Out go the cathode's resistor and cåp - In goes the BATTERY

[zhyla]

I don't have anything technical to offer in this thread but I just read the other day that early tube amplifiers actually used batteries to supply bias (I *think* it was in the context of power tubes). It said they last for years that way because they don't end up supplying much current. Other bias techniques obsoleted this (my guess is batteries were expensive back then).

Maybe batteries can be like bell bottoms for amps?

[Enzo]

Well, early tube amplifiers did use batteries for bias. They also used batteries for the B+, and they used batteries for the heaters. They used batteries for the whole thing. it was only later as houses were wired for power that they changed over to line voltage.

The heaters ran off the A battery, the bias battery was the C battery ( and yes it would last a lot longer than the others), and the high voltage was the B battery, which is why we refer to it as B+ today. In a lot of circuits, there was a large rheostat in series with the heaters, in fact there were rheostats for individual tubes. They would turn the gain of the tube up and down by varying its heater voltage. That was in the day of direct heated cathodes - the heater WAS the cathode.

[mooreamps]

No it does not. The thermal noise generated by the cathode resistor across the very high 1 meg ohm input impedance of the gain stage, is the major contributor to the hiss factor of the gain stage. I know this because I am currently writing a doc thesis on the subject. Secondly, I would imagine quite the opposite, with the open bandwidth of the led, allows for a fuller , fatter tone and quite honestly my intent of the gain stage is a flat frequency response; thus allowing any tone shaping to be done inside the tone stacks.

-g

[Chrome Dinette]

Instead of using batteries, couldn't one just build a heavily filtered 1.5v supply, and never have to worry about charging, leaking, etc?

[Guitarist]

For DIY purposes it is really easy to swap in a couple AAAs. If it means lower noise and higher gain (we'll soon find out) then that is amazing. Batteries are in all kinds of gear out there; depending on the amp, heat may not be a serious issue. Hey, I do many wild and dangerous things all the time - believe me I wont lose any sleep over an improbability like this ;]

There is the mentioned tradeoff of not having the tone shaping you get with .47uF and the like.

[mooreamps]

Instead of using batteries, couldn't one just build a heavily filtered 1.5v supply, and never have to worry about charging, leaking, etc?

You could, and it would not need to be heavily filtered since it feeds a 1 meg ohm input impedance {if applied to the grid leak} ; but an led gives the same effect.

-g

[Chuck H]

Still, if someone intended to use multiple stages with this arrangement it would be more cost effective to use a 1.5 volt supply than multiple LED's. Because, as you noted, it wouldn't need to be hevily filtered. But from what I've built I can only see the occasional need for such a circuit. An effects loop here, reverb recovery or summing amp there... So an LED makes more sense in these lower use applications you see in guitar amps.

Chuck

[mooreamps]

As a designer, it's your call. I use the led's in all my gain stages, including the power tubes ; especially for the power tubes. They make for great bite lamps.

-g

[Chuck H]

You know whats loads of fun for me (you care, right?) is the little hints of how different designers have come up with ways to make preamps "behave" and power tubes "perform".

I haven't used LED's in the power amp yet. But I may. The amps I've been working lately are of the 2XEL84 type and I'm using a cathode bias with a zener to "fix" the bias at the onset of distortion. + the drive limiter made popular by Paul Ruby and a tiny conjunctive filter. The combination allows me to drive the piss out of a pair of EL84's with only a tiny (inaudible) amount of crossover distortion. I'm actually thinking I may want more crossover distortion to add a more familiar character and some swirl. But I really have these tubes jumping through hoops for me right now. Fun. I should share it with the guys on the AX84 site but it's kind of "proprietary" right now.

Chuck

[mooreamps]

You know whats loads of fun for me (you care, right?) is the little hints of how different designers have come up with ways to make preamps "behave" and power tubes "perform".

I haven't used LED's in the power amp yet. But I may.

Chuck

Not really. I've had a few too many old fender combo's come through my shop with bad power tubes, and the first thing I do is add the PA bite lamps to them. Having bite for preamp tubes is OK, but it seems to be the PA tubes that go out first ; and when they do, it's usually only one, and just just don't know what's wrong with it ; without the benefit of built in test.

-g

[CRU JONES]

I like the battery idea!

Ironically I was about to wire in a -8 v supply in my 6sn7 PA tonite... I think I won't, just quite yet... Thanks Guitarist! I'm stoked!

I have tried very hard to nail down safe trickle charging currents for NiMH and have read literature from battery manufacturers that say between 1/16 and 1/25 of capacity.

so I have a 9.6 V NiMH that is rated at 325mAH.
1/16 of 325maH is 20.3mA.
300V (B+) * 20.3 = 6 watts

It's still only trickle charging at 6Watts of output power!

I wonder if the battery will die if I don't play loud enough?

well... Off to solder a 9v battery snap into a tube amp!
Cru

[Guitarist]

I just swapped in two AAA in my Carvin TS100 stereo power map (anyone have a schematic?). Now I have a fair bit of noise and hum (especially on one channel). This may be a ground loop -pc board layout is skewed towards on channel- or the 22K cathode resistance it replaced is actually required Perhaps the battery requires star grounding on its own tube circuit. But it does sound better. Maaaaan...

[6267]

Sounds like a good application for a supercap paralleled with some bypass caps on a 1.5v supply. The supercap will act like a battery, with no thermal noise. The bypass cap will remove any hash and "fix" any esr problems at higher frequencies the supercap might have. They can charge/discharge millions of times before mean failure, I believe, maybe hundreds of thousands. Most have a max rating around 1.5-2v, perfect for first stage preamp which is often around 1v. The 1.5v supply will keep this "battery" charged. Hell, you could even bypass a cathode resistor with it and get a stable DC supply for even fairly extreme transients, like unplugging the tube and plugging it back in .

[mooreamps]

But it does sound better. Maaaaan...

Ya, funny how that works. Then, you just need to tune the input and output coupling to handle chords, and that's about it.


-g

[Guitarist]

I'm going to add a 6.8K resistor in series to reign back the gain a bit.

With rechargeable cells: would it work to power the heaters with a digital power supply such as this Nokia 5.7 volt @ 800mA I've got here and add some 9 Volts r.ch'ables to the B+ as well? Meaning will the heaters power up the whole tube this way? I'm itchin' to add a Pete Cørnish style pedal buffer to the pedalboard. Such a preamp doesn't require much gain so perhaps 10 and 9 Volts yielding 90Volts across a 67K plate resistor and a 12at7 would be kickin. But that adds up to 10" x 5.5" (250MM x 130mm) in real estate. Approx. to and a bit wider than an Ernie Ball volume pedal, dang.