Another 5f2h question - Bruce?

[Wrongdog]

I am buiding Bruces 5f2h harp amp with Weber choke and OT but I am using a doner 305-0-305 PT. It does not have a 5 v winding so I am using SS rectifier. The 6.3v winding does not have a center tap to do the cathode set up as shown in the schematic. So the filiment heater wires are grounded via a 100ohm virtual ground for hum reduction.

So what do I do without the center tap to that 100 ohm risistor before the 270 cathode resistor and bypass cap???KT66 Cathode?? What does that do exactly?? I am confused...

https://taweber.powweb.com/store/5f2h_schem.jpg

[Tom Phillips]

...The 6.3v winding does not have a center tap to do the cathode set up as shown in the schematic. So the filiment heater wires are grounded via a 100ohm virtual ground for hum reduction.

So what do I do without the center tap to that 100 ohm risistor before the 270 cathode resistor and bypass cap???KT66 Cathode?? What does that do exactly?? I am confused...

You would need TWO 100 Ohm resistors. Connect one from each side of the heater supply to Point "D" (The power tube cathode) For convenience you can mount them both directly on the power tube socket. You would then have a virtual center tap elevated by the cathode voltage.

[Wrongdog]

Thanks Tom - I do have two 100 ohm resistors but I just have them to ground. I have another homebrew with ac noise set up my way but it was not 100% affective. Maybe I will try that on that one too. Does the elevated cathode voltage help with the hum reduction more?? how? Aren't ac and dc votages present?? confused again as usual!

[Tom Phillips]

Thanks Tom - I do have two 100 ohm resistors but I just have them to ground.

OK. You have created a virtual center tap. To elevate the heater you should disconnect the junction of the two 100Ω from ground and connect that point to the power tube cathode.

Does the elevated cathode voltage help with the hum reduction more?? how? Aren't ac and dc votages present?? confused again as usual!

The “elevated heater supply” helps in some cases. It also does not hurt anything. Therefore, if it doesn’t immediately help then it is OK to leave it connected that way. It may help later when you change tubes.

The reason elevating the heater supply can reduce hum is because the positive voltage floating on the heater supply attracts free electrons back to the heater element rather than letting some of them reach the cathode where they will cause hum.

The DC voltage is just used to elevate the heater supply. There is no DC current flowing and nothing smokes because the AC heater circuit is not connected to ground. As you said there is AC & DC voltage present. That’s OK. There is DC and AC voltage present in lots of other places in the amp too. The DC just raises the AC component up above ground. The 6.3V AC still does all the work to run the tube heater.

Cheers,
Tom

[Wrongdog]

Cool! That explains it. BTW - are you in S. Ca.? I am in Ventura.

[Tom Phillips]

I'm in SB

[Wrongdog]

Glen Phillips brother??!

[Tom Phillips]

He's a nice guy but we are not related.
Tom

[Wrongdog]

I need some help!

I got the amp up and running but my voltages are off. That PT (from an old donor hifi dual EL84 amp) put out 305-0-305 AC but and with the varying power tubes I subbed in it drops to 290 AC.

Then at SS rectifier I see 245 vdc, after choke to OT 237 and next 209 with 14 vdc at pin 8 cathode. Wth 6l6 removed I get 305ac, 392dc ,388 and 340 at the last filter cap.

If I bypass the choke with 6l6 in I get 305ac, 320dc, 311, 275 and 19vdc at Cathode. It does not add any hum without choke nor does it sound any different. The choke reads 120 ohm.

WHat is wrong?? I am tired at staring at this thing and going over everything 20 times but not seeing any build issues. All spec out, wired right and grounds are proper as far as I can tell. Cathode resistor and bypass cap are fine. Can the giant choke be too much draw on the old PT?? it was designed to handle 2 6bq5's and 4 preamp tubes so it should have 70+ ma rating I assume.

Thanks to anyone who offer any thoughts - Scott

[MWJB]

You have a filter cap to ground right off the rectifier, before the choke? Did the PT put out 305VAC under load in the old EL84 amp? Sounds high for an old EL84 amp?

If you have only seen 305VAC unloaded, then that will drop when loaded up with tubes.

I would start by biasing the 6L6 cooler (with 330/470ohms) and see what happens to your B+. Or put an ammeter between rectifier & the choke, in place of the wire there now (clip in place, & remove, with the amp unplugged from the wall), to measure/confirm that the appropriate current is being drawn through the B+ rail.

Some old amps were borderline with respect to current capacity, a big draw might be enough to pull down B+, but starting with a EL84 PT means that it's not likely to be very high to start with.

[Wrongdog]

hi - no rectifier>choke>40ufcap/OT>2.7k/cap etc. I will try the messing with the bias - I think it is a 270 now.

I am suspect of the PT being too wimpy. I was building this out of junque laying around regarding PT Chassis lugged turret board etc. Just crap lying around for 2-4 years! I decided to pony up for the budget Weber choke OT and a SS alnico 10. Looks like my original $50 project will be a $300 project between iron, speaker, sheet of Baltic birch 15mm ply etc!!

I am thinking a Hammond P-T272BX needs to be ordered...or an Edcor equivalent.

I have another uber Frankenchamp that is killer - made with junk parts except a big sweet Edcor OT that this new one was to outshine. It is a BF Champ basically with no NFB, fat caps and higher power tube voltages but with browner lower voltages to the preamp tubes. Works great for guitar with a 12ax7 and superb dimed with a hot vintage Shure harp with 5751, 12at7 and even 12av7/5955. That is a tough one to top - so far at least!

Thanks for help - Scott