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Old 24th April 2011, 09:00 AM   #1
Skorpio is offline Skorpio  Denmark
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Default Drive for Simple SE with EL34?

Hi,

I want to use 12BH7A tubes for the driver instead of the ECC81 used. The reason is my good experience with 12BH7A and CCS/LED bias (15mA) in a F4 driver.

The gain in this combo is just above 20dB (22-23dB), will that be enough for driving the simple SE output stage in this configuration:
-EL34 in triode connection
-5K output transformer
-No cathode feed-back

My source gives 1.2Vrms at 0dB (DAC) and giving 23dB gain it gives 17Vrms or 24Vpeak...will that drive the outputstage?

Another issue:
A tube amplifier must always have a load connected. How to best implement a speaker switch between two set of speakers? The will be momentary disconnected between switching?
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Old 24th April 2011, 10:25 AM   #2
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Ill let others chime in, but i think tubelab has the thing biased at 40 volts, so 24 is 1/4th the power.

I would not recommend your switch. With no load all the internal voltages are going to shoot to the roof. If your caps cant handle the voltage then they are going to wear alot faster or just pop. Same thing for your tube plates.
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Old 25th April 2011, 08:18 AM   #3
Ian444 is offline Ian444  Australia
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Just turn the volume down to zero before switching speakers.
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Old 25th April 2011, 12:42 PM   #4
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Im not going to get into this much more since this is over my head alittle.
But without feedback and no load the amp will go into infinite gain mode. So there is a good chance that either the output tube will blow, or the output transformer will short. I

If you had a make before break switch then you would be ok.

"If the transformer is not connected through a circuit on both its primary and secondary taps, it will feed back on itself and generate a large voltage spike at its connected end. What happens with too high a load connected to the transformer's secondary is that it acts like an open connection and causes feedback within the transformer. Since the primary of the transformer is connected to the plates of the output tubes, this voltage spike hits them first. As long as it doesn't exceed their limit, you're OK, but if it does, you can short them, which WILL destroy your amp."
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Old 25th April 2011, 02:20 PM   #5
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The 12BH7 has about half the gain of the 12AT7. I have used them in the SSE and they can be made to work, but my DVD player could not drive the amp to clip with some CD's. They do sound nice, and some users like them.

The speaker switch issue is a bit more complex. The rule of thumb says to never operate a tube amp without a load. It is true that amps can be destroyed by doing this. I have seen it happen, and often with flaming consequences. There is however a lot of misinformation and supperstition about what happens when you do this. Since I am known for blowing stuff up, of course I have fried a few parts in the name of science.

Quote:
If the transformer is not connected through a circuit on both its primary and secondary taps, it will feed back on itself and generate a large voltage spike at its connected end.
How does a transformer "feed back on itself". A large voltage spike IS generated, but it has nothing to do with feedback. It has to do with the basic principle of inductance.

Operating a non - feedback tube amp without a load and without an input will generally not cause any damage. Operating a non - feedback tube amp at low volume without a load will generally not cause any damage. Operating a tube amp that uses feedback at zero or low level without a load will generally not cause damage IF the feedback system is COMPLETELY STABLE under a no load condition. Some feedback systems may become unstable without a load causing the amp to oscillate. A perfect feedback system will hold the output voltage constant preventing damage. Perfect systems do not exist.

Operating nearly ANY tube amp into clipping without a load WILL cause generation of serious high voltage spikes in the OPT. It is also possible that an amp that operates without clipping at a moderate power level will clip when the load is removed causing failure. WHAT IS HAPPENING?????

An OPT is basically two coupled inductors. An OPT without a load is just a single inductor. To understand what happens, we need to understand inductance, so I consult Google, and the first hit takes me to the Wikipedia. I quote the first line of the definition:

"Inductance is the property of an electrical circuit causing voltage to be generated proportional to the rate of change in current in a circuit."

In normal operation the tubes cause a constantly changing current through the primary inductance of the OPT. The current changes at an AUDIO RATE. This results in a changing magnetic field that causes a voltage to be generated in both the primary and secondary inductances, causing a current through the load.

When we run the amp into clipping the current through the output tube abruptly cuts off. Even if the current in the primary abruptly ceases, current will still flow in the secondary coil through the load. The abrupt cesation of primary current does generate some high frequency energy (distortion) that can damage tweeters.

If there is no load on the secondary there is no current flow through the secondary. If the audio level is such that there is no clipping there will still be a changing current flow through the primary inductance at AN AUDIO RATE. When we run the amp into clipping the current through the output tube abruptly cuts off. The RATE OF CHANGE can be near infinite! Since the "generated voltage" is proportional to the RATE OF CHANGE the generated voltage can be NEAR INFINITE! This is how the ignition coil works in your car.

I have measured several kilovolts across the primary of an unloaded OPT when the amp hits clipping. If the OPT was perfect the voltage would be near infinite. Cheap OPT's with lots of winding capacitance have lower voltage spikes since the capacitance slows the rate of change. Usually the high voltage spikes will cause an arc inside the OPT eventually destroying it. Sometimes it can damage the output tube. I have also seen burnt tube sockets, and even a blown power transformer and a blown 12AX7. How? The plate of a 6L6GC is pin 3, the heater is pin 2. An arc jumps across the pins of the tube socket turning the high voltage loose on the heater circuit (Fender Bandmaster).

SO....The rule of thumb says DON'T DO IT. That said, I swap speakers all the time with the amp on, just turn the volume ALL THE WAY DOWN first.
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