Hi tube guys!
I have just almost finished my second amp build. It's a Silvertone 1471 clone that I've built into an old Zenith radio cabinet using the original chassis only. With the exception of the fact that I wired it for a grounded 3-prong cord and I got rid of the "death cap", it's true to the schematic. I'm pretty much finished except that I have a bit of hum that I'm trying to squelch. The odd thing is that the hum level changes as I operate the volume pot - but not linearly. The hum increases with the volume until about 75% volume then it decreases noticeably from there. There is almost no hum at full volume but at half-volume, it's unacceptably loud. According to my scope, it's 60Hz. My first build was pretty much this same amp but I'd moved the volume from where it is in this schematic to between the two stages of 12AX7 (a'la Champ F51) and that amp is hum-free. I owned an original Silvertone 1471 and there was no hum - and I'd like to figure out why I can't make it work as they did.
Thanks in advance.
Silvertone 1471 build - WEIRD HUM Query
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ServiceBob
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Silvertone 1471 build - WEIRD HUM Query
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Stevem
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Re: Silvertone 1471 build - WEIRD HUM Query
If your grounds for the volume pot and both of V1s cathode resistors are daisy chained together the open that chain and run a separate ground wire for those 3 points back to a ground on the PTs mounting bolts, or drill a new hole for a long 6/32 machine screw and do the same.
Solder on ring lugs to do this.
This assumes that you already have the filiment wire runs twisted tightly and you have r14 and 15 wired in place right.
I would make a steel plate to close off the chassis bottom that the amp can sit on and be sure it grounds out to the amp chassis.
Also post a a few close up pictures of the guts of your build.
Once you get it right I would experiment with bypassing r11 with up to a 150uf cap, be sure its rated for a minimum of 25 volts.
Solder on ring lugs to do this.
This assumes that you already have the filiment wire runs twisted tightly and you have r14 and 15 wired in place right.
I would make a steel plate to close off the chassis bottom that the amp can sit on and be sure it grounds out to the amp chassis.
Also post a a few close up pictures of the guts of your build.
Once you get it right I would experiment with bypassing r11 with up to a 150uf cap, be sure its rated for a minimum of 25 volts.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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ServiceBob
- Posts: 30
- Joined: Fri Sep 25, 2020 12:12 pm
Re: Silvertone 1471 build - WEIRD HUM Query
Hello Stevem!
First, let me say that if this were 500 years ago you'd be burned at the stake as a witch - and I'd light the first match! (It seems like witchcraft that you I.D.'d the problem exactly!)
I took a few photos of the inside of the chassis and as I was editing them to post here as you requested I noticed what looked like a cold solder joint on R14. I went back into the amp and poked at that resistor with my chopstick and I'll be darned if the lead didn't pop right off as though I'd never soldered it. Actually, I had but the PTFE tube I'd put over the lead had slipped down covering the end of the lead before I'd soldered and there was no connection between the resistor and ground. I corrected the problem and then plugged the amp back into the variac, powered it up, and VOILA no hum!
THANK YOU!
Now, about adding a bypass cap on the 6V6 cathode resistor. I actually did that on the other build, 25/25 because I saw that on virtually every other amp schematic I've looked at. And I'd read that it would increase the gain. My other amp sounds great (at least to my ears). I was trying to stay closer to the original circuit with this build. Aside from the potential increase in gain, would I notice any difference in tone? Is there any other benefit?
First, let me say that if this were 500 years ago you'd be burned at the stake as a witch - and I'd light the first match! (It seems like witchcraft that you I.D.'d the problem exactly!)
I took a few photos of the inside of the chassis and as I was editing them to post here as you requested I noticed what looked like a cold solder joint on R14. I went back into the amp and poked at that resistor with my chopstick and I'll be darned if the lead didn't pop right off as though I'd never soldered it. Actually, I had but the PTFE tube I'd put over the lead had slipped down covering the end of the lead before I'd soldered and there was no connection between the resistor and ground. I corrected the problem and then plugged the amp back into the variac, powered it up, and VOILA no hum!
THANK YOU!
Now, about adding a bypass cap on the 6V6 cathode resistor. I actually did that on the other build, 25/25 because I saw that on virtually every other amp schematic I've looked at. And I'd read that it would increase the gain. My other amp sounds great (at least to my ears). I was trying to stay closer to the original circuit with this build. Aside from the potential increase in gain, would I notice any difference in tone? Is there any other benefit?