Silvertone 1474 Chassis Question

[jrdamien]

Just scored a complete chassis of the bay. Chassis only, working, no tubes, knobs, cab or speakers. I've been looking for a combo so this will do nicely after I build a pine cab.

Questions: it comes with 2 impedance matching trannys for the speakers. Was this to keep the stock Jensens from coming apart? Can I lose them if I use speakers appropriately rated?

This amp splits the 4x 6L6 output into two loads, one for each speaker. Is there a way to combine this load to one speaker or pair of speakers and is there any benefit in doing so?

[Enzo]

They basically made two power amps, one for each speaker. This let them use the same output transformer they used for smaller amps instead of making a larger one. As far as speakers are concerned, two 25 watt amplifiers powering two 25 watt speaker is no different from one 50 watt amplifier powering two 25 watt speakers.

Can you combine them? Sure. then you would be more or less just building a new power amp in the chassis.

[jrdamien]

They basically made two power amps, one for each speaker. This let them use the same output transformer they used for smaller amps instead of making a larger one. As far as speakers are concerned, two 25 watt amplifiers powering two 25 watt speaker is no different from one 50 watt amplifier powering two 25 watt speakers.

Can you combine them? Sure. then you would be more or less just building a new power amp in the chassis.

Makes sense. So what about the impedence matching transformers on the speakers themselves. Are they unnecessary?

[g1]

Tube amps require output transformers between the tubes and the speakers. Sometimes instead of being mounted in the amp chassis, they are mounted on the speakers. They are necessary.

[jrdamien]

Tube amps require output transformers between the tubes and the speakers. Sometimes instead of being mounted in the amp chassis, they are mounted on the speakers. They are necessary.

The amp in question, the Silvertone 1474, has impedence balancing transformers mounted to the speakers themselves in addition to the output transformers mounted on the chassis that, as you said, are required for it to make sound.

So I am wondering if these speaker mounted trannys are required if not using the original speakers.

[g1]

Well that's odd. I found some pics online that showed the chassis with the PT mounted, but no OT's on the chassis, just on the speakers. So I assumed they would be OT's, no impedance matching transformers.
Sorry.

[jrdamien]

They're hard to see in pics because they're behind the power tubes on the right side.

[52 Bill]

I think he's talking about the chokes that are added across the speaker loads. I've seen these on this model Silvertone and some Danelectro amps.

I never knew if they were used to change the tone or to change the impedance load.

[g1]

Ok, I saw another pic and it shows them as only having 2 wires (chokes) and connected in parallel with the speakers. So, like 52Bill said, they could be for tone, or impedance, and I'm also wondering if they could be for power handling.
As you will be using speakers that can handle the power, I would try with and without, and choose whichever sounds best.
If they are being used in lieu of proper OT impedance tap, I don't think the mismatch without them would cause any issues.

[jrdamien]

OK just saw a close up pic, two wires so they must be chokes. Never saw or worked on an amp with chokes on the speakers so if anyone has anymore info on why they're on there, I'd appreciate it!

[52 Bill]

...they could be for tone, or impedance, and I'm also wondering if they could be for power handling.

Seeing that you have a set of these chokes, if you take a few measurements for us we may be able to answer the question as to what they are there for.

Let's see, dc resistance of the winding, Inductance. What else?

[Enzo]

Shunting low freq across the speaker to keep it from getting muddy and boomy and woofy.

[nevetslab]

Silvertone 1474 Combo Case with spkrs & coils

I didn't stop to characterize the coils in the 1474 I restored for a client a couple years back, but here's a couple pic's of the speaker compartment with the coils in place:



I also included a couple images of the top view of the chassis, showing the two output transformers. In the first image showing the cabinet with the spkrs & coils, that funny tube is their 'back kitchen door spring' that they used as the reverb spring. worthless.

I did document the original wiring of the 1474, but I haven't yet turned it into a formal schematic in CAD. I'll have to go thru my files on the project, and put together the stock circuit as we all haven't ever seen in print. And, one rendition of adding a traditional Accutronics reverb tank circuit.

The only place I found to mount the new tank was down below with the speakers. It had to be mechanically 'tuned' by height to find the minimum resonance without feedback.

[drewl]

I've used the reverb circuit in a bunch of these as a killer gain channel for clients that don't want reverb since the "screen door spring" reverb is usually bad or missing on these amps.

[nevetslab]

Silvertone 1474 preamp/driver schematic as built

I went thru my project files from the Restoration project of the 1474, and before doing any of the work, I took the time to trace out the actual preamp circuit, since all the available schematics didn't match it. I've only drawn the schematic as far as the driver stage. There are differences in the power supply and output stage that aren't included here. But, this should clear up a lot of the mystery.

I made modifications to the 1474 to add a working Reverb tank, and will draw up those changes soon.

1474 Preamp Schematic-1.pdf