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Old 10th June 2013, 09:50 PM   #1
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Default el34 output transformer, 3 or 5 wire ~?

I have a push pull el34 valve/tube amplifier for which the design shows it needs a 5 wire primary.
I have found a cheap 3 wire primary transformer, can I use this ?
I obviously need to connect grid2 to this anode which currently is done using 270 resistors.
Will this be ok ?

What other options are there ?
Triode mode ? pentode mode ?
What grid 2 resistor value should I use ?
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Old 10th June 2013, 10:51 PM   #2
Enzo is offline Enzo  United States
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OK, look up "ultralinear" power amp circuits. They use an output transformer with the standard center tap and the end wires going to the plates, but they also have taps part way between center tap and the end for each screen grid. That is your five wire primary.

To use a common center tapped primary, you have to then make an alternate power source for your screens. Look at just about any tube guitar amp schematic for examples of that. Well, except ultralinear ones.
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Old 10th June 2013, 11:04 PM   #3
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It seems I will get lower gain in triode mode.
However I have plenty of gain in the amplifier so hopefully that will make up for it.
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Old 11th June 2013, 03:52 AM   #4
Enzo is offline Enzo  United States
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Triode mode?


Whether ultralinear (5 wire) or conventional (3 wire), we are still wired as pentodes. We are NOT connecting the screens to the plates. The screens go to their own B+ node. We generally use 1k 5w resistors from each screen to the power supply.
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Old 11th June 2013, 04:06 AM   #5
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It seems that you have wired the EL34s up as triode-strapped pentodes, that will lower the output power quite a bit, if you are looking for more power, then you should create a separate node on the power supply after the B+ to supply the screen grid directly. Just make sure that its voltage is lower than the plate voltage at idle. If you know the specs for your output transformer, we can give you more feedback (no pun intended ;-).
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Old 16th June 2013, 06:13 PM   #6
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First a silly question: Is the 3 wire transformer suitable for EL34s?
Typically 3400 ohm primary impedance, rated for about 50W output power?
What voltages are we talking about? In pentode mode (both screens are being fed from the same HT supply) most designs call for 400-450V for both screens and anodes. In Ultralinear mode (screens connected to taps on the output transformer) some designs go up to 500V. The sound will be cleaner, but at the expense of a little less maximum power. Triode mode is even cleaner, but you loose half of the power compared to pentode mode. Preamp gain cannot compensate for the loss of gain in the output valves: it is the maximum output power that is limited.
Take good care of the g2 HT supply: classic designs use a choke to filter the g2 supply. Two reasons: the g2 supply needs to be clean. Any power supply ripple will be amplified and show up at the speakers. The other reason is that the g2 current in EL34's increases a lot when the signal goes up. When the g2 supply sags under signal conditions, that will increase distortion. You have to find out yourself if it is the kind of distortion that you like or not.
Beam tetrodes like the 6L6 draw a lot less g2 current, so there you can filter with a big resistor instead of an (expensive) choke.
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Old 16th June 2013, 06:20 PM   #7
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I am using 4 el34's with a 100 watt Fender output transformer.
The B+ is currently 350VDC.
I intend to use pentode mode, with g2 connected through 1k to 350VDC.

I might fit a switch for switching between pentode mode and triode mode.
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Old 16th June 2013, 07:13 PM   #8
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Sounds fine
With 350V and the slightly higher primary impedance of that Fender OPT you can run the EL34s almost into class A if you want. Nowhere near 100W, but you should be able to get a very nice tone.
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Old 16th June 2013, 07:21 PM   #9
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I would like to get as near to 100 watts as possible.
I am currently using 120R cathode resistors shared between 2 valves.
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Old 16th June 2013, 09:51 PM   #10
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Do you know the primary impedance of your output transformer?
My guess would be around 2800 ohm, since Fender mainly uses 6L6 outputs.
Four EL34s running at 350V into 1700 ohms using cathode bias will give you around 70W. If your OPT is indeed 2800 ohm, the power would be closer to 50W than to 70W.

First step to get more power is to change the cathode bias to fixed bias.
After that, the quick 'n' dirty way to get more power is to use 8 ohm speakers on the 16 ohm tap or 4 ohm speakers on the 8 ohm tap. This translates to halving the primary impedance that the EL34's see. Assuming the OPT is 2800 ohms to 16/8/4 ohms, you could use it as 1400 to 8/4/2. According to the Telefunken datasheets, this would give you 88W. This is with 350V on the anodes. If the OPT has a high dc resistance you will loose some precious Volts there.
Of course your power transformer must be able to deliver enough current: 0,6A continuously at full power, more when you overdrive the EL34's.
My advice: Just try and see where it takes you
The other way to get to 100W would be to get a 450V supply.
One other thing you need to ask yourself is: do you really need 100W. I have seen '100W' Marshalls that put out no more than 80W.
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