Output tubes warm not hot.

[Axtman]

What would cause output tubes to be just warm and not scorching hot? Usually output tubes run so hot that you can heat up leftovers on them. I am working on an amp where you can hold the output tubes all day and not get burned.

Would drifted screen resistors or a high negative bias cause this effect?

[Enzo]

Well, get out your volt meter and find out.

[g1]

Can you see that the heaters are lit?
That's about all you can tell without opening and doing some measurements like Enzo said.

[kanks]

Well tubes just with the heaters on will get to hot too touch after awhile so there is a slight hint.

[Randall]

Depends on what filament voltage you have.

[lowell]

It's weird, because it WAS working fine.....

Perfection Randall

[Enzo]

And first step, make sure the 120/240v mains switch is properly set.

[nosaj]

And first step, make sure the 120/240v mains switch is properly set.

Before going too far. what is really the problem? Don't think I've even been brought an amp that was described as couldn't cook breakfast on the outputs so lets fix that ok?

Seriously though what other issue are you trying to address here?
thanks,
nosaj

[nosaj]

Before going too far. what is really the problem? Don't think I've even been brought an amp that was described as couldn't cook breakfast on the outputs so lets fix that ok?

Seriously though what other issue are you trying to address here?
thanks,
nosaj

Also what amp are we talking about?

nosaj

[Axtman]

The amp is a Fender Super Six Reverb. The cabinet was chopped to only contain two 10 inch speakers instead of six. Besides the output tubes only being warm (not hot) the amp would go into distortion really early. Also the amp was not very loud at all.

I was hoping that you guys would clue me into something so that I did not go chasing my tail in circles....which is what I did. I checked everything associated with the output tubes. The plate voltage (475 V) was fine, the 470 ohm screen resistors were fine (actually slightly lower resistance than 470 which is odd because carbon comp resistors drift high with age). The 1.5k grid resistors were within spec. All the resistors on the bias circuit was fine, except the bias was high (low?) at -75v. I thought maybe it was wired from a hum balance to bias adjustment or maybe it was wired wrong attaching to the eyelet board. Maybe the power transformer was fried. I checked everything and everything was fine.....except....the resistor directly from the bias tap on the power transformer. The value was listed on the schematic as 1k but in actuality was a 470 resistor! I changed out the resistor and the amp is louder with more headroom, the bias is -52v and the output tubes are HOT!!

Here is the crazy thing. The resistor was stock. It was wired from the factory that way. The solder joints were untouched. Thus the reason why I measured it but did not compare it against the schematic. So I figure this amp was sold and resold many times because it just sucked! Now it is not as loud as a Twin and breaks up a bit early, but that is because it has old inefficient 10 inch speakers instead of efficient new 12 inch speakers.

[Axtman]

Below is the bias board with the new resistor attached and the old wrong resistor next to it unsoldered.

[nosaj]

Below is the bias board with the new resistor attached and the old wrong resistor next to it unsoldered.

[ATTACH=CONFIG]44026[/ATTACH]

How old is that Bias Cap? Do not believe I've ever seen one that color by sprague.
Not trying be smart or anything, but how can you tell a solder joint has never been touched since it left the factory?
A quick hot iron will leave almost no trace and a good joint.
nosaj

[Justin Thomas]

It's "period correct," if not original. Turn it a little bit, we can read the date code on it...

Justin

[Tom Phillips]

It's "period correct," if not original. Turn it a little bit, we can read the date code on it...

Justin

I have never seen that style cap in a stock Fender. Just a comment FYI.
Tom