Function of 470pf/220k R on v1b?

[ontariomaximus]

What is the purpose of the 470pf/220k on the grid of V1b? Thanks in advance.

[talbany]

Changes the frequency response.. What I hear is a tighter more focused low end..Also cuts back on the gain a bit as well as smooths out the overall tone....


Tony VVT

[dogears]

Bizarre stuff..... The posts seem to have disappeared!!!

To recap, a grid resistor with bypass cap will preemphasize the highs. You restore highs lost to miller effect. More clarity and articulation.

Tony points out that no grid resistor at all or a small size grid resistor results in more gain.

[Structo]

Man, the server must have crashed again.

There were about 6 posts with graphs and info lost.

[wjdunham]

Here's the plots that were lost - they show graphically what's already been explained. Three plots are - constant grid resistor, vary the cap from 100pf to 500pf, constant cap, vary the resistor from 0 to 500k (shows how the cap pins the HF response gain), and no bypass cap vary the grid resistor from 0 to 500k (shows how the gain changes). Overall effect of the cap is to add some "sparkle" between 1 and 3KHz...

[wjdunham]

One other note that was lost - these plots were generated by a spice model of both stages of the clean channel of a d-style amp. Measurements taken at the plate of V1B.
Bill

[ontariomaximus]

Thanks for all your input fellas. Now what is the function of the 47pf/470k network in front of V2A? A little more gain from 5khz and up? That's what it seems like, kinda opposite the previous filter discussed.

[Structo]

What layout or schematic are you looking at?

[ontariomaximus]

ODS 101 HRM

[talbany]

The grid resistor on the preamp stages typically ranges from 0 to 68K, although very large values, such as 470K, are sometimes used in high-gain preamps to shape the frequency response (Set up the knee) and prevent "blocking" distortion in the preamp section under heavy overdrive conditions. The Miller capacitance (As Scott stated)of a typical 12AX7 is around 151pF, so the upper frequency response -3dB cutoff point of a stage using a 68K grid resistor is around 15.5kHz. The frequency response drops to around 2.2kHz if a 470k grid resistor is used. This "free" response rolloff can be used to tame the "buzziness" of high-gain preamp stages without having to add additional rolloff capacitors. .Basic function of Grid resistors in preamp tube amps..Grid resistors also help prevent stray radio signals from entering the grid (Ever see Spinal Tap!! Sex Farm) Nigel could have used some grid resistors that night... He more than likley took them out to squeeze out more gain..This preamp tube goes to 11...LOL! AM signals or powerful CB mainly

Tony VVT

[jelle]

Tony,

I think he means the HRM OD entrance network.

Yes this adds highs to the OD entrance.

Jelle

[talbany]

Tony,

I think he means the HRM OD entrance network. Very Happy

Yes this adds highs to the OD entrance.

Jelle

Yeah I got that...Doesn't matter... the math still applies!!!

Jelle

Check your inbox in a few

[dogears]

The 470K that feeds the OD is different math. Miller effect is not coming into play since the 470K is not a grid resistor.

Tony,

I think he means the HRM OD entrance network. Very Happy

Yes this adds highs to the OD entrance.

Jelle

Yeah I got that...Doesn't matter... the math still applies!!!

Jelle

Check your inbox in a few

[ontariomaximus]

The 47pf across the 470k is essentially a bright cap. The combination of the 470k and 220k in series and the od entrance pot creates a large gain drop (it's a voltage divider), except for high frequencies above 5k, where the drop is about 9db less, due to the 47pf cap. I just find it interesting it's near the opposite of the previous filter where the gain was in the 2khz to 5khz range

[talbany]

What my ears hear is higher series resistance = higher frequency roll off. The math is probability different there since it's mainly used to set up the knee..!! .. (I assume it raises the Miller capacitance, changes input impedance, altering the 22 giggawatts the flux capacitor needs to produce those fragile harmonics extracted in a non crystal lattice thermonuclear device)LOL!!...
Raising the value of OD trim entrance pot (not the tail) also rolls off highs even set low.. To my ears... This becomes obvious If you raise the tail the opposite occurs.. More gain and the tone gets brighter...In some cases avoiding the need for the 47pf cap...(The 47pf allows highs to pass around the 470k. But, at a really high knee, without it I hear more low end and less harmonic content)...

Taking it a step further...In some of the Non-HRM's I like the sound of a 250Ka drive pot.. The increased resistance creates a HUGE!! roll off..in some amps avoiding the need for the snubbers..or the famous Treble bleed..If you use the 250k drive and like a lot of gain you may need to lower the .01 coupling cap V2a..(This is especially true in lower powered 50 watters or D-Lite) the low end can get out of control.. I built a 2 KT-88 Non HRM with Traynor YBA-1a transformers with a 250k drive axed the stubbers and left the .01 and the thing ROCKS!!...

Tony VVT