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Opinion on EL84 Schematics

6A3sUMMER

Member
2016-06-07 6:50 am
Remove the 1N4007 from the screen. Replace with a straight wire, or a 100 Ohm grid stopper resistor if you prefer.

Remove the other 1N4007 from the B+ (that connects to the other channel).

By the way, the link says the 6BQ5 and the EL84 are the same.
The specs are similar or are the same.
But a True 6BQ5 is a Beam Power tube with Beam forming elements connected to the cathode.
And a True EL84 is a Pentode tube with a real Suppressor Grid that is connected to the cathode.

The negative feedback:
Depending on the quality of the output transformer, it may cause the square wave shape to be degraded.

Do not be afraid to try Triode wired mode. Just wire the screen to a 100 Ohm resistor, and the other end of the resistor to the plate.
 
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6A3sUMMER

Member
2016-06-07 6:50 am
zdenoeddie,

Push Pull:
While in the class A region of push pull, each tube's current change (and each tube's plate resistance, rp) is effectively in parallel.

8k plate to plate primary has 2k for each half of the secondary (impedance is proportional to the number of turns squared; 8k full turns, 2k for half turns; 1/2 x 1/2 = 1/4).
That means with the plates effectively in parallel, each tube plate 'sees' 4k. Or to look at it another way, 2k of turns sees two plates in parallel, making it 'look' like 4k.
You will not get lots of power, but the damping will be ok, and distortion should be ok at the lower power level.

When you leave the class A region (larger signal level), you go to class AB. One tube is cut off during the 'positive' alternation, and the other tube is cut off during the 'negative' alternation.

Single Ended
If you are doing Single Ended with an EL84, I ask what mode is the EL84 operating in, Triode mode, Ultra Linear, or Pentode mode?
I am of the opinion that for EL84 in triode mode, and single ended that 8k is too high.
But transformers rated 8k Raa is a push pull transformer, and can not take the DC of Single ended operation.

For an EL84 in triode mode, the plate resistance, rp, is approximately 1.7k.
All class A single ended has to be in class A. Any other than class A will be badly clipping (sounds bad).
 
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6A3sUMMER

Member
2016-06-07 6:50 am
zdenoeddie,

In general . . .

The difference in plate resistance, rp, is very far apart from Pentode versus Triode mode.

The difference in plate resistance, rp, is nearer from Ultra Linear mode versus Triode mode. It depends on the UL tap percent; I have used 40% for push pull.
(50% and 80% for single ended, which is not what you are doing).
The higher the percent (%), the more UL mode is like triode mode.

Of course, the voltage gain of the output stage is highest in Pentode mode, lowest in triode mode, and in-between for UL mode.
With a given bias voltage, that limits the maximum power out, lower gain means lower power out.

In order to predict the results of switching from Pentode mode to Triode mode, some knowledge of what rest of the circuit is like is needed; then some generalizations can be made.
Your mileage may vary.

Please post the circuit that you are going to use.
That would be helpful to generalizing the results you might get with Pentode versus Triode modes.

In general, most applications of Pentode mode requires quite a bit of negative feedback, Schade (output plate to driver plate); Global from the output transformer secondary; output plate to driver cathode; etc.

Often, Triode mode can be used with little or no negative feedback.

Ultra Linear requires a transformer with UL taps, and generally needs at least a little negative feedback too, often global from the output transformer secondary.

I have been considering using EL84 in push pull with a 10k output transformer I have. Nothing definite yet. I have too many tube amplifier ideas waiting to be done.
Right now, I am designing and building a push pull 7591A amp. The topology will not be like what you typically see on the web, Japan's MJ magazine, etc.

One thing to remember is that you need to set the Pentode / Triode mode switch with the amplifier powered off and B+ discharged).
The same also goes for a Ultra Linear / Triode mode switch, do that with the amplifier unpowered (and the B+ discharged, so there is no current in the output transformer primary). Same principle.
If you switch that mode switch with the amplifier powered, it will cause a very large transient voltage that can be bad for tweeters, output transformer primary, and the output tube.
The temptation is to do a quick listening comparison, and change the mode switch setting with the amp playing music. Just remember the risk to tweeters and the amplifier, your mileage may vary.
 
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