390-0-390 for 50w marshall?

[rdrgtr]

I guess the subject title sums it up. The power tranny I am looking to use has right at 780 unloaded (390-0-390).  The schematics I have been looking at call for 325-0-325.  I plan on using el34s so my plates should be able to handle it right?  How will it effect the rest of the build?  I do not want to  end up trying to drop my b+ to keep my amp stable ( no zeners ).   I am trying  to keep building in a bad economy.  Any input would be very helpful.

Thanks

[LooseChange]

Solid state rectifier ~540vdc on the plates. Use EL34's or 6550's as Marshall did and all will be good.... And loud!

[The_Gaz]

Bad idea, dude. That kind of plate voltage is only acceptable with a relatively low screen voltage, and high primary impedance on the output transformer, either of which you haven't mentioned. The spec sheets are very contextual - 800vdc max especially! Check the Mullard spec sheets or others.

ampcabinets, he did not say the xfrmer has 390 after the rectifier, but instead, 390-0-390 VAC. 390vdc would be a pretty conservative B+ if that were the case.

I definitely wouldn't use a bridge rectifier for that transformer. Instead, wiring that transformer as full wave you'll get about 550 volts unloaded (no tubes in) plus twice the current of a bridge recto. Like LooseChange said, and you will 'be loud', but it will not 'be good'. You screens will be glowing by the time you hit '3' on the gain knob, and those poor tubes will be dying. Those Marshalls LooseChange referenced were infamously mis-spec'd by Ken Bran of Marshall for the wrong primary voltage, which in turn made the secondary voltages too high, and those amps "tube eaters".

That voltage on the tubes is not economical in the long-run because you will be going through tubes left, and right. And tubes are not cheap like they used to be! I have a Marshall type amp that has 480vdc on the plates and just a few volts less on the screens. The only reason the tubes do not self-destruct (red-plating, screen glow, etc.) under heavy signal conditions is because the power supply sags 100v volts full-tilt, putting the amp in the safe zone of operation. If your transformer and power supply is too stout, and doesn't allow the voltage to sag that much, your tubes will definitely glow. Trust me, I speak from experience! You will most definitely be trying to lower your screen voltage with that transformer, and there's not to many elegant good sounding solutions that will make it work.

Magnetic Components and Weber both offer Marshall style transformers at very reasonable prices if that's the amp you're set on. If not, you may be able to use the transformer if it has another HV secondary that's slightly lower, which you could run the screens and preamp from. You could also use a tube-recto to drop the voltage, use resistor instead of choke to lower the voltage a bit more, and largish screen resistors to drop it a bit more, and keep the screen from over dissipation. At that point the amp will have less power, and may be too squishy and soft for your taste, but maybe not. I would also use something lie a JTM45 output transformer with a 6k primary, or something else in that realm.

Please let us know the other specs of the transformer, and perhaps someone can point you a good power supply for it.

[The_Gaz]

The only thing I said  was that his transformer was not putting out 390vdc, which is what you stated here:

390 should be well within it's limits. My book says anything up to 375 is normal, for 35 watts.

15 volts is not that much more.[/quote] you seemed to be confusing the AC secondary rating.

I was only telling the OP to take LooseChange's cavalier attitude towards voltages with a grain of salt. I never said that the datasheet didn't say 800vdc max either, what I said was that those ratings are contextual. Why don't you try putting 800vdc on the plates and screens of a EL34 with any 'ol output transformer and see what happens?

This forum is hosted by Doug, of course, but people talk about ordering parts from Mouser all the time. By all means, order them from Doug! If my post came off strong, that's because I didn't want someone to destroy expensive tubes, or waste their time and money.

[navdave]

Loose Change is on the money 780 VAC after a full wave rectifier will give you that much B+ under load dude I mean bro I mean RDRGTR     
 

Solid state rectifier ~540vdc on the plates. Use EL34's or 6550's as Marshall did and all will be good.... And loud!

[The_Gaz]

 

[Merlin]

Under load you probably won't get more than 480V anyway. You could even use a GZ34 for more loss, if it bothered you.

[LooseChange]

Merlin, your a wizard (at this stuff)  Thanks!

[phsyconoodler]

You guys are not understanding the transformer with a center tap.The 390-0-390 rating is each leg is 390v loaded.
  So using this formula; 390 x 1.414 = 551v at the first filter cap.Now loaded with all the tubes in it might be around 500v,maybe as low as Merlin says,maybe not.Either way it will work fine.Just adjust the current to the power tubes accordingly and the amp will be very loud and have tone out the wazzo. Keep in mind that filter caps with 500v ratings will be at the ragged edge(not good),so use the best you can buy.
  Understanding rectifiers is necessary here.A full wave rectifier does not give you 800v unless you use a full wave BRIDGE rectifier.Look in Dougs' library of information on rectifiers and it shows the different types.
  You only need a FULL WAVE and NOT a FULL WAVE BRIDGE rectifier.The two are very different.
 Overall,you are best to lower the voltage or get the right power transformer from Hoffman.

  The tubes cannot take the CURRENT but the voltage is not a concern.Upping the screen resistor value will keep the screens under control.
The things that concern me are the filter caps at that voltage.Hiwatt amps have run 550v on the plates and they live as long as you can get the bias voltage to up around -65v or so.
  Loosechange dosn't have a 'cavalier attitude about voltage' just knows what do do with it.The OP is probably better off getting the right PT.

[LooseChange]

Personally, I wouldn't want that PT in my amp.  I don't like building amps with B+ over 450v due to filter cap maximum voltage issues.

[rdrgtr]

Hey guys,  After further review, the power tranny does have a 5v tube rectifier tap on it.  This would bring the the votage down 30 to 40volts with a 5urgb right?  The 390-0-390 reading was not from a spec sheet it was the actual ac voltage of the transformer unloaded (780vac total).

[dynaman1]

I say, go for it. Mebbe adjust a resistor in your power supply string to get the preamp where you want it. No sense in tossin' out good iron.

You could use a cheapo low voltage transformer before the PT to "buck" some wall voltage. I made myself a couple of boxes with these trannys so I could run 110VAC stuff without altering the old equipment. Give a holler if you want one. I never use 'em anymore. Course, you'd have to carry around the little box all the time or make a permanent setup in your amp.

Bah, just cook the tubes. They're cheap.

[rafe]

It was most likely removed by our moderators and rightly so. There are many reasons this is a great forum
two of the greatest are civility and respect for each other. There are always going to be different opinions on things here, and different approaches on various problems. I for one am totally grateful for the various advise, help and guidance  I have recieved here many many times over the years from different folks, I am a total novice and have slowly been learning to solve all problems on my own amps and I try to give back freely when I can . And I buy nearly all my supplies (not very much really) from Doug to help support this site, and they are great parts at a great price and are delivered faster than imaginable
 

[navdave]

I totally agree if it wasn't for this forum I wouldn't know anything about building or working on tube amps.
I still need to learn so much more so I'm very grateful for Doug and everyone else here at the Hoffman Forum.

[The_Gaz]

Yeah, that was the craziest thing I've ever read. It was deleted by the moderator, and rightfully so...

Anyway, I should elucidate on my concern for using those voltages. The voltage on the plates should be no problem with a high enough primary impedance on the OT (say 4-5k), but if the screens are seeing around 500v, or even 480 they might not be too happy. This week I've seen two amps with 480 on the screens - they would both glow along with the signal inside the plate structure. This is not a normal operating condition for the tubes, and means the screen is over dissipating.

In both amps, raising the screen resistors significantly or creating another pi filter in the B+ line to drop the voltage down got rid of the glowing. The downside, of course, was that the amps lost power - about 10 watts clean - and became very compressive.

Both of these amps had very stout PTs, only dropping 30v or so on the plates and screens under load, which did not put the screens in the safe zone. Altering the main B+ filtering did not effect the sag. Conversely, in a 100-watt amp with the same 480vdc or so B+ there is no glowing whatsoever. This is because the amp sags 90 volts under load, bringing the voltage down into the tubes acceptable limits. I ran tests with both E34L and KT77 tubes, FWIW. There wasn't much I could do in these amps, however changing the tubes got rid of the glow, but it was only a short period before it came back again.

  Loosechange dosn't have a 'cavalier attitude about voltage' just knows what do do with it.

Bah, just cook the tubes. They're cheap.

Really, guys? I don't think I'm out of line here telling the OP to be cautious with 540 B+ on modern tubes, which do not like the have their ratings exceeded. Biasing in a tonally acceptable range doesn't aid the screens from drawing too much current, and it's only a matter of time before they melt down.

[rdrgtr]

So, If I use the 5vac tube rectifier tap there would not be enough B+ drop to give the screens some relief?  Has anybody used copper caps in the rectifier socket to drop the b+?

[phsyconoodler]

Ok,with a rectifier tube the voltage will be down around 468-487v or so.Totally acceptable.And as far as burning down the screens,why not just drop the screen voltage down with a larger resistor?
  I agree that new production tubes don't like high plate and screen voltages but just drop them down and play the amp.
 Resistors are cheap and they don't get overly hot when dropping low current devices like screens.
    Don't worry about the rant.He's on the edge sometimes.He'll get over it.
We're all easy going here except a few high-strung individuals.

[rdrgtr]

I have been doing some research.  Since I have 5v rectifier taps, I can really get the b+ down by using a tube rec.  I have used tube rec. in two of my previous builds but they were more fenderesque.  This would be my first Marshall type build with one in it.  I understand the sag effect but what about the whole tonal effect with a pretty hot front end?  oh yea, does this guys b+ drop numbers look accurate?




http://www.300guitars.com/articles/rectifier-tube-voltage-drop-chart/

[The_Gaz]

I'd say that dude's (if you will) chart is pretty accurate. You could also "buck" some voltage using the 5v taps, which is a trick I picked up from Gil Garnet's book. I'll have to draw a picture for you if I can't find the schem somewhere online - I think it'd drop about 20 volts...

[navdave]

I built a bass amp using Marshall Major iron this thing runs under load about 670VDC!
Put a quad or Sovtek 6550's in this thing in 2007 and the are still wrakin!
I have it biased @ 30mA's using 3K on the screens but this doesn't do much to drop
the screen voltage. I say don't look back and let it ROCK!!
JJKT 77's cant handle 800VDC on both the plates and screens what a kick ass toob!!!!

[darkbluemurder]

With a B+ of 550V, KT88 tubes are probably the only tube type that will work safely.

Best to save that PT for a Hiwatt bass amp clone project IMHO.

Cheers Stephan

[phsyconoodler]

Use the tube rectifier.The amp will be more like a JTM45 with a little bit of sag.Not enough to cause any concern.A little squishier,a little softer.That's it.Use it.

[The_Gaz]

I built a bass amp using Marshall Major iron this thing runs under load about 670VDC!
Put a quad or Sovtek 6550's in this thing in 2007 and the are still wrakin!
I have it biased @ 30mA's using 3K on the screens but this doesn't do much to drop
the screen voltage. I say don't look back and let it ROCK!!
JJKT 77's cant handle 800VDC on both the plates and screens what a kick ass toob!!!!

Not sure if you even read my post, but JJ KT77s cannot handle 800 volts on the plates and screens, that is totally insane! I was getting the screens to glow bright orange with only 480 on the plates and a few volts less on the screens. You may think that's okay, but it means the screens are literally melting. 800 volts is the max voltage that can be applied to the elements, but that info on the datasheet doesn't take into account the OTs primary impedance. Those tubes would red-plate at any bias setting under those conditions. The whole point I was trying to make earlier was that the datasheets are contextual, and it's not just about voltage, but it's relationship to current and power (this is the whole ohms law works!). The tubes will draw too much current, and over dissapate. People are always throwing around the '800vdc max for an El34', but that would require about half that voltage on the screens, and an 11k primary for those tubes not to self-destruct.

I know you're not saying to actually apply those voltages, but you're implying that everything would be A-OK running them like that in guitar amp with a typical linear power supply, I assume, which is not the case at all. 

To the OP: The last two posts offer some sound advice. Don't be afraid to use the transformer, just take this as an opportunity to learn how power supplies and output stages work so you don't have to second guess yourself, or take any one's word for it. There's lots of crazy stuff out here on the interwebz! You might want to start here: http://valvewizard1.webs.com/se.html

[navdave]

I'm not saying hit em with 8hundo... You could get your power tubes glowing RED with 400VDC if they ain't biased rite.
The PI for a M Majors OT is 1.9k, like it or not those 6550's are running nice and strong with almost 700VDC.
Come on 250 to 800 is a BIG window 550 to 600 piece of cake http://www.jj-electronic.com/pdf/KT77.pdf

[The_Gaz]

http://ampgarage.com/forum/viewtopic.php?t=10502&highlight=redplate

Check out the 2nd page of this thread, where Randall Aiken explains why a guy's Marshall is redplating at any bias setting. It's a really nice explanation if you've never read how to draw load lines. Voltages are meaningless without knowing the primary Z of the output transformer, or the voltage on the screens, that's all.

[phsyconoodler]

The gaz said: "Not sure if you even read my post, but JJ KT77s cannot handle 800 volts on the plates and screens, that is totally insane! I was getting the screens to glow bright orange with only 480 on the plates and a few volts less on the screens. You may think that's okay, but it means the screens are literally melting."

  Who's talking about having 800v on this amp anyway?With a tube rectifier it will not have more than 480 or so.
And the current can be lowered with the bias pot and the screens can be handles with a larger resistor.
  The existing PT is a bit much with diode rectification but acceptable with a tube.That's basically the end of it.

[imaradiostar]

I hope I can add some positive stuff to this discussion. I think it's probably a great transformer- what you want to do with it is totally up to you. I have a ton of old iron around and I've been learning a few things with this little test rig I built. I most recently tested a transformer very similar to yours- a 400 VAC one from an old TV. Here's what I found.

First, I'd download Duncan's power supply designer, measure the DC resistance of the primary and secondary then start simulating some different designs.

For that matter, would you mind posting the DC resistance of the primary and high voltage secondary?

I assume 390 is the unloaded voltage.

Plenty of amps use very high plate voltage happily. I think it comes down to tube choice. With fairly traditional Fender/Marshall style output design you should be able to fairly easily run 6L6's or KT77's. You might have to increase the screen grid resistors to prevent them from melting when the amp is played loud or overdriven. This way you won't need any kind of crazy screen voltage regulation. I'd start with 1k 5 watt resistors per tube on the screen node. I'd also be tempted to use a resistor instead of a choke for the screen node. You can play with using different value dropping resistors and see how it effects the tone and volume of the amp. If you're worried about not having enough filtering you can always increase the capacitance on the screen node.

Stick with relatively low load impedances- too high an impedance with high voltages like this on the plate and screen can result in overloaded screens. I'd imagine the normal Fender 4k value is a good place to be.

Does the transformer have a 5 volt winding? If it does you can use a 5U4 or 5ar4 tube rectifier. I've found that a 5u4 that's loaded up in a 40 to 80 watt amp will give a DC output not too much higher than the unloaded AC voltage at the transformer, assuming it's a relatively high impedance transformer from the 50's or 60's that was intended for tube rectifier use. A 5ar4 has much less voltage drop but still much more than a solid state diode.

If you absolutely must use EL34's, here's how I'd do it. They have much more fragile screens so you'll need to lower the screen voltage. Use a second set of diodes into a choke input filter (normal Fender style choke will do). This will give your normal 500ish volt plate supply and a separate 300ish volt screen supply. You may need to use a resistor to ground from the output of the screen supply to keep the voltage from rising too much but this is an acceptable and affordable old-school design convention- I believe there is even a section in RDH4 about it though it's easy enough to arrive at a good resistor value by trial and error.

Since the screen voltage is low and plate voltage is high the max current the tube can draw will be less, saving the screens and creating a situation where a higher load would be helpful. I'd start around 6k and see how it goes.

Yes, I've built all of the above stuff and it was fun. It always feels good to use old parts and build something cool!

jamie