PT rectifier centre as B+

[Philr228]

Hi all,

Looking for some help and assistance.

I'm building a 5E3 (yeah another one of us) and intended using my power transformer rectifier centre tap as my B+ supply. If I do this and want to try a plug in solid state rectifier instead of my 5Y3, I'm assuming that it won't work, is that right? My PT is a 300-0-300

Please all your help and comments would be appreciated.

Cheers guys

Phil

[txstrat]

I don't get what you mean. Could you post a sketch or schematic?
There are ways to substitute a tube recto with a diode rectifier such as the Weber copper cap.

[Philr228]

Sorry I haven't got a schematic. I wanted to use the rectifier CT for the B+ to the OT and first filter cap. That will work, but if I then plug in one of the Sovtek plug in solid state rectifiers will that be wired for the heaters and hence the rectifier CT give no output - that's a mouthful I understand if that's not clear again!!

Phil

[txstrat]

I think we use different terms (abbreviations) for things.
The rectifier has no Center Tap (CT), the Power Transformer (PT) has. The CT of the PT goes to ground. The other ends of this winding go to the rectifier tubes anodes.
The 5v winding (no CT) feeds the cathodes/filament of the recto tube and serves as your B+ (only with a rectifier tube plugged in).
Heres a schematic:
http://www.schematicheaven.com/fende..._5e3_schem.pdf
I don't see a way (other than sort of a copper cap) to plug a diode rectifier into the tube socket of the rectifier tube.

[MWJB]

Do you mean that you have a Hammond PT and will be connecting the rectifier heater CT to the B+?

You could try GZ34 or 5V4 rectifiers and see how you go with those before the SS?

Alternatively use a regular BF Deluxe PT & a 330ohm cathode resistor.

Converting to fixed bias (or switching between cathode & fixed) might be a better way to tighten up the sound with an otherwise stock amp?

[Philr228]

Thanks txstrat,

My power transformer has a rectifier supply (5v) centre tap and I'm told this can be used as my B+ rail and is better than off the rectifier. Just wanted to know, will these plug in rectifiers work wire as I suggest?

[Philr228]

That's it MWJB,

Yeah I had thought about alternative valve/tube rectifiers just was considering the plug-in solid state types before I wire it up!

Phil

[txstrat]

I don't know the exact circuit in these but they simulate a rectifier tube.
You could also wire your tube socket up with diodes/resistor in parallel to the tube. That would leave you with a diode rectifier when when the tube is pulled and a mix of both when the tube is in. Don't have the schematic around.

[Philr228]

I'd love to see that schematic if you can find it TXSTRAT or anyone else if they have it!

Thanks TXSTRAT and MWJB for you help with this so far, you guy's never fail!

Cheers

Phil

[Steve Conner]

Soldering diodes to the socket won't work. Because they rectify so much more efficiently than a tube, they'll always do all the work and your rectifier tube will just be there for looks.

All rectifier tubes (except for little dinky ones like the EZ80 and 6X5) have the heater pins connected to the cathodes. So, a Copper Cap will work. Depending on how it's wired, the B+ current might flow through only one side of your heater winding, but I don't see that being a problem. (it would also happen with a GZ34 or similar indirectly heated tube)

[Philr228]

Thanks for your help with this too Steve.

I think I'll probably wire for the B+ off the rectifier supply centre tap, start with a 5Y3GT with my Hammond 300-0-300 power transformer and see what the voltages are like as it's rated at 100mA. Then if I fancy a change and maybe increasing the B+ I'll try a different valve rectifier as indicated by MWJB.

Thanks again for all your help guys.

Phil

[txstrat]

Originally Posted by Steve Conner

Soldering diodes to the socket won't work. Because they rectify so much more efficiently than a tube, they'll always do all the work and your rectifier tube will just be there for looks.

When you put a resistor (sort of 250 ohms @ 10w) in series after the diodes, woudn't that share the load between the diodes and the tube?
I'm asking, cause I read about this sort of mixed rectifier in Gerald Weber's book.

[Philr228]

Additionally to this would using the rectifier supply CT as the B+ on a 300-0-300V output power transformer mean that you actual get nearer 302.5V?

Sorry for so many questions, it just occurred to me?

Phil

[MWJB]

It's not really relevant, whether it does or not. Once that 300, or 302.5VAC is rectified, you then get a multiplication factor for the dc produced, based on the rectifier type...

5Y3 = 1.1
5V4 = 1.26
5AR4/GZ34 =1.3
SS = 1.41

...then you lose whatever voltage drop is caused by the current load on the B+ & 6.3VAC heater supply. As a 5E3 is cathode biased and runs relatively high idle currents, compared to fixed bias, you probably won't get voltages as high as the multiplier suggests, and +/-2.5VAC is such a small increment in the grand scheme of things it's not worth thinking about (your wall AC probably fluctuates by more than that).

[Steve Conner]

It depends on which way round you wire it, one way it'll add to the B+, the other way it'll subtract. Probably not as much as 2.5V either, because the voltage is distributed along the length of the filament. I bet you wouldn't notice the difference, either way.

The rectified B+ is 1.4 times the transformer AC voltage with silicon diodes, so 420V. A tube rectifier knocks a fair amount off that, depending on how much current the load draws. It could end up around 350-370V. But not 302.5...

[pdf64]

Are you saying that your PT 5V winding for the rectifier heater has a centre tap (ie so it's 2.5-0-2.5)?
If so just leave the CT not connected, ie sleeved off.
If your B+ is connected to pin 8 of the rectifier, then there's no interaction between the rectifier heater 5V, and the B+. The B+ just acts as a dc reference for the 5Vac.
But if the B+ is taken from pin 2, then the 5V would be a ripple on the B+ (because the rectified B+ current would be flowing along the heater).
Peter.

[Steve Conner]

But that's the whole point of a directly heated rectifier. The heater is also the cathode, so the HT current is emitted by it, and hence flows in it alongside the filament current.

So the centre tap on the heater winding helps directly heated rectifiers run completely balanced. Without the center tap, one side of the rectifier works a little harder than the other.

With indirectly heated rectifiers like the GZ34 it does the opposite, unbalancing them.

In all cases the difference is probably hardly worth bothering about, but this is the internet where we take trivia to new levels of pedantry.

[Philr228]

Thanks Steve and MWJB with this one. I think the answer is yes then but, and I realised this the extra voltage is pretty insignificant.

From what you all indicate I think going for a 5V4 instead of a 5Y3 and using my rectifier supply centre tap ( I like balance - always have done!) is the way to go - but of course I could change my mind, 'specially if you suggest something else, hehe!

Thanks again.

Phil

[tubeswell]

For directly heated rectifiers, running the B+ off the CT of the 5V winding (if it has one) is supposedly quieter, but I've never been sufficiently annoyed to be bothered finding out. FWIW Merlin B has a schematic at the bottom of this page

The Valve Wizard

[pdf64]

'But that's the whole point of a directly heated rectifier. The heater is also the cathode, so the HT current is emitted by it, and hence flows in it alongside the filament current.'

I don't see how the HT current should flow alongside the heater current.
The HT and heater both happen to share pin 8, but there's no circuit between them.
Because the heater current flows between the cathode (pin 8) and pin 2. And the HT current flows from the cathode. The HT will only get 'corrupted' by the rectifier heater supply if the HT is taken from pin 2, the other end of the heater.
I acknowledge that my analysis seems to differ from everyone else's, but I can't see where I'm going wrong.
A 5V4 will give a 'harder' tone with more power output, than a (NOS) 5Y3.
Peter.

[Steve Conner]

Originally Posted by pdf64

I don't see how the HT current should flow alongside the heater current.
The HT and heater both happen to share pin 8, but there's no circuit between them.

That would be true in an indirectly heated rectifier. But in a directly heated tube, both of the filament pins are also cathode connections. The electrons pushed through the filament to heat it are the same ones that boil off it to produce the HT current, and they come from whichever filament pin has the most favourable voltage at that instant in time.

[pdf64]

Thanks for your patience - I've now got my head around directly heated cathodes, ie the cathode is the heater. Peter.

[Philr228]

Well, certainly a lot info buzzing about, I may give a Sovtek 5Y3 not a NOS 5Y3 a go once I've measured the output from the transformer and wire the B+ from the centre tap of the rectifier supply.

Thanks again for all the discussion and help.

Phil