Ampeg B15 high plate voltage?

[pontiacpete]

I was wondering if the plate voltage on a b15 at about 500v is too high? The schematic calls for around 430v. I biased the tubes to 27.5 and the grid voltage is at -52.

[Gtr_tech]

As long as the voltage the filter caps are seeing is not over thier rated voltage, I wouldn't worry about it. Bias within safe operating dissapation and use *good* 6L6s.

[pontiacpete]

I replaced the first filter cap that was a 30uf / 600v with a 40uf/500v cap. I was hoping to do a cap job but finding 600v caps, there aren't many options. If you were doing a cap job on one of these would you put 2 47uf/500 in totem for that first one or some variation like 2 100uf/350s?
I changed the 47K resistor that is paralleled with 100uf/100 bias cap. With it the 6l6s ran pretty hot at over 35ma but the plate voltage was lower at 479v. So I changed the 47k to a 51k and that got me the 27ma. Should I go back to running the tubes hotter?
Anyway thanks for help gtr tech

[txstrat]

35mA at 479volts is about 60% acording to the weber bias calculator (Weber Bias Calculator). Not hot IMHO.
That way you could leave the 500v caps in there. A totem pole of 2x100uF/350v would give you a total of 50uF at 700v. Don't forget the parallel resistors.

[pontiacpete]

thanks txstrat, I seem to get hung up on this is over and over again. Why is there such difference between the the weber bias calculator and say this bias chart: http://www.diyguitarist.com/Images/BiasChart-6L6.jpg and for that matter the house of jim chart?

Anyway this thread had me back at the aiken article on why 70%? for a re read, Idle Current Biasing - Why 70 percent?

[MWJB]

To bias to 70% dissipation we first have to know the max plate dissipation of the tube in question....not easy with current production tubes as some listed plate dissipations are plainly wrong, most are unknown, some hedge their bets with "30W tube if you run it at low B+". So, a safe-ish assumption is that, unless you have reliable testimony otherwise, that 25W is a realistic plate dissipation for EL34/6L6/5881 style tubes.

In reality, the tube will function "normally" over quite a range of plate dissipation, fine tuning exact plate current depends on taste. Find your 70% mark, then try lowering current & running back up to 70% again...play in between and decide for yourself what sounds best. At lower settings make sure that tube & cap voltage limits are not exceeded. I know harp players, looking for lower gain/more fuzz who like to run idle dissipation as low as 15% in some amps, in other applications someone might want a slightly cleaner tone and find 50% to 60% works better for them. Some old amps have had the wall AC rise since they were designed & may end up running more like 80-90% (a bit of a stretch for bigger tubes like 6L6/EL34but some sturdier EL84 & 6V6 at lower voltages may take it fine? - seek specific advice if contemplating exceeding 70%!).

Most amp manufacturers did not supply current figures, just negative voltage figures, plus they often biased tubes just cool enough to ensure they didn't expire under warranty.

To be told that there is a "correct" spot for your idle dissipation, that does not concur with what you are hearing (as long as it's not overstressing anything) is akin to being told what treble and bass settings you "must" use.

[pontiacpete]

Thanks for that explanation, MWJB , very helpful.

[Rhodesplyr]

There are lots of metallized polypropylene motor-run capacitors that will take >600VDC--like the ones rated 370VAC. (They will take a lot more DC than AC.) Of course, they may be physically too large. The manufacturers can usually tell you the DC rating if you ask.

Cornell-Dubilier - 21FD3730-F - Passive Components - Capacitors - Allied Electronics

I've always wondered quite how manufacturers managed a 600V electrolytic capacitor. Most accounts I've read say that the basic chemistry of electrolytics gets dicey over 450V.

Depending on a number of factors, you can cause B+ voltage to increase by increasing the capacitance of the first filter stage. If you put two 68uF caps in series, you'd get 34uF.

*NOTE: Motor-run and Motor-start capacitors are not the same. Only Motor-run types are appropriate for power supply use.

[Steve Conner]

Maybe all of the 600V ones were two units in series internally. A rubber bung in both ends is a giveaway for that.

The highest I've seen in a single can is 500V.

The motor run caps are BIG compared to even old electrolytics. But I have seen amps built with them. Hi-fi guys love them, on the grounds that the bigger and more expensive a component is, the more authentic the sound reproduction.

RG also liked them as part of his "Immortal Amplifier" concept. They don't wear out like electrolytics do.

[Rhodesplyr]

The motor run caps are BIG compared to even old electrolytics. But I have seen amps built with them. Hi-fi guys love them, on the grounds that the bigger and more expensive a component is, the more authentic the sound reproduction.

RG also liked them as part of his "Immortal Amplifier" concept. They don't wear out like electrolytics do.

These days, if you use Sprague Atoms, the price difference between them and motor run caps is minor :-) Of course, that may simply reflect that Atoms are over-priced....

I like the indefinite lifespan and the low ESR of the film caps. I confess, my main Hi-Fi tube amp (a VTA-70 modded ST-70) has an all-polypropylene power supply. It's still smaller and lighter than most guitar heads. I've only used the film caps for Hi-Fi--thus far--with the exception of rebuilding Leslie speaker crossovers with them.