5F2-A biasing

[Leevi]

I finished my 5F2-A build and wonder a relative high cathode current which is about 46mA.
I have followed the original schematic. Plate voltage is 385V and I'm using Hammond 125DSE OT at 10K/8 Ohm.
Should I increase the 470 Ohm cathode resistor in order to get the current lower?
Btw. the amp sounds excellent,

http://www.freeinfosociety.com/electronics/schemview.php?id=802

/Leevi

[Tiny_Daddy]

So is the cathode measuring around .046 x 470 = 21V? If not, maybe it's because of capacitor leakage. Disconnect the cathode bypass cap and see. Maybe the cathode bypass cap is backwards? The + end goes to the tube. This is not indicated on the schematic.

[John]

If the cathode cap is the right way around, I'd try sticking in a 700'ish ohm restistor and see if that makes a difference. What I have found though is that when I increase the cathode R, the plate voltage goes up. But that should still lower the dissipation somewhat.

It seems one of the most common questions is about plate voltage too high, I always have the problem too. The next PT I actually buy instead of take off a carcass is gonna be a 250-0-250! 

[HotBluePlates]

Guess what? You're right where you probably should be, given modern wall voltage and your specific setup.

The 5F2-A Princeton layout and schematic do not have voltages listed. Despite a somewhat different power supply, we should look at the 5F2 Princeton layout and schematic as a guide.

The 5F2 shows 19v across the 470 ohm resistor. You have ~21v (if our math matches yours). 19v across 470 ohms would indicate ~40mA of cathode current.

But your plate and screen is at 385v; the 5F2 schematic shows 305v (mostly due to the choke ahead of the plate node). The extra 80v is consistent with the extra 6mA of cathode current. It's also consistent with actual voltages/currents in blackface and silverface Champs and VibroChamps (regardless of what the schematic says).

Screen current is not a constant percentage of cathode current, but there is a rough ratio by which cathode current splits into plate and screen current. Judging from the 6V6GT datasheet, screen current is around 1/10th of plate current, or 1/11th of total cathode current. Your cathode current is .046A, so your actual plate current is probably ~42mA.

385v-21v = 364v
364v * .042A = 15.3w

Your 6V6 is running on the warm side; so did ALL the Champs/VibroChamps/Princetons that left Fender. Does it redplate at idle?

If yes, follow PRR's suggestion and get a pack of 100 ohm resistors, tack 10 of them in series, and use a jumper to select your total resistance. Parallel another 100 ohm across an existing 100 ohm to get 50 ohm in-between values. Find the resistance that works for your tube.

If no, forget about it. The tube will run cooler when passing signal than at idle. Your next tube may idle a little warmer/cooler; do you really want to go back in to change the cathode resistance?

[Leevi]

Does it redplate at idle?

No

Maybe I increase the cathode resistor little bit.

Thanks
Leevi

[Fresh_Start]

FWIW my first build was a 5F2-A.  Plate voltage was around 360 initially.  Dropping it down to around 310 sounded much better to me.  Warmer sounding breakup, if that makes any sense.  That voltage level is consistent with Tweed Champs and some Tweed Princetons.  You can adjust bias by changing plate voltage - it's just not the easiest thing to do.

If you are using a new "5Y3" rectifier tube, swapping a real NOS 5Y3 tube dropped 20 volts right off the bat.  After that, I experimented with dropping resistors between the rectifier and the first filter cap.  If you try that, make sure that you use a resistor with a high enough power rating.  We can help with the math if you want.

You said that the "amp sounds excellent", so you may choose to follow the homily "If it ain't broke, don't fix it!"

Respectfully,

Chip

[Leevi]

If you are using a new "5Y3" rectifier tube, swapping a real NOS 5Y3 tube dropped 20 volts right off the bat

I'm using Sovtek 5Y3. Since there was a low ground hum in the amp I added an extra filter stage after the rectifier which dropped the voltage by 5V.

Since the high voltage is often a problem I'll choose next time a transformer with lower secondary (e.g. 250-0-250).

/Leevi

[Fresh_Start]

If you are using a new "5Y3" rectifier tube, swapping a real NOS 5Y3 tube dropped 20 volts right off the bat

I'm using a Sovtek 5Y3. Since there was a low ground hum in the amp I added an extra filter stage after the rectifier which dropped the voltage by 5V.

Since the high voltages are often a problem I'll choose next time a transformer with lower secondary (e.g. 250-0-250).

/Leevi

Despite the label, a Sovtek "5Y3" isn't a 5Y3 either in terms of the voltage drop, sag, or even the basic structure of the parts inside the tube.  If you are in the U.S., you can get an NOS 5Y3-GT for about $20.

You also could try increasing the resistance in the pi filter right after the rectifier.  I'm surprised that adding a pi filter between the rectifier and the plate node on the power rail only dropped your plate voltage by 5 volts.  What size resistor did you use?

FWIW I stuck a small choke in between the rectifier and the plate supply node like the 5E1 Champ.  That lowered the noise level a lot for me.

Hope that helps,

Chip

[Leevi]

You also could try increasing the resistance in the pi filter right after the rectifier.  I'm surprised that adding a pi filter between the rectifier and the plate node on the power rail only dropped your plate voltage by 5 volts.  What size resistor did you use?

I just put a 47 Ohm resistor there since I was just aiming to eliminate the hum. Of course I could double that and drop the voltage by another 5V. Btw. the hum disappeared.
/Leevi

[stingray_65]

Guess what? You're right where you probably should be, given modern wall voltage and your specific setup.

The 5F2-A Princeton layout and schematic do not have voltages listed. Despite a somewhat different power supply, we should look at the 5F2 Princeton layout and schematic as a guide.

or look at the "harvard 6G10" looks like a princeton 5F2A if you ask me, except it has voltages called out.

http://www.el34world.com/charts/Schematics/files/fender/HARVARD_6G10.pdf

I see this print lots of places, but have never seen any mention of fender making one.

Ray

[sluckey]

or look at the "harvard 6G10" .... I see this print lots of places, but have never seen any mention of fender making one.

I had the good fortune to restore a Harvard 5F10. It was a small P/P fixed bias amp. Quite a ballsy sounding amp! Then Fender made it even smaller (and cheaper) with the 6G10. Who knows why? Then it just disappeared and the Princeton won out. Probably an economic decision, but I always thought Fender should have pumped the Harvard and dropped the Princeton. I still wonder why.

[PRR]

> I added an extra filter stage after the rectifier which dropped the voltage by 5V.

Drop 50V here. That gets your plate voltage back in a sane zone. Now you can leave the bias and current alone. 330V*40mA is fine plate dissipation. 330V/40mA is 8K working resistance, a fine match for typical "7K" OTs.

[HotBluePlates]

... or look at the "harvard 6G10" looks like a princeton 5F2A if you ask me, except it has voltages called out.

Damn good eye! Yep, spot on 5F2-A circuit, with voltages. Note the 20v for the cathode resistor.

I see this print lots of places, but have never seen any mention of fender making one.

Do you mean the Harvard in general, or the 6G10 version?

Fender Amp Field Guide has pics of the 5F10 and 6G10 Harvard versions, both in tweed.

I don't recall ever seeing a 6G10 Harvard in person, but I know I've seen and heard the earlier push-pull 5F10 version. It is interesting how the Harvard devlolved from the fixed-bias push-pull output of the 50's amp to the big-box Princeton in the 6G10 version, but it probably filled a gap in the model line, considering the tweed Champ remained essentially unchanged during the same time.

[Fresh_Start]

> I added an extra filter stage after the rectifier which dropped the voltage by 5V.

Drop 50V here. That gets your plate voltage back in a sane zone. Now you can leave the bias and current alone. 330V*40mA is fine plate dissipation. 330V/40mA is 8K working resistance, a fine match for typical "7K" OTs.

IOW a 470 ohm resistor, based on a 47 ohm resistor dropping 5 volts.  Your 6V6 should be happier there.  I'd use at least a 10-watt resistor there even though the math doesn't seem to call for it.

Sorry about my mistake on the resistor power rating.  Since a 680 ohm resistor dropped 25 volts, the minimum rating would be 2 watts

(25*25)/680=0.92, then add an extra watt for a safety margin.

Chip

[sluckey]

I'd use at least a 1-watt resistor there even though the math doesn't seem to call for it.

My math says that a 470 ohm resistor that is dropping 50V is dissipating 5.3W (P = E²/R) I'd rather use a 10 watt resistor.

Or did you mean use a 47 ohm 1 watt?

[Leevi]

I found a 680 Ohm/22W resistor and connected it to the first filter stage
and succeeded to drop voltage by 30V. After a 10 minutes playing I could still touch the resistor.

The values are now:

Plate: 360V
Cathode with 470R: 20V
Screen: 325V

I think I let it be now especially I had looked at the 6G10 schematic where the voltages
are on same level.

/Leevi

[tubeswell]

Or you could chuck a JJ6V6S in there.  They are pretty reliable 14W tubes (probably even capable of 16W!) that can easily handle above 400V, and they sound good.

[Leevi]

They are pretty reliable 14W tubes (probably even capable of 16W!) that can easily handle above 400V, and they sound good.

Yes, this is one option. But I think the high voltage is not a problem with the JJ's 6V6GT I'm currently using.
I'm more worried about the high current.
/Leevi

[tubeswell]

If the JJ isn't red-plating it'll probably be fine methinks

[Leevi]

Ready!
I added a NFB switch to the panel. There is also master if you wonder the number of pots.

http://i218.photobucket.com/albums/cc183/kz14100/RikstoneC5Pfront.jpg

/Leevi

[Fresh_Start]

Looks nice!  Did you build the cab yourself?

Cheers,

Chip

[Leevi]

Thanks Fresh_Start,

Did you build the cab yourself?

Yes, I did, the tolex is synthetic not woven.
The cabinet is 18mm pine and the baffle 12mm plywood
/Leevi

[Leevi]

I built another 5F2-A with PT having 0-250v secondary where I used
SS rectifier. Furthermore I added a VVR to it.

The voltages are now (on max B+):

Plate: 330V
Cathode: 18V

/Leevi