5 Why? 3

[jeff]

 I recently wrote about a Champ with too high voltages. I was getting about 420V B+. Now that I think of it maybe the 5Y3 is to blame. I used a Sovtek 5Y3 at first and my voltages were about 460V. After reading up I found that Sovtek rebrands other rectifier tubes as 5Y3's. So I bought a NOS RCA 5Y3GT 5Y3WGTA. This brought the voltage down to 420V. There is a schematic that has a Champ running at 420V but I just noticed its transformers AC is 365-0-365.

I'm using a 330-0-330 transformer. In the schematic using a 320-0-320 transformer with a 5Y3 the B+ is 360V.

Are some brands of 5Y3's more efficent than others?
What's the least efficent?
What brand did Fender use originally?

[phsyconoodler]

It's likely you aren't using the right power transformer.An NOS 5Y3 is not much different from brand to brand.I have used a great many NOS ones in many brands and have not noticed much difference at all.The Sovtek is not a real 5Y3 and the voltages are indeed about 40v higher.

[jeff]

 The amp is schematically the same as the one using a 365-0-365 PT but I'm getting the same B+ using a 330-0-330.

Is there a difference in 330-0-330 transformers? For example can two different transformers with 330-0-330 give different voltages in the same circuit?

What brand of 5Y3 has worked best for you?

[mresistor]

What brand PT? Is it a Hammond rated for 115V? If so and your wall voltage is 120V you're going to have higher than rated output voltages.

[jeff]

 Well I'm reading ~330VAC from the red wires on my meter when its running.

Actually it's a 5Y3WGTA and it's got 5 pins.

[stingray_65]

what may be happening Jeff is that your transformer may have a higher current rating.

Your B+ rail will drop under load.

So if the original PT is rated at 365V but lower current  rating, it will drop more underload than your 330V transformer with a higher current.

Just a guess at this point without knowing more details

Ray

[OldHouseScott]

Wall voltages have crept up over the years. Back in the tweed days, 110 vac was the expected norm. Fast forward to today, and you might see 120 to 130 vac for a nominally rated 117 vac circuit. If your transformer is rated for a 110 vac primary, you'll see an increase on the secondary in the amount of the difference in wall voltage times the step-up ratio. I believe Hammond has begun to address this with a line of transformers with primaries rated for today's higher wall voltages.

[jeff]

what may be happening Jeff is that your transformer may have a higher current rating.

Your B+ rail will drop under load.

So if the original PT is rated at 365V but lower current  rating, it will drop more underload than your 330V transformer with a higher current.

Just a guess at this point without knowing more details

Ray

OK I didn't know that thanks

[jeff]

Is the 5Y3WGTA different then a 5Y3? I can't find the specs on http://www.mif.pg.gda.pl/homepages/frank/vs.html

[phsyconoodler]

No.

[stingray_65]

Jeff,

Do a simple experiment,

measure your B+ at the first filter cap with your tubes plugged in.

then unplug your tubes and measure there and see the difference for yourself. (leave the 5Y3 in)

this is most noticeable in class A amps like your champ that draw maximum current all the time regardless of what your input is

Ray

PS always unplug your amp and when you swap tubes in and out, do not "Hot swap" tubes, they don't like that.

[jeff]

Is this just to show me there is a drop when current is being drawn?

 I'm not so sure I wan't to do that. I assume that with no load the B+ will be 1.4 times the AC or 462V, my caps are only rated at 450V. But this is OK because the 5Y3 needs time to warm up and by that time the power tube warms up and draws current and never reaches that voltage.

I'm more concerned with if the 5Y3GT Fender used drops more than a NOS RCA 5Y3WGTA. The RCA dropped more than the Sovtek I'm wondering if there is a 5Y3GT that will drop more than a RCA 5Y3WGTA.

[tubeswell]

You can drop the B+ with zeners. If you use 5W axial zeners, you can put them in series (cathode/banded end pointing to ground) - mount them on a tag strip away from the chassis where they won't pick up excessive heat. 2CW

[jeff]

We can totally disregaurd wheather my transformer was meant to run on 110V 115V or 120V. Whatever it is doesn't matter. What matters is that I'm seeing 330VAC on the secondaries. Yes if I ran it on 110V it would be less but let's work with what I got.

Looking at the Fender schematics, a transformer with 320VAC yeilds 360VDC and 365VAC yeilds 420VDC. DC equals about 1.12 to 1.15 times AC respectively. Using 330VAC I would expect to see ~375VDC.

I'm using 330VAC and getting 420VDC which is about 1.27 times AC. It just seems to me that the WGTA is more efficent that the GT.

[sluckey]

I'm using 330VAC and getting 420VDC which is about 1.27 times AC. It just seems to me that the WGTA is more efficent that the GT.

I still think the main reason your B+ is high is because your PT is rated for more current than the old PTs that Leo used in those cheap amps. Is your PT physically larger than the original PT?

Higher current rating = lower internal PT losses (resistance) = less internal voltage drop = higher B+. I don't think WGTA vs. GT is much of a factor when talking about NOS.

[prs_matt]

In all my experience with rectifier tubes, the 5Y3 yields secondary PT voltage * 1.2-1.3 while 5U4 yields 1.3-1.4 depending on the tube. SS yields 1.4. So... it looks like all voltages are to be expected from your PT.

The tubes will load the power supply and drop the supply voltage a bit so you will see a drop.

Just a note and I am sorry if I somehow missed this bit of info. Have you checked your wall voltage?

[phsyconoodler]

that's with new production 5Y3's.The NOS versions are closer to 1.09-1.1 ratio drop.
  A 5AR4 is 1.3 and diodes are 1.4.

Never yet seen any tube come close to diodes.
5y3-1.09
5U4-1.2-1.25
5Ar4-1.3
Diodes-1.414
 I agree with Matt about the voltage coming from your power transformer.The only way to get that high a voltage at the plate is to start with high primary voltage.

[dynaman1]

I recently wrote about a Champ with too high voltages. I was getting about 420V B+. Now that I think of it maybe the 5Y3 is to blame. I used a Sovtek 5Y3 at first and my voltages were about 460V. After reading up I found that Sovtek rebrands other rectifier tubes as 5Y3's. So I bought a NOS RCA 5Y3GT 5Y3WGTA. This brought the voltage down to 420V. There is a schematic that has a Champ running at 420V but I just noticed its transformers AC is 365-0-365.

I'm using a 330-0-330 transformer. In the schematic using a 320-0-320 transformer with a 5Y3 the B+ is 360V.

Are some brands of 5Y3's more efficent than others?
What's the least efficent?
What brand did Fender use originally?

Yeah, I had the same experience using a Sovtek 5Y3. I was a little annoyed, as I specifically chose the 5Y3 to drop DC voltage. Vintage 5Y3 did the job.

[jeff]

I'm so sorry. I'm such a dope. I was working on this amp and saw my RCA5Y3WGTA on the table next to me. I checked again and somehow I swapped it out. I was reading voltages with the wrong tube in.

Thousand appologies. Sorry Sorry Sorry
       Jeff.