Low Power 2 x KT88 amp (low plate voltages)

[bruno]

Hi guys, been a long time since I last built an amp.


I've been thinking about building a 2xKT88 with a very low power rating. This is a bit counter productive because I'm not taking advantage of my money per tube, but I really do love the sound of KT88 and can't handle an amp with more than 25w.


I was thinking about getting a power transformer that would give me around 250 volts on the plates and probably try to bias it as close to class A as possible, so around 125 ma per tube? Which means I'll probably need a transformer with around 200-0-200 vac and 300ma, right?


For the preamp I'm probably going to copy 65 amps, ventura preamp, that uses an EF86 in the front end, coupled to a 12au7, for an extra gain stage and cathode follower.


Anyone built something similar, any ideas?

[shooter]

Here's a thread that might help;
http://el34world.com/Forum/index.php?topic=19777.0

I built a 23W that started out with KT88's, but I re-worked it for EL34's
basically I melded the preamp from dougs bj to a matchless clubman, but I used an inter-stage tranny as my PI. great sound, and through my 2x12, it CAN be loud, but sound sweet at lower volumes

[PRR]

250V and 200mA total leads to a 2.5K happy-load. Can you find a reasonable 2.5K winding in reasonable size?

_I_ would go 400V and 6.6K load, just over 100mA total. I down-rated a too-loud VT-40 this way and it worked well, with any of the larger octals (metal 6L6 on up through 6550). This also leaves 350V for your small tubes. I got 19 Watts very-clean with low line voltage. Would easily rate 25W with proper line voltage and guitar THD.

[bruno]

Hi PPR,


as usal thanks for the input, you're right I wasn't thinking about the 350v I will be needing for my preamp tubes. Edcor did seem to have a 2.5k primary that would be a good match, but I think I'm going to follow your suggestion. As it makes more sense then what I was getting myself into.

I will have a read at the topic you posted shooter, see if I can follow it, maybe get some ideas as well.

Thanks

[shooter]

will have a read at the topic

SG's build is an SE, but all the math n science will help in a PP amp

[PRR]

6.6K and ~~400VDC is a very common iron-set for higher-power 2*6V6 gear.

The only trip-up is: does it have the extra Amps for the big heaters?

[DummyLoad]

250V and 200mA total leads to a 2.5K happy-load. Can you find a reasonable 2.5K winding in reasonable size?

_I_ would go 400V and 6.6K load, just over 100mA total. I down-rated a too-loud VT-40 this way and it worked well, with any of the larger octals (metal 6L6 on up through 6550). This also leaves 350V for your small tubes. I got 19 Watts very-clean with low line voltage. Would easily rate 25W with proper line voltage and guitar THD.

cathode or fixed bias?


--pete

[DummyLoad]

PT:
https://www.edcorusa.com/xpwr044
-or-
https://www.edcorusa.com/xpwr139

OT:
https://www.edcorusa.com/cxpp30-ms-6_6k

--pete

[DummyLoad]

PT:
http://www.classictone.net/40-18095.pdf
use 560V taps.

OT:
http://www.classictone.net/40-18064.pdf

--pete

[bruno]

Thanks for all the great replies, the suggested value make a lot of sense. And I'm definitely going to pick edcor transformers as they seems to have a good choices and transformers with enough mA to suit the KT88.


I'm usually a big fan of 6v6s and el84s, not sure what to expect from the kt88, but they seem to have some characteristic tonal properties, from the amps I've heard, even though I'm sure a lot of it is from the circuit.


I'll keep you posted.

[PRR]

> cathode or fixed bias?

My VT-40 hack-up was cathode bias. I wanted minimum (NO!) future repairs; and 35 years on there was no point in a LOUD amp when modern transistor toys do 2X the Watts in 1/4 the weight. So 19-25 Watts out of two 25W tubes suggested cathode bias, and no-trouble suggested some self-limiting when a tube went bad.

If he "likes the sound of KT88", he's probably only heard them in fixed-bias (to get maximum power per tube dollar). He might explore fix-bias even at these low power levels where it "is not needed", just to approximate the different overload action. This can be decided when the amp is 90% built and can be test-played.