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Old 22nd August 2005, 11:15 PM   #1
Sherman is offline Sherman  United States
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Question Pentode to Triode Switch

Today I "finished" soldering up my SE EL84. It is pentode connected and I think it sounds pretty good though I haven't tried it with any good speakers yet.

Since this is the first amp I've "designed" and since I've used almost all spare parts for it I'm planning on doing some experimenting. One thing that seems easy to do would be to put in a switch to go from pentode to triode connection. It appears I can simply wire to poles of a DPST switch to the B+ and two to the anode and run the center two to G2.

Is that really all there is to it? Putting it into SE Amp CAD it appears that G2 is connected directly to the anode.

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Old 26th August 2005, 12:27 AM   #2
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Yes, that is usually all that you need, but I usually add a 100 ohm resistor from the switch to G2. Mount the resistor right at the tube socket. Remember to NEVER flip the switch with the power on. Trust me bad things will happen. I know that the old Chinese KT88's will spark out big time. I was dumb enough to do it twice, with the same result both times.

If your transformer has ultralinear taps you can use a 3 way switch to go from triode to UL to pentode mode. I put a switch like this in the guitar amp that ate the previously mentioned KT88's. 38 Watts in pentode, 30 Watts in UL and 18 watts in triode mode with about 400 volts B+.
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Old 26th August 2005, 12:35 AM   #3
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Sorry, I'm trying to post during a hurricane, and I lost the connection just as I hit the submit button. Hey the cable is out, there is nothing better to do. I am sure the power will go out before the night is over.

You need a DPDT switch for an SE amp. The center goes to G2, one end goes to B+, and the other end goes to the plate. The second set of poles is for the other channel. I usually add a 100 ohm resistor from the switch to G2. Mount the resistor right at the tube socket. Remember to NEVER flip the switch with the power on. Trust me bad things will happen. I know that the old Chinese KT88's will spark out big time. I was dumb enough to do it twice, with the same result both times.

If your transformer has ultralinear taps you can use a 3 way switch to go from triode to UL to pentode mode. I put a switch like this in the guitar amp that ate the previously mentioned KT88's. It was a push pull amp, 38 Watts in pentode, 30 Watts in UL and 18 watts in triode mode with about 400 volts B+. You can't push your luck with those tubes since they are known for fireworks.

Flipping the switch with the power on causes the tube current to drop to almost zero and then return as soon as the switch makes contact. You will at the least get a loud pop in the speakers. Not good for your tweeters.
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Old 26th August 2005, 08:59 AM   #4
Fuling is offline Fuling  Sweden
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I had a triode/pentode switch in a DIY 6AQ5 PP guitar amp and nothing happened when I switched during operation. Of course, the 6AQ5 worked at only 250V
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Old 26th August 2005, 01:06 PM   #5
Sherman is offline Sherman  United States
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Quote:
Originally posted by tubelab.com
Yes, that is usually all that you need, but I usually add a 100 ohm resistor from the switch to G2. Mount the resistor right at the tube socket. Remember to NEVER flip the switch with the power on...

If your transformer has ultralinear taps you can use a 3 way switch to go from triode to UL to pentode mode...

Thanks for the reply! I think I'm going to go ahead and wire in the switch. This is a sort of test amp. My first ever with my "own" schematic and values and I would like to get as much info and different listening experiences as I can from it.

Unfortunately I'm using Hammond 125ESE OPTs so I don't also have an ultralinear tap or I would wire in a rotary switch to let me pick any of the three modes. On the plus side the 125ESE OPTs sound amazingly good for $37.

I sort of assumed I shouldn't flip the switch with the power on. I didn't know if it would be bad for the tubes but I thought the 425V B+ would arc across the switch eventually either burning it out or fusing it.

I also have a pair of SE KT88 monoblocks which are running in ultralinear mode. I may put switches in those to switch between the three modes as well.
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Old 26th August 2005, 06:16 PM   #6
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I have flipped the switch with the power on while using a "sacrificial pair" of cheap 6L6's and nothing bad happened. I did however get a loud pop in the speakers. This won't hurt my "500 watt" guitar speakers, but is not good for delicate tweeters. Those 6L6's are still alive after years of abuse, but 2 different Chinese KT-88's died a fiery death from flipping the switch with the power on. Only one tube (not matched pairs) in the push pull amp died each time. Since I abused this amp with my guitar on a regular basis, I never put expensive tubes in it.
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Old 30th December 2007, 01:03 PM   #7
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Quote:
Originally posted by tubelab.com
You need a DPDT switch for an SE amp. The center goes to G2, one end goes to B+, and the other end goes to the plate. The second set of poles is for the other channel. I usually add a 100 ohm resistor from the switch to G2. Mount the resistor right at the tube socket. Remember to NEVER flip the switch with the power on. Trust me bad things will happen. I know that the old Chinese KT88's will spark out big time. I was dumb enough to do it twice, with the same result both times.

If your transformer has ultralinear taps you can use a 3 way switch to go from triode to UL to pentode mode. I put a switch like this in the guitar amp that ate the previously mentioned KT88's. It was a push pull amp, 38 Watts in pentode, 30 Watts in UL and 18 watts in triode mode with about 400 volts B+. You can't push your luck with those tubes since they are known for fireworks.

George-
I'm just finishing wiring my SE KT88 amp, and I'd like to clarify the switching arrangement you describe above.
Is the 100 ohm resistor always in the circuit? I was under the impression that it's only needed between the plate and the grid when in Triode mode. It sounds like you have it between the switch and the grid, which means it's always in the circuit. Did I misinterpret your explanation?
Glenn
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Old 30th December 2007, 02:33 PM   #8
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Gents -
For the sake of simplicity; let's consider a SPDT switch for one tube. This switch would be an 'On-On' sort, and it would have three solder tabs on it. You would wire it in something like this:

--
| |--- Wire from UL Tap
| |--- Wire to Grid
| |--- Wire to 100ohm resistor & 1N4007 soldered to the cathode
--

So, when the switch is in the #1 position, the amp would be in UL mode (so the switch connects the grid to the UL tap). When the switch is in position #2, it connects the grid to the cathode via the 100 ohm resistor and diode.
More on the diode - wire it in series with the resistor, having the cathode (the stripe end) on the grid pin

Hope that helps !
Steve
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Old 30th December 2007, 03:01 PM   #9
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Why would you connect the grid to the Cathode? Do you mean Anode (Plate)? This answers my question about the resistor though. It's only connected to the grid when in Triode mode.
I've also seen people use the diode in similar applications.

I also want to run this amp in Pentode mode, so I need an additional position on the switch to connect the grid to the B+.

Thank you for the diode suggestion, I'll probably add that as protection.

Glenn
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Old 30th December 2007, 05:24 PM   #10
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Yes; sorry - momentary brain fart - connect it to the ANODE
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