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Old 27th April 2014, 11:34 PM   #1
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Default Found some Sunn Transformers for 2xKT88!

Hi All,



I've been trying to scratch an itch to build something with KT88s. I've been looking for a big, loud, clean (but not necessarily sterile) amp for my tube preamps. I actually started a thread a few months back to help me get the specs right for a set of Heyboer transformers specifically for the project. Well, I was picking through my parts pile (it's a big pile) and found a box… and in the box was 2 transformers I'd pulled from a beat up old amp a friend had given me in trade 10 years ago (someone had gutted and rewired it if I remember right so I chose to salvage rather than repair at the time). I dug up the cabinet and realized that it was old Sunn head, most of which ran 2 KT88s for about 60W.

The transformers are 28-1360 and 28-2360, one is marked WTA 74. Some googling leads me to believe they are original and that the specs are right for what I'd like to do.

Click the image to open in full size.[/URL]

I put them up on the bench and the power transformer taps all yielded expected voltages in the unloaded condition… 830vAC w/CT, Heater and 5vAC for tube rectifier. The OT was trickier… first I checked the resistance between taps (20ish ohms from CT to UL taps, another 40 from UL to Anode taps) and then wired up a little 6.3vAc power transformer to the 16 ohm tap. I got 110vAC Across the anode winding… looks to be a working 4500:16/8/4 transformer.

I cleaned them up after with a wire brush and mineral spirits, almost painted them but nostalgia go the best of me.

Click the image to open in full size.[/URL]

And here they are rough placed on a 24X8" chassis. The OT is centered and the 2 KT88 will be 4"[100mm] apart (per the data sheet) aligned with the OT.

Click the image to open in full size.[/URL]
Click the image to open in full size.[/URL]

And the schematic for a Sunn amp using these transformers:

http://www.4tubes.com/SCHEMATICS/BY-BRAND/Sunn/100s.gif

I'd follow a Sunn 200S schematic but I don't like to mess with hard to find or unique tubes (like 6AN8) so I'll be doing something different for the phase splitter/driver. Given enough driver gain will a split load phase splitter w/ a 12AU7 be able to squeeze everything from these KT88s? I know these older amp companies wouldn't throw away away any useable power but not always where it might require one more triodes…. I on the other hand am willing to do more. So is there a better/more robust approach? I was thinking of using a 12AX7 for the driver/splitter and buffering it with 12AU7 cathode followers.

So, before I dive in, any feedback on the project so far?
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Old 28th April 2014, 08:37 AM   #2
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Looks like a nice project. In the schematic, I am not familiar with the connection from the output feedback circuit to the screen-grid of the lower KT88 (marked 'E'). The usual feedback still goes to the cathode circuit of the pre-amp pentode. Is this extra connection to the KT88 screen-grid to ensure high frequency stability perhaps?
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Old 28th April 2014, 09:58 PM   #3
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Quote:
Looks like a nice project. In the schematic, I am not familiar with the connection from the output feedback circuit to the screen-grid of the lower KT88 (marked 'E'). The usual feedback still goes to the cathode circuit of the pre-amp pentode. Is this extra connection to the KT88 screen-grid to ensure high frequency stability perhaps?

No clue! Looks like some hi-fi wizardry to me. Given the magnitudes of the signals I would think the UL tap would have more impact on the feedback loop than the other way around but I'm not sure. I've heard old Sunn amps were derived from Dynaco hifi designs so I'd hazard to guess it's purpose was linearity or bandwidth, neither of which are an overt priority for me. Looks pretty easy to add/remove anyway.

I answered my own question on drive signal... GEC datasheet says 67vACp-p for tetrode mode, 140vACpp for UL!. Looks like that cathodyne splitter w/ 425vDC should be able to just about get to that 140 theoretically... almost like someone "engineered" that stage for maximum output without going any further than necessary.

If I want to get any overdrive in the KT88 I'll probably have to consider some post phase-inverter gain stages.... but then again I probably don't need any clipping when running in UL - kind of conflicting ideas. Looks like if I can work out a tetrode mode then the 140vACp-p will be more than enough to clip the output but I'll need a 300vDC rail capable of some current. The simplest might be an amplified zener approach using a pass mosfet - not a regulator per se, just a series drop to bring the 560vDC rail down somewhere that won't annihilate my screens but still breathe with the B+.
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Old 28th April 2014, 10:21 PM   #4
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Or, how about changing to a long-tail-pair phase splitter?
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Old 28th April 2014, 11:37 PM   #5
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My understanding is that a cathodyne is capable of just as much signal swing as a LTPI... and typically more gain when you account for one triode as a driver/amplifier.
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Old 29th April 2014, 12:13 AM   #6
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Here's where my head is at for a driver stage:

Click the image to open in full size.[/URL]

I used a 12AX7 for the driver/cathodyne because I didn't want to use a weird tube like a 12DW7 or 6AN8. The AX7 can make a decent cathodyne and boot strapping back to the plate of the previous stage should max out the gain there. The driver would be biased for about 1.3mA to get the requisite 75vdc for basing the cathodyne. The 12AU7 is to buffer the cathodyne to prevent imbalanced loading and allow for smaller than normal bias feed resistors. I tried a DC coupled stage (w/ negative rail) in my last amp and didn't really think it was worth the trouble. I'm thinking on this one that the larger than normal grid stoppers on the KT88 ought to eliminate grid current at clipping and the small bias feeds should be able to keep the coupling caps at a stabile voltage without too much wandering. The AU7 followers prevent loading on the cathodyne.

Negative rails and DC coupling are cool but I think they aren't worth the trouble if you don't have a stabile dedicated winding so I'm skipping this time around.

I'll probably add a nice big grid stopper on the cathodyne too.



Brian
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Old 29th April 2014, 08:15 AM   #7
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Hi Brian,

That looks like a very nice design.
You are right about the gain of the cathodyne with driver.
I don't want to be 'picky', but just in the spirit of sharing information, I think an LTP can give a bigger voltage swing than a cathodyne, because after you have taken away the voltage across the tail (which doesn't have to be too big) all the remaining B+ is available to each triode. In the cathodyne, each output only has half the B+ to play with.
As I say, just a point for discussion, I think your approach will work very well.

Malcolm
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Old 29th April 2014, 11:36 AM   #8
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Malcom,



Thank you for the complement. In truth, I'm as guilty as anybody of regurgitating what I hear on the internet and mistaking it for knowledge. Here is the thread that convinced me a cathodyne could keep up with the output of a LTPI:

AX84.com - The Cooperative Tube Guitar Amp Project

I think the tail resistor loss and the and gain stage's inability to saturate to the lower rail are in both play on an LTPI. These are generalizations though and need to think about my application. They were discussing a theoretical 200vDC rail whereas I will be working at 2x that.


Thanks for the help.
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Old 29th April 2014, 12:23 PM   #9
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Hi,

Thanks. A very interesting thread. There is more to it than I thought (as usual)!

I don't know if your have Merlin Blencowe's pre-amp book:
'Designing Tube Pre-amps for Guitar and Bass' (second edition) 2012.
At the beginning of chapter 9 (page 154): "It (the LTP) has the advantage over the cathodyne in that it can provide greater output swing for a given HT and load, making it the circuit of choice to drive 'big bottle' power valves."
I think it was also Merlin who was part of the AX84 discussion?

Anyway, there does not seem to be much difference in output swing, and by loading
the cathodyne with cathode followers you avoid the problem of unequal output impedances.
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Old 29th April 2014, 07:22 PM   #10
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I actually don't have Merlin's book (Just ordered it on Amazon), I do frequent his website though. I do think it's is the same Merlin that was part of the discussion.

I'm coming to the same conclusion that it looks like a wash between the two: In UL moade the cathodyne can get me exactly what I need for full, clean output power - in tetrode mode it will be more than enough to get past full output and into power tube clipping. Now I'm starting to wonder what the signal looks like of the plate of the pentode... with an anode voltage of 75v I'm thinking the clipping is hugely asymmetrical and quite large?

Do you know if the grid signal of a cathodyne needs to be equal to the split signal magnitude or 2x the output magnitude? In other words, will 140Vp-p signal at the grid yield two 140Vp-p signals a the out... or do I need a 280Vp-p signal at the grid? I'm thinking the latter.

That makes me wonder if the cathodyne will clip when the incoming signal exceeds the cathode voltage (90ish volts) and if that means you really can't get enough p-p signal from the cathodyne for full clean output after all.

Perhaps it's time to start down the road of building...
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