How is this amp biased?

[jukelemon]

http://www.freeinfosociety.com/media.php?id=3208

1958 Gibson GA-85

Hi all.

I cannot understand how this amp is getting its bias. It should not be fixed but I don't see a Cathode Resistor.

Does the 5V6 driving the OT have anything to do with it?

Thanks all.

[tubeswell]

The 5881 bias is cathode biased from the 27k Rk.  The heaters from the 27k || with the heaters for V1 and V2 (which) are also powered from this voltage.


(Edit: thanks for being quicker on the uptake Sluckey and others)


The 5V6 is regulating the 5881s' screens

[kagliostro]

I don't see anything strange 

The 50uF + 2.7k resistor that are between the 220k resistors and the cathodes are the cathode resistor + bypass cap

What is a bit odd to me is the way they have draw the inseries connection of the heaters of V1 and V2 that are connected

to ground on one side and to the cathodes on the other side (may be a way for elevated Heaters ?) and the Standby SW

that acts in a strange way (to me seems strange) on the grids

K

[jukelemon]

Thanks all.

I guess I was looking for a smaller/high watt resistor for a Cathode resistor on a pair of 5881's i.e. like a 470 or something.

Damn this amps sounds good...very interesting tone from the "speaker box"

[HotBluePlates]

2.7kΩ seems improbably high. I'd recommend confirming with an original GA-85, or using a resistor that seems more typical if you're scratch-building.

Duh.  Sluckey has it below.

[sluckey]

2.7kΩ seems improbably high.

The bias resistor is the two series filaments of the 12AX7s. 12.6v/150mA=84Ω. Times 2 is about 168Ω, a more realistic cathode resistor value. The 2.7K is probably just there to fine tune the total resistance or maybe to allow some current to flow should one of the 12AX7 filaments opening or being pulled. That's just a guess, but the filaments are the cathode resistor. You also get nice free dc voltage for the preamp filaments.

[HotBluePlates]

2.7kΩ seems improbably high.

The bias resistor is the two series filaments of the 12AX7s. 12.6v/150mA=84Ω. ...

You got it, but ignore calculated resistance.

12AX7's with series-wired heaters (pins 4 & 5 only) = 12.6v. Two of em = 25.2vdc. At 150mA.

So the 5881's see 25.2v of bias, and the pair idles at 150mA of total cathode current (75mA per tube). Voltage chart says 22.5, which was probably the voltage the 5881's settled on to pass 150mA total.

[PRR]

> I don't see a Cathode Resistor.

(As has been said) The Cathode Resistor is the 12AX7 Heaters.

Two 12V heaters together need 25V at 150mA. Or maybe 22V at 135mA... 12AX7 is not fussy. Ah, chart shows 22.5V across both heaters.

The 2.7K sneaks another 8mA through the 5881s but not the heaters. It's a fine-trim, and probably pointless. (Might limit cathode-cap abuse if a 12AX7 is pulled.)

The 5V6 (as has been said) reduces the screen voltage. If allowed to go high, the cathode bias would have to be larger, and we'd need some odd fraction of 12AX* heaters to do the job (or an added power resistor). This loading does not need high screen voltage, so they reduced it. A resistor won't work because 6L6/5881 screen current varies a LOT.

It is _5_V6 because this tube's cathode sits way up at 350V, far too high to be heated from the groundy 6V winding. However this is reasonably close to the 425V at 5V4 heater. This is 5V so they used the 5V6. (Also 5V6 is an odd-duck and maybe they got it cheap, or by mistake.)

But overall, I think Gibson was just having a bad day.

If I squint the chart I think I read 485V at 5881 plates. However it is clearly 425V at 5V4 and 418V at 5V6. 5881 plates are surely less than the 425V at the rectifier. Might really be 405V, though that's a large OT drop for a geetar amp.

At first I thought the 5V6 was "tremolo". I have seen a scheme with huge choke and extra tube to wobble the final stage screens. However this is not that.

[kagliostro]

The Cathode Resistor is the 12AX7 Heaters

That is odd but comprehensible and intriguing  .................

Can the fact that the current on the cathode resistor (the heaters) vary, have an effect on the sound of V1 and V2 and so on the tone of the whole amp ??


---


The value of the pots with intake on the tone control is 1.8M+200k or 1.8k+200k ?? (I'm not able to read it correctly)


Looking to the other pot that are 2M I suppose the value is 1.8M+200k, but I'm not sure at all

Franco

[sluckey]

Can the fact that the current on the cathode resistor (the heaters) vary, have an effect on the sound of V1 and V2 and so on the tone of the whole amp ??

The current thru those filaments is pretty constant. There's a 50µF cap in parallel to insure that it does.

The value of the pots with intake on the tone control is 1.8M+200k or 1.8k+200k ?? (I'm not able to read it correctly)

That's correct. Another way to describe that pot would be "it's a 2meg pot with a 200K tap" or "it's a 2meg pot with a 10% tap".

[kagliostro]

it's a 2meg pot with a 200K tap

I like this version 


Thanks


Franco

[jukelemon]

Thanks all for the info!

[PRR]

> Can the fact that the current on the cathode resistor (the heaters) vary, have an effect on the sound of V1 and V2

If the load impedance gives a full-roar current that is same (or similar) to the idle current, heater power does not change.

If power stage current changes a LOT from idle to roar, in cathode-resistor bias, the bias shift WILL change the sound. So we usually avoid any large change in cathode-resistor bias.

If the power amp "never distorts much", such as music re-production, the above observations are true.

If driven to gross distortion with the varying impedance of a loudspeaker, current will vary some, and heater current will drift (slowed by 50uFd, but that's only a few milli-seconds storage). So preamp sound might change a little, but "behind" all that gross distortion in the abused output stage.

This is not a high-gain amp. This is not a Hard Rock amp. When designed, guitarists might "blip" a little distortion on the loud notes, but not distort ALL the way through a solo as is common now. With "mostly clean" guitar, the power stage current is very constant.

[kagliostro]

As always Thanks for the explanation PRR

K