MODS

[EricS]

Just curious what everyone's favorite mods are to the stock circuit?

[Jazz P Bass]

I prefer factory correct circuits.

[tedmich]

[Steve Conner]

I can haz mods?
http://scopeboy.com/scopeblog/wp-con...a-innards1.jpg

[hasserl]

My favorite mods would be 4.7uf cathode bypass on the input stages, .022uf coupling caps in the pre, .047uf coupling caps to the power tubes.

[loudthud]

There are a few updates that everyone takes for granted. The power cord is updated to a 3 wire with safety ground. The "death cap" and ground switch are removed and replaced with a standby switch. The heater wiring is updated to a balanced type with a virtual or actual center tap. The center tap can be connected to the 6V6 cathodes for a little lower hum. The 16uF caps in the power supply are sometimes increased to 22uF for availability reasons.

There are several mods to the coupling caps that are frequently done because of excessive or flabby bass. See Hasserl's post above.

Grounding can be an issue. Everyone seems to have their own opinion on how to get it right. Good luck with that!

Tube swaps can be done to change the gain and/or headroom.

[Bruce / Mission Amps]

I can haz mods?
http://scopeboy.com/scopeblog/wp-con...a-innards1.jpg

Man Steve, it's a good thing you aren't a plumber...

[deci belle]

The center tap can be connected to the 6V6 cathodes for a little lower hum.

Please tell me about that, Loudthud!

[loudthud]

Preamp tubes can inject hum from the heater circuit into the signal for a number of reasons. One is that the heater wants to emit electrons (because it's a hot glowing metal) and those electrons can be attracted to the cathode. Not a problem if the cathode is grounded or fully bypassed to ground with a big capacitor.

If the cathode is always negative with respect to the heater, the cathode will repell those electrons. This is accomplished by making the heater slightly positive, about 20V. This reduces the 60Hz hum that sounds like a low organ note, not the 120Hz buzz that comes from the power supply. The amount of reduction will vary from tube to tube.

[deci belle]

Ok— now I know why, Loudthud!

So on a 5E_, the center tap shares the ground connection with the cathode/resistor bias circuit?

Is this the green/yellow 6.3v center tap?

[loudthud]

Ok— now I know why, Loudthud!

So on a 5E_, the center tap shares the ground connection with the cathode/resistor bias circuit?

Is this the green/yellow 6.3v center tap?

Your wording makes me a little nervous. In this thread, post #4, I posted a layout of an amp with elevated heater supply: http://music-electronics-forum.com/t12663/

Different output tubes were used but it's basically a 5E3. Note how the green yellow striped wire goes to the plus side of the cathode bypass cap of the output tubes. The layout also shows a good grounding scheme.

In this thread, post #10, I posted a schematic of a 5F1. That transformer didn't have a center tap on the heater winding so a pair of 100 ohm resistors were used to create a virtual center tap. http://music-electronics-forum.com/t16311/

[deci belle]

Different output tubes were used but it's basically a 5E3. Note how the green yellow striped wire goes to the plus side of the cathode bypass cap of the output tubes. The layout also shows a good grounding scheme.

Yes, Loudthud. I see the ctr tap in the first example (5E3) and the two 100Ω in the second example (5F1) connecting at the same spot: the positive side of the cathode bypass cap for the power tube(s).

I did not understand which side of the cap you meant, which IS a good reason to be worried. I've only been at this for two years and am just in the middle of my first scratch build. Thank you so much for including the links to both examples!

In my case, even though I have a heater ctr tap, I want to ground the 100Ω resistors per example two. My PT came with a warning NOT to use the heater ctr tap unless it was on my schematic (since I drew my schematic it isn't there haha!!).

[loudthud]

Use one or the other, not both. If you hook one to the cathodes and ground the other one, you will short out the bias for the power tubes! (That's very bad!)

[deci belle]

Haha! I'm joining the ends of the 100Ω Rs and that'll go to the positive side of the pwr tubes' bypass cap. I'm lifting the heater CT.

[Bruce / Mission Amps]

Adding to this... you can just lift the filament winding's center tap (usually green/yellow here), then solder a 1/2 watt, 47 to 100 ohm resistor on the end of it and solder the other end of the 47 to 100 ohm resistor right to the positive end of that same cathode bypass cap previously mentioned.
You don't really need the 1/2 watt resistor but I always do it in case there is short from lug 2 to 3 somewhere on one of the power tube sockets.....and my small brain makes feel like the 1/2 W resistor could at least blow open if too much Hi-V goes into it.