Correct cap to use here? Bias

[Merlinb]

What are the disadvantages to using a capacitor coupled bias supply?

You can't draw much current from it, and you always need at least some load current on the HV winding.

[R.G.]

FWIW... What are the disadvantages to using a capacitor coupled bias supply? I think it was discussed here once, but I can't locate the thread. I actually have a project I'd like to use it on. This assuming that the cap coupled bias is the ONLY way to make a bias supply with a bridge rectifier.

Since the power is microscopic, one way to do it is to put a one- or two-VA transformer on the heater supply, but run backwards. 120Vac to 6Vac transformers in small powers are easy to find. For instance, this one is in stock at Mouser, costs under $6.00, and is about the size of an ice cube. It would give you out about 100Vac at 10-20ma for a bias supply when connected "backwards" with its secondary running from the 6.3V heaters. A full wave rectifier and a filter cap and you have a bias supply. No funny capacitive current limiters or dividers needed.

[J M Fahey]

Fine. Will work, no doubt.
Yet the capacitor is simpler and cheaper; Mouser sells overkill .047x1250V Panasonic capacitors for $1.40
Only 2 terminals to solder and can be easily added to any PCB design, even easier in eyelet or turret boards.
In fact, it's the same as using the classic 220K resistor, only that it's AC coupled and works on any rectifier configuration.
No wonder all above mentioned comercial manufacturers have been using them for ages.
I wind my own transformers and even so find it a (not so minor) disturbance in having to reset the machine for the new diameter and then add extra 180 turns of fine wire , (easier to break) just to have a bias tap.
Now, I agree that if you have some other use for those -50V , as in powering an SS EQ (Mesa Boogie) or preamp, then it's more justified.

[Chuck H]

Thanks guys. What I have is an amp project for a friend. I thought I had all the parts in my salvaged inventory but the PT is from an old Traynor PA (Traynor=bridge rectifier and capacitor bias supply). It's good for a pair of EL34's and four preamp tubes. I'll be adding a fifth preamp tube as is. That's already 4.5A on the 6.3V wind. Not sure I want to tax that wind any more or even have my bias supply depend on it. I know the capacitor coupled bias has been phoeyd here before but I've never actually heard of any problems related to the cap coupling or low current capability. Any trouble using a bias vary trem with cap coupled bias that I should know about??? TIA

[J M Fahey]

Well, the bias modulation tremolo *does* add extra load, and not only on the bias supply but also in the main supply, since it causes *huge* variations in power tube current.
I find it a very primitive and crude method of tremolo generation; maybe justified when there was no other choice, but after the LDR was invented .....
Even if it supplied some tiny extra sound quality (whatever that might be) compared to modern ones, I don't find it worth the trouble.
Others may differ, of course, and I respect that.

[Chuck H]

I was thinking the same thing. That's why I asked. I'm looking into modulating a preamp tube cathode instead. Unless that sounds bad.?. I've never been a trem guy so I haven't researched it.

[Intergalactic Tourister]

So I'm wondering if closer inspection will show the cap in the original post to be .022 ?

Yes, it's a .022 cap.

[Gingertube]

Modulating a tube preamp cathode instead? - Like this vintage Ozzie Moody GA40 Amp I traced.

http://www.ozvalveamps.org/moody/moo...tremolocct.gif
http://www.ozvalveamps.org/moody/moo...tremosccct.gif

Cheers,
Ian

[Chuck H]

Ha.!. That looks remarkably like the schem I penciled just an hour ago. Except I did a typical cathode follower circuit with Rk on a split resistance with the bottom end bypassed followed by a series resistor. My idea being that I could keep the modulated triodes cathode circuit more independant of the trem circuit when the trem is off. Does anyone know how it sounds?

[Gingertube]

"That looks remarkably like the schem I peniled just an hour ago".

You want to watch that - you could go blind.

I recall that it sounded OK but was'nt in the league of an AC30 Tremlo/Vibrato.
I thought it lacked depth. I've seen similar on an old Tiesco (running all 6AV6) and in that case I remember that there was a preset pot which needed setting very carefully to stop it blocking instead of modulating.

Cheers,
Ian

[Chuck H]

Thanks. Post edited BTW

I won't need a lot of depth. It's sort of fun as an instant gratification effect to make the trem OFF/ON/OFF/ON, whomp, whomp, whomp... But who ever uses it like that? I'll try modulating a preamp tube, but leave my design open for a change if needed. I'll even leave room in the design to add another transformer if I decide to go with power tube bias vary trem (sorry JM, gotta go with what I know). Most people seem to think the power tube bias vary trem is the best. Like JM, I think it's a sort of clumbsy way to do it. Like tilting the room to pour water into your glass instead of just tilting the bottle.

[J M Fahey]

[Chuck H]

Oh come on!
Tell me you guys don't do it too?