• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

KT88 SE Abdellah (modified by Alex Gendrano) + AK..

To Choke Or Not To Choke?

This is my first build and my first thread on this site. There are 4 schematics that I have seen on the 6N1P/KT88 SET amp by Michael, Alex, Glenn, and Kegger. I am planning to build mine in stereo operation with a Hammond 375-0-375V 175ma PTX, 5U4-G, CLC network, and James 6123HS OPT.

Just a few questions about the recent mods:

1. Does the circuit need the additional LC on the driver plate?

2. Replacing a 5H 250R choke (which I can't find) with a 1H 26R choke (Hammond 193S) or 5H 5.8R choke (Hammond 193L) require any other value changes in the schematic?

3. To switch between triode and ultra linear modes, should the triode mode engage a 100R between grid and plate, and in ultra linear mode engage a 1200R between grid and 43% tap on OPT?

4. If I wanted to use a LM317 to control driver cathode bias (will use both halves of 6N1P), is my calculation correct: 1.25/0.008A = 156ohm or 150ohm resistor placed between the "Adjust" & "Out" pins with the "In" pin to cathode and "Out" pin to Gnd?...

5. By adding a 1200R between grid and 43% tap decrease dynamics or power output?

Thanks!
 
Last edited:
88man said:
This is my first build and my first thread on this site. There are 4 schematics that I have seen on the 6N1P/KT88 SET amp by Michael, Alex, Glenn, and Kegger. I am planning to build mine in stereo operation with a Hammond 375-0-375V 175ma PTX, 5U4-G, CLC network, and James 6123HS OPT.

Just a few questions about the recent mods:

1. Does the circuit need the additional LC on the driver plate?
(NO, Just an RC stage is fine, like I'd posted earlier)

2. Replacing a 5H 250R choke (which I can't find) with a 1H 26R choke (Hammond 193S) or 5H 58R choke (Hammond 193L) require any other value changes in the schematic?
(Not really needed, just use an RC stage an make it easy on yourself)

3. To switch between triode and ultra linear modes, should the triode mode engage a 100R between grid and plate, and in ultra linear mode engage a 1200R between grid and 43% tap on OPT?
(Yes, that would be the best way to run it)

4. If I wanted to use a LM317 to control driver cathode bias (will use both halves of 6N1P), is my calculation correct: 1.25/0.008A = 156ohm or 150ohm resistor placed between the "Adjust" & "Out" pins with the "In" pin to cathode and "Out" pin to Gnd?...
(your on your own there an not needed as far as I'm concerned)

5. By adding a 1200R between grid and 43% tap decrease dynamics or power output?
(No)

Thanks!
Click to expand...

I would use the 5V4 rectifier instead of the 5U4 myself, 5AR4 for more voltage.
 
88man said:
This is my first build and my first thread on this site. There are 4 schematics that I have seen on the 6N1P/KT88 SET amp by Michael, Alex, Glenn, and Kegger. I am planning to build mine in stereo operation with a Hammond 375-0-375V 175ma PTX, 5U4-G, CLC network, and James 6123HS OPT.

Just a few questions about the recent mods:

1. Does the circuit need the additional LC on the driver plate?

2. Replacing a 5H 250R choke (which I can't find) with a 1H 26R choke (Hammond 193S) or 5H 5.8R choke (Hammond 193L) require any other value changes in the schematic?

3. To switch between triode and ultra linear modes, should the triode mode engage a 100R between grid and plate, and in ultra linear mode engage a 1200R between grid and 43% tap on OPT?

4. If I wanted to use a LM317 to control driver cathode bias (will use both halves of 6N1P), is my calculation correct: 1.25/0.008A = 156ohm or 150ohm resistor placed between the "Adjust" & "Out" pins with the "In" pin to cathode and "Out" pin to Gnd?...

5. By adding a 1200R between grid and 43% tap decrease dynamics or power output?

Thanks!
Click to expand...

Concerning the LC plate filter, this is the choke and capacitor I am going to use. My B+ is calculated to be right at 400V.

http://angela.com/hammondchoke155h.aspx
http://thetubestore.com/ca-fp-ca-au-10000uf-630v.html
 
Thank you Kegger for such a complete response! I will stick with an RC stage for the driver plate - it will be more economical, lighter, and occupy less space. The 2 diodes for the cathode bias are pure magic! :) I'll experiment with a 5V4 rectifier as well...
 
Hi,

I noticed on the KT88 SE schematic on your webpage you changed your grid leak resistor to 220K. Did you notice any change in the operation? I realize this appears to be the recommended value.

Thanks, Scott

vfr800fiman said:
Kegger-
You've got me interested in a few of these changes. I'm interested mostly in what the effect is of dropping the grid leak resistor to 220k on the KT88? I've seen this value on TubeLab's Simple SE design for a similar amp.

Glenn
Click to expand...
 
I personally can't tell that anything is different, however that's from a listening standpoint. I really have to take some measurements to see what's going on with the circuit. I just felt more comfortable using a recommended value that's in a lot of the tube data sheets. I also see that TubeLab uses this value in his Simple SE amp, and I trust his judgment :D

I did order a lot of 200 red LED's per Kegger's suggestion, so I'll be trying these in place of the cathode R/C on the driver tube shortly.

Glenn
 
You shouldn't really notice any difference in sound or measurements with the 220K,
it is just a safety factor for any suspect output tubes, an doesn't hinder the circuit.

Many people run higher grid resistors with no ill effects, but it doesn't need to be as
high as it was in THIS circuit for it to function correctly so might as well lower it then. :)
 
Well, I just finished installing the LED's for the bias of the 6N1P.
I will say that this is a very noticeable improvement in the sound! After sorting through about 25 LED's and getting the same bias current (about 3.32mA), I decided just to go with the 3.32mA value (at least the manufacturer is consistent). I was previously running at around 4.2mA with the R/C on the cathode per the schematic.

I'm definitely leaving the LED's in-place while I review some more music, but so far I'm really liking them!
Thanks for the tip Kegger!

Glenn

LED_bias.jpg
 
I'm glad they worked out, :)

Curious though, the 3.32 your reading is voltage not current correct?

If it is voltage then your current has gone up a bit, check your plate V
you should see with the LED's the plate voltage is bit lower then before.
(when drawing more current you'll drop more voltage across the loads)

Also if you have less voltage on the cathode with the LED your drawing
a bit more current, which is what we wanted to do, just a bit more here. :thmbsp:
 
Oh yeah, I calculated it for the voltage drop across the former 1k resistor, so it used to be direct reading V/R. Now that it's across a diode, I'm a little unsure of how to calculate current. I suppose I could insert the meters in series and measure directly.
Thanks for pointing that out. I'll also check the plate voltage.
Thanks again for suggesting the LED's

Glenn
 
You can calculate it at the opposite end, measure the voltages at the top sides
of the plate load resistors (I believe that 47k was used) subtract the voltage on
the plate of the tubes (bottom side of resistor), an divide that by the plate load
resistor value to figure the current through the tube. (voltage drop across load)

Something like, 395v - 165 = 230, 230 / 47000 = .00489, pretty close 5ma then.
(those are just arbitrary numbers, and I'm sure you didn't need the math on this)

Now if you add the plate to plate/grid feedback I posted makes it more difficult. :)

-----

You are welcome..:thmbsp:
I'm glad you got a chance to try the LED's, I find quite often they work rather well.
 
(Wow that was pretty close)

I'd be curious if you have any 6DJ8's to try them out in there.. :)
 
Hi All,

One simple question: why do you use two 1.7 V LED and not one 3.5-4 V. The matching business get much easier.

Thanks,

Davide
 
looks very intresting this led biasing , what would be the correct leds for ecc88,e88cc /6n23p,is there a general value [2volts????]i am still tunning my amp and trying by tube rolling to get the right sonic configuration at this momemt its tesla e88cc tesla [the real old ones not jj wichts is not tesla]tesla el34 brown based ,5u4 svetlana [sint petersburg produced 11/1960 black plate] the factory nowadays is called winged c ,what nowadays is markt as svetlana is made in the reflector factory wich is a total diferent quaility of tubes , greeting ko
 
Nikon1975 said:
Hi All,

One simple question: why do you use two 1.7 V LED and not one 3.5-4 V. The matching business get much easier.

Thanks,

Davide
Click to expand...

What we want in THIS circuit is 2.9 - 3.4 volts so we get a bit of extra current,
RED LED's have a typical voltage drop of 1.50 - 2v depending on LED, RED have
been shown to have very low dynamic resistance and a good choice for biasing.
so using 2 1.66v RED LED's in series work well here, not difficult to match them.
But any LED combo to get the right voltage will do a fine job in this amp circuit.
 
seel34 said:
looks very intresting this led biasing , what would be the correct leds for ecc88,e88cc /6n23p,is there a general value [2volts????]i am still tunning my amp and trying by tube rolling to get the right sonic configuration at this momemt its tesla e88cc tesla [the real old ones not jj wichts is not tesla]tesla el34 brown based ,5u4 svetlana [sint petersburg produced 11/1960 black plate] the factory nowadays is called winged c ,what nowadays is markt as svetlana is made in the reflector factory wich is a total diferent quaility of tubes , greeting ko
Click to expand...

We would need to know your overall circuit design an voltages to make a suggestion.
But the single RED LED of 1.66v is usually fine for the ECC88/6DJ8 based driver tubes.
(don't "need" much bias voltage for that tube)
 
Kegger said:
We would need to know your overall circuit design an voltages to make a suggestion.
But the single RED LED of 1.66v is usually fine for the ECC88/6DJ8 based driver tubes.
(don't "need" much bias voltage for that tube)
Click to expand...

Thanx ,this works great , I made a switch with 1 led for each side of the tube for the ecc88 and the anode is 87 volts[cathode minus 1,7]
and when I switch I have 2 leds in series for each side for the 6n1p with a anode of 178 volts [cathode minus 3,5 ] sounds great ,:banana:

My b+ is 350 volts, when I measure on the el34 in triode I measure 328v anode, cathode neg 25,2 with a resistor of 560 ohm with a 220uf
cap over it , does it make sense lowering this resistor to 370 ohm?? Or can I just leave it like this ?

kind regards

ko
 
You should be able to run the ECC88 with 1 or 2 1.66v red LED's, try both ways here.
(it may sound better 1 way or the other)

Well at the low voltage your plate dissipation on the EL34 is only 13.5 watts on a 25w
tube so yah you have plenty of dissipation to spare, going 370ohm might be just right.
(it will work fine the way it is but your output will be lower then it "can" be though to)
 
You must log in or register to reply here.
Back
Top Bottom