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Old 28th April 2013, 03:18 AM   #21
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I am not too fussy about a hi fi amplifier.
I was going to use it as a guitar practice amp and I am into rock music so some healthy distortion wont go amiss.
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Old 28th April 2013, 10:42 AM   #22
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Originally Posted by TheGimp View Post
They could have used a pentode configured as a current source in the tail with another (-)200V supply, but probably would have thought that was just plain silly.

They also had Current Sources in the form of a Barretter (RDH4, p1212), or current regulating tube.

If one searched hard enough I have no doubt someone tried it with 'tubes back in the day'.
A couple of months ago, I saw a variant on the Dutch historical radio forum (Nederlands Forum over Oude Radio's) where the current source had been replaced with a resistor to a tap of the transformer secondary that ideally carries the same signal voltage as the midpoint between the cathodes. That gives you the same small-signal behaviour but somewhat different distortion characteristics, because the midpoint between the cathodes and the transformer tap do not carry exactly the same signal anymore when the amplifier is driven close to clipping.
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Old 29th April 2013, 01:03 AM   #23
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Quote:
Originally Posted by TheGimp View Post
Junm, the second output tube does more than balance the DC current.

Because the fixed current is in the tail of the two tubes and is shared, when one tube conducts more, the other tube has to conduct less.Therefore, as one anode goes in the positive direction, the other anode HAS To GO in the opposite direction by a balancing amount.

This is what Push Pull does by definition.

It is not an SE output using a second tube for DC balancing to allow it to use a PP transformer.
Thanks for the explanation...I'm just new to tubes and still learning...just focus my eye on the single input..since most design here for push pull has phase inverter.
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Old 29th April 2013, 04:00 AM   #24
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since most design here for push pull has phase inverter.
The input stage is SRPP, but could be anything that furnishes enough drive. The output stage IS the phase inverter. It is a classic LTP and functions just like the LTP's used as phase inverters, except instead of feeding output tubes, it feeds an OPT. This is sometimes called a self split output stage.

Being an LTP the gain is about half what you would expect, meaning you need twice as much drive voltage as an EL84 would normally need. The CCS in the tail forces class A operation. This can be the deal breaker for a guitar amp. The smp remains very clean up to the point where the CCS can't give enough current for both tubes, then some rather ugly distortion sets in.
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Old 29th April 2013, 04:38 AM   #25
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Quote:
Originally Posted by tubelab.com View Post
The input stage is SRPP, but could be anything that furnishes enough drive. The output stage IS the phase inverter. It is a classic LTP and functions just like the LTP's used as phase inverters, except instead of feeding output tubes, it feeds an OPT. This is sometimes called a self split output stage.

Being an LTP the gain is about half what you would expect, meaning you need twice as much drive voltage as an EL84 would normally need. The CCS in the tail forces class A operation. This can be the deal breaker for a guitar amp. The smp remains very clean up to the point where the CCS can't give enough current for both tubes, then some rather ugly distortion sets in.
I have added a decoupling capacitor across the SRPP lower cathode just in case I need more gain. It can be used or left out as desired on the pcb.

Its a strange output stage as both valves are on for a portion of the time.
The outputs uses 80mA and at some point this can be 40-40 in either tube meaning it is pulling and pushing at the same time !
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Last edited by nigelwright7557; 29th April 2013 at 04:45 AM.
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Old 29th April 2013, 04:46 AM   #26
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Quote:
Originally Posted by Junm View Post
Thanks for the explanation...I'm just new to tubes and still learning...just focus my eye on the single input..since most design here for push pull has phase inverter.

don't let the 2007 date into thinking that this is a new topology, it has been done before in the 60's or even earlier, i saw that in a popular electronics magazine years ago....
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Old 10th May 2013, 10:50 PM   #27
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Finally got it built.
Only put out about a watt !!!

Traced fault down to the input valve not being grounded, my fault not changing netname to GND.
Works great now and very loud.

Click the image to open in full size.
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