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I had been looking for a higher power amplifier design to act as a test bed for trying out EL34s, KT88s, etc. when I stumbled across this KT88/6550 push-pull amplifier in Tube Kingdom Vol. 38.
As a mono-block it’s designed to generate 40-44W/ch with manageable ~150mA @ ~415V B+. The schematic is here:


I liked this amplifier because the design uses triodes in the input stages and a long-tail-pair splitter yet it is simple enough to be implemented with point-to-point in one chassis.

Power Transformer Considerations:
The mono-block HK10 is designed to produce 40-44W/ch from B+ of 415V with total quiescent current of 150mA

The obvious candidate for the PT would be Hammond 278CX with 400-0-400 @465mA. Except that this amplifier requires filament current of 1.6A per each KT88 which exceeds this Hammond's filament winding capacity of 6.0A. Also, 278CX does not have a bias tap.

Hammond also has a Fender replacement unit, 290FX that is rated to supply 450mA at 320-0-60-320V. It appears to have the identical (well, almost..) mounting dimensions as the Dynaco PA060-S. It is relatively inexpensive at $81 at Angela. Alas, its heater winding is rated at only 5.5A at 6.5V. Not enough to power 1.6A per power tube times 4.


Another possibility would be Antek AN-4T360 with whopping 360-0-360 @1000mA capacity. With two 6.3V filament windings of 5A each, it easily accommodates a quad of KT88/6550. The problem was at 4.9” diameter and 3.1” height, I could not easily fit into a 2” high chassis that I prefer.

Edcor turned out to have a great candidate, XPWR016 that is rated to deliver 450mA at 350-0-70-350V and has two heater windings each rated at 5A @6.3V. Alas, I did not know its existence until I completed the project. Oh well.
http://www.edcorusa.com/Products/ShowPr ... spx?ID=593

In the end I settled for Triode PA060-S since it has all the needed voltages and filament windings except that its high voltage capacity
of 300mA at 360-0-360 is marginal at best for a 40W/ch stereo amplifier. The amplifier is designed to operate as a class AB1 and it is not supposed to consume significantly more current in louder passages.
I would compensate for the lack of capacity with larger PS capacitance.

Output Transformer Considerations:
The Hammond 1650N, 1650P, the Triode A431S, or the Edcor CXPP60-8-4.2 will do nicely.
I wanted to try the Edcors,

Output Tube Considerations:
I had two sets of matched Sovtek KT88 pairs that I have been saving for a yet-to-be-determined amplifier project.
I also had a matched quad of Svetlana 6550Cs.
They were fine for this project.

Driver Tube Considerations:
I had various 12AU7As and 6CG7/6FQ7 on hand. They would do nicely to start with.

Chassis Considerations:
Since this amplifier is to be a tested for trying out different configurations such as UL, triode strapping, and tube rolling, I wanted
to make it as compact as it is transportable to and from a work bench.

In the end, I selected the Hammond 1441-30BK (17"x10"x2") steel black textured chassis.
As it turned out, the chassis was not quite rigid enough to support the weight of all the iron. I borrowed Ed Brown's solution - an aluminium L-beam from Ace Hardware.

How It is Implemented:
I made a minor modification to adding a CCS to the long-tail-pair.
I also made these bias adjustment pots accessible to make tube rolling relatively painless.

"As implemented" schematic:








With 6550s:


It sounds as good as a Stereo 35 with more authority in lower frequencies.
It sounds louder, too.

/b
June 15, 2015 : Added KT88 bias circuit for the second channel

I am impressed! That is one of the nicest point to point wiring jobs I have seen. I am envious. A well thought out layout inside also. The trannys are a bit cramped but your arraingment is the best that can be done. Very very nice job sir.

Brik wrote:It sounds as good as a Stereo 35 with more authority in lower frequencies.

More authority indeed. With all those big power tubes, I can only imagine. Nice looking piece of work there. :)

Wow - beautiful job. It must weigh about 50 pounds !

Dude, that's one nice looking amp. Great work! I bet it will look good in monoblocks too. How much did you spend on the project, if you don't mind me asking? -Min

Great amp! Ditto on the point to point wiring. Puts my skills to shame.

My only quibble is the PA-060 for the power supply. 300 ma for stereo PP KT88s is pushing it. How hot does the PA-060 run?

I'm looking at the specs on your power supply. I'm thinking you're running right at the edge of the PIV rating on those 1N4007. According to PSUD2, those diodes are going to see just a hair over 1kV on every cycle. I dunno if that's going to be a problem or not.

The ST35 seems to be fine with 1N4007 rectifiers, but it has a lower voltage power supply. The original schematic you show has a 640VCT power transformer, which shouldn't exceed 900V across the diodes, so I'd think it would be OK too.

In other designs that I have seen where the builder used solid state rectification with a 720VCT transformer, a diode with a 1200V PIV rating was used. Maybe something like the 747-DSEI12-12A from Mouser?

or replace each single 1N4007 with two UF4007's in series.

Hi All,

I really appreciate all the comments. Thank you.
I think I spent ~$850 on the project including wires, bolts, nuts, tools, parts, and odds and ends.

Here is a partial BOM:
http://home.att.net/~Brik/HK10/HK10_BOM.xls

Regarding the diodes' PIV rating, I usually prefer to have the PIV rating at least twice the peak-to-peak voltage.
In this case it's 360V(RMS) * sqrt(2) * 2 ~= 1010V which exceeds the PIV rating of the 1N4007s.
It may be wiser to use a diode with a higher PIV rating in the spirit of a conservative design.

Now that I know the amplifier works reasonably well, I ordered two custom power transformers from Edcor with the following parameters:

Based on Edcor XPWR101:

Primary:
120Vrms, 60 Hz
Secondary:
350V (1.0A), (instead of 240V)
60V (0.1A),
6.3V (5A), (instead of 4A)
6.3V (5A) (instead of 4A)

The price is $109.43 each plus a one time design and setup fee of $20.00.
I am planning to use bridged hexfreds.

/b

Nice job. You really put some work into that.

Those trannys really are nose to tail. I've never seen them that close to one another.

Change your rectifiers to 1N5408. And ya that thing must weight 50 pounds.

Don

Hi all,

You can buy an off-the-shelf Edcor power transformer that is appropiate for a 60W/ch stereo amplifier.
The Edcor XPWR071 is meant for a Stereo Amplifier with solid state bridge rectification with B+ of up to 480 volts at current capacity of 600mA.
I like the idea of using a troidal power transformer from Antek for such duties, but I have not been able to find a way to fit one into a low-profile
chassis that I was hoping to use.
The transformer is slightly bigger than the Hammond 278CX and significantly heavier at Price of ~$110 each.

Edcor charges a one-time "set up fee" of $20 for a custom design, and once it is designed, you can buy an existing custom design without this charge:
http://www.audiotransformers.net/forum/ ... opic=198.0

One regret I have is not to have specified to a ceter tap for the filament windings.






/b

Brik wrote:You can buy an off-the-shelf Edcor power transformer that is appropiate for a 60W/ch stereo amplifier.
The Edcor XPWR071 is meant for a Stereo Amplifier with solid state bridge rectification with B+ of up to 480 volts at current capacity of 600mA.
I like the idea of using a toroidal power transformer from Antek for such duties, but I have not been able to find a way to fit one into a low-profile
chassis that I was hoping to use.
The transformer is slightly bigger than the Hammond 278CX and significantly heavier at Price of ~$110 each.

600 mA?!? Wow. That's a lot of nuts. You're talking 288 watts of idle dissipation. You could almost feed two quads of KT88 off that. Not quite enough 6.3 volt, though. I'm thinking big monster PPP 100 watt per channel behemoth. Lots of hernia potential there.

"Slightly" bigger? I think you could safely hide that Hammond underneath the Edcor, and you'd never know it was there.

Very nice amp! I'd still change the rectifier s to something a bit heftier than the 1N4007s, uF5408s are good, or go with some 6A 1600V HEXFREDS, to make it totally bulletproof.

A couple years ago, I bought an all-tube labratory grade pulse generator, which yielded a LOT of very nice Philips PQ tubes, and a hefty power trannie, with two 120VAC primary windings, and a 360VAC, 500mA secondary, and a 6.3VAC @ 14A filament winding. Not too shabby for a total outlay of five bucks. It is a beast, vertical mounting and at least as big and heavy as Triodes PA-060, The HV winding is not CT, so it will need a 4 diode FWB, but that's no biggie Yellow_Light_Colorz_PDT_03

With that trannie, I could power up a very hefty PPP ST70, which would be closer to an ST100...

Then there is another "werid" Harris Radio plate tranfrormer, which has a 220/240VAC, 50Hz rated primary and various tapped HV secondary voltages from 600 to 1200V, on 120V, figure half that, though I dunno just how well the current will hold up on "half rations". I could always run a dedicated 240VAC line, if necessary...

It was really cheap, that and a huge "mystery" choke cost me five bucks combined, and that works out to somewhare around 12 cents per pound,,, :o That plate trannie may be as usegful as turtle teats, but it was an irrestible bargain Yellow_Light_Colorz_PDT_05

Don't overlook that old boat anchor test equipment at the hamfests and flea markets (and even the town dump).

Some of the older tube reg power supplies have been known to be loaded up with Tung-Sol 6550s, so be alert! Yellow_Light_Colorz_PDT_11

(Be alert, the world needs more lerts)

/ed B

Thank you Ed.

I suppose the power supply of a tube amplifier is not a good place to go sentimental...

I have this pack of 100 1N4007s which was on sale for $2.99 at the infamous "You-Do-It Electronics" in Needham, Massachusetts well before my 17 year-old daughter was born. It's the same store I met Kenneth Olsen (the co-founder of Digital Equipment Corporation, for those of you who are under 30) by chance 22 years ago. Ken was there looking for a good soldering iron, I think. I was hoping that using the 1N4007 from the store could add some aura of engineering excellence by proxy...

Anyhow, l have this feeling that the Triode PA 060-S transformer is fairly conservatively rated. Before I started building this amplifier, I put six series-connected 25W/120V light bulbs across the 720V HV taps. The measured voltage across the taps was 728V AC. Which implies that, in a center-tapped configuration, the transformer can supply 400mA AC to the rectifiers at the rated voltage.

/b