Sears Silvertone Twin Twelve 1484 Major Restoration Project [3/6]

Here in part 3 we continue with preparation with turn-up by installing the pot needed for the hum balance which replaces the filament center tap. After this a dry run without tubes is conducted just to be sure everything is ok. If the tap cant meet 6.3 without tubes it cant be the right tap and i have to switch to the next one. Given the one at around 7 volts its time to add the tubes to the amp.
With all of the tubes in the amp we come to find that the final filament voltage is 5.9. It looks like another turn or two will be needed out of that transformer. A retest with 2.5 wraps shows 6.2 volts at 120vac.
Following this The temperature gun will be used to measure the heat off of the transformer after 30 minutes or so. No heat issues were found. To celebrate, the transformer covers were stripped and painted, as well as the rewiring of the reverb tank.
I break out the impedance bridge to test the cabinet and confirm the impedance at 8 ohms.
I found an extra speaker jack that was inappropriately added to this cabinet that I could remove to place the hum pot into. After that, its time for the first full smoke test.
I wish this amp had an adjustable bias..... also I discover at this time that the standby circuit is garbage. I end up having to use several tubes from Cornerstone music to find something that operates within safe parameters.
This brings the project into the noise isolation phase. and there is no shortage of noise in this amp; all sorts of different types of noise from different stages.
This starts from the finals backwards to the phase inverters. Where things are already looking strange.
#retrotechandelectronics #vintagetubeamprepairs

[@jnoreen]

Well done! you are one of the few on UA-cam that actually knows how to troubleshoot electronics. I enjoy your videos.

[@retrotechandelectronics]

I appreciate that!

[@jjsant3250]

Changing those grid leak/bias supply resistors is not a way to adjust bias. There is no current flowing there so no opportunity to “drop volts.” Cathode resistors under cathode biasing can be changed to change bias since current is flowing thru them.

[@retrotechandelectronics]

This is almost entirely true! However, those resistors are still out of specification and warrant replacement.

[@dhpbear2]

9:26 - How about installing that pot *inside* the chassis?

[@retrotechandelectronics]

I would have, if not for that extra useless jack made available to me.

[@xnotx2]

Time to rig up some bias pots lol
Can't wait for the next video on source of noise and the fix :)
Oh, how's the moped ignition coming along?

[@retrotechandelectronics]

Bias pots dont work in this configuration. You would need to first produce an overabundance of negative voltage that you could then sent to ground in a metered fashion via a pot. This setup produces 'just enough' negative voltage, there is little to meter.
The moped is awaiting more Soviet parts and is not forgotten. Thanks for asking it will also continue this winter

[@xnotx2]

Ok, makes sense.
Thanks

[@MikeHammer1]

Your output transformer is definitely not the original. The one in your amp is significantly larger. A question regarding changing the speaker lead to a 1/4" jack. Can the jack be safely grounded to the chassis or must it be isolated?

[@MichaelLloyd]

I had to change a lot of out of tolerance resistors in my Silvertone Twin 12... Ah... now I remember... Standby isn't truly standby on the Silvertone. That's why I did this:
"Here’s another quick mod. The 1484’s standby switch is different than most: It does not disable the high voltage-it simply cancels the signal going to the output tubes. Try removing the switch and replacing it with a 1 Meg linear pot. Connect the two wires that were attached to the switch to the pot, one on the wiper and one on the CCW leg. Turning the pot all the way down will yield the same result as the original standby switch, while turning it up will control the amount of signal going to the output tubes."

[@retrotechandelectronics]

I understand the intended result of the addition of the pot as opposed to the switch, but what is the benefit where volume is used to meter the output to the finals anyway?

[@MichaelLloyd]

@@retrotechandelectronics You can crank up the first stage and approach or achieve distortion there but limit the volume from the amp so the neighbors don't call the cops on you...

[@retrotechandelectronics]

AAAAH Im a Hifi amp guy... That hurts to think about... I gettcha though..

[@dhpbear2]

17:20 - Where are the cathode bias resistors? Weird!

[@MichaelLloyd]

There aren't any... it's a grid biased, double ended amp. The output section is similar to Fender Princeton Reverb amp. You can adjust the values of R35 and R36 to make up the difference in the tubes (I wouldn't :o) ) or use a matched set of tubes and make sure the grid bias resistors are close in value. You have to use the center tap to plate resistance reading for each tube and plate voltage from each tube to calculate plate current, unless you have a nifty Eurotube socket thingy.

[@retrotechandelectronics]

I believe, though, I can't fully recollect my displeasure in the setup for the bias in this amp at a few junctions. There are a couple of key items here that leave you wondering how many of these amps didn't have chronic issues out the door. Either that or Sears had amazing matching skills. Furthermore, The voltage supplied by the transformer for the grid bias is somewhat anemic, leaving little room to work with.

[@srtamplification]

The standby switch on those amps don't do what most standby switches do in other amps. All it does is shorts the PI outputs. It is kind of cheesy.

[@srtamplification]

Sorry, I made this comment before I got to the part of the video where you saw that on the schematic. LOL