JTM45 KT66 wiring - grid stoppers and screed grid resistors

[PharmRock]

JTM45 build with GZ34 rectifier and KT66's.  The original did not have grid stoppers, but many of the later reissues and the countless clones of this amp do.  Planning on mounting the grid stoppers directly on the tube sockets across pins 1 and 5. I know there isn't a whole lot of current passing from the PI to the power tube grids, but was thinking that a 1W resistor would be good here due to the heat it will be subjected to directly above the tube socket.  Also, is anything from 1k5 to 5k6 ok?  the Hoffman 5F6A uses 1k5, and the Plexi 50 uses 5k6. 

As for the screen grid resistors...the original calls for 470R/5W, which is what I have.  However, some circuits utilize a 1K or 1K5 (2W) coming from the choke and then to each of the 470R's on the screens (I have a 750R/5W and an 820R/5W wirewound on hand I could use for this purpose).  I did not include this "shared" resistor on my board layout and wonder how critical it is...would the 470R/5W resistors be sufficient by themselves, or would that end up being an $80 experiment to fry my power tubes?

Thanks and Happy New Year!

[pdf64]

KT66 have a rep as being more prone to instability than 6L6, I read that its longer electron path leads to increased phase shift at higher frequencies  So whilst I acknowledge that the 60s JTM45 tended not to have grid stoppers, having had some experience with KT66 and an RS DeLuxe OT in a JTM45 type build, I strongly suspect that they were not unconditionally stable. So I regard 5k6 grid stoppers as a minimum, 10k would be better.
I regard the 1k shared screen grid (g2) resistor as being essential. It provides a degree of local NFB, linearises the high power waveform, and reduces g2 dissipation. Without it, unless operating at a lower HT, I think heavy power overdrive will cause significant g2 overdissipation, especially if using an 8k OT.
Also an RS type choke, 20H with about 500ohms resistance, is very beneficial in that regard. A Vox AC30 60s type choke should have a similar spec, so should also be suitable.

[66Strat]

GEC recommended 10k ohm to 50k ohm grid stoppers. See the text under the installation paragraph on pages 4 and 5 of the attached.

[HotBluePlates]

... KT66's.  The original did not have grid stoppers ... As for the screen grid resistors...

A lot of years ago I took an amp that had been working fine for years with 6L6s, and installed a pair of KT66s.  I immediately had problems with oscillation until I added grid stoppers for the control grid & screen grid (what you're calling "screen resistors").

While the screen resistors performed their normal function (keep the screen from overheating by dynamically dropping voltage), it was also important to mount the resistor right at the socket pin to kill the oscillation.

Might not happen with every KT66, but that was a lesson learned for me.

[PharmRock]

Thanks everyone for your replies.
The choke I have is 7H/150ma (based on the old MetroAmp build specs). 
Per your suggestion, I'll go with a 1K/5W shared resistor off the choke filter to each of the 470R/5W screen resistors, and a 10K for the grid stopper.
As for the grid stoppers directly across the tube sockets...is a 1W rating ok here?

Regarding pin 1, the GEC diagram shows pin 1 as internally connected....but isn't this pin not connected to anything in the KT66, similar to a 6V6?  I was going to use this as a "tie-in" for the grid stopper over to pin 5. (Or does IC and NC mean the same thing?)

[Willabe]

Regarding pin 1, the GEC diagram shows pin 1 as internally connected....but isn't this pin not connected to anything in the KT66, similar to a 6V6?  I was going to use this as a "tie-in" for the grid stopper over to pin 5. (Or does IC and NC mean the same thing?)

I thought; IC = internal connection, NC = no connection?

I think pin 1 that says IC is the beam forming plates connection. Many power tubes have the beam forming plates/grid 3/suppressor grid IC to the tubes K. EL34 is 1 of the few exceptions that I know of that has a separate pin out for the suppressor grid/beam forming plates.   

The link for a KT66 spec sheet that 66Strat posted shows the pin out as NP not NC for pin 6. I don't know what NP stands for. 

[pdf64]

NP = no pin 

A g1 grid stopper shouldn’t dissipate more than negligible power, so I’ve never used anything more than 1/2W.
I fit the 1ohm cathode current sensing resistor between 1&8, as it’s most convenient for my 0V scheme, and because I wanted EL34 compatibility.
I suggest to use a resistor to make up the choke resistance to around 500ohms. Apparently some KT66 brands typically draw more g2 current than others.

Note that no schematic seems to show the 1k safety resistor that was always fitted across the OT secondary on the originals. I think that if Marshall had carried on fitting them, a lot of OT failures might have been avoided, especially given the awful load impedance selectors used up to the JCM era.

[PharmRock]

I suppose a simple continuity check between pins 1 and 8 would answer the question, correct?
Will try later today.

The old MetroAmp build guide (for Kt66) shows to connect 1 and 8 and then a 1-ohm resistor to the tube socket mounting bolt.  I didn't understand the rationale for this unless it was to make the amp compatible with EL34's, as you have also done. 

[Willabe]

NP = no pin 

Seems right.

[PharmRock]

Here's the datasheet for the Gold Lion reissues (New Sensor) that I have.

[dwinstonwood]

"No Pin" makes sense because the JJ data sheet doesn't even include a pin 6 in their diagram.

[PharmRock]

It seems then from the New Sensor and JJ datasheets that pin 1 is unused on the KT66, as the beam forming plate/suppressor grid is internally connected to the cathode (unlike the EL34), and I can use pin 1 is as a connection point for the grid stopper over to pin 5.

[PRR]

Pin 1 is the metal shell or base. Has been that way, standardized, since the first Octals.

Yeah, many Octals lack any external metal. _I_ still think that using Pin 1 for anything except Chassis Ground is risking mystery trouble. Is a terminal strip that expensive?

[PharmRock]

You're right that a terminal strip is cheap and addresses any concern about pin 1.  I could do this but it would be getting a little crowded around the tube sockets with nearby wiring and the circuit board right up next to them, as well as a terminal strip I'll need to use for the 1K/5W resistor running to the screens.

Just realized I have some insulated turrets that may do the trick...could potentially use them in place of the nut for one of the tube socket mounting screws. However I think they are #4 and the tube socket screws are #6. 

Looking at how Fender wired their 6_6 amps, they seem to all have the grid stopper across pin 1 to pin 5 (and screen grid resistors across 4 and 6).  As long as I'm using tubes without a metal base, implementing the Fender-style approach shouldn't be an issue, correct?

[shooter]

As long as I'm using tubes without a metal base, implementing the Fender-style approach shouldn't be an issue, correct?

just add a note in the Docs to the future so they don't add metal based tubes and the world will be happy 

[PRR]

Wouldn't 6550 sound good in that amp?

[sluckey]

CAUTION! DO NOT USE EL34S

Write it on the chassis next to the power tubes. 

[PharmRock]

CAUTION! DO NOT USE EL34S

Write it on the chassis next to the power tubes. 

I could definitely see this being a problem if I were to ever sell the amp.  Later JTMs used EL34s, and someone might want to swap tubes for fun.  This might be the best reason to mount the grid stoppers on a nearby terminal strip or insulated turret, and tie pins 1 and 8 together to ground. 

[sluckey]

I'm very fond of Fender's method of mounting those resistors. Very nice and tidy. I even used it in my 6V6 Plexi. So did Hoffman. If some dumbass comes along after me and decides to plug in EL34s, well, let him deal with it.   

     http://sluckeyamps.com/6v6plexi/P-6V6_05_big.jpg

[PharmRock]

Good point Sluckey! 

I've frequented your website many times, and used your Plexi6V6 (and other projects) pictures and documents for ideas on how to lay out my own board for components and wiring. 

When I realized my board real estate was getting pretty crowded, the socket-mounted grid stoppers as you have them in the Plexi6V6 (and Phoenix) seemed liked the perfect solution.