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Old 29th October 2010, 08:29 PM   #1
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Default Help complete the 300B schematic

Currently have a Karna-type amplifier running for a year or so now. Interested in losing the first IT and ideally going DC coupled. Power supplies might limit me somewhat, however.

Care to fill in the missing pieces with your recommendations? Not opposed to using DN2540 current sources, but space is limited, even for making -Vcc supplies. I can add them if they really help improve the performance.
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Old 29th October 2010, 09:04 PM   #2
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Hi Kurt,

How about replacing the input transformer with a LL1674 for a 1:4 step up, the first tube with something with a bunch of gain (6sl7 perhaps) then DC couple to a pair of mosfet source followers. You can use a servo to keep the sources at a pretty similar voltage allowing them to drive the interstage transformer. Indeed, with a decent servo, you could replace the interstage transformer with a high level 600 ohm type (Cinemag CMOQ-1H for instance), or even one with a bit of step-up (run it at 150:600) if you want more gain.

-d
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Old 31st October 2010, 11:33 PM   #3
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Not the worst idea; I have considered going with a power drive type of circuit, though I would probably remove the IT and go direct to the 300b grid.

Real happy with my Tribute input tranny, so that will stay. Perfect gain and phase balance up to 100 kHz; quite amazing, really. I tried to keep the step up to a minimum in order to keep preamp drive requirements to a minimum; at present I have a high pass RC network (not shown) which imposes some load on the pre.

I have determined the sound improvements for this specific amp are had by installing a Lundahl OPT and getting rid of the first IT. Ideally I want to DC couple the first and second stages.
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Old 1st November 2010, 11:35 AM   #4
45 is offline 45  Italy
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Quote:
Originally Posted by zigzagflux View Post
Ideally I want to DC couple the first and second stages.
I would use the same tube for both. The 6H30 is very good for this because it works happily down to low plate voltages (about 75-80V/5-6mA with 27K to 39K anode resistor, for example ) so you don't need to have high supply voltage.
230-260V total supply voltage should be enough. Then it's more about finding the right combination for the best sound.
You don't need to buy expensive 6H30P-DR pre-1986 versions, the Sovtek EB version or the EH gold pin are very good....

45
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Old 1st November 2010, 08:16 PM   #5
waltube is offline waltube  Italy
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One of the best solution, in my opinion, is to use a Sowter 3575 OCC version; it is 1:1CT.
It is configurable for use as bal/bal or bal/unbal.
I don't suggest to use a step up input transformer because may be possible to meet trouble to the connection with the source.

Reagarding the 6H30 I don't sugget the use at low voltage; the value written are not correct.
For my experience the best sound with this tube is obtained with 10 mA minimumof anode current (better with 15 mA) at 200 Vdc (max), this mean about 10k on anode.
With the 300B the only solution to dc connection between the stages is the use of negative rail for driver circuit.

Ciao

Walter
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Old 1st November 2010, 09:04 PM   #6
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Quote:
Originally Posted by waltube View Post

Reagarding the 6H30 I don't sugget the use at low voltage; the value written are not correct.
I am afraid I just use it this way (input + cathodine splitter) and it works very well. With 80V/6 mA you have approx. -4V bias which is enough for an input tube. It is more linear than an ECC88 in same conditions and has less problems with grid current (my 6H30's have got gold plated grids). Maybe the tubes you have are not so good....

Last edited by 45; 1st November 2010 at 09:07 PM.
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Old 1st November 2010, 09:21 PM   #7
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Quote:
Originally Posted by waltube View Post
With the 300B the only solution to dc connection between the stages is the use of negative rail for driver circuit.

Ciao

Walter
He is not asking for a dc driver. This amplifier has got two interstage transformers + input phase splitter. He just wants to get rid of the first interstage between the input stage and the driver stage. The driver uses the remaining IT to drive the 300B. So if you have 2 6H30's in cascade (one at the input and dc coupled to the other) you just need about 260V total supply, 75-80V for the input and 165-170V for the driver.
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Old 1st November 2010, 10:39 PM   #8
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Right, just looking to DC couple the first and second stages. Currently, the first stage is using the 6N6P, which isn't all that different from the 6H30 (a lot less money tho). I have also considered the triode-connected 6W6 for the driver stage- currently using 46, which is fine too. The 6W6 can operate a little better at the lower voltages.

So I can either put CCS in the input tail and/or as plate loads, DC couple to the driver, which may or may not have CCS in the tail. That DC coupling has its challenges with idle conditions is no secret; any experience with good control without servos?
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Old 1st November 2010, 11:28 PM   #9
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Zigzagflux,

How about making a direct reactance drive type of input/driver stage?
You can use the primary of the first IT as centertapped plate choke for the 6H6; the 6H6 is direct coupled to the driver tube grid.
You can run this stage on the same power supply voltage as the 300B power stage, which makes power supply things easier.
Thanks by the way for the kind words on the input transformers.

Pieter
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Old 1st November 2010, 11:32 PM   #10
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The "centertap" of the IT primary should have separate connections for each phase of course.
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