My first build: a Silvertone 1471 - Champ hybrid

[ServiceBob]

This is my first build and I should note that when I began I knew almost nothing, but a few months later and with this one amp under my belt I realize how much more there is to know and how much more I have to learn.

It began with a Sears Silvertone 1471 that a friend asked me to replace the original unpolarized cord and plug with a 3 prong plug - and I said, "Sure!"
He hadn't asked because I play electric guitar (which I don't) or because I'm amp tech or even an electronics "guy". He asked because I'm old and handy and I can use a soldering iron. He gave me the amp and I looked on the internet and learned about the "death cap" and the proper way to wire the cord, fuse, and switch. While I was in there I was fascinated by way the whole thing was put together and resolved that I could make one of these. If I could find the parts. And if I knew what all this stuff did.
I could read a schematic - sort of. I knew what all of the symbols were and what the components those symbols represented looked like. I knew Ohm's law and I understood a very basic circuit. That was it. Nothing more.
But three months later I have learned a lot. I've been taking courses at Kahn Academy in Electrical Engineering (it's FREE and amazing!) and I've watched countless videos on Youtube by generous and smart guys explaining vacuum tubes and the electronics around them. And by visiting forums like this one and reading other folks' problems and the answers they'd found.
And because I'm handy I wanted to start building something. The obvious place to start was that Silvertone 1471. I found the schematic online and discovered that I had a transformer in my vast collection of junk that came with stuff I'd bought that looked like it would work. According to the tag tied to it, it was a 260-0-260V with a 6.3V tap for heaters. And the 1471 schematic said that it needed 260 VAC so it looked like I was good to go. Like I said - I knew NOTHING. But I went ahead and ordered all of the parts that it would take to build the amp and when they arrived I built it using the parts that I'd bought. I was happy when it actually produced a sound - but it sounded awful and thin and well, just not very good. I have an old SG that I once fancied learning which sounds great when plugged into a good amp - but this was not so good.
It was at this point I thought about measuring voltages and what I found was that all of the voltages measured were high. B+ measured about 320 and all other voltages were proportionally higher than shown on the schematic. I figured that was my problem.
I considered just buying a new transformer - say about 450V CT - and just replace the transformer. But I'm old and cheap and I figured it made more sense to try and adapt the circuit to the voltages at hand. So I began to look for circuits that used the tubes I had on hand and the voltages I measured in my amp. Without much looking I found the schematic for the Champ 5F1 and it looked "close enough for jazz". The biggest diiferences are probably the larger cathode bias resistor on the 6V6GT, the addition of a screen grid resistor, and the relocation of the volume control. I left the Silvertone tone control where it was and it works much better than it had in the original configuration. I think it sounds pretty good playing through a 10" speaker salvaged from an old electronic organ. The speaker is marked 20W at 8 ohm.
So anyway, the schematic shows what I ended up with and the voltages when it's turned on, warmed up, and no signal at the input. According to my calculations the 6V6GT's plate dissipation is just below 10 watts. Does that look right?
I'm willing to admit that I got lucky with this and that I may well have 10 problems or something that will kill my tubes or my transformers in a week or two. Or something that is pointless or that could be improved, or something it needs. I'd love to hear your analysis and to learn from you.

[xtian]

RE: your schematic, there are two errors I spotted:

You have the volume wiper connected to both the V1b grid (OK) and the 0.01uF cap leading to the 6V6 (not good); and the plate of V1b (marked 115V) should be connected to the 0.01uF cap.

You have no grid reference on V1a. Usually we use 1M to ground.

Why did you opt for two input jacks, identically wired?

[ServiceBob]

Hi, thanks for the comment. The wiper on the volume pot problem was with the schematic - not the actual amp. It was my first time using that software and I screwed up. I have repaired and reposted the schematic.

I assume (because I don't know) that "V1a" refers to pins 7-8-9 on the 12XA7. It's pretty much identical to the Champ schematic. The Silvertone had pin 7 connected to the wiper of a 1 meg volume pot. And both amps had identical matching inputs, so I followed suit. Where would the 1M resistor go? And why.

Thanks for pointing out that error!

[xtian]

Since there were no labels to refer to, V1a means first triode, V1b is second triode, same tube.

Triode grids require a voltage reference to ground (otherwise signal can cut off unexpectedly and unpredictably). Yours does have a reference: if you plug into one jack, the V1a grid can still find a path to ground thru the other 68K resistor. But this is weird: you've created a voltage divider with those two 68K resistors, so if you plug into either jack, you're only getting half of your guitars amplitude at the V1a grid. Intentional? Or were you trying to emulate Fender's Hi/Low inputs?

[ServiceBob]

Thanks for the explanation. Really!
No, the "voltage divider effect" was not intentional. I guess I wasn't understanding the necessity of the reference to ground and didn't interpret the other leg of the Silvertone's 1 Meg volume pot as that part of the circuit. I have made a change to the schematic but won't have a chance to actually put iron to wire until later this evening or tomorrow. I hadn't considered treating the two jacks differently, mostly because in the circuits I've been considering thus far that hasn't been done. I will look into it. Do you have a suggestion as to what amps I might look at for inspiration?

Thanks again!

Champtone '71.png

[sluckey]

I would wire the two jacks just like a Fender AB763 Deluxe Reverb. This gives you a "high" input and a "low" input. The low input gives you exactly what you have now but the high input gives you full guitar signal. Here's the schematic...

https://el34world.com/charts/Schematics ... _ab763.pdf

And if you have questions here's a brief explanation of how the jacks work. Look on page 11.

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[thetragichero]

1M resistor to ground should go from the connection of the tip of one of the jacks and the 68k resistor to ground. this is your "high"il input. the other jack "low" input should have only the 68k resistor connected to tip, nothing on the middle (shunting) lug, and ground connected to sleeve. having the low input tip shunted to ground means that when you're only using the high jack the effective resistance to ground is 68k, attenuating signal (turns the high to a low input)

[sluckey]

1M resistor to ground should go from the connection of the tip of one of the jacks and the 68k resistor to ground. this is your "high"il input. the other jack "low" input should have only the 68k resistor connected to tip, nothing on the middle (shunting) lug, and ground connected to sleeve. having the low input tip shunted to ground means that when you're only using the high jack the effective resistance to ground is 68k, attenuating signal (turns the high to a low input)

That's not the way Fender does it.

[ServiceBob]

Thanks, Sluckey, for the idea and the explanation! Same parts that I had, but more intelligently arranged with much more utility. And your Amp Scrapbook is a great find. Thanks!
I've attached the new version with changes based on your suggestion.

Champtone '71b.png

I still haven't made the changes to the actual amp - but now I'm glad I've waited.