November tranny getting warm!!! How much current for 2 EL84?

 :

9/10/2000 10:22 AM

guitaguy

November tranny getting warm!!! How much current for 2 EL84? Hi Ampagers!     I finished my November plexi clone last night. I realized the power tranny was getting warm just by playing with the volumes turned all the way down.     I checked for short circuit, everything okay. The heaters on 6,6V, a bit high, though...     I took the tranny from a crappy push-pull ECL86 (one triode and a seven watts pentode in one bottle) Watkins amp. 240V secondary making it around 340V B+.   I guess the tranny doesn't supply enough current, does it?     How much current do you reckon for two EL84/6BQ5 push-pull class AB? 150mA?     There also was a significant voltage drop after the choke (- 9 volts!)     I guess I order a bigger PT...     Regards     guitarguy

 

9/10/2000 5:40 PM

Bruce

Re: November tranny getting warm!!! How much current for 2 EL84?/Ramble My EL84 amp with two 12AX7s will sit at idle with about a 64ma load on it.   That's measured across a 1% 10 ohm resistor right at the first B+ node after the first filter cap.   All the current from the high voltage supply has to flow through this one resistor before it goes anywhere and it gives me a stable place to measure the entire HV current load on the PT.   I have built this same amp up using a very small 260v/50ma tranny running at about 315vdc, biased around 7 watts/22ma per EL84 tube, just to see what would happen and it ran for countless hours at idle without ever seriously over heating the PT.   Fine for small 10-12 watt EL84 PP amp.   My production amp has a 270v/120ma secondary and under full tube compliment load, gives me a similar 340vdc to the plates.   SO, how did you measure your 340vdc?   Was that with no load or no tubes?   If under load your 240vac tranny can still run at 340vdc, I doubt the thing is underated.   You might just be feeling normal heat from the PT.   If however, your B+ under load is dropping down much closer to the 240vac input, I also suspect yours is too way small for the job.   A 9v drop across the choke just means it has a little higher DC resistance then you thought.     Bruce

 

9/10/2000 8:03 PM

Carlos

Thanks Bruce!     I've got two EL84 and three ECC83. I guess 2.5 amps are enough for the heater supply, aren't they? Or do I need more. I think the tubes live longer if the heater voltage is BELOW 6.3 volts. What do you think?     You know, I can have a PT custom-made quite cheap as long as the PT is under 50VA!!     You said your production amp has a 270V/120mA supply. That should give 380V at idle. My caps are rated 350V/380V. I'd better stay at 260V/130mA.     Regards     Carlos

 

9/11/2000 1:28 AM

Richie{~}==:::

And just what brand PT is this? Just wondering   I know the New Sensor PTs seem to get very hot,i've even talked to there tech about it, he says they haven't heard of any problems. But I think every one i've used has got really hot. With or without a load on them.These have all been the PTs used for fender amps, Just wondering if anyone else has noticed this? The Real Fender PTs just get warm..normal ..The NS get so hot you can hardly touch them.   Richie{~}==:::

 

9/11/2000 4:22 AM
Troy	I've found some PT's to get really hot too Richie. I've got an old Sunn Model T and the PT gets really hot after about an hour set; now if I stick a low speed fan on it, it's cool to the touch. I've found this also to be true when using my Fender PA100 (a PA version of a Twin Rev) so a fan helps it too. Both amps work perfectly.     Troy

 

9/13/2000 11:50 PM

Richie{~}==:::

I Know Sunn had lots of trouble back in 77..cause i went through 5 different sunn model Ts..until they found that all of them were bad.. the power transformers..they called it a bad run of transformers.It didn't stay on long enough to get hothot.. it would run about 20 mins and poof..take out the rectifiers.. All of them i tried..new out of the box.. But i have others that work fine..seems like they fixed the problem.   Richie{~}==:::

 

9/13/2000 8:35 AM

Bruce

Funny you should mention the NSC iron.   I used a couple of their SF Deluxe Reverb PTS in some of my amps and found that they got much hotter then the Magic Parts reissue SF DR PT.   BTW, "hot" to some folks might be just getting warm to me, but when I say they are hot, rest assured they are hotter then a $5 Saturday night special in a New Orleans back alley.   I never used any of the NSC power trannies after that.   I don't know if they were bad ones or that's just the way the all are.   Others here have used them with no concern though.     Bruce

 

Page 1 of 2

Next>

Last Page>>