PI for KT88's

[clyde]

Will a 12AT7 using standard later Fender Silverface values (47k, 330k) have enough swing to drive a pair of KT88's or 6550's at 575va?  It will probably be 6550's and the screen voltage will be 100v less than plate.  I need some math here and a decent method to determine this.  TIA. 

[6G6]

If in doubt, copy a schematic from something you know works.
I think the 6550 screens will want to be at or below 300V.

[clyde]

Thanks for the replies.  It looks like 6550's are out because a dual rail supply will be needed for the screens (max 450v) while the plate voltage is around 575-600-too large a difference for just a dropping resistor.  Because this is a combo amp, the KT88's that were in weren't standing up to the speaker vibrations.  Having some ANOS Tung Sol 6550's around I thought it might be the ticket, guess not.  I might try 7027A's or just replace the PT and go with 6L6's or 7581A's.
Still interested in a PI design tutorial, quick and dirty works best!    

[Willabe]

Hi Clyde,

It might be easier for the guys to help you if they new a little more of what your trying to do. From your last post it sounds like something is already built and you already have the PT/OT?

But for clarity;

1. Is this a new build or is it a re-working of an older amp?

2. What is it?

3. Do you already have the PT/OT?

4. What kind of amp do you want/need as far as power?

5. Is it for guitar or bass?


                 Brad      

[DummyLoad]

use UL and screens will be OK. 12AT7 will drive KT-88 to full power but depends on design. 6SN7/6FQ7 would be a better choice.

--DL

[PRR]

With most audio power tubes, with most driver designs, you will have ample drive if the driver is fed a supply nearly as high as the screens.

G1 swing needed is approximately G2 voltage divided by Mu(g2) of the power tubes. Mu(g2) of EL34, 6L6, 6550, 6V6, KT88 etc is about 10 (EL84=18, 6146=4). A decent driver scheme can deliver 20% of supply as signal swing. Therefore EL34..KT88 will have more-than-enuff (EL84 tends to too-much, 6146 tends to fall short).

Now you ask if you want to run 12AT7 at 575V-100V= 475V. Not all of this will appear across the 12AT7, so we may not violate the 300V-330V rating. However "Silverface" to me means long-tail driver. We often drop 100V in the tail. That leaves 375V across tube and 47K resistor. The worst-case for tube dissipation is half-voltage, 187V across tube (OK!) and 187V across resistor. Resistor and tube dissipation are equal at 0.75V each. I think this is OK for 12AT7, clearly the resistor needs to be far larger than a default 1/2W part.

> 47k, 330k

Is 330K the power tube grid resistor? Note that MAX on 6550 and KT88 in fix-bias is 50K (100K for smaller tubes, often 500K in Self-bias). Yes, every g-amp maker violates this. However if there is a history of killed tubes, I would aim lower. With 47K plate load, 100K grid resistor will still drive well, and 100K will bleed-off a lot more grid-trouble leakage without excess un-biasing than 330K.

Agree that at this voltage, Ultra-Linear connection is worth consideration. When tube current is high, screen voltage drops. It is less likely to go nuts and squirt more current than is safe.

[clyde]

Thank-you PRR, thank-you very much, exactly what I was after!  Sorry guys, I thought everyone would figure out that later Silverface is LTP, 12AT7, 330k grid resistors and 47k plate resistors.  Earlier it was 1M grids and 82k, 100k plate.  Yes the power tube grid-leaks are 56k, although I was using higher with the KT88's because the JJ data sheet said it would be fine.  It is a re-working of a head that ran trouble-free for 6 years but now in combo configuration has some issues.  I do have a pentode-UL switch on the back, but the UL tone leaves a lot to be desired.  I was running the KT88's at 605 va, 603 vs, 100R screen resistors, which I have now upped.  Using the unused 5v rectifier tap in boost configuration I was able to get about 30v increase in B+ over straight in, but now am running in buck for 30v under normal.   Pt is 425-0-425 (monster) and OPT is Hammond 1650R.  Thanks again, you rock!

[Ritchie200]

Before you warm up the soldering iron, I have a question.  What KT88's were you running?  I have some brand suggestions that may hold up better if you want to stay with the current configuration.  I have spent many $$$$ finding a brand that will work in my beast.

Jim

edit:  OOPS!  I just saw your JJ reference.  I have not had good luck with JJ's.  Pick up a pair of winged C, the true Svetlana's.  I have beat them to death for years in this amp and they have not let me down.

Good luck!
Jim

[clyde]

Hi Jim, I'm no fan of JJ's either but they ran 6 years without a hiccup until one went microphonic in the combo setup.  In went another set which didn't last long.  Fortunately I had a matched quad of Winged C's here.  Well 3 out of 4 right out of the box went over-current in my Amplitrex tester.  Major bummer as I've had them a few years so Mojo won't do shit.
I think all new production tubes suck.