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Eico HF-81 Tube Amp to Restore

I have a whole drawer full of 1% 360 MF 3 watters. Seems to put the tube in a sweet spot for my ears, and it sure don't strain them. (one per channel of course)
 
I have another set of EL84's I'm going to try next, then I'll check the measurements.
I have to get a new set from Jim McShane.

Tube
 
NOSValves said:
Yup for some reason your output tubes are drawing near maximum idle plate dissispation right now on one channel and still too hot on the other. pin 3 voltage is the important one in this respect.

When developing 12.8 Volts on the cathode resistor it basically equates to over 35 mA per tube in one channel.

323V - the 12.8 volts on the cathode = 310.2 effective plate voltage x .036 (current) = 11.16 watts of plate dissipation and this takes for granted that the tubes are reasonably close matches which they most likely are not unless you bought them from a reputable professional tube seller that burned them in and then matched them. Those rusky mil spec tubes are always wildly mismatched... I know I used to buy them buy the 100 lots years ago. So if they are not matched one tube could be drawing way more then the other in the channel which is real bad...I'd be upping the cathode to at least 220 ohms if those are the tubes your going to use...

Where did you get the tubes?
Click to expand...

One thing I've done is to put a meter port up on top of the chassis for each tube (with the R to ground trick), it makes matching them in circuit so easy, especially when you are buying bulk like that.
 
V7
Pin#3 11.92v
V8
Pin#3 11.92v
V9
Pin#3 12.09v
V10
Pin#3 12.09v

these are the other tubes I have EL84's. I got the 6P14P's off a dealer on ebay a few years ago.

Tube
 
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12v is pretty good, with about 325v on the plates. With 180 ohm resistors, that's about 10.8 watts dissipation. That would normally be relatively high (90% of max dissipation) for other tube types- but that's routinely done with EL84s, IME.

Also, the ratings for the EL84 are 12w plate AND 2w on the screen. Since at least some of the measured cathode current is going to the screen, that means the plate is actually dissipating even less than 10.8 watts...

Regards,
Gordon.
 
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Gordian,

You're absolutely right but wrong at the same time. In the case of HF81 the power transformer is painfully over taxed to the point that they regularly fail. Since no sonic benefit exists running the output tubes near max idle current and the fact that they = 90% of the total b+ current the amplifier consumes they are the best canidate to cool down and prolong the transformers lifespan..

Cheers Craig
 
Well this morning I decided to hook up the eico, Might as well see if it works before
going further and! I switched it on and heard a little buzz that came from the transformer
at first turn on but it went away, now with a slight hum only when I touch it, and you know
it really sounds great it's pretty smooth and the center image is dead on in the center
now I know what everyone likes about these guys. Last night I did order some PIO
caps .033uf 600v's to replace the four disc caps if need be.

Tube
 
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Okay the 200ohm resistors finally got here, I ordered six and matched two at
198 ohms the others ones are over or under but not matching I'll try these out.
So when I get these in what am I looking for in the voltage's

Tube
 
tube-a-lou said:
Okay the 200ohm resistors finally got here, I ordered six and matched two at
198 ohms the others ones are over or under but not matching I'll try these out.
So when I get these in what am I looking for in the voltage's

Tube
Click to expand...
You should not match the resistors, but rather the idle currents, otherwise what's the point of having separate resistors for each tube? You may want to consider running the EL84s cooler per NOSValve's suggestion, as well as adding screen grid resistors to protect them.
 
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Dandy, it really sounds great it just opens up. It totally different sounding than the
Fishers, it must be those EL84's and everything else, the last EL84 tube amps I had
were the Scott's and they didn't sound as good as this.
Tube
 
The only sad thing is that we can't share the sounds too. I've just dug out my restored Heathkit AA-50 and diggin' that too. It's fun when you've worked through the whole thing. But IMHO, there is special magic in those EL84s.
 
Guy's I was playing the Eico tonight for about two hours and all is well, It was
pretty cool for about a hour or so but two hours it got hot and there was no hum
from the transformer, it was comparable to the Fisher KX200 I have in heat.

Tube
 
Thanks for trying to post a movie Tube! If the voltages all check out, the transformer is probably doing fine. My Heathkit's PT is also hot after a couple of hours. You can put your hand on it, but don't want to leave it on too long.
 
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