DC Heaters..

[didit]

Works if you have that 14VAC available. Going to 12V means you can power three 12AX7's within the 1A capability of the LM7812.

You need to have a custom wound transformer and plan ahead to get that 14V tap.

A 12.6VAC "filament" winding should be sufficient. Depends on minimum Vin of your regulator & low dropout variants run just ~2V above target DC so 14.5VDC, conservatively requiring ~11.5VAC at bridge input (11.5 x 1.3 = ~15) to handle reasonable but low line voltages.

For example https://www.onsemi.com/pub/Collateral/NCP7800E-D.PDF.

Best .. Ian

[romberg]

Soooo,
This is a very interesting subject! Now, please allow the dunce to ask the burning dumb question: What is the most convenient and practical way to convert (at least for the preamp tubes) the AC heater circuit to DC. Even as a dunce, it seems fairly clear that heating with DC could alleviate some of the noise/hum issues associated with using AC for the purpose.
Please Inform The Uninformed?
Gene

An alternative to full DC heaters is to simply elevate the existing AC heaters by referencing them to a DC offset above ground. This will saturate the current (signal) that may be leaking between the heater filaments and the cathode effectively removing the noise. Elevated heaters have the advantage of being much easier to implement as you can create a reference supply off of the B+ rail with a simple voltage divider and cap. No need to supply enough current to drive the heaters (AC still does that). So, cheap and easy to try.

Merlin covers this in his book. The section on this he has online:

http://www.valvewizard.co.uk/heater.html

Full DC heaters have the lowest noise potential. But elevated heaters are worth trying before going full on DC in my opinion.

Mike

[Tony Bones]

I'll add that if the DC supply isn't well filtered then it can inject a lot of high frequency crap. I find that more objectionable than 60Hz hum.

[Cathode Ray]

I'll add that if the DC supply isn't well filtered then it can inject a lot of high frequency crap. I find that more objectionable than 60Hz hum.

I'm guessing that explains a question I had about why all the caps in this little circuit.

[Cathode Ray]

So, in an amp with DC heaters for the pre-amp tubes, why would the phase inverter be on AC heaters

I understand this tube isn't doing any amplification... but that just seems odd.


Thanks guys

[martin manning]

Depends on the type of PI. A typical Fender/Marshall LTP has plenty of gain, just not as much as a preamp stage. The thing is that the signal to noise ratio is lowest at the input, so most of the benefit from noise reduction is realized in the first couple of stages.

[Cathode Ray]

Depends on the type of PI. A typical Fender/Marshall LTP has plenty of gain, just not as much as a preamp stage. The thing is that the signal to noise ratio is lowest at the input, so most of the benefit from noise reduction is realized in the first couple of stages.

Thanks Martin

Yeah, it's a hot-rodded 2204 circuit. 3 pre-amp tubes.

[pompeiisneaks]

Yeah everything I've read/heard is that IF you use DC Heaters they're really mostly needed for the first gain stage or two. After that the signal is so strong it rejects most ac heater noise. OTOH Shielded wire on the signal path near the higher gain stages is probably more effective at rejecting heater hum than most anything.

~Phil

[Cathode Ray]

Yeah everything I've read/heard is that IF you use DC Heaters they're really mostly needed for the first gain stage or two. After that the signal is so strong it rejects most ac heater noise. OTOH Shielded wire on the signal path near the higher gain stages is probably more effective at rejecting heater hum than most anything.

~Phil

Makes sense.

Appreciate all the in-depth help, guys