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Old 01-20-2015, 05:04 PM
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1958 RCA SHF-4 Record Player Amp

Hello everyone, Happy 2015. Got a few questions about restoring the amp on my RCA SHF-4 player. I recapped it and tested/replaced all out of spec resistors. Tested tubes and fired it up (after recapping tuner as well). Plays great, sounds awesome. Noticed that after 20 minutes the power transformer was getting pretty toasty on the amp. Not boiling hot, but hot enough that I could not keep my hand on it after 15-20 seconds. I have a second SHF-4 that I restored some time ago and that one runs cool as a cucumber. So I pulled the amp and gave it a once over. Everything underneath checked out ok. So figured I'd test the 5Y3 and the two 6V6s again. 5Y3 tested ok, but swapped in a different one (also testing good) for the heck of it. Now the two 6V6s. I tested them on two different testers and both showed no shorts but buried the needle at the top of the "Good" zone. Does anyone know what this indicates? When the tube tester needle gets buried at top of good range? So I found two other 6V6s in my stash that had normal "Good" readings and put them in. Reinstalled the AMP and running MUCH cooler now.

Could the readings on the 6V6s indicate something that would have caused my power transformer to run hot???

Thx.
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Old 01-20-2015, 05:08 PM
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Why don't you measure the cathode current of the 6V6s and see if it looks okay? It shouldn't be more than about 35 - 50 mA per tube if everything is normal. If there is a common cathode resistor, double that figure. If the cathode current is in spec, then you can rule that out.
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Old 01-20-2015, 05:11 PM
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I will check that out. Thx. Any ideas on what the buried Good Range needle on the 6V6s might indicate for those tubes? Had not seen that before.
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Old 01-20-2015, 06:05 PM
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what brand are the tubes? If you swap output tubes between the two record players, does the problem follow the tubes?
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Old 01-20-2015, 06:12 PM
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What does 'buried good range needle' mean? Assuming you are talking about the scale on a tube tester, are you trying to say the meter is pegged above the highest good range reading?
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Old 01-20-2015, 06:17 PM
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The "super" good tube may be asking for more bias current than the amp can deliver.
I'd try another known good pair.
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Old 01-20-2015, 09:28 PM
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Right, needle is off the top of the Good Range scale on my tube testers.

After an hour and a half of constant play, transformer is at 118 F.
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Old 01-20-2015, 10:20 PM
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After almost 2 and a half hours of constant play amp transformer temp at 128 F.

Holding at this temp at almost 3 hours.
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Last edited by bigaudioal; 01-20-2015 at 10:59 PM.
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Old 01-21-2015, 01:21 PM
Olorin67 Olorin67 is offline
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If i recall right, this unit uses a split level power supply to provide grid bias to the output stage. Check the voltages on on the output tubes to see if everything is oK. -or as suggested, check the current in the output tubes.
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Old 01-21-2015, 07:27 PM
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128F sure doesn't sound unprecedented for a power transformer in tube equipment under load.
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Old 01-21-2015, 07:30 PM
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Quote:
Originally Posted by maxhifi View Post
128F sure doesn't sound unprecedented for a power transformer in tube equipment under load.
That's with the cool-running tubes installed. TBH I'm surprised that its that low.
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Old 01-22-2015, 12:06 PM
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I am curious what those "hot running" 6V6s actually are. In any case good job keeping the old RCA alive.
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