18WPC Schematic Help

[EWBrown]

The two electrolytics and two diodes in the AC primary circuit seem kinda "strange" Yellow_Light_Colorz_PDT_09 . I'd ditch them, and use one of the Keystone (or similar) CL-90 inrush current limiters. (in place of the NTC, unless it is similar).

EL84s are happy with around 360VDC on the plates, 35 mA idle plate current.

THis circuit definitely needs some up-front gain, a minimum would be as in the "compact" amp design, better would be to emulate the VA / PI design from the ST-35, or even the 832 amp (using a 12AT7) that I posted recently.

Now for my bit of "plinker" engineering:
If 7247s or 12DW7s, 12AX7s and 12AU7s are too costly, just use a 6AV6 for the VA stage (equivalent to half a 12AX7) and a 6C4 for the phase splitter (equivalent to half a 12AU7). These tubes generally are low priced, even for NOS. For 12AT7, a 6AT6 serves as half of a 12AT7.

/ed B in NH

[Blair]

Thank you Ed. The NTC is an inrush limiter. It is a thermistor, and it does work pretty well. It takes about 10 seconds to see the tubes slowly reach power. We have these tubes running on about 330-340vdc using a 500 ohm resitstor on the B+ line. My friend has a preamp circuit he is using for his that has a medium - high gain factor that should compensate his amp. I am going to use a 12AU7 for mine, or possible a few single triodes to keep things balanced looking on the chasis<--Yeah I'm anal retintive! This should balance the circuit and allow this amplifier to run at proper levels right? By the way, after several troubleshooting attempts with the grounding, this amplifier is sounding very sweet according to my friend. Very quiet and well behaved with rich bass and dynamic midrange if I am quoting him correctly.

Thank you again everyone,

Blair

The two electrolytics and two diodes in the AC primary circuit seem kinda "strange" Yellow_Light_Colorz_PDT_09 . I'd ditch them, and use one of the Keystone (or similar) CL-90 inrush current limiters. (in place of the NTC, unless it is similar).

EL84s are happy with around 360VDC on the plates, 35 mA idle plate current.

THis circuit definitely needs some up-front gain, a minimum would be as in the "compact" amp design, better would be to emulate the VA / PI design from the ST-35, or even the 832 amp (using a 12AT7) that I posted recently.

Now for my bit of "plinker" engineering:
If 7247s or 12DW7s, 12AX7s and 12AU7s are too costly, just use a 6AV6 for the VA stage (equivalent to half a 12AX7) and a 6C4 for the phase splitter (equivalent to half a 12AU7). These tubes generally are low priced, even for NOS. For 12AT7, a 6AT6 serves as half of a 12AT7.

/ed B in NH

[EWBrown]

The single tube Decware line stage is most likely a cathode follower, which has no voltage gain, but it does impedance matching, probably 100K input and a few K output impedance.

In order to get voltage gain, you need a linestage like this one on Gary Kaufman's page:





http://www.the-planet.org/images/6SN7_3.jpg

if the picture doesn't automagically open


It will have more than enough voltage gain and drive to power your amp just the way it is. In place of a 6SN7, a 6CG7 / 6FQ7 will work as well.
The B+ doesn't have to be "exactly" 370VDC, just tap it off the existing B+ in your amp.

/ed B in NH

[Blair]

How about this with the 12AU7:

The single tube Decware line stage is most likely a cathode follower, which has no voltage gain, but it does impedance matching, probably 100K input and a few K output impedance.

In order to get voltage gain, you need a linestage like this one on Gary Kaufman's page:



It will have more than enough voltage gain and drive to power your amp just the way it is. In place of a 6SN7, a 6CG7 / 6FQ7 will work as well.
The B+ doesn't have to be "exactly" 370VDC, just tap it off the existing B+ in your amp.

/ed B in NH

http://img.villagephotos.com/p/2005-12/1115293/18wpcStereoAmpversionBlair.JPG

[erichayes]

Hi All,

With the values you have assigned to the plate and cathode resistors of that 12AU7 stage, that tube's going to last about 15 minutes. Triple the resistances of the plate and cathode resistors and either lower the capacitance of the cathode bypass cap to something below 100µF or eliminate it; a value that high will be bypassing everything, so why bother?

I'm still curious as to why you want to do your phase inversion in the output stage. You severely limit your options for reducing distortion, and don't really have any advantage over upstream phase inversion to balance the books. If you're doing it just to be different, that's cool, but if you want an amp that sounds (or can be made to sound) good, go with a more traditional circuit. There are reasons they're still around.

[Blair]

I have recently changed my mind and will be building one of Shannon's ST35 boards. Thank you all for trying to explain all of this to me. I appreciate your time.

Blair

Hi All,

With the values you have assigned to the plate and cathode resistors of that 12AU7 stage, that tube's going to last about 15 minutes. Triple the resistances of the plate and cathode resistors and either lower the capacitance of the cathode bypass cap to something below 100µF or eliminate it; a value that high will be bypassing everything, so why bother?

I'm still curious as to why you want to do your phase inversion in the output stage. You severely limit your options for reducing distortion, and don't really have any advantage over upstream phase inversion to balance the books. If you're doing it just to be different, that's cool, but if you want an amp that sounds (or can be made to sound) good, go with a more traditional circuit. There are reasons they're still around.

[erichayes]

Excellent choice. Shannon's board is flexible enough to do experimenting if you want to go off the beaten path just to see what makes things tick. Given high quality components to go with it, it also makes an excellent amplifier with no modifications.

Don't stop asking questions, though.

[mesherm]

Thats not a bad way to go. You pretty much have all the major parts already and if you pick up a DIY chassis those Edcors might fit with a minimum of effort.

[dhuebert]

With the values you have assigned to the plate and cathode resistors of that 12AU7 stage, that tube's going to last about 15 minutes.

That's interesting, I recently went thru a design cycle using the equations at Aikenamps.com. With a supply of 250V and well within the power rating of the 12AU7 I came up with a plate resistor of 20K and a cathode resistor value of 470 ohms which gives a saturated current of 12.5 mA and a quiescent current of 6 mA. For the above circuit we see 300V across 15K which gives 10mA quiescent current or 1.5 Watts (150 volts x 10 mA), well within the 2.75 watts spec'd for the tube. Where have I gone wrong here?

Don

[erichayes]

Hi All,

Misspeak, Don. I saw different numbers than what were actually there. Just came in from the lab after hooking the basic circuit up to a power supply and it's drawing a very polite 9.5 mA (measured) at 300 VDC (measured).

I'd still want to have larger plate and cathode resistors in there to get a better AC voltage swing, though. Either that and/or use a tube with more gain than a 12AU7 has . . . that was one RF tube that should have stayed in RF, as far as I'm concerned, for a variety of reasons.

[dhuebert]

that was one RF tube that should have stayed in RF, as far as I'm concerned, for a variety of reasons

OO, OO, OO (Arnold Horshack) Tell me! Tell me!

Curious Don

[EWBrown]

That 12AU7 input stage would probably be happier with 750-1000 ohms for the cathode resistor and 47-68K for the plate, as in the VA portion of the 6SN7 line stage. Won't beat on it as hard - 9.5 mA is still beating upon the rented mule way too hard Yellow_Light_Colorz_PDT_07

A 6CG7/6FQ7 or 6GU7 can be subbed for the 12AU7, just feed the 6.3 VAC filament across pins 4 and 5 and don't use pin 9

Or a 12BH7 / 12BH7A can be plugged in w/o any wiring changes.

===============================================

Best yet, gow with the "blue board" use your Edcor trannies, and your EL84s, and 12AX7 & 12AU7, 0r 5751 & 12BH7A (or 5814A) for the input and phase splitters.


If you don't want or need the individual EL84 adjustable tube biasing, things can be simplified by using a 400 ohm, 2W resistor for each EL84 cathode. This will necessitate matched (or near-matched) quads to keep things on an even keel.

The rev D board also has provisions for using an LM317 and 36 ohm resistor for "automagic" biasing of each EL84, though this works best in triode-strapped mode. It will work for Ultralinear mode, though some folks may not prefer this approach.

I'm working up a blue-board driver feeding PP 6S4As, with 25 mA "automagic" biasing, which will require some etch surgery around the four power tube sockets, and re-adjusting the NFB circuit (if it is even required at all). The B+ will have to be reduced to 300-330VDC in order to keep the 6S4As from melting down. :o


HTH

/ed B in NH