5E3 build voltage problems- zener?

[flatfive]

I just finished a 5e3 built from parts. The Mojotone PT seems to be putting out some big voltage. I'm getting 380v plate, and 22v across the 250 ohm cathode resistor. So, if I = E/R, that's like 45ma per 6v6 tube. Seems pretty high.

So I see that some people use a higher cathode resistor, like 300 ohms. Also, I see that there's a zener diode (50v?) that lowers the B+. So which is the best way to go? If the zener, how do I go about it, and which one do I use?

I'm using an NOS 5Y3 rectifier.

[lowell]

yeah that does seem very high... even JJ 6V6 could only take 25ma per tube at that voltage, if you're using the 70% of max dissipation rule. Those zeners are pretty expensive... try the geofex way which is to put a 45-50v low voltage zener in parallel w/ a power mosfet... much cheaper. see: http://www.geofex.com/Article_Folder...osfetfolly.htm
scroll down towards the bottom...

[Bruce / Mission Amps]

Which MOJO 6V6 PT is it? 761 series?
Most 6V6GTs are are rated at 14 watts these days and I think JJs might be closer to 15-16 watts.

Actually, 380v at the first filter cap with a 5Y3GT is not terribly high!
With these amps, for classic tone you want the power tubes running at around 90% of rating or more if you are brave.

I've worked on a number of vintage 5E3 amps with their old power tubes running at 14 to 16 watts each and the owners have told me they'd been using these amps like this for years.
Most of these amps just needed main filter and bypass caps.... because the power tubes checked out fine, the amps still sounded great and they were still making around 10-12 watts of clean output.
So,
With decent free air cooling, I think you should be able run a pair of JJ6V6s in cathode bias at around 12-14 watts each for a long time... I do.

However, if you want to lower the B+ rail a little, pick up some $1.00 five watt 10v zener diodes and use a one, two or three of them in series with the red/yellow center tap wire of the PT before it is grounded.... the zener diode's cathodes pointing to ground.
Each 10v zener will NOT drop 10v but it will drop enough with a few to get you a little lower at very low cost.
Once the B+ is such that your JJ6V6s are running a little cooler, and you are satisfied, I'd just leave it alone and play.

[flatfive]

Bruce:

I don't know what series the PT is. It has the two green filaments, a green/yellow, red/white (I guess it's the center tap for the 120v), a brown, a brown/white (paired together and twisted to the white wire from the power cord) a black, a black/white (paired together and twisted to the black wire from the power cord, and an orange wire. I'm not using the orange wire. I think this may for Europe.

I'm grounding the green/yellow, the red/white, and the green wire from the power cord all together.

So I guess I would find these zener diodes (where?) and put them between the end of the red/white wire and ground, right?

Thanks.

[macdillard]

What is wrong with 380 v. on a 6V6? You see 6V6's running at over 450 volt on the plates in other Fender circuits..Get the bias right with the cathode resistor and let her bump.

[Bruce / Mission Amps]

OK, you'll just have to take my word for it on this... I actually have some experience with these amps.
B+ of 360v to a max of 380v is right at the limit for these with a 250 to 270 ohm biasing resistor.
If you use a cathode biasing resistor that is more then 330 ohm (to an absolute max of 360 ohms) will just make the amp start sounding and feel completely different then the classic 5E3.
Yes, you can run very high B+ and use a high resistance biasing resistor to keep the tubes idling at or below their 14 watt DC rating....but it won't have the classic 5e3 sound.
The trick 5E3 set up is to bring the B+ down, use no more then 270 ohms and a pair of matched power tubes that are not overly soft, (that means wanting to draw high quiescent current with the same bias and B+).

[flatfive]

My big problem seems to be the 45ma.

[tubeswell]

My big problem seems to be the 45ma.

But if you lower the B+, the current drawn across the tube will decrease.

[MWJB]

"My big problem seems to be the 45ma." It's not certain that your tubes are drawing 45mA. This is not a particularly accurate way of measuring plate current, you haven't allowed for screen current, which could be 2 or 3 mA. If you really have <40mA of measured plate current then I wouldn't worry.

As Tubeswell & Bruce say, drop the B+ a few volts if you are really uncomfortable. I've seen 50mA from a stock '59 tweed Deluxe at rather more voltage than you have, previous owner ran it like that for years with no failures.

[Gee]

However, if you want to lower the B+ rail a little, pick up some $1.00 five watt 10v zener diodes and use a one, two or three of them in series with the red/yellow center tap wire of the PT before it is grounded.... the zener diode's cathodes pointing to ground.
Each 10v zener will NOT drop 10v but it will drop enough with a few to get you a little lower at very low cost.
Once the B+ is such that your JJ6V6s are running a little cooler, and you are satisfied, I'd just leave it alone and play.

Bruce is exactly right. I followed his advice and reduced my voltage from about 378V to 350V using a string of 5W zeners mounted on a short vintage tag board. They work flawlessly and I have used the amp for up to about 4 hours fully cranked over the last 15 months, without any heat issues. My local store only had 12V and 15V 5W zeners but with some trial and error I found the right number and values to drop the B+ to where I wanted it to be.

[macdillard]

Also if you are using a Sovtek 5Y3GT for the rectifier your voltages will be high. Change it out to a NOS and you will probably be right where you want to be.

[Bruce / Mission Amps]

Also if you are using a Sovtek 5Y3GT for the rectifier your voltages will be high. Change it out to a NOS and you will probably be right where you want to be.

He said in the very first post he is using a NOS 5Y3GT.

[flatfive]

So I put a string of 3 10v zeners with cathodes pointed to ground on the center tap. Now I have lowered the plates to about 360v, the current to 39ma. Subtracting 2ma for the screen, that gives me 13 or so watts of dissipation. Better, but I guess I should have used 5. It's hard to find room for all these diodes. I probably need a small board to mount them on.

[Gee]

So I put a string of 3 10v zeners with cathodes pointed to ground on the center tap. Now I have lowered the plates to about 360v, the current to 39ma. Subtracting 2ma for the screen, that gives me 13 or so watts of dissipation. Better, but I guess I should have used 5. It's hard to find room for all these diodes. I probably need a small board to mount them on.

I used 3 from memory, 2 15V and 1 12V which adds up to 42V but dropped about 28V. I used a 5 tag vintage strip with the center tag acting as the grounded mounting stud.