Choke positon in a BF Champ

[tyru007]

Can a choke be placed before the power supply to the plates in a BF Champ build?

I'm considering a power supply using the following:

> Hammond 270DX (275-0-275 at 90 mA) followed by
> solid state FW rectifier consisting of two UF4007
> 40uf Cap + 220K resistor to ground
> 4H, 90mA (110 ohm) hammond choke
> 20uf filter cap with tap to OT/Power tube plate (35 mA at 367V)
> 1K resistor
> 20uf filter cap with tap for power tube screen (2.9 mA at 362V)
> 22K resistor
> tap for preamp tubes (2.4 mA at 310V)

voltages were calculated using PSU designer II

This places the choke prior to the tap for the OT. PSUII shows a current spike with a max of 400 mA in the first 1/10 of a second but settles down to less than 90 mA.

Any issues doing this instead of placing the choke after the OT tap?

PS: Thanks for all the help so far. I ordered almost all the part for a modified BF champ build in a head configuration. Decided I would wait till later or second build to add a reverb.

[MWJB]

A BF Champ has a plate voltage of over 400vdc, so 330-350VAC at each secondary leg would seem more appropriate. Either way 367v at the plate seems optimistic with a 275-0-275 winding?

If you place the choke between the OT & the screen supply you are going to have to use a large screen grid resistor (>2K?) to drop the voltage below plate voltage (tolex Champs & Princetons rarely achieve this, so if you want to keep plate voltage above screen, even with the stock circuit, you still need a screen grid resistor).

If you want to use the 270DX PT I wouldn't bother with a choke at all. You don't want anything that could pull down B+ further.

[tyru007]

I'm using a fullwave SS rectifier (not tube), this gives me an B+ of approximatley max of 1.4 x 275 = +385V, if what I have read is correct. The schematics for the BF champ shows B+ of +360V to the plate on the power tube and +320V supply voltage to the preamp tube. PSU-II tends to confim my calcs. Is the fender schematics incorrect?

If you place the choke between the OT & the screen supply you are going to have to use a large screen grid resistor (>2K?) to drop the voltage below plate voltage

I was going to place the choke prior to the OT supply as shown on the older champs and princetons (5F2). Then use a 1K power resistor along with a 470 ohm resistor at the screen grid.

If you want to use the 270DX PT I wouldn't bother with a choke at all. You don't want anything that could pull down B+ further.

The choke only has a 110 ohm resistance. Seems like it would be better to use the choke than a power resistor in this position to achieve an RC filter.

[loudthud]

The choke will really knock down the B+ ripple. PSU designer should show the ripple as a 120Hz quazi-sine wave of a couple of volts. The 20uF on the output side of the choke will have an effect on the tightness of the bass response. Experiment to find what you like.

[tyru007]

Using PSU-II, the ripple is about 7V before the choke and 0.12V after the choke (on the OT/Plate) and about .012V (on the power tube grid screen).

The ripple is 1.2V on the OT using a 470 resistor instead of the choke. About a 10X decrease in ripple using the choke instead of a resistor.

I don't know if this would have any audible effect on the amp.

[MWJB]

"Is the fender schematics incorrect?" It 'may' have been correct when it was published, using whatever power supply was the norm in the factory, but if you have ever heard a BF champ played today, that amp was running considerably higher voltage than the published schem...as do all 50's & 60's Fenders.

Your calculation has not allowed for any voltage drop under current load...I very much doubt that you will see 385vdc on the power tube plates.

B+ ripple hum is probably not a concern...you could just use a 40-50uf main filter at the B+ node in the stock champ power supply and hum will be inaudible. Far more likely to come from the 6.3VAC supply/layout/grounding errors.

[tyru007]

Thanks for the comments and feedback.

I am running PSU-II with loads of 1.6 mA for the 220K drain resistor to ground, 35 mA for the 6V6 plate current, 2.9 mA for the 6V6 screen current and 1.9 mA for the 12AX7. The Hammond 270DX would give me a B+ of 368V using a SS FW rectifier under load.

Most of the champ replacement transformers I can find run about 320-0-320 at 70 to 90 mA. Using a 5y3-GT FW rectifier, this gives a B+ of 385V under the above loads.

I could jump up to a Hammond 272BX with 300-0-300 at 100 mA. This would give me about B+ of 400V under load.