Super Rocky

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Cressy Snr
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Super Rocky

Post by Cressy Snr » Mon Oct 28, 2019 1:19 pm

Went to my mate Clive’s place last Saturday evening and we got talking about an upcoming class reunion in November, commemorating 50 years since we all started at the now long since demolished, Mexborough Grammar School, September 1969 (yikes!)
It also came up in the conversation, that it’s coming up on twelve years since I built him a Rocky amp for his 50th birthday.
The thing hasn’t missed a beat in all that time, apart from needing one pair of replacement EL34s.

It also happens that I want a basic amp for the craft room I’m setting up in the spare room next year, so given the recent multiple power supply shenanigans, a Rocky amp is the obvious candidate for pimping. Be nice to get re-acquainted.

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Re: Super Rocky

Post by RhythMick » Mon Oct 28, 2019 1:35 pm

Watching with interest...

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Re: Super Rocky

Post by pre65 » Mon Oct 28, 2019 2:03 pm

I'm still using the KT66 (as a triode) SET monoblock amps that Mike H helped me with circuit details. :)
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ed
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Re: Super Rocky

Post by ed » Mon Oct 28, 2019 4:24 pm

interesting times we live in....that we should arrive nearly where we started....way to go Steve...

I was going to suggest an alternative, based on the fact we fairly well know what rocky does....when I built mine I was torn between the simplicity of the angela circuit and that proposed by Claus Byrith. I loved Claus' write up as it revealed a lot of the secrets and explained how the mystical things worked, and together with Bruce Rozenblit' offerings I thought I could conquer the world...ho hum, nostalgia again
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Re: Super Rocky

Post by Cressy Snr » Mon Oct 28, 2019 4:53 pm

ed wrote:
Mon Oct 28, 2019 4:24 pm
interesting times we live in....that we should arrive nearly where we started....way to go Steve...
Yes, the OTL will remain in the main system and the SuperRocky will end up in the craft room system. I could, I suppose, have just put the NVA A20 in there, but I just like making things, and the Rocky seemed to be the best fit. There’s going to be an electronics area and a writing area in there. A man cave is something I’ve never had and it’s about time I got myself one.

I too was once tempted by the Claus Byrith circuit but somehow I got sidetracked into plate-to-grid feedback pentodes, omni speakers and a boatload of bloody grief. :lol: :lol: I’ll have to look at his 8W single ended article again. I have it somewhere

So with the SuperRocky, it’s back to innocent days when the flat earth had been banished, valves were exciting, glowy things, I’d crawled, blinking in the sunlight out of a black hole and the world was in colour.

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Re: Super Rocky

Post by Cressy Snr » Mon Oct 28, 2019 7:35 pm

Interestingly the Hammond 1627SE output transformer I’m planning to use, has a single 4 Ohm winding and a tapped 8 Ohm winding on the secondary, plus a 43% ultralinear tap on the primary. Normally both secondary windings are hooked up in various series and parallel arrangements to give 4,8 and 16 Ohm configurations.
B79EDABD-1E00-4884-B702-6D5C6DA3D3A3.jpeg
BUT....
This opens up the interesting possibility of using SE ultralinear operation of the output stage, and using the single (grn blk) 4 Ohm secondary at the bottom right of the above diagram, as a cathode feedback winding to the output stage, without putting DC on the speaker.

Attach the speaker to the upper 8 Ohm winding and the bottom of the output stage cathode resistor to green on the lower winding and black to earth.

What you end up with is this:
FD162DEC-9EB0-445B-85B1-F9A1A5318BD0.jpeg
Ignore the ‘out’ terminals because the speaker will be on the separate transformer secondary. (blk/yel - yel)
I did something similar with a single KT120 a good few years ago, but had to include the speaker in the cathode feedback path as the James OPT only had a single, tapped secondary.

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Re: Super Rocky

Post by Cressy Snr » Tue Oct 29, 2019 8:49 pm

Schematic, showing cathode feedback setup:
221A7E25-B6F7-481A-BDB1-2290E652EBC5.jpeg

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Re: Super Rocky

Post by IslandPink » Tue Oct 29, 2019 9:08 pm

I did have a look at it - first my brain hurt, then I got the gist of it.
Is the feedback required to lower the output impedance, and/or reduce distortion ?
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Re: Super Rocky

Post by Cressy Snr » Tue Oct 29, 2019 9:37 pm

IslandPink wrote:
Tue Oct 29, 2019 9:08 pm
I did have a look at it - first my brain hurt, then I got the gist of it.
Is the feedback required to lower the output impedance, and/or reduce distortion ?
It’s primary function is to lower the output impedance. I dug out the Claus Byrith SE EL34 Ultralinear amplifier, the one I think Ed was talking about, off the Lundahl transformers website.
https://www.lundahltransformers.com/wp- ... r_8wse.pdf

It was the section on cathode feedback that gave me the idea of using the separate 4 Ohm secondary winding on the 1627SE as a feedback winding. I won’t know if it’ll work until I try it. If it disappoints, I can always revert to triode mode; that’s a known quantity with these amps.

The really interesting bit for me, will be the power supplies. The OTL was transformed by the triple power supply setup. I can’t see any reason why the same won’t apply to this one.
Last edited by Cressy Snr on Tue Oct 29, 2019 9:52 pm, edited 1 time in total.

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Ray P
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Re: Super Rocky

Post by Ray P » Tue Oct 29, 2019 9:48 pm

Cressy Snr wrote:
Mon Oct 28, 2019 1:19 pm
Mexborough Grammar School, September 1969
s'funny old world Steve - September 1969 I turned up for my first day at Price's Grammar School (in Fareham, Hampshire - yes, I'm a proper southern softie!).

I remember we had a PE Master from Yorkshire; made us play Rugby league instead of the proper game and transported his wife all the way to Yorkshire so his offspring could play for Yorkshire CC (they had a daughter :lol: ).

Anyway, I digress....

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Re: Super Rocky

Post by simon » Tue Oct 29, 2019 10:27 pm

Thems were the days, when you had to be born in Yorkshire to play cricket for the county :-)

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Ray P
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Re: Super Rocky

Post by Ray P » Tue Oct 29, 2019 11:02 pm

simon wrote:
Tue Oct 29, 2019 10:27 pm
Thems were the days...
and before ultrasound scans too!

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Re: Super Rocky

Post by simon » Tue Oct 29, 2019 11:21 pm

Pedant alert!

YCC only removed the birthright requirement in 1992. Michael Vaughan was born in Manchester (I think) but grew up in Sheffield and was allowed to play. Sachin Tendulkar was the first overseas player then too I think.

Pedant alert 2!

I'm not sure the birthright requirement had been absolute before anyway, but I'm no historian.

Anyway, apologies Steve, as you were.

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Re: Super Rocky

Post by steve s » Wed Oct 30, 2019 9:02 am

Cressy Snr wrote:
Tue Oct 29, 2019 9:37 pm

The really interesting bit for me, will be the power supplies. The OTL was transformed by the triple power supply setup. I can’t see any reason why the same won’t apply to this one

I'm sure it will Steve. My px25s are not alot different to the rocky amps, just the different valves make a difference.

I'm pretty sure now the sound differences between coupling, using chokes/capacitors/transformers were actually the effect of more or less inter valve smoothing offered by the different topologies.
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Re: Super Rocky

Post by Mike H » Wed Oct 30, 2019 6:31 pm

Image Watching.
 
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