KT77 with optional UL mode

[pjd3]

Hey everyone,

As a trial in a project I am currently doing, I'm very interested in grabbing a pair of JJ KT77's for one side of the power section (this is to be a stereo guitar amp for clean only performance).

I'm very interested in hearing what the difference will be between running this in regular pentode mode (if I have that description right) and then in UL mode as well.

Should both have their place depending on musical performance material, I would be interested in having the output modes switchable.
Have any of you embarked in creating a situation like this and, if you did, what are the things to look out for, pitfalls, non-optimal configuration, stressful switching events, etc.

I have the preamp board all build up and most of the power section, just need now to grab a proper OT and pair of KT77's.

Thanks, and very much look forward to your comments.
Best
Phil D.

[LOUDthud]

I built a four 6550 bass amp with a Pentode-UL switch. Didn't like the UL mode, switched to Pentode and left it there. Suggest you wire up UL mode and try it before you commit to a switch.

[martin manning]

You will have very different scenarios when switching from pentode to UL mode, and some tube types are better suited to one or the other. KT77 at say 450V Va and Vg2 the same or a bit below would need Raa about 4k to place the load line just below the knee on the Vg1=0 curve. In typical UL with 43% distributed load the screen voltage drops significantly, and they will need about 8k Raa for similar placement of the load line when Vg1=0, You could solve that by miss-matching the load, but output power will be reduced by almost half. Bottom line is you have to do more than just connect the screens to the DL taps.

[pjd3]

Thank you for your input on this.

I hadn't yet taken a deep look at how the presence of UL would effect the impedance that the tubes would see. This implies that it may be fine leaving it in a pentode configuration. I'm sure that will be fine, as I've heard many a desirable clean tone with that kind of output config.

Maybe someday somewhere I'll play with UL to hear exactly what it does.

Thank you again,

Best,
Phil D.

[JD0x0]

I've built a few amps with switchable UL mode. Nothing with KT77's though. The difference is in UL mode it takes some of the (to my ear) more offensive harmonics out when the power amp starts to overdrive. It's basically adding NFB to the output tubes and IMO sonic results are fairly similar to adding NFB to a preamp stage. Cleaner, more even, has that sort of clean compressed thing going on. Overall, I'm a fan, but I know a lot of people who claim to dislike it. I guess it depends what you're trying to do. The amps I have it in either are more clean oriented amps, intended for pedal drive, or channel switching amps which get their distortion from the preamp.

[pjd3]

Thank for that JD0x0.

This particular amp, which is to be (if I'm successful, so far so good) is to be a 2 channel 50 watt per side clean channel from the Bogner Shiva. It is specifically for a guitar I am building now this soley for acoustic finger style performance on a baritone electric.

So, I am looking at a very clean system here, but, not super sterile Fender clean. What I loved about the Bogner Shiva clean channel is that while it stays very clean with big bold dynamics, it also has an element of that British sauce to it, not as much as a plexi, but, not an element found in generally in a Fender blackface type amp.

Unfortunately, there isn't alot of information on the Bogner Shiva clean channel, just some handdrawn schematics of those that attempted to unravel the circuit by inspection. I'm finding a couple of differing circuits but, they have enough overlap to give something to start on. I'll experiment from there. Based on the application of this "guitar/amp system" I had to wonder if having a UL option would benefit some songs and performance types of material. But, from where I stand, I have no experiential way to know which one (pentode or UL) would be best, or if both are desirable for different materials.

As a side note here, during covid when we had alot of home time, I decided to use that time to search for new (new to me) kind of guitar approach that was both beautiful, different and challenging at the same time. I stumbled upon people like Stephen Wake, Pierre Bensusan and Tony McManus that offered material that really fit that bill. And so spent hours everyday learning what I thought was a few of their most enchanting and challenging tunes. At some point, I thought that having and electric system designed to best serve this kind of material might be really fun, and open up to do say, some Celtic power trio instrumentals with bass guitar and drums. So I fell in love with the Bogner Shiva clean channel. It would have been easier falling for a Hiwatt since there is so much good info on those amps. But, I'm about 2/3 of the way assembling the Shiva clean channel on my ugly "Tube amp design station board" so, I'll see that through to hear what a single channel sounds like. The reason for the amp eventually being a 2 channel is to use good quality stereo effects to get a big, bold glorious stage sound, with very specific ambience and effects combos for each song, or even different parts of the same song. Yeah. I'm really planning to pig it up on this venture. And very excited about it. So, for all I know, I wouldn't even like the UL configuration. Might think its too clean in tandem with the other global negative feedback. I could get an Edcor output transformer to experiment on my design board, then I would know. And I will probably do just that.

Thanks for sharing your experience with UL. Very interesting to hear.

Best,
Phil D

[sluckey]

You may be interested is the old Sunn amps. They were all UL, powerful, clean amps. Easy build if you don't need reverb and tremolo. Worth a little research time. You might pick up some info that can be applied to your build.

[pjd3]

Thanks Sluckey,

I did go about to listen to some Sunns, Hiwatts and others that purportedly have very nice clean channels. And they definately do, probably something I could live with for this project. But I keep coming back to the Shiva clean channel. It has both a Fender like transparency mixed with some of the texture heard in the British amps and the others. Although the Sunns and Hiwatts and others have nice clean presentations, they lacked a particular open shimmer I hear in the Shiva. I'm probably crazy. But, thats what I'm hearing. (65 year old ex-rocker with a tested dip in the 2k-5k range. The said I have "moderate hearing impairment". Great.

Thanks. Those Sunns sound really good. I recall a friend who used to have a Sunn combo with two inner angled speakers. His tone was loud and bold.

Best,
Phil D

[bepone]

You will have very different scenarios when switching from pentode to UL mode, and some tube types are better suited to one or the other. KT77 at say 450V Va and Vg2 the same or a bit below would need Raa about 4k to place the load line just below the knee on the Vg1=0 curve. In typical UL with 43% distributed load the screen voltage drops significantly, and they will need about 8k Raa for similar placement of the load line when Vg1=0, You could solve that by miss-matching the load, but output power will be reduced by almost half. Bottom line is you have to do more than just connect the screens to the DL taps.

Sorry, i started to read, but, what did you wrote here?
What does it mean:
"In typical UL with 43% distributed load the screen voltage drops significantly, and they will need about 8k Raa for similar placement of the load line when Vg1=0.."?

and why you need to change load line for double?