ground a grid for single ended?

[Guitarist]

Hi I just tried this, yeah I know reckless as all, and it works, so far...

I just switch in a .33@600 volt cap to shunt the grid of an el84 to ground. It is a Laney VC30 with cathode bias. The volume drops to 1/2 and you get a bit of that SE hum (which a guy would never want full time).
There is minor change in tone, not sure if I hear more of the 2'nd harmonic et al...but the volume drop is great.

OK, it was too easy so I'm thinking, why is this bad?


-By the way here some mods that worked very well:

add 2uf to V1 cathode piggy back on the stock.

C4- drive channel cap, thin up to .0022
R12- plate resistor, bleed some highs to B+ with a 680 pF cap. (I know that leaves little bandwidth but oh well. Probably remove this after the CFilter below.
V2a- grid of drive stage, add resistor/cap combo ala Marshall. I just grabbed a 280K carbon and paralleled it with a 500pF ceramic.

I will be doing the conjunctive filter later today, God willing, ala Dr. Z and The Ahola. ~.033 into 10K @ 10 watt across the OT primary. The Skot OT in these amps look pretty puny so this will be a huge improvement.

Wish I had the schematic. There is some minor heater's hum. Two black wires come out of the PT and go to a fused ground. Even if a guy were to lift the heater center tap and feed it the cathode bias, the few volts there wouldn't do much good 'don't predict.

[cbarrow7625]

I don't know that amp but, even with cathode bias, I can't imagine that it is a true Class A amp. Here is a quote from their website describing the "Class A" nature of this amp:

The VC30 generates very little cross over distortion because it runs in pure class A mode - its valves are always hot and ready to roll, which gives it that unmistakable class A sound and feel.

Well, if it actually was Class A, it would create NO crossover distortion because both tubes are conducting all the time. This is likely just a hot Class AB like a Vox, not really Class A.

That being established, what you basically now have (with one power tube grid AC shunted to ground) is an output signal that goes into cutoff for about 1/3 of the time. If you scope it your out you will see what looks like an asymmetrical clipped signal.

At low levels it may sound OK but as you crank it up (and get into the cutoff region of the bias with the drive signal) it will not sound very nice.

You won't really be hurting anything but, unless you are playing at low levels, I wouldn't expect this to sound terrific. I know this from experience, I have a post PI MV on an amp I built made from two seperate pots. I can completely shut off the signal to one power tube grid (without loading down the PI with a cap and likely creating additional "bad" distortion like you have done). At very low levels it sounds OK, but trun it up & it sounds harsh.

[kg]

Hi I just tried this, yeah I know reckless as all, and it works, so far...

I just switch in a .33@600 volt cap to shunt the grid of an el84 to ground. It is a Laney VC30 with cathode bias. The volume drops to 1/2 and you get a bit of that SE hum (which a guy would never want full time).
There is minor change in tone, not sure if I hear more of the 2'nd harmonic et al...but the volume drop is great.

OK, it was too easy so I'm thinking, why is this bad?

it's not "bad."

it might work better if you disconnected the coupling cap instead of shunted the grid to ground, to avoid loading down the PI as cbarrow stated.

[Guitarist]

I was surprised to hear that the player, often in a jazz trio, now leaves the amp on half power all the time. I know it hums more, as might he, but hey that is how he prefers it.
I remember Seymour Duncan saying he gives players what they want. Sounds like a good principle.

[Enzo]

Well, if it actually was Class A, it would create NO crossover distortion because both tubes are conducting all the time.

Only if the PI was perfectly accurate.

[cbarrow7625]

The devil is always in the details, isn't it? Do you think their marketing department would admit to any crossover distortion if the amp was really a true class A? Probably not. But, as usual, you are technically correct.

[Guitarist]

"to avoid loading down the PI as cbarrow stated."

If I did that then the grid would be floating right like an non-shorting pre but probably no sound? Is there a negative to this loading down other that tube wear?

Or do both? I'm about to do this again to my new Carvin rack amp (see kt66 post) and it has been a while since the last "SE" mod

Is the new Brad Paisley CD out yet?