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There seems to be a love-hate relationship among tube builders over UL?
Some give it a big....'thumbs up', while others feel it is the worst of possible combinations.
Using the center tap of a pair of Edcor GXSE5-5.5K-8 OPTs, I put together a pair of small 6aq5a/6ab4 SEUL mono blocks.
This is my first attempt at UL, and I'm surprised at how well it turned out sonically?
Steve
Follow Ups:
Yes - 6550s in UL with an EF86 pentode driver. Loop feedback from OT secondary. Choke Input power supply.
I like it - a lot.
Of course I also like Single-Ended Pentode with VR regulated screens. ;)
I'm giving it a try. Good sound so far, and very easy to implement. Once I work out all the bugs in my 6l6 amp I'll start on a more serious project.
I have a wee Kt66 amp. The OPT is a custom Heyboer with a 2k primary, 40% U-L tap and a 10% E-Linear tap for the EF184. There is a 6CG7 up front, cascaded for adequate gain from a Geetar pickup. Sounds amazing on an OB and a Wolverine 8" speaker. Going to built it a proper chassis( it is in a small box, and fed from a Lambda PS for B+ and heater ). It will get DC for the gain stage heater as it hums a bit on AC.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Douglas
I would be interested in seeing how you implement the 10% tap into the circuit. I have been throwing ideas around for an all DHP e-linear amp.
There doesn't seem to be much information out there on this type of amp
so anything you can throw at me would be appreciated.
Cal
The 10% tap is where I get the EF184 pentode driver's plate supply. It is just a separate tap...and I will likely try it with the KT66's g2 hung there too. or maybe a switch between the 10% tap and the 40% tap...musicians like adjustments.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Edits: 04/07/15
Thanks Douglas
One more question, I am assuming varying the tap for the driver plate supply will also vary the amount of feedback. So, lower percentage tap, less feedback higher percentage tap, more?
Thanks Cal
At the time this one got designed I way usually running the usual U-L tap location( between 20 and 50% ), but for a guitar amp I figured less FB and more gain was the order of the day.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Hi PakP, have you tried DC bias on the heaters? If your filament is center tapped it is easy to inject some DC into the heater circuit by tapping off the B+. It worked great in one of my amps. cheers, Dak
The next iteration will just run DC; I don't want to mess with it...just a guaranteed, no heater noise...the last two stages will be fine with AC( elevated no doubt, just as you suggest ).
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
If so, I would guess that the difference in output impedance would be the biggest change with triode lowest, then UL and of course the highest output impedance in zero feedback would be pentode mode. Also, for the same amount of feedback(in dB), the UL should be(if properly designed) lower in distortion. With triode wiring, you may be able to use zero feedback but, power levels will be much lower.
No negative feedback.
For me, the extremely low parts count, made it an attractive DIY build.
Small chassis size (6" x 10" x 2" times 2) and budget limitations dictated small and inexpensive but good quality (Edcor USA) iron.
Steve
"No negative feedback."
What levels of distortion are you seeing? Have any commercial designs used UL alone, without other feeback?
--------------------------
Buy Chinese. Bury freedom.
I am not sure?
My old dumpster-found 1960's Tektronik O-scope, gave up the ghost a few years back.
I actually found the schematic (originally using 6V6 and 12AT7 tubes with a UX-80 tube rectifier) on THIS FORUM, dated March 2001.
As far as any commercial designs, I have NO idea??
Steve
I'm neither a fan nor a detractor. The downside to UL operation for me is the reduction in output power compared to non-UL. For a high power amp, it wouldn't matter too much, but a 6BQ5 or similar in UL doesn't quite have the ooomph to drive my speakers. If I had a nice set of horns, I'd probably use UL exclusively for all my pentodes, just to ease the external NFB requirement.
--------------------------
Buy Chinese. Bury freedom.
My speakers are 95 db EV 8" FR drivers in vented cabinets.
Doesn't take much to drive them to pretty insane sound levels.
Steve
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