Silvertone 1471

[dkevin]

Morning all!
Got some questions about a Silvertone 1471 that I am wrestling with. A fiend of mine asked me the difference between a 1471 and 1481. The 1481 has an 8 ohm secondary and the 1471 has a 3.2 ohm secondary. Then he asked me what the functional difference would be...and I said...uh....I don't know. Would the secondary make any difference in the tone/volume/etc??? What say you??
Also, the 1471 has an unusual design. The volume pot comes after the input jack and before V1A. The tone pot is between V1A and B. Is there an advantage to leaving it here or relocating it between the stages of V1? I have also heard of mods that advocate moving the volume pot after V1 to drive the 6V6. Any opinions out there?
Lastly, the amp takes an unusually long time to warm up. Voltage shows up quickly on the main electrolytic caps but no sound will emerge for 45-60 seconds. This amp came to me with a pair of diodes soldered on the 6X4 pins and barely breathing. Could the higher voltage of the diodes have fatigued the caps? They seem to lose their voltage a lot more quickly than my other amps when the amp is turned off.
So many questions; so little time....
Thanks for your time!
dkevin

[JHow]

Are the advocates of moving the volume control also boosting the gain? As I read the stock schematic, the 12ax7 looks like fairly low gain set up, unbypassed cathodes, relatively low voltage (143V).

Regarding filter caps, according to schematic they are rated at 450V. The voltage after the 6x4 is supposed to be arund 260V, even if you added about 50-60V by using diodes, you're still within reasonable voltage range.

You didn't say if the caps are old, orginal, or replaced? They could just be old?

[dkevin]

More 1471...

Yes, I believe the purpose of relocating the volume pot is to achieve more gain from the amp. The 12AX7 is not bypassed (stock) but there again, they advocate several different approaches. One is to fully bypass the first stage with a 20uf/25 vdc cap but leave the 2nd half of the 12AX7 stock. The 6V6 is said to benefit from a rather massive 100uf bypass cap. Other sources suggest modest bypass caps on the first and second stages and, again, 100uf on the output tube cathode.
The main filter caps are 450WV and are original in a multicap can. When I received the amp it had diodes soldered from pins 5 to 7 and 1 to 7 of the 6X4. By moving the PT plate from pin 6 to pin 5 I suppose the 6X4 became a pretty paperweight (non-functioning).
Also, when the amp got here, the output transformer had bare (non-insulated) wires from the secondary that were lying against the speaker frame (a 10 inch speaker- the original is 6inch, I believe) and also grounding against each other.
I am wondering if perhaps, the OT might have suffered some damage from the aforementioned monkeyshines. Now might be a good time to replace the 3.2 ohm OT with an 8ohm and work from there. I am considering an Edcor OT 5.5Kprimary/8ohm secondary at about $18 plus shipping.

[RWood]

I had great results adding a 10uF bypass cap to the first stage, but I made it switchable using a push-pull pot on the tone control. Everything looked stock but pulling the pot engaged the bypass cap for extra gain. I put a 100k resistor across the switch terminals to keep it quiet.

Otherwise I stuck with the original circuit, but wound up replacing every capacitor and most of the resistors. Fortunately, there aren't many of either!

Lots of opportunities for you to experiment here, and you can't really hurt anything. Easy point-to-point construction - a great amp on which to learn. You might even try the tube rectifier if you want to lower the voltage some. Are you sure the OT is blown? Secondary leads that look bare can actually have varrnish on them. Check it out with your voltmeter. You can also run a low AC voltage through it backwards to measure the turns ratio and see for yourself which speaker impedence would work best with it.
Output Transformers

RWood

[dkevin]

1471 progress update

The amp is now breathing fire! I had been wondering why the output was so weak. I checked the voltages and tested tubes but everything seemed in the ballpark. I did notice that V1 had raspy static when the tube was wiggled in its socket. But after chasing most of the other possibilities down to no avail, I finally replaced the 12ax7 socket. Now the amp has plenty of volume, even with a well-used Jensen speaker.
This sort of brings me back to my earlier question about the OT secondary. Would I experience a change in the amps performance if I swapped the 4ohm OT for an 8 ohm OT? I've seen a few reports on the internet where they link the limited headroom of this amp to the 4 ohm OT.
As regards the suggestions for a switchable 10uf bypass cap on the 12AX7 cathode, that sounds like a cool idea! Now I need to be thinking about a speaker. I cut a replacement baffle from plywood to mount an 8 inch speaker. The Jensen alnico is a little overmatched. I hear that a ceramic Weber is a mighty fine addition. Let me know what you are using.
Thanks for all your help!

[RWood]

Glad to hear you ID'd your problem.

About your OT question, you should get similar performance results when the correct impedance speaker is matched to the correct OT secondary. What I suspect you were reading about the Silvertone OT was not that it is 4 ohms, but that it is rather small, and replacing it with something bigger like a tweed Princeton OT would capture more of the tube's output (especially in the lower frequencies). With either output transformer, though, you want to start with a speaker/OT impedance match (even though it is really a moving target, frequency dependent, etc).

I always thought part of Silvertone's charm (and tone) was because of it's underspec'd components such as the OT. This was definitely the case with the model 1482 I grew up with in the 60s. So when rebuilding my 1471 I not only used the stock OT but I also found a prinstine replacment speaker, a 1056 (Fisher) at 6" and 4 ohms that came from a console. Same as the bug-eaten one that was in the original amp. I did cut a new baffle, though, lol. Btw, have you noticed that the grill cloth is very washable? It is easy to work with, too, being sandwiched in place instead of stapled.

RWood

[dkevin]

More 1471...

The grillcloth (?) is wonderful! It was my pleasure to discover that it was easily removable and reusable.
To do justice to the internet posts I cited, they mentioned matching the primary of the original OT but replacing it with an 8ohm secondary. They were actually lamenting the missing "rattiness" of the old 4 ohm OT. I am assuming that replacing the OT with a 5K primary/8 ohm secondary may affect the overdriven aspects of the performance but may provide a little more clean headroom. It may prove worth the $20 investment to experiment and see for myself.
The speaker I choose will follow the OT I select. Using an 8 ohm secondary would simplify my options since I have many similar amps with a variety of alnico and ceramic speakers to swap in and out. It is a bit deceiving to try and "voice" the amp with a suspect speaker. That's kinda what I'm dealing with now.
Anyway, thank you all for your help. These Silvertones are marvels of low-cost production. They operate at conservative voltages and the tubes seem to last forever. And they are relatively easy to repair. Add to that, they sound incredible!!! I wonder why, given their simplicity of design, modern manufacturers seem determined to re-invent the wheel when it comes to tube amps. The Valve Jr. was priced excellently, but sounded like feet (unmodified). A whole lotta sound, but most of it unbearable.
Viva la Silvertone!!