Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Scsoul on December 30, 2012, 04:42:46 pm
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I have a pt i pulled from a old record player that ran a pair of 6v6s, the hv winding have no center tap, is there a type of voltage doubler i should use? does anyone have a diagram? im measuring 140v on hv winding and 6.7v on heater windings, im hoping to get 300v for a pair of el84 Scsoul
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(http://s.eeweb.com/members/piyush_itankar/answers/1300724102-dubler.gif)
If your AC secondary is 140VAC, the DC output of the doubler will be ( approximately ) 140 x 1.4 x 2 = 392VDC
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If your AC secondary is 140VAC, the DC output of the doubler will be ( approximately ) 140 x 1.4 x 2 = 392VDC
At half the output current. Might not be enough current to get full output for the PP EL84's?
Brad :icon_biggrin:
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I have a pt i pulled from a old record player that ran a pair of 6v6s
Do you know anything more on the record player circuit ?
I think that 196v (140v * 1.4) that is the voltage you obtain with a SS rectify is not an usual B+ voltage for a pair of 6v6 (to me it is a bit low)
are you sure there was not a doubler in the old circuit ?
K
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PT was very likely powered 2 x 6V6 in P-P for mono or stereo in SE via doubler PS. 392V will be significantly less when loaded, especially in a doubler topology. do you have the output transformer(s)? it/they will work w/ EL-84s as well.
--DL
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kagliostro I pulled the transformers and tossed the chassis i did'nt really make note of the doubler circuit but i took some voltage reading and 6v6s were running @ 330 plate voltage, which i thought would be just right for a pair of el84, this pt has no centertap so i can't use a tube recto so after the VD i then use a grounded fullwave (4 diodes with one tail grounded)? what value diodes and electrolytic caps do i build the doubler with? thanks Scsoul
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kagliostro I pulled the transformers and tossed the chassis i did'nt really make note of the doubler circuit ...
I wish we could get guys to know this earlier: Always make careful notes of the original amp's circuit before removing stuff like transformers.
The power and output transformers probably worked okay in their original circuit, may not work okay in a typical guitar amp circuit, even using the same output tubes. By that I mean there were probably cost-cutting measures (such as the lighter, cheaper PT using a voltage doubler) that were balanced out in the various tradeoffs in that piece of equipment.
I learned this the hard way too, cause in my early days I also thought, "well it's an amp running XXX tubes so it's all the same, right?"
Then I learned things are generally not all the same. :l2:
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I wish we could get guys to know this earlier: Always make careful notes of the original amp's circuit before removing stuff like transformers.
Yep.
This should be in the arcives, yes?
Brad :icon_biggrin:
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this pt has no centertap so i can't use a tube recto
That isn't the real reason you can't use a Tube Rectifier, the real reason is you are missing the 5v winding for the filament of the rectifier
if you can supply the filament of a Tube Rectifier you can use also a HT winding without CT, you can use an hybrid solution to have a bridge, a pair of diodes plus the rectifier
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The diodes you can use in the voltage doubler (use the schematic posted by Darryl that is fine) are the usual 1N4007, for the electrolytic capacitor 250v (or 300v better) 220uF
Happy New Year
K
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I could use the 6.3 winding for a ez81 i guess, do i use a rectofier after the VD or does the VD do the job? Scsoul
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If I'm not wrong ez81 has a drop voltage of near 60v, you must consider this
K
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Stick to solid state diodes!
There is a reason the original was built the way it was; further argument for why I say not to rip parts like transformers out of a donor chassis.
this pt has no centertap so i can't use a tube recto
That isn't the real reason you can't use a Tube Rectifier, the real reason is ....
No, the real reason is the arrangement of the two diode in the voltage doubler. You'd need 2 individual rectifier tubes, because the individual diodes are arranged anode-to-cathode.
Most dual-diode rectifier tubes share a single cathode inside the tube. Therefore you can't wire them up as a doubler.
This is the same reason you don't see tube rectifiers used in a bridge circuit in a guitar amp (or most consumer stuff). It would take 3 rectifier tubes (one dual-diode, 2 individual diode tubes) and 3 heater windings (because the heater is generally at cathode voltage).
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> Might not be enough current to get full output for the PP EL84's?
On the HV side, there's not a dime of difference 2-6V6 or 2-EL84. They are both 300V 12W tubes.
Do note that the Heater demand for EL84 is much higher than 6V6. This goes-with the EL84's higher gain (smaller bias and drive). Same control voltage on a larger cathode means more current-swing. But a larger cathode takes a bigger fire to heat.
> Always make careful notes of the original amp's circuit before removing stuff like transformers.
Yes, I was too tired for this rant last night. Keep all the stuff together!! The PT and OT(s) were designed to be used *together*. Commercial gear is more engineered and more-proven than any pile-of-parts build. Note also the approximate size of the power bottles and the way the rectifier is configured, how big/many filter cap(s) off the rectifier(s).
This one is just unusual. What if you scavenged one of those strange SilverTones with *two* windings and doublers stacked-up? When power crystals were new, that cobbled-up design was a penny cheaper than any other (and SilverTone was about the pennies). If you are an experienced historian, you have seen many oddballs and can make good guesses. If not, you really want to stick with what obviously worked in the amp's first life.
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... for the electrolytic capacitor 250v (or 300v better) 220uF
Is this a typo(220uF) or does a doubler need bigger caps than normal (20~40uF)?
I have a preamp I was thinking of using a doubler with because of the low voltages. It originally used a half wave rect, but it has has no seperate 6.3 heater winding. Instead it used a 6.3 tap, so the ground was common(in the above schematic picture a tap between A and B. So B to A is 140V, B to tap is 6.3V). would it be OK to use that tap with the doubler? This would elevate the 6.3Vs "ground" to half the B+. And does it make a difference it it is on the diode side or the cap side?
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I was gonna build a prototype VD but all i could find is 220-160v e-caps, will these work for now? ill have to order a batch of higher voltage caps, I have a couple amps on bench with VD they use 100-150v caps in the doubler as do most ive looked up on net is there a reason for the higher value? Scsoul
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The 220uF isn't a typo
About the voltage in your amp you have 140v so you must use caps rated higher than your 140v
I think 160v is higher than 140v, but there is no safety margin
K
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... for the electrolytic capacitor 250v (or 300v better) 220uF
Is this a typo(220uF) or does a doubler need bigger caps than normal (20~40uF)?
Not a typo.
Full-wave rectifiers (regular or bridge) pass current during both halves of the incoming a.c. cycle, and have 120Hz hum (in the U.S.).
Half-wave and voltage doublers effectively pass current during only half of the incoming a.c. cycle, and have 60Hz hum. The lower hum frequency requires more filtering than 120Hz, becuase the power supply components are essentially less effective filters.
The generalization about the doubler above is technically wrong: During one half of the incoming cycle, one of the 2 caps is being charged, while the other gets charged during the other half-cycle. But since you could say that the total charge of the filter cap totem pole happens only once during a cycle, you could probably argue the filter is effectively only charged half as often as in a full-wave rectifier. Hum frequency is 60Hz regardless.
So the filter caps also need to be bigger than "normal" because current is being sucked out faster than it is pouring in if the rectifier were full-wave. In other words, 2 reasons to use bigger value caps. This is the reason doublers are usually only used in low-current supplies: the current is sucked away from the caps such that if the current draw is too high, the voltage drops considerably.
I have a preamp I was thinking of using a doubler with because of the low voltages. It originally used a half wave rect ...
Do you want an output voltage twice what you had with the half-wave rectifier, with poorer voltage regulation (maybe as much as 100v sag in some cases) and needing bigger caps for same/higher hum? If so use the doubler.
They good circuits, but not usually a great choice for most applications. If you're just power a few 12A_7's, then you're probably alright.
It originally used a half wave rect, but it has has no seperate 6.3 heater winding. Instead it used a 6.3 tap, so the ground was common(in the above schematic picture a tap between A and B. So B to A is 140V, B to tap is 6.3V). would it be OK to use that tap with the doubler? This would elevate the 6.3Vs "ground" to half the B+.
I'm having a hard time visualizing what you're describing, because I'm thinking a couple statements are incompatible with each other.
If this is a preamp, and only powering a few 12A_7's, it sound like the designer did something tricky. It's best not to mess with something tricky without all the information that went into designing it. So what is this preamp?
You also can't have a tap that's part of the secondary that only outputs 6.3vac, while at the same time having one side of the 6.3v at half-B+. Wihtout any other info, I'd say don't do what you were considering. Either use the PT as in the original preamp, or post more info on the preamp or its schematic so we can figure what's happening.
I was gonna build a prototype VD but all i could find is 220-160v e-caps, will these work for now? ...
No.
... I have a couple amps on bench with VD they use 100-150v caps in the doubler as do most ive looked up on net is there a reason for the higher value?
You have to be guided by the transformer you're using, cause the others on the 'net may be different and with different ratings.
You measured 140vac on the transformer winding. 140v * 1.414 = 198v peak. You need a cap that's rated for at least 200vdc, and higher would probably be a good idea (for days when your wall voltage rises).
You'll be stacking 2 of these in series, for a total rating of 400vdc, which you need to ensure caps don't fail at turn-on. Even with that rating, your B+ in use will probably drop to 300-350vdc (I'm banking on the lower end of that).
Here's how the doubler works:
When point A is negative compared to point B, D2 is forward-biased and charges C2 to 140vac * 1.414 = 198vdc. On the next half-cycle, the polarity of the seocndary changes, and point A is positive compared to pont B. Now D1 is forward-biased and charges C1 to 140vac * 1.414 = 198vdc.
But C1 is stacked on top of C2, so the B+ is 198v + 198v = 396vdc.
Until you draw current, in which case the voltage sags.
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Here's how the doubler works:
When point A is negative compared to point B, D2 is forward-biased and charges C2 to 140vac * 1.414 = 198vdc. On the next half-cycle, the polarity of the seocndary changes, and point A is positive compared to pont B. Now D1 is forward-biased and charges C1 to 140vac * 1.414 = 198vdc.
But C1 is stacked on top of C2, so the B+ is 198v + 198v = 396vdc.
Until you draw current, in which case the voltage sags.
Very good explanation HBP
Thanks
K
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HBP,
Aren't there some full wave doubler circuits out there? I know I've seen some using 4 diodes and two caps....O'Connor talks about one in one of his books, though which one I don't remember right now. Merlin talked about it too I think in his power supply book, though I will have to go back and look now....
I used a doubler circuit in my Bogen build since that is what it had and just increased the caps size.....it works well and the amp sounds and plays great. I also used a doubler in another build since again that is what it used. That one isn't done, but the power supply portion of that amp works great. They both regulate pretty well. In both I used four diode circuits.
Greg
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I've also seen solutions using 2 bridges
(http://www.webalice.it/crapellavittorio/electronic/duplica.gif)
the text say 2000-3000uF for each A - resulting ripple within 15-30% in respect to the max output voltage
http://www.webalice.it/crapellavittorio/electronic/duplica.htm (http://www.webalice.it/crapellavittorio/electronic/duplica.htm)
K
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There are a LOT of voltage multiplier circuits (http://en.wikipedia.org/wiki/Voltage_multiplier) out there. The link is just a small snippet of them all.
We were evaluating the one circuit presented, not every possible circuit. Most circuits possible are rarely, if ever, seen in tube electronics (or at least audio tube electronics).
I used a doubler circuit in my Bogen build since that is what it had and just increased the caps size.....it works well and the amp sounds and plays great.
But the amp was originally built with that, so it stands to reason it would work in that case.
My point was expanding Willabe's point that you get half-current. There's no free lunch: if you double the voltage output of the transformer through a doubler, you can only draw half the current rating of the transformer.
So that means current is pumping into each cap half as fast as it would in a full-wave circuit, which will lead to voltage sag faster above some critical current level (Bogen obviously accounted for this some way in their amp). If you try to draw more than half the transformer's rated current, you'll get voltage sag feeding the caps even before draining the caps faster, or voltage sag from two directions.
Quote from NEETS, Module 7 Solid State Devices and Power Supplies:
"VOLTAGE MULTIPLIERS are used primarily to develop high voltages where low current is required. The most common application of the high voltage outputs of voltage multipliers is the anode of cathode-ray tubes (CRT), which are used for radar scope presentations, oscilloscope presentations, or TV picture tubes. ...
If you have studied transformers, you may have learned that when voltage is stepped up, the output current decreases. This is also true of voltage multipliers. Although the measured output voltage of a voltage multiplier may be several times greater than the input voltage, once a load is connected the value of the output voltage decreases. Also any small fluctuation of load impedance causes a large fluctuation in the output voltage of the multiplier. For this reason, voltage multipliers are used only in special applications where the load is constant and has a high impedance or where input voltage stability is not critical."
They didn't include voltage multipliers in the Vacuum Tube Module, probably because of the difficulty of constructing them with tube rectifiers.
You can download NEETS Modules here (http://jacquesricher.com/NEETS/).
Bear in mind these points are not unique to doublers. Choke input power supplies have very good regulation (little voltage change with load/current changes), but the output voltage from a transformer is typically 0.9 * secondary RMS volts.
But that's probably a good feature if you have a transformer with very much more than the needed voltage output, and a healthy current rating.
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I didn't remember where I've read this, but I'm sure I've read some guys talking about the use of Voltage Doubler in tube guitar amps
One of the things that was told and that I remember is that a Voltage Doubler isn't so bad in a guitar amp because of his tendency to SAG, something like if you have a tube rectifier instead of SS rectifier
So with a Voltage Doubler you can save a SAG resistor
Of course, as HBP say, all this is not exceding the 50% request of current that the circuit can supply if the Voltage Doubler is not present
K
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One of the things that was told and that I remember is that a Voltage Doubler isn't so bad in a guitar amp because of his tendency to SAG, something like if you have a tube rectifier instead of SS rectifier
So with a Voltage Doubler you can save a SAG resistor
But you might get 10w out of an amp you expected 30w from. Maybe not bad, but be sure you know that before you start.
I warn about this stuff because readers have a tendency to generalize, and think there's no downside to using a doubler or that it will work well in every situation.
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Of course, the disposable current is halved because the W of the transformer is defined and didn't change if the output voltage is doubled
K
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I don't think any power loss is as extreme as you mention. Merlin talks about it in his power supply book and shows some charts with the conditions as to its performance. It actually holds it own very well providing certain precautions are taken into account. Personally I would never use a half wave doubler unless it was in a bias supply....but full wave doublers are another story. I've built several amps with full wave doublers and they perform nicely and do give a sag that is similar to tube rectifiers. One of the amps has less power than you might expect (four 7868's are only about 55 watts RMS instead of the possible 88 watts) but that is due to the undersized output transformer more so than the use of a doubler. I've only used doublers in amps that were designed to use them however.
Greg
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I think HotBluePlates mean that if you have a 200mA HV winding,
and you use a Voltage Doubler,
when planning the amp you must remember that it will supply max 100mA
so the available power is considerably reduced
respect those you can have if the available current stay on 200mA
a completely different scenario where you can obtain only half the power
I think that if you have a 200mA winding and use a Voltage Doubler
plan your amp for max 100mA consumption and you'll obtain the planned power
Darryl has a large experience on the use of the Voltage Doubler
and would be able to confirm, or not, what I'm thinking
K
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I think HotBluePlates mean that if you have a 200mA HV winding,
and you use a Voltage Doubler,
when planning the amp you must remember that it will supply max 100mA
so the available power is considerably reduced
respect those you can have if the available current stay on 200mA
a completely different scenario where you can obtain only half the power
I think that if you have a 200mA winding and use a Voltage Doubler
plan your amp for max 100mA consumption and you'll obtain the planned power
Darryl has a large experience on the use of the Voltage Doubler
and would be able to confirm, or not, what I'm thinking
K
Maybe that is what he meant, and of course that is correct. Theoretically, a full wave doubler should give 2.8 times the secondary without a load. So I have an amp that gives 525v without a load using a voltage doubler....that would be a secondary of 187.5. Under load the amp is about 450v on the plates of the power tubes. I forget at the moment what the current rating for the power transformer was....it was a Foster transformer out of a Conn organ....but it is well known that using a transformer when connected as a doubler, you have to expect half the current at about 2.8 times the voltage. If you use a tripler, then you get half the current again, with even poorer regulation. As long as you use large enough cap values in the doubler, then doublers regulate pretty well. You can expect that you will need to use twice the normal size for your caps at half the voltage though. I've had good results using larger than normal caps in my doublers...a couple 220uF caps makes for a 110uF cap and that amp works well with it.
Greg
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Triplers aren't a good thing
there is tendency to saturate the core of the transformer
May be is better to use a full wave rectify and stack on it a capacitor coupled rectifier (bridge), this will give 1xHV + 2xHV so the resulting B+ is 3xHV
K