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Old 25th May 2004, 12:58 AM   #1
Hesky is offline Hesky  United Kingdom
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Default 6C33C schematic

Hi everyone,

For the last few months I've been looking around for a nice monobloc project to do over the summer holidays. I've had a little experience with valve amps, built and tweeked a KT88 PP integrated and presently building a triode SE headphone amplifier. I'm not quite ready to play around with those rather high voltage transmission tubes (as much as I would like to though) so Iíve started looking for a lower voltage triode project. I originally considered a PP 300b amplifier (my speakers are rather insensitive ) but 4 good quality 300bs would be rather costly, also I wanted to use a less commonly used tube. Then I stumbled across the 6C33C indirectly heated triode. The thing that appealed to me most was the rather low plate voltages just over 200V and also the fact you can use a very low primary resistance on the OPT giving good bandwidth.
At the moment I have 4 well matched 6C33C tubes and a set of good quality teflon sockets.

After searching for a while I found this quite nicely laid out schematic IMO.

http://www.jogis-roehrenbude.de/Lese...chematic-1.gif

http://www.jogis-roehrenbude.de/Lese...chematic-2.gif

I've emailed the chap that created this circuit to ask for permission to use it but I have had no reply. As itís from a German web site I think my translated English was clearly rubbish.

Just wondered what you guys thought of it.
Thanks!

Hesky
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Old 25th May 2004, 01:39 AM   #2
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Hi,

Quote:
I've emailed the chap that created this circuit to ask for permission to use it but I have had no reply. As itís from a German web site I think my translated English was clearly rubbish.
If it were me, I wouldn't worry about that aspect too much.
As long as it's for private use that is.

As for the amp itself, it seems straightforward enough, with a little change you can even drive it from a balanced source and the 6C33-C are great sounding monsters.

Just be aware of the high heater current which tends to carbonize the pins on the sockets. I was advised by (316A I think) to wrap the wire around the pin(s) a few times to increase surface area and provide some cooling surface area.

Cheers,
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Old 25th May 2004, 02:14 AM   #3
Hesky is offline Hesky  United Kingdom
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Hi Frank, yes this project will just be for my own musical pleasure

Quote:
with a little change you can even drive it from a balanced source
This was my thinking also, as there monoblocs a balanced connection would help a lot to reduce the chance of ground loops as I may have to use nasty 1-2m interconnections. Also with the removal of the interstage/phase splitter transformers a little more money can be spent on better decoupling capacitors and good quality resistors.
The transformer attenuator I'm building will have balanced connection which is convenient.

Thatís a good tip for increasing the heat transfer on the filament connections, Im also glad this circuit has manual biasing for both output tubes.

Hesky
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Old 25th May 2004, 05:27 PM   #4
316a is offline 316a  England
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Default 6C33

Quote:
Originally posted by fdegrove
I was advised by (316A I think) to wrap the wire around the pin(s) a few times to increase surface area and provide some cooling surface area.

Cheers,
Hello ,
The Russian military used to replace the sockets more often than the valves from what I have heard ! The wrapping is a good idea if Russian / Chinese ceramic or British teflon B7A sockets are used , just strip away about 40mm of insulation then wrap the wire around the socket pins a few times before soldering , this only needs to be done with the filament pin contacts . I found this out the hard way and ended up with some sockets with crusty , burnt contacts , my fault really for running the 6C33 at 55W . US Johnson sockets are apparently immune to this problem but are rather expensive . Some advice : Running at 55 watts I also had a 6C33 outgas and split on me , quite a spectacular little firework display inside the thing but also a complete P.I.T.A . Run the things at around 40-45 watts and they'll last and last and don't run any hotter than this with fixed bias .

ps Just one last thing : make sure the 6C33 pins are cleaned before usage , Brasso wadding follwed by a rinse with isoprop and cottonbud does the trick

cheers

316a
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Old 25th May 2004, 06:27 PM   #5
Hesky is offline Hesky  United Kingdom
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Thanks 316a, I think using that technique and rather thick gauge wire the sockets will live as long as the tube.
55W is a heck of a lot of anode dissipation Kt88 extreme maximum is some where in the region of 42W I think.
I know this maybe a little hard to answer as there are many circuit topologies, but what sort of sound characteristics do these indirectly heated triodes have? I've heard the BAT PSE tube amp which uses the same tubes but only for a very short time

Hesky
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Old 25th May 2004, 07:05 PM   #6
316a is offline 316a  England
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Default 6C33 ctnd...

Quote:
Originally posted by Hesky
Thanks 316a, I think using that technique and rather thick gauge wire the sockets will live as long as the tube.
55W is a heck of a lot of anode dissipation Kt88 extreme maximum is some where in the region of 42W I think.
I know this maybe a little hard to answer as there are many circuit topologies, but what sort of sound characteristics do these indirectly heated triodes have? I've heard the BAT PSE tube amp which uses the same tubes but only for a very short time

Hesky
Hello ,
Yes , the 6C33 is a hot ****** with a greedy filament , make sure the chassis is large enough , remember that the filaments take 36 watts alone . I can't really compare what I built to a standard SE amp as I used stacked 6C33 in SRPP topology . This had a standalone 380V 250mA PSU per channel , and was big , ugly and very innefficient . It would heat my workshop better than an electric heater in the winter Mains toroids were used as output transformers which surprisingly work particularly well with low Ra valves I did notice that at low volumes , probably a few watts , the sound was remarkably clean which would suggest that if sensitive speakers are used these would be an ideal match . Bass was also clean and not peaky like many SE amps , probably as a result of improved speaker damping . The only thing I did find was that medium mu valves such as 6SN7 don't really have the legs to drive these things adequately . After first trying E182CC and 6SN7 then trying 6BX7 , 6BL7 I eventually plumped with 2C50 which is a 12.6V filament double triode with mu of 10 .

cheers

316a
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Old 25th May 2004, 07:07 PM   #7
316a is offline 316a  England
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Default Re: 6C33 ctnd...

Quote:
Originally posted by 316a


Hello ,
Yes , the 6C33 is a hot ****** with a greedy filament ,

...blimey , you can't say 'bug-ger' around here !

cheers

316a
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Old 25th May 2004, 07:28 PM   #8
grataku is offline grataku  United States
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What is 'brasso wadding'?

Out of curiosity how hot do the pins get?
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Old 25th May 2004, 07:34 PM   #9
316a is offline 316a  England
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Default Brasso

Quote:
Originally posted by grataku
What is 'brasso wadding'?

Out of curiosity how hot do the pins get?
Hello ,
Brasso is a brand of British metal polish . Wadding is mildly abrasive spongy stuff impregnated with the polish . Seems to do the trick . Maybe this is a British thing like Bovril that doesn't appear anywhere else I haven't measured the temperature at the pins but running at 55 watts the solder at the tags starts to go crispy within a few months with the Russian sockets

cheers

316a
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Old 25th May 2004, 07:42 PM   #10
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Default 316a

Quote:
Mains toroids were used as output transformers which surprisingly work particularly well with low Ra valves
Hi 316a,

Can you please tell me/us what kind of transformer (Plitron, other) you've used, and what the voltages where?

Thanks in advance,


Audiofanatic

P.S. I have a schematic of a PP 6C33C amp. with 6922, EL84 and the monster 6C33C and that amp. sounded really nice.
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