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Choosing 6550 PP Output Transformer
Choosing 6550 PP Output Transformer
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Old 9th May 2012, 03:08 AM   #1
mbeards is offline mbeards  United States
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Default Choosing 6550 PP Output Transformer

Hi everyone,

I am working on a push pull build using a pair of 6550's in AB1 (pentode operation) and I am working on selecting the right output transformer.

First, off I wanted to confirm that my calculation of the transformer primary impedance is correct.

The amp has a plate voltage of 450V. At full power this will be down to around 400V. (I used the 400V for my load line.) The screens are regulated to 310V so there will be no sag at full power.

Using GE tube data, I drew my load line to intersect with the knee of the zero grid voltage curve. This point was estimated for 310V screen based on the curves for other screen voltages.

I found an ac voltage swing of 330 volts (400 - 70) and an ac current swing of 425 mA.

330/.425 = 776.5 ohms

776.5 * 4 = 3.1K

So does 3.1K sound right based on the operating conditions?
Also how much is it "safe" to vary the impedance by with minimum effects?

Second, what are some good brands or well liked output transformers?

I have seen edcor as a popular choice but I am wondering what experience people have had with them as well as other higher quality transformers.

Thanks to all
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Old 9th May 2012, 05:06 AM   #2
roline is offline roline  United States
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6.6 K Raa are easier to drive with less feedback, I've used 6P3S at 360v, 5881 wafer bases, 6550's and KT88's at 450V.

4.3k Raa require a little more feedback and lesser tubes like the 5881's will run out of steam at volume. When the tubes switch from class A to class B the impedance will appear to be 1/2 of class A operation so the tube will see about a 1.1k load. The 4.3K 60 watt Hammonds with Svet 6550=C= tubes make a fine combo.
SO many tubes, SO little time!!!
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Old 9th May 2012, 06:19 AM   #3
Chris Hornbeck is offline Chris Hornbeck  United States
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For a good example of load lines in push-pull output stages, the old Radiotron Designers Handbook has served many generations. I can't follow what you're doing from the description, but it doesn't sound complete. I strongly suggest making two copies of the plate curves and superimposing them (with one copy inverted) to begin your process. And I really can recommend RDH4 for a decent working explanation.

All good fortune,
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Old 9th May 2012, 08:02 AM   #4
nazaroo is offline nazaroo  Canada
Join Date: Feb 2012
If this is a 'hi fi' amp rather than a guitar amp,
I'd strongly suggest considering Ultralinear operation,
rather than straight pentode/tetrode,
as it offers lower distortion at higher wattages,
and a more triode behavior without significant power loss.

If you do so, you'll need UL curves to analyze your loadline,
rather than tetrode-curves.

voltages look reasonable, but you'll need at least a 100 watt output transformer, and your power transformer must be able
to easily deliver the current to feed 6550s (over 200-250 mA at rated voltage for one pair).

Your primary impedance looks way too low,
because almost all audio power tube pairs require a plate-to-plate of between 2.6k and 7k.
While your looking at static transfer curves,
also have a look at recommended operating conditions,
or typical operating conditions for 6550 /KT88s.
There you'll see suggested loads for a pair.

6550 Datasheet GE
Attached Images
File Type: jpg kt88b.jpg (115.9 KB, 383 views)

Last edited by nazaroo; 9th May 2012 at 08:15 AM.
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Old 9th May 2012, 08:53 PM   #5
Chris Hornbeck is offline Chris Hornbeck  United States
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SY just posted this link in another thread and it's excellent:


and the first part is at:


All good fortune,

Last edited by Chris Hornbeck; 9th May 2012 at 09:00 PM.
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Old 10th May 2012, 05:05 AM   #6
mbeards is offline mbeards  United States
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Thanks for the great info, you guys have lots of great resources. I am rethinking that I may want to go Ultralinear.

I can possibly add a switch to change to a regulated pentode operation. I have to investigate since switching to pentode may make the plate load wrong since it will be optimized for ultralinear operation.
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