PT getting hot

[Ed_Chambley]

I have a SE, 2 EL34 parallel a 12Ax7 and an EF86 tube in a customer amp.  The additional EL34 was added recently.  It was brought to me to get rid of some noise.  That was easy.

The problem is I noticed the PT getting too hot IMO.  It is an old Stancor 260-0-260 with 90ma.  Is this enough ma for these tubes?  How can one determine the total ma needed from the data sheet?

[kagliostro]

you must pay attention to the Heater consumption one more EL34 is 1.5A more consumption, not only the few mA in B+

K

[Ed_Chambley]

you must pay attention to the Heater consumption one more EL34 is 1.5A more consumption, not only the few mA in B+

K

The heater is 4a, 6.3v.  Should be plenty for 2 EL34 and a couple of preamp tubes.

[HotBluePlates]

What's the primary impedance of the OT? (you'd need to know this to get a guesstimate of plate current drawn from the power supply)

And how close are EL34's to the PT? Is there a tube rectifier? Is it close to the PT, too? (tubes could radiate heat making the PT seem hot)

[Ed_Chambley]

What's the primary impedance of the OT? (you'd need to know this to get a guesstimate of plate current drawn from the power supply)

And how close are EL34's to the PT? Is there a tube rectifier? Is it close to the PT, too? (tubes could radiate heat making the PT seem hot)

The OT is multitap.  FTCO-M Mercury.  9.2k at 16 ohms.  Has 8 ohm and 4 ohm.

The chassis is large and there is plenty of room between tubes.  SS rectification.  I have 347 vdc plates.  When he had on one EL34 tube in it he ran an 8 ohm speaker.  I think the PT should be fine, but I have never used a 90ma tranny.

[Ed_Chambley]

K you stole my comment.  If you have a filament transformer lying around, wire it in. else shut it down.  I found a 1959 catalog that shows there is only 3 amps available on the 6.3 windings.  Model  number pm8404. here's the link http://www.tubebooks.org/file_downloads/stancor_1959.pdf  see page 15,

If I am reading your post correctly you are drawing about 3.5 amps on the heaters
1.5x2 for the el34's, 0.3 for the the 12ax7 and 0.2 for the EF86. 

The PT is PC-8420.  Specs 260-0-260, 90ma, 6.3v 4.0a.  No 5v winding.

[HotBluePlates]

25w/350v = 0.071A  ->   x2 is 140mA idle.

You might assume placing the 8Ω speaker on the 16Ω tap to get 4600Ω primary (for the 2x EL34). If there are 300v to swing across this, that's only 65mA peak for the pair of tubes to draw above their idle amount, or 205mA peak.

The current draw from the PT should still average the 140mA at idle, but this is more than 50% over the rating. That seems like it should cause the PT's voltage to sag. 260vac * 1.414 = 367vdc, and you say you have 347vdc. Maybe the PT voltage is sagging due to too much current draw.

I don't know that you're presently idling as hot as what I'm showing up top, but maybe a PT with 150mA capacity would be a good thing to get.

[Ed_Chambley]

25w/350v = 0.071A  ->   x2 is 140mA idle.

You might assume placing the 8Ω speaker on the 16Ω tap to get 4600Ω primary (for the 2x EL34). If there are 300v to swing across this, that's only 65mA peak for the pair of tubes to draw above their idle amount, or 205mA peak.

The current draw from the PT should still average the 140mA at idle, but this is more than 50% over the rating. That seems like it should cause the PT's voltage to sag. 260vac * 1.414 = 367vdc, and you say you have 347vdc. Maybe the PT voltage is sagging due to too much current draw.

I don't know that you're presently idling as hot as what I'm showing up top, but maybe a PT with 150mA capacity would be a good thing to get.

Or like I suggested. Getting rid of one of the EL34.  That should work shouldn't it.  Placing an 8 ohm speaker on 1 EL34 should you still use the 16 ohm tap or should it be a 4 ohm speaker connected to the 16 ohm tap.  SE OT confuse me as I don't mess with them much.

I think it goes like
16 ohm tap to 8 ohm speaker halves the reflected impedance and that is the 4k6.  If you attached 4 ohm speaker would it reflect 2k3?

[Ed_Chambley]

I did a search for the Mercury FTCO-M and  the only reference shown is a fender tweed amp. no other info, which suggests 5c or 5d series of amps.  You stated this was an SE.  Is it possible that the O/T is a PP?

Not Push Pull.  It is an oversized Champ style transformer.  According to Patrick at Mercury, it is designed up to 15 watts.  FTCO means Fender Tweed Champ Output.  The m on the end means multi tap.

[HotBluePlates]

Or like I suggested. Getting rid of one of the EL34.  That should work shouldn't it.  ...

Yep, that'll work.

...  Placing an 8 ohm speaker on 1 EL34 should you still use the 16 ohm tap or should it be a 4 ohm speaker connected to the 16 ohm tap. 

Yeah; in fact, I guess 2kΩ would have been closer to ideal for the original case of 2x EL34's. But now that you're thinking of going back to one, use your 8Ω speaker on the 16Ω tap. Reflected Pri Z drops by half to 4.6kΩ.

Or 4Ω of speaker load on the 8Ω tap.

[PRR]

How hot is hot? If you can hold it more than 10 seconds, give a 90 day warranty and take your chances. If you can't hold it a second, you MUST get that current way down.

> put the 8 ohm speaker on the 4 ohm tap?  Your power draw should drop.

ONLY at full roar.

Full-output power consumption is about loading.

Idle power consumption is only bias; load does not matter (isn't doing anything).

Generally the idle and full-roar currents are very-near the same.

So he could test the full-roar current, then change the bias to a similar current.

Or do the math.

If you have 347V and max 90mA....

Assume ~~5mA for little tubes. 85mA in power tube.

Part of the 347V is lost in cathode resistor. Pencil 325V across the tube.

EL34 screen current tends to be 10%. So 90% of 85mA is 76mA in plate.

325V/76mA gives 4,300 Ohm optimum load (aim low, 4K).

325V*76mA is 24.5W dissipation in plate (one EL34 is working 99% of conservative rating; two EL34 at 38mA each are loafing half-rating each).

38mA in one EL34 at Vg2=~~350v means about -30V grid-cathode bias. 800 ohms cathode resistor per tube.

76mA in one EL34 at Vg2=~~350v means about -24V grid-cathode bias. 320 ohms cathode resistor.

You could maybe use one common cathode resistor at 400 ohms and let him use one _or_ two tubes to taste.

Power output is near 10 Watts either way.

[Ed_Chambley]

How hot is hot?


[/quote]
How high is up?
Thanks for the instruction.  In reality I know the PT is overloaded.  I can put my hand on it, but if you grabbed it it would burn.  I have a PT that will do the job as he wants, but it is way overkill.  I tried it last night and I cannot tell any difference between one and 2 tubes.  Maybe a tad fuller and mid volume.

The problem with the 90 day warranty is he has been running the amp stressing the PT for a few days.  He says he is not looking for more wattage, just "rounder."  I plugged in a 6l6 and a 6550.  The EL34 actually sounds more fender like which I thought of as strange.  I tried many different tubes.  Old and new alike.  To me the largest change was replacing the EF86 with an NOS Tesla and a RCA 12Ax5 Black plate.  Not a lot of difference in power tubes but had a nice increase in 2nd Harmonics.

If you were designing this amp with this tube lineup, EF86, 12Ax7 and 2 EL34, what would be the total ma's needed to provide and not be overkill and stiffen the amp too much?

[jojokeo]

How high is up?

If you were designing this amp with this tube lineup, EF86, 12Ax7 and 2 EL34, what would be the total ma's needed to provide and not be overkill and stiffen the amp too much?

Nice! - How was the 6550 we discussed? I personally like the new "Mullard" EL34 & KT77 as far as sound/tone goes. They make great sounding '34s IMHO. Tried many others and went back to those.

Going by the book you should plan 2mA+3mA+100mA+100mA=205mA @ ~250Vdc minimum and use a 1K to 2K OT 15watt minimum. But as HBP said, you could get away w/ a high voltage PT @ 325-0-325 and drop the mA to around 150mA and use the commonly available 50watt Plexi/JCM PT. But if it were me, I wouldn't run those two tubes in parallel and would rather go class AB1 w/ cathode bias which would keep power down, give the fuller tone w/ more headroom he's looking for. You'd be making better use of those tubes & iron as a result. If you want to get more control & state-of-the-art, install a vvr along with it and you're set. The OT would be more appropriate as far as it's rating but not sure if this a pp type or not? So you'd need a similar rated one but not SE if that's what it is?

[Ed_Chambley]

How high is up?

If you were designing this amp with this tube lineup, EF86, 12Ax7 and 2 EL34, what would be the total ma's needed to provide and not be overkill and stiffen the amp too much?

Nice! - How was the 6550 we discussed? I personally like the new "Mullard" EL34 & KT77 as far as sound/tone goes. They make great sounding '34s IMHO. Tried many others and went back to those.

Going by the book you should plan 2mA+3mA+100mA+100mA=205mA @ ~250Vdc minimum and use a 1K to 2K OT 15watt minimum. But as HBP said, you could get away w/ a high voltage PT @ 325-0-325 and drop the mA to around 150mA and use the commonly available 50watt Plexi/JCM PT. But if it were me, I wouldn't run those two tubes in parallel and would rather go class AB1 w/ cathode bias which would keep power down, give the fuller tone w/ more headroom he's looking for. You'd be making better use of those tubes & iron as a result. If you want to get more control & state-of-the-art, install a vvr along with it and you're set. The OT would be more appropriate as far as it's rating but not sure if this a pp type or not? So you'd need a similar rated one but not SE if that's what it is?

The 6550's sound ok, but the EL34's just seem more balanced.  6l6 is the same.
This is going to get to you, but I prefer original Mullard XF2.  Remember, I told you I scored 2 cases of them years ago.  They are in matched pair boxes.  I fried one a couple of years ago so I sold him the other one for cheap.

I have a lot of =C=.  Real ones which are durable as hell.  One of my old Marshalls would eat new production tubes so I started buying =C= when they held up.  I have the same quad in the amp and they are running fine after 8 years.

The OT is an SE.  He wants a single ended amp.  I built him a 30 watter Push Pull using EL34's, but no VVR.  It has the LarMar master which I prefer over a VVR.  I have never had much luck with a VVR.  A VVR seems to cause the bottom end to get too loose for my taste.  It is probably me not doing it correctly because it seems a lot of people like them, but a 30 watt speaker running at 1 watt sounds like chocolate cheese.

[jojokeo]

Chocolate cheese?!
Nah, the vvr doesn't go down that low or at least it doesn't have to. That can be set easily by changing a resistor. But forget about that. You know what you & he both are after. I don't play my vvr but down about halfway only. As it goes down further the grid starts to become a lot more sensitive anyway so the preamp signal will blast it easily. I just find my happy spot and I'm good to go. These are only suggestions & feedback anyway. I can run 6L6 & others too but I also like EL34 or KT77 in there and just leave it be. It's also got a nice Brittan Heerleen 12ax in there too through a Celestion Blue. Perfect set-up...for me anyway. My gain structure sets three ways two stages at a time and w/ a tone stack tweaked bypass if wanted, it's got all the versitility I need and then some.

PS - all this talk about finding the "perfect" reflected load z on this type of amp is just that - a load of manure (no offense to anyone). The over-concern for this is way overkill and unneccessary. Once you get in the "Mr Roger's" neighborhood you won't hear the difference no matter what tap you plug into.

[HotBluePlates]

I still believe you want to put that nominal 8 ohm load on the 4 ohm tap. ...

That reflects a double-normal primary impedance. I bet Ed knows the primary is 9.2kΩ by either measurement or asking MM. 8Ω of speaker on the 4Ω tap makes the primary impedance 18.4kΩ.

"Normal" for a SE EL34 would be in the range of 3-5kΩ for a single tube. So my suggestion of 8Ω on the 16Ω tap still only provides a good load for 1x EL34; 2x EL34 would be more-properly loaded by 2-3kΩ.

... Lets look at power draw, given the 1:1.4:2 voltage ratio typical of a 4-8-16 ohm output.  Since power = volts*volts/resistance, you would have
1 squared over 8 or 1/8, versus 2 squared over 8, or 1/2. ...

The problem is the voltage ratios only hold if the same power is put through the OT in all settings. That is, if there is 15w of power output through the OT, the ratios of voltage between the taps is 1:1.4:2 but only if a 4Ω load is on the 4Ω tap, or an 8Ω load on the 8Ω tap, etc...

When you change the loading/mismatch, you change how the load impedance seen by the output tube, which changes its power output before you consider the effect of differing voltage at different taps.

There is also a potential danger in an extremely high load impedance presented to an output tube driven hard: it makes the loadline cut way below the knee of the plate curves when pushed, and screen current can be excessive. It's enough of a problem that RCA's design procedure for output pentodes/beam power tubes was to draw multiple lines at different slopes radiating from the knee of the plate curves, and do the math to see which provides the best power output vs distortion.

... I agree with PRR that the power draw goes up as we crank up the gain (volume) ...

Power drawn from the power supply stays the same from idle to full-roar because SE mandates class A operation. There will be a little bit of rise in current at full power, but that is due to distortion and rectification effects (in the output tube).

What actually happens is the tube dissipates the most power when it sits idle, but dissipates much less at full power output, because of the power transferred through the OT to the speaker.

[DummyLoad]

The problem is I noticed the PT getting too hot IMO.  It is an old Stancor 260-0-260 with 90ma.  Is this enough ma for these tubes?

no. not even close. waste of time w/ 2 x EL34 - suggest a downgrade to 2 x 6V6 or upgrade PT.

--pete