SUNN Sorado Project

[AHeck]

Good evening,
I've Been working on this bass amplifier for a few weeks.  I've replaced quite a few components and reworked the ground scheme, replaced nonfunctioning boost switches, reflowed some solder, etc. In working on it, I noticed where the schematic did not match the actual wiring. I will post schematics. I was wondering if you all might help me determine if the schematic is wrong and it came to me as per the factory, or if the schematic is right and someone had been in the chassis before me and made some mistakes.

[AHeck]

So the first boost circuit does not match the wiring. Here is the first tube schematic followed by the circuit as it was wired.

[AHeck]

This is the second tube, and the lil' capacitor was in a different spot than the print.

[AHeck]

Also, while I got you...
It still has a loud hum. With signal applied with a frequency generator, if I flip the power switch, The hum goes away immediately and the signal tone is left to fade out with a dead quiet background.  The way the heaters were run is terrible, wires not twisted and very long parallel runs with signal cable.  That's what I'm betting is the problem and I'm going to redo that, too.  Is this reasonable?

[sluckey]

I would think your "as wired" schematic is correct. I would not mess with the heater wiring. Sunn knew what they were doing and the way they ran the filaments works just fine. Look to the power supply and/or the power tubes for your loud hum.

BTW, the Sunn Sonaro (EL34 version of the Sorado) was the first guitar amp I ever built back in '68.

[kagliostro]

Hi guys

Please can you explain me what happen around V1b ?

The presence of the switch with the 10M resistor in parallel confuses me

If the switch is open I don't think the signal will pass ......

Decisely there is something that I don't arrive to understand

..... OK, the signal from V1a feeds V1b via the .002 capacitor ..... is the other cap with the pot present as a tone control ?

Franco

[sluckey]

There is a .002 coupling cap between V1A plate and the top of the volume control. When you close the switch you place a .0075 cap in parallel with the .002 cap for a total capacitance of .00095 (just call it .01). This allows more low frequency signals to pass. It's called a LO BOOST. The 10M just prevents any popping noise from the switch.

[AHeck]

Thanks, I thought the schematic was incorrect.  I especially appreciate the explanation, now I understand how that works.

[sluckey]

I think either way will work but your "as wired" schematic just seems more politically correct.

[kagliostro]

The way it was wired is much easier to be understand, the other way, as per the schematic, is still confusing

But as wired all is clear

Thanks

Franco

[kagliostro]

I asked to a friend to help me to understand better and he confirmed what Sluckey told, both arrangement are functional

So an idea come out

Why don't use an SPDT with the center pin connected to the .002 cap and the other pin connected one to the wiper and one to the pin of the pot that isn't connected to ground

So you can easily compare and decide which arrangement is better for you

Franco