Some advice on a Champ 5F1 build (from Mojo)

[d. spree]

I am helping a good friend and his 13 year old son build a 5F1 (kit from Mojo). I've built 3 5E3s. We've made a good start despite abysmally bad instructions from mojo with only 4 unclear photos and a wiring diagram that is poor. That said we are doing okay. I have a couple of questions and because we are only just laying out the components I don't yet have pictures to post so hence the long-winded description that follows. I hope it is clear enough.

My first question relates to grounding. I put in a couple of grounding lugs in when we mounted the PT so that the ground from mains and the center tap (red/yellow) ground along with it to one of them.

I presume Mojo is suggesting a star grounding scheme but they offer no detail. So following my 5E3 build I cut and drilled a brass plate and have it mounted with the two input jacks and the on/off/volume pot. I plan to run two grounds - from pin 3 on the 12AX7 preamp tube via a 1.5k resistor to ground the right side of the plate (i.e. nearest the first input jack). The filter caps ground I plan to run to the left side of the plate. Also on the Mojo scheme they show the grounds for the filaments coming off of the pilot light terminals via 2x 100 ohm resistors. Is it okay to ground these onto one of the lugs I installed with the PT?
Trying to avoid any crazy ground loops. Does this sound like a plan? Also the Mojo drawing doesn't show any grounds wires coming off the input jacks. Is this because simply mounting them is sufficient. Ditto for the on/off/vol pot. On the third pin there is a ground shown simply to the back of the pot. Is this sufficient.

Second questions relate to whether or not to install an additional capacitor in parallel to the 1500 ohm resistor to ground off of the 12AX7. The Mojo drawing shows no cap there but they did ship one. I notice some versions of the amp do show a 22uF cap here (or a 25uF). Just wondered how omitting or adding the cap shapes the sound of the amp?

Thanks in advance.

Spree

[Jazz P Bass]

First off, the mains ground should be all by itself.

As to the 12AX7 bypass capacitor, the amp will be tighter without it.

[loudthud]

Second questions relate to whether or not to install an additional capacitor in parallel to the 1500 ohm resistor to ground off of the 12AX7. The Mojo drawing shows no cap there but they did ship one. I notice some versions of the amp do show a 22uF cap here (or a 25uF). Just wondered how omitting or adding the cap shapes the sound of the amp?

Without the bypass cap, the amp loses some of it's tubieness, the even order distortion that makes a tube amp sound like a tube amp even before clipping. It pushes the voltage gain higher, but you can always turn it down with the volume control. I recommend you install the cap. If the amp sounds too bassy or muddy, replace the cap with one of lower value, 4.7uF or perhaps 2.2uF. You may have a hard time finding a cap with long enough leads. Just solder a short length of bare wire to one of the leads.

[bob p]

Cathode Bypass Capacitor Calculator

[spsimmons]

Spree, I'm glad you posted this. I'm building a Princeton from Triode's 5f1 kit right now. They also omit the 25uf cap off the pre-amp tube. I'm adding it back in, but it was more of a gut decision than anything else. Now I understand better why it's there.

I'm wondering about the grounding scheme you're using. Triode's layout shows the mains ground going to one PT bolt and all other grounds being attached to another (different) bolt on the PT. Would it be desirable for me to locate this grounding point closer to the inputs? Could this be accomplished by using the tabs/lugs that Triode provided and simply bolting them to the chassis at a spot near the inputs?

-Shannon

[d. spree]

Without the bypass cap, the amp loses some of it's tubieness, the even order distortion that makes a tube amp sound like a tube amp even before clipping. It pushes the voltage gain higher, but you can always turn it down with the volume control. I recommend you install the cap. If the amp sounds too bassy or muddy, replace the cap with one of lower value, 4.7uF or perhaps 2.2uF. You may have a hard time finding a cap with long enough leads. Just solder a short length of bare wire to one of the leads.

Thanks, as always, loudthud. I think I'm gonna install it but as a switchable mod - put a little toggle switch in the circuit like I did my mods on the last two 5e3s I built.

I do have another (perhaps stupid) question. It's about the on/pff volume pot - on the back it has two terminal lugs -



like this

The Mojo wiring diagram shows this - which of the two lugs is which?

[d. spree]

Spree, I'm glad you posted this. I'm building a Princeton from Triode's 5f1 kit right now. They also omit the 25uf cap off the pre-amp tube. I'm adding it back in, but it was more of a gut decision than anything else. Now I understand better why it's there.

I'm wondering about the grounding scheme you're using. Triode's layout shows the mains ground going to one PT bolt and all other grounds being attached to another (different) bolt on the PT. Would it be desirable for me to locate this grounding point closer to the inputs? Could this be accomplished by using the tabs/lugs that Triode provided and simply bolting them to the chassis at a spot near the inputs?

-Shannon

The reason I posted originally was I was not sure of which ground scheme to use. The Mojo scheme shows what needs to be grounded but not how or where. As of now I plan to have the ground from the power cable along with the red/yellow center tap grounded to a lug I installed on one of the PT bolts (bottom left corner). The Mojo scheme has grounds running from the green filaments leads (running from the pilot light as in the picture I posted above, with 100 ohm resistors. I'm gonna run those grounds to a separate lug I installed on the top right corner bolt of the PT. I fabricated a brass plate which I will install under the two input jacks and the on/off volume pot - using lock washers and very tight to get a good ground connection for them. I have run two ground leads to each corner of this brass plate - one for the cathode bypass and one for the filter caps. I was taught (when building 5e3s) to use a similar ordering of the grounds to avoid ground loops. I'm hoping the experts here chime in.

Good luck with your build.

[Jazz P Bass]

At the risk of repeating myself, "The mains ground wire should be placed by itself."
Plus it should be 'bonded' to the chassis.
Using the power transformer bolt is not recommended.
We are talking about a safety aspect here.
Please do not treat it lightly.

[loudthud]

Thanks, as always, loudthud. I think I'm gonna install it but as a switchable mod - put a little toggle switch in the circuit like I did my mods on the last two 5e3s I built.

I do have another (perhaps stupid) question. It's about the on/pff volume pot - on the back it has two terminal lugs -

[ATTACH=CONFIG]26049[/ATTACH]

like this

The Mojo wiring diagram shows this - which of the two lugs is which? [ATTACH=CONFIG]26050[/ATTACH]

It doesn't matter, but you should route the wire from the center lug of the pot on the side of the pot closest to the input jacks. Keep that wire as far as possible from the Mains and heater wiring. It has been suggested that in many cases a separate switch for the mains helps reduce hum because it keeps the mains wiring away from the volume pot.

[d. spree]

At the risk of repeating myself, "The mains ground wire should be placed by itself."
Plus it should be 'bonded' to the chassis.
Using the power transformer bolt is not recommended.
We are talking about a safety aspect here.
Please do not treat it lightly.

Thanks Jazz - I take seriously what you are saying but where should I ground it then?

On the 5e#s I've built (one from tube depot and two from mission) the ground from mains was routed to a lug at the PT bolt and all the wiring schemes I've seen for the champ do the same (the ones from Mojo shows the center tap and mains ground together and the Ceriatone and Weber show the use of the PT bolt). So this is confusing? What is the safety issue just so I can understand? I presume that your advice would involve drilling the chassis and bolting a ground lug? Can the center tap (red/yellow0 from PT be on the same ground lug as the grounds from the filaments?

[Jazz P Bass]

What you have to fear is 'what if it comes loose or corroded'.
That is the main thrust behind it being bonded.
You want the lowest impedance connection that you can realistically achieve.
You can solder it to the chassis, which requires a high wattage iron.
You can also use stainless steel nut, bolt, star washer setups.
In my opinion, if the kit guys are recommending setups as you say, they are way off base.

[d. spree]

What you have to fear is 'what if it comes loose or corroded'.
That is the main thrust behind it being bonded.
You want the lowest impedance connection that you can realistically achieve.
You can solder it to the chassis, which requires a high wattage iron.
You can also use stainless steel nut, bolt, star washer setups.
In my opinion, if the kit guys are recommending setups as you say, they are way off base.

Thanks. I guess I'll drill the chassis and use a star washer set up for the mains ground lead.

[Bruce / Mission Amps]

Cathode Bypass Capacitor Calculator

Something wrong with that. plug in 100pF and what happens to that graph of dB and +6dB... etc.

Also, do everything like you did on your 5E3.... they are so close in the major ways that you can't go wrong now you have a brass plate to act as the large ground plane/Star ground.

[d. spree]

Something wrong with that. plug in 100pF and what happens to that graph of dB and +6dB... etc.

Also, do everything like you did on your 5E3.... they are so close in the major ways that you can't go wrong now you have a brass plate to act as the large ground plane/Star ground.

Thanks Bruce - as always you cut to the point and as I have never had a problem with anything you've advised before I am gonna follow your advice one more time!!! By the way any thoughts on where these should be grounded? I'm thinking of one of the bolts for the PT?

[Bruce / Mission Amps]

Thanks Bruce - as always you cut to the point and as I have never had a problem with anything you've advised before I am gonna follow your advice one more time!!! By the way any thoughts on where these should be grounded? I'm thinking of one of the bolts for the [ATTACH=CONFIG]26121[/ATTACH]PT?

Run a single wire from their junction over to the positive end of your power tube's cathode bypass cap.