4 power tube amp 1/2 power options - Pentode/Triode OR lift ground to 2 tubes?

reverberocket2 · May 16th, 2007, 06:54 PM

I know both methods will reduce power, but how does each option effect tone, headroom, feel etc.

Also I know that using a switch to lift ground off 2 tubes is the same a pulling 2 tubes which changes the impedance. When switching from Pentode to Triode is there also an inpedance mismatch?

The Radium King · May 16th, 2007, 07:50 PM

i've got both in my twin. according to o'connor, triode mode changes the impedance of the tube, but i've yet to get the definitive answer on exactly how much. if i recall, a review of tube data for A-A impedance for pentode vs triode mode yielded a 1.25x multiplier, but i could EASILY be wrong here. i thnk most just treat it as a 2x and are done with it.

tonally, neglecting the effects of impedance mismatch (compression) cathode lift just reduces power by a factor of 1 or 2 notches on the volume dial - still painfully loud to get any breakup/saturation in a big iron amp. otherwise no tonal change. triode mode drops power by about 2 notches on the volume dial (more than pulling two tubes, or more than 50% reduction) and rolls off the highs and compresses the sound. i find the compression works well with a disconnected nfb, as it negates the volume increase and limits the bass excursions but allows the power tube harmonics to come through louder.

reverberocket2 · May 17th, 2007, 09:49 AM

Thanks Radium King,
Sounds like triode is what I'm looking for.

I have a big iron BF Showman head. Currently the ground switch is wired as a cathode lift on two tubes.

With the 2 tube operation the Showman as basically the same volume - little thinner sounding (slightly less bass response) and a bit less head room.

Triode definitely sounds intreaging

The Radium King · May 17th, 2007, 12:21 PM

go for it. i'd keep the ground lift switch and add a triode switch also (this way you have the ability to run two tubes at 40%; turn your showman into a 20 watt amp, but watch the impedance mismatch). i actually incorporated the ground lift into my standby switch (use a dpdt centre off - up is no grounds lifted, down is two grounds lifted, middle is all grounds lifted - standby). if you don't blow fuses very often, you can move your fuse holder inside your amp and use that opening, or your external speaker opening so as to not drill any holes. o'connor details the switch install in tut1 and tut2, but just consider the high voltages (450v); you'll need rated wire and a large-bodied switch. o'connor also recommends twisting the wires to each side of the switch (ie, don't twist the screens together and the plates together; twist the screen and plate for one side together and the screen and plate for the other side together).

Tim Swartz · May 17th, 2007, 02:10 PM

Triode mode will knock down the power substantially, but it really shaves a lot of the top and bottom. Great if you're going for a Tom Scholz/Boston sound that is all mids... but that's about it IMO.

The Radium King · May 17th, 2007, 02:55 PM

i've heard triode tone described as 'throwing a wet blanket over the amp'. the peak frequency attenuation (high/low rolloff, compression) can be addressed at the tone stack. if you're looking for the same clean tones at lower volumes, it won't do it for you. the good thing about showmans and twins is that they sound just fine at 1.5 on the dial. in my case, i use triode mode to drop power so that i can get the amp into saturation range without upsetting the cats. in this case i want the bass and treble pulled back a bit to stop the ice picks and bass flab. conversely, more mids provides the standard distortion tone that we are used to (marshall).

if you do it properly it is a reversible mod that gives the player more tonal options. my two cents. you're right though, it depends on what you want out of the amp.

reverberocket2 · May 17th, 2007, 05:47 PM

Thanks guys

So triode will lower the headroom and roll off highs and lows leaving pronounced midrange. If it pushes the Showman in more of a 50 watt Marshall normal channel direction that would be spectacular. I typically run it with the te bright switches off, I assume that in triode mode putting the bright switches on should help put some top end back in.

reverberocket2 · May 17th, 2007, 06:58 PM

One more thing

I'ts an 8 ohm Showman (not dual). When using the cathode lift on two tubes what is the correct ohm cab I should run the head with? Can I fry the tranny if I run it on 2 tubes into an 8 ohm cab?

Thanks again everyone

The Radium King · May 17th, 2007, 07:05 PM

think of each power tube as a 32 ohm resistor, and each is in parallel with the other. 4 tubes and you get 8 ohms. 2 tubes and you get 16. to match impedance with 2 tubes you need a 16 ohm speaker load. you can hit that impedance by reconfiguring speakers (ie, two 8 ohm speakers in parallel is 4 ohms, in series is 16 ohms, or run one speaker for 8 ohms provided they can handle the power; note that you'll get tonal variation between parallel and series configurations; series tends be be a bit more 'flabby'). conventional thinking is that an output transformer can handle a +100%/-50% mismatch; ie, your 8 ohm showman ot shouldn't eat itself for loads from 4 to 16 ohms. an impedance mismatch within this range will give you even more compression, and some people like the the sound of it.

May 16th, 2007, 06:54 PM	#1 (permalink)
reverberocket2 Tele-Meister Join Date: Mar 2006 Posts: 351	4 power tube amp 1/2 power options - Pentode/Triode OR lift ground to 2 tubes? I know both methods will reduce power, but how does each option effect tone, headroom, feel etc. Also I know that using a switch to lift ground off 2 tubes is the same a pulling 2 tubes which changes the impedance. When switching from Pentode to Triode is there also an inpedance mismatch?

May 17th, 2007, 02:10 PM	#5 (permalink)
Tim Swartz Tele-Afflicted Join Date: Mar 2003 Location: Michigan - Tweenst the Great Lakes Posts: 1,881	Triode mode will knock down the power substantially, but it really shaves a lot of the top and bottom. Great if you're going for a Tom Scholz/Boston sound that is all mids... but that's about it IMO. __________________

May 16th, 2007, 07:50 PM	#2 (permalink)
The Radium King Tele-Meister Join Date: Aug 2006 Location: Canada Posts: 106	i've got both in my twin. according to o'connor, triode mode changes the impedance of the tube, but i've yet to get the definitive answer on exactly how much. if i recall, a review of tube data for A-A impedance for pentode vs triode mode yielded a 1.25x multiplier, but i could EASILY be wrong here. i thnk most just treat it as a 2x and are done with it. tonally, neglecting the effects of impedance mismatch (compression) cathode lift just reduces power by a factor of 1 or 2 notches on the volume dial - still painfully loud to get any breakup/saturation in a big iron amp. otherwise no tonal change. triode mode drops power by about 2 notches on the volume dial (more than pulling two tubes, or more than 50% reduction) and rolls off the highs and compresses the sound. i find the compression works well with a disconnected nfb, as it negates the volume increase and limits the bass excursions but allows the power tube harmonics to come through louder.

May 17th, 2007, 09:49 AM	#3 (permalink)
reverberocket2 Tele-Meister Join Date: Mar 2006 Posts: 351	Thanks Radium King, Sounds like triode is what I'm looking for. I have a big iron BF Showman head. Currently the ground switch is wired as a cathode lift on two tubes. With the 2 tube operation the Showman as basically the same volume - little thinner sounding (slightly less bass response) and a bit less head room. Triode definitely sounds intreaging

May 17th, 2007, 12:21 PM	#4 (permalink)
The Radium King Tele-Meister Join Date: Aug 2006 Location: Canada Posts: 106	go for it. i'd keep the ground lift switch and add a triode switch also (this way you have the ability to run two tubes at 40%; turn your showman into a 20 watt amp, but watch the impedance mismatch). i actually incorporated the ground lift into my standby switch (use a dpdt centre off - up is no grounds lifted, down is two grounds lifted, middle is all grounds lifted - standby). if you don't blow fuses very often, you can move your fuse holder inside your amp and use that opening, or your external speaker opening so as to not drill any holes. o'connor details the switch install in tut1 and tut2, but just consider the high voltages (450v); you'll need rated wire and a large-bodied switch. o'connor also recommends twisting the wires to each side of the switch (ie, don't twist the screens together and the plates together; twist the screen and plate for one side together and the screen and plate for the other side together).

May 17th, 2007, 02:55 PM	#6 (permalink)
The Radium King Tele-Meister Join Date: Aug 2006 Location: Canada Posts: 106	i've heard triode tone described as 'throwing a wet blanket over the amp'. the peak frequency attenuation (high/low rolloff, compression) can be addressed at the tone stack. if you're looking for the same clean tones at lower volumes, it won't do it for you. the good thing about showmans and twins is that they sound just fine at 1.5 on the dial. in my case, i use triode mode to drop power so that i can get the amp into saturation range without upsetting the cats. in this case i want the bass and treble pulled back a bit to stop the ice picks and bass flab. conversely, more mids provides the standard distortion tone that we are used to (marshall). if you do it properly it is a reversible mod that gives the player more tonal options. my two cents. you're right though, it depends on what you want out of the amp.

May 17th, 2007, 05:47 PM	#7 (permalink)
reverberocket2 Tele-Meister Join Date: Mar 2006 Posts: 351	Thanks guys So triode will lower the headroom and roll off highs and lows leaving pronounced midrange. If it pushes the Showman in more of a 50 watt Marshall normal channel direction that would be spectacular. I typically run it with the te bright switches off, I assume that in triode mode putting the bright switches on should help put some top end back in.

May 17th, 2007, 06:58 PM	#8 (permalink)
reverberocket2 Tele-Meister Join Date: Mar 2006 Posts: 351	One more thing I'ts an 8 ohm Showman (not dual). When using the cathode lift on two tubes what is the correct ohm cab I should run the head with? Can I fry the tranny if I run it on 2 tubes into an 8 ohm cab? Thanks again everyone

May 17th, 2007, 07:05 PM	#9 (permalink)
The Radium King Tele-Meister Join Date: Aug 2006 Location: Canada Posts: 106	think of each power tube as a 32 ohm resistor, and each is in parallel with the other. 4 tubes and you get 8 ohms. 2 tubes and you get 16. to match impedance with 2 tubes you need a 16 ohm speaker load. you can hit that impedance by reconfiguring speakers (ie, two 8 ohm speakers in parallel is 4 ohms, in series is 16 ohms, or run one speaker for 8 ohms provided they can handle the power; note that you'll get tonal variation between parallel and series configurations; series tends be be a bit more 'flabby'). conventional thinking is that an output transformer can handle a +100%/-50% mismatch; ie, your 8 ohm showman ot shouldn't eat itself for loads from 4 to 16 ohms. an impedance mismatch within this range will give you even more compression, and some people like the the sound of it.