Laney Klipp 60

[EstynEstyn]

My Klipp is giving me odd readings.

I've made a few modifications to it (here's the schematic
http://www.prowessamplifiers.com/sch...s/klipp_60.jpg)

The modifications I've made are
- Moved the bias supply to the otherside of the switch
- Individual Cathode resistors
- Upped the bias cap to 47uf
- Upped the Grid Resistors to 1k
- Installed a 100k trimmer in place of the 56k Bias Resistor

My problem now though, is that I'm getting different readings off the Cathode resistors when trying to bias it. One side is reading around double the mv of the other. Any ideas?

Thank you kindly.

[Alex R]

If you mean the individual cathode resistors you've added to the power tubes, then it isn't so unusual to see one tube drawing twice the current of the other at idle. They're not matched, is all. it will work ok so long as it doesn't hum too much for you.

What value cathode resistors did you fit?

[EstynEstyn]

Hmm... I've used 10r Cathode resistors and a matched pair of JJ KT77s. I'm measuring 0.027 over one and 0.058 over the other, with a plate B+ of 603v, that makes an Idle Plate Dis. of 16.2w and 34.9w. Does that not make one of then idling way too hot?

[Alex R]

Yes 34W is too hot. 10W isn't necessarily too cool... but if they are a matched pair then that is odd. Check plate, screen and grid voltages on each tube next: pins 3, 4 and 5.

[Gtr_tech]

I'd swap the output tubes and see if the one that was passing too much current continues to behave this way in the other socket. Don't keep it on long....just long enough to get a reading. At that current the tube will be toast in no time.

Maybe before that verify that the bias feed resistors are ok and make sure the solder connections are not cracked loose. Have a look at the sockets themselves as well. Clean the contacts with a toothpick (break off the pointed end) and some contact cleaner solvent. Pay special attention to pin 5....make sure its tensioned enough to grab the tube pin securely.

[EstynEstyn]

Tube 1 measures...

Plate @ 605v
Screen @ 596v
Grid @ -64v

Tube 2 measures...

Plate @ 610v
Screen @ 601v
Grid @ -64v

Tubes were matched at...

Plate Current @ 90
Transconductance @ 10

I've also been told by my mate (the owner of the amp), that he's lost 2 tubes in the past 3 months in the hot socket.

[EstynEstyn]

So, I tried switching tubes and it does change with the tube. Matched Eh? What can I do now? Maybe install a Bias Balance Trim?

(Also, I think the 2 previous Kaput tubes were down to them not handling the high B+, the JJs have a max. of 800v so should be fine this time.)

[Gtr_tech]

Sounds like a tube gave it up or its *really* out of match. 16w static diss. is just fine for 34s at that plate voltage. I've used the EH EL34s in high plate v amps and they seem happy to be there. Screen voltage seems a bit high tho. 34s don't need a high screen voltage to wake them up like a 6L6 or similar beam tube does. And...the datasheet for a Mullard 34 specs the screen voltage max at 500v. So....maybe think about dropping the screen voltage a couple hundred volts to keep things under control. I bet that thing was a *real* tube eater with 470s on the screens and the elevated voltages. Even with good quality tubes back then....wow.....

[EstynEstyn]

Cheers for the reply. The tubes I measured those figures with are brand new "matched" JJ KT77s. Looking at the spec sheet (http://www.jj-electronic.com/pdf/KT77.pdf) they should take the thrashing, but obviously they're not running as efficient as I'd like. Would it be worth installing individual bias trims? And if so, what values should I use? Maybe like a 100k Trim in Parallel with a 2.2uf just before the 220k, on each side?

[EstynEstyn]

Here's an updated schematic of the changes I've already made, and I've included the individual bias setting I described. Does it make sense? Do I need to do something about 56k Bias resistor? Thanks.

http://i55.tinypic.com/15ph995.jpg

[Steve Conner]

Are you in the UK? Who did you buy the tubes from? I've seen a few threads here concerning people who bought tubes from one UK tube vendor that were supposedly matched, but turned out to be completely unmatched.

[EstynEstyn]

Hi Steve. Tubes are from HotRox.

[Steve Conner]

Well, see what you think of this: http://music-electronics-forum.com/t17462/

(Disclaimer: I personally bought a matched pair of tubes from Hot Rox once, and they were properly matched, but one swallow doesn't make a summer)

[Alex R]

Hot Rox usually are ok for me too. Maybe they'll change them if you explain?

[Sweetfinger]

I bet that thing was a *real* tube eater with 470s on the screens and the elevated voltages. Even with good quality tubes back then....wow.....

I've got two Klipp 100s. IIRC they are a zappin' 630 ish. You can almost hear the tubes scream in agony. I need to do some mods to both in order to get them to run without exploding but they've been way, way back on the back-burner. The cobbler always has the worst shoes in town........