GEC Audio Tube Data book/KT88 100W

[Ears]

"The GEC Audio Tube Data book reprint has a circuit for a 100 watt fixed-bias KT88 ultra-linear amplifier. The transformer spec. calls for a power transformer capable of 325mA supplying the two KT88s, a 6J5 and a 6SN7. "

Can anyone help me locate this cct? Or even a similar cct runing KT88s with 560V on the plates.

Also I've the RCA tube manual and handbook and would like to obtain the GEC book, is it available on-line anywhere (as RDH4 is)?

Thanks

[dobbhill]

How about the Dynaco MKIII? not 100 watts, but a starting point....
HTH, D

[gearhead]

Not what you want, but here's KT-66 ultra-linear amp application notes:

http://www.dc-daylight.ltd.uk/Valve-Aud ... 201977.pdf

[mhuss]

Here you go...

It *is* a hifi amp, spec'd and voiced accordingly.

--mark

[Ears]

Thank you all for these.

Mark, what immediately strikes me is the bias cct (including its 20H choke) delivering a voltage range beteen what looks like -50V to -100V. This agrees with Genalex data sheets which give -34V as typical bias for straight tetrode connected AB1 and -75V for the ultralinear configuration at same plate supplies. Which has served to spur me into further reading on ultralinear as I'd little appreciation before of the magnitude of the difference. I now wonder how ccts that switch in/out of U-L get around the differing drive requirements if they wish to maintain ability to deliver max power.

Anyone have ideas on that or can instead point me to a suitable discussion of that in a reference?

Did the KT88 100W cct have any accompanying description?

[mhuss]

I hadn't heard of that very large difference in bias requirements before, and it seems odd to me, as UL really only affect the AC operation of the circuit, I don't see how it would affect the static DC operation. It seems to me from a DC point of view UL isn't much different than using a small screen resistor; perhaps it acts more like a "pseudo-triode" mode (screen connected directly to plate) connection?

There's a couple of pages of descriptive text in the book, but it basically just talks about the main power supply and has a couple of graphs. Forgive me, but the booklet is bound with staples, and I don't really want to mash it too badly on the flat bed scanner!

--mark

[David Root]

I'm no expert on UL but I believe it does operate "in between" pentode and triode mode.

There are a few other old hi-fi OT tricks, check out early '50s McIntosh and the Quad II, the Quad II also uses KT66s but is a bit weak in screen/grid protection, like many amps of its day. These amps were big, discrete advances in their day and still sound real good built with modern components, especially good filter caps.

[Ears]

I'm no expert on UL but I believe it does operate "in between" pentode and triode mode.

There are a few other old hi-fi OT tricks, check out early '50s McIntosh and the Quad II, the Quad II also uses KT66s but is a bit weak in screen/grid protection, like many amps of its day. These amps were big, discrete advances in their day and still sound real good built with modern components, especially good filter caps.

I've found another take on the same design: http://www.plitron.com/vtvkt88.asp
again the bias is -80V.

I attach the Genalex KT88 data with typical operating conditions. I've rechecked and it still says -34V bias pp tetrode vs -75V bias for p-p UL .

It sure surprised me.

Edit : I shouldn't really though - UL is a form of negative feedback and a higher drive requirement is to be expected for same output, but the magnitude of the difference is telling - and what to do about it if you want to provide the option of UL switching.

[PRR]

> says -34V bias pp tetrode vs -75V bias for p-p UL

Look at the G2 voltage!!!

> I hadn't heard of that very large difference in bias requirements before, and it seems odd to me, as UL really only affect the AC operation of the circuit,

There's one big effect on DC operation: the G2 must sit at Plate voltage. With Pentode operation, G2 voltage may be very different from Plate voltage.

It is VERY convenient to set G2 just a bit below plate voltage, and when you take medium-big power from medium-big tubes, that can be accomodated in tube design decisions.

But when you want BIG power from medium-big tubes, you go in a direction which suggests setting Plate far higher than a sane G2 voltage. 8417 likes 600V Plate 300V G2. 6146 likes 500V-600V plate 150V G2. KT88 is in this class: it has a high plate voltage rating but not a very high plate dissipation rating. We can run high voltage, but not high current. So we have to keep current down, and we do this with grid voltages.

KT88 has a pentode ("Tetrode") suggestion for 300V on G2 and a rather small G1 bias. But in UltraLinear, the G2 "must" be way up at plate voltage. (OK, there is a trick with a tertiary winding but it is a lot of trouble.)

Another way to look at it: in pentode mode, the G2 stays at 300V through the audio cycle. In UL mode, the G2 tracks 40% of plate voltage and dips low just when we need maximum current. To get the peak current, the G2 idle voltage must start high (and plate DC voltage is natural). But if G2 voltage is high, G1 voltage must be large or that silly little 40W plate will melt.

Yet another way to look at it (Ears already saw this): Triode and UL are Feedback Amps. Compared to Pentode, they have less gain and lower distortion and output impedance. The improvement is about proportional to loss of gain. UL is usually aimed at about half the gain of Pentode: enough to improve output, not enough to strain the driver. So we need twice the G1 swing, but G1 still can't go (conveniently) positive, so somehow we are going to wind up near twice the bias.

> I now wonder how ccts that switch in/out of U-L get around the differing drive requirements if they wish to maintain ability to deliver max power.

If the Pentode amp was optimized for G2 voltage nearly equal to plate voltage: bias stays essentially the same.

If the Pentode amp load and G2 voltage were optimized for Power Output: switching G2 to 40% taps is gonna cause a loss of power due to low G2 voltage at the pull-down peak of the cycle.

But the difference between Naked Pentode and Half-Triode operation, especially in low-NFB amps like many guitar amps, is more impressive than a few (dozen) Watts more or less. There's nothing like the raw sound of an overdriven pentode, love it or hate it.