Tube rectifier AND SS rectifier

[jeff]

 Is it possible to take the transformers secondaries and use a tube rectifier to have a power supply to run your power tube and also use SS diodes to make another power supply to run the preamp at the same time?

 Right now I'm using a tube rectifier but could I build a seperate power supply, one tube rectified(power tube) one SS diode rectified(preamp)? With a 300V transformer I could run the power tube at 330V(300 X 1.1) and the preamp at 420V(300 X 1.4)

I'm just thinking out loud. Every thing I've read says that power tube distortion is more pleasing than preamp distortion. If I can raise the voltage on the preamps they will stay cleaner longer shifting the ratio of preamp to power amp distortion.

[tubenit]

Every thing I've read says that power tube distortion is more pleasing than preamp distortion

.

That's a matter of opinon of course. For example, Dumble amps have a reasonably clean power amp and the preamp is where the overdrive comes from. Same thing is true with the Tweed Overdrive Special amp that Geezer and I worked on together.

Having said that,  if you use a VVR then you can lower the power amp to a lower voltage while keeping the preamp at a similar voltage (it actually goes up a few volts)

With respect, Tubenit

[jeff]

 Not sure what a VVR is.

But is it possible to run the two power supplies?

[sluckey]

But is it possible to run the two power supplies?

yes

[phsyconoodler]

Quote:"Not sure what a VVR is."

Variable voltage regulator.It allows you to lower the voltage of the power amp section and then the preamp will drive it harder,producing your power tube distortion at lower volumes.
  Very pleasant sounding.

[jeff]

Cool but I don't want to dercease the power tube supply I want to increase the preamp


Now I have to figure what value plate resistor and cathode resistor values will give me the most headroom for a 12AX7 with a 420V B+ power supply.

 Is it true that, set up correctly, 12AT7 will give a bigger, cleaner output than a 12AX7 though it requires a bigger input signal? In other words it takes more to drive it but a 12AT7 has more headroom than a 12AX7 at the same output level?

[sluckey]

Now I have to figure what value plate resistor and cathode resistor values will give me the most headroom for a 12AX7 with a 420V B+ power supply.

Just copy a Twin Reverb.

Is it true that, set up correctly, 12AT7 will give a bigger, cleaner output than a 12AX7 though it requires a bigger input signal? In other words it takes more to drive it but a 12AT7 has more headroom than a 12AX7 at the same output level?

Not sure what you're asking? 12AX7 just has higher voltage gain than a 12AT7. 12AT7 can drive a heaver load though.

[tubenit]

Cool but I don't want to dercease the power tube supply I want to increase the preamp

Use a solid state rectifier to increase ALL of it and then lower the power amp with the VVR and you will have what you are needing, I think.  And as I mentioned when you lower the power amp voltage the preamp voltage does increase somewhat.

http://www.el34world.com/Forum/index.php?topic=6899.0

Dana Hall's VVR information: http://hallamplification.com/main.html?src=%2F#2,2

With respect, Tubenit

[jeff]

 Are you suggesting the VVR because this won't work?

 I just don't see the point of raising the voltage just to knock it back down. If I can get away with keeping a tube rect for the power amp and raise the preamp with a SS rect all I'd need is two diodes soldered to the tube socket. Much easier.

[tubenit]

I am simply suggesting the VVR because I know I can make it work that way based on experience and it's inexpensive and allows one to have an option of a cranked amp tone at lower volumes.


What you're suggesting to do may work great. Can you draw or edit a schematic and post it to show exactly how you're going to do this?  I'd be interested in seeing it.

With respect, Tubenit

[TIMBO]

Hi guys, Something i was thinking of too. The preamp of the Mesa boogie DR has some high voltages to the 12AX7s which i was hoping to copy.The power amp stage i was going to copy the marshall 18w. As i see it i would like to have a bit of sag in the power amp and a bit stiffness in the preamp.Jeff, is this what you were think of??  The PT i was wanting to use is a 290-0-290 and to get the voltages i need for the preamp the diodes could achieve this. I know the VVR works well and does what Tubenit says which is a good option. Hope this helps.

[FYL]

Are you sure you want to run EZ81s from a 5 V heater tap? They are 6.3 V valves...

[sluckey]

And why two of 'em. Is this a 'cost plus' job? 

[FYL]

And why two of 'em. Is this a 'cost plus' job? 

This is a small Dual Recto...

 

[TIMBO]

Its just an experiment.I've always wondered if the the dual rectifier was a gimmick or did it acturaly do something. ( i thought it was to cope with the sag)Jeff the mesa boogie blue angle has a half diode/ rect tube rectifier.

[FYL]

Its just an experiment.I've always wondered if the the dual rectifier was a gimmick or did it acturaly do something. ( i thought it was to cope with the sag)Jeff the mesa boogie blue angle has a half diode/ rect tube rectifier.

Gimmick. Two 5U4Gs in // behave more or less like a single 5AR4 but draw twice the heater current - quite silly by any engineering standards. The Blue Angel is IMO a cruel joke.

[TIMBO]

I would like to hear your reasons, because i don't know the technical thoughts behind these builds i can only look at these schems a think that if they have gone to the trouble of designing the amp that way it must be there for a reason. Thanks

[FYL]

Mesa amps are designed by marketing guys and finalized by bean counters. Their features are as worthless as Smith's patents.

[TIMBO]

Thanks for the reply FYL , But this is getting away from jeffs idea. Jeff i can't see that what you are suggesting would be a problem.If you are able to do a build that would allow for future mods i think thats the way to go.Thanks

[PRR]

Mesa Dual Rect has BIG power tubes worked HARD.

Common rectifiers will sag under the load.

I have a shelf. It sags. I put another board on it. It sags less.

A single pair of EL84 is not a heavy load to any bottled rect, but they all sag some.

Pair-EL84, '4007 - one EZ81 - two EZ81 = 410, 360V, 320V

In any rational product planning, you would NOT bash another hole and stock twice as many bottles for a mere 12% difference. Another 15V on each HV winding is cheaper than another bottle.

In DIY (and costly-brand) guitar amps, there is little reason to be rational. Put in four rectifiers, that's gotta be better than any mere Dual Rectifier. Anyway, winter is here and my toes are cold.

[Shrapnel]

I can see using multiple bottles of rectos IF the current demand is larger than just what one bottle can produce. (i.e in a 100+W amp) However SS Diodes are cheaper and give less sag... but back a couple or so decades, multiple bottles were the only solution in such a case as not to blow a single bottle in a high power amp.

[jeff]

I am simply suggesting the VVR because I know I can make it work that way based on experience and it's inexpensive and allows one to have an option of a cranked amp tone at lower volumes.

I see what you're gettin' at now.

[jeff]

Pair-EL84, '4007 - one EZ81 - two EZ81 = 410, 360V, 320V

Wouldn't 2 rects in parallel sag less that one?

[jeff]

 Thanks for the schemo. Assume the switch is always on the tube rects and that's exactly what I was thinking.
Tube rect to power, SS rect to preamp at the same time.

The whole point of this line of questioning was to change the ratio of preamp distortion to power amp distortion.

 Ideally I like to have a preamp with no or little distortion presenting the power tube with a big enough signal to drive it into distortion. Does that any make sense?

[TIMBO]

Jeff, Part of the reason i am wanting to use two EZ81 is that i have two spares and i've over spent my allowance The other reason is that my current draw (if i am correct in thinking this)is over the range of a single EZ81.If this can be confirmed that would be great. Thanks

[FYL]

Wouldn't 2 rects in parallel sag less that one?

For the same current draw, yes.

[phsyconoodler]

Ok,what you want is POWER TUBE DISTORTION right? You get that by cranking the volume to 10.
  By using VVR it jsut effectively makes the POWER TUBES into lower power units,almost like changing a 6L6 to a 6V6.With the 6V6 the power tube distorts earlier,so your ears are not ringing.Same glorious POWER TUBE DISTORTION.

  Raising the preamp voltages increases the clean headroom before the power tubes distort.
Just for your information,the power section still distorts with lower preamp voltages.You think that because the voltage in the preamp is lower that you are hearing mostly preamp distortion.Wrong.It simply means you are adding a bit more preamp distortion to your output distortion.It can be very nice and it does not sound like having the preamp gain cranked on a two channel high-gain amp.
  You still have to crank the volume up to get distortion.
High preamp voltages sound less pleasant when the power amp distorts.

To get the best blend of preamp and power tube distortion try a JCM800 preamp.you can have a really nice blend of both or none at all.

[FYL]

The other reason is that my current draw (if i am correct in thinking this)is over the range of a single EZ81.If this can be confirmed that would be great.

The EZ81 is rated at 500 mA peak, wich translates to around 150 mA continuous. More than enough for a pair of small power bottles and 3 preamp toobs.

[jeff]

 If I use a VVR and drop the voltage of the power tube don't I need a different transformer?
I thought a 6V6 at 250V requires a 5K primary while at 300V closer to 7K. Lower voltage lower primary, No?

[PRR]

> Wouldn't 2 rects in parallel sag less that one?

Yes. You knew what I meant to type.

Pair-EL84
'4007 = 410V
one EZ81 320V
two EZ81 360V

[Merlin]

If I use a VVR and drop the voltage of the power tube don't I need a different transformer?
I thought a 6V6 at 250V requires a 5K primary while at 300V closer to 7K. Lower voltage lower primary, No?

No. The whole point of a VVR /Power Scaling is that you don't have to change anything as long as the voltages in the PA are all proportionately scaled.

[tubenit]

If I use a VVR and drop the voltage of the power tube don't I need a different transformer?
I thought a 6V6 at 250V requires a 5K primary while at 300V closer to 7K. Lower voltage lower primary, No?

Merlin has already answered the question but to give you a visual of what has to be changed to VVR the power amp and phase invertor ............

If you have not read it yet, I suggest you look over this link that I posted earlier in the thread. Perhaps it will answer some of your questions/concerns.

http://www.el34world.com/Forum/index.php?topic=6899.0


With respect, Tubenit

[HotBluePlates]

If I use a VVR and drop the voltage of the power tube don't I need a different transformer?
I thought a 6V6 at 250V requires a 5K primary while at 300V closer to 7K. Lower voltage lower primary, No?

For maximum power. But the point of the VVR is non-maximum power, often very, very much less than maximum.

Have you ever cranked a Twin Reverb or 100w non-master volume Marshall until they were very distorted? If you did without good earplugs, you probably can't hear what your drummer is saying to you from across a quiet stage. Hence the VVR to make an output stage act like it is very small wattage.

But the key thing to know:
We see a lot of recommendations for specific impedance OT primaries with specific tubes and specific power supply voltages. However, those are specified in order to make maximum output power, and with low (or a particular type of) distortion. If you reduce the supply voltage with the VVR, you will not change the OT primary impedance or type of distortion, but you will dramatically reduce the power capability of the output stage. 1/2 the voltage results in 1/4 the power output.

Ultimately, the idea is output tube distortion at much lower volume in the room, so that a big-tube amp can mimic the volume of a small-tube amp in a smaller room (or bedroom).

[jeff]

> Wouldn't 2 rects in parallel sag less that one?

Yes. You knew what I meant to type.

Pair-EL84
'4007 = 410V
one EZ81 320V
two EZ81 360V

I assumed that's what you meant but wanted to be sure. I trust you guys more than I trust myself.

[jeff]

 This is what I had in mind.
Would it be better to use two seperate caps like in A or put the cap on the preamp side of the switch as in B?
Is it bad for the cap to sometimes see 300V and sometimes see 420V?
Also what happens if its charged at 420 then is switched back in parallel with the other caps?

(it's a single ended amp so I don't have a phase inverter)

[Merlin]

Would it be better to use two seperate caps like in A or put the cap on the preamp side of the switch as in B?

Either would work ok, but you will need at least one extra filter stage for the preamp when in "SS mode". One cap will have too much ripple on it. I would choose A,  but move the switch so that it comes immediately before the second resistor.

Is it bad for the cap to sometimes see 300V and sometimes see 420V?

With modern caps I wouldn't sweat it.

Also what happens if its charged at 420 then is switched back in parallel with the other caps?

You'll get a big pop sound.

[PRR]

> I thought a 6V6 at 250V requires a 5K primary while at 300V closer to 7K.

Why?

Do math.

FOR MAXIMUM CLEAN POWER OUTPUT:

Work tube at maximum plate dissipation. In DIY, 6V6 can be run at 14W,

Pick a voltage. Find current needed to hit dissipation goal.

14W/250V = 56mA
14W/300V = 47mA
14W/350V = 40mA
14W/400V = 35mA

(for pentodes) Load *approximately* with V/I.

250V/56mA = 4,464 ohms (try 5K)
300V/47mA = 6,430 ohms (try 7K)
350V/40mA = 8,750 ohms
400V/35mA = 11,430 ohms

BUT.....

> power amp distortion.

What you seem to want is a strong driver and less-strong power amp, get more strained-6V6 sound. I approve. But it means you can VIOLATE "good" design rules for power amps.

Fer example you could run 250V and 7K. 250V/7K = 36mA. Anything much more than 36mA is a waste. Plate dissipation is only 9 Watts. Hey, why pay for a 14W bottle and only use 9W of it? Do you buy 14 ounces of beer and drink 9 ounces? Oh, WTH.... maybe 9oz is all you wanted, but the price was the same anything up to 14oz. (Try to buy a small amount of KY jelly. I need a few schmears for a filter gasket. The store only has a big jug, enough for dozens of horse-suppositories.)

Anyway 5K or 7K is quite a small difference. Unless you get paid to read a meter, maybe no difference at all. Maybe some "sonic" difference how it interacts with speaker impedance (+500%/-20%) and your music.

[RicharD]

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