Too high B+ for Alembic/BF-style preamp?

[bluemonk]

I'm building a preamp based upon Alembic F-2B (basically a one-tube AB763 design). It is said to work at 300V B+.
My PS gives 375V+ and a plate voltage of around 250V.

Is it too much or is it safe?
Maybe good to tame it by a series resistor?

Thanks in advance.

[loudthud]

It should work OK at that voltage as long as all capacitors are within their rating. A series resistor in the power supply will take the edge off and mellow the tone a little. IIRC there is just one 12AX7, something like 22K to 39K should work.

[bluemonk]

Hello Loudthud. Filter caps are near their border line (rated 400V)... though I noticed that safer parts are most in our DIYer's worries than in commercial branded gear.
I could get rid of voltage doubler and get about 260V B+ or so.
Would you suggest it?

Thank you for your kind and helpful attention.
Best regards.

[ThermionicScott]

Several blackface amps had a B+ of more than 400V feeding preamp stages. (Even more had 375V+.) I wouldn't worry about it. Maybe use 450V-rated caps if it makes you feel better.

- Scott

[bluemonk]

A series resistor in the power supply will take the edge off and mellow the tone a little.

Something like a rectifier tube sag?
Thanks.

[ThermionicScott]

Something like a rectifier tube sag?
Thanks.

The series resistance resembles the effect a tube rectifier has, but remember that preamp sections draw a nearly constant amount of current, regardless of the attack of the note. So you won't get any neat audible "touch-sensitive" effects here.

- Scott

[bluemonk]

preamp sections draw a nearly constant amount of current, regardless of the attack of the note. So you won't get any neat audible "touch-sensitive" effects here.
- Scott

Yes you're right, I just tried it. Played a few minutes and that series resistor in PS makes very little (or no) difference. Just first impression.
That preamp sound nice but has a very high output volume. I have no place for an output pot.
How to tame output?
A couple of padding resistors (which values)?

[ThermionicScott]

Yes you're right, I just tried it. Played a few minutes and that series resistor in PS makes very little (or no) difference. Just first impression.
That preamp sound nice but has a very high output volume. I have no place for an output pot.
How to tame output?
A couple of padding resistors (which values)?

I'm looking at the Alembic F-2B schematic... did you leave off the volume pot?



If you have one, you could try a 12AY7 or 5751 instead of the 12AX7.

- Scott

[bluemonk]

I'm looking at the Alembic F-2B schematic... did you leave off the volume pot?

I saw it and put in the right place
(On the other hand should it be a decoupling resistor before C8? I also believe than mid pot value is wrong and 50uF bypass caps is too much).

The issue I was referring to is not the overall gain (different tube will help) but the high output volume which I'd like to tame by a switchable -10db or -20db switch at output or input (or I will have to find place for a Master pot).

Thanks.

[ThermionicScott]

I saw it and put in the right place
(On the other hand should it be a decoupling resistor before C8?

No, that 300V is for both stages. They just needed to put the filter cap somewhere on the schematic.

I also believe than mid pot value is wrong and 50uF bypass caps is too much).

25k would make more sense for the mid pot, but 50uF for the cathode caps isn't excessive, it's just enough to make sure the cathode resistor is bypassed for all frequencies, not unusual for hi-fi.

The issue I was referring to is not the overall gain (different tube will help) but the high output volume which I'd like to tame by a switchable -10db or -20db switch at output or input (or I will have to find place for a Master pot).

Thanks.

Shouldn't turning down the volume knob get your "output volume" to the right level? If you want a "master volume" for it, I'd replace R9 with a 1MA pot.

- Scott

[bluemonk]

No, that 300V is for both stages. They just needed to put the filter cap somewhere on the schematic.

There are three filter caps before C8, each stage decoupled by 1k5 resistor. So I was wondering about the lack of a resistor before C8.
Anyway "original" schematic doesn't show that C8 cap and perhaps it isn't needed (I haven't put it yet): http://www.geofex.com/FX_images/alembpre.gif.

Shouldn't turning down the volume knob get your "output volume" to the right level?

Yes, volume knob works correctly but it has a very short action before saturation (especially when connected to the input of another amp and even power amp). Thus I am thinking about a -10db switch. I believe I could add a voltage divider at output but what resistor range should I use? 10k-1k, 100k-10k, 470k-47k, 10M-1M?

Thanks for your explanations.

[Steve Conner]

The voltage divider at the output is tricky. It needs to be able to drive the cable capacitance, so you want the values as small as possible.

But to minimize loading on the plate, you want them as big as possible.

100k/10k seems like a reasonable compromise to me.

To be pedantic, that gives a voltage division of 1/11 which is 20.8dB of attenuation, not 10. But that's probably about what you need, though the answer will depend on how much "tube drive" you want.

If I was building this, I'd probably use a master volume pot. You can regulate the amount of tube drive that way: to get more, you turn the volume up and the master down, and to get less, turn the volume down and the master up.

I'm working on a JFET preamp somewhat like the F-2B just now, and that's what it has. Bass, treble, volume and master.

[bluemonk]

Well, a pot in place of R9 is the best way.
Thanks a lot