Tube output stage is getting hot

Hi,
i built myself R47 preamp and found out that the output tube (E88CC) runs on limit of dissipation, and also get very hot. Anode resistor (8k2) also get too hot to touch.. Anyone else experienced this problem?
It sounds very nice this way, but i'm worried about burning the tube too fast if i leave the circuit as it is.
Is there anything i could do about it and preserve the sound? Maybe use lower B+ or more resistance on anode?

Another problem is finding non-inductive anode resistor, 8k2. Which easy to get values i should use in paralell to get 8k2R and at least 6 to 7W?

Miha

Don't know that preamp design at all---but be aware that resistors can run quite hot and be happy. When semiconductors are too hot to touch for more than brief moments you may have a problem. Resistors can run very warm---as long as they don't heat things in the vicinity too much you are usually fine.

just to make sure.. you did use at least a 5W resistor for 8K2?
You could use 33K with 11K (10K and 1K in series), but searching for a single 8K2 5W resistor is probably easier. Farnell has them for instance.

also you can check the voltage at the top of the 8K2 resistor and check the current going through it. Should be around 290V and 18,5mA.

cheers,
Corneel

[quote author="Nele"]just to make sure.. you did use at least a 5W resistor for 8K2?
You could use 33K with 11K (10K and 1K in series), but searching for a single 8K2 5W resistor is probably easier. Farnell has them for instance.[/quote]

Hi Nele,
thanks a lot for help. I knew farnell had this resistor, but max voltage seems to be 200VDC max, power 7W. Does this mean that only 200V is allowed at max power, but if i have less, than it will survive higher voltage?
My voltage is exactly 290VDC before 8k2, and about 1mA less current, so i'm ok. I used G9's psu with some adds. I like it very much.
What do you think about having so high dissipation at E88CC? Datasheet says this tube should work at least 10000 hours at full power, but i'm not sure if that's truth. At the moment i use NOS frame grid, gold pin Tesla (with sword logo, best tube of this kind they made). Since i have enough heather current i will also try Russian version of E88CC. They might last longer...

Regards from Miha

i built one redd, and of course the 8,2k is hot, it have to dissipate around 3 W, i use a 7w resistor and i've no problem.

the 18,5 ma are for the two triode, if they are strickly the same, it's around 9.25mA for each triode, that is the the value given in the datasheet of the E88cc, so it's normal condition

Anything getting that hot should be installed with its own diameter higher than the PCB. thats what the long leads are for. youve probably cut them already, so I would replace and "hang" the resistor above the pcb, definetly not touching the pcb. use those insulator rings on the exposed resistor legs for safety.

Hi Miha,
You could try this one. I haven't built the redd yet, though it is on my list.. (but then, what isn't.. ), but if I look at the datasheet for the e88cc it should be capable of handling 15mA so 9mA shouldn't be a problem. I take it your heater voltage isn't too high?

adios,
Corneel

> Does this mean that only 200V is allowed at max power, but if i have less, than it will survive higher voltage?

No. 200V max.

Which does sound awful low.... 300V is more traditional on non-teeny resistors. Maybe with the decline of tube products, the resistor companies don't bother to test higher?

In fact, "300V" 0.5W carbon-composition resistors "do" stand 500V. They just drift more and faster. If I wanted 10% value for 10,000 hours, I would stay below 250V. But it is not an emergency.

A plate resistor on a 290V supply tube-amp will be dropping 90V-150V; the tube drops the rest. A "200V" part is safe, unless the tube shorts-out, and then the 4X heat will probably kill the resistor long before "excess voltage".

> Anode resistor (8k2) also get too hot to touch..

Resistor temperature is Watts/size. Size is money. Temperature is limited only by smoke or change of resistance. You could use a teeny bit of NiChrome wire running red-hot, but resistance would double from cold to hot. You could nail hundreds of pencil-leads to a board, get a very cool-running resistor as big as the studio wall. Commercial resistors run as hot as they can to reduce size and cost, using low-tempco materials for stable value.

Oh, electronic resistor temperature is traditionally limited to less than the ignition point of paper, though some very-small hi-power parts must push this concept awful hard.

Boiling spit may be normal. Burning Kleenex is mighty hot.

You reduce temp by using a larger part. 10W is readily available.

An 8K resistor as an audio load probably does NOT need to be "non-inductive". It would take a lump of iron to get inductive reactance in the 8K range in the audio band. And classic wire-wound resistors (the type which cause trouble in low-Z points) are now obsolete.

It is unlikely you need 8K2 exactly. Probably work just as good with 10K.

Yes, simple high-performance tube circuits do run hot. A lousy 12" TV set could throw over 500Watts. Heat means it is working.

You can buy tube coolers, or use the metal shields, although some folks claim the metal shields shunt high frequency audio to ground, slightly changing the frequency response.

Can you hold your finger on it for "one Mississippi" ?

That's medium rare on the Bogner King -O- Matic outdoor Bar B Q, the same grill that Hank Hill uses.

You can get a situation where the circuit oscillates above your hearing range due to positive feedback problems, this can heat a tube and resistor well above their normal operating point, since the HF contains a lot of energy, like a microwave oven.