ODD Request....install 0.1uF cap in series with Guitar Jack inputs to preamp tubes

[nevetslab]

This past month, while what began as a simple minor mechanical fix to the Power and Standby Switches on a Hiwatt Little Rig head, they decided a 0.1uF film cap was necessary in series with the input grid of the 1st tube stage of their amp. I did notice from their photos they had mistakenly eliminated the 1M Grid Bias resistor to those tubes, so they corrected that, but still insisting that that cap is necessary. Strange.



I've not seen this needed before, and don't find it on any of the conventional Tube Amps we all work on. Claimed to prevent what tiny DC potential there is at the grid from going back to the guitar. The Volume control that follows this tube stage, in their 1/2W Output setting, you can hear the volume control making noise. This was their 'fix'. Oh well......that's what was requested.

[mhuss]

That just seems silly. I guess it would further protect against DC offset from a driving pedal or active guitar, but how often do we run into that problem?

[The Dude]

I agree, that's just goofy.

[Leo_Gnardo]

There are primitive guitar amps that have grounded cathode preamps, and block DC offset generated at grid from the guitar by passing the signal thru a capacitor. There's always a resistor from control grid to ground too, typically 1M to 5Mohms. Just worked on an early 50's Fender Pro a couple days ago, had that circuit.

[loudthud]

A low plate Voltage on a preamp tube causes grid current. If the 1/2W mode messes with the B+ in the preamp, that's why the cap becomes necessary.

[mozz]

Are the cathodes grounded or is there a resistor there?

[nevetslab]

Are the cathodes grounded or is there a resistor there?

Each have 2.2k resistors. The Brilliant Ch has a 47nF cap across it, noting on the other input channel.

[nevetslab]

A low plate Voltage on a preamp tube causes grid current. If the 1/2W mode messes with the B+ in the preamp, that's why the cap becomes necessary.

As of yet, I still don't have a schematic on the 1/2W Power Mode. It does lower the power supply level....might just be on the output stage, though. I'm only going to have this amp here for another couple days so I might get a chance to look further.

I agree....strange to see input caps suddenly put into the product. They don't do that on any of the other products.

[Enzo]

Grounded cathodes. Look up "grid leak bias".

[Greg Robinson]

Insurance against gassy tubes, or as speculated above, B+ manipulation for 1/2 power mode possibly.
But I'd definitely want a high value resistor to ground on the other side of the cap rather than leaving it floating during pedal switching, that's just asking for pops to be louder than necessary. 10Meg would solve that.

[nevetslab]

A low plate Voltage on a preamp tube causes grid current. If the 1/2W mode messes with the B+ in the preamp, that's why the cap becomes necessary.

Good call. I just finished up with the last mod's on this amp, and powered it back up. The 1/2W Output Mode both lowers the Output Stage supply voltages as well as the Preamp stages.

The Preamp supplies drop from 237VDC down to 108VDC in the low power mode. Plate voltage on the input tube drops from 130VDC to 66VDC, and the voltage at the input grids change from -0.7mVDC to -0.7VDC. I hadn't noticed that, nor looked at the voltages on the amp, not having a schematic for it....a project that might have happened at some point.

I just looked at the other Little Rig amp, which doesn't have this mod, and the 1/2W Output mode has -0.4VDC & -0.2VDC on the input jacks, which then drop to less than 0.7mVDC in the 20W mode.

Greg Robinson suggested adding 1M to 10M at the input jacks ahead of the coupling caps to discharge them. I plugged my Fluke DMM into the jacks and powered it up in both power modes, and it has a 10M input resistance. You'll still see charging voltage pulse that does drop down...still in the moderate mV range until it fully discharges to 0VDC.

So, that does justify the input caps that I was told to add, as they have just made that change at the new factory up in Canada.

[nevetslab]

Insurance against gassy tubes, or as speculated above, B+ manipulation for 1/2 power mode possibly.
But I'd definitely want a high value resistor to ground on the other side of the cap rather than leaving it floating during pedal switching, that's just asking for pops to be louder than necessary. 10Meg would solve that.

Good suggestion on that. I'll pass that along to them.