Silvertone 3902 P/P 6BQ5 Console amp

[Pio1980]

Thank you very much!! I especially needed the design voltage at C316C. The value of resistor R324 will need to be changed to compensate for the absence of the other chassis' load.
The really good news is that the output transformers are rated for a standard 8 ohm impedance, no need to change them out for full power into standard loudspeakers.
You can see how simple this amp is and why I am enamoured of it's elegant design.

 

[gogofast]

no problem.

i have a few questions/confirmation:

1. i understand that those two .02uf disc caps from input lines are to be taken out and the each input is going straight to pin #2 of 12ax7's. also those two resistors connected to the ground of input lines will remain?

2. as for your main power supply mod, i think i get the picture, but what do you mean by "replaced 10k 2watt power resistor"?

.... A 0.1uF/400V plastic film cap is paralleled with the 100uF capacitor at a convenient tie point and another is added to the other end of the replaced 10K 2 watt power resistor at a convenient point with the other end to chassis ground......

so this is roughly what i understood.... except for that last 0.1uf/400v cap.

 

[Pio1980]

That looks like it all right.
The 10k 2W resistor is a replacement for the 4.7k 2W resistor on the can's bottom to adjust the voltage downward at that point, it can be anywhere from 6.8K to 10 k as long as it results in a voltage with the same proportional relationship to the other PS voltages as indicated in the diagram. Base them on the actual voltage at the 'new' plate take-off.

 

[gogofast]

oh, i see.

thank you!

my amp is in pretty bad shape with electorlytic cans leaking with that distinctive smell and the chassis is covered with half inch of dust and dog hair.....wish me luck.

oh, and the need for R324 value to go up - what value are you thinking?

 

[Pio1980]

"1. I understand that those two .02uf disc caps from input lines are to be taken out and the each input is going straight to pin #2 of 12ax7's. also those two resistors connected to the ground of input lines will remain?"
The resistors stay.

"oh, and the need for R324 value to go up - what value are you thinking? "
'Round roughly 6.8 to 8.6k.

 

[Pio1980]

BTW, Keep us informed on your project.

 

[gogofast]

few more dumb question:

is it ok to use the external speaker terminals as a main speaker out? use those two yellow wires for + and use closest ground for - ? and just cut off ones coming out from the opt?

i have black and black/white cloth wires coming from the transformer that are connected to ac outlet i'm gonna get rid of the outlet and connect power plug. do you know which one should be connected to hot side of power plug? i'm thinking black but not sure.

thanks again!

 

[Pio1980]

Sure, cut the speaker wires at the transformer terminals and use the external terminal strip.
Yes, code is black hot, but I would go for the polarity that shows the lowest AC leakage to chassis when powered up.

 

[gogofast]

i've worked on mine on and off for the last few days. here's what it looks like.

thank you so much for your help!

one question remaining...the .1 film cap that's to be connected to replaced 10k resistor across the can cap... the resistor is connected between two 40uf sections of the can cap. now, the .1 film cap is going to any one end and then to the ground?

 

[Pio1980]

Use two 0.01uF film caps, one from each end of the replaced resistor to ground.

 

[bigbargain]

New acquisition!

Not sure if I should start a new thread or add to this one, but the amp in this thing is exactly the one PIO modded. Let me know if I should start a new thread.

I am a noobie to tubes, so really bought it to get my feet wet. Of course, I'm expecting I'm going to do the same mods described in this thread.

But I also want to find out how to allign the FM tuner and tune up the pre-amp (right now the tone controls need bass @ 9 o'clock, treble @ 3 o'clock.

I'm listening to it right now, sounds pretty good but speakers I'm using are not too great and it probably needs some attention.

Pictures aren't the greatest, but it is in REALLY nice shape - walnut. Gonna go into my office. Once I'm done, I'm gonna be looking for some bookshelf speakers that are era correct or at least walnut (I have some old jenson that have nice walnut cabs, but no drivers and need a refinish - maybe, but too big for bookshelves)

Anyway, check out the pics and of course, I'm VERY interested in input and hand holding on the rebuild.

 

[bigbargain]

1 more thing...

Original Silvertone tubes - all good (glowing, at least).

 

[Pio1980]

Proper IF alignment of the the FM side of the tuner is not possible without an instrumented bench set-up tho' the 'front end' may be spruced up via "seat of pants" by someone who understands what is supposed to happen and how it works. The AM side is pretty much the same story, I just can't recommend randomly twiddling the alignment adjustments at all. Doing it "blind" can leave you with a deaf radio that is way off the dial markings if it works at all. Let that go for now and just replace signal path and tubular/audio bypass caps in the tuner while checking resistors for more than ca 20% deviance from indicated value for replacement. De-Oxit for the noisy/ intermittent switches and controls. Do the amp as per this thread for now, leave in the ancillary cabling for the tuner/preamp of course.
The better the speakers, the better the results. Reasonably efficient larger (something not mini-cube-cute Minimus-7 F.I. ) with some bass extension to ca 50-60Hz at least) bookshelf speakers with audiophile cred would not be wasted on this amp. Think NHT, PSB, ect, that class speaker. Otherwise the Jensens may be "E-Wave-able". See that thread in the speaker forum, my secondary system is running E-Waved Advent-2 bookshelfs and I like what they do with this amp.

That is a really nice package, no repackaging needed. Must be a rarity version.

I think OEM Silvertone tubes are Sylvania. Anyone? Mine still had it's OEMs and they looked 'young'.

Lastly, I like one-stop-shop threads on a given model series as a very helpful resource for present and future prospective persons of interest.

 

[triode]

I have what looks to be an exact same amp. I was debating whether to take it up as a project or not but from the notes in this thread I think I am encouraged.
Btw, mine is chassis# 528.69240. The schematic posted by gogofast looks same but I just wanted to confirm. And also, gogofast, where did you get the schematic from ? I'd like to inquire about mine too.
Thanks!