I like the sound of amps utilizing Ultra-Linear connections on the screen grids. For a new amp, I'm using KT88's in push-pull configuration and the voltages I'm using are way too high for the screen grids (600V max). I'm trying to think of a way too still use (something like) Ultra-Linear, but I'm drawing blanks. Any options??
The only way you can do ultra-linear the way you want is to lower B+ voltage to an acceptable level or have a custom OPT made that has a dedicated CT winding for the screen grids. Then you can apply a lower voltage to that winding for the screens. Try Heyboer for the custom wind; it will probably be pricey.
I see you are in Europe, so find a custom transformer winder near you.
I see you are in Europe, so find a custom transformer winder near you.
By the voltages you are using ,do you mean that your power tranny is putting out too high raw voltage DC? If it is a multi-tap primary, can you wire it up for a different voltage to drop the secondary like putting a lower wall voltage on a higher voltage connection? How much over are you on voltage? Is the Zener diode trick not an option, zeners in series with B+ to drop it? Or in series with the center tap of the sec. There are various places to do this. As danFrank says, lower the B+ somehow. If you are not severely over,maybe the pri. impedance of your OT will still be OK with your new voltage. Hopefully.
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What voltages are you using?
KT88 is good for 800V plate and 600V g2, at least the high quality ones.
TDSL Tube data [KT88]
There is AB1 UL operating point for 553V plate voltage, 2x50mA bias, and 100W power. Isn't this enough?
KT88 is good for 800V plate and 600V g2, at least the high quality ones.
TDSL Tube data [KT88]
There is AB1 UL operating point for 553V plate voltage, 2x50mA bias, and 100W power. Isn't this enough?
Thanks, I'll look into that. i have access to custom wound toroidals, so that should be fine. The thing I;m not getting; If you use dedicated tap for the screens, won't that impact the amount of signal on them??
My current build is a 125W amp. 4 x kt88 with fully regulated 470V B+. Ultra-linear connected OPT. Sounds awesome. I tested the amp with both UL and pentode configurations and preferred UL, so I'd like to stick with it. But I want more power! 😀. 300W is my next goal. Simulations show 6 x KT88 with a B+ of 700V and 450V/500V on the screens will get me there. So lowering the B+ voltage is not an option, unless I use more KT88's, which Im trying to avoid (this is how sunn did it btw). But that's why I was wondering if there are ways to retain UL with the given voltages.
Any connection to the output transformer will not drop the DC voltage to the screen, after all it is connected to the anodes and any good quality transformer will have a low DC voltage drop in the windings. Try using a 2k2 10W wire wound resistor in line with the screens. That may drop enough voltage.
Remember about the more power aspect of it. Doubling your power only gets you 3db more volume. The changes you are wanting will only give you 3.8db more loudness. So really the extra power only gives you more clean headroom, something that the UL operation gives already, cleaner sound. It seems that all of the design hassles outweigh the extra power aspect, but it's your amp! Just my two cents.
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I agree partially. 'Twice' the volume shouldn't be taken too lightly. Twice the money on my bank account or twice the speed limit on my motorcycle have quite the impact too 😀. In all seriousness, I do have too weigh the pros and cons. And your point about actual 'clean' headroom is a very valid one.... but besides all the hassle and weight, 300W would be awesome! Ow and did I mention I already have a 300W OPT laying around?? 😉
Hi Guys
UL with KT-88s at 600V is fine provided you use proper screen-stop values, as TUTs show.
1k-5W per tube.
You see 6550s used this way as well, and also with high fixed Vs, but always with proper screen-stops.
As TUT2 details, it is NOT the screen voltage that is the issue, rather the impedance of the screen circuit.
Have fun
UL with KT-88s at 600V is fine provided you use proper screen-stop values, as TUTs show.
1k-5W per tube.
You see 6550s used this way as well, and also with high fixed Vs, but always with proper screen-stops.
As TUT2 details, it is NOT the screen voltage that is the issue, rather the impedance of the screen circuit.
Have fun
Ah I see, so it's more a current issue instead of voltage (arcing etc)?
My simulations show I need a B+ of about 685V to get to the full 300W output. I recon that's a bit of a stretch....
My simulations show I need a B+ of about 685V to get to the full 300W output. I recon that's a bit of a stretch....
Hi Guys
For KT-88 685V is no problem. Look at The 400 chapter in TUT6, which examines the power limits of the popular big-bottle tubes. You can use a pair of 6550s or KT-88s to get 150Wrms, although the KTs will be happier with a UL connection at these voltages than will the 6550s.
All limitations are ultimately based on power. There are absolute voltage limits, like the 2kV arc-over for EL-34.
Have fun
For KT-88 685V is no problem. Look at The 400 chapter in TUT6, which examines the power limits of the popular big-bottle tubes. You can use a pair of 6550s or KT-88s to get 150Wrms, although the KTs will be happier with a UL connection at these voltages than will the 6550s.
All limitations are ultimately based on power. There are absolute voltage limits, like the 2kV arc-over for EL-34.
Have fun
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