tube rectifier

[dscottguitars]

Is there anyway to have a tube rectifier with a power transformer that does not have a center tap?  I found a PT with several different voltage taps that looks good for having a switchable high/low power amp but I don't know enough about rectifying to figure this out. 

[kagliostro]

Yes, you can





Here a bit of documentation

http://www.valvewizard.co.uk/bridge.html

K

[stratele52]

+1 kagliostro

[smackoj]

nice work Italiano   

[kagliostro]

Isn't flour of my bag

I learned this from Merlin and our friend Geezer

one more thing

the use of a full bridge allows to use also single diode tube rectifiers  

K

[HotBluePlates]

one more thing

the use of a full bridge allows to use also single diode tube rectifiers  

Not really allows, but requires.

The two diodes which you have shown as being replaced by a dual-rectifier tube in your diagram is the only spot where a dual-rectifier tube can be used.

The remaining two diodes (shown as solid-state in your diagram) must be single-diodes. Each of those also must have their own filament winding, separate from the dual rectifier and not shared between single diodes.

That's right, a full tube bridge requires 3 tube envelopes and 3 rectifier heater windings (or transformers).

[sluckey]

3 indirectly heated cathode tubes such as EZ81 can share the same filament winding.

[kagliostro]

When I told a single diode I was thinking to this

as to maintain the sag due to the vacuum rectifier

(using a direct or indirect cathode heated tube diode rectifier)

K

[dscottguitars]

Thanks for all the replies... It looks fairly 'simple' in diagram.  I'm assuming these diodes are the same ones used in a full wave rectifier.

If I wanted to use that PT and have a high/low power option:

Could I use the 0v - 690v taps for the high power and then switch between to the 70v - 610v taps for the low power?  I figure this would give 488v to the amp on high and 389v on low.

What tube would work for this setup?

What would the Rlim1 value need to be?

Lastly, the filter capacitor; what would be required?  With a 5U4GB is use two 220uf/350v caps in series.

Here is a drawing of what I have in mind... (the tube drawing is not correct for the B+)

[sluckey]

Could I use the 0v - 690v taps for the high power and then switch between to the 70v - 610v taps for the low power?  I figure this would give 488v to the amp on high and 389v on low.

Something is wrong with your figuring. 690VAC will give 1.414 x 690 = 976VDC unloaded.

What tube would work for this setup?

Most any of the popular rectifier tubes, 5U4, 5AR4, 5Y3, EZ81, etc. Depends on what amp you will build.

What would the Rlim1 value need to be?

I wouldn't even use it.

Lastly, the filter capacitor; what would be required?  With a 5U4GB is use two 220uf/350v caps in series.

That's too much. I'd use 100µF.

[dscottguitars]

Thanks Steve,

I'm a little confused about the figuring for voltage.  For a center tap PT, I use the voltage between the two secondaries and multiply by 0.707; ex: 360v-0-360v would be 720vx0.707=509v.  It must not be the same then for a non center tapped PT, correct?

•So using your formula, I would then want:

high power- 0v and 370v taps
low power- 690 and 410 taps

Is this okay to do?

•About the Rlim:

This is a quote from the link kagliostro posted:

"Minimum Limiting Resistance: A valve rectifier must have a resistance in series with each anode. Many 'classic' amps do not include these when they ought to, and rectifier failure is common in these amps."

•The filter capacitor:

Are you suggesting two 100uF caps in series for a total of 50uF?  My setup of two 220's gets 110uF in series.  Also the voltage is 700v in series.

Thanks...

[kagliostro]

About the limiting resistor you must remember that the transformer winding has a resistance

here you can find the math http://www.valvewizard.co.uk/fullwave.html


If I've a 250v-0-250v I multiply 250 * 1.4 if I use a SS rectify, using a tube you must subtract the voltage dropped by the Tube Rectifier (each Tube Rectifier has a specific drop voltage value depending on the model of the tube and the current that is crossing it)

Assuming the 0-370v and a SS you'll have 370 * 1.4 = ~520v

using the 690 and 410 tap you have 280v then 280 * 1.4 = ~390v

to me there is a very big difference between the two options

(you must remember that this voltages will drop after the Tube Retifier)

K

[sluckey]

Is this okay to do?

Yes. When using a bridge you will use 1.414 as the multiplication factor. That's exactly double your .707. But remember, this gives the unloaded voltage. As soon as you put a load on the B+, it will drop.

•About the Rlim:

K just covered that. But remember... The vast majority of amps built in the last 60 years did not use a 'Rlim'. Many are still alive today. The one's that are dead probably died of causes other than 'Rlim'.

Are you suggesting two 100uF caps in series for a total of 50uF?

Yes. Your 110µF input cap is gonna be hard on that 5U4 and will likely contribute to early failure.

[dscottguitars]

I see my confusion in the figures I presented for voltages.  0v to 250v times 1.4 is the same as -250v to 250v times 0.707.

kagliostro, that is right where I want the voltage difference to be.  I'm looking for the 'brown sound' Eddie Van Halen got by putting a Variac on the plug into the amp running it at 90VAC, but not affecting the heater voltages.  That's 25% less and 390v is also 25% less than 520v.  I realize this is the unloaded voltage.  I'm looking for about 300v on the 6L6 plates.

Steve, I use a 5U4 in my amps now with that 110uF/700v filter as suggested in Gerald Weber's books.  Is your comment different because of the bridge rectifier?

Thanks to you both for helping me.  This PT is $23 less and I don't need to use a high watt resistor that gets really hot to drop the voltage for the low power setting.

[sluckey]

Steve, I use a 5U4 in my amps now with that 110uF/700v filter as suggested in Gerald Weber's books.  Is your comment different because of the bridge rectifier?

no

[dscottguitars]

Can you please expand on that 'no'?

It seems you disagree with Weber's advice.

[kagliostro]

The 5U4 tube drop 44v at 225mA

so you can aspect a drop of less than 44v in your amp as the consumption is lower

to have 300v on the plates you require near 235v AC, if I'm not wrong

K

[dscottguitars]

Thanks K,

When I get to that point I'll need to experiment with the taps and see what works for the plate voltage target. 

[sluckey]

I'll take a backseat to Mr. Weber. You're in good hands when he's driving.

[dscottguitars]

Thanks everyone, I've got a much clearer picture of my next build...

[Ed_Chambley]

I'll take a backseat to Mr. Weber. You're in good hands when he's driving.

I am not so sure about that.

[Willabe]

When you first turn the amp on there is a surge current because the filter caps are empty. The bigger the filter caps the larger the surge current. The larger the surge current the harder it will be on the rectifier tube. Each rect. tube can only pass so much current.



               Brad