I'm testing tubes for my Sherwood S-5000 which uses 7189 (4), 7199 (2) and 12ax7 (5). I'm using a Hickock model 800A which tests for mutual conductance. There are two settings for testing the 7199's labeled as "Pent. Section" and "Triode Section". These tubes are old and some reach their mutual conductance spec on either one or the other "Sections".
My question is, does the Sherwood S-5000 use both sections of the 7199 tube? If not, can I use a tube that tests bad for the section it does not use?
I also have a Dynaco St 70 and SCA-35, both of which use the 7199 so my question applies to them as well.
I'm not an electronics guy at all, just a music and audio gear lover, so I hope my question is not a dumb one.
My question is, does the Sherwood S-5000 use both sections of the 7199 tube? If not, can I use a tube that tests bad for the section it does not use?
I also have a Dynaco St 70 and SCA-35, both of which use the 7199 so my question applies to them as well.
I'm not an electronics guy at all, just a music and audio gear lover, so I hope my question is not a dumb one.
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Yes, the 7199 is fully used in the Sherwood, as the power amp's input amplifier (pentode section)
and phase inverter (triode section). Same use in the ST70 and SCA35.
If the pentode amplifier section is marginal, sometimes you can reduce the value of the screen resistor
and perk it up. For the triode phase inverter, the DC voltage on the cathode should be close to 1/3
of the DC supply voltage, if the triode section is working ok.
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Hold on to them even if they're marginal, since they are very hard to find these days.
The 7199 is essentially extinct. AAMOF, all of that sort of pentode/triode is no longer being produced. Supplies of the 6BL8/6GH8/6U8 bunch are still relatively reasonable and all 3 types (which pin out identically) are reasonable electrical substitutes for the 7199. I suggest you rewire the sockets to take tubes that both work well and are available.
That's good to know. I'll certainly need help with that when the time comes.
BTW, the 7189's in the S-5000 are labeled "Sherwood" and "Made in W. Germany", not sure who made them but they test strong. Can I use them in an amplifier that was designed for 6BQ5 ?
That's good, burn it in with the Variac for several hours, and then it should run normally
directly from the wall outlet.
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Mr. Flintstone (bedrock):
I've seen (heard) old amps that didn't hum but the electrolytic filter caps were toast. The aluminum can actually got warm (that's bad). Sometimes they even snap, crackle, pop. Even if they are quiet and cool chances are they need replacing. Old coupling caps can get leaky (electrically). If you love the Sherwood and want to use it regularly show it some love and have a tech go through it and check all caps and resistors. I did that to an old Bogen PA amp I repurposed for guitar use. Replaced a lot of caps and a few resistors. Added an inrush limiter and a small wire wound resistor in the power transformer primary 'cause my line voltage is often above 120 volts. Experimented with the primary side resistor so the filaments didn't exceed 6.3 volts. A lot of old gear was designed for 110 to 115 volts and todays higher line voltages can stress them out a bit, especially the tubes.
Steve
I've seen (heard) old amps that didn't hum but the electrolytic filter caps were toast. The aluminum can actually got warm (that's bad). Sometimes they even snap, crackle, pop. Even if they are quiet and cool chances are they need replacing. Old coupling caps can get leaky (electrically). If you love the Sherwood and want to use it regularly show it some love and have a tech go through it and check all caps and resistors. I did that to an old Bogen PA amp I repurposed for guitar use. Replaced a lot of caps and a few resistors. Added an inrush limiter and a small wire wound resistor in the power transformer primary 'cause my line voltage is often above 120 volts. Experimented with the primary side resistor so the filaments didn't exceed 6.3 volts. A lot of old gear was designed for 110 to 115 volts and todays higher line voltages can stress them out a bit, especially the tubes.
Steve
The amp does not appear to have had any extensive work. All of the caps look original. Since it is working, and I would like to use it, is it safe to run it as is?
Are there any simple tests I can do determine if the Sherwood will be safe to run for a while? I'd like to get my feet wet in electronics (that sounds dangerous) so I bought a nice B&K multi-meter to replace my cheap analog Radio Shack one and a Weller soldering station.
Advice well taken. After considering my last post, I'm thinking I should start with a cheaper and simpler amp to work on. The Sherwood is just too nice to experiment with.
I've had very good luck with older audio equipment. If it seems to work well, I'd use it as normal
until there's a good reason to mess with it. You could check the output tube current by measuring
across the 12 ohm cathode resistors used for each pair of output tubes, but that won't show how
well matched the tubes are.
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