Silvertone 1471 in a Champ

[hackworth1]

Lacking power resistors. I made the turret board and designed the layout. The mounting holes align with the chassis.

After this prototype is complete and checked out, the Silvertone 1471 (aka Monkey Wards Airline) will be available in kit form from Boot Hill Amps.

[JDO]

no volume control? is that normal for those amps? does the tone pot also act as a volume pot of sorts?

[Telenut62]

Volume control is coming off the inputs, look hard left.

[hackworth1]

Yes there is a volume control. On that blurry schematic (far left side), see the 1M volume control come directly after the jack-mounted 68K resistors. The wiper (center lug) on the volume pot connects to V1's grid on pin 7.

[JDO]

didn't even think to look there. that's a little odd. you gonna post a clip when you're finished? look forward to hearing it. it'd actually be nice to hear a clip of this, the gibson you built and one of your stock champs. be interesting to hear the differences.

[hackworth1]

By boothillamps at 2012-04-11
Cardboard Templates are helpful in amp building.


By boothillamps at 2012-04-11
Insert (#6) screws for your output transformer first - before mounting the board. If you do not, you will need to remove the board later to do it. It is good to have on hand - the output xformer you plan to use. Many sizes of output xformers can be facilitated in these chassis.


By boothillamps at 2012-04-11


By boothillamps at 2012-04-11


By boothillamps at 2012-04-11


By boothillamps at 2012-04-11


By boothillamps at 2012-04-11


By boothillamps at 2012-04-11

[muchxs]

You may want to use the Champ value of 470 ohms for the bias resistor. Dano got away with the 270 ohm resistor because they used a very low supply voltage. Even better, toss in a 680 in with the parts set as well. People like to think they're customizing stuff.

...the pot is in a different place in the signal path. There's really no good reason for it to be where it is besides "that's the way they did it". What you're doing is putting another pot in series with your guitar volume control. It's redundant. Now that the pot is ahead of V1 you need to think about the proximity of your signal path to the AC switch on the pot. She may crosstalk a bit. I wouldn't run the signal back from the pot so close to the plate resistors. Compromise. Use a bit more wire, run the return up over the input jacks then tuck it down next to the V1a cathode resistor its way to the grid.

Originally Posted by hackworth1

boothillamps at 2012-04-11
Insert (#6) screws for your output transformer first - before mounting the board. If you do not, you will need to remove the board later to do it. It is good to have on hand - the output xformer you plan to use. Many sizes of output xformers can be facilitated in these chassis.

Mount your circuit board, punch the screw location on the board. Blow a 10mm hole straight through the board. That gives you access to that fastener no matter what. I've been thinkin' about doin' that for a while. Why not?

Measure twice, drill once. That way your hole is in exactly the wrong location. That's the way I did it.

[hackworth1]

Thanks much! That dark red preamp resistor is 10K ohms. That will reduce voltage to V1. How will that affect the circuit in total? I see my 22772 PT B+ will be 130v higher than stock.

[muchxs]

Originally Posted by hackworth1

Thanks much! That dark red preamp resistor is 10K ohms. That will reduce voltage to V1. How will that affect the circuit in total? IIRC, the voltage specs of my 22772 Fender Replacement PT are within 50 Volts of the original PT for the Silvertone 1471.

It's a Champ PT. Use the Champ cathode resistor value. Your tubes will be happier.

[hackworth1]

Thanks for the excellent tips. I'll reroute that signal wire the way you have suggested to get it away from the plate resistors.

Since the only thing remaining now is the AC power cord, I'll fire it up tomorrow and see how it goes first. (In the spirit of redplates and experimentation.)

[hackworth1]

By boothillamps at 2012-04-12
398 volts on the plate, loaded. Replaced the 270 ohm 5 watt resistor with a 600 ohm unit.
Amp sounds great and works well. Not noisy with existing config. Improvement will be relocating the volume control wire, but interestingly, it is not necessary. The shielding on the volume control pot is apparently effective.


By boothillamps at 2012-04-12
Sucess.

[Telenut62]

How does it sound compared to the 5F1 as it doesn't have negative feedback

[hackworth1]

Let me get it in a cabinet and give it a thorough sound check. It is different.

[slotrod65]

This is veeery interesting.

[hackworth1]

Much experimentation has been done on this circuit. It did not have the output of a Champ.

1.) Changed the 600 ohm Cathode resistor to a 300 ohms. Slight Improvement.

2.) Changed the second power resistor in the filter section from 90K ohms to 22K ohms (Champ value) More improvement. Preamp plate Voltages were increased by 40V. Still can't hold a candle to a Champ for output.

3.) Changed Preamp Cathode Resistors from 2.2K ohms to 1.5K ohms. More improvement. Still not great.

4.) Changed the input and volume pot to Champ arrangement. This did the trick. Woke it right up! The volume pot - a variable resistor - eliminates the need for that fixed 220K resistor. The 1m volume pot is a more component when installed in this manner.

...but now it's a Champ with no Neg Feedback, a 25uF bypass cap on V1 Pin 8 Cathode resisitor and .1uF coupling caps vs. .022uF caps in the Champ.

The Silvertone puts the input wire off the jacks' 68K resistors to the input on the volume pot.

The Champ puts the inputs off the 68K resistors direct to V1 grid.

The Silvertone puts on v1's pin2 grid a wire between the first coupling cap and a 220K resistor that goes to ground.

Sidebar: Changing the value of that 220K resistor might "fix" the Silvertone circuit. Any ideas?

Does the Silvertone circuit warrant further study? Or is it essentially a design that is outdone by the Champ?

[Robster]

I have had a lot of the 1471/1481 Silvertones and they never had the volume or the good sound of a Champ amp. That volume pot where they put it was just odd. I would not go out of my way to build one...

[hackworth1]

Well, there you go.

This amp sounds fantastic in a Champ cabinet with the .10 coupling caps and an 8 inch vintage Rola Speaker.

I changed the first bypass cap on V1 to 10uF. The second bypass cap is 25uF. I changed the cathode resistors on V1 also. 1750ohms each.

The amp oscillates when the volume is wide open. it makes a rhythmic chirp.

The amp can be almost dimed. But the dime gives the chirp.

What can rid that chirp other than a neg feedback R?

[hackworth1]

By boothillamps at 2012-04-18

By boothillamps at 2012-04-18

[celeste]

There is a huge reason not to put your volume control in front of your first tube, worse signal to noise ratio everywhere but with the pot wide open.

If I understand your chirping, it goes chirp chirp chirp chirp 1.5 to 2 times a second? Sounds like a form of "motorboating". That is almost always feedback through the power supply. Try doubling the size of the caps on the preamp and or screen node.

[hackworth1]

Changing the preamp filter cap from a 10uF Taiwanese to 16uF F&T fixed the motorboating chirp. Big improvement in the entire operation of this hot rod Champ. Many Thanks.