260 to 300 is fine ... but really the answer is more like "80% of plate ±5%" ... which was determined empirically from tubes in service. One of the reasons for the "ultra linear tap" on many an output transformer. Transformer "ultralinear" is also quite reactive - meaning, that the screen voltage jumps about quite a bit in response not just to signal passing through the EL84, but also from reverse-EMF from the speakers stepping up into the autotransformer-connected primary. A nice Zener of about 20 volts is a great solution, though rarely used. Could also use 6 ea blue LEDs in series. About the same voltage, and gives that cool-blue glow.
GoatGuy
GoatGuy
Hi Bas,
I built an RH84 many moons ago and it sounded great to me, the best of any SE EL84 I had built before (and there have been many
)
I agree 250V - 300V should be fine. Going from memory, my plate voltage was a little over 300V, but seem to remember using a 10k resistor in place of the 2.2k. I never regulated mine but had all intentions to do so. You could always try a zener to drop the B+ and connect that to the screen as a "regulator", add a cap in there as well to kill noise if you think its necessary. If you used a 51V Zener then you should end up with ~ 250V on the screen or as GoatGuy suggested maybe a 20V Zener. I have found these smaller tubes need the voltage on the screen to be on the higher end to produce any power. Just watch the total dissipation of the EL84.
Andrew
I built an RH84 many moons ago and it sounded great to me, the best of any SE EL84 I had built before (and there have been many
)I agree 250V - 300V should be fine. Going from memory, my plate voltage was a little over 300V, but seem to remember using a 10k resistor in place of the 2.2k. I never regulated mine but had all intentions to do so. You could always try a zener to drop the B+ and connect that to the screen as a "regulator", add a cap in there as well to kill noise if you think its necessary. If you used a 51V Zener then you should end up with ~ 250V on the screen or as GoatGuy suggested maybe a 20V Zener. I have found these smaller tubes need the voltage on the screen to be on the higher end to produce any power. Just watch the total dissipation of the EL84.
Andrew
By the way. Regulated the screen to 250v. Pretty big improvement in sound!
Triode electronics TO-9105-137-7216 5k. Bought more than a decade ago for 10usd a pop. (Not sold anymore) They are pretty minute. So the bass is not deep and groundshaking. But I'm so suprised at how well they perform!
Then again the design lends itself to small ot's. This from the RH807 page.
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Want bass? Try this. http://www.lundahl.se/pdf/1663.pdf SE 50mA version.
I have a pair. I replaced a pair of Hammond 125ESE's with them.
Little more expensive though
What would you say the improvements were?
The sound becomes "fuller"
I'lI've seen a post about someone on some other forum going for the 1663...could have been you under a different moniker? Like you said...a little more expensive.
But I've promised myself to first build stuff with the transformers I have lying about...as mentioned earlier...I've had these little critters for more than 10 years..LL1682
Bas, I posted about using this OT (5500/5) a few years back on the K & K forum. It makes a really nice amp. I switch it back and forth between pentode and triode strapped. My speakers are efficient enough that the triode mode gives adequate power, I think that triode may be a little better sounding, though not night and day. I am using a 5965 as the driver, I leave the 100k plate to plate feedback resistor in place for either mode. I did run with the screens regulated, but I really couldn't tell any improvement over the original design. I left the regulator tubes in place and use them to regulate the driver stage. I have built 6 of these amps (gifts for my kids, one in the office, and one for the main listening area) using Hammond 125ese, Edcor Gse 15, and the lundahl ll1682. While all of them sound good, the lundahl is better.
Bas, I posted about using this OT (5500/5) a few years back on the K & K forum. It makes a really nice amp. I switch it back and forth between pentode and triode strapped. My speakers are efficient enough that the triode mode gives adequate power, I think that triode may be a little better sounding, though not night and day. I am using a 5965 as the driver, I leave the 100k plate to plate feedback resistor in place for either mode. I did run with the screens regulated, but I really couldn't tell any improvement over the original design. I left the regulator tubes in place and use them to regulate the driver stage. I have built 6 of these amps (gifts for my kids, one in the office, and one for the main listening area) using Hammond 125ese, Edcor Gse 15, and the lundahl ll1682. While all of them sound good, the lundahl is better.
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