"Mullard 5-20 KT88 PP blocks!

[taj]

Originally Posted by SY

Well, not from the ones shorted together- they'll just need one. So for each monoblock, you'll have 8.

I think that's how I got 9. I guess you mean to also short pins 4-5 on the 12AT7 and use 1 cap for them?

They go right to the chassis, not to the ground wire. That's usually easy to do, since the tube sockets are generally mounted on the chassis, and you can either add a solder lug under one of the mounting screws or use the type of tube socket that has integral solder lugs.

Yeah, I realize that. I was thinking more about to how to draw it on the schematic. See attached pic.

..Todd

[SY]

Yes, with the understanding that each cap is individually returned to chassis, not to a common lead which connects to the chassis. The Building Valve Amplifiers reference will illustrate this beautifully.

You DO have Building Valve Amplifiers and Valve Amplifiers, right?

[taj]

I've only been thinking about tubes for a couple weeks now, I'm still reading the basics about how they work (the Crowhurst series book 2 currently). Once I've made it through that, I'll tackle [some of] the rest of the books on P. Millet's website.

I'll Google these books you mentioned. Why do I get the impression you've written them?

..Todd

[SY]

I wish I had.

These were written by Morgan Jones, a former engineer at BBC. IMO, they are far and away the best currently-available books on the subject. I reviewed them on my website. Though it's no secret that Morgan and I are good friends and that we share many points of design philosophy, he wrote the most recent edition before we had met and I have no financial stake in the books' sales- I just honestly think that they're terrific and extremely helpful. In VA, Morgan does a very detailed and thorough analysis of the Mullard 5-20.

[taj]

I think this schematic better illustrates the reality then.

Is the bias voltage divider correct in this diagram?

..Todd

[Ultima Thule]

Originally Posted by SY

Problem there is Miller from the next stage, 50pF or so, since the pentode (with feedback to its cathode) will have an extremely high effective plate resistance. With a source impedance in the megohms, you'll have a pole right where you don't want it somewhere in the 200-2kHz range. You'd need to add a cathode follower between input stage and phase splitter.

Wouldn't the set gain decide how big the Miller guy looks like on g1, though plate output impedance is in the MOhm region the gain is set by the external Rp and the gain goes down and so do the effective Miller capacitance seen on g1, gain times Cpg, right?

EDIT: checked little bit closer ECC81/12AT7 Philips datasheet, Cag 1,6 pF, with other words we would need a gain of 30 to come up to 50 pF.

Cheers Michael

[Johan Potgieter]

Originally Posted by taj

Is everyone married to a CCS there?

I am not. No problem with CCSs, but you specifically ask "there". The CCS only guarantees equal outputs from the phase inverter, not the KT88s. Then there are the OPT internal capacitances. So in practice ....

This type of circuit has been successfully built with a tail resistor often enough, and the resistance is constant with frequency (CCS is not necessarily). I cannot get to earlier measurements now, but the impedance on the combined cathodes is low, so a 27K is already comparatively high. Regarding unequal tube ageing, I have never had a problem with triodes in the same envelope in multiple builds.

But having said that, it will help to have a small balancing pot in the anode loads for final exact equalising on the KT88 anodes.. In the judgement of some that is equal extra trouble. So some personal preference creeps in ....

[Geek]

Originally Posted by Johan Potgieter

The CCS only guarantees equal outputs from the phase inverter ....

Mmmmmm.... no. Close, but no

Cheers!

[Ultima Thule]

tubelab, all valid points there, I can fully understand that when you are providing PCBs for the DIY community it's good practice to add "too much" than "too little".
We'll shall hope the phono stage have some rumble filter as OPTs tend to saturate quite easy at very low frequencies, one R-core transformer I have been looking as a possible choice for my KT88 amp has an impressive 800+ H a-a but a few mA and it drops like rock down to 100 H, but R-cores are very special in that they do suffer the same overly sensitivity to DC components as do toroids, other cores that comes in 2 halves or so when mounted together still have micro-small gap and that makes a BIG difference believe it or not and that's why such transformers can take more DC components without dropping so much in inductance.
While on the matter DC and ULF as I noticed Johans advice for a more exact equalising on the KT88 anodes I would tend to say the current design with separate trimmers to g1 is very cumbersome to use, one can not make any quick trimming without doing it in small steps on both trimmers forth and back forth and back... I would really prefer one trimmer that set the rough and quick trimming on both KT88 and a small trimmer which is splitting up to each grid on the KT88 pair and makes the fine equalising Johan is talking about, but this is on the grid side and not on the anode as Joahn talked about.

Thinking here on the matter of adding some HP filter function in the first tube stage in an attempt to lower DC/ULF sensitivity I am pondering here for some practical places to add a pole and while there may be couple of places to do that but to mention one I was thinking one could add a R+C in parallel with the current Cathode resistor, the values will be chosen such that the single resistor sets the gain below cf while the R+C sets the final gain for the whole audio band and up, any drawback on that idea?

Cheers Michael

[Eli Duttman]

A 12AT7/ECC81 LTP yields a voltage gain of about 17X. C_Miller is not much of an issue, when 'T7s are employed. Remember the type's VHF front end heritage.

If Dr. Miller's "pet" causes lack of sleep, definitely use the 6GK5 voltage amplifier. That type's high gm and low R_P will swamp LTP capacitances.

A 150 KOhm grid to ground resistor in combination with a 68 nF. cap. sets F₃ at a very nice 15.6 Hz. BTW, this is the place to use a PIO part, should that tickle your fancy. The I/P cap. is outside of the GNFB loop and attempts to control voicing here have good prospects of success.