HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Keepsake10 » Thu Aug 14, 2014 3:29 pm

Hi Birk,
In the mean time I did some more work on this amplifier... hum still present, nevertheless I am now sure it comes from the first stages... maybe bad grounding but I will take a break for the moment.. here are some pics maybe helps... The wiring is not the best one even if such small place available.... Maybe I will redone this part. I made as well some additional measurements.. With 1Khz / 1,2V at input I get on a 8 Ohm dummy load (100W) a little over 59 Vpp before clipping... anyway not the power is my target but the quality of the sound reproduction... I consider this setup rather conservative and I think will be in the benefit of a very long life for the tubes

Best regards and happy building....

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HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Keepsake10 » Fri Aug 15, 2014 3:43 am

Hi Birk,
Last night I couldn't hold myself and experimented a little... If I short the first stage grid to the ground the hum is almost gone but not entirely - nevertheless quite acceptable...
If I simply take out the first tube I still can hear some hum, therefore I suspect the most of this noise comes from the first stage, amplified and added to the noise from the splitter. I'm not sure the wiring is as bad as it looks - I did applied a small NFB as a test (firstly 20K and than 12K and finally 9,1K without any bypass C)... hum reduced but still present, nevertheless the sound when NFB applied is not any more so detailed - like was coming almost around the corner.... All this make me think this all have to do with the setup of the first stages... all connection's are kept very short, heaters are DC... Hmmmm ... after the grid stoppers (1K for each section) very close to the socket I used small ferrite beads to avoid HF oscillation - but as far as I know this material is magnetic .. Could be this the reason ???
The final stage alone is quiet as a stone.. one thing I could see in the dark was a blue glow with in the power tubes - just as a remark.
Well, it looks like I have to "fight" for a while to find this mistake.
Best regards,
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Keepsake10 » Wed Aug 20, 2014 1:37 pm

Dear Brik,
the last days I tuck some time to "meditate" on this issue ... finally I could only laugh on myself... The mistake I made is a beginner mistake...
The heaters (even if DC used) where not elevated to a certain voltage nor grounded... Today after work I simply added from the pin 9 (central tap of the heaters) a small 120 ohm R to the ground and the hum is gone. Thus, if I "stick" my ear in the speakers I can barely hear some noise... but something is still there... One of the next days I will see if elevating the heaters to about 70 to 90 Volts will give better results. After all, with the actual setup the cathode of the LTP see 136V but I can't go so high because the first tube is sharing the same heater supply.
The amplification of the first stage is still high... I have to do some measurements when I have the time... alternatively I could make some experiments on a small board and try a new LTP made out of two E86C single triodes. It might be even better as this little tube have some really impressive data....
Best regards,
Dan
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Joris » Thu Nov 06, 2014 8:12 am

Hello,

I am new to this forum and my name is Joris. I live in Belgium in a small town called Putte. I am new to tube amp designs but I have already expierence with tube radio repare. I am 32 years old so the tubes are new to me. I would like to build this amplifier but I found out that Edcor has stopped building the XPWR071. I have a hard time to find a replacement tranny, but I did find a Edcor XPWR002. Could I use 2 tranny's (XPWR002) to build the power supply? But my guess is that I would have to build 2 seperated powersupply's then?

Can I use the Edcor CXC100-5H-200mA for the choke?

I am sorry for my english but it is not my native language. I'll do my best.

Thanks in advance,

Joris
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Brik » Thu Nov 06, 2014 3:06 pm

Welcome to the board, Joris!

It appears that Edcor no longer makes some of the larger power transformers that are appropriate for this amplifier.

I looked up Edcor XPWR002:

Power transformer for a 120V, 60Hz. or 240V, 50/60Hz. line to 720V (360-0-360) at 220mA center tapped,
60V at 800mA, 6.3V at 7A and 5V at 4A.


Unfortunately, Edcor XPWR002 has insufficient current capacity at B+ since this amplifier would need ~360mA at B+
for a clean output at 40~45W/ch.

Instead, I would recommend the Euro version of the power transformer for the Dynaco ST-70 called PA521
which can supply 360mA at 360-0-360VAC without difficulty:
http://triodeelectronics.com/pastpotr24.html
Image

Or, you could use Classic Tone 4-18069, although you would need to use a full-wave-bridge-rectifier set up since it only has a single winding:
http://triodeelectronics.com/40-18069.html
Image

Can I use the Edcor CXC100-5H-200mA for the choke?


Yes.

Since this amplifier dissipates less than 200mA in the class-A region of its envelope, it should be fine.
You could opt for a device with a greater current capacity if you are so inclined.

Good luck to you. (y)

Brik
/b
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Joris » Thu Nov 06, 2014 3:54 pm

I wanted to use 2 Edcor tranny's. 1 per channel. Or am I not correct?
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Brik » Thu Nov 06, 2014 5:19 pm

Using one XPWR002 per channel will work fine, especially if you are planning to build the amplifier as
a pair of two mono-blocks, since the original was designed that way.

Image Image
https://translate.google.com/translate? ... 0info.html

Yes, you will have to duplicate the power supply circuit for each channel which would make building it into one chassis a challenge.
/b
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Joris » Thu Nov 06, 2014 10:20 pm

A challenge because of its size, or hummm?
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Brik » Fri Nov 07, 2014 6:37 am

Hi Joris,
Joris wrote:A challenge because of its size, or hummm?


For me, the major design constraint is the maximum chassis size that I consider "reasonable for home use."

Where I live, the largest available "off-the-shelf" chassis is 17" wide x 12" depth x 3" high (432mm x 356mm x 76mm).
When I build a full-sized amplifier for home-use, I try to get all the components into that 17" x 12" footprint.

The combined footprints of two power transformers, two chokes, in addition to two output transformers,
four driver valves and four KT88s/6550s would be too large and heavy for a single chassis design, I think.

Another alternative to a single power transformer solution is to use the Antek AN-4TK360 toroidal transformer:
http://www.antekinc.com/an-4tk360-400va ... ansformer/
Image

In this case, you'll have to be a little creative in the B+ secondary connection to derive the -90V bias current.
/b
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby aros_71 » Thu Jun 11, 2015 8:04 pm

Thanks for this - very informative. The PSU will have a lot more headroom with the extra capacitance.

I am assuming in the PSU you duplicated the highlighted section?? (the bias supply for the KT88s):

Image

Also - the bias supply for the 2nd channel - did you give that its own diode, or was that also fed from D3?

Finally - it's been driving me crazy. What do you use to draw the circuit diagram?

Thanks!
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Brik » Fri Jun 12, 2015 5:33 am

Welcome to the forum!
And thank you for the reply!
aros_71 wrote:I am assuming in the PSU you duplicated the highlighted section?? (the bias supply for the KT88s)

Yes, you are absolutely right.
I regret the oversight, and I will update the diagram as soon as I get back to town. :$
aros_71 wrote:Also - the bias supply for the 2nd channel - did you give that its own diode, or was that also fed from D3?

It is also fed from D3.
aros_71 wrote:Finally - it's been driving me crazy. What do you use to draw the circuit diagram?

For this particular diagram, I used Microsoft Paint to cut-and-paste circuit elements from the original
scanned image. :{)

For others, I use a free software called CircuitMaker (Student Version) that I downloaded more than 10 years ago.
It is not particularly good at drawing transformers or tetrodes, but I should not complain too much since it was free.
I did not use it for this project since the free student version I use did not allow more than 50 circuit elements,
and it does not have a symbol for the tetrode KT88.

I have used it for other smaller projects however.
Examples are:
Image
Image
Image

The version I use is similar to this one:
http://my.ece.ucsb.edu/York/Bobsclass/2C/Simulation/circuit_maker.htm

I see that there is a much newer version of CircuitMaker that may or may not have the same limitations.
It is worth exploring, I think.
/b
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby aros_71 » Mon Dec 12, 2016 5:42 am

Uh-oh. All the images are broken :(

Came back here to admire your wiring job
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Re: HK10 - Stereo KT88 / 6550 Push-Pull Amplifier

Postby Brik » Thu Dec 15, 2016 1:16 pm

Sorry about the broken links to the photos and graphics.

My Internet provider decided to drop the support for personal web pages where I parked most of the photos.
When I find an affordable service provider that allows me to keep the directory tree structure for the photos, I will put them back online.

Flickr and other sites won't allow me to keep the file names and the directory structure by 'hashing' the files. (666)
That makes using these sites a major pain to update each and every URL reference in all the posts. :'(

Please give me some time to sort things out. *)

Thanks.

Brik in Cairo, Egypt.
/b
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