Screen grid resistors

[Bob M.]

I have a  60's 4x KT 88 Sunn head. It's a 100S but it bears a great schematic resemblance to a 2000S (and not to the 100S schematics I've seen). I use it as a bass amp and utility amp (I'm going for a clean and rock solid sound; not a distortion amp). I'm updating the amp, as it hasn't been serviced in about 6 or 7 years (it doesn't get alot of use; only occasional use). It has an output transformer with the ultralinear taps (like most Sunn amps). Like the 2000S (and many other Sunn amps), it has 47 ohm/5 watt resistors in series between the plates and the output transformer primary and it has 47 ohm/2 watt resistors between the screens and the UL taps. The 5 watt plate resistors measure and look fine but a couple of the 2 watt screen resistors have drifted in value. I want to replace these but I have a few questions first. Most screen resistors on tube amps are usually a much larger value (and a higher wattage, as well). On the later model, the Sunn Model T, which has a power amp section and UL taps that are very similar to my amp, the plates have the same 47 ohm/5 watt series resistors but the screen resistors have been updated to 1.5K/5 watt devices. I'm wondering if I should do the same to my amp, or stick with the 47 ohm/2 watt resistors? How about it all you Sunn amp tech? Do you think that they changed screen resistors on the Model T for better stability and reliability and to keep that screen voltage farther below the plate voltage? Why did they change the screen resistor value and voltage?
All comments welcome but I'd especially like to hear from the techs out there. Thanks!

[Greg Z]

Bob
I've been wondering the same thing.
 I have a '70 1200s which right now has svetlana 6550c's in it.
I'm getting ready to retube,and was going to go to the EI-yugo KT-90's.
Last time I was in there I noticed like you that the screen resistors had drifted
a little. Perhaps someone will shed some light on this.

Greg z
to thine own sound be true

[Bill Matthews]

Don't worry about the little bit of drifting of resistor values.. that is what is supposed to give some of the vintage amplifiers that " Magic ".. Anyway, if you are going to retube the 1200S, and similarly equipped KT88 amps,  step up to the plate and find yourself the Genelecs that came in it when it was new.. YES it'll be PFE ( pretty freakin expensive ) but when you hear the tone..you won't believe it..    PS my 67 1200S ( Dynaco Power and output trannys, and built before they put reverb and tremelo in ) still has EVERY original component in it.. including the can cap..  Good luck, and if you need any help finding the parts.. drop me a line.. Bill

[GrannyGremlin]

One of the screen resistors in my 1200s cracked all the way through so I need to replace it.  I am being given advise to up it to 1k 5W.  The 5W seems like a no brainer good idea, but is Bill saying to leave it at 47R vs upping to 1K?  Why - that seems super low compared to just about any other amp out there?

[GrannyGremlin]

Heh, I seem to have the answer; in response to that 1K recommendation there's another post (on another forum):

"This is an ultra linear amp, rather unique with the plate stoppers which don't serve the traditional purpose that preamp plate resistors do. As well, the 47R resistors on the screens are an added safety feature as well, but again, they don't do the same thing that screen resistors do in pentode output stages. I wouldn't change the values. 1K will strangle the amp and make for thin, lifeless tone.  … Most ultra linear amps don’t have screen resistors at all.  If they’re there, it’s not to limit current like pentode mode but rather to control oscillations and as a backup safety “fuse”.  The tap, usually 43%, on the output transformer limits current and reduces output impedance and distortion as well as giving a slight increase in power at the optimum tapping point, which varies with different tubes.  Suggesting that higher value resistors would make the amp more controlled and useable nullifies the original designers’ knowledge and intent. "

Hope this is useful to others.  I keep forgetting that 6550/KT-88s are tetrodes not pentodes.

[Soundmasterg]

Hope this is useful to others.  I keep forgetting that 6550/KT-88s are tetrodes not pentodes.

The 6550 and KT88 are listed as both beam power pentodes and beam power tetrodes on datasheets, old and new, depending on where you look...see the links below. Tube manufacturers played loose and fast with those terms from the beginning. Both the 6550 and KT88 are similar to the 6L6 in style of construction. That was the first power tube that used beam forming plates instead of another grid like a pentode. They are more efficient than a true pentode.

The info you said about the screen resistor replacement is spot on...a high value resistor in either the screen or plate circuit with restrict the power and tone of the amp and is not advised.

Greg

http://www.svetlana.com/pdf/genalex/kt88-genalex.pdf

https://frank.pocnet.net/sheets/164/k/KT88spec.pdf

http://www.tungsol.com/tungsol/specs/6550-tung-sol.pdf

http://tubedata.tigahost.com/tubedata/sheets/127/6/6550.pdf

http://www.svetlana.com/pdf/svetlana/6550c-svetlana.pdf

http://www.tubeampdoctor.com/images/File/Specifiaction%206550%20JJ.pdf

[GrannyGremlin]

Cool.  I've already done it, and that got rid of the problem I was hearing (pops on transients - the resistor would vibrate open for a microsecond on the low notes; it litterally sounded like turning your chain off in the wrong order).... but now I know why the resistor died in the first place.  So where I say above to upgrade to 5W (originals are 2W), don't - Sunn used 5W wirewound Ohmites on plate and shitty 2W carbons on screen for a reason (other than cost savings), mentionned in the bit I quoted above; it's an additional safety fuse.

After putting in 2W metal resistors the amp runs great .... for about an hour to an hour and a half.  After that point it gets insanely hot (power tubes and power tranny mostly) and there is a weird fuzzy drone in parallel with the signal and seems like a loss of volume/headroom too.  Basically that shitty screen resistor did it's job and failed to save the power tranny (and whatever else).  Not sure what this problem is, but it appears the power section is drawing too much current.  Took it to the tech as this is a bit above my head.

[Soundmasterg]

You likely have a power tube problem, a filter capacitor problem, or perhaps an arced tube socket. A good tech should be able to spot these issues and get the amp working again in no time. Hopefully your tech is good.

Greg

[GrannyGremlin]

Tested the tubes (as well as swapped out the only iffy one of the quad), so not that, but yeah - he'll find it. Thanks.