Tubes for 72 OR80?

drbob1 · Post by **drbob1** » Fri Dec 13, 2013 9:57 pm

The caps are in, handle fixed, bias adjust pot neatly installed… Now I have to decide on tubes (came with a dogs breakfast of assorted modern tubes). Will my Siemens EL34/6CA7s (Germany) handle the B+? I do have some Mullards but I'm kinda saving those for a rainy day… Also, any suggestions about preamp tubes? I've got ANOS long and short plate Mullard, Amperex, Telefunken, Phillips Heerland and a few others, as well as a LOT of different american tubes. I"m assuming short plate for V1 to prevent micro phonics and long plate for V2?

drbob1 · Post by **drbob1** » Sat Dec 14, 2013 6:42 pm

No one has any ideas? Over at TGP the amp/tube experts are suggesting (as I thought I'd heard before) that the RFT/Siemens tubes aren't happy at more than 460+ plate volts. And several have suggested that an OR80 plugged into modern wall voltage is running closer to 550. Anyone?

yossarian83 · Post by **yossarian83** » Sun Dec 15, 2013 1:22 am

Since vintage oranges were setup for 110 input voltage, the internal voltages are gonna be higher than nearly any EL34 is spec'd for. My solution was to get a step-up transformer (30 bucks) and run the amp off of the 240 tap. This brought my OR120's plate voltage from 520 to 470, which is where the old schematics say it should be. This extended tube life significantly and you should be able to run just about any el34 at that voltage.

Gladmarr · Post by **Gladmarr** » Sun Dec 15, 2013 1:37 am

Another option would be to swap around the primary windings til you get close to the right secondary voltages. Usually the difference between the 110 lead and the 240 lead is close enough.

yossarian83 · Post by **yossarian83** » Sun Dec 15, 2013 1:44 am

Interesting point, I hadn't thought of that. I'm gonna try it!

yossarian83 · Post by **yossarian83** » Sun Dec 15, 2013 2:34 am

This is probably a dumb question but how would I go about this?

Gladmarr · Post by **Gladmarr** » Sun Dec 15, 2013 3:12 am

You'd have to hardwire it that way, bypassing the mains select switch. Make sure the fuse is still in the primary circuit, though. Someone sent this in to the OFG. I still gotta upload this along with a lot of other updates that need to go in there.

drbob1 · Post by **drbob1** » Sun Dec 15, 2013 5:10 am

I'm drawing a blank on what you actually mean here… The primary windings are not independent? just different taps off the total primary winding? So if you wire one end of the 240v winding to one side of the cord, and the same side of the 110v winding, you're in effect using a 130v winding?

Gladmarr · Post by **Gladmarr** » Sun Dec 15, 2013 7:06 am

The primary windings are not independent. Each mains voltage selection is just one tap on the primary. So, if you put your fused primary line on the 240 tap and your neutral at the 110 tap, it's like a 120-130VAC primary. You'd have to insulate the line you took off the neutral wiring and make sure you aren't connected to the mains selector anymore. You'd have to test your secondary voltages to make sure they're correct before you try to use it for real.

drbob1 · Post by **drbob1** » Sun Dec 15, 2013 9:33 pm

Have you seen this before:

http://www.geofex.com/Article_Folders/v ... ntvolt.htm" onclick="window.open(this.href);return false;

Using a 10v AC transformer to shave 10v off the input voltage dropping the B+ by around 10%?

nlimbaugh · Post by **nlimbaugh** » Mon Dec 16, 2013 2:02 am

SInce you only have two powers and two pres, I think NOS or ANOS is the way to go. Triple mica black plate GE or Sylvania for pres IMO. For power, I'd like to get Mullards but haven't had the chance yet. The JJ kt77's I have for powers are ok, but I won't get them again.

Gladmarr · Post by **Gladmarr** » Mon Dec 16, 2013 2:15 am

nlimbaugh wrote:The JJ kt77's I have for powers are ok, but I won't get them again.

May I ask, why?

jontheid · Post by **jontheid** » Mon Dec 16, 2013 3:21 pm

Will my Siemens EL34/6CA7s (Germany) handle the B+?

They work fine in my Orange with 500V on the plates.

Have you actually measured your plate voltage yet?

drbob1 · Post by **drbob1** » Mon Dec 16, 2013 4:19 pm

Headed over to the tech with some tubes and my variac later this morning. With no tubes in place he measured 540v so I'm guessing in excess of 500v with the tubes in there. It's apparently a bit of a gamble with Siemens tubes at 500v, and at $100 a pair, not sure it's a gamble I want to take. The transformer idea is looking better and better!

Gladmarr · Post by **Gladmarr** » Mon Dec 16, 2013 4:30 pm

The reference I use is the filament voltage. If you have much more than 6.3v on your filaments with tubes in place, your transformer is expecting to see 110VAC and seeing higher in reality. If that's the case, you want to do something to keep the B+ voltage in check and the filament voltages in check too. High filament voltages will decrease your filament life and by extension your tube life. Some tube data sheets have life expectancy curves based on filament voltage. As volts go up, life expectancy goes down.

Tubes for 72 OR80?

Tubes for 72 OR80?

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