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

ecc88 led biasing

helo ,thanks for the advice :thmbsp:, my driver in this moment is the voskhod 6n1p ve
wich sounds prety good with 2 leds on 187 volts , i tryed a 6n23p but on 2 leds but the anode current was 138 where it should be 90 or less , i also tryed a tesla [old production] e88cc where the anode current was also above 130 on 2 leds , and i tryed my favoriete tube a valvo ecc88 grom 1973 philips heerlen
wich went also above the 130 , on 1 led they al stay arround 80 volts ,i am
afraid that the higher current will burn up the tubes quickly ,i don,t mind burning up some 6n23p but the valvo
and tesla,s i would not like to burn ,what is your expierience with ecc 88 on a 130 volt anode ? kind regards ko
 
One thing to remember with those maximum values is that is with everything at max.

Meaning that if quite under max dissipation an max current then voltage has leeway.
(any of the combos work this way to some extent with tubes)

In this case with voltage rating of 130v, 25ma rating and 1.8w rating, were just fine.

Call it 140v plate, 5ma an .7w, that's well under on the other 2 category's, yur good.
(that's running the tube fairly conservative an should last long in there)

---------------

Also your mentioning anode current as volts, anode current is around 5ma the anode
voltage is the 135v and 85v volts your reading on the plate, that is not your current.

---------------

Now since you have the switch to run 1 or 2 LED's I would run it at whatever sounds
best to you with whatever tube your putting in there. (they all should be just fine)
(I like the switch, good idea and should give you a decent tweak to the sound)..:thmbsp:
 
thank you for all the good advice
here are some pictures of my amp
2jfg48o.jpg

2mgwavk.jpg

rqytk6.jpg

kind regards ko :music:
 
Nice!!

There are other tubes that can drop right into the front of this amp as well, no changes.

The 6BK7, 6BQ7, 6BZ7 family of tubes are but just one, there nice and cheap tubes to. :)
 
Fabulously Modded !

:thmbsp:
Kegger said:
Also myself I would run that circuit a bit different as well. :)

I'd drop the grid resistor down on the output tube to something in the 220k range,
I'd add a 1m grid resistor to the input tube for volume pot wiper safety, I would be
running in UL with feedback coming from the output plate to the driver plate like I
have it on the attached schem, I'd remove that bypass cap/resistor on your driver
tube an replace it with (2) 2v red LED's in series, also 1.2k screen resistor on KT88.

(actually 1.66-1.7v LED's instead of the 2v ones here to gain a bit of current now)

That is the basic circuit I've been running on SE Pentode amps running in UL mode.
(on the original amp I'd at least drop that KT88 grid resistor down to 220K)

'
'
Click to expand...

Re-wired mine to Kegger's mod and was stunned by the fabulous sonics!

However, I tried to achieve maximum plate dissipation by using a 330R for
the cathode R but merely got only 32W despite an idle current of 90mA!?

Probably, my B+ is too low, 384v at plate.

Nevertheless, it still sounds great. :thmbsp:

Kegger ... thanks for sharing.

Zekk
 
Glad it worked out for you,

Yes that B+ is low, 400-425v would get you closer to achieving max dissipation.
 
Certainly won't hurt anything, give it a whirl and tells us how you fair..:thmbsp:
 
Kegger said:
Also myself I would run that circuit a bit different as well. :)

I'd drop the grid resistor down on the output tube to something in the 220k range,'
Click to expand...

Hi Kegger,

I just wanted to say thanks for your (and everyone else on this forum) suggestions. I finally finished the amp, and when my OPT shipper said "up to 80 days to arrive" he meant it!

I used the 220K grid leak, and switchable 330K feedback resistor for ultralinear, although I really like the triode mode the best. My B+ is 430V. I stayed with AC filaments and RC for both driver & power tube bias using Elna Silmic II and metal film. I used the LC circuit (5H/10uF) for the driver B+. I've been listening for about 20 hours so far and I have to say the clarity and focus far surpasses what I have been used to with solid state. No semiconductors at all. I measure 0.7uV hum & noise at idle. There is actual silence between the notes. My tubes are EH 5U4GB, NOS 6N1P, & EH KT88. I now realize that all the hype about SE Class A operation is not hype at all, just pure music.

Scott
 
Nice to see you are enjoying it,,:thmbsp:

scitizen17 said:
Hi Kegger,

I just wanted to say thanks for your (and everyone else on this forum) suggestions. I finally finished the amp, and when my OPT shipper said "up to 80 days to arrive" he meant it!

I used the 220K grid leak, and switchable 330K feedback resistor for ultralinear, although I really like the triode mode the best. My B+ is 430V. I stayed with AC filaments and RC for both driver & power tube bias using Elna Silmic II and metal film. I used the LC circuit (5H/10uF) for the driver B+. I've been listening for about 20 hours so far and I have to say the clarity and focus far surpasses what I have been used to with solid state. No semiconductors at all. I measure 0.7uV hum & noise at idle. There is actual silence between the notes. My tubes are EH 5U4GB, NOS 6N1P, & EH KT88. I now realize that all the hype about SE Class A operation is not hype at all, just pure music.

Scott
Click to expand...
 
Along with the alternate driver tubes I mentioned there are a host of outputs to "roll"

You can use kt88, el34, 6l6gc, kt66, kt77, 6550, ect..
(few more tubes there with same pin-out as well, voltages an currents should be fine)
 
Kegger said:
Along with the alternate driver tubes I mentioned there are a host of outputs to "roll"

You can use kt88, el34, 6l6gc, kt66, kt77, 6550, ect..
(few more tubes there with same pin-out as well, voltages an currents should be fine)
Click to expand...

Many thanks for this :thmbsp:. On another note: Have you ever experimented with a CCS in the output stage?

AM
 
Last edited:
There both the same basic amp/schematic, just minor changes and driver tubes.
I'd have a hard time saying which 1 I'd like better "maybe" the one in this thread. :)
(I usually use a preamp though, so the pot wouldn't be needed here on this amp)
(there are a few more tubes to roll for the driver in this circuit, can be beneficial)

---------

And no haven't found a use/need for a CCS in an output stage.
 
screen grid resistor for ultra linear tap

Kegger said:
Nice to see you are enjoying it,,:thmbsp:
Click to expand...

Kegger,

I used 1.2K from the screen grid to the ultralinear OPT tap per your suggestion and could not be happier with the sound. I have searched high & low searching for how the value of this resistor is calculated and have not been able to come up with anything. Could you possibly point me in the right direction concerning this?

Thanks,

Scott
 
That resistor can be thought of and or used in a few ways, it can be a "grid stopper"
which is needed on some tubes for preventing oscillation (needed often on tv tubes)
and it can be used to drop voltage as well which is what it's doing here.

When you look at the output tranny that has UL taps, follow the voltage coming into
the tranny and where the voltage goes to the plates, you see the UL tap is between
those two, well there is a certain resistance in those windings the farther the voltage
travels the more voltage will be lost/dropped, so the UL tap will have more voltage on
it then the Plate tap does.

The UL tap is connected to Screen/G2, "technically" you never want screens to have
more voltage on them then what the plate does so we use a resistor to drop some of
that voltage on the screen, the screens typically draw very little current so a decent
sized resistor is used to actually get some voltage to drop there.

I picked up a bunch of 1.2k resistors a while back and have used them in this fashion
they tend to drop just enough voltage on most tubes to get the same screen voltage
as the plate voltage an that is why I spec that value, when your building an amp you
can measure those 2 voltages and adjust that resistor if you want, but 1.2k "should"
do the trick pretty well, lower screen voltage is fine but I try an keep the resistor on
the lower side of what is needed to get the job done.
 
Excellent. Thank you very much for the explanation. In this case it appears that 1.2K is just about correct considering that on my build the plate voltage is 397V and the screen/G2 voltage is 396V.

Scott

Kegger said:
That resistor can be thought of and or used in a few ways, it can be a "grid stopper"
which is needed on some tubes for preventing oscillation (needed often on tv tubes)
and it can be used to drop voltage as well which is what it's doing here.
Click to expand...
 
I've gone thorugh this and the other thread I mentioned earlier but seem to be unable to find some info.

What are the input (V rms) requirements for this amp?

Thanks, AM.
 
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