Harvard Amp Mod

[TurboGuitarMelton]

Hi


I am building a fender tweed harvard 5f10 style amp. I opted to use a 12ax7 instead of the 6av6 that it uses for the first preamp stage. I dont want to mess with tubes that arent as easy to get as a 12ax7.


I had the idea to use the extra triode to have two styles of input triodes. One being the standard cathode bias and the other to be the old style grid leak bias.


Would this switching method I drew out work okay? I was thinking I could just use a single dpdt switch to go back and forth between the two different input triodes. I am concerned that it would cause oscillation with those input/output wires meeting at the switch. Would mixing resistors before the volume pot work?

[sluckey]

Maybe hard wire the inputs together and only switch the outputs. Or use separate input jacks and mix the outputs together and don't even use a switch.

[TurboGuitarMelton]

Thank you for the reply sluckey!


I like the idea of having no switch and having separate input jacks!


Would I just use the fender standard 220k mixing resistors before the volume pot? Like this?

[mresistor]

6AV6 is plentiful and cheap and 7 pin sockets are available.   I think you might regret not using one.  I really like my 5F10 clone in stock configuration.

[sluckey]

I like the idea of having no switch and having separate input jacks!

Would I just use the fender standard 220k mixing resistors before the volume pot? Like this?

yes

[DummyLoad]

See attached - You need a grid leak connected at all times or you'll fry that upper triode.

--Pete

[TurboGuitarMelton]

See attached - You need a grid leak connected at all times or you'll fry that upper triode.

--Pete

Really?   That upper triode is a typical cathode biased setup like most other fender amps. I know they usually have the 1M to ground at the jack (which I forgot in my schematic) but I didnt think it was needed. The gibson GA5 has that resistor omitted.

[bmccowan]

I think that might be because the GA5 (the one with 12Ax7 - 6V6) has one switched and one unswitched input. Which ever one is used, there will be a reference to ground through a 47K resistor?

[sluckey]

Actually, when the HI input is used, the grid must rely on the components in the guitar to provide a ground reference. Cheap engineering! Much better to just put a 1M resistor inside the amp.

[bmccowan]

Thanks Sluckey. I will add that to my GA-5. I suspected there might be an "actually" out there. Reminds me of my nephew at 8 years old who told me, "well actually they eat Krill." Turned out he was also right.

[PRR]

You may not need a 1Meg if you rely on what is inside the guitar.

But since you can switch away from the guitar, it may be wise.

Also, didn't most Fenders have 1Meg in circuit all the time? Leo hated to spend 12 cents, so he musta thought it was important.

The upper triode will not "fry". With 100K in series it won't even get over-warm. However the un-biased plate voltage may be 100 volts different than when biased right. This will charge the plate coupling cap. When you switch to the upper triode, BANG!  The 10X over-loud transient will scare the crowd. And if you cheaped-out on your "guitar" speaker, throw the speaker cone out of the box.

[printer2]

The input pentode has a switch that changes from grid leak to cathode bias, in case you do not need the two inputs.

[TurboGuitarMelton]

Is there any real difference in putting that 1M resistor to ground at the jack (like most fenders) or connected directly to the grid after the 68k grid stopper resistors?


My understanding is that for my purposes if I use my original idea (with the switch) putting the 1m to ground after the grid stoppers would


1) stop a big pop when i flip the switch
2) stablize the bias of the triode


Am I understanding that correctly?

[sluckey]

According to your original diagram the 1M needs to go between the switch and the grid. This insures the grid always has a path to ground regardless of which position the switch is in.

[CascoSieg]

Is there any real difference in putting that 1M resistor to ground at the jack (like most fenders) or connected directly to the grid after the 68k grid stopper resistors?

Not sure how well it will suppress switch pop, but putting it after the grid stopper creates a voltage divider that will reduce your input signal slightly, ~ 5%. May not matter to you.