Scott 299C question
SB9090
I've been restoring a Scott 299C and I'm needing some advice... I've noticed
that the power transformer runs, well, HOT! After about 30 minutes you can't
touch it. So far I've replaced all of the PS electrolytics -- all were leaky.
I've replaced the bias supply rect., and cap, I've checked all the coupling
caps for leakage. The strange thing is that the amp seems to be drawing too
little current and, given the hot transformer, I'd expect the opposite. (I
measured total PS current across an in-rush resistor that goes from the 5AR4 to
the first cap. and it is about 105ma).
So my question is, for those with experience on this amp... is it normal
for the transformer to get so hot? If not, any ideas? The thing sounds very
good. I suspect the lower PS current is due to weak 7591's?? Thanks for the
help. Steve Brown
Oh yes, FYI, I measured the plate current on each 7591 and they average
around 24ma.
Chris Terry
In my experience, all this old tube stuff runs way hot.
Add in the fact that they were designed to run at 110V, 115, or 117V,
and now my Voltage gets up to 125V at night in my house. This starts
to run into saturation of the core of the transformer and heat
follows.
They probably designed the transformers to run right at the knee of
the iron curve to give a little bit of regulation for voltage
variations. Now all our outlets are at the high end of the original
tolerances. 110 V +/- 10% = 121 and 99 V.
Per usual, just educated guessing.
Chris
SBench
>
>I've been restoring a Scott 299C and I'm needing some advice.
>the power transformer runs, well, HOT! After about 30 minutes you can't
>touch it.
>The strange thing is that the amp seems to be drawing too
>little current and, given the hot transformer, I'd expect the opposite.
> If not, any ideas?
Hi Steve,
Are the voltages by any chance subtly low coming from
the power transformer? It may be a low probability, but
perhaps the transformer has a shorted turn in it.
You could test for this by unloading ALL the windings
by pulling all the tubes etc and turning the thing on. If
is STILL gets hot, bad transformer.
Best Regards,
Steve
Ned Carlson
On 8 Mar 1998 02:36:39 GMT, sbe...@aol.com (SBench) wrote:
>Hi Steve,
>Are the voltages by any chance subtly low coming from
>the power transformer? It may be a low probability, but
>perhaps the transformer has a shorted turn in it.
Just to rectify (pun intended) any misunderstanding,
most transformers on old tube gear do run pretty hot,
how much depends on how conservative the
manufacturer was. Mike will know better than I, but I'd
take a SWAG that the average rise over ambient
would be like 50 degrees C: plenty warm enough
to be too hot to touch. And apparently what
manufacturers spec'd was all over the map,
ST-70's are pretty notorious for running awful hot,
but I've had on occasion this Ampeg chassis sitting
next to my desk cranking 50W into a dummy load for
a couple of hours and the power tranny barely gets warm.
(IMA: The Ampeg upgrade kit for this amp consisted,
as far as anyone remembers, 6550's to replace the 7027's
and a new set of tube retainers, no instructions to
rebias or anything else.So apparently the factory
was pretty confident in it)
I think this effect gets amplified (another pun intended)
by the proximity of the power & rectifier tubes,
in a Scott 299C, they're scrunched in there together
pretty tight...
I'd also note that I've had defective rectifier tubes cause
that problem, exactly why, I'm not sure, since they tested
good, but the B+ was low and the transformer was humming
like hell.
>You could test for this by unloading ALL the windings
>by pulling all the tubes etc and turning the thing on. If
>is STILL gets hot, bad transformer.
I think it'll get a little warm, anyway, but it shouldn't be like
*hot*.
Ned Carlson Triode Electronics,2225 W Roscoe Chicago, IL, 60618 USA
ph 773-871-7459 fax 773-871-7938
12:30 to 8 PM CT, (1830-0200 UTC) 12:30-5 Sat, Closed Wed & Sun
http://www.triodeel.com
Text file catalogs:Catalog 'Bot at cat...@triodeel.com
william bittle
SB9090 wrote:
> Oh yes, FYI, I measured the plate current on each 7591 and they average
> around 24ma.
Hi,
At 24 ma per tube, you are well with in the ratings of the 7591. The
Scott 299C should have a plate current of 30 - 35 ma per tube. There is
one thing I am curious about. Do the output transformers get hot too? In
the 299C, if the bias is set wrong and the tubes are drawing too much
current, the output transformers will get hot too. If your amp is
running correctly, the power transformer should be getting warm after
about 1/2 hour of operation at about 1/3 volume. If your 299C's power
transformer is getting so hot it is uncomfortable to touch after only
1/2 hour of operation, something is wrong. Did you change the
electrolytic capacitor that is used in the first filter stage? it is the
one closest to the power resistors and behind the power transformer.
This one had a mild short in both my 299C's. It caused the plate voltage
to be low, and the power transformer to get real hot. The amazing thing
was that the amp actually worked. What is the B+ voltage at this
capacitor? If it is below 470v using a digital multimeter something is
wrong. Try replacing the 5AR4. I have had them go bad and cause some
really wierd problems. How are you measuring these currents? here's a
ball park figure for the bias on a 299C. the actual voltage for the -
grid bias should be around -21 volts relative to a plate voltage of
465v. These are measurements taken off of one of my 299C's.
How does this amp sound? does it hum? do any of the tubes have plates
that glow even slightly red?
Let me know..
Bill B.
CERREM
>I've been restoring a Scott 299C and I'm needing some advice... I've noticed
Yea,,,,Thats about right.........
That little bugger runs soo bloody HOT that you could fry up an egg on it....
a western omlette preferebly.....
In fact the 24 mA per valve you mentioned sounds about right if memory serves
correclty.....
If you want to do a qiuck check on this tranny...heres what to try...
First ...pull the rectifier valve as to open the secondary winding...
measure the AC voltage on each side of secondary with respect to
center-tap......this should be identical voltage on each half of winding...if
not then you may have a shorted turn....Even if you have a shorted turn ,,don't
sweat it...I know of many amps still running with a few shorted turns and they
are doing OK , but a little hot... :) Just don't leave the house with the
darn thing unattended....
The other test you can due ..to check the flux density..is to leave the
secondary open and connect the primary mains to a variac..
Now put an AC milliameter in series and a AC voltmeter directly in front of it
across the primary mains leads....
Now in increments of 10 volts write down the volts and current at each
increment.... go up to 140 V or 160V..but briefly..
Now take each co-ordinate and divide volts by current to get reactance.... now
divide that by 377 if your mains is 60HZ...
This will give you primary inductance......
Now multiply current and volts at each co-ordinate also...to get watts....
Now you have inductance vs. watts for a series of points......
Since I am not sure of what grade lamination is used....I know that the
inductance will peak then start dropping off acording to the BH curve of the
steel........Most of these amps used M17 for the powers and M6 for the
outputs....So get a few BH curve sheets and match up the inductance with
permeabilty based on the Area and Magnetic path length of your particular
lamination....Once you have this info you can see the operating points and
limits of your transfromer....
If you do not have access to this data...send me your measurements and I will
do the rest...
CHEERS
C.M.
Jim Coulter
In article <19980307220...@ladder02.news.aol.com>, sb9...@aol.com says...
>
>I've been restoring a Scott 299C and I'm needing some advice... I've noticed
>touch it. So far I've replaced all of the PS electrolytics -- all were leaky.
>I've replaced the bias supply rect., and cap, I've checked all the coupling
>caps for leakage. The strange thing is that the amp seems to be drawing too
>little current and, given the hot transformer, I'd expect the opposite. (I
>measured total PS current across an in-rush resistor that goes from the 5AR4 to
>the first cap. and it is about 105ma).
> So my question is, for those with experience on this amp... is it normal
>for the transformer to get so hot? If not, any ideas? The thing sounds very
>good. I suspect the lower PS current is due to weak 7591's?? Thanks for the
>help. Steve Brown
>around 24ma.
Steve,
I have a 299c also that runs hot in the power transformer as well as the output
transformers running hot (in their case due to the proximity of the 7591s).
The previous owner adjusted the plate current to be 35 ma. for each tube.
Mine also sounds very fine. I finally resorted to buying two small DC fans
to cool the thing down. Even a little forced-air cooling goes a long way in
this situation, I found. I have no case or cover over mine and I just prop
up the little fans at the edge of the chassis to blow on the transformers.
It makes a tremendous difference on the exterior temperature of these transformers,
and should prolong their life. I believe it will also increase the life of the output
tubes. The fans are not terribly noisy.
Good luck,
Jim Coulter
Tim Reese
CERREM <cer...@aol.com> wrote:
#....So get a few BH curve sheets and match up the inductance with
#permeabilty based on the Area and Magnetic path length of your particular
#lamination....Once you have this info you can see the operating points and
#limits of your transfromer....
Neat idea CM - thanks. ciao tr
--
---------------------------------------------------------------
Tim Reese, MGH NMR Center re...@nmr.MGH.harvard.edu
Old Engineering Proverb : never calculate what you can measure.
Buffstereo
Hello! I know some Pass equipment uses a fan pushing air through hoses to
specific points of concern to the designer. If a fan was installed in an
outboard box with some sort of pipe running to the areas to be targeted then
the fan bearing would keep nice and quiet because the amps heat wouldn't touch
it. Amps with cages could hide such an arangement but I don't think I'd want
such a look on a 'tubes exposed' version. However, if the tubing was chromed
like hotrod exhaust pipes..........hmmm!!! Robert Thompson
"Use a transistor, go to jail !!!"
----------------------------------------
(VTV bumpersticker)
Tim Reese
Buffstereo (Robert Thompson) <buffs...@aol.com> wrote:
<snip>
>outboard box with some sort of pipe running to the areas to be targeted then
>the fan bearing would keep nice and quiet because the amps heat wouldn't touch
>it.
could be hard to get them to run at half speed without a 30Hz supply?
I also recall that the larger fans are quieter - turbulence at the blade
tips is noisy, and small diameter means higher speed/air-volume ratio,
hence noisy.
Fan cooling has to have some diminishing return as the ambient air
reaches the room temperature - also, there have been claims recently that
tubes are cooled by radiative cooling, rather than convectively as I
would suspect. This would reduce the effect of fan cooling and make
diminishing-returns come on sooner ...
cheers tr
Sheldon D. Stokes
In article <6e69gu$p...@larmor.nmr.mgh.harvard.edu>,
re...@larmor.nmr.mgh.harvard.edu (Tim Reese) wrote:
> Buffstereo (Robert Thompson) <buffs...@aol.com> wrote:
> <snip>
> >outboard box with some sort of pipe running to the areas to be targeted then
> >the fan bearing would keep nice and quiet because the amps heat
wouldn't touch
> >it.
> I recall reading somewhere that half-speed muffin fans are pretty quiet...
> could be hard to get them to run at half speed without a 30Hz supply?
> I also recall that the larger fans are quieter - turbulence at the blade
> tips is noisy, and small diameter means higher speed/air-volume ratio,
> hence noisy.
>
> Fan cooling has to have some diminishing return as the ambient air
> reaches the room temperature - also, there have been claims recently that
> tubes are cooled by radiative cooling, rather than convectively as I
> would suspect. This would reduce the effect of fan cooling and make
> diminishing-returns come on sooner ...
The plates of the tubes themselves dissapate heat primarily via ratiation,
but that heat gets radiated to the surfaces "in view" of the tubes. Hense
the heat gets transfered to the amp body and such. Plus there is all the
heat generated by the other circuit elements. So I'd say a fan would be
pretty effective at reducing the temp of an amp. Whether it's nessesary
is another question.
Note: while the primary heat transfer mechanism from the plates is
radiation, the glass envelopes are cooled by convection and conduction as
well. so forced convection (a fan) would be quite effective at lowering
the bulb temp.
Sheldon
--
Remove SPAM_BE_GONE. from my address to reply to me.
SB9090
First, I want to thank the group for all the help I received -- I knew I could
count on you all! Hey, the problem turns out to be a bad Radio Shack bridge
rectifier that I had used to replace the original one. Ironic that a 30+ year
old rectifier is better than Radio Shacks... anyway, the thing was drawing
about an amp through the bridge with no load. I replaced it and now the power
transformer runs cool even after three hours with the case on. As for the
outputs, no problem there, either.
Thanks again for all the help.
Steve Brown
Sheldon D. Stokes
In article <19980312043...@ladder02.news.aol.com>, sb9...@aol.com
(SB9090) wrote:
> First, I want to thank the group for all the help I received -- I knew I could
> count on you all! Hey, the problem turns out to be a bad Radio Shack bridge
> rectifier that I had used to replace the original one.
I got a bad bridge rectifier from them too. I was fixing my Dad's battery
charger a while back. Just another data point.