Tube rectifier emulation

[pdf64]

There's a thread on fenderforum.com which has some crossover to this
http://www.fenderforum.com/forum.htm...-10-2905:39:52
My contribution was to measure current draw with 6V6s and 6L6s with 5Y3 and GZ34, excert regarding 6V6s below:-

'6V6s, bias them to 19mA, B+ 458Vdc. OT can now supply a 8k or 4k load, depending which tap the 8ohm resistive load is on.
No signal - 60mA
8k Max sine - 160mA (14V into 8ohms)
8k Max square - 243mA
4k max sine - 167mA (11V into 8ohms)
4k max square - 257mA

Replace GZ34 with 5Y3, bias to 21mA, B+ 412Vdc.
No signal - 62mA
8k Max sine - 135mA (11V into 8ohms)
8k max square - 190mA
4k max sine - 137mA (9V into 8ohms)
4k max square - 198mA'

So, when you crank a pair of 6V6s, they can draw a lot of current and so you need to plan for 250mA to avoid your rectifier turning into a smoke machine. Peter.

[tubeswell]

Hi Matt,
the voltage drop would be the same in both cases, only I don't understand why you put four diodes instead of two, as they're shown wired in series....this way the second diode of each series does nothing but adding its Rd to the equation as the first diode already rectifies the incoming sine wave....Only two diodes can suffice with the configuration you have drawn, simply connect the anodes to the HT secondary lugs and the cathodes together ( one resistor ) or each cathode to its dropping resistor and the resistors together on the other side ( two resistors ).
Bob

Yep two diodes can suffice, and if they are 1000V 1A diodes, they probably will. But putting a couple in series like tx strat had shown does spread the load between the series diodes and offers backup if one of those diodes shorts (if that is a concern), and (FWIW) if you put a 600ish V film cap (like .01uf) in parallel with each diode in series, you can spread the load more evenly.

[Robert M. Martinelli]

putting a couple in series like tx strat had shown does spread the load between the series diodes and offers backup if one of those diodes shorts

Hi TW!Hi Peter!
I agree about the "backup" issue, OTOH using 1N4007s ( 1Amp continuous -1000V ) a diode failure seems a very unlikely event....

What I don't understand is how can this series configuration spread the load between the series diodes, as they're crossed by the same current, as soon as the first diode has cut half of the sine wave the second only adds its series Rd and its typical 0.6V drop to the equation.

The second "series" diode of each pair will work at conditions which are only a tiny bit better than the first, because voltage will be, say, 1-2 V less, but the current will be the same, so I still think it's not really needed.

Peter, thanks for adding your current measurements, surely using a 250 mAmps figure ( the one I used initially ) puts Matt on the safe side, so all that can happen is that the drop is less than expected if the current is less than that ( the "simulator" will "sag" less ) but there will be no safety issues.

Best regards

Bob

[Steve Conner]

Hello all,

The reason for the two series diodes is that the required PIV in this circuit is equal to the sum of B+ and the transformer's peak output voltage. This works out roughly twice the peak output voltage when the power supply is lightly loaded.

So, if the B+ rail gets over 500V unloaded (say you turned the amp on and never used the standby, so the heaters are cold and the power tubes draw no current) then the diodes will see more than 1kV peak, hence 1N4007s may blow up.

When you put diodes in series, the PIVs add. They share voltage in series because each diode leaks a little current when its PIV is exceeded. So for 50 watt amps, we want two 1N4007s in series to get 2kV PIV.

Just to reiterate, the diodes are placed in series to share the reverse voltage. In the forward condition they both conduct and drop about 1V, and the 2V total drop across them is just as negligible compared to 450V as the drop of a single diode would have been.

[Robert M. Martinelli]

Hi Steve,
I understand your point and I agree with you, having two diodes in series sums the peak voltage they can withstand, what I was questioning was that having two diodes was helpful in spreading the load between the diodes....the diodes are crossed by the same current and fed approximately with the same voltage.

I don' t think the extra diodes are a "must" in this particular design ( 6V6s work at lower voltages than 6L6s or EL34s ), but they cost very little so adding them would be advisable and would add "peace of mind" to the design.

Best regards

Bob

[txstrat]

Peter,

I understand your post about the peak voltage and I'm a little concerned. I'm now thinking about using resistors with higher wattage. I didn't know that a peak could go up that far. My recently bought tansformer has a secondary of 2 x 330v CT at 120ma. Do you think it can deliver peaks at 250ma without blowing a fuse? I was thinking of a fuse at 1A slo blo.

Matt

[tubeswell]

What Steve said is what I meant by 'share the load', I was just too lazy to elaborate :-)

Sorry about the confusion it caused

[Robert M. Martinelli]

Hi TW,
maybe I owe you some apologies too, I should have understood you were concerned about the PIV rather than the "load", but, as I told already, maybe my English is not as good as I thought, and this affects my understanding, ( or maybe I was too lazy to elaborate on that too ).

Hi Matt, a 1 amp slo-blo seems a little bit too much IMHO, I would start with a 500 mAmp slo-blo and maybe move to 630 mAmp if needed.

Well, I think we have analyzed Matt's problem pretty thoroughly, so now he should have the means to go ahead and come back to us with the results....

Good luck Matt!

Enjoy your build!

Bob

[pdf64]

Re the transformer winding, 120mA seems a bit low to me. Best to confirm with the transformer vendor that it cope in your circuit. BTW those currents aren't peak, but continuous steady state - when you're driving the output stage hard, those are the kind of currents you will be dealing with.
Regards the fuse rating, is that 1A for the primary or secondary? Secondary is implied, and I would go with along Bob's suggestion of 0.5AT.
Don't use resistors anywhere near their full power rating, as most all types get worryingly hot and smelly, get them at least 50% over your max calculated dissipation. Peter.

[txstrat]

Peter,

the transformer is a Hammond replacement for the deluxe/deuxe reverb amp with original specs plus an additional 50v tap for the bias supply. You might have a look at the Hammond website - it's the BX290/BEX290 transformer.
They don't have the specs in their data sheets but on the transformer is a sticker with the values (660v CT @120ma, 5v@3A, 6.3v@3A).
I always thought 120ma is enough for a PP 6V6 amp.
The amp runs a GZ34 rectifier, two 6V6 and 3 12AX7. I believe it should work at least like the original (with all the disadvantages the original might've had).

I thought of putting a 1A slo blo in one of the primary taps and 0.5A slo blo in the B+ line right before the stand by switch.

Regarding the resistors: since there are not all kind of values (wattage) available I believe I'll go for 25W ones to be on the save side.

Matt

[pdf64]

Matt, that should be fine then. The spec on the lable most likely means that with higher current draws it will lose regulation ie voltage will drop = power supply sag, but it should be plenty robust. The traditional / best place for the B+ fuse is in series with the B+ CT ground return.
Other best practice protection I like to fit is a fuse in the cathode ground return for each power tube, and a 1/4watt 47R resistor between the heater CT and ground. The good thing about the cathode fuse is that it can be quick blow, and as it is specific to each tube it can be smaller than the B+ fuse, so offering much better protection from the most common failure mode (tube shorts etc). The heater CT resistor will blow should there be a plate to heater short inside a power tube.
25 watt resistors tend to be the metal cased type which need heatsinking, might be better with 2 x 11 watt ceramics. Or just buy a non plug in copper cap from Weber? Peter.

[Steve Conner]

Wow, that's a lot of fuses.

The cathode fuses and B+ fuse are fairly redundant, since the same current flows in the plate circuit as the cathode. Granted the cathode fuse can be half the rating, since the current divides in two. Or one-quarter for a four-tube amp.

The suggestion to fuse the heater CT is a little weird. If the heater ever did short to the plate, then the heater CT fuse would blow and the heaters would go up to B+, taking out the heater-cathode insulation in all of your tubes. So I think you want to keep the heater circuit solidly tied to ground, which would blow the HT fuse and minimize collateral damage.

I prefer to put the fusible resistor on the bias tap, since burning out the bias tap would trash an otherwise perfectly good transformer.

[txstrat]

Or just buy a non plug in copper cap from Weber?

The more I think about it I tend to buy one and be satisfied.
On the other hand I was lately thinking of putting the diodes and resistors in parallel to the tube recto, the way I read about the Port Arthur rectifier. That would reduce the "stress" on the GZ34 and if the tube fails to rectify you could pull it and still play with the diode rectifier.

My idea of the fuse in the B+ line was not only for a short inside a tube but also for a cable short inside the chassis caused by vibration.

Matt

[Victor Denance]

I built a Deluxe style amp (more like a 5E3) w/ a 120mA sec, and used a sag resistor.

I think there's a lot of variables so it's pretty hard to guesstimate its value.
In my case, I ended up w/ a 250 ohm 10W that dropped the B+ from 420V to 390V (IIRC, that was long ago, and I sold the amp since).

I think 100 ohm is a good start. Quite like a 5U4. I would try that and start from there.
I would also use the big 25W encased resistors, they're easy to screw on the chassis and can handle massive power.

[Fretts]

Why not just go ahead and use the Weber? He's already done all this grunt work, plus you can pretty much order any voltage drop and current limit you want.
He uses a piece of copper pipe to house the unit.
I read last year a complaint from a guy who claims it overheated and dies, but he may well have been using the wrong part for the job...