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Old 12th March 2013, 05:52 PM   #1
MelB is offline MelB  Canada
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Default KT120 "triode" and Hammond 1650N

Opinions please.
Hammond 1650N is a 60 watt 4300 ohm push pull output transformer with 4-8-16 ohm option.
Best I can measure my speakers are around 6 ohms impedance over most of the lower frequency range.
I'm doing a triode connected push-pull project (attached) Already had it running with KT88's many years back but never had the tools to measure anything.
Sooo...4 or 8 ohm tap? Ya I'll measure it when I get it built....
I'll post my progress on this project. So this post will be it for the project for all those interested.
I'm using a new power transformer for my KT120 version over what I used with the KT88 version (Hammond 290HX). PSU Designer II gives a B+ of ~460v. It has a 420mV rating so it was my choice for the project. My previous transformer only had a 250mA rating and sagged a wee bit as I recall.
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Old 12th March 2013, 06:21 PM   #2
palmas is offline palmas  Portugal
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I suggest you listen to bothe 4 and 8ohm, and you'll see the difference can be quite audible.
Just choose what you prefer
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Old 13th March 2013, 02:12 AM   #3
tomlang is offline tomlang  United States
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Have you built and tested the bias circuit feeding off the tap of the power transformer? Just today I was modeling a similar circuit and it does NOT work. I think it is because there is no direct ground reference for the bias circuit. The ground reference is created by the full wave bridge rectifier. If the transformer had a ground tapped into one of its windings it would work.
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Old 13th March 2013, 06:35 AM   #4
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Quote:
Originally Posted by tomlang View Post
Have you built and tested the bias circuit feeding off the tap of the power transformer? Just today I was modeling a similar circuit and it does NOT work. I think it is because there is no direct ground reference for the bias circuit. The ground reference is created by the full wave bridge rectifier. If the transformer had a ground tapped into one of its windings it would work.
It's not obvious, but as long as the grid is not pulling current (and it shouldn't) the ground reference is through the FWB and the bleeder resistors. No current, no voltage drop across the two 330k resistors in parallel.

It should work.

Oops - I neglected the 30k voltage divider to set the bias... That could be a problem...

OK, this got me curious... The 290HX has a separate 100V winding that is likely being used for the bias. If so, all is good
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Old 13th March 2013, 01:50 PM   #5
tomlang is offline tomlang  United States
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As long as he is using the separate 100v winding all is fine. If he is coming off the center tap of the secondary as the diagram suggests it won't work in my opinion.

And indeed, looking at the 290HX data sheet shows a 100v 50 ma winding which I suspect is being represented by "98v 50ma" tap in the schematic.
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Old 13th March 2013, 01:54 PM   #6
MelB is offline MelB  Canada
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No I have never used this Hammond model transformer before. However the bias winding is a separate winding as shown here:
http://www.hammondmfg.com/pdf/EDB290HX.pdf

I was just being lazy drawing it out....my bad.
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Old 13th March 2013, 02:04 PM   #7
tomlang is offline tomlang  United States
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...I would never have noticed it except for having thought of trying to do that just today...

I would consider (for less ripple but probably not really necessary) a full wave bridge for the bias supply.
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Old 13th March 2013, 02:52 PM   #8
MelB is offline MelB  Canada
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Quote:
Originally Posted by tomlang View Post
...I would never have noticed it except for having thought of trying to do that just today...

I would consider (for less ripple but probably not really necessary) a full wave bridge for the bias supply.
Ya I will use a full wave bridge in the bias supply...this ain't no stinkin' guitar amp! Thanks!

Attached are the now stripped down previous KT88 project chassis.
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Old 13th March 2013, 03:18 PM   #9
GoatGuy is offline GoatGuy  United States
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First ... you have not designed a "triode-mode" amplifier. Sorry. You're using pentodes, as pentodes. [And I've got no problem with that. Good work!]

Second ... I'm glad you put in the separate B- supply - the first drawing wouldn't have worked as you expected, I think. [Then again, with an additional capacitor it would have worked ... as a voltage multiplier does! but that's a different story]

Third ... its not clear why you're using that poor little 12AX7 as the phase splitter ... when at full power operation the output tubes may well want current draw on their grids. I recommend at least considering socket-and-pin compatible higher current relatives, such as the 12BZ7, 12AU7, etc. The BZ is a particularly sweet tube, overall. You would need (and want) to drive it with more current though, choosing 11K cathode/anode resistors.

Fourth... since you're using a pentode ("cascode on the cheap") in the front end - always kind of ballsy in an audio forum site like this - why not keep on with the pentodes? They work great where the 12AX7 is in the circuit. Ultimately, the phase splitter is acting as a cathode follower - where the anode resistor is "along for the ride" (and thus accomplishes the phase splitting/inversion). So... why not use a tube whose IV characteristics are so much squarer and less dependent on cathode-to-plate voltage? Dunno... I would sub.

Your work looks good. Keep it up.

GoatGuy
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Old 13th March 2013, 03:46 PM   #10
MelB is offline MelB  Canada
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The phase splitter is a 12AU7. Other schematics (By noted "designers") I have seen are using 47k resistors....I went with the 33k's which I will use as my starting point and see how it measures ...thanks for the tip.....it would appear I can go to 16k there easily with my 12au7. The 12BZ7 sounds like a tube I must try! Thanks!
I don't understand your first line...."You are using pentodes as pentodes." ? (Help!)

When this was first running as a KT88 indeed I was using the EF86 as a pentode and the KT88's ultra-linear with negative feedback....I spent a lot of time screwing around with the circuit and arrived at what you see. This was ~15 years ago when I first started playing with tubes. I didn't measure anything and just went with what sounded better to me. It will be fun to play around with the circuit now and see where I end up. As you can see I have a lot of options to try with my current tubes and transformers.

Thanks for the input.
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