Issue with Kt66 close to speaker magnet.

[FranciscoPerez]

Hey guys, I built a KT66 single ended amp which recently started squealing and oscillating badly. Everything started after a friend installed the amp inside a 1x12 combo cabinet. The KT66 hangs upside down close to the speaker magnet, which makes me think of that being the reason why it's oscillating. Could this be possible???


The amp worked great several months until it was loaded into the cab.
I don't have the amp here to unload the chassis from the cab and check if something else is going on, but would like to check with you guys if placing a tube close to a speaker magnet could be an issue.


Asked my friend to do some basic troubleshooting, and removing the power tube stops the oscillation. With the tube in, it squeals regardless of the volume and master settings. The amp is basically a champ using a 6sl7 + Kt66




Thanks!!
Fran

[PRR]

I would suspect mechanical vibration more than "magnet".

[FranciscoPerez]

Thanks PRR,
The amp squeals with no guitar plugged in. I'm taking a look at the amp today, let's see what I find !

Cheers
Fran

[FranciscoPerez]

Seems like the KT66 got hit during the chassis installation into the cab. I replaced the tube with a 6l6GC and the amp is quiet again.
   Too bad, because it was an old Marconi Kt66 which tested new, and seemed worth a couple hundred dollars on eBay.
What brand/ tube would you recommend as a replacement for the broken Kt66, that won't break the bank?

[smacbride]

I've been running the Shuguang kt-66 in my JTM-45 build and I've been very happy with the sound.

[eleventeen]

Having rare, fetish-grade tubes freaks me out...I bought an organ amplifier with a rare metal-base Amperex GZ34 and I want to get it out for sale on ebay ASAP. I bought the amp mostly for that one tube. I have seen those silly tubes for for well over $200 and if you look at "completed auctions" that was/is not a one-time event.

[FranciscoPerez]

Yeah...Agreed. I got it for free as a throw in from a local guy long time ago. would've loved to put it on eBay sometime though, and get some extra cash. Wish I had done that by now. :)

Thanks for your suggestion smacbride!

[kagliostro]

KT66 is octal, if it was a noval tube to be replaced for that problem

a good candidate will be the 6P14P-K (6П14П-K)

that is specifically buil to resist to high shock, so perfect for combo amps

but I don't know if exist an octal tube specifically planned for high shock resistance

Franco

[FranciscoPerez]

Thanks Kagliostro,


I'll have a look at those russian tubes!! The 6P14P-K is an el84 equivalent, is that right?

[kagliostro]

Yes they are

http://tubes-store.com/product_info.php?currency=EUR&products_id=49

Franco

[Stevewdewitt]

FWIW, JJ makes a very rugged kt66. I have used them and they hold up well

[FranciscoPerez]

 

[HotBluePlates]

Hey guys, I built a KT66 single ended amp which recently started squealing and oscillating badly. ...

... I replaced the tube with a 6l6GC and the amp is quiet again. ...

I once tried some modern KT66's in a 6L6 amp I have, and had occasional squealing.  I was able to resolve the problem with larger grid stoppers.  For whatever reason, they seemed more prone to oscillating than 6L6-type tubes.

[FranciscoPerez]

Thanks HBP, I'll give it a try. Right now the Kt66 grid stopper is 47K

[alerich]

Having rare, fetish-grade tubes freaks me out...

Not me. I bought a used Peavey Triumph 120 combo on CL for $125. The thing weighs a metric ton and even with some mods it still sounds like dookie... but hiding inside was a quad of Peavey branded Sylvania 6L6GC STR-387 power tubes. No eBay for me. I sandbagged them even though most of my amps are EL34 based. Installed them in the Soldano Hot Rod 50 I eventually bought. Jeepers, they sound good.

[eleventeen]

I did precisely that same exercise with a Peavey MX head (the road-case one) that was for sale locally non-working for $50. And it had exactly those same Syl STR-387 tubes you describe, and they were in top condition. The amp had blown its heater fuse so it was a classic 50 cent fix. Not that I have any use for a flaming loud 100 watt head, but I could not resist the price. I myself don't consider those tubes to be "fetish" tubes like Amperex metal-based GZ34s or Gold Lion KT66 or real Genalex KT66...those are tubes that folks will pay $100 or even $200 for. I also notice that even RCA 6L6GC have softened in price. I have always liked Sylvania tubes and have always had good luck with them. If I can buy a used piece of gear with 6L6GB/GC tubes where the price of the piece is $10-15 per 6L6, count me in. No hesitation. Not metal ones.


Those Peavey Triumphs, if not the best sounding, are rather decent amps and are often sold pretty cheap, well under the parts cost. Well worth looking for. Built like tanks, too.   

[HotBluePlates]

Thanks HBP, I'll give it a try. Right now the Kt66 grid stopper is 47K

I think I also had to add a screen grid stopper right at the tube socket (if you are not already mounting screen resistors there).

[FranciscoPerez]

Using 1K here, right at the tube socket. (actually two 2K resistor wired in parallel)
First data sheet I found using google (https://frank.pocnet.net/sheets/086/k/KT66.pdf) states: 


100R - 300R screen resistor  and 10K to 50K grid resistor to prevent parasitic oscillation.
Do 1K and 47K make sense?


Thanks!

[HotBluePlates]

100R - 300R screen resistor  and 10K to 50K grid resistor to prevent parasitic oscillation.
Do 1K and 47K make sense?

Yes, those should be fine.

It's hard to know if this tube will have ringing due to oscillation or microphonics.

[FranciscoPerez]

Great!
I recently replaced the tube with both an el37 and a kt66 and problem solved. The amp works great now, it was definitely the tube.


Thank you all for your help! You're awesome 
A couple of pictures of the amp, hope you like it.

[kagliostro]

Nice Cabinet !


an EL37 tube ... WOW, that is Story 


Franco

[FranciscoPerez]

   glad you like it kagliostro! That el37 is driven really easy, love the way it sounds!!


Cheers,
Fran

[Willabe]

Glad you found what was wrong. 

Amp looks great!
 

[p2pAmps]

Hey guys, I built a KT66 single ended amp which recently started squealing and oscillating badly. ...

... I replaced the tube with a 6l6GC and the amp is quiet again. ...

I once tried some modern KT66's in a 6L6 amp I have, and had occasional squealing.  I was able to resolve the problem with larger grid stoppers.  For whatever reason, they seemed more prone to oscillating than 6L6-type tubes.

Same here.  I moved the grid stopper on my KT66 Deluxe Reverb to 5.6K and all was WAY better.  1.5K was just to small in my case.