What If Filaments?

[plexi50]

What if all your filiment voltages were running at 5.75VAC per tube per pin4-5 & 9. DVM chassis ground and pos to each pin-s

Would this cause the cathode voltages in each preamp tube to be abnormally high?

Normal=1+VDC / Abnormal= 3.45 VDC

[Geezer]

I'm not understanding your question....... 

Are you asking: "If my filament voltages are too low [or too high, not sure which yours is], then will it affect the cathode voltage/cause it to be abnormally high?"

If not, could you re-state the question?

What is the filament voltage "pin to pin"?

Can we see what amp you're working on? (schemo?)

[plexi50]

Measuring across pin 4&5 to pin 9 there is 12 VAC. Each pin 4&5 or 9 to ground read 5.75 VAC

Could a higher filiment voltage (not lower) be the cause of a higher cathode reading?

This is the Seymour Duncan Convertible 100 that i removed the original board from and built a board i made with a plexi/2204 layout

The tube filiments are very night light bright

[sluckey]

Measuring across pin 4&5 to pin 9 there is 12 VAC. Each pin 4&5 or 9 to ground read 5.75 VAC

Could a higher filiment voltage (not lower) be the cause of a higher cathode reading?

Maybe. Probably. For certain, you're gonna kill those tubes if they are 12A_7s!

[plexi50]

Yes thats what i fear Steve. I have a small board i am thinking of building a small DC regulated bridge. Man it's one thing after another with this volkswagon sized PT. Just tell me to put it away until i get another PT.

[RicharD]

12V filament supply..... use pins 4&5 and ignore pin 9.  Dunno about your power tubes.  Series pairs?

[plexi50]

Filaments wired series from power tubes through all preamp tubes. What do you mean by ignore pin 9? Remove lead from pin 9 no filament action

[stingray_65]

Filiments wired series from power tubes through all preamp tubes. Should i ground pin 9 heater lead?

If you run the heaters with 12V you do NOT ground pin 9

pin 9 is an internal tie point between the separate heaters

6v___ 4_/\/\/\/\_9_/\/\/\/\_5___6V    series 12v supply

3v___4_/\/\/\/\_9____3v            parallel 6v supply
     |_5_/\/\/\/\_|


Ray

[plexi50]

Thanks Ray. Will try it now

[stingray_65]

I learned this lesson the hard way when I built a 12v heated champ

I had a small Stancor tranny with 2  matched 6V taps. Tied the taps together in series and used the connection as the CT.

One 12AX7 and a 12V6 and I was in business.

Ray

[Fresh_Start]

Sorry guys, I'm not following the solution - especially in regard to a power tube with only two heater pins.

Is there any advantage to a 12 volt filament supply?  Less current draw maybe?

Chip

[plexi50]

The PT i am using was originally setup to operate the preamp moduels at 10 volts. My brain just quit working and i am thinking how to wire it in parallel

Im trying to get 3VDC but anyway i look at this im still gettin 5.74 VDC per pin 9 and pins 4&5

Im sorry i think i am wired up in parallel already. One heater lead to pins 4&5 and the other to pin 9 correct?

[stingray_65]

The PT i am using was originally setup to operate the preamp moduels at 10 volts. My brain just quit working and i am thinking how to wire it in parallel.

Im sorry i think i am wired up in parallel already. One heater lead to pins 4&5 and the other to pin 9 correct?

NO!
one lead to pin 4 the other lead to pin 5

pin 9 NOT connected

Chip,

12V heater supply will use less current, but  it's all about supplying what the tube wants. the 12A_7 is versatile in that it CAN be wired 6 or 12.
 The 12 A_7 is designed to run at 12V therefore it has a 12 in the first position of its name.
the A_ is the tubes actual name or type. the 7 is how many internal parts there are

so a 12V6 is the same as a 6V6 except it has 12V heaters (used in ancient car radio's) (and dirt cheap too)

Ray

[plexi50]

One lead to pin 4 and one lead to pin 5  5.75 VDC. Measured from ground to each pin
Measured across pins 4 & 5 11.90 VDC

[sluckey]

Looks like this on a schematic...

[stingray_65]

One lead to pin 4 and one lead to pin 5  5.75 VDC. Measured from ground to each pin
Measured across pins 4 & 5 11.90 VDC

BINGO!

It's different so its weird and sometimes hard to wrap your head around.

But you got it now and that's important

Ray

[plexi50]

Here how the schematic looks. Back to work. Ribs that is /

[Fresh_Start]

Thanks guys!

A wise gentleman once said "A schematic is worth 1,000 words!"

Chip

[plexi50]

Ok preamp tubes wired parallel and cathode voltages have come down to between 1.800VDC and 2.145VDC +- from a whopping 3.45VDC

Now explain the power tubes wiring. Presently the EL34 tubes are wired one lead to pin2 and one lead to pin7

This is repeated with the next set of EL34 tubes as a complete chain. This looks wrong on the schematic

[PRR]

What was wrong with the original plan?

The part you cut-off to the right is DC power for your first and second stages. In hand-wired work, DC on guitar-amp stages is not essential; but in PCB work it kinda is needed. Since you have (or could quickly restore) the DC supply, and the PT winding was designed to give some good clean 12V_DC_ power, you may as well use it.

You don't want DC power on all your tubes. Just the ones with weak signals. Your first stage and your reverb recovery stage have signals as low as 0.02V, your post-Volume and post-tone stages just a little bit higher. After these stages the signal is boosted to around 2V; finally to 20V at EL34/6L6 grids. This is 100-1000 times larger! So 99%-99.9% of your heater hum sneaks-in at just a few stages. All the others have so much larger signals that they won't be a problem.

And DC heat is expensive. Up to 0.5A it isn't too bad. Going for 5A (what the whole amp needs) gets you into a much larger (costlier) rectifier and filter caps. So you want to be selective.

As Seymor did. EL34 on AC heat, and I bet these V1-V3 tubes are after all preamps and tone/volume stuff (which IIRC are on plug-ins and probably fed the 12V DC).

The original DC rectifier and cap(s) should be fine. If they have been modded-out, a "6A 100V" (or better) full-wave-bridge module onto a 3,300uFd 16V cap will give you DC. I think it will give about 14V of not-very-clean DC. So add 1 ohm 5W in series, and a second 3,300u cap to ground. Adjust the 1 ohm for 12.0-13.0VDC with all small-signal tubes lit.

Note carefully: the ground on the heater system is at the DC end. The AC side is NOT hard-grounded. This is correct. Rectifier-action will semi-balance the AC side. Not perfectly, so there is a cap on the AC side to steady things while the rectifier is cut-off twice per cycle.

> EL34 tubes as a complete chain. This looks wrong on the schematic

The secret goal is to make DC heat. There's like 2V of waste in the process. At 6VDC this hurts bad; at 12VDC not so much. For 12V DC you want to start around 11V-13V AC. So your AC-heater tubes MUST be 11V-13V heaters. Two 6V heaters in series IS a 12V load. Since we have pairs of 6V tubes, this works out great.

Seriously: Mr Duncan may have some advanced feature-rich ideas which you don't appreciate, but this heater-power system IS correctly worked-out IF you follow it exACTly as it was. Don't use "common knowledge", don't improvise, play it literally as-written.

[plexi50]

PRR i removed the original board and built a different board. The DC heater block was not incorporated on the board i have built

I understand better the difference between series vs parallel heater wiring and the voltage reduction

Heater wiring always mess's with my head. It should be the easiest part of building an amp but for me it drives me crazy winding and soldering down the tube paths

This is the amp now




 

[eleventeen]

12V heater supply will use less current, but  it's all about supplying what the tube wants. the 12A_7 is versatile in that it CAN be wired 6 or 12.  The 12 A_7 is designed to run at 12V therefore it has a 12 in the first position of its name.
the A_ is the tubes actual name or type. the 7 is how many internal parts there are.

so a 12V6 is the same as a 6V6 except it has 12V heaters (used in ancient car radio's) (and dirt cheap too  

This is *almost* always true, easily 99+% of the time but not always. One notable exception: a 50L6 or a 25L6 or a 35L6 is much more akin to a 6V6 in terms of physical size, eg; straight-sided, power handling/dissipation, and NOTHING like a 6L6.

A 35W4 is a minature 7 pin rectifier tube used in AA5 radios. A 6W4 is an octal rectifier.

Carry on!

[plexi50]

Wired all up correctly and now the tubes have a nice soft filament glow. Amp is much quieter as well as much hum was being injected from having too high a voltage on the filaments. Thanks you all

[stingray_65]

Love it when a plan comes together!

[PRR]

> 50L6 or a 25L6 or a 35L6 is ... ... NOTHING like a 6L6.

Good point.

Rumor has it that RCA deliberately picked the "L6" designation so some of the glamour of 6L6 would rub-off on kitchen-radio tubes. But the small-radio tubes are good for a couple Watts, not 9W like one 6L6 might do.

Also not that-similar to 6V6. These are low-voltage HIGH current tubes. To get the high current at low voltage they have BIG heaters.

6V6 : 6.3V 0.45A = 2.8W heater
50L6 : 50V 0.15A = 7.5W heater
35L6 : 35V 0.15A = 5.2W heater
25L6 : 25V 0.3A = 7.5W heater

Two or three times the heater power, and a bit less plate dissipation.

Like a race-horse versus a plow-horse. Horses for courses. If you have >250V you sure want the 6V6 or 6L6 type tubes: thrifty heaters and more power output. (In recognition of this fact, the 25/35/50L6 types are made with 150V-200V plate-stuff, because you'd never use such heater-hogs when you had ample plate voltage.) OTOH when you can save a buck with a 110V DC plate supply, and when heater power is practically free (transformerless radios), the 25/35/50L6 are the cats-meow.

[stingray_65]

Thanks for that insight! (PRR & 11teen)

there are always exceptions to the rules and those tubes are common enough that I should have that info to keep in mind.

Ray

[eleventeen]

Zactly, PRR. I always kinda wondered about 50L6, 35L6, of course, hardly enough to really care since these are (as you say) series-filament tubes across the AC line that I'd never use for anything. A pile came with the tubes in the garage of doom so got me to thinking "hey, these aren't big heater volts 6L6's by any stretch!'

So...why aren't they called 50V6? 35V6? Enquiring minds don't care that much!