. That's definitely not a fair assumption, because those are usually high wattage amplifers. 100 watt amps were common in the 70s, sure, if you don't mind throwing your back out trying to move a twin reverb, or you want to play your Marshall stack. I guess the AC30 is not so super high wattage, so yeah, you could dig up something with EL84.

Anyway, the difference between something like a 50 watt Marshall and a 100 watt is there, but it comes down to slightly less headroom and still an extremely loud amplifier by the time the poweramp breaks up. Same would be true of other 4 tubers, but you only shed a few db by halving power and 2 or 3 db less on the breakup threshold while looking for poweramp distortion is not a huge difference. So that might not be a big deal, but it certainly won't fit the OPs goal of distortion at reasonable volume.

And the real point -- Most guitar amplifiers have two tubes. At least anything 50 watts or less. You can't just yank a tube from a 2 tube PP amp, switch the impedance selector, and expect it to work nicely. Amps like this with a low power switch often do things like switching to Triode mode, which is complex and changes the tone and response of the amp. Or other folks will use something like a VVR, which is a mosfet voltage reduction circuit that drops the B+ on the poweramp (or the whole amp) and has to track the screens and the bias voltage on a fixed bias amp so you don't go too hot as you adjust it. VVRs work, but they're not a minor mod. There are other ways, but they usually aren't worth the hassle.

And you're thinking very bass centric. OP specifically said Tube Guitar Amplifier. He wants a guitar amplifier he can drive into distortion at a reasonable volume, so I am assuming guitar amplifier, not all the bass amps you're listing.

My advise remains the same. Just get an appropriate low wattage amp for when you want guitar dirt. There are scads of them available, and some damned good ones to be had at a reasonable price if you keep your eyes open.

agedhorse likes this.
20. ### Martin Beer

Dec 4, 2004
Some guitar amps reduce the output power with a pentode/triode switch, though opinions vary on how useful this is. It should be a cheap, simple mod to have done if the amp is suitable though.
On certain guitar amps, a well implemented master volume is as useful as any power reduction scheme, though that applies to some amps more than others.

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