Fender 5F8A Bias

[martin manning]

I think I might reduce those capacitor values to trim some bass a la the Tweedle Dee approach. Is there any predictable math on differing the separated V1cathode components to maximize even-order harmonics when jumping channels? Maybe since they're parallel, they ought to be the same?

This is only about the gain vs. frequency for the stage. You're correct, if you want the same as you have, double the R and halve the C (to hold RC constant). If you want to trim some bass, keep the R (to maintain the bias point), but reduce the C. See: http://www.valvewizard.co.uk/OtherStuff.html

[ViperDoc]

I think I might reduce those capacitor values to trim some bass a la the Tweedle Dee approach. Is there any predictable math on differing the separated V1cathode components to maximize even-order harmonics when jumping channels? Maybe since they're parallel, they ought to be the same?

This is only about the gain vs. frequency for the stage. You're correct, if you want the same as you have, double the R and halve the C (to hold RC constant). If you want to trim some bass, keep the R (to maintain the bias point), but reduce the C. See: http://www.valvewizard.co.uk/OtherStuff.html

Thank you sir. So When splitting the cathodes, go 1K5 on each cathode and maybe 1 uF on the bypass cap. Wondering how much sacrilege I'll be committing.

[martin manning]

Somewhere in there, 1u to 10u. Try a couple of values and see what enraptures you.

[ViperDoc]

Somewhere in there, 1u to 10u. Try a couple of values and see what enraptures you.

Love it. Definitely want more rapture! So I split the cathodes and put the original and 2x 1K5/1uF 25V on a dpdt switch and only fired it up quickly last night, it sounded quite sterile. I'm going to go for 2 x 10 uF and see how that settles out, perhaps more like a 5E3 if it's loaded with a CC 25uF setup.

[Reeltarded]

Coming from a non EE view from a guy who has drank more than a case of beer in 90 minutes..

You might like double more than less than half!

You say 'sterile'. How sterile? What sterile? Hey.. don't be afraid of jacking with the plates too. some more is fun, twice as much is forward and facey. A brighter plate and a bigger bypass need to be examined by feel..

Sterile.. where did the NFB end up?

[ViperDoc]

Dehydration be damned, I say. Let thy thirst be conquered! (Where's the Viking-of-Valhalla-wearing-a-beer-bong-helmet-emoji?)

With the stock 250uF V1 bypass cap, I've got the bass turned down to about 2 like a Marshall to avoid too much flab. When I flick it to the dual 1 uFs, it sounds very thin. I'm going to adjust the bass at home (it was too late to crank it last night) and see if I can get settings I like. I'll probably try some 10 uF caps to see if I like that better.

The NFB is at 100K right now off the 8 ohm OT secondary. Stock is 56K off of the 4 ohm tap. So the equivalent is ~79K. I put in an 82K earlier and still wanted less feedback/more gristle.

Keep it Reel, bro!

[ViperDoc]

I’ve been entertaining converting this 5F8A to cathode bias. I’ve got -40 VDC on the PT grids, 398 VDC on plates and 396 on screens. I’ve also been tempted to lower the screen voltage with a pot, just haven’t done it. How would you calculate the appropriate cathode cap and resistors?

[sluckey]

I would just start with a 250Ω shared resistor and 50µF. Adjust to taste.

[ViperDoc]

I'm looking at dropping the B+ in my 5F8A. I've been attenuating the wall voltage to 117 VAC and get 399 VDC on the PT plates. I want to design the power supply for 120 AC. I've seen various techniques of doing this. I'm mainly interested in minimizing heat to the chassis, if at all possible. I've seen various methods on previous threads, mostly naming components that are presently outdated or obsolete.

What is the best way to drop 10-15 VDC on the rectifier output? This is what I've read:

1) Drop the HT voltage on the HT CT using a 25W zener diode. This drops the ground reference voltage?

2) Use zener diodes in series with the rectified DC output either prior to or after the first filter cap.

3) Use a Mosfet circuit like the one shown here after the rectifier (courtesy of mhuss):

Simple Voltage Reducer circuit.gif

4) Drop HT secondary voltage straight off the transformer, but this seems flawed. Maybe it all is. What do you think?

For reference, here are my latest voltages:

Wall AC = 117 VAC (attenuated)
B+ = 399
V1 = 161 / 0 / 2.6 : 167 / 0 / 2.6
V2 = 193 / 0 / 1 : 324 / 0 / 194
V3 = 248 / 23.5 / 38.5 : 236 / 25 / 38.5 (This is with 82K on V3B plate, 100K on V3A plate)
V4-7 = 399A / 398S / -41G / 37mA K

Thanks.

[martin manning]

Another option is to use a MOSFET in the CT ground. Works the same as a Zener, but easier to get and the voltage drop is adjustable: https://ampgarage.com/forum/viewtopic.php?f=6&t=27035

You could also reduce the line voltage input using a bucking transformer. A bit more portable than a Variac.

[ViperDoc]

Another option is to use a MOSFET in the CT ground. Works the same as a Zener, but easier to get and the voltage drop is adjustable: https://ampgarage.com/forum/viewtopic.php?f=6&t=27035

You could also reduce the line voltage input using a bucking transformer. A bit more portable than a Variac.

Thank you, Martin!

I'm still scouring the thread, but the Aiken document mentions that the voltage drop achieved is the sum of the zener diode voltage rating and the gate-to-source drop of the mosfet. In his schematic, there are two zener diodes. Which diode value determines the diode voltage drop. or do they both?

aikenzenerctdrop_175.pdf

[EDIT: I found another document in that thread and it seems to imply that the 1N4252A zener diode is the one that specifies the drop voltage, am I right? such as this: ]

MOSFET B+ Reducer board.pdf

[martin manning]

You've got it right. The 12V Zener is just there to protect Vgs. If you make the other one 12V you should get the ~15V drop you are looking for.
if you have room, an easy and compact layout can be made using a 3-lug terminal strip.

[ViperDoc]

Thanks, Martin.

Is it appropriate to still place the heater CT and first filter cap ground with the ht ct/mosfet circuit ground or no?

[martin manning]

Is it appropriate to still place the heater CT and first filter cap ground with the ht ct/mosfet circuit ground or no?

Yes, ground those two as usual, just put the reducer between the chassis ground and the PT CT, and run the reservoir ground lead to the ground lug on the terminal strip. I added the reservoir ground to the drawing above. Note also you will have to adjust or modify your bias circuit since its reference will be pulled below chassis ground. Your bias voltage will be lowered by the same amount as your B+.

[ViperDoc]

Very good as always.

The spec sheet for my PT does not show the HT secondary current. YAY! A 5F8A Heyboer PT shows 250mA on the secondary. So when choosing power rating for those zeners, would I be safe to calculate (.250A)15V = 3.75W for minimum zener diode power rating? So 5W is OK? I note that a 10W zener diode looks like something designed for the neck of Frankenstein.