Did I give too much "work" to my PT ?

[archaos]

Hi guys,

             I had some questions concerning my PT (Hammond 290NX) voltages. Yesterday evening I had been playing with my new built from scratch AC30 (no more crackling) for +/- 2 hours until it suddenly stopped to work  :shocked: No more sound ; the jewel light was off. At first, I thought my PT had blown since both fuses (3A & 0.5 A) & power light didn't blow themselves. Nonetheless, the PT was hotter than a horse fever, I could have fried eggs.
I was scared & sad. 1/2 an hour later it worked again, I was therefore even more surprised. 
Then stg came to my mind. Indeed, I live in France  where AC outlets give usually 220V, so I had used the 220V tap of my PT, but while measuring the AC outlet whith my DMM, it read 231V.

So here's at last the questions : were this breakdown & the overheat due to a too strong voltage causing too much work for the PT ?
                                               If so, would you recommend me to use either the 230V or 240V tap ?
                                           

Thank you for your answers...

[phsyconoodler]

you may have some fluctuation of wall voltage,just like it is over the big pond,but to get the PT so hot that you could fry eggs on it is not a good thing.It may be you are drawing too much current.Check the voltages in the amp and work out the bias of the EL84's.
 Does the amp have adequate ventilation?Or is it in a cab like the original AC30's were that basically cooks everything inside?
And many PT's run pretty hot regardless.They need to be hot enough to burn your hand befor eit's considered too hot.
  The cutout bother's me though.You need to check those operating voltages especially the heater voltages.If this is a new build,you should have done that before just randomly choosing the 220v tap.

[jojokeo]

My two pennies thinks not about voltages but rather current being used. With higher voltages less current is being drawn - all things being equal and less voltages more current is needed for the same power output. Trannies heat up because of current being drawn from them - and also it's pretty obvious that if they don't have a way to dissipate the heat that it will lead to possibly a run away condition. Is this tranny running very close to it's limits current draw-wise either w/ heater current, B+ current, or both?

[rzenc]

I don't think it will answer your questions, just food for thoughts...
Hammond lists 290NX as a drop in replacement for vintage AC30 and specs H.T. as 580VCT@ 0.262A. It also shows 220VAC primary winding - which is what you are using??
Hammond data sheet: http://www.hammondmfg.com/pdf/EDB290NX.pdf
EL84 data sheet says maximum cathode current is 0.065A`s. So if all 4 EL84`s were drawing full current - and I believe they won`t - this means = 4*0.065= 0.260A. But at suggested conditions per data sheet (300V @ plates, 300V @ G2, cathode resistor 130 ohms, O.T. load 8000 ohms) it would be dragging 4*(46+11)=4*(57)=228mA`s = 0.228A`s.
How many preamp tubes do you have on your amp??
Check your voltages against data sheet and compare.

Hope this helps
With Respect
Best Regards

Rzenc

[archaos]

you may have some fluctuation of wall voltage,just like it is over the big pond,but to get the PT so hot that you could fry eggs on it is not a good thing.It may be you are drawing too much current.Check the voltages in the amp and work out the bias of the EL84's.
 Does the amp have adequate ventilation?Or is it in a cab like the original AC30's were that basically cooks everything inside?
And many PT's run pretty hot regardless.They need to be hot enough to burn your hand befor eit's considered too hot.
  The cutout bother's me though.You need to check those operating voltages especially the heater voltages.If this is a new build,you should have done that before just randomly choosing the 220v tap.

Is this tranny running very close to it's limits current draw-wise either w/ heater current, B+ current, or both?

OK, here's the valve voltages : http://www.el34world.com/forms/valvedata/JMI%20AC30_1.htm
Yes, indeed you can consider my amp as a baking oven since it's built with an original JMI cab...
Concerning the heater voltages the DMM reads 6.89V exactly like my other AC30 TB, using the stock 240V Drake PT. How yet can I work out the EL84's bias, please ?

[jojokeo]

I"m going to try to go by memory for the sake of time & testing my memory,
Measure the output tube's actual cathode resistor and write it down (don't go by value)
Measure the plate voltage, measure the cathode voltage.
calculate current though resistor I=V/R
Calculate power dissipation, P=VI

Let amp heat up for a minute or two before taking measurements:
say you measure 7V at cathode, 148 ohms at resistor, 330V plate V
I=7/148
I= .047 Amps for resistor current

Power dissipation
pD=330 * .047
pD=15.5 watts

This is too high as dissipation from an EL84 should be 12w or less so choose a larger resistor and re-measure again. Another possibility is if you can lower plate voltage or not.

[archaos]

It also shows 220VAC primary winding - which is what you are using??

Yes.

How many preamp tubes do you have on your amp??

3.

Measure the output tube's actual cathode resistor and write it down (don't go by value)
Measure the plate voltage, measure the cathode voltage.
calculate current though resistor I=V/R
Calculate power dissipation, P=VI

Here's the new valve voltage (240 V tap used) : http://www.el34world.com/forms/valvedata/JMI%20AC30_2.htm
Not sure @ all to understand your explanations about power dissipation, as far as the cathode reads 8.85V, the plate 308V, but the resistor 47 ohms...

[Geezer]

8.85 cathode voltage / 47Ω Rk = 0.188A (188mA for all 4 tubes)

0.188 / 4 (tubes) = 0.047A (47mA) per tube

.047*300v (plate voltage ~308 MINUS cathode voltage ~8v = ~~ actual plate voltage) = 14.1W dissipation...too high

Try raising the cathode resistor from the current 47Ω up to ~~75Ω (or even slightly higher?) then re-measure the voltages on the plate & cathode of the EL84's. We want to get closer to 12W dissipation...

G

[jojokeo]

Thanks Geezer, I was pretty close just shooting from the hip and remembering what you told me about 5 months and two amps ago.

[Geezer]

Not sure @ all to understand your explanations about power dissipation

Jojo, you were right on.....I was just trying (in the short amnt of time I had) to show how & what was going on with figuring the dissipation....

[archaos]

Got it ! Thanx a lot guys. I actually had some doubts myself, & had multiplicated the resistor value 47 ohms x 4, but wasn't sure. 
I will therefore try another res. value & will report ASAP...

[archaos]

We want to get closer to 12W dissipation...

As far as Wa limiting value is 12W for JJ EL84's, should I be very close from 12W, i.e. 11.85W for instance using a 56 ohm res. or below, like +/- 9W using a 75 ohm res. ?

[rzenc]

I would try both, but before installing your chassis back to cab, run it for a couple of hours to be sure it is not replating. Check both in order to hear how it will sound. Crank volume and let the beast roar!!
Original JMI AC30 were famous for overheating inside it's cab. Maybe you can find a way to install a small fan to ensure ventilation and cooling.

With Respect
Best regards

Rzenc

[jojokeo]

Jojo, you were right on.....I was just trying (in the short amnt of time I had) to show how & what was going on with figuring the dissipation....

No, I'm still like little Kwai Chang Caine and you're Master Po. I only gave an example for one tube and forgot to subtract the cathode voltage from the plate voltage.  

[PRR]

> My two pennies thinks not about voltages but rather current being used. With higher voltages less current is being drawn - all things being equal and less voltages more current is needed for the same power output.

That can be true if the load is Constant-Power.

This is nearly true for PC power supplies and electric saws. The PC supply will pull just what is demanded by the 3.3V CPU and other stuff. The CPU and other stuff does not know what the wall-voltage is. Saws pull just the power needed to cut as fast as you push the wood in.

Now look at an electric heater. Say it is a 10 ohm resistor. At 100V it will pull 100V/10r= 10 Amperes and dissipate 100V*10A= 1,000 Watts. At 200V it will pull 200V/10r= 20 Amperes and dissipate 200V*20A= 4,000 Watts.

Tubes are not constant-power, unless you get very-very clever. In most bias schemes, they are very-nearly constant resistance.

> I had used the 220V tap of my PT, but while measuring the AC outlet whith my DMM, it read 231V.

Transformers have a Rated Voltage. If you go low or a little high, the output voltage changes in proportion. Since most loads draw current in proportion to voltage (constant-resistance), the current will be low or a little high in proportion. This is usually OK.

If you go a LOT over the rated voltage, like 220V or even 150V on a "115V" winding, you have BIG trouble. If you measure the DC resistance of a transformer primary, and calculate what the actual voltage/current ratio is when working on AC, you see that the primary "must" have high impedance for AC. This happens because the iron core fights AC variation. But only to a point. If the AC voltage is too high, the iron stops working like iron and acts more like air. The primary starts pulling BIG excess current, hums loudly, and melts quickly. However there is always a safety-factor. "120V" outlets may be 108V to 128V or even 132V for a short time. Hammond has been in the business a looooong time and is known for good stuff. I would assume their "115V" windings can stand 130V day and night, and 140V for hours. Your "220V" winding should stand 250V easily.

[archaos]

> I had used the 220V tap of my PT, but while measuring the AC outlet whith my DMM, it read 231V.

Transformers have a Rated Voltage. If you go low or a little high, the output voltage changes in proportion. Since most loads draw current in proportion to voltage (constant-resistance), the current will be low or a little high in proportion. This is usually OK.

If you go a LOT over the rated voltage, like 220V or even 150V on a "115V" winding, you have BIG trouble. If you measure the DC resistance of a transformer primary, and calculate what the actual voltage/current ratio is when working on AC, you see that the primary "must" have high impedance for AC. This happens because the iron core fights AC variation. But only to a point. If the AC voltage is too high, the iron stops working like iron and acts more like air. The primary starts pulling BIG excess current, hums loudly, and melts quickly. However there is always a safety-factor. "120V" outlets may be 108V to 128V or even 132V for a short time. Hammond has been in the business a looooong time and is known for good stuff. I would assume their "115V" windings can stand 130V day and night, and 140V for hours. Your "220V" winding should stand 250V easily.

Thanx a lot PRR, your explanations are a light bulb hanging over my head  .

[archaos]

Hi there !

               I've swapped my former 47Ω power cathode res. with a 56Ω, & got the following measurements (V) :

                 - k/g3 =   9.53
                 -   a   = 315
                 -  g2  = 309

               It actually seems that whereas I've increased the res. value, the voltages have increased too, thus the dissipation remains the same : +/- 14W. 
               Therefore, here's my main question : is there any link between that resistance & the tube voltages ?
               Would it be then judicious to install another res. with a higher value ?

Thanks.

[jojokeo]

I get 12.99 watts power dissipation. Go up to 68 ohms and this should put you right at 12w. But remeasure everything to make sure then button her up.

[archaos]

I get 12.99 watts power dissipation. Go up to 68 ohms and this should put you right at 12w. But remeasure everything to make sure then button her up.

OK, thanx. 

[archaos]

I get 12.99 watts power dissipation. Go up to 68 ohms and this should put you right at 12w. But remeasure everything to make sure then button her up.

68.9 Ω cathode resistor, & this time :

                                                    - k/g3 =   10.48V
                                                    -   a   = 325V
                                                    -  g2  = 319V

 

[jojokeo]

power dissipation = 11.95 watts = perfectomundo!

btw, cows on the beach!? Last time I saw that was when I was on the Peninsula de Nicoya in Playa Samara, Costa Rica

[archaos]

power dissipation = 11.95 watts = perfectomundo!

:toothy12:

btw, cows on the beach!? Last time I saw that was when I was on the Peninsula de Nicoya in Playa Samara, Costa Rica

Here it's in Corsica, where cows are free...