New schematic for Review, Criticism & Errors

[Howlin' Mad Mac]

Hi ya'll... got bored & drew this up. Let me know what ya'll think.

[octal]

Looks like it'll work, just a few drawing errors (input jack, transformer windings of HV and filament are continuous!)

Are you using the choke in a swinging manner to lower B+ or something, or is that a drawing error?

Nathan

[Howlin' Mad Mac]

Looks like it'll work, just a few drawing errors (input jack, transformer windings of HV and filament are continuous!)

Are you using the choke in a swinging manner to lower B+ or something, or is that a drawing error?

Nathan

I'm not seeing error on input you mentioned. Transformer windings are not continuous, just drawn wrong. Regarding supply 100R/10W resistor is dropping B+ voltage, choke is for filtering & to tighten up the bass a bit. I placed it before the 1st filter cap because I wanted a larger reservoir (hence the two 16u caps in parallel) rather than split them up. I'm also considering adding a 2nd 18K resistor on a switch across the 1st in the tone stack which should allow me to switch between mid scoop & mid peak sound. Perhaps a conjunctive filter network across the OT primaries as well. Ideas anyone?

[NorCalTuna]

I'm guessing the choke input is intentional so I won't ask, I'm guessing you want (very) low voltage. But the resistor? Not really doing much, that choke is gonna be like 450 ohms. You have no class or bias or voltages specified so it's impossible to tell if it will lose a lil voltage on idle and TONS on load- class ab, or just sit there dropping- class a.

There is a spec on the choke, 50 ma, and everything but the heaters is running through that. Thats not a lot of current for the whole shebang; your pre will take about 10ma there, screens around the same... bias will take about half the remaining current... see where i'm going? choke input is pretty kooky. It limits your voltage as well as total current a LOT! There's nothing wrong with using a choke, but it's usually placed at the screens as large chokes are very expensive.

Doubling the caps is expensive and pointless; a 30 or 40uf cap will be at least 50% less costly than two 16uf and take half the time to install.

It'd (possibly) run and may even be interesting but it'd be nice to see some transformer specs up there. PP amps can be built tiny (princetons have champ power transformers, for instance) but the choke input and tiny current are a bit split-personality. Thats where I'd put some time into.


Oh yah, also? You have a dead short on your scheme. fix that before you have to start replacing fuses. It'll never turn on like it's drawn. Hint: the short will happen 120th of a second after the switch is thrown.

[Dave H]

As octal said there are a few drawing errors, the input and output jacks need a connection to ground, the transformer secondary windings should be separated and the junction of the two 0.022u caps in the tone stack is not normally connected to ground.

Dave H.

[Enzo]

A choke instead of a resistor in many amps may have the effect you desired, but WHERE you added your choke converts your powr supply to a "choke input" power supply, and that substantially reduces the voltage.

[mooreamps]

Would you consider using a DPST switch, for your power switch, instead of the SPST ?? It would absolutely guarantee all voltage is disconnected going into the chassis...

-g

[Wilder Amplification]

Would you consider using a DPST switch, for your power switch, instead of the SPST ?? It would absolutely guarantee all voltage is disconnected going into the chassis...

-g

So does an SPST switch on the AC mains hot leg.

[Enzo]

Only if the wall outlet is properly wired, and we can never assume that.

[kg]

i always wonder why people do a cathode biased split load inverter when it is so simple and elegant to direct couple to the preceeding stage? three less components, less phase shift (can be handy if you loop NFB) plus it allows a bit of grid current...

[mooreamps]

So does an SPST switch on the AC mains hot leg.

Not Good Enough... Go take a multimeter to some of these clubs and measure the voltage from A/C neutral to ground. You'll see for yourself.

-g

[kkillebrew]

Assuming the minor bugs mentioned above get addressed, there's gonna be hell of a lot of gain in that thing. If it were me I'd eliminate the third gain stage and use and use that triode half to create a schmidt/long tail pair phase splitter instead of the Williamson style you have. You'll get more clean headroom and better sensitivity from the power amp and you still have plenty of preamp drive to create distortion.

[Bruce / Mission Amps]

Here is the way I 'd do that power supply....

[Howlin' Mad Mac]

Redrawn with the most grievous errors fixed (I think) although input/output jacks still not showing connection to ground so you'll have to use your imagination there. See the error of my ways & fixed placement of input choke. Also once I get to the building stage (maybe this weekend) will have to see just how much voltage is showing following rectifier & either remove/adjust value of B+ dropping resistor. Oh & someone asked about transformer specs... hate to disappoint you but they're just generic "Princeton" replacements made in ChiTown.

12 Watt Dual 6V6GT.pdf
12 Watt Dual 6V6GT - Power Supply.pdf

[octal]

Re: DPST mains switch. I agree that there are some advantages to doing it this way, but I'm almost positive that in the US, switching neutrals is not UL approved. (I had to convert some Euro equipment to US spec and was required to jumper the neutral leg of the DPST switch.)

Here's a good "real world" example of the problems with switching neutrals. I had an old bench Variac which had a DPST on off switch, built in volt and ammeters. I shut the power switch on the Variac off. The voltmeter read zero, so I thought the (three prong) amp plugged into the Variac would be safe to work on. I caught a nasty AC shock from the amp. Turns out the "hot" side of the DPST switch had welded shut contacts so it was possible for everything to appear off (due to the switch still interrupting the neutral) and safe to work on, when in fact there was still 120VAC on the hot leg referenced to ground! Lesson learned.

Of course, this is probably an amp you're building for yourself, so UL compliance does not enter into it. You'll have to weigh the risks of switching one or both sides of the AC power. Safest bet is to not touch anything in an amp unless it's unplugged anyway!

Nathan