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

Quote: Originally Posted by AmadeusMozart
"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?"

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

Should be about 1.5-1.8v depending on which driver and or output used
an whether 1 or 2 leds are used for your drivers cathode circuits as well.
(since this one is using a volume pot any source should have plenty gain)

Do away with the pot an any passive or active pre should be fine as well.
 
Not quite sure of your exact question. The schematic states B+ of 400V. Mine ended up being 425V, works fine. Current draw at 120VAC is 1.17A.


AmadeusMozart said:
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.
Click to expand...
 
Kegger said:
Quote: Originally Posted by AmadeusMozart
"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?"

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

Should be about 1.1-1.3v depending on which driver and or output used
an whether 1 or 2 leds are used for your drivers cathode circuits as well.
(since this one is using a volume pot any source should have plenty gain)

Do away with the pot an any passive or active pre should be fine as well.
Click to expand...

Now I understand the question, my bad. My signal in is about 2VRMS max, no volume control, 100K to ground, with no issues using RC cathode bias.
 
seel34 said:
i just orderd a 5ar4 to higher the b+ ,i will post the results when i get it
grtz ko
Click to expand...

tryed the 5ar4 and the b + went up by about 40 volts
i am now using a philips 5u4gb when i use el 34 , with the weber bais caculation
the 44ma bij 330v anode over the plates is perfect
thnx for the url of the weber :thmbsp:
and i use the i use the 5ar4 for when i get a pair of kt88,s with 400v b+
i am also very intrested in using the rusian gu50 tube
grtz ko
 
Interesting thread. I was just looking at 6n1p ==> 6BQ5 SE idea using LED bias. For the VAS I was thinking that one red LED, 300V B+ and 22k plate resistor might be a good combo (estimating around 7 or 8mA) but it sounds like you have had better results biasing cooler (two LEDs). That might be more forgiving of variance in tube parameters too. If using 12AU7 instead I suppose the options would be similar no?

mike
 
The thing about the 6N1P it isn't as close to a 6DJ8 as many lead on, and in that
regard it likes a higher plate voltage, so yah on a 300v supply with 22k plate load
I would still suggest (2) 1.66-1.7v red leds in series to keep the plate voltage up.
(your little low on the plate load there, but not much you can do on 300v supply)

Now the 6N1P has a bit more gain then the 6DJ8 an since the 6BQ5 really doesn't
need it a 6DJ8 with the same 22k plate load and (1) 2v red led for bias is another
good option with a 300v supply. (nice operating parameters for the 6DJ8 there)

For the 12AU7 and led bias with the 300v supply, I'd go 33K plate load and the 2
1.66-1.7v series red leds to get decent current through the leds and decent load.
BUT
Much has been made of lack of clean signal/bad distortion from the 12AU7, I have
been lead to believe, running them on low current like 2-3ma with around 150v on
the plate actually produces a pretty darn good stage. Though the led's will not be
operating that well at the low current. So I would suggest a resistor/cap bias. :(

Now if you wanted to run 12BH7 or ECC99 or 6CG7 with led bias I'd look at a circuit
something like the 12AU7 operating parameters I first suggested.(should work well)
"For the 12AU7 and LED bias with the 300v supply, I'd go 33K plate load and the 2
1.66-1.7v series red leds to get decent current through the leds and decent load."

6DJ8 would make the 6BQ5 not need much in the way of a preamp to swing so the
volume "could" get touchy using preamps with a fair amount of gain, so if using an
active pre the 12BH7, ECC99 or 6CG7 may be a better choice there.

OK my 2 cents.. :)
 
Thanks Kegger, lots of good info there. One idea I had for using LEDs at these lower currents is to simply tie the cathodes together and let them share a LED (or LED string). Since the dynamic resistance of the LEDs is pretty low I would suspect that there wouldn't be much detrimental interaction between channels.

In this application the input will be driven directly from sources like an iPod or portable CD player so having some extra gain on tap might not be a bad thing.

mike
 
Led, battery or ?

Kegger

There are also some suggestions of people who prefer to use a battery in the cathode lead to earth.

However what is attracting me a bit more is a rather old approach where the grid has a battery to create the negative grid voltage. Main reason to use battery in the old days was to create this negative voltage without introducing hum. Perhaps this is something else you may want to try out?

AM
 
Yes as of yet I have not tried battery bias, whether on the cathode as pos or grid for neg.
(I have a feeling it would be to my liking, just haven't tried it yet)
 
Kegger said:
Yes as of yet I have not tried battery bias, whether on the cathode as pos or grid for neg.
(I have a feeling it would be to my liking, just haven't tried it yet)
Click to expand...

All the parts have now finally been collected, yesterday the James OPT choke and power transformer arrived. Seldom seen such a thorough packing and the finish is top notch.

Rather than using a fixed resitor for the output tubes I've bought some 500 ohm / 12.5 watt / 160 mA rheostats (variable wirewound pots) on oBoy. These will be put in series with some 150 Ohm resistors. This will allow me to use some different output tubes and bias them differently. This is a far more compact setup than using some switches. Can't belief how tiny these rheostats are, about the size of a full sized dual volume pot.

Am having difficulty to decide if I shall make two holes or just a single hole for the driver tube (in case I want to use a different driver setup with two tubes rather thant he single tube). Making a second hole after everything is mounted will not be easy and one idea that has arisen is using the second hole with a second tube socket and use that to measure the different volatges (basically a centralised test point). From HAM radio days I have a plug that will fit and has a nice cover protecting the user from the dangerous voltages.

AM
 
If I were to use 2 driver tubes instead of one (one for each channel) I would certainly
be leaning towards the 6EJ7/EF184 triode strapped, I've use them on my KT88 SE, UL
amp, there inexpensive, perform great and look way cool to boot! Quite a nice driver..
 
Keggar .. when using 2 driver tubes instead of one , do you wire it pararall per tube?

Also, am I OK with this figures of my 6DJ8 ?
420v B+ ~ 47K load ~ 130v at pin #1 / #6 and cathode is 3.6v with 510R.

Thanks.
 
The driver I mentioned is a single tube in a bottle, so you need 1 for each channel.

And your just fine with the operating parameters on your 6DJ8's, less then half the
rated plate dissipation, you could lower bias resistor a bit, it'll give yu more current
and drop the plate voltage, say 430-470 ohm, but still yur fine by me with the 510.

Enjoy!
 
Kegger said:
The driver I mentioned is a single tube in a bottle, so you need 1 for each channel.

And your just fine with the operating parameters on your 6DJ8's, less then half the
rated plate dissipation, you could lower bias resistor a bit, it'll give yu more current
and drop the plate voltage, say 430-470 ohm, but still yur fine by me with the 510.

Enjoy!
Click to expand...

Hey Kegger!

Once again, many thanks for your great advices!

With vintage Tungsol 6550, it sounds superb, not even my expensive GE KT88 nor Mullard EL 34 comes close!

WOW! What a journey!
 
You know the more I look this thread over the more I think that this amp is probably exactly the right amp for this project. I may just build a variation of this. I will probably use SS rectification since this will be a portable system and I want to keep the heat in check.

Kegger I am having a hard time wrapping my head around how to choose an LED from data sheet info. The voltage is obvious and I presume that we want a V-I curve that is as vertical as possible so that bias is stable WRT to current but what about dynamic resistance? Is there a way to determine that from the sheets?

I took a look at mouser and my head is spinning with the choices. If you want to PM me with some suggestions or sources I would be grateful.

mike
 
mashaffer said:
.... I will probably use SS rectification since this will be a portable system and I want to keep the heat in check.....
Click to expand...

Mike

if you use solid state rectifiers then I suggest you use a delay for the B+ to come on. If you don't then the tube life gets considerably shortened.

regards, AM
 
Hey Mike,
Sorry I'm battling a killer cold, I do not know how to determine dynamic resistance,
I know you keep it low by using low luminescence output LED's.

Yes, you are correct with the V-I curve, I go scower ebay for bulk LED's that have
the correct voltage drop and low luminescence, I'm just not up to it at the moment
to go there looking for some, the recent ones I've suggested are gone, I would need
to track more down, if yu find some your thinking about an want to either post them
or pm me I'll be happy to look them over and give my opinion on them.

There is also a lot of good info regarding the use of LED's for audio on DIYAUDIO, you
will probably find all the info you could ever want on them over there, I spent lots of
time trying to digest all that info, I won't pretend to say I understand it all either.

mashaffer said:
You know the more I look this thread over the more I think that this amp is probably exactly the right amp for this project. I may just build a variation of this. I will probably use SS rectification since this will be a portable system and I want to keep the heat in check.

Kegger I am having a hard time wrapping my head around how to choose an LED from data sheet info. The voltage is obvious and I presume that we want a V-I curve that is as vertical as possible so that bias is stable WRT to current but what about dynamic resistance? Is there a way to determine that from the sheets?

I took a look at mouser and my head is spinning with the choices. If you want to PM me with some suggestions or sources I would be grateful.

mike
Click to expand...
 
Thanks Kegger, coincidentally the old cold virus is doing a pretty good number on me at the moment. I found a couple of interesting candidates at mouser but I will check the bay too.

mike
 
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