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KT88, (ect) Single ended amp, SEUL

CC stands for Carbon Comp,

They make the best grid resistors, Corbon Film will work but I prefer CC here...

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Looking Good.. :thmbsp:
 
I builded one (left) channel yesterday. Power tube so far 5Z3PAT. 5AR4 tube is coming. B+ 402 V. KT88 bias current 72 mA. EF184 driver plate 210 V. EF184 bias 6.6 mA and cathode to ground 2.76 V.

PSVANE KT88. Old EF184. 4 ohm PHILIPS speaker hooked. CD player as a source, no volume pot in use.

No hum, but the sound volume is very very low. Just audible.

I will wait for 5AR4 and give more B+ , but I am a little bit nonplussed of the low power. KT88 should give much more? Should it be question of the B+ or bias, or other matter?
 
It's another matter. If you have 402 V and 72mA, it should be plenty. Check all other connections for other issues.
 
I did not use any preamp, only CD player line out connected to input.
Connecting finger in the input, quite loud hum was heard. Thus it seems promising, that everything in connections are ok.

Do I need a preamp with this KT88 ?
 
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Couple of things..

Remember yes this is an KT88 amp but it's still single ended, and only about 10w output.
You do want to run some efficient speakers to get some decent volume, say 94db an up.
(you can run Lower Efficiency speakers, but not going to get a real loud volume though)

The B+ isn't going to change much, it is "possible" that CD-player your using has an low
output an would want a preamp to up it's output a bit, the amp wants Over 1 volt input.

I would suggest try some other "speakers", run both channels, and another cd-player to.

Other then that all the voltages do look good.. :thmbsp:
 
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Good news, friends.

I found the error in my wiring. I had put the last driver filter capacitor (33uF/3.3uF) after 39k plate resistor. No wonder, that sound was neglible.

Sounds good even with those old PHILIPS speakers, B+ now only 360 V and bias 66 mA. I made little modification to power supply. RC- anf LC - stages added to driver supply line to clean driver voltage. Simulated ripple only 3 nV after 60 sec. No hum at all. I paid a lot of attention to layout and ground wiring.

Thanks Kegger and others for lot of advice here. I will post some figures in some time.
 
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I'd say you're in a good place there with 360V & 66mA - should sound pretty decent. The only hiccup is a misplaced cap, I'd say you're doing VERY well, good job.

Hows about a pic or 2 of the chassis!!??


Cheers,
Bob
 
Chassis not ready, but you can maybe imagine the look of the figures. I have a plan to make wooden sides. The aluminium base plate stands on 8 mm bolts. That was IMHO a good idea since there was lot of room to make solderings and this construction gives later many possibilities to tune look of the chassis.

The main idea in layout was to keep everything symmetric to the center ground tap (Solen C1 ground) and keep wires as short as possible. Power supply components are in the center of the chassis. Grounding is tree-like and radially symmetric.

In and out terminals are not ready yet. I would not like to wire input wires from back to volume pot in front but maybe I have to make compromise in this and wire those along sides of the chassis.

http://imageshack.com/a/img842/4779/s8w6.jpg

http://imageshack.com/a/img835/2235/2d7w.jpg
 
I like your idea of building with the top plate and a jig with legs to hold it while building. I'm building one right now with a top plate and wooden chassis but using the wooden chassis to hold top plate while wiring up and I've got to be VERY careful not to scratch my nice new wood chassis!! Looks good, what are those coupling caps you're using there ??


Cheers,
Bob
 
I have listened the amp now some time. Sounds nice. I got 5AR4 and tested it. Not a big difference to 5Z3PAT. B+ 390 V and 360 V respectively.

Driver (EF184) currents between channels (R/L) differ quite a lot. I have now Siemens pair and Valvo pair, bias currents differ from 0.5 mA to 0.9 mA (about 5 mA level) and driver plate voltages differ 20 - 30 V accordingly (are say 170 - 200 V, 39 k plate resistors). I can not hear any difference in sound quality or volume, but this bothers me a little bit.

Is there any possibility to set the driver bias currents for both channels same? I tried to change bias resistors but it did not give logical effect. Or is it necessary at all have the same bias currents and plate voltages? Very well matched pair is of course a solution, but it seems that EF184 tubes differ quite a lot. I ordered on eBay a lot of four Russian tubes, we see how they are.

KT88 channels are quite equal.
 
The Driver, yah it'll happen when yur drawing that much current, not much you can do
except like you said try a few different tubes till you get a well matched pair to plop in.

It bothers me to when I see it, but truth be told it won't make much difference in sound
or in the performance as your still only 15% different of each other in the grand scheme.
(with the original schem and "adjustment" in the cathode you could dial it in for a pair)
 
Also make sure that swapping the driver tubes between channels the
voltages and currents Follow the Tubes and not the Channel there in.
 
Yes, I have followed tubes, not channels. I will make some minor adjustments and wait for new tubes. Then I will select best matches. One thing still is that different tubes principally have a little bit different sound.
 
Chassis almost "ready".

http://imageshack.com/a/img842/98/jec3.jpg


jec3.jpg
 
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