Lowerimg B+

[andrew]

I want to lower my B+ 40vts. What value power resistor would accomplish this? I have a 340vts, 150ma. PT. Thanks for any advise.

[Luthierwnc]

We'll need a bit more info:

rectifier tube (which one) or SS (full wave or bridge)
first filter cap stage values
is the 340 per side AC or 340VDC on the plates
fixed or cathode bias (and source of -volts if fixed)

Off the cuff I'd say use a reverse series zener on either the center tap or negative of the bridge. No use turning power into heat. Be prepared to change your bias resistors.

Skip

[Phil_S]

In general, it's not that simple. Some people use a Zener on the CT.

I saw an interesting design that uses an IRF840 or IRF820, here:
https://tubeamparchive.com/viewtopic.php?t=4798

Read here on the Zener trick. Note that is requires a heatsink.
http://www.aikenamps.com/BackBiasing.html

[andrew]

The " specs" on this PT is 340-0-340. What I want to do is get close to a 300-0-300 PT. Would a 5Y3 do what I am looking for. The first cap is 40uf.

[Cygnus X1]

I was entertaining a similar idea.
I built an SEL that uses a KT88 for a 15 watt output.
I'm happy, but it's still too loud for bedroom levels.

Well, it appears that by dropping the B+ would accomodate a 6V6 for a 5 watt. Plus some other minor changes, of course. There's also a 2 watt 6SN7 I'm looking at.

Mine is SS full wave bridge rectified.
Similar question, just wondering.

[dansamp]

I dropped the B+ on a PT by connecting one 39ohm resistor on each side of the ac wires going to my rectifier tube
it went from 345 volts to 330 volts
i was using a hammond transformer in an AC30 type amp and the B+ was a bit high for the EL84's

Dan........

[Andy Le Blanc]

that zener is used to regulate bias....... back bias....... raising the center tap
of a PT above ground V with a resistor will reduce the HT by the same amount
it depends on power suppy choice...... and the entire current of the supply
will be returning thru what ever you put there....

[Phil_S]

that zener is used to regulate bias....... back bias....... raising the center tap
of a PT above ground V with a resistor will reduce the HT by the same amount
it depends on power suppy choice...... and the entire current of the supply
will be returning thru what ever you put there....

Right you are, Andy. I referenced the wrong Aiken article.

This, I believe is the correct link:
http://members.optusnet.com.au/eyes.onl ... r%20B+.pdf

[Phil_S]

I dropped the B+ on a PT by connecting one 39ohm resistor on each side of the ac wires going to my rectifier tube
it went from 345 volts to 330 volts
i was using a hammond transformer in an AC30 type amp and the B+ was a bit high for the EL84's
Dan........

I suggest this is not the best way to do it, though each method has it's drawbacks. Have you got a laser temperature probe? I'd be curious to know just how hot your PT gets and how hot those resistors get.

[Phil_S]

The " specs" on this PT is 340-0-340. What I want to do is get close to a 300-0-300 PT. Would a 5Y3 do what I am looking for. The first cap is 40uf.

It depends...the 5Y3 might drop as much as 60V. This would be my first choice if the PT has a 5V 2A winding. I am thinking you will get close enough. Even if it drops the full 60V to 280, that is much closer to your targe than 340.

[dansamp]

I dropped the B+ on a PT by connecting one 39ohm resistor on each side of the ac wires going to my rectifier tube
it went from 345 volts to 330 volts
i was using a hammond transformer in an AC30 type amp and the B+ was a bit high for the EL84's
Dan........

I suggest this is not the best way to do it, though each method has it's drawbacks. Have you got a laser temperature probe? I'd be curious to know just how hot your PT gets and how hot those resistors get.

actually I did notice the PT got quite hot when I had one resistor after the rectifier (DC) it was a 10 watt resistor at a higher value
it didn't seem to get hot IIRC with the two resistor on either side of (AC)

not sure I understand why it would get hot either way ??

I know it is not the best way
I was just trying ways to lower the plate voltage since the logos on the JJ EL84's are pretty much cooked to a dark brown color

I did try using a zener on a different amp a while ago but that didn't really seem to drop the voltage much
on that amp I ended up using a choke and some capacitors
something I got from Torres amps a long time ago

Dan..

[skyboltone]

I'm going to jump in here and make a small point. There are a couple of posters here that really need to take the time to absorb some basics from any number of different texts or sources. An understanding of Ohms Law would be a good place to start.

If I had a 340-0-340 transformer and I wanted a 300-0-300 transformer I would buy a 300-0-300 transformer or select another amp design. We can apply the bandaids to these things all day but the results are usually less than pleasing.

If you insist, for sentimental reasons, on using this 340-0-340 transformer, the zener or mosfet trick on the center tap will work but you will need to look carefully at the bias of every tube. This in itself is a worthwhile learning experience I think. Zeners of sufficient power rating are as rare as hen's teeth. I posted a long while back, a Darlington power multiplyer for low cost 1 watt zeners, but on the whole, I think that the mosfet trick is a better implimentation.

No offense intended.
Dan

[Phil_S]

Dan you are absolutely right about this. For all the BS you go through to tweak B+, for not very much money, you can buy one of these: http://musicalpowersupplies.com/4.html or you might find some decent used iron for cheap here: http://www.oldradioparts.com/pg2a23.htm

We are aptly reminded, get the right tool for the job.

I've pondered this very question a number of times, but have never taken the approach of attempting to lower B+ with either a Zener or the Mosfet.

[jaysg]

Zeners of sufficient power rating are as rare as hen's teeth.

They are not hard to find. Here's a 45V 50W jobber...

http://www.mouser.com/Search/Refine.asp ... _Wildcards

They used to be cheaper...so goes the world. You have to understand that mouser stocks a pile of things that are not in the printed catalog. These are available in most values between 3.3V (NTE511AK) and 120V (NTE5158AK). I saw someone's implementation recently, with a ceramic cap across it. That's probably a good idea.

[skyboltone]

Thanks Jaysg: That is I think a factory drop ship setup. NTE is a great company that way. I think they make an equivalent to just about every oddball semiconductor ever made. Anybody need a germanium power transistor?