stereo SE kt88 build ... abdellah diyaudioprojects design

[jmillerdoc]

Also, check Sonic craft's site too, they have quite a bit of pieces of BG that I have found others didn't carry. Very friendly and helpful people at Sonic craft too.
Jeff

[88man]

I am going to run the SE KT88 in stereo configuration with a single PSU - all on one chassis. Porkchop and Sherman use 350-0-350 transformers in their schematic. Sherman mentions that for a single PSU, the power transformer should be capable of 200mA.

The Hammond 273BX is rated @ 175mA - I don't want to run out of headroom. The only 200mA transformer is the Hammond 278X 400-0-400 transformer. Will the voltage be too high, if so by adding a 250R/20W @ the end of the PSU get the voltage down to 400V?

-OR-

Would it be better to use an Edcor XPWRO86 350-0-350 @ 200mA with 12.6/6.3/5.0 taps?...
EDCOR - XPWR086
Then that means I have to use Edcor choke too to aesthetically match.

[jmillerdoc]

Edcor does not have a choke with these specs at the moment unless you want to pay the extra fee to custom build one. I recently checkd. I am using the Edcor with 360-0-360 @ 200mA, I didn't know they had a 350! Must have missed it on the list.
Jeff

[athos56]

I used the Hammond 278cx for my stereo kt-88 build but it ran a little hot and buzzed a bit with my C-LC-LC-RC filter. I'm converting to mono-blocks and changing the filter to C-LC-RC. I'm also making the switch to SS rectification. I plan on adjusting the first cap to raise/drop the B+ vs. having a large resistor. For example 1.5uf for ~400v or 2.2uf for ~450v (according to PSUD)

[jmillerdoc]

Originally Posted by athos56

I used the Hammond 278cx for my stereo kt-88 build but it ran a little hot and buzzed a bit with my C-LC-LC-RC filter. .............. (according to PSUD)

My gosh! A what filter? can you tell me what the values are of your current filter, I am curious.

I have played around with PSUD2 and the PSU for this amp. I am also curious, do you use a CS or resistive load for the modeling on PSUD2? What values do you use for the load? I have modeled it using a CS with about 125-135mA for the est. value of this amp. Also, what restifier tube are you using, a 5U4-G/GA/GB? I have modeled it with my 360-0-360 Edcor with a CLCRC with 47u/10h/80u/8.2r/82u and it pulls the current at IFMR of 0.88. I went ahead and ordered the 5U4-GB today to accomodate this higher IFMR.
Jeff

[porkchop61]

Originally Posted by 88man

I am going to run the SE KT88 in stereo configuration with a single PSU - all on one chassis. Porkchop and Sherman use 350-0-350 transformers in their schematic. Sherman mentions that for a single PSU, the power transformer should be capable of 200mA.

The Hammond 273BX is rated @ 175mA - I don't want to run out of headroom. The only 200mA transformer is the Hammond 278X 400-0-400 transformer. Will the voltage be too high, if so by adding a 250R/20W @ the end of the PSU get the voltage down to 400V?

-OR-

Would it be better to use an Edcor XPWRO86 350-0-350 @ 200mA with 12.6/6.3/5.0 taps?...
EDCOR - XPWR086
Then that means I have to use Edcor choke too to aesthetically match.

As you said I'm using the 278X, and I don't have any issues with running out of headroom. I'll have to measure current draw on the HT to see where I'm at, you've got me curious now I will say that the transformer never gets to the point where you can't leave your hand on it, It also does not buzz.
I agree that you should always have a little more power than you actually need, the transformer will run cooler
I'm trying to remember how I came up with the current requirements for the HT on the amplifier. I'm pretty sure I asked this question here and was given the answer that 175mA would be fine (which it looks like it is in my case YMMV). Try searching the forum.
Good luck.
Glenn

[athos56]

Originally Posted by jmillerdoc

My gosh! A what filter? can you tell me what the values are of your current filter, I am curious.

I have played around with PSUD2 and the PSU for this amp. I am also curious, do you use a CS or resistive load for the modeling on PSUD2? What values do you use for the load? I have modeled it using a CS with about 125-135mA for the est. value of this amp. Also, what restifier tube are you using, a 5U4-G/GA/GB? I have modeled it with my 360-0-360 Edcor with a CLCRC with 47u/10h/80u/8.2r/82u and it pulls the current at IFMR of 0.88. I went ahead and ordered the 5U4-GB today to accomodate this higher IFMR.
Jeff

I was using 6d22s tube diodes to 4.7uf - 10H - 100u - 10H - 100uf - 68r - 100uf for somewhere between 450 and 500v (not sure I ever measured it). The buz in the supply was mostly a bad damper diode and a plate I had over the them to keep the cats off 'em (the metal plate would vibrate in the chokes field).

Now I'm going with 1n508(?) diodes and around (1uf ~ 2.2uf) - 18H - 50uf - 10r - 600uf. I also have a big russian bleeder resistor 47k I'm thinking of puting across my giant 600uf ACS cap I have at the end, but I'm not really sure where that would put my final voltage (math alergy)...

I use a CC load on the PSUD, I set it for ~ 90ma (80ma for the output and 10ma for the input)

[88man]

Sherman and Porkchop seem to use the 273BX 350-0-350 transformer. If I go with the Hammond 278X 400-0-400 200mA transformer:

1. Will the voltage be too high?...
2. Will a 250R/20W resistor be enough at the end of the PSU to bring down it down to 400V?...
3. By adding a resistor, will the cap and choke values have to change?... (I can't use PSUD2, so I don't know).

Thanks Guys!

[alexg]

Originally Posted by 88man

Sherman and Porkchop seem to use the 273BX 350-0-350 transformer. If I go with the Hammond 278X 400-0-400 200mA transformer:

1. Will the voltage be too high?...
2. Will a 250R/20W resistor be enough at the end of the PSU to bring down it down to 400V?...
3. By adding a resistor, will the cap and choke values have to change?... (I can't use PSUD2, so I don't know).

Thanks Guys!

Get PSUD 2 from duncamps.com and model the PSU. If you use a rectifier like, 5r4 it should drop more voltage than the 5u4gb that was on the original Mikael Abdellah's circuit and it should come down to the 400VDC that you need. If you use 350-0-350, you can use 5ar4 and voltage will be around 400VDC.

If your voltage is a bit higher than 400VDC, it is still ok, but use 560ohms on the KT88's cathode, instead of the 470ohms.

This amps sounds excellent, I have built a few of them for my friends.

[88man]

Thanks Alexg! I'll use a 560 ohm resistor on the KT88 cathode when using the 400V transformer. This project is a great evolution with many people having contributed to it's design. This is my first build. My experience with tube circuits amounts to only 3 months now, mostly from this forum. Honestly, beyond V=IR and P=[I*2]R, I am really shooting in the dark when I stray from an original circuit. However, I feel that 175ma is not going to give the headroom for stereo operation, and that is why I am aiming for 200ma.

I've tried to figure out PSUD2 for the first time for a 400V transformer @ 200ma. Here are my graphs for the 5U4-G and 5AR4:

Power Supply:
-Transf: Hammond 278X. Prim: 115VAC; Sec: 400-0-400 @200ma
-Caps: ASC Motor Run, 440VAC (600VDC)
-Choke: Hammond 193J 10H, @200ma, 82 ohm

Circuit:

#1: 400V - 5U4G - 40uF - 10H - 50uF - 680R/25W - 50uF in CLCRC: Vmax = 415VDC

#2: 400V - 5AR4 - 40uF - 10H - (2)50uF in CLC: Vmax = 554VDC

#3: 400V - 5U4G - 40uF - 10H - (2)50uF in CLC: Vmax = 471VDC

Questions:
1. I am getting a higher voltage with the 5AR4 (554VDC) than with the 5U4-G (471VDC). Did I do things correctly?...
2. Did I design circuit #3 correctly with a 680R/25W resistor between 50uF(C2) and 50uF(C3) caps?... How will this alteration affect the sound or electrical characteristics of the amp?...


I just checked the Hammond website just after just now, and the 273BX is now listed as 201ma. Everywhere else the Hammond 273BX is rated @ 175ma. Is it a typo or a new transformer?...
Here is the link: Hammond Mfg. - "Classic" Power Transformer - (263-282 Series)