• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

The Red Light District - another PP EL84 amp

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paulster

Member
Joined 2011
Sy

I apologize if I missed it somewhere in the depths of the thread but you said you'd tested Agilent HLMP-6000 LEDs and they performed well, although you ended up using the cheap ones you found for your build.

Since the Agilent ones are readily available and make for an easy off-the-shelf solution do you still have any notes on how many of them you'd need in the cathode dropping string?

I know it kind of defeats the fun of scouring for batches of old LEDs to buy in bulk cheap, but it's nice to have a workable solution that can just drop straight in!

Cheers
Paul
 
SY

SY

Ex-Moderator
Joined 2002
SYclotron.com
If I had to do it over again, I'd just use the Agilents and not worry about testing surplus diodes. In any case, their forward voltage drop is close to identical to the surplus ones. For most EL84, a series string of six should work, giving about -10-11V.

In my most recent build (I think I mentioned this but it bears repeating), I went way over ratings on B+ (if memory serves, maybe 450-470V), and dropped the idle current to stay within plate+screen dissipation. The distortion performance was still about the same, but I'm getting over 25W output.
 
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phildcamp

Member
Joined 2006
Ok guys. Got a question on heater wiring. Im running three lines from the supply to the tubes. Left for two el84's. Center for driver tubes. And right for two el84's.

When wiring just one side (right or left ch) of the el84's. Do they go in series or parallel with each other? I was thinking parallel but the schematic has me confused.

Peace
Phil
 
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phildcamp

Member
Joined 2006
ok i got music outta this thing! :D whoop whoop

but i do need to make a ground lift with parallel diodes (right now i have three 100R resistors in parallel stuck in there). in a 69 configuration as Sy describes. but i dont have squat for diodes on hand. just a couple left over uf4007's from the bridge rectifier. i know nothing about what to look for when selecting diodes for this.

any pointers??? ima make a run to radio shack for em if i can.

thanks

phil

oh and the amp sounds so sweet. only had 1/2 hr w it in the mid of the night last night. but oh boy :D:D:D
 
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phildcamp

Member
Joined 2006
oh ok cool.

im ironing out the kinks and things are coming along nicely. but one thing has me confused. :confused:

i was using one screen regulator for all four el84's cuz i have matched quads. i set it to get 85mv across the 1R cathode resistor while measuring the left ch. and then switched to the right ch to confirm and am only getting 72mv on that one. :( i tried swapin the led boards n regrounding things but still low on the right ch. so i put in the other screen reg i had on hand and started playin around with it that way. i have to just about max out the thing up to 292v to get the 85mv across the cathode resistor on that right side. while the left is at 280v for the same 85mv.

no idea what the deal is. :confused:

oh also my heaters are raised to 108v. is that to high? B+1=350v B+2=458v

thanks for the help guys. still gotta big ole smile on my face cuz i got music outta this thing! :D

phil
 
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phildcamp

Member
Joined 2006
ok. i was running 300k and 100k resistors in the voltage divider. that gave me the 108v. so i switched to the 200k - 50k combo. that brought it down to 95v. now please understand, i slept through or got kicked outta a whole lotta math class in school! :D:D:D

so any pointers on what resistor values to go with so i can get closer to that 70-75v mark. its just that they're a huge pain in the :mad: to change out cuz i put em in a tricky spot.

peace
phil
 
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JZatopa

Member
Joined 2003
zatopa.com
SY said:
Poobah, no, it's AB1.

And the currents are indeed balanced if you use reasonably matched output tubes. It is my experience that overload recovery is the single greatest factor separating really good amps from also-rans, much more so than the piddling imbalance introduced by 2 or 3 mils of mismatch (with toroidal output transformers possibly excepted).

Given how cheap TIP50 and LM317 are, if you're really worried about output stage balance, you could build a separate regulator for each screen.
Click to expand...

Could you explain to me why using LED bias leads to AB1 biasing instead of class a? I would think that an LED would act like a CCS and bias into class A
 
J

JZatopa

Member
Joined 2003
zatopa.com
SY said:
Just the opposite- the LEDs provide constant voltage at low impedance. A CCS provides constant current at high impedance.
Click to expand...

I think that bit of info was missing from my databanks, now I get the difference. Have you tried constant current and constant voltage for biasing the input tubes? I have not tried constant current on the input tubes and I am not sure if it offers much over a simple led.
 
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