how bright should the 7159 tubes glow?

[andyboy]

Hello,

I picked up a heathkit vacuum tube audio integrated amp this weekend
that uses
the tube compliment 12AX7's, 12AU7's, 7199, 5AR4 rectifier, EF86 and
four 7159 power tubes.

The first time I turned on the amp, I noticed the 7159 power
output tubes were glowing very brightly, too bright it seemed to me.

The amp did play, but not real well but knew it was going to be
marginal
anyway.

I noticed the thin metal housing around all the
filaments, cathodes etc. was actually glowing orange
over almost the entire height of the tube guts. I thought
this looked unusual.

So I shut it down to investigate. All four 7159 tubes in the
amplifier were doing it. The tubes also felt incredibly hot after the
little 2-min session.

So what do you think? is that normal or is the amp
over jucing the tubes?

thanks,

Andy Rein

[Raymond Koonce]

Hi Andy,

Don't plug it in again!!!!! What you describe is *not* in any way
normal. Your output tubes are experiencing runaway which can occur for
any number of reasons. It will destroy the tubes in short order. If
you don't feel competent to troubleshoot and repair the amp, you should
take it to a tech.

Good luck,

Raymond

andyboy wrote:

>Hello,
>
>I picked up a heathkit vacuum tube audio integrated amp this weekend
>that uses
>the tube compliment 12AX7's, 12AU7's, 7199, 5AR4 rectifier, EF86 and
>four 7159 power tubes.
>
>The first time I turned on the amp, I noticed the 7159 power
>output tubes were glowing very brightly, too bright it seemed to me.
>
>The amp did play, but not real well but knew it was going to be
>marginal
>anyway.
>
>I noticed the thin metal housing around all the
>filaments, cathodes etc. was actually glowing orange
>over almost the entire height of the tube guts. I thought
>this looked unusual.
>
>So I shut it down to investigate. All four 7159 tubes in the
>amplifier were doing it. The tubes also felt incredibly hot after the
>little 2-min session.
>
>So what do you think? is that normal or is the amp
>over jucing the tubes?
>
>thanks,
>
>Andy Rein
>
>

[andyboy]

"Steve O'Neill" > wrote in message >...
I put in the new diode and it did not fix the problem. Any other suggestions?


Andy

> Hi:
>
> From your description this sounds like the Heath AA100. In this amp, Heath
> uses a fixed bias scheme. Since the bias supply is common to all four 7591
> output tubes, your experience indicates something amiss in it. As already
> posted, don't operate the amp till you've corrected the problem: permanent
> damage to the power transformer and/or output transformers and/or 5AR4
> and/or 7591s could result.
>
> The bias supply picks up AC off a tap on the power transformer secondary and
> uses a selenium half wave rectifier and electrolytic cap for filtering. If
> stock, the selenium rectifier is probably bad resulting in low bias voltage.
> It should be replaced with your choice of modern Si rectifier. You 'll
> probably want to replace the electrolytic cap too.
>
> Even if this isn't the AA100 I'd bet that the problem is in the bias supply
> because Heath produced lots of amps with different model #s where the only
> real differences were updated cosmetics.

[Steve O'Neill]

Hi:

Next step: Check the voltage betw pin 5 (cathode) and pin 6 (g1) on each
7591. Pin 6 should be somewhere betw minus 17 and say minus 22 VDC WRT pin
5. If you find this voltage then your problem probably lies elsewhere: dirty
socket contacts etc. If you find some other voltage then your problem
remains in the bias supply. Could be bad cap or voltage divider resistors
or cold solder joint(s) or ...??? Trouble shooting these things is the hard
part, esp by remote control. Replacing the defective components is easy by
comparison.

--
Steve


andyboy wrote in message
>...
>"Steve O'Neill" > wrote in message
>...
>I put in the new diode and it did not fix the problem. Any other
suggestions?