5f1 build questions

[nop]

I'm building a 5f1 champ as a head using the ceriatone layout. Why is the nfb only connected to the 4 ohm tap on the impedance selector? I'm omitting the impedance selector and just using 3 output jacks. Is it essential to use shunting jacks? Is it bad to use an iec inlet with an integrated fuse holder? Should I mount the OT in roughly the same place as combo builds?
Thanks for your time.

[martin manning]

I'm building a 5f1 champ as a head using the ceriatone layout. Why is the nfb only connected to the 4 ohm tap on the impedance selector?

The different taps have different voltages on them, so sticking with then4-ohm tap keeps the amount of feedback constant.

I'm omitting the impedance selector and just using 3 output jacks. Is it essential to use shunting jacks?

In this case you do not want to use shorting jacks or wire a switching contact to ground the outputs when no speaker is plugged in. Otherwise, the output will be shorted unless all such jacks are "occupied." Using non-shunted jacks for all three is an added risk for accidentally running without a load.

Is it bad to use an iec inlet with an integrated fuse holder? Should I mount the OT in roughly the same place as combo builds?
Thanks for your time.

No, an IEC with an integral fuse is fine. The relative positioning of the OT wrt the PT is important for minimizing hum in the speaker due to magnetic coupling. Keep the OT in the same spot or check for hum in an alternate location. See "headphone trick."

[Phil_S]

NFB: the choice of the NFB resistor changes with the impedance of the transformer secondary. Simply run the connection to the 4 Ohm jack.

Jack type: I use simple non-shorting jacks. I thread a common sleeve connection through all three, usually with bare wire. Just remember, if you plug into two jacks simultaneously, the connections are parallel. This means, for example, if you plug in a 16 ohm cab and an 8 ohm cab at the same time, plug the 16 ohm cab to the 8 ohm jack and the 8 ohm cab to the 4 ohm jack. If you do the math concerning parallel resistors, you will arrive at this solution as the correct one. In short, using two jacks at the same time makes each cab worth half it's stated ohm value.

IEC inlet with integrated fuse sounds good to me. If I had one, I wouldn't hesitate to use it.

If you are using a tested layout (OT), stick with it. Heads are typically more forgiving because your chances of unwanted magnetic coupling with the speaker are considerably reduced. Make sure to line the bottom of the head cab with a metal plate or foil (or use a bottom plate) for hum reduction.

[nop]

I finished my build, but I'm not getting any sound until the 9 oclock position on the potentiometer. I get cleans from 9-12 and breaks up after 12 to max. Is this normal?

[Mark]

Yes, what voltage are you getting?

[RWood]

No sound until 8-9 O'clock is right and 5f1 should stay somewhat clean up to about 12-1 O'clock.

I built with 5-15w OT, so sometimes I use Copper Cap Recto, 6L6, through 12" high efficiency 102db 175w speaker which stays clean to about 3 o'clock. The bass is tighter with less sag using this setup, but it is not really Champ sounding but sounds great with Les Paul HBs.

Other times I use 5y3, 6V6, and less efficient extension cab w/92db 30w speaker which starts to break up nicely at about 12-1 with good sag which is more champ-like with Stratocaster.

I like both setups so I'm going to build another PTP Champ so I can have both without changing tubes.

All in all I think the 5F1 likes to be driven hard and saturated like Dwayne and Eric did on Layla recording.

[Mark]

The output transformers are down right puny in Champ style amps, you should opt for one that gives you a better bass response and then chose your speaker. Using a 12" with the standard output transformer is a bad option IMHO.

Speakers such as the Weber Silver Ten have a significant bass response, but I don't think that speaker is needed if you have an upgraded output transformer.

[bluesfendermanblues]

A couple of tips regarding the 5F1 ....

I've build a champ 5F1 combo. Wanted it as stock as possible as a small recording amp. However, don't underestimate 3 watt. The amp can hold up its own at band (jazzy/bluesy) rehearsals on it s own. If you mike it up theres no limits, off course.

Sweet spot. I play strats and using input 1, I've found the 7-8 (goes to 12) on the volume is the sweet spot for me.

PSU MOD/ Tuning. Only mod is the dropping string resistor to the 12ax7. the schmo says 22k, but I found that 68k gave the right voltages on the plates - aim at 150-180 VDC. The exact dropping string resistor value will depend on the actual TUBE.

I've used standard hammond transformers in mine (PT: 290AEX 125P1B
022772. OT: 1750C 125A35A 022905). Plenty of power.

Speaker. Regarding speaker unit, I went for a Weber Sig 8s. Its really great - after a burning period of 10-50 hours, its really great.

All in all a very nice little amp, that delivers all the champ charm I read about for many years. Great recording amp.