20 V too high for my TOS

[stingray_65]

Damn my eyes.

I ordered a Magnetic Components "Super Reverb" power tranny for my TOS build to match the Mag Comp OT I had new sitting on a shelf.

When I read the specs I swear it said 335-0-335, but as I can clearly see now that is not the case.

So when the tranny arrived at my door yesterday I threw it on my bench and took down the readings.

I thought I had my Variac set wrong when I read 355-0-355, I did not  

I'm wondering if I have to worry about this?

Quick math tells me this should be about 400V on the plates right? and maybe 375 on the grid?

6L6's live with this kind of voltages and 5881's are designed to hold up at like 20% higher ratings

I'll need to increase the value of the dropping resistor between nodes B and C to bring the rest of the voltages back in line.

But that’s roughly an 80V drop from 400V to about 320V and multiply that by say 7mA is only about .6W so I won't be having undue heat issues there.

My other concern is running those 5881's at 400V is they may be sucking as much as 125mA each and my tranny is rated at 200.

Suggestions anyone?

Thanks Ray

[eleventeen]

125 mils is definitely 50% overcurrent for *a single* 5881. According to this data sheet http://www.drtube.com/tubedata.htm you could run these up to 205 ma for a push pull pair in AB2 w/360VDC on the plates and -22.5 volts bias, but that's running them pretty danged hot in my book.

[tubenit]

So when the tranny arrived at my door yesterday I threw it on my bench and took down the readings.  I thought I had my Variac set wrong when I read 355-0-355, I did not 

If I take a voltage reading of any of my PTs with NO tube in the amp, all of them read higher than spec'd. They only read accurately for me with tubes in.  I think you might only get an accurate reading with the tubes in?  Maybe someone else can chime in here.

You certainly are NOT stuck with using 5881's either.  I have used 6V6's and 6L6's in the amp.

And remember IF you are using tube rectification (as I do with the TOS & COS) that you can use a 5Y3 with 6V6's ........ or a 5V4 or GZ34 with 5881/6L6.

My PT is only rated for 150ma. It's about 280-0-280 with wall voltage high & I am using a GZ34. 

Take a look at Geezer's schematic. He has voltages posted on it & compare.

With respect, Tubenit

[eleventeen]

Agree with Tubenit on no-load voltage readings. Although, I am building a tube matcher where the PT runs from a Variac and the filament tranny runs direct (two AC power cords, no waiting) and the fil trans is putting out about 7.4 volts which I do not like. Because sometimes I'll have 2 power tubes in there and sometimes 4 and the tranny is rated 6 amps. So I don't think 2 tubes will drag the voltage down much, and in fact, though the project is not complete, I've thrown some tubes in there and indeed the fil voltage is about a volt higher than I would like. I suppose the answer is to use the both-ways diode trick but of course I can't use 1N4007s of which I have a load, because total filament current is beyond their specs.

Also agree, a 5Y3 vs a 5AR4/GZ34 will knock off somewhere between 10 and 35 B+ volts, will it not?

[jjasilli]

Ditto to measuring voltage under load.  Also:

1. Primary voltage: PT's tend to be spec'd for 115VAC primary voltage, even though 120VAC or higher is more common in today's reality.  120/115 = 1.0435 X 335 = 349.  Search for "bucking transformers" if you want to knock-down your wall voltage (or maybe use a small variac).

2. 20% tolerance.  20% tolerance is good for an off-the-shelf tranny, unless a tighter tolerance is specified.  335 + 20% =  402!!!  Your 355 is w/in 6% of 335; this is as good as it gets.  

3.  The PT secondary voltage is AC prior to rectification.  After rectification the forward convserion factor for B+VDC is 1.414 (for SS diodes), less the impedance of a rectifier tube.  

[stingray_65]

No 5 V heater taps on this tranny so no tube recto.

advertised output is 355-0-555 and thats what I measured no load.

I forgot all about that 20% tolerance. I keep forgetting that building a tube circuit is a lot like hunting with a shotgun, just point it in that general direction

So I'll worry bout this till after the Power Tubes are in.

thanks guys,

[tubenit]

TOS =  Tweed Overdrive Special.

It's in this post with 2848 hits:  http://www.el34world.com/Forum/index.php?topic=9638.0
 

With respect, Tubenit

[stingray_65]

Fleabay came thru with a new matched pair of Sovetek 6L6WXT's for a tic under $25 inc shipping.

Just hoping my PT is up to it.

[Shrapnel]

hehehe... to add to the confusion TOS is also used to abbreviate Terms of Service.

Obviously it also is a long way off from how it is used here.

[FYL]

TOS?

top of stroke?

what thu?

I no understandy abbreviations, or texting?

TOS = The Original Star Trek Series
 

 

[mresistor]

TOS means many things to many people. Can't believe there's a wiki page on it...  

http://en.wikipedia.org/wiki/TOS

[PRR]

> In the oilfield, it is Top Of Stroke

In high-speed engines (cars etc), revved past redline, TOS is when the connecting rod will break.

Because the rod is short and angularity is high, the piston goes over the top quicker than it goes around the bottom. Stresses are highest over the top.

Also while we assume metals are strong in tension and compression, short squat rods are a bit stronger in compression.

There's two different TOSes. Top of compression stroke is cushioned by, uh, compression. Top of exhaust stroke, both valves are open, no cushion.

So an over-revved rod will uually break at TOS of exhaust stroke.

Thinking that a rod-pump will also break at TOS. Alternatively, at mid-stroke where fluid flow rate is highest.

> In the oilfield,....

In the 1950s I bopped around San Diego and Bakersfield. I loved watching the rocking birds. Different motions, mechanisms. graceful.

Look at Wikipedia: http://en.wikipedia.org/wiki/Pumpjack

"...more power to move the heavier lengths of sucker rods..."

That seems wrong to me. The rod is counterbalanced; anyway the rotary motion returns energy to the system. The actual "Power" is lifting the liquid (plus Friction, probably tons of the stuff).

There's a comments page on Wikipedia articles; I added a comment.

http://en.wikipedia.org/wiki/Talk:Pumpjack#.22more_power.22.2C_but_not_in_.22rods.22