"Mullard 5-20 KT88 PP blocks!

[Johan Potgieter]

Brethren,

Wish you would not post so energetically in my night!

Regarding the choice of tube for driver: It has to deliver the highest signal voltage output of the stages preceeding the power stage. Occasionally it has to deliver a little power or alternately have a low rp: This is when for fixed-bias output stages it must, apart from its own load resistor(s), also cope with a next-stage grid resistor of maximum 100K (and should you go push-pull parallel, very likely 50K). And for cleanest results, the 'loading' of said 100K through a capacitor should follow an even lower anode load resistor. (To best show these load lines one needs an Ia - Va graph - perhaps later.) Then, sometimes the ability to reach a low Va at Vg1 = 0. (This is where the 12AX7 mostly fails.) Then, as Eli showed, distortion pattern is of consequence; again because it operates at relatively high output signal.

Regarding direct coupling of the LTP:

Mmm .... Eli, a capacitive coupling to LTP input need not necessarily upset the stability 'Applecart". One can have a high next-stage grid resistor (several meg). I am thinking of where one can hardly avoid it because of different optimal operating points for the respective stages. (You will be shocked to learn that in my hi-end 100W design I cope with 5 l.f. poles - OPT included. And that is not counting the power supply - semi-regulated though. The Ø-band-limited pole in the G2 supply of my pentode input stage helps a lot to save the day, also I had to build a special 0,1Hz - 30Hz sine wave oscillator to test down there! But off-topic.]

Getting further technical; does one know that other triodes are worse than the 12AT7 in LTP? Will one not find that other similar triodes exhibit the same, if not identical traits? ....although that is probably what you are saying - they do not.

Tube-Mack,

Using the 12AT7 as input will not have the same 'pattern' as in phase inverter, as its distortion then goes through 'as is', unaffected by the p.p. topology.

Eli said:

The very last thing wanted is to satisfy Barkhausen's criterion for oscillation.

Ah, imagined that basic rule of amplifiers would come up sometime:

If you want an amplifier, build an oscillator. If you want an oscillator, build an amplifier. (Guchelsnack's basic rule 1 of audio. G. is the son of Murphy, only worse.)

[zigzagflux]

Possible option?

Really great sound, should mate with just about any output tube you choose.
Plenty of headroom from a low Rp driver.

[Johan Potgieter]

TubeMack,

No, as said (we are posting so rapidly), for 400V one can re-examine and bring the EF86 anode voltage down to give extra headroom for the 12AT7. Also, naturally the KT88 set would have a lower G1-signal swing at that h.t. So it should be quite feasible - that can be looked at. If it is decided to keep the 460V (or possibly even raise it) to use the KT88s to full value then there is no problem. Also, one needs to recalculate the load for the KT88s with 400Vht; it could still be made to give high power (lower Vac, higher Iac signal). But then we come to a new (lower) OPT primary impedance, and so on. That can also be calculated. As I previously said, we have to do with several different sets of acceptable conditions; it might depend on what is available.

It would appear that an OPT was available. What is the position regarding a power transformer? You guys will have to decide; I cannot comment from down here, it is not my market convenience.

[Eli Duttman]

"Sand" can be of great help in keeping LTP gain up, while allowing the use of comparatively small KT88 g1 to ground resistors. Keeping g1 to ground resistance reasonably low avoids destabilizing contact bias effects.

ZVN0545A source followers DC coupled to the LTP plates will accomplish the task. As small a value as 27 KOhms for the net grid to ground resistance is easily handled by the little FET. Just remember to size the coupling cap. to get a F₃ <= 5 Hz.

Johan is correct about very careful design and execution allowing multiple high pass poles in the signal path to work well. Being a "belt and suspenders" kind of guy, I prefer to avoid the issue, from the outset.

[boywonder]

Originally Posted by taj

I am confused (as usual). Is there a concern about the choice of the 12AT7 and KT88 combination based on their operating voltages? I don't yet understand the implication of the numbers on their datasheets.

It sounds to me, based on comments here, that optimizing one tube's comfort zone throws the other into a less comfortable zone. Is this correct? I'd greatly appreciate an explanation in lay terms.

Another question I have is about something Eli mentioned regarding the grid resistors on the KT88s needing verification. Can someone elaborate? We should fix it if something needs fixing. (sorry, I don't have the exact quote.)

..Todd

I'm also unclear over the constraints the 12AT7 LTP puts on the B+........seems like 460V or so is the lower limit based on the discussion but I have no idea why.


Taj: I can answer your question about the max value of the grid leak resistors. There are conflicting requirements for them, on one hand we want them large to minimize loss of gain since they form a potential divider with the output R of the preceding stage, and large value R's also allow smaller coupling caps to be used for a given lower cutoff freq. However, gas currents from residual gas in a tube need an easy path to ground to get rid of + charged ions when the residual gas molecules are slammed by the traveling electrons. That's why they are called "grid leak" resistors. When the resistance is very large (including the source resistance all the way back to the bias winding), the grid cannot stay -ve enough when lots of grid current is flowing, and this "less -ve" grid voltage causes more anode current to flow, blasting more +ve ions free, etc causing the tube to run away and self-destruct.

Typically the fixed bias max value for grid leak R is about 1/10th that of cathode bias, since cathode bias has some self stabilization capabilities that fixed bias does not. Interestingly, for the KT88, it lists different values for fixed bias above 35W and below 35W. So the 100K value is the most stringent of all operating conditions, which is listed for fixed bias above 35W.

With all of that being said, I'm still to much of a tube knucklehead to be able to calculate the source R all the way back to the bias winding, but it won't take much to exceed 100K ohms.

[boywonder]

Originally Posted by TubeMack

Here is the criteria that I feel must not be violated:

1. Monoblocks
2. KT88 in PP Ultralinear
3. 50 to 60 watts
4 Mullard style LTP topology

No problem here, either. That's what caught my interest in the first place.

[taj]

Originally Posted by boywonder

For PP load lines, look here (assuming that you haven't seen this site yet-look at the load matching sections):

education+diy

and post #11 here:

PPP OPT clarification

I've been recently trying to figure load lines out as well, and I haven't found much coverage for output tube load lines in MJ 3rd ed, but the above links pretty much cover it.

Great links, boywonder, thanks muchly. What is MJ 3rd Ed?

..Todd

[boywonder]

Originally Posted by taj

Great links, boywonder, thanks muchly. What is MJ 3rd Ed?

..Todd

One more link for loadlines.....Steve Bench's site

Steve's Tube Pages

MJ 3rd edition = "Valve Amplifiers" 3rd edition by Morgan Jones. There is an accompanying book by MJ called "building valve amplifiers" as well.

[taj]

Oh. I see. I have VA by MJ 3rd Ed. Didn't connect. Thanks again.

[boywonder]

Taj: Check out MJ VA 3rd ed pp 80-81 for the choice of grid leak resistor.