Silvertone 132.878 to Princeton Conversion

[NoDough]

Hello all! I have recently received a project to convert an old Silvertone radio into an working guitar amp. Here is a picture of the chassis and enclosure:



I plan on using the princeton schematic from Weber Speakers, as I have had success with building from this before.

Here is the schematic and layout:



So before I embark on this journey there are a few questions that I really want to get some clarity on.

1) The chassis is metal and the enclosure is metal. Is that a problem?
So this may seem like a silly question, but I just want to make sure that I won't run into any big issues with a metal enclosure. I did some google searches on metal enclosures for guitar amps, and came up with very little.

2) Knob/input/fuse/pilot lamp/ac switch placement. Can any of these be placed in the back?
So the Weber layout calls for all of these to be placed in the front. I would like to keep the face of the old radio as close to original as possible. That being said, I will likely drill through for the Knobs and input as I don't want to run into noise issues by placing them in the back. But could the Pilot lamp, ac switch, and fuse be placed in the back without any problems? I will likely keep the three tubes in their current locations near the back of the original chassis.

3) Where should I place the OT? Or should I source a smaller output transformer?
My plan is to knockout the left side of the chassis to fit the new power transformer, but the output transformer is not as straightforward. This kind of ties into the previous questions concerning placement of knobs/etc. The limited space really only allows me to place the OT in the front right of the chassis. If I place the input and knobs in the back, that would get them away from the output transformer, but will this introduce unwanted noise issues? Should I potentially try to get a smaller output transformer for this build?

4) What to do with the old radio dial?
So the hole in the top right of the enclosure is for the old radio dial. In an ideal world I would be able to reuse this as the tone knob, but I assume this would be a poor design layout with the wires running close to where the new power transformer will go. Are there any other ideas as to what I could do with the old radio dial? Maybe a preamp overdrive? Or should I just glue it down and leave it for cosmetic purposes?

Thanks in advance for your help and wise direction in this project! 

[WimWalther]

Where do you plan to put the power & opt transformers? There's not much room on that chassis. Are you planning to install a speaker as well?

[NoDough]

I'm thinking they would slot in at those spots. Basically take off the the old variable capacitor to the left and dremel out a hole for the pt.

The right side with the OT, I'm a little less sure what to do, but should be able to just bolt it down to the chassis and make holes with gromets for the wires in a perfect world. Of course the questions surrounding that still stand with unwanted noise interference.

[PRR]

I was gonna say.... but no, the 132-878 wasn't even a good RADIO unless you were a block from the transmitter. Attaching schem for reference. Note hot chassis (tied to wall-power).

Trying to jam a 5-Watt amp in that case may be uphill all the way (or downhill, depending how you look at it).

[WimWalther]

I was gonna say.... but no, the 132-878 wasn't even a good RADIO unless you were a block from the transmitter.

Heh, wow.. talk about cheap & cheery. No RF front end, no IF amp, just an SA7 converter running straight into an SQ7 audio / detector stage.. with gridleak bias, and then to the L6 output tube. Strangely enough, it does have AGC, as if you'd ever need it!

But I guess it's a step up from a Volksempfanger VE301, though not by much.

[NoDough]

Are you planning to install a speaker as well?

The plan is to install a new speaker where the old speaker currently sits.

I was gonna say.... but no, the 132-878 wasn't even a good RADIO unless you were a block from the transmitter. Attaching schem for reference. Note hot chassis (tied to wall-power).

Trying to jam a 5-Watt amp in that case may be uphill all the way (or downhill, depending how you look at it).

Yes I have that schematic. The reason I decided to go the "stuff an amp into an old silvertone" route was because I figured it wouldn't be worth trying to outfit everything with an isolation transformer that would only result in a poor, noisy amp.

I figured this should be doable as Tube Amp Doctor sells a champ amp chassis that is 9 in wide X 4 in deep.
https://www.tubeampdoctor.com/en/amp-chassis-tweed-champ-style-5f1


The Silvertone chassis is smaller at 8 in X 3 in, so an even tighter squeeze, but I don't mind spending some time time trying to figure it all out.

So have I completely lost the plot here PRR? These are all learning experiences, so feel free to be as honest as possible.

Thanks again for the education 

[bmccowan]

I think its a cool looking cabinet. So have you thought about alternatives? A micro amp with one tube for example? A small extension speaker? or?? I have a few cool looking radios that don't work - they just sit there looking cool. Nothing wrong with that in my book

[PRR]

It IS a terrific cabinet but it ain't no Fender. Improvise a little 1 Watt transformer powered guitar amp in it and let it be what it is. (IMHO case and color can be more important than circuit.) The 50L6 and OT would be fine, but I think you will be happier with 6V heat, which leads to 6Y6 (big octal) or an underpowered 6AQ5, or the sweet 12FX5/60FX5, or even that PRR Third-Watt on 6AU6 bottles.

[WimWalther]

It IS a terrific cabinet

Nice color for sure. So what's it made of, anyway - painted Bakelite or some blue thermoplastic?

[PRR]

Nice color for sure. So what's it made of, anyway - painted Bakelite or some blue thermoplastic?

The chassis is metal and the enclosure is metal.

[NoDough]

It IS a terrific cabinet

Nice color for sure. So what's it made of, anyway - painted Bakelite or some blue thermoplastic?

Yes as PRR said it is metal that has been painted blue.

It IS a terrific cabinet but it ain't no Fender. Improvise a little 1 Watt transformer powered guitar amp in it and let it be what it is. (IMHO case and color can be more important than circuit.) The 50L6 and OT would be fine, but I think you will be happier with 6V heat, which leads to 6Y6 (big octal) or an underpowered 6AQ5, or the sweet 12FX5/60FX5, or even that PRR Third-Watt on 6AU6 bottles.

Prr, any resources (schematic,layout, etc.) you might have for a 1 watt transformer powered guitar amp. I found this build:

https://www.youtube.com/watch?v=IAqMCp7-oGQ



I am heavily leaning towards that as it has a SS rectifier, which eliminates the need for 3 total tubes. It also has a smaller OT transformer, which would help me relocate it to where the current rectifier tube is. Also since it is modeled after a princeton I can still use a lot of the parts I already ordered when I was thinking of going the 5w 5f2a route.

I'm still concerned about where to place the the volume and tone knob as the real estate on the front panel is very limited. I think I will definitely place the on/off switch in the back, and maybe drill out a hole for a pilot lamp up front.

I have also looked pretty extensively at Rob Robinette's Champ Micro, and that seems like a pretty solid contender as well.

So....yea....any advice is appreciated. 

[WimWalther]

It IS a terrific cabinet

Nice color for sure. So what's it made of, anyway - painted Bakelite or some blue thermoplastic?

Yes as PRR said it is metal that has been painted blue.

Ah, metal.. excellent! Perfect choice for a set with the chassis tied to the AC line.

[sluckey]

Ah, metal.. excellent! Perfect choice for a set with the chassis tied to the AC line.

It's not connected directly to the chassis. There's a 330K and parallel death cap between the AC line and chassis.