[BPW666]

Hi All,

I'm working on an amp based on the Friedman pink taco.  I found a plan on the internet which i based my design on and ordered all the parts from tube town.  A friend is building the TT PL18 kit so I've seen the TT PL18 schematic and it matches my plan with one or two minor changes.

My (first) problem is that the power tubes are running too hot.  I'm measuring 403V on the plates and 9V across the cathode resistor.  If I'm calculating things correctly the tubes are running at 116% maximum dissipation,  and i can see they are starting to red plate.

I'm using 120r cathode resistor,  i can see that has been upped to 150r in the TT PL18. I'll try upping that value  when the new one I've ordered gets here,  but don't think it will be enough to reign it in.

My power transformer is rated for 36W with a 600V secondary and centre tap.  I have no idea what voltage the original Friedman transformer putts out?

Anyone have any suggestions what i can try?

Thanks in advance, Ben.

[Dirk]

Anyone have any suggestions what i can try?

Yes: stay calm and make sure that the maximum power disipation of the tube is not exceeded.

Best regards, Dirk

[BPW666]

Hmmmm i might start by putting in the correct output tubes.  On closer inspection it turns out I've got a pair of EL844s rather than EL84s.

[BPW666]

Ok i don't understand this, can anyone please help me?

From the Tube Town schematic plate voltage target is 355v, cathode resistor 150R with 12.95V dropped to earth so cathode resistor current of 87mA.

Split in 2 gives,  43mA per tube x 94.55% = 41mA anode current.

P=IV = 41mA x 355V = 14.54W plate dissipation > 12W max dissipation from tube data sheet.

What am i getting wrong? It feels liked this thing is designed to melt output tubes.

[cca88]

Ok i don't understand this, can anyone please help me?

From the Tube Town schematic plate voltage target is 355v, cathode resistor 150R with 12.95V dropped to earth so cathode resistor current of 87mA.

Split in 2 gives,  43mA per tube x 94.55% = 41mA anode current.

P=IV = 41mA x 355V = 14.54W plate dissipation > 12W max dissipation from tube data sheet.

What am i getting wrong? It feels liked this thing is designed to melt output tubes.

You hav e two small errors in your calculation

• You have to subtract the kathode voltage from the plate voltage
• You have to subtract the screen grid dissipation - atm you are calculating with cathode current not plate current

regards
Jochen

[BPW666]

Apologies,  1 yes minor error,  2 is that not what I'm doing by multiplying my cathode current by 94.55%?

Anyway the problem remains. Using https://robrobinette.com/Tube_Bias_Calculator.htm and remembering to subtract the cathode voltage. I still get an idle dissipation of 123% max for each tube. This seems far too high unless the datasheets are not to be trusted?

[Laurent]

Hi,

To 2: no. ATM you are calculating Ik = Ia + Is.
If you have a screen grid resistor, with Ohm's law, measuring the voltage across that resistor will let you know Is.
If you don't have such resistor in place (what I strongly recommend), you can estimate it. The relation between Is and Ia Is more or less constant for each tube. Take the relation out of a datasheet and extrapolate it.
But better to add anyway a screen grid resistor to save the life of the tube.

But coming back to your case, which voltages do you have now at your both EL84? I cannot figure out where is your issue and if there is one at all...
The best would be that you post a small schematic from your output section with parts values and voltages.

Cheers
Laurent

[Laurent]

PS: needless to say is that when you perform all those measurements that you need to have a proper load on the secondary of the output transformer (speaker or big resistor as dummy).

[cca88]

If i take your B+, and cathode measurements - and - the sitation at the screen grid form TTs TT18 i get 11,85W Plate dissipation...

I'd advise you to take an actual measurement of the screengrids in your amp - and do the complete calculation
I don't know, what Rob advises - his page is not reacable atm

regards
Jochen

[BPW666]

Thanks for the help so far, it's really appreciated!

I'm running an output transformer with a 7371R primary into an 8R load via. the 8R tap.
schematic and values here. https://ibb.co/qsbvsjL
1V across 1K screen resistor gives a screen current of 1mA

At the moment I'm only trying to understand the schematic.

Ben

[BPW666]

My mistake (again)
B2 = 362
So 362-354 = 8V
8/1K = 8mA

So the tube is running at 96% max dissipation?
Still feels too high but i guess i can live with it for now.

Thanks
Ben

[cca88]

My mistake (again)
B2 = 362
So 362-354 = 8V
8/1K = 8mA

So the tube is running at 96% max dissipation?
Still feels too high but i guess i can live with it for now.

Thanks
Ben

 
Yes fairly hot... Similar to an AC30...

But - the 6V6 in my tweed deluxe run under the same conditions... approx 14 years now - i think about 2000 - 3000hrs on them

No problem

regards
Jochen