TERRIBLE Noise in new build (cathode biased 84's)

[jcat5503]

I recently finished an amp really similar to the Ceriatone Creme Brulee

Nik uses a 290-0-290 PT and a GZ34, and I used a 330-0-330 PT with a 5Y3

I had it up and running just fine and I noticed that the 3-way bright switch wasn't working so i flipped the chassis over to have a look and I noticied that I forgot the lead coming from the first lug of the volume to the center of the DPDT switch. I figured no biggie I'll fix it later, then I flipped the amp over and turned it bacl on and now it's making a really really bad racket. It sounds terrible, growls and pops and such

It's really loud and the pots don't have any affect on it. It makes the noise if a guitar is plugged in or not.

The tubes were glowing an awful lot when it was working. and every time I struck a chord they'd get a good bit brighter

Any thoughts? Maybe the higer B+ cooked the 84's? I figure I may need to increase my cathode resistor to around 220ohm? Versus the 120ohm where it is now

If i pull the PI it stops, but not if i pull the ef86 (V1)

Thanks everyone!

[Alexo]

Sounds like a bad solder joint after the EF86 and before or upon the pi. I'd poke around with the old chopstick until you find the joint that makes the noise.

...although those glowing power tubes sound pretty worrisome. Do you have a schematic?

[Structo]

What value is your cathode bias resistor?

[M Fowler]

What is your plate voltage using 330-0-330 into a 5Y3? Sounds to hot to me.

The other issues sounds like solder or wiring problem. You need to go back over the whole amp and make sure everything is wired as it should be.

No such thing as being to careful with building your amp.

[20to20]

Nik uses a 290-0-290 PT and a GZ34, and I used a 330-0-330 PT with a 5Y3

An amp designed to need a GZ34 cannot use a 5Y3. The current handling capacity of a 5Y3 is only half of a GZ34. Since your PT is sending higher B+ you are probably over driving the EL84s and overloading the 5Y3. I'm suprised your amp hasn't caught the garage on fire.

[Alexo]

Nik uses a 290-0-290 PT and a GZ34, and I used a 330-0-330 PT with a 5Y3

An amp designed to need a GZ34 cannot use a 5Y3. The current handling capacity of a 5Y3 is only half of a GZ34. Since your PT is sending higher B+ you are probably over driving the EL84s and overloading the 5Y3. I'm suprised you amp hasn't caught the garage on fire.

You can totally use a 5Y3 with a pair of EL84's. Maybe not in a GZ34-equipped Plexi, but just because an amp is using a GZ34 does not mean that it's drawing 125ma+ of current through it.

...although you don't want that first filter cap to be much beyond 20UF in a 5Y3 power supply.

But even with a 5Y3, a 330/0/330 PT is pretty hot for EL84's. Probably seeing something like 375 on the plates, depending on bias - hot but not insanely so, just above what you'd see in a 1974.

..Probably want to cool the screens off a little though - bet that's what's glowing under signal.

[jcat5503]

What value is your cathode bias resistor?

It's 120 ohm, I'm wondering if I need to increase it

I will check the plate here in a few

I just got a new set of 84's to see if the higher voltage killed them in a short amount of time and that's what is causing all the problems

[20to20]

You can totally use a 5Y3 with a pair of EL84's.

Of course, if that's it. Not familiar with the amp so as I said, if the amp was designed to "need" a GZ34...

Also since the voltage drop of a 5Y3 is about 40v more than the GZ34, there will be a lower rectified voltage to partially compensate for the higher PT secondary output. If the EL84s are biased OK for the plate voltage, the tubes shouldn't go red.

[20to20]

What value is your cathode bias resistor?

It's 120 ohm, I'm wondering if I need to increase it

I will check the plate here in a few

I just got a new set of 84's to see if the higher voltage killed them in a short amount of time and that's what is causing all the problems

I'm betting your plate voltage is 330v or higher. If so, then you need to get the plate dissipation down under 12W. That will be about 35ma. or less if you want full power Class A. If you want to run your PP in AB your going to need about 30ma. or less.

[labb]

If he is using a Sovtek 5Y3 I would bet that his plate voltage is close to 390 VDC.

[rp]

I learned to stop bringing amps up the first time with the spec'd CR. If it calls for a 120 I might use 180, and if 250 I might use a 360. Like setting the B- to max on a fixed bias amp. Once all the voltages look ok and it sounds ok then I work out the dissipation and try lower CRs and different tube sets. Best to avoid the freak out of seeing the plates glowing red.

[Colossal]

Very sensible RP!

I like to start with a base value for the CR, but add a mil-spec 2W 100R to 1k linear pot in series (letting the base resistor do most of the dissipation). Then do what you do; bring the amp up to see what the full B+ looks like, then adjust the pot to appropriate bias and measure the total resistance. Then when all is well will substitute in a fixed value for the CR, removing the pot/base resistor rig.

[Alexo]

I learned to stop bringing amps up the first time with the spec'd CR. If it calls for a 120 I might use 180, and if 250 I might use a 360. Like setting the B- to max on a fixed bias amp. Once all the voltages look ok and it sounds ok then I work out the dissipation and try lower CRs and different tube sets. Best to avoid the freak out of seeing the plates glowing red.

Heh a couple of weeks ago, I powered up an El34 amp with a pair of 6V6's, kinda haphazardly, figuring the voltages, resistances and load were close enough that it might fly.

Well, it sounded AMAZING - but after about 5 minutes of playing, I read out the bias - 21 watts per tube!!! No red plates though, gotta love those old RCA's.. Better to play it safe with oversized Rk's, but most tubes won't be damaged by overdissipating long enough to check the bias.

[M Fowler]

Here is the Creme brulee.

Pair of EL84s 290-0-290 with a 130R cathode bias resistor.

Might have to use a reverse Zener diode to reduce that secondary PT voltage.

[jcat5503]

Here is the Creme brulee.

Pair of EL84s 290-0-290 with a 130R cathode bias resistor.

Might have to use a reverse Zener diode to reduce that secondary PT voltage.

How would I do this?

Is there a possibility it's the recifier tube?

I just tried a brand new pair of 84's with no luck

Double checked all my connections and found no col solder joints...