Motorboating Silvertone 1484Motorboating Silvertone 1484 Try

[Blindog]

Post up some decent res pics and we'll help you get this thing going

Mark

[Micajah]

Started checking voltages - here's what I found.
Voltages on pins 1 and 6 of preamp V1 should be 100 - pin 1 is 74v, 6 is 134

on the Rectifier array the B+ should be 480V, its 370V
On the output tubes V7 & V8 pins 3 should have 475v its 480.
I've attached schematics - the original from the manual and a re-drawn.
I'm thinking the PT may be going bad. Anyone have any suggestions?
Thanks in advance.

[rp]

475V! They made the cab out of 1/2" chipboard, the baffle out of Masonite, covered it in contact paper, used a reverb pan the size of a cigar but ran almost 500V on the output tubes - bless their twisted econo-hearts.

Your output tube plate voltage is dead on, did you measure wrong off the rectifier? You sure you re-stuffed your can correctly? Given the difference btwn 1964 wall voltage and today you are lucky you don't have 525V on the plates!

You prolly don't need screen grid resistors, they took care of that with a 135v difference btwn the plate and screen.

Your V1 is OK too, the discrepancy is likely an imbalanced tube, no big deal, or the 220ks have drifted, or if silver band 10% there you go.

I had one of these, they are magnificent, reverb aside. The cab and those budget small voice coil Jensens are essential to the greatness, so don't sneer at the dinkyness. I keep promising myself i will one day clone a super shitty cab like this and hunt down some old lightweight Jensen/Heppner/Magnovox speakers for it. Relive the joy - maybe.

Makes you really wonder if the original designers were extra mindful and carefully tweaked the voltages, circuit, and cab to perfection or just threw some parts together randomly and hit it out of the park?

[rp]

Question, on V1 what's the 1M across the input and pin 6 do?

[martin manning]

Question, on V1 what's the 1M across the input and pin 6 do?

Some kind of LNFB, but I think it must be drawn incorrectly. I can't believe there is supposed to be high voltage on the tip of the input cable.

[shane]

Its drawn wrong on the second schematic. If you look at the original it comes after the coupling cap (thankfully!).

Cheers
Shane

[Stevem]

I do not recall my 1484 having plate voltages that high as even with the big ass 50 rated OT I have in it , the amp with pristeen RCA Black plate 6L6 tubes only outputs 32 watts RMS!
I will pull my head out tonight a check that plate voltage for you.
Mean while what is your bias voltage on pin 5 of the outputs? also what to you have for cathode voltage on all the preamp tubes?

[Micajah]

On the output tubes V7 & V8 pins 3 should have 475v its 480.

Sorry - Typo -...pins 3 should have 475v its 380

[Micajah]

Question, on V1 what's the 1M across the input and pin 6 do?

Actually it's there in the original - however the re-draw shows a 1Meg going from hot to ground on a switching jack. Creates a high and low gain input across the 2 jacks. This is explained in an article from Vintage Guitar "How Input Jacks Work -Interactive - Volume Controls" by Dan Torres.
I've attached the PDF

[rp]

Sorry - Typo -...pins 3 should have 475v its 380

Bet you a beer 380V is correct and spot on, the schematic wrong. I owned one, I was already monkey poking then, had recapped mine, and should have noticed scary Vp, but it was 20 years ago. As stevem noted it didn’t sound like 50W amp, 32W sounds about right, if that. Otherwise it would have shot those 1”(?) jensens right across the room.

Actually it's there in the original - however the re-draw shows a 1Meg going from hot to ground on a switching jack.

Wrong 1M. I’ll go w/ MM that it’s LNFB, not sure why they thought it necessary, might be a hifi touch like the 6CG7 PI, but it’s an awesome amp so who's to argue. The wrong schematic sure had me curious, that thing should be pulled before some sophomore clones it and becomes the human on/off light.

[Micajah]

Checked the the wiring in the voltage doubler stacks and the positive side of C27 was mis-wired and that's where the voltage drop was. Now it's back to about 475V. I knew it wasn't the Cap can C25 - I replaced all those. Also I put in a 3-wire AC cord and removed the unnecessary death cap and ground switch.
Now the plate voltages on V1 are 107 and 118. Bias on the output tubes Pin 5 are V7 - -32.42, V8 -32.53.
Now all I got to do is trace down a big hum on channel 1. It's silent when the volume is down but very discernible when you turn it up.
Channel 2 is great no hum like 1- tremolo works, but the signal is not being fed to the reverb. If I knock on the tank with the reverb up I can hear the spring through the speakers but no feed from the output. back to the schematic and magnifier.

Thanks for all your input.

[Micajah]

Tracked down channel 1 hum to the volume pot. Re-cleaned it and replaced the 100k resistor and then re-flowed the solder on all the connections. Now it sounds great - the hum is still slightly there but it matches channel 2 and is probably as good as it going to get. Going to try to get the reverb running. I know the output from the tank is working because when I turn the reverb control up and knock on the tank I can hear it spring through the speaker. It just doesn't seem to be getting any feed.

[Stevem]

Watch out with that low bias voltage on those tubes!
With anything over 450 on the plate and 300 on the screen you want closer to 37 volts coming in on pin 5.

While you are still tweakin the amp I would place in serise with each output tubes pin 8 a 3 watt 1 ohm 1% resistor so you can directly read the amerage on a DC set voltmeter.
For now be sure that in a dark room that the outputs are not red plating at idle and for sure when you play the baby.

Those reverb pans sound like crap anyway so dont kill yourself in regards to time.

That being said, I have a somewhat bigger reverb pan from a solid state Univox U65 model, and these do not sound too bad as far as cheap pans go.
Its a bit bigger than the silvertone pan and I have not checked its impeadance yet, but one of these from a dead and cheap Univox might be a up grade so to speak.
I will keep ya posted if I get to it over the weekend!
Oh, congrats on the trouble shooting and repair on the amp!

[Micajah]

Watch out with that low bias voltage on those tubes!
With anything over 450 on the plate and 300 on the screen you want closer to 37 volts coming in on pin 5.

Yeah - I got a bias Tube scout from TubeDepot for this and future use - the bias is actually closer to .039 on V8 and .038 on V7 so I think I'm good.

Those reverb pans sound like crap anyway so dont kill yourself in regards to time.

That being said, I have a somewhat bigger reverb pan from a solid state Univox U65 model, and these do not sound too bad as far as cheap pans go.
Its a bit bigger than the silvertone pan and I have not checked its impeadance yet, but one of these from a dead and cheap Univox might be a up grade so to speak.
I will keep ya posted if I get to it over the weekend!

Yes I do believe the piezo's on the input side are kaput. I know it's getting signal and i can hear it when I knock on it so I'd be interested in seeing your suggestion with the Univox tank. Also seeing a few other suggestions to adding a Hammond style tank with a mod of the reverb circuit to accommodate an Ampeg type circuit that uses the same tubes. See the attachment. Would love to hear what you think.
Thanks