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Old 13th September 2010, 08:16 AM   #11
athos56 is offline athos56  United States
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Quote:
Originally Posted by kevinkr View Post
More when I next work on this.. It strikes me that this might not be a great design for anyone else to build although I will share it once I am sure it does not blow up.. (at least)

I think the main problem with this design is actually the pentode driver.

Sonically speaking I have no idea what to expect. Sine waves look fine..
The reason for my interest was that I was going to make the exact same circuit except with an 6SJ7 or EF86 driver. I'll be interested in your listening impressions. I was hoping it would end up a little more "Hifi" than "Guitar"

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Old 13th September 2010, 10:51 AM   #12
Cassiel is offline Cassiel  Libya
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Why pentode? Because of Schade feedback? Getting 1% distortion at 5 watts without feedback is possible, someone did it tweaking the 12ax7 plate and cathode resistor values. 6V6 gives 5% so a lot of distortion cancellation was achieved.

Gnfb in a 6V6 SEP sounds good to me - better than the Schade loop in Alex Kitic design.

I would use a triode driver.
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Old 13th September 2010, 04:53 PM   #13
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by athos56 View Post
The reason for my interest was that I was going to make the exact same circuit except with an 6SJ7 or EF86 driver. I'll be interested in your listening impressions. I was hoping it would end up a little more "Hifi" than "Guitar"

Athos

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It's a stereo hi-fi amp, can't play the guitar to save my life.

From what I have measured so far with the exception of the excessive buzz on the output (which I will fix) the amplifier should perform decently at a couple of watts output. 20Hz - 20kHz performance at full power is possible with careful layout, once I think I am close I will do an FFT and see what the performance is like.

I will probably crank the B+ up to 275V or so, right now it is sitting at almost exactly 250V.. Hope to get at least 3Wrms out of this at under 2%, not sure I will get there or not.
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Old 13th September 2010, 05:00 PM   #14
kevinkr is offline kevinkr  United States
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Originally Posted by stalker View Post
Why pentode? Because of Schade feedback? Getting 1% distortion at 5 watts without feedback is possible, someone did it tweaking the 12ax7 plate and cathode resistor values. 6V6 gives 5% so a lot of distortion cancellation was achieved.

Gnfb in a 6V6 SEP sounds good to me - better than the Schade loop in Alex Kitic design.

I would use a triode driver.
Interesting question, my answer is no better than I have never designed or built an SE pentode amp before, and decided I would do all pentode for a change. Might as well go totally off what passes for the beaten path currently. Same reason I used the 6J7, that and it seems to be surprisingly linear..

Actually this isn't a Schade at all - the feedback from the output is applied to the cathode of the 6J7 and is a form of global nfb from the transformer primary. As in the original version I have borrowed heavily from I am probably going to trade some additional gain off and add a loop including the secondary of the OPT.

I have no idea how this is going to sound, but I did finally get it stable into resistive loads at least. As a precaution I am probably going to add zobel networks across the transformer secondaries to make sure the amp is loaded above 30kHz or so..
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Old 14th September 2010, 02:09 AM   #15
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Curious as to why you went CRCLC instead of CLCRC.
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Old 14th September 2010, 04:00 AM   #16
kevinkr is offline kevinkr  United States
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Curious as to why you went CRCLC instead of CLCRC.
Funny you ask.. Expediency - I did not have a choke onhand that I wanted to place on top of the chassis and originally I was using zener regulated screen supplies.. I decided that this tiny little 12H/400 ohm 30mA choke would be ideal for making a very clean screen and driver stage supply - and in fact this is the case.

I've ordered a small Hammond 5H/105 ohm/150mA choke and will convert to CLCLC supply once it is in.. The ripple on the CRC filtered supply is rather excessive and can be heard quite clearly through the Half Chili Changs this amp was designed for. (Yes it is running!)

Ripple on the CRC is >5Vpp and is quite audible unfortunately, converting to CLC will reduce the supply ripple amplitude by roughly 26dB (20X) based on simulations and this should be good enough to get the ripple down to a couple of mVrms on the output.

I made a number of changes in constructional details which have made it quite stable.

I have shield all of the input wiring coming from the RCA jacks and the cables to the grid caps are also shielded, and right at the cap I have placed a 10K resistor covered with heat shrink.

The sound is interesting to say the least - and warm up unexpectedly long. This amplifier is extremely detailed, dynamic, and fast - the bottom end despite good measurements is largely non-existent. It images like crazy and has good depth as well. (Currently listening to Antifogmatic by the Punch Brothers) It is particularly good on plucked strings and is not easily confused. I would not however say this a "nice" sounding amplifier, but it seems like a very good match for these speakers - it plays quite loud actually. (Overall I would say they have never sounded better.) This is in no sense akin to triode sound, and I suspect that my inexpensive transformers might be responsible for the lack of low end or it could be my design.. It has a hint of that good old pentode hardness as well - a warm sounding speaker might help, not how I would describe my HCC..

I used good parts and was careful in its construction, despite this it has proved problematic in a lot of ways. The bias batteries definitely do work..

I decided to use some of my used metal Kenrad 6J7 - surprisingly they sound very good, and are immune to the external pick up issues that my glass NOS ones do exhibit.

The sound does have me wondering about using a pentode driver in my next dht amp, and based on my experience with the 6J7 that is probably what I will use. Could it be the poor man's WE310?

I will warn anyone contemplating building something like mine that getting it to work correctly is less trivial than I would have expected for the relative lack of sophistication.
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Old 14th September 2010, 05:32 AM   #17
kevinkr is offline kevinkr  United States
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Here are the schematics in rough draft (not sure when if ever I will get around to drawing the real thing..) and a picture of the functional amplifier.

I have reinstalled the Canadian made GE 6J7G and think I might like them better. The metal Kenrads are quite good, but these sound a bit smoother.

The amplifier is sounding a bit smoother.

Note the gain is a bit excessive and you can probably use this with a passive line stage. Gain was calculated (not modeled) at ~18dB and it is at least this - oddly enough I did not actually measure the gain. Still more work to be done.

It is either smoothing out or I am getting used to it..

Build at own risk, potential is there, but work will be required, and good OPTs.
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Old 14th September 2010, 05:45 AM   #18
kevinkr is offline kevinkr  United States
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Another picture of the amplifier, this time with the 6J7Gs installed - and yes it is running, and I am listening to it as I write this.
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Old 14th September 2010, 05:59 AM   #19
kevinkr is offline kevinkr  United States
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Once I get the supply issues sorted out, I'm going to look at the square wave response and determine whether additional compensation is worthwhile or not. I played around briefly with a zobel on the primary of one of the OPTs and will spend some further time seeing whether this improves HF stability or not.

Some point I may look at reducing the open loop gain and commensurately reducing the closed loop gain as it is a bit on the high side.
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Old 14th September 2010, 10:44 AM   #20
45 is offline 45  Italy
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Kevin you could use a mixed bias: 9V batteries and some cathode bias with 22 or 27 or 33 or 39 ohm resistors. This should result in approx. -10 to -10.5 V bias (as you don't get enough current at -11V).

Cheers,
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Last edited by 45; 14th September 2010 at 10:50 AM.
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