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Old 15th November 2009, 04:02 PM   #1
pointy is offline pointy  United Kingdom
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Default transistor AC blocker

a little while ago there was a post about using 2x transistors in place of the anode resistor to block AC, then there was a post about a 6V6 anode follower amp.
I was thinking about the 6V6 amp designs of the 70s and the best one was a anode follower with major smoothing for the B+ (370v). then before the primary of the out-put transformer they used one of the 2x transistor AC blockers, this was the only way they could get good treble.

so here's the question -

is this type of AC blocker still used at this point in other types of amps much now days and if not why not....?
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Old 16th November 2009, 01:18 AM   #2
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"a little while ago there was a post about using 2x transistors in place of the anode resistor to block AC"

link?
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Old 16th November 2009, 01:36 AM   #3
kevinkr is offline kevinkr  United States
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Do you mean a CCS? (constant current source) If this is what you mean they are widely used. The general consensus around here seems to be that cascode types based on current IXYs mosfets are amongst the best that can be realized.
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Old 16th November 2009, 02:14 AM   #4
pointy is offline pointy  United Kingdom
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sorry iv'e been unable to find a link to the original post, and yes it would be one of the ccs,s.
it's just I hadn't seen any with transistors feeding the output transformer, may be the mosfets are just so much better.
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Old 16th November 2009, 02:29 AM   #5
pointy is offline pointy  United Kingdom
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they used transistors as they were good at stopping AC.

which may mean less cost for the L-Cs on the dead hampster wheel.
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Old 16th November 2009, 02:38 AM   #6
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Do you mean AC noise feedthru from the power supply? The CCS will stop noise from the B+ from getting to the plate, but aren't usually used on the output stage. Did the output stage then have a cap between the plate/CCS connection and the output tranny? There was a thread recently on using toroidal power transformers for OTs that did this.
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Old 16th November 2009, 02:52 AM   #7
pointy is offline pointy  United Kingdom
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yes I think they may have used a cap between the connection, it's so long ago it's hard to remember

I will have a look for the post using toroidal transformers it sounds about right if they are using CCS to output transformer

thanks
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Old 16th November 2009, 02:58 AM   #8
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Strange idea for an output stage - comments requested ?
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Old 16th November 2009, 03:02 AM   #9
pointy is offline pointy  United Kingdom
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thanks alot now ive got my reading cut out for the rest of the night
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Old 16th November 2009, 04:45 AM   #10
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Quote:
Originally Posted by pointy View Post
a little while ago there was a post about using 2x transistors in place of the anode resistor to block AC, then there was a post about a 6V6 anode follower amp.
I was thinking about the 6V6 amp designs of the 70s and the best one was a anode follower with major smoothing for the B+ (370v). then before the primary of the out-put transformer they used one of the 2x transistor AC blockers, this was the only way they could get good treble.

so here's the question -

is this type of AC blocker still used at this point in other types of amps much now days and if not why not....?
You could be talking about either a CCS or an active decoupler. Either one can be implemented with transistors. These days, active decouplers are done with power MOSFETs. These puppies are a good deal cheaper than ripple filter chokes, and often are used in their place. The active decoupler is connected as a source follower.

Active plate loads are often turned the other way around. Since you mention two transistors, I suspect you're talking about an active plate load.
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