PT Getting Rather Hot...

[ericnuke]

Here's my dilema (possibly?)

Built a Matchless Lightning into an old laney amp I had using a mojo tone PT for an 18 watt Marshall. 290x2 secondaries. It's what the layout I used called for. Well that PT is getting super hot after no more than 10 or 15 min of playing. So I assume that I'm drawing too much current through it.

I'm using a tube recto, GZ34 and putting out 354v or so B+

I fear my problem may be the fact that I used 47uf filter caps in place of the 33uf caps the layout calls for because I couldn't find the 33's.

I'm also getting some weird distortion when I hit low notes, happens the most on a C or D# on the E or A strings. But I think this is just a bad soilder joint somewhere I havn't found yet, or maybe something else???

Any help? I've looked through the forum already and couldn't find anything that helped. Thanks.

Here's the layout.

http://www.ceriatone.com/images/layoutP ... iatone.jpg

[jaysg]

1) remove all tubes, turn amp on, check for hot PT, turn off immediately if it starts to heat.
2) if it's good in step 1, verify voltages for the PT
3) add the rectifier and repeat from 1
4) if it's good in step 1, check the value of the 120 ohm, 10W resistor.
5) were the output tubes starting to redplate?

[Structo]

Have you checked your bias current?
That could tell the tale as far as what you are hearing.

Usually inadequate filtering causes the ghost notes if that is what you have.

Do you happen to know what the specs on the PT is as far as current?
For some reason nobody is publishing specs on the 18 Watt PT's......

You can take all the tube heater current ratings and add them up to see what you are drawing.
For instance, each 12ax7 draws .3 or 300ma of current.
So 3 x .3 is 900ma.
Each EL84 draws 750ma.
There is 1.5 amps.
So yo have 2.4 amps draw on the heater winding.

The GZ34 draws 2 amps on the 5v winding.
The GZ34 specs say you can have a first filter cap of 60uf so you are fine there.
So you're looking at almost 2.5 amps just on the heater winding.

[M Fowler]

The last Heyboer Marshall 18 watt PT I used was 290-0-290 (120mA)
5V 2A and 6.3V 3.25A if that helps?

Black/white 120V and black is common. Black/Red is the 240V main.

Mark

[ericnuke]

Thanks for all the input guys. I figured out the distortion/weirdness problem, of course it was an input jack bad solder joint and dummy me just forgot to solder one side of the slope resistor in. Always surprises me how I can go over and over my work and still miss something obvious like that. Oh well.

So the amp is working and sounds pretty darn good. PT is still getting pretty warm. I wanna say warmer than any of the other amps I've built so far so that concerns me a bit but I'm not gigging at the moment so it's not a pressing thing to need to have the amp on for hours at a time yet.

I pulled all tubes and checked HV and it came in at a hair over 300 which I guess was fine with no load on it. With just rectifier tube it came down a bit and then with all tubes in in came down to 294 which I assume is close enough to 290. B+ is still right at 354.

And I'm a little embarased to admit this but I've just never really taken the time to really learn how to bias a cathode biased amp. I built an 18 watt as my first amp and it sounded good and worked fine right off the bat so I never really messed with it. And the rest have been fixed bias which are, to me at least, much more straightforward, to bias. If anyone could point me toward a good learning example that would be awesome. Thanks.

[jjman]

Does the PT get warm with no tubes in? 300vac (per HV side) with no load sounds too low to me for a setup with 294vac under load.

I don't know what is should be but I remember seeing significant spreads in this department in the past on my own amps. Maybe your setup has a bleeder resistor that creates some load?

[Structo]

I copied this and will paste it here.

Usually for me, I just measure the voltage across the cathode resistor and divide that by the value of the resistor. That doesn't take in to account the screen current but that is usually only a few ma.

Here is the full meal deal:

Cathode Bias:

I will start out with the easy one first. The most common cathode biased amp is one that uses EL84 power tubes. It really is a fairly simple operation to check the plate dissipation using the following calculations.

Using a digital multi-meter, determine the cathode current using this formula:
Cathode Current = Cathode Voltage/Cathode Resistor Value (Ohms)
This will give you the cathode current in milliamps.

Again, using your digital multi-meter, determine the screen current by using this formula:
Screen Current = Voltage Drop across Screen Resistor/Screen Resistor Value (Ohms)
This will give you the screen current in milliamps.
(If no screen resistors are present, you will have to determine the screen current directly by using your digital multi-meter)

After these two steps have been completed, you can determine the plate current by using this formula:
Plate current = Cathode Voltage/Cathode Resistor Value (Ohms) - Screen Current
This will give you the plate current in milliamps.


Finally, you can now calculate plate dissipation in watts by using this formula:
Plate Dissipation in Watts = (Plate Voltage - Cathode Voltage) x Plate Current


Assuming that you have a cathode biased amp using EL84s, you do not want to dissipate more than 12 watts, which is the quoted "design center" value on every EL84 data sheet that I have seen. Exceeding this value can dramatically shorten the life of the power tubes and possibly cause major problems if one should fail while in operation. It's not a bad idea to at least check the plate dissipation when replacing the power tubes in a cathode biases amp - no two power tubes are exactly the same. Some may tend to move more current than others. If you find that the plate dissipation is over 12 watts, increase the size of the cathode resistor until the plate dissipation is at 12 watts or under - 10.5 to 11.5 watts is a pretty good operating range and will give longer tube life. Cathode bias is generally very forgiving due to its very nature. However, don't assume that just because your amp is cathode biased that it is bulletproof - you just never know what you are going to get with new manufacture tubes.