cannikin said:Finally here is the fruit of my labors for the past 2 years (mostly searching for parts... especially the XFRMRs). I got the circuit from a 1954 RadioCraft magazine, hence the name "1954 Limiter".
I wanted this project/unit to be grand in size -- big chassis, big meters, big knobs -- just like the old days before everything was optimized to be small and compact -- With each unit in a seven space rack -- I think I achieved that goal.
Bear with me as I try to describe what's going on in the circuit.. still learning:
The limiter uses UTC A-12 (600:80K) input transformer and 2 6SK7s. The thing I liked is that it implements 250K pot to adjust the screen voltage on only one of the 6sk7 to get the 2 6sk7s screen voltage in balance. The bias is controlled by a 500K pot (input attenuator) and the grids even have a 500 ohm pot. The 12AT7 amplifier is used ahead of the 6AL5 rectifier to get a rapid attack - the 6AL5's cathodes are biased positive with respect to their plates and will not conduct until the output signal is fed thru the 12AT7 to have a limit level. Control of both bias if the 12AT7 and the screens of 6sk7s is obtained with the OD3 voltage control tube. Adjusting the 25K pot on the OD3 changes the limit level. the attack and release time is fast (for tube limiter). I really love the sound of it, it uses a UTC A-27 (100K:600) for the output -- sh*t with these transformers how could it sound bad? The switch is a 3 position switch, limiter "on" "off" and "Balance"-- balance makes is the unit a linear amplifier with no limiting.
The 2 6SK7s use DC heater power (I got the DC heater circuit from a Marshall amp that uses DC heaters on the input tubes), the other heaters are AC and have a hum balance pot -- just in case I'm getting some 60Hz in them. (everything the orange cap and couple of resistors to the right on the picture is part of the limiter on/off/balance switch)
I used a bus ground scheme and the only ground point is near the input... seemed to have served me well.
The meter circuit just toggles between input and output, I tried to implement a compression meter circuit (like the la2a or federal) but the result was a disaster - I burned 2 nice meters in which the needles on the meter melted and now are bent and always point down ;-) oh well I learned not to do that........
The Dual B+ power supply is 250V (for each channel) which I put in its own 2 space rack chassis to hopefully have ZERO AC hum issues (which worked!! in addition to the ac heater balance pot and dc heaters on the input tubes. 60Hz/120Hz hum has been my nemesis since my first project). The power connects to the units via a barrier strip =B+/GRND/Heater1/Heater2. I used a 6AX5 rectifier and since it eats 1.2amps, I had to add a triad filament transformer to power everything else. The first two filter caps are grounded to the center tap of the power transformer and chassis, the last Filter cap is grounded at the buss bar of the limiter circuit. This gave me a good ground point (zero volts).
The unit doesn't have a ton of gain like a federal limiter but the limiter starts at -30dbm on the input (input attenuator at max) when the output reaches -20dbm. I might have to build a line amp to put on the backside of the unit but I will test it the studio before I start another project....
scott2000 said:Sweet!
Would the 250k pots be more of a threshold?
EmRR said:Nah, you're drawing is great, at least you know how to draw something. Organic is great, we don't need robot overlords dictating our aesthetic. It's not draftsman quality, but what is? A drawing from a draftsman who went to school for that 50 years ago? They don't even teach it anymore, not with pencils, I don't think.
You definitely want to change the attack control approach.
I would swap the power tap points. You want the output section on the 1st and higher tap.
A cathode current balance control on the 6SN7 could prove useful, especially with the Edcor output which isn't spec'd for DC current.
The GR meter could be a 200 microamp for more range possibility, just changes the resistor.
davemascera said:I want to swap to the PSU tap points as well, but first stage seems extremely sensitive to changes in B+. It really has to be on the mark to get great gain reduction without overwhelming the regulator.
scott2000 said:I wonder how much voltage could be gained with a cap before the choke???
or are you saying the od3 needs a steadier power???
DaveP said:If you look up on Franks site,
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/0/0D3W.pdf
Tung Sol 0D3W, on page 4 it gives you the formula for calculating the correct dropping resistance for your application,
Its quite straightforward.
Good luck
DaveP
Edcor make a transformer for this job, I think its called an XPP1-15k:600, (pp for push-pull). Its rated 1W which is about all you will get from that tube. (audio current)EDIT: Can the Edcor handle this or do I need a new transformer?
DaveP said:Good voice!
All sounded good to me.
DaveP
davemascera said:we were in a rush to get out of there.
scott2000 said:+1
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