My 5F11 Build (aka The amp who wouldn't be)

Look at a AA964 princeton schematic. See how the bias circuit is done on that amp. Also note how similar the circuit is other than the tone stack and some of the voltages. You will need to adjust resistor values to get the bias voltage in the ballpark (rink?), but the basic idea is there. I recommend putting a potentiometer in so you can adjust the bias easily.

 

Clear your schedule and spend the weekend reading this website.

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#High_Voltage_Tap_Adjustable_Bias

The people here are generous with their knowledge, and none more than this guy.

 

I've noticed that the tone cap in Mojotone's 5F11 schematic (and 5F10, for that matter) is connected to the third terminal of the volume pot, as opposed to the tone pot's housing as in the original Fender schematic. Is this simply a different way of grounding, or does it give a tonal difference too?

 

Yeah, don't replace the transformer. Where Mojo has the brown bias tap going to the board

• take your bias voltage from pin 4 or 6 of the rectifier (EDITED from pin 8 earlier - my mistake)
• change the first resistor before the diode to 100k
• use a bias pot and set the bias change the bias resistor from 56k to somewhere around 18k-22k

As stated above, reference the Princeton Reverb layout for all you bias info for this. http://www.prowessamplifiers.com/schematics/fender/princeton_reverb_aa1164_layout.pdf

 

Jason, you know more about the 5F11 than anyone, but do you mean pin 8 or pin 4, like the PR you mention? Maybe either? I’ve definitely heard of tapping B+ for bias, but I’ve always just used the HT feed. (Note tho if I fit recto protect diodes, I tap pin 3 before the diode.)

BrucerBC, another excellent source for this 'PR style' bias is Doug Hoffman’s classic page, where he discusses the PR specifically.

 

Thanks for catching me on that! Yes, I definitely named the wrong pin for some reason. I edited my response above.

 

It is just a different (better) ground scheme. You will notice the cap attaches to the volume terminal that is attached to the input jack ground.

 

@brucerbc Did you catch my correction - and @King Fan good eye - about how to get the rectifier voltage for bias? Just follow the Princeton Reverb layout, and I use pin 6 on a Tweed amp because it's closer to the board.

 

You're doing great. Looks like a nice clean build.

 

Nice workmanship. Here’s something I hope is constructive. In this pic we see neutral to switch and hot to fuse side.



For technical reasons, like what gets hot if fuse fails or rat eats insulation, most experts prefer hot to fuse tip, fuse side on to switch and then to primary A, and neutral spliced to primary B, with A and B being arbitrary. From Rob's 5E3:



That brings up a question, just to check. The pic parallax and the Hammond overspray make it hard to see; which black wires are your primaries? Where do they go right now?

 

If potentiometers are installed without insulators there is possibility for ground loops when you also have a ground loop soldered on pot cases and obviously all those circuit return currents are made to run thru chassis when you don't have a solid ground bus.

DC Current follows kirchoff law but when there are also AC on amps because on impedances things get complicated but it is not good to use parallel ground loops which there comes when circuit has many grounds. And then if return current is made run thru chassis it can become what ever...

Tube amplifiers are often very simple and single ground bus for all stages is enough! Star ground is very good method but it does not use chassis for return current either!

Think chassis as a shield which protects circuit from electromagnetic stray fields.

Chassis is huge and it gets over circuits so it does not make sense to run current in it because then it can act as antenna and spread electromagnetic stray field to circuit.

There is need for only two electrical connection to chassis. First is mains cable earth wire very near mains input and it makes amp electrically safer. Second=last is the end of ground bus very near the input jack which connects shield and ground where ground bus current is lowest.

Screw mounted lug makes good contact but non insulated input steel jack can be used because there does not run any current and if more than one jack is used I think all can be mounted without insulation because current between them is very small.

I think loudspeaker jacks should be insulated although there is AC and no return current runs there but when they are easy to obtain and are cheap this removes noise possibility.

 

As @2L man pointed out, you definitely have ground loops now. A better (more likely quieter) ground scheme can be chosen. It would less messy to change the grounding from the back of the pots now than to change it later.

 

^^^ true. I was gonna mention this, but BrucerBC, since you’ve already done the soldering (another suboptimal item Mojo shows) I also figure you might be able to leave it. Yes, Merlin mentions it creates (small) ground loops, but tons of homebuilds as well as factory builds do it; Marshall etc. did it after all, and even Merlin basically admits those little loops must not create much noise. A practical problem is soldering to pots, but you've done that already. The bigger problem, as LLC says say, is *unsoldering* pots if they need replacement. If you want to upgrade now, a standard separate ground bus is shown in many builds and in many of Rob's diagrams. If you decide to stay with the current plan, and do it right, your amp will still work.

 

Possibly you can use that wire you have already soldered without problem when you continue it to power supply Negative IF you connect ground bus to chassis ONLY at input jack. Then no return current runs thru chassis and all thru ground bus.

If you connect power supply negative to chassis and that partial ground bus to chassis at input jack then some current flow first towards input jack but some flow thru potentiometers to chassis and it is called ground loop.

Practical ground bus is installed to turrett board and it begins at power supply negative and ends to input jack and every ground points in between "feeds" amp stages.

Power supply negative and chassis should not be connected together because negative is where all the electrons flow to the system and chassis is very grong place to let them escape!

 

When potentiometer are mounted without insulation like most of us do and pot case is not used as active circuit no current flows there and then pot cases act as a shield around pot resistors almost like chassis makes a shield over amplifier.

Do not connect power supply to chassis!!! If you do and you also connect this partial ground bus to input jack negative then all this current returns to power transformer thru chassis and it passes input jack proximity and it is not good because there instrument signal is weak and amo input impedance is high.

Yes mains cord earth is connected to chassis and I believe it is written to law.

Ground bus is connected to chassis to prevent potential difference only at input jack because it is best place and when it is only place there does not flow any current. I think all input jacks can be without insulation IF it is only place ground meets chassis and no current can flow thru and between them.

If you use the wire between potentiometers as ground bus finish it at input jack. But do not connect any other active circuit part to chassis. Connect beginning of this ground bus to power supply negative.

If you want you can make own grounds for power stage and phase inverter which you also connect to PS Negative and it becomes Star Grounding but they are not connected to chassis, only to PS negative.