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Tube Replacement Signs

AudioScout

AudioScout

New Member
I have a Scott 380 receiver, and maybe my hearing is getting worse, but I think I am cranking the volume farther to hear the music at the same level I was just the other month.

The tuner and phono preamps are also very low, but they have been like that since I've had the receiver.

So, what are the signs that tubes need replacing?
 
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My first thought given your thread title was something like this:
upload_2016-2-23_9-59-12.png

Actually I don't know that particular Scott, but I would think that if the tubes are starting to weaken, I'd look in the power amp section--either the output tubes or possibly the 6U8 pentode section. Gain on that 6U8 pentode can change quite a bit when that tube is old and tired. It would be nice to narrow it down at least to either the power amp portion, or somewhere else.
 
I have had bad tubes kind of make a high pitched noise. I have also had them cause some distortion on voices and other things. I think it would be worth it to get a good tube tester. I have one and I can see when I suspect a tube what is going on.
 
Hey Audio Scout, since your vintage Scott power tubes were once subjected to red plating (from your other thread) , maybe one or two of the quad are weak.
The 6U8 for that rig can also be subs with 6gh8.

Try to get them tested or drop in some new Electroharmonix 7591A (matched quad $99). Get them from a reputable store that burns them in first.

Your rig is already set up (power tube bias-wise) to operate on a matched quad of Electroharmonix. No need to mess with extra resistors or re-bias..etc...

I doubt that the subbed tubes in the tuner section ( (2) 6HS6 - V6, V8 subbed w 6au6) has anything to do here.

Last case scenario would be to re-clean the volume pot and other pots and switches.


Most expensive,
Next-to-last (also fairly rare)
Scott Tube Receiver design;

Once again Scott engineers creatively married past designs:

This time Scott engineers combined:
the 320/355 AM/FM tuning gang,
with the 340-B's Auto-Sensor switching MPX section,
with a 7591-based amplifier section.


The 380 can also be identified by:
headphone outputs,
input indicators (like the 340-B),
speaker switching, and Auto-Sensor FM-stereo switching
"Extruded Panel" styling,
AM/FM slide-rule dial tuning,
traditional tuning meter
solid state power supplies,
all on the tall "G" chassis.
It is easily confused with the (FM-only) 340-B.

Note: two different multiplex section designs were produced; S/N 234531 and below used ZMX-6, 4-tube, MPX sections)

21 or 22 tubes
(depending on MPX design):

Amp section:
(4) 12AX7 - preamp - V1/101, V2/102
(2) 6U8/6GH8 - splitters, V3/103
(4) 7591 outputs, V4/104, V5/105

Tuner section:(1) 6BS8/6BQ7 - fm tuner - V401
(1) 6BA6 - am tuner - V402
(1) 6BE6 - am tuner - V403
(1) 6U8 - fm tuner - V404
(2) 6HS6 - V6, V8
(1) 6AU6 - V7
(1) 6AV6 - V9

MPX section (3-tube, design):(1) 12AT7 - V501

(1) 6KE8 - V502
(1) 12AX7 - V503
or
MPX section (4-tube, Z-MX-6 units, S/N 234531 and below):(1) 6BL8 - V501
(1) 12AU7 - V502
(1) 12AU7 - V503
(1) 12AU7 - V504

http://hhscott.com/receiver.htm
 
Just wanted to add that an acquaintance who has a Mac 7591 based power amp had the similar symptoms until he bought new power tubes. He was surprised at the difference.
Best to check the power tubes to avoid shorts later.

best regards !

one other thing, the H.H. Scott does not have a fast rise volume gain. Unlike the Fisher 500C and 800C, the volume gradients on the volume pot are very gradual and sonically perhaps better.
The Fisher gets louder faster on the volume pot segments. Maybe meant to impress the 'loud-crowd'.

check this out also

http://audiokarma.org/forums/index.php?threads/remember-to-regularly-wiggle-them-tubes.703310/
 
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I have a tester, but it only lights up "Bad" or "Good," so it's not terribly specific about the life left in the tube.
The new Russian made tubes seem to have fine reviews, but to my understanding they will not fit with the wooden enclosure around the receiver?

Should I just take a leap faith and get a quad set of power tubes listen for the results? (Should probably have a spare set anyway...)
Looking on Ebay, it looks like there are some old RCA and Sylvania quads, how do these compare to the original Scott 7591a quad I have?
Are there any new tubes that compare to the old stock and fit with the case?

Also, regarding the volume potentiometer....
The volume isn't balanced in both speakers/headphones until about 1/5 of the way.
Not a big deal, as I use the balance, but is it supposed to be like that?
 
The Tung-Sol tubes are physically a better match for old stock if that is a factor for you. The EH ones are taller and fatter. Some amps have physical fitment troubles with that.
 
Audio Scout, the problem with some tube testers is that they are best suited for small tubes.
Power tubes need higher voltage in order to simulate "in circuit" operation.
Other variables can also give false readings like leakage/gas can spike transconductance readings.
EBAY is a crap shoot for that very reason. You need a high quality tube tester to really give you all the facts like a computerized one. I bought a matched quad of NOS American 7591 ($350) once from a reputable (non ebay) vendor. Had them tested with a computerized tube tester and returned them to the vendor. Yup variables can give false readings! Just wanted to add that sometimes ya get lucky on ebay. I have recently collected a quad of 6L6GC GE NOS from antique-tube-picker that turned out great on the computerized tube tester. v good price.

Anyway the Tung Sol is a good drop in. Jim McShane has alot of reviews in this forum. He's a good one.
A few other vendors also get good reviews. Do a search. Your rig used to operate on the ElectroHarmonix power tubes. Ask McShane or vendor whether the Tung Sol can also drop in. 'KVFlyer' is a member with lots of HH Scott experience. Search in this forum for this subject.
I believe he said that in general 'stock' HH Scott rigs did not need the resistor 'fix' to accommodate modern 7591 tubes. (Yours has been 'restored'.)

If you buy tubes you might as well buy two 6HS6 for the tuner to see if its better than the subs.
Also try to clean the rca inputs for the phono. That might help. Make sure your patch cord is good.

I recall reading somewhere on audiokarma or other about an anecdote of someone with slight uneven balance at low volume. I think it was to be expected for the particular tube amp. I don't recall much else but no one was alarmed by it pointing to something wrong. Off the top of my head, I think my own 380 has a bit of that. I think its the right thats a bit more.I just position my listening place at low volume. I don't use headphones so I have not observed it that way. (I usually listen at a bit higher volume.) My Luxman solid state amp did that also at low volume. My Fisher 800C did not do that, I believe, or my Mac. Maybe its due to the gain curve of the volume pot...??? Also some recordings emphasize the right channel for some reason.... variables..
You can always try cleaning with de-oxit cleaner and follow up with lube. Use the correct de-oxit product. some pot cleaners can damage the materials inside the pots. Never use WD-40 !!
Let the pot cleaner dry at least 1 hour before powering up. Sometimes pots need up to three cleanings at a time to get them tip top..
It helps to periodically turn the pots back and forth quickly about 40 times to keep the sweeps clean.

doh ! just thought.. any imbalance might also be the power tubes going south...
 
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OK I inquired about the tuner tubes with a tube amp tech friend in Florida. Archangele audio. http://home.windstream.net/wbittle1/index.htm


"The 6HS6 has a frequency bandwidth that is such that it can not be subbed with a 6AU6 in many RF circuits. When he has them, Dale, at Radio Electric Supply sells them for $24 a tube, which is not bad. I see they are really gouging the price on that tube on Ebay. This tube is normally only found in the AM section of Scott tube tuners. If it is bad, the worse that will happen is that your AM will not work. I believe the 399 uses two of them. Seems to me I remember picking up one or two a few years ago as spares for my 399."

These guys are good vendors. I have bought small tubes from them in the past. They offer this tube for $16 each
http://www.esrcvacuumtubes.com/vacuumtubes_tubelist_tubes_6d4-6jz8.html


So evidently, the other tube tech was not correct or maybe not far off in subbing with the 6au6. Probably did not hurt, just hampered the tuner reception.
I have no idea whether the phono section is affected. RF sections are all different. As in other circuitry, some tube subs work, but others don't. Its not black and white across the board.
 
The 6AU6 does work sorta as a sub for a 6HS6, its just not as good. At worst, your tuner will not perform as well. This will have no effect on sound through the aux inputs or phono stage.
 
Sonytubes; Found that the 6AH6 is an almost perfect match for the 6HS6 and it also works well in place ofthe 6HR6. Most places have them and less than a LINCOLN. It's a lot better than the 6AU6 as a replacement / sub.

Larry
 
AudioScout here is some info re: phono volume. It may be overwhelming but don't despair. Just study it out and get familiar with all the jargon and concepts.
Try cleaning the function selector dials also and retest each of the two stages of the 12AX7 tubes.

http://www.vinylengine.com/turntable_forum/viewtopic.php?f=85&t=49364

http://www.antiqueradios.com/forums/viewtopic.php?f=13&t=267685

(I believe that Don Cavey is KvFlyer on this forum-he is an HH Scott wiz)

The 380 has solid state rectification......

http://audiokarma.org/forums/index.php?threads/hh-scott-340-receivers.534813/
 
larryderouin said:
Sonytubes; Found that the 6AH6 is an almost perfect match for the 6HS6 and it also works well in place ofthe 6HR6. Most places have them and less than a LINCOLN. It's a lot better than the 6AU6 as a replacement / sub.

Larry
Click to expand...

6485 (Computer rated 6AH6) will also sub with a slight degradation in sensitivity over the 6HR6/6HS6.

Just look for the 6485 alone and not vendors playing games saying 6AH6/6485.

The 6485 has tighter tolerances and a lower noise floor than the 6AH6.

And its a Bargain.

EDIT: Had to rack the brain and its blonde so forgive me.

I bought up some Russian 6J5P's (Not the same as the USA Octal 6J5) to mess about with and in my 800C the initial results are promising before I had to put the 800C away to protect it from the falling ceilings here.

http://www.radiomuseum.org/tubes/tube_6j5p.html

Frannie
 
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