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 Post subject: Resurrecting An HH Scott 299C
PostPosted: Jul Mon 16, 2018 12:55 pm 
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Location: Fernandina Beach, FL
One of my goals was to assemble a collection of HH Scott vacuum tube amplifiers. I have several (maybe more :wink: ) and I decided to start working on them. My first candidate (on the top of the pile) is a 299C. This amplifier is first in Scott's 299 line to start using the 7591 as an output tube. Previously, the 299 line utilized the 7189, also a very respectful tube. Mind you, there are about 7 minor variations of the 299C. So, it just may not match the schematic that I have. Also, the 299C is very much like the kit version, the LK-72.

I started by testing the first filter capacitor and surprisingly, it was good. Next, I tested one of the coupling capacitors and it too was good, on value, no leakage. Mind you, by this time, Scott may very well have been using either Mylar or paper/Mylar capacitor types. I ramped up the voltage slowly whilst monitoring the current. When I got to 117 VAC, no fireworks, no redplating and no electrolytic warming. I got absolutely no output. However, if I rocked the phase inverter tubes, there was lots of noise. I always preach (to myself and often do not heed :cry: ) "Try the obvious first"!

So, I got out the DeOxIt and using a small brush, treated each and every contact in the tube sockets. Guess what? That was it! Lovely sound emanating from my test speakers, Bang & Olufsen S45-2. But there is a weak point in these amplifiers. It is the Siemens Selenium bridge rectifier. They supply the DC voltage for the preamlifier tube filaments as well as the bias for the output tubes. It was not putting out the required -40 VDC. Replaced with a silicon bridge and the first resistor was changed from an 18ꭥ 1-Watt to a 47ꭥ 5-watt and the voltages were very close.

So, hopefully the take away from this as I start to go through the "pile" is message to self:

- NEVER overlook the obvious
- Tube sockets can be a problem!!!

Now, cosmetic cleaning is next as well as pictures.

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Don


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 Post subject: Re: Resurrecting An HH Scott 299C
PostPosted: Jul Mon 16, 2018 6:52 pm 
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Joined: Aug Sat 01, 2009 2:23 pm
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And...never trust a selenium rectifier!

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John Stone
Antique Radio Club of Illinois


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 Post subject: Re: Resurrecting An HH Scott 299C
PostPosted: Jul Mon 16, 2018 11:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Boston, MA USA
Particularly when it's providing bias!

-David


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 Post subject: Re: Resurrecting An HH Scott 299C
PostPosted: Jul Tue 17, 2018 12:28 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 13895
Location: Fernandina Beach, FL
jmsent wrote:
And...never trust a selenium rectifier!

I agree. Sometimes they are "OK". But they can cause catastrophic damage to the output tubes and output transformers when not up to snuff.
dberman51 wrote:
Particularly when it's providing bias!-David

Yes David, I did monitor the current drawn by the amplifier when idling. When it was still original with the Siemens bridge, the current was much higher than when I replaced the bridge. Now, the 7591 tubes have good negative bias and it works good.

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 Post subject: Re: Resurrecting An HH Scott 299C
PostPosted: Jul Tue 17, 2018 6:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4152
Location: Boston, MA USA
Not sure how you tested the filter capacitors, but it is important to check not only for capacitance and leakage at full voltage, but also ESR (Equivalent Series Resistance). A common failure mode for electrolytic capacitors is increased ESR, which can be described as adding a resistor in series with the capacitor. In a filter capacitor, excessive ESR causes increased hum and reduced power supply current capacity. The amplifier performance will suffer perhaps without your being aware of it.

I assume the filter capacitors are the Mallory FP type (metal can)? In my experience resurrecting old equipment after years of disuse, these capacitors may appear to work OK for awhile, but after a couple of weeks of use the capacitors start to fail noticeably. I find this very common with the low-value high-voltage filter capacitors used with tube equipment. I have not had this experience with the high-capacitance value low-voltage filter capacitors used with solid-state. These seem to last decades.

If it were my 299C I'd restuff those cans!

-David


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 Post subject: Re: Resurrecting An HH Scott 299C
PostPosted: Jul Wed 25, 2018 12:29 am 
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Joined: Mar Mon 05, 2012 6:35 am
Posts: 302
Location: Chicagoland, 60194
In addition to everything else already mentioned I bypassed the filter slide switches and removed the headphone cable assembly. Both features can cause issues so if you don't plan on using them the amp will sound better without.


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