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Triode-Pentode-How's That Work?
Old 25th September 2012
  #1
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🎧 5 years
Triode-Pentode-How's That Work?

I looked around for the answer to this. I got alot of full versus half power answers some about ohms and cabs. But what I want to know is what happens in the tubes when you switch between triode and pentode. Is is something to do with grid, 2x grid switching one off? Or does one tube cut out of circuit, does part of the preamp cut out or jeez, I haven't understood what's going on. I have a Bugera V55. I like it alot. The overdrive on the triode is creamier than on the pentode, but I can't blast it in pentode because I already have hearing loss (too much GnR). Thanks
Old 26th September 2012
  #2
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There are a number of ways to cut power in a pentode power tube:

1. Pentode vs Triode: the extra two grids are grounded. This makes the tube less efficient but changes the character of the tube to a softer, more even harmonics sound. If you want country chicken pickin', you want Pentode! There isn't a change in output impedance with this. Figure dropping power by 1/3?

2. Cathode Bias-instead of setting the idle current to a hard point with a resistor to ground, it's set relative to the cathode voltage by a resistor/cap. This results in more sag and ultimately less power output. Again, perhaps a 1/3 drop in power with more compression.

3. Turning off two out of 4 tubes: this does result in a change in output impedance, but amps that have 1/2 power switches often compensate by switching other things out as well. If you do it yourself by removing 2 tubes, you move your speaker up to the next rating (so you plug your 8 ohm speaker into the 16 ohm socket)
Old 26th September 2012
  #3
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🎧 5 years
The Bugera has 3 preamp tubes and 2 6L6s with a triode/pentode switch. So if the amp goes from pentode to triode does that mean that grids in each power tube are grounded? Or are they bypassed? I can't find info anywhere on the Bugera site. Seems many Marshalls also have triode/pentode. I read some people complaining that their manuals didn't talk about needing to switch driver output when cutting one to the other.
Old 27th September 2012
  #4
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🎧 10 years
Drbob1's 1st answer is the one that pertains to you. I can't say for sure, but I'd bet they are grounded. It's the easiest way to bypass something in a circuit. Well, it involves a cheaper switch than bypassing the whole thing, and with stuff like this, usually your switches and other large pieces of hardware are where you spend a lot of your production budget. But I haven't seen the circuit, so I don't know. Why exactly do you need this information?

I've owned several Marshalls with the pentode/triode switch and currently have a Bugera V22 with the same switch. I always set them to pentode. The difference in volume is almost negligible (maybe 3dB's) but I certainly notice a difference in tone. In triode mode, all those amps seemed to loose some of their dynamic range and bass output. It's a sound that I could see some people liking, but not me. Personally, I'd just use my ears and go with whatever sound I liked better.
Old 27th September 2012
  #5
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I promise, you don't have to switch anything to use triode mode. Tubes are tolerant of any changes in output impedance of that size. Just switch it and see which sound you like better!
Old 27th September 2012
  #6
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eve69's Avatar
 
🎧 5 years
Cool, I like both modes. Each takes a bit of adjustment. What do I need the info for? I'm not sure exactly. I suppose knowledge is power to adjust tone in this instance.
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