capacitor across dropping resistor - VOX AC15TBX

[Intripped]

hello everyone,
i'm servicing a VOX AC15TBX, and found inside some little mods previously made:

There are some HV ceramic capacitor (unknown value) in parallel with some smoothing/decoupling/bypassing electrolytics (C1, C2 and C4); and a 10nF HV ceramic capacitor across the dropping resistor R6.

while the caps across the electros don't bother me, i have doubts about that 10nF cap across R6. what is it for? does it alter the performance of the amp in any way? should i remove it?

thanks,
Cristiano

[Helmholtz]

Generally a cap in parallel with a dropper resistor is a bad idea, because it impairs (HF) filtering and decoupling.

Ceramic caps across filter caps do the opposite, i.e. they can improve HF performance of the supply, but are typically not needed in tube amps.

[Intripped]

Mmm ok, so probably I should remove it.
Do you have an idea of why they put it there? Perhaps trying to solve an HF oscillation issue?
Do you think that this capacitor could impact on the sound of the amp?

Thanks

[Helmholtz]

I would remove it.
I's more likely to support HF oscillation than to stop it.
Has no (beneficial) influence on sound.

Not everybody who mods amps knows electronics..

[Intripped]

i will proceed and remove it, thanks a lot!

Cristiano

[Helmholtz]

I think the cap is/was for a high frequency cut- maybe or maybe not to prevent oscillation. A small value cap across the plate resistor is almost the same as a cap from that point to ground because the node filter (electrolytic) cap is very low impedance (AC) to ground.

It's not connected to a plate but across a PS dropper, both sides connecting to PS filter caps/nodes.

Edit: Ok, it does connect to the plate of the CF but there's no signal.

[The Dude]

Thanks for pointing that out! You're right. I guess I didn't look close enough at the schematic. I initially thought it was across a plate resistor. I'll delete my post above to avoid confusion.

[SoulFetish]

I would think that’s almost certain to be a mistake and was probably meant to bypass R7

[Helmholtz]

I would think that’s almost certain to be a mistake and was probably meant to bypass R7

Ever tried a 10n cap across a 100k plate resistor?
(LP corner frequency might be around 400Hz.)

[SoulFetish]

Ever tried a 10n cap across a 100k plate resistor?
(LP corner frequency might be around 400Hz.)

I probably wouldn’t put it it my amp, but it isn’t a completely outrageous value. Especially considering it driving a DC coupled grid will minimum input capacitance and no Miller effect.
of course, I’m not arguing the merits of the design, just that it’s a more reasonable location than where it ended up

[Helmholtz]

. Especially considering it driving a DC coupled grid will minimum input capacitance and no Miller effect.

Miller capacitance of gain stages is typically between 100pF and 150pF resulting in corner frequencies above 10kHz with typical cicuit impedances.

A corner frequency of 400Hz would mean signal being down by a factor 2 at 800Hz, a factor 4 at 1.6 kHz and a factor 10 (20dB) at 4kHz.

[SoulFetish]

Miller capacitance of gain stages is typically between 100pF and 150pF resulting in corner frequencies above 10kHz with typical cicuit impedances.

right.

In that gain stage, if the 2.2k cathode resistor is bypassed, then with the 100k resistor bypassed with a .01µF cap, the gain at 1kHz is around 24 (≈28dB). The gain at 400Hz is about 39 (32dB). That is not outrageous when considering bandwidth relevant to guitar. Especially when you factor in amps featuring several gain stages resulting in gains at 10kHz and above in the thousands. But, I reiterate, I'm not advocating for this approach. You dig?

[J M Fahey]

hello everyone,
i'm servicing a VOX AC15TBX, and found inside some little mods previously made:

There are some HV ceramic capacitor (unknown value) in parallel with some smoothing/decoupling/bypassing electrolytics (C1, C2 and C4); and a 10nF HV ceramic capacitor across the dropping resistor R6.

while the caps across the electros don't bother me, i have doubts about that 10nF cap across R6. what is it for? does it alter the performance of the amp in any way? should i remove it?

thanks,
Cristiano

Clearly not original and WRONG.

Not surprised, the Net is chock full of brain dead "mods", NO SCHEMATICS INVOLVED,to the tune of:
"replace FR46 by 10k, R102 by 1M, C44 by 120pF, clip C33 legs ..." and so on.

Since even exact same schematics change parts ID version to version,simply by counting from a different start point, the potential for damage is IMMENSE.

Or maybe a YouTube University graduate thought: ".001uF is often paralleled to 100k plate caps to tame oscillation ... then 10k needs .01uF ... after all it IS in series with plate"

Oh well.

[Intripped]

Who Knows...
what is sure, is that the amp now has not that 10n cap installed anymore, and is working fine and sounds very good!

There were many, MANY broken solder joints on all the 3 PCBs, and they could cause every sort of issues.
ALL the solderings have been redone now, but probably in the past someone tried to solve one of these issues, not addressing the real cause.

thanks again guys for your help and your time,
Cristiano