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Old 27th April 2013, 03:09 AM   #11
TheGimp is offline TheGimp  United States
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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.
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Old 27th April 2013, 03:10 AM   #12
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I built the KT77 version and biased it for the 6v6 tubes (I had a bunch laying around) and I used 8k trans and I run the 6v6 tubes at a lower voltage, I think roughly 270 volts(it's been a while) but they are JJ's. It's an easy build and gives great results. Sure it's not a 3000 dollar amp but it makes a great workshop amp and I have to have about 200 hours so far on the same tube set. JJ 6v6gt and the new Tung sol 6sl7 I like my octals

Mike.
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Old 27th April 2013, 05:41 AM   #13
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Quote:
Originally Posted by egellings View Post
The impression I get from this is that the driven output tube operates as a common cathode type, while the bottom tune operates grounded grid. The upper tube inverts, and the bottom tube doesn't, so you do get phase inversion. Problem is, the grounded grid bottom tube has a very high plate resistance as compared to that of the upper tube, so I would think that there would be a serious impedance mismatch problem.
I think you are making a classical mistake here. The grounded-grid stage (if you want to regard it as such) is driven from such a low impedance that it hardly has any local loop gain. As a result, when you calculate the impedance from its anode to ground, it is only twice the internal resistance. The common-cathode stage (if you want to regard it as such) sees some local series feedback from the common-cathode stage. As a result, when you calculate the impedance from its anode to ground, it is also twice the internal resistance. When you calculate the differential impedance between the anodes, you again find twice the internal resistance of a single valve.
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Old 27th April 2013, 10:32 AM   #14
DF96 is offline DF96  England
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Yes, both output valves have the same anode impedance in this circut - twice the usual value. This is because each cathode sees the other cathode.
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Old 27th April 2013, 10:42 AM   #15
AJT is offline AJT  Philippines
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classA push-pull power amp...
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Old 27th April 2013, 10:44 AM   #16
SY is offline SY  United States
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It's worth looking at RDH4 and the comments made about circuits of this type.

The label on the schematic notwithstanding, this isn't an SRPP. Distortion and overload recovery will not be inspiring.
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Old 27th April 2013, 11:24 AM   #17
godfrey is offline godfrey  South Africa
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Well, it's got an SRPP input stage. That doesn't count?
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Old 27th April 2013, 12:31 PM   #18
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Quote:
Originally Posted by SY View Post
It's worth looking at RDH4 and the comments made about circuits of this type.

The label on the schematic notwithstanding, this isn't an SRPP. Distortion and overload recovery will not be inspiring.
the comments in RDH4 are not really appicable since they never had the CCS which are on offer now, and which allow this circuit to behave respectably compared to the historic implementations.

Shoog
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Old 27th April 2013, 12:53 PM   #19
TheGimp is offline TheGimp  United States
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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'.
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Old 27th April 2013, 11:38 PM   #20
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I have ordered the parts to build this amplifier.
I have laid out a pcb and sent it off.

I am using 2 back to back transformers.
The first gives 6v3ac and the second steps up the 6v3ac to 170vac.
I got a 8k 25w output transformer off ebay.
I have fused the output transformer just in case of problems as it was 36.
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