UA 175, or how much power can a 12BH7 stand ??

I looked into a 12BH7 data sheet: it says Plate dissipation 3.5Watts maximum rating.

Working conditions, from the UA175 schematics:

Cathode Voltage 11V. Shared cathode resistor 330R for both halves of the 12BH7. Current thru each triode: 1/2 * 11V / 330R = 16.7mA

Anode voltage: 295V. Power dissipated in each triode:
(295V - 11V) * 16.7mA = 4.7 Watt.

What's wrong?

JH.

You also have the 4700R anode resistors dropping some voltage..

[quote author="gyraf"]You also have the 4700R anode resistors dropping some voltage..[/quote]

What 4700 anode resistors ???

JH.

The schematic link at Waltzing Bear is dead, do you have another link?

On my schematic the hand written voltage looks like maybe 245. Not clear, but even then the tube is 3.9W. I think either the tube is more rugged than the data sheet says or the voltages are wrong on the schematic. My guess is the voltages are wrong.

Oops... I was looking at the input stage.. Sorry..

The 12BH7A is rated 3.5W plate dissipation for each section...

Jakob E.

[quote author="thomasholley"]On my schematic the hand written voltage looks like maybe 245. Not clear, but even then the tube is 3.9W. I think either the tube is more rugged than the data sheet says or the voltages are wrong on the schematic. My guess is the voltages are wrong.[/quote]

I checked the plate courves - the combination -11V Grid, 280V anode and 17mA current seems to be consistent.

JH.

Well, it wouldn't be the first time a tube was run outside its design center maximum in a commercial product. The abuse inflicted on poor 6V6G's in blackface Fender Deluxes comes to mind...

[quote author="gyraf"]Oops... I was looking at the input stage.. Sorry..

The 12BH7A is rated 3.5W plate dissipation for each section...

Jakob E.[/quote]


... and if these voltage redings in the schemos are right, they even run _both_ triodes at 4.7 Watt !

Very strange.

JH.

Some EL34s can take a good wack too. I've run a lot of current trough 6J5s, more than 30mA, and they sound peachy even after some six years. So, where's that UA175 schematic then?

[quote author="NewYorkDave"]Well, it wouldn't be the first time a tube was run outside its design center maximum in a commercial product. The abuse inflicted on poor 6V6G's in blackface Fender Deluxes comes to mind...[/quote]

Ahh - I see.

How does this affect the _sound_ ? It will obviously increase the output
level, that's not what I mean. Is there any magic benefit of running tubes that way, or was it just an economic thing?

JH.

I spend a lot of time finding the sweet spot. Quite often it's with a lot of current. I use 12B4A, WE417A, 5687, 6J5/6SN7 & 2C22. They all sound best with a good deal of current running trough them. The operating point is more important than the tube, ime.

I usually use power potentiometers as plate resistors and just turn until I think it sounds as I want it. I measure it AFTER that. And yes, I have blown a few tubes, but they sure can take a beating! Silicon suck at this :roll:

[quote author="sismofyt"]So, where's that UA175 schematic then?[/quote]

Downloaded it a long time ago. I just sent it to you via email.

JH.

Got it thanks :thumb: Isn't the shared cathode resistor 270r and not 330r?

[quote author="sismofyt"]And yes, I have blown a few tubes, but they sure can take a beating! Silicon suck at this :roll:[/quote]

Is there a rule of thumb, how much overheating a tube can stand?

JH.

No. Some seem to take more than others, but you can be sure it's going to shorten life. How much? Dunno, I'm only 29 :wink:

12BH7, 12B4A, 5687 and all of the 6J5/6SN7/7193/2C22 family are pretty strong, ime. ECC8X stuff is not and not very good sounding either. That is my highly biased opinion though

The schematic may very well be correct...

[quote author="sismofyt"]Got it thanks :thumb: Isn't the shared cathode resistor 270r and not 330r?[/quote]

Ooops, yes. The 330R is in the 176 schemo. I'll send you this as well.

270R (with the same 11V across of it!) would even mean 5.8Watt per
triode. Very strange.

JH.

Looking at the curves, 11V, 20mA & 295V is right on allthough it is almost a hundred percent overwattage :shock: This is not seen in a lot of commercial gear, lotsa tube exchange :wink: But the 12BH7 is a tough little bastard. You can use it as a poweramp as well :grin:

[quote author="sismofyt"]No. Some seem to take more than others, but you can be sure it's going to shorten life. How much? Dunno, I'm only 29 :wink:

12BH7, 12B4A, 5687 and all of the 6J5/6SN7/7193/2C22 family are pretty strong, ime. ECC8X stuff is not and not very good sounding either. That is my highly biased opinion though [/quote]


And I'm happy to hear your opinion. I also plan to build a HiFi amp from tubes soon, and I still look around. At first I wanted to use ECC83's for the Phono preamp, but several people have convinced me that this is no good idea. But I disgress.

The schematic may very well be correct...

Click to expand...

The 270R can't be right - at least not with th every same voltage readings as the 330R version. But there are other obvious errors in these schemos, too.

JH.

Try to get another copy. It's not so rare so I'd think someone here has it. Maybe ask UA.. HAHAHaaa.. arh not. Maybe Winston O'Boogie?

Did you download it from Waltzing Bear?

ECC83 blow. Kein geil oder spass. Life's too short to suck :wink:

I'd rather use a 50 cent 6J5 than a 50 dollar <> TFK '83. They're _overrated_.