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PSE EL34 OPT

6A3sUMMER

Member
2016-06-07 6:50 am
Take an optimum SE EL34 design, including an optimum output transformer.

Now design an optimum SE Parallel EL34 design, including an optimum parallel tube output transformer.

The difference between the single EL34 output transformer and the parallel EL34 transformer will be this:

Single tube primary impedance = X
Parallel tube primary impedance = 1/2 X

Single tube xfmr laminations = Y
Parallel tube laminations = 2 X Y (more laminations, more power).

Single tube xfmr power rating = W
Parallel tube xfmr power rating = 2 X W

Using 1 EL34 tube versus 2 EL34 tubes on a single transformer model will be less than optimum for one of those amplifier configurations.

I can not make it any simpler than that.

The other completely different direction is to use 2 EL34 tubes in Push Pull; that is another thread.
 
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6A3sUMMER

Member
2016-06-07 6:50 am
You could use a dual impedance primary, for example a 3.5k and 5k, as long as the transformer will withstand the DC currents without saturation, and has enough laminations for the output power levels and lowest frequencies you will need.
These are not optimum ratios, but there are a few models like this out there.

The problem I see, is that if you use parallel plates on 3.5k, the unconnected extra windings that give 5k are "hanging and banging" in free space. Just an unterminated resonator waiting to resonate.

By the way, from 3.5k to 5k is only ~ 20% more turns. (i.e. 2000 turns, then 400 more turns added in series). (They do not wind both a 2000 turn primary, and a 2400 turn primary).

The square of the turns ratio gives the impedance ratio. T ratio squared = Z ratio
1.2 x 1.2 = 1.44

Remember that the magnetic field is amp x turns. More turns has to have less current, to prevent saturation, or use more laminations.
Generally, a 3.5k/5k dual Z transformer can have 20% more DC current in the 3.5k connection than the 5k connection.
 
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6A3sUMMER

Member
2016-06-07 6:50 am
I have used the 1600 Ohm UBT-1 in several different amplifier topologies.
It is a good transformer when properly used.

Some use it as a 3200 Ohm transformer, by connecting an 8 Ohm load on the 4 Ohm Tap(or 16 Ohm load on the 8 Ohm tap).
But the UBT-1 primary inductance is only 8 Henry.
8 Henry is 3200 Ohms at 64 Hz.
The result of using the transformer this way is the low frequency bandwidth is not very good.

The Randall amplifier uses 3 EL34 tubes in parallel, and the tubes are wired in 'super triode' mode (the suppressor grid, screen and plate are connected to each other). The transformer is used in the intended 1600 Ohm mode (8 Ohm load on 8 Ohm tap, etc).
8 Henry is 1600 Ohm at 32 Hz.
 
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Huntom

Member
2020-01-29 1:41 am
I have used the 1600 Ohm UBT-1 in several different amplifier topologies.
It is a good transformer when properly used.

Some use it as a 3200 Ohm transformer, by connecting an 8 Ohm load on the 4 Ohm Tap(or 16 Ohm load on the 8 Ohm tap).
But the UBT-1 primary inductance is only 8 Henry.
8 Henry is 3200 Ohms at 64 Hz.
The result of using the transformer this way is the low frequency bandwidth is not very good.

The Randall amplifier uses 3 EL34 tubes in parallel, and the tubes are wired in 'super triode' mode (the suppressor grid, screen and plate are connected to each other). The transformer is used in the intended 1600 Ohm mode (8 Ohm load on 8 Ohm tap, etc).
8 Henry is 1600 Ohm at 32 Hz.

I found one from hammond what they recommend with parallel 6ca7
It’s the 1640SEA

https://www.mouser.co.uk/datasheet/2/177/1627-1642-1389902.pdf

Could this make the job?
 

6A3sUMMER

Member
2016-06-07 6:50 am
Bas Horneman,

You are correct.
I believe Super Triode Mode is when you drive the screen and sometimes also the control grid, right?
I should have not used the term Super Triode mode for the Randall amp.

The EL34 is somewhat unusual for a Pentode. It is about the only 'modern' design power pentode that brings the Suppressor Grid out on a separate pin.
Most 'modern' and most 'old' design power pentodes connect the Suppressor Grid directly to the cathode.

The Randall amp takes advantage of the separate Suppressor Grid, so they connect the Suppressor to the plate along with the screen to plate connection.