[Vintage-Audio] Re H.H. Scott Stereo Amplifier

[Roy Morgan]

At 02:56 PM 5/22/02 -0400, you wrote:
>
>I am told that getting two matched pairs of 7591 is very unlikely, very
>expensive if located. Any thoughts?

The AES price for those tubes is $65 new and $45 used.  That is $230 for a 
set of four. Matching is more likely with new tubes.  The amplifier does 
have DC balance controls for each channel.  Matching may not be 
necessary.  You may be able to make money if any of the tubes in your free 
amplifier is any good.  Then modify the thing to use 5881's like the Chuck 
suggested.

I have one of these amplifiers. Only one of the four tubes is much 
good.  The case leaves absolutely no extra space above the tubes, so the 
5881 is likely the only tube that will fit other than the 7868 compactron 
that Chuck mentioned.  The AES price of the 7868 is now $42.

Another problem is the rectifier: it is a GZ-34/5AR4. These are very 
expensive also and not common.  No other tube will fit in the space and do 
the job. None.  I do NOT recommend you use a cheaper new production tube 
from places with bad reputations for tubes.  Using solid state rectifiers 
will raise the B+ even more.  The manual mentions the slow-turn-on 
characteristic of the 5AR4 rectifier.  Current AES prices are: Chinese, 
$11; Japanese, $32; New Old Stock U.S., $61; Same, used $32.

>How about the multi-section electrolytics? Any sources you know of?

The capacitors are rated at 500 volts.  Measured B+ in my unit was 505 
volts on the output tube plates.  I have no idea where to find replacement 
can capacitors.  AES does sell a single section 20 microfarad 500 volt can 
cap for $19 but I have no idea if it is short enough to fit in the 
case.  The amplifier uses two can caps each with two sections of 30 
microfarad at 500 volts. There are also two cans with 20 microfarad at 450 
volts sections.  There is essentially no room on the chassis to put extra 
can capacitors.  The room underneath the chassis is very limited because it 
is very shallow.  Modern tubular electrolytic caps might fit but I have not 
tried to do this.

At least one of the caps in my amplifier started shorting out after I got 
the thing running, so I have never used the thing.  I could rebuild the can 
capacitors but I have had other things to do.

In my amplifier all four output tubes ran red hot in the plates until I 
replaced the selenium bias rectifier bridge.  The bias level must repeat 
must be checked before applying plate voltage to these amplifiers or damage 
can result.  The final tubes run with fixed bias not self bias and will 
certainly destroy themselves unless the bias supply is working right.

More about capacitors: The coupling capacitors to the output tube grids and 
in other places are point one microfarad. These must be checked for leakage 
or the final tubes will overheat.  The filaments of the preamp tubes are 
run on direct current filtered by four 75 microfarad at 75 volt capacitors. 
Even if these can caps are bad, small replacement tubular caps should be 
easy to find for mounting below the chassis.

I belive that this amplifier should never be run without a fan. It get so 
hot normally you cannot pick it up. It literally destroys itself with the 
heat generated.  It is too bad this amplifier has so many challenges to 
getting it running reliably. It likely sounds very very nice.  The power 
output of 30 watts per channel is more than most any one needs.  Changing 
the output tubes to 5881's seems like a very good idea.  New 5881's can be 
gotten with good quality.  They are likely short enough to fit inside the 
case. Running them at much lower power will produce a very fine sounding 
amplifier and the thing will last a lot longer.

The good news in the deal you have is that the tuner is nice and works well 
and has no real difficult parts.  The tuning indicator is a normal moving 
pointer meter, not a hard to find eye tube.

My advice is this:
1) get the amplifier and tuner  Do not let anyone who does not thoroughly 
know what he/she is doing even plug it in.
2) sell the final tubes and the rectifier on Ebay for whatever you can get 
for them, no matter how bad they may test in a tube tester.  Be very 
careful to find out if any tube in either the amplifier or tuner says 
"Bugle Boy" on it. DO NOT RUB OR EVEN TOUCH THE MARKING.  Sell these tubes 
also on Ebay. You will be amazed at what they sell for.
3) change the amplifier to use 5881 tubes running a lower power
4) replace all capacitors that need it.
5) use solid state rectifiers
6) rig up a fan to keep it all cool
7) use a line bucking transformer to keep it even cooler and reduce the 
plate voltage to reasonable levels.

Darn.  All this has got me inspired to do all that for my Scott amp and 
tuner.. I wonder where they are buried!

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
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