36 watt lite issues

[phsyconoodler]

I've built quite a few 18 watt lite's but this is my first 36 watt version and I'm having issues.
first question:do you have to use extra grid stopper resistors or will the 8.2k work for a pair of EL84's?
Also do you need to use extra screen resistors for each EL84 or will a single 1k service all 4 tubes?
It squeals and oscillates as is and turning the tone control induces a nasty oscillation.
I don't think it's lead dress,as I've built quite a few amps.I THINK it may be the grid stoppers,or lack of in my case.I've tried reversing plate leads.Nada.
Never had a tone knob cause an oscillation!
Any insights would be appreciated!

[snoof]

Gotta have the grid stoppers. One thing to try is mounting the 8.2k-10k grid stoppers (one for each tube)right on the tube socket. That might stop the oscillation. So far, every time i've had oscillation it was remedied by either swapping the OT primary (even on non-NFB amps) or lead dress.

Max screen current is 11ma per tube, so total of 44ma. So if my math is right, you'd drop 66 volts (300 volts supply) across one 1.5k screen resistor. That voltage drop calls for 2.9 watt resistor, so 6 watts for safety. Looks like you'd have to use a 10watt resistor. Using one 1.5k for each pair, that's a 33volt drop, .7watt resistor, so 2 watt would work for each two tubes. That solution looks better! But it also depends on how much screen voltage you wanna drop.

[zaphod_phil]

Using one 1.5k for each pair, that's a 33volt drop, .7watt resistor, so 2 watt would work for each two tubes. That solution looks better!

An additional 100 ohm screen resistor for each tube, soldered directly onto the socket pins will also help prevent oscillation. So for each PP pair of EL84s you would have a single 1.5k (or even 1k) followed by a pair of 100 ohms connected to the socket pins.

[phsyconoodler]

thanks guys,I'll give it a try tonight.

[snoof]

Interesting Phil, I didn't know the screens could contribute to oscillation. Learn something new everyday i guess!

[phsyconoodler]

All problems solved.Thanks again guys!

Now to get the feedback to actually do something.....it doesn't seem to work worth a fart.
Hmmm........

[phsyconoodler]

Duh! I guess actually hooking it up to the output jack would work!
Me dumb!

[phsyconoodler]

Ok,this freakin' amp is LOUD!
I know it's only 3db louder than an 18 watt,but my ears are ringing!
I should have done a 36 watt a long time ago.
Now,I have some hash on the bass notes when cranking it a bit that are kind of funny.It sounds like a sqwack of some sort.Not a cold solder joint kind of noise,but still some kind of oscillating type noise.
I didn't put the 100 ohm resistors on the screens yet,and i'm hoping they are the issue with this noise.
I used a Hammond AC30 OT and it is fantastic sounding!
What a nice amp!This amp can easily keep up to any 50 watt Marshall and has way more touch sensitivity.I'm using a 5AR4 rectifier tube and the B+ is 359v with 325v on the screens.I'm thinking those 100 ohm resistors are the key to getting a noise-free crunch,but we'll see.
Thanks again to the guys for the steer in the right direction.

[snoof]

just a thought, in addition to looking at oscillation, You might try shaving a bit of low end off by lowering a coupling cap early in the crkt. That might clean up the bass distortion a bit.

[zaphod_phil]

Also worth trying the huge cap trick - a 1000uF or bigger cap at the cathodes of each PP pair.

If the problem still persists after that you might also need to increase the size of your filter caps. Hopefully not. You're allowed to have up to 60uF for the first filter cap, after a 5AR4.

[malt]

Ok,this freakin' amp is LOUD!
I know it's only 3db louder than an 18 watt,but my ears are ringing!

My ears are ringing to just finished mine and it's LOUD as H.ll . hoked mine up to a 4x12 marshall cab not good for my tinnitus.

[phsyconoodler]

It definitely doesn't have too much bass.It's full and round and the top end is lovely.
I will try the Zaphod 1000uf bypass cap.It has helped me with other EL84 based amps.
I have 22uf there now.a pair.I have my tubes idling at a conservative 11 watts right now and they seem just fine.I'm using JJ's and Tung-Sol preamp tubes.
With this amp a single tone control is enough to cover any guitar from a bright tele to a dark Les Paul easily.
What a great,simple circuit.I like the negative feedback and the paralleled input.The lo input is very usefull as well.All in all,a versatile amplifier.
Now to build one for ME.

[snoof]

a cut control also works really well in this crkt. it's hard to explain, but it works on a bit diff freqs than the tone control.

[phsyconoodler]

ok,I tried the 1000uf caps and they are nice,but the squawk was still there,so I added 100 ohm 5 watt resistors on each tube screen,pin 9.The squawk is totally gone and it's marshall grind at it's sweetest.
Nice call Zaphod!
Sometimes it takes a little tweak here and there to tame the beast.
And believe me,this is a little beast!