Scott 340B Over Heating

[AudioScout]

I have an old Scott 340B I've been looking to get back up and running.

But there's two issues:

1. There is a loud hum in both channels.
2. The 7591 Scott Power Tubes in the back glow red how and look like they are about to explode or catch fire or something....(That's when I turn it off)
3. There is one tube missing, it looks to be a "6AU5?" The print is barely legible.

Is this just a matter of replacing the tubes, or is there something else going on?

 

[sony6060]

The power supply capacitors are likely in poor condition causing the hum. A bad tube can cause this or incorrent biased power tubes.

The power tube bias is likely out of spec due to suspect leaky coupling capacitors and or bias power supply issues. This condition causes the power tubes and over time power transformer over-heating.

The missing tube is likely a 6AU6 7 pin miniature tube from the tuner IF section.

Do not operate the Scott as service is needed plus can ruin the expensive power tubes and audio transformers.

 

[Palustris]

As Sony6060 says, these are the classic symptoms of bad power supply capacitors and bad coupling caps: that unit needs a complete rebuild.

 

[Dadbar]

You probably also need a new bias rectifier to bring the bias back to "code."

Don't use your 340B until it's had a complete recap. You stand to wreck your transformers and power tubes which will be big bucks to replace. The recap is relatively cheap by comparison.

 

[kvflyer]

You have been given accurate descriptions of the most likely cause of problem. I would follow that advice before connecting power again.

Hum is often caused by power supply filter capacitors. They may either go open (cause hum) or become very leaky (cause overload to the power transformer). This hum will not be affected by the position of the volume control.

Red plate on the output tubes is most likely caused by incorrect bias to the output tubes either by leaky coupling capacitors (.01 uFD White American Radionic Ceracap) or as mentioned, a bad Siemens Selenium bias bridge. It is a silver colored square item, mounted on the side of the chassis with four flat leads. It provides DC for the bias to the output tubes AND DC for the filaments of the 12AX7 preamplifier tubes.

I am surprised that the fuse did not blow. Did you check to see if the correct on is installed?

 

[AudioScout]

The fuse looks fine, didn't blow.

Thanks for the info guys.

Is this something I might be able to repair myself (Amateur DIYer)?
Or should I go straight to a technician?

If so, any recommendations on tube audio repair services in Upstate, NY/VT?

 

[valve dude]

repair

I have an old Scott 340B I've been looking to get back up and running.

But there's two issues:

1. There is a loud hum in both channels.
2. The 7591 Scott Power Tubes in the back glow red how and look like they are about to explode or catch fire or something....(That's when I turn it off)
3. There is one tube missing, it looks to be a "6AU5?" The print is barely legible.

Is this just a matter of replacing the tubes, or is there something else going on?

i would be testing all the valve
i would be checking all the capacitors with a ESR meter
2nd of all if its humming better if you use a series globe to save your transformer it sounds like a nasty power supply problem
i would remove all tubes check power supply caps and resistor and test the power supply choke if it has one
then i would fire her up on the series glob chech EHT