The "$250" Amp? - 6LU8 Compactron SE, UL

[Lavane]

Well I went ahead and built it! I followed the Skunkie Designs video series and schematic. My thoughts post build are that this is much better than I expected. It drives my Advent Graduates, and my Polk 50 monitors very well. Neither of those have bass below about 50-60Hz, so I cannot comment on bass power, but to my ears, Santana's Black Magic Woman has a great bass rift at the end, lots of dynamics, detailed percussion, guitar, and organ. I was not expecting this amp to be capable of powering the Polk Monitor 50's at 89dB sensitivity, but did a great job of it.

As far as any regrets, I wish I had bought a more powerful power transformer. The one I got delivers 104mA, andI think the amp is drawing just about that. It gets hot to the touch, but reaches a thermal equilibrium after a while and gets no hottoer, so no runaway. I suspect that I will upgrade the tranny at some point. The pair of tubes I got were not well matched and the sound stage is a little lopsided, hard to tell how much is due to tube and how much due to speaker placement as even with my Sony AVR it is a little off center, but not by this much. I have a spare 6LU8 I purchased,and will try to see how well it is matched.

dand of course pics. My amp, and my listening space with the 6LU8 in the center on the console. The Advent Graduates are a recent refoam.
View attachment 3006660View attachment 3006661

Nice amp! Those little Edcore output transformers sound pretty good.
I built a 5.5wpc 6LR8 SEUL amp with them. I really like this little amp.
I recently built a passive preamp with input selector and a CNC phono preamp to match. I use it in a second system next to my bench. Hopefully its just the tube causing the imbalance for you. I would have built that 6LU8
amp, but I have too may amps I've built already. Its on to tube preamps and phono preamps now.

 

[RogerA]

Nice amp! Those little Edcore output transformers sound pretty good.
I built a 5.5wpc 6LR8 SEUL amp with them. I really like this little amp.
I recently built a passive preamp with input selector and a CNC phono preamp to match. I use it in a second system next to my bench. Hopefully its just the tube causing the imbalance for you. I would have built that 6LU8
amp, but I have too may amps I've built already. Its on to tube preamps and phono preamps now.

Thanks. I am building a preamp with selector switch myself. The chassis and switch arrived yesterday. I ordered a $10 phono preamp board from amazon, just to get me going. I will upgrade to something like a CNC or VSPS type active pre stage with op amps.

Really enjoying this hobby.

Roger

 

[Lavane]

Thanks. I am building a preamp with selector switch myself. The chassis and switch arrived yesterday. I ordered a $10 phono preamp board from amazon, just to get me going. I will upgrade to something like a CNC or VSPS type active pre stage with op amps.

Really enjoying this hobby.

Roger

Originally I was just going to put a cheap Pyle P-999 into that chassis.
The CNC is a really nice phono preamp. You have to source different caps and inductors from the BOM since it's several years old. I tried different OP amps and liked the ones with JFETs inside the best.
Post some pictures of yours when your done.

 

[mschmitzusa]

hey-Hey!!!
Don't want no Locker...they eat tires waaaaaay to fast. Torsen OTOH...

The 6LU8 is about a 12AT7 triode in the same envelope with a 6AV5 pentode. These two parts will make up the voltage amplifier section and power section respectively. First things first, download the 6LU8 sheet from Frank Philipse's site.
http://www.mif.pg.gda.pl/homepages/frank/sheets/123/6/6LU8.pdf

The pentode section "may" not work so well rigged Ultra-Linear. The high screen voltage would offer characteristics that may be hard to utilize with 40% tap location. It should work with a 60-70% U-L tap, but that gets us into custom OPT land. As a triode it has a mu of 6.5 and a plate Z of ~750 Ohms. That will work just fine. Now with a 5k plate load, a 40 mA idle current, and B+ of ~350 we'll want a solid g2 supply sittiing at ~100 V. That is easy with a 0C3 VR tube in a shunt regulator( tentative parts selection leaves us a bit of current to burn ).

So possibly a choice between triode or pentode rigging for the power section. The triode-strapped power stage working with a 5k plate load will not need any negative feed back; the amp will run open loop. With the pentode connection we'll need some FB. The RH84 offers a very well designed FB circuit( somebody please follow up with a link to that schematic ). ( UL should be tested though )
cheers,
Douglas

Hello PakProtector, can you assist me with the UL configuration and transformer specs for a 6LU8 PP amp? It looks like everybody uses 8K PP transformers made for EL84 PP with the 6LU8 PP design. I ordered a 8K transformer and the winder offered me to add a custom UL tap to it. I read here that you suggested 60 to 70% UL. So 65% seen from B+ or 35% seen from the plate is a good middle of the road solution? I have to let the transformer winder know by Thursday. Can you help me out with your knowledge?

 

[PakProtector]

As I suggested, the power section is rather like a 6AV5. Soooo...it seems to me either run pentode with a regulated screen, or triode. It could be you misinterpreted; a 20-30% tap could be used...the effect on plate current is high with varying screen voltage. Don't get too carried away with B+. Let me dig through its curves today and I will see if I can't help with a screen tap recommendation that is better.

Douglas