rocket head room

[martin manning]

You have something that has degraded over time, and that would most likely be tubes. Have you tried a new set of power tubes? I'd try swapping them out first to see if that cleans it up, then check the idle current and dissipation.

To check the bias, it's easy to measure the total idle current by measuring the voltage across the cathode resistor. Divide that by the resistance (50 Ohms), and divide that result by 4 to get the average cathode current per tube. Then, measure the voltage across the four screen resistors, average it, and divide that by 100 to get the average screen current. Subtract that from the average cathode current, and multiply the result by the plate voltage to get average plate dissipation in Watts. You should get about 70% of 12 (the maximum), or ~8.4.

[M Fowler]

Did the overall volume of the amp decrease as well? I am wondering if the OT is bad?

Check over the output jack grounds and make sure all the OT secondary wires are soldered well. In my above post I was thinking that one triode was not working and I had a heater wire with bad solder joint, once fixed the volume came back up.

Mark

[brodie900]

ok might be onto something here, i have previously swapped all the tubes out but i pulled a used set of el84's from another amp NOT a new pair.. duh. i just measured my cathode current and if i did it right then it looks like my bias is way hot at 50 ma

cathode resistor has 10v across it

10/50= .2

.2/4= .05 per tube

[martin manning]

You should probably increase the cathode resistor to ~60 Ohms, as a guess. What are the plate and screen voltages? A better estimate can be made with that information.

[brodie900]

ok my screen voltage (pin 9?) is 320 and the plate voltage (pin 7?) is 332. iv never had to trouble shoot this far before and im really appreciating all your help

[brodie900]

does that mean my average plate dissipation is 5.9?

[brodie900]

i worked out the average screen current to be .068

[martin manning]

The 332 plate voltage is a bit high compared to the expectation on the voltage chart, and the difference between that and the screen voltage seems high... it should be only a couple of volts.

Can you post the Plate and screen supply voltages (B+1 and 2), the cathode voltage (top of the cathode resistor) and the voltages on all power tube pins 2 (grid), 7 (plate), and 9 (screen)? Take care due to the high voltages involved, and measure these all at the same time so that they are all consistent.

[brodie900]

b+1 = 330
b+2 = 320
cathode V = 10.3

power tubes;

1. pin2 = 20mv, pin 7=320v, pin 9=311,

2. pin2 = 20mv, pin 7=320v, pin 9=312,

3. pin2 = 18mv, pin 7=322v, pin 9=311,

4. pin2 = 18mv, pin 7=322v, pin 9=311,

[martin manning]

OK, so

Cathode current is 10.3V Vk/50 Ohms=0.206A or /4=0.0515 per tube

Avg. screen current is (320-311.25)/100 Ohms=0.0875?? This doesn't look right, like 10x too high, and not real because its more than the cathode current. The screen resistors are 100-Ohm, correct? Measure them with the power off and the caps drained to be sure.

In any event, based on the cathode current and 321-10.3=320.7 Va-k (sorry, forgot to mention subtracting the cathode voltage above), the dissipation is 320.7*0.0515=16.5W! Even if screen current is ~10% of the total that is well over the maximum.

These are known-to-be-good power tubes? All the other tubes were in when you made these measurements?

[brodie900]

wow... im in way over my head now! yea they were brand new when i put them in, thought id give a set of new production mullards a go

[brodie900]

oh and yea all the tubes were in when i took those measurements. maybe i did something wrong ill double check tomorrow. my screen resistors are 1k not 100r. thats the stock value for the ceriatone clones

[martin manning]

...my screen resistors are 1k not 100r. thats the stock value for the ceriatone clones

OK that makes more sense, screen current is 0.00875 then.

Plate current alone is 0.0515-0.00875=0.0428, and that times 320.7 is 13.7W, or 114% of max.

The supply voltages are high compared to the norm, which explains the high plate dissipation using the "standard" 50R cathode resistor, and it's probably the reason for the 1k screen resistors. Sounds like other Ceriatone owners have had problems with this too. You definitely need to turn the wick down with a larger cathode resistor or do something to lower the B+ by 30V or so.

You could get some inexpensive 5W resistors, say 10R and 20R, and put them in series with the existing cathode resistor (so you can make 60, 70, and 80R total) to find a value that gets you about 70% of Pa max, and then buy a new 25W to match.

[brodie900]

awesom, thank you so much martin you'r a legend mate!

perhaps i should switch to the 260V tap on my pt rather then the 300V tap then?

and does this mean those power tubes are probably fried by now?

[martin manning]

Yes, do that and measure the voltages again. I didn't know you had that option, but I looked at the Ceriatone layout just now and see the 260/290 switch. Is that to provide for say 220 and 240VAC mains? Where you are, the 260 setting will definitely be better. The tubes may be ok if they've only run at those voltages for a short time.