OD trim voltage value

[stiltamp]

Hi,

How about is the maximal voltage value at the trim potentiometer taper for best od tone?

[bluesfendermanblues]

Hi,

How about is the maximal voltage value at the trim potentiometer taper for best od tone?

I set the trimmer 'by ear'

My trimmer measures 23Kohm from ground to center pin of the pot.

[butwhatif]

i think the 'classic' sound has the wiper tap near the center of the 20-30k ohms from ground ballpark--that way there is a very controllable od breakup w/nice bottom end--also depends on what the entrance resistance value is, and whether there is an entrance hi pass cap, -my pref is for no cap

[ChrisM]

On my #124 build my trimmer is set at 26K.

[stiltamp]

Thanks to all.
The trim pot setting is quiet clear now, but the problem is that there are different trim pots and series resistors depending on the used circuit.
That's the reason for my question regarding the signal voltage value which feeds the OD-stage.

[bluesfendermanblues]

Thanks to all.
The trim pot setting is quiet clear now, but the problem is that there are different trim pots and series resistors depending on the used circuit.
That's the reason for my question regarding the signal voltage value which feeds the OD-stage.

Its true that 100k/1.5k on V1 delivers more output into the OD trim, than a High Plate version.... realised that once I recorded the amp with first high plates and then later with low paltes. The difference was a staggering 4 dB on the meter in Cubase 5.

Still, you've got to set the trim by ear. Besides V1 configuration, the choice of pickups (Humbucker vs. singlecoils), also has a lot of impact on where you want to set yoyr trimmer.

[stiltamp]

If the guitar player uses single coil and humbucker pickups, then it would be helpfull to have a trim potentiometer switch respectively a gain switch for the clean section.

[bluesfendermanblues]

If the guitar player uses single coil and humbucker pickups, then it would be helpfull to have a trim potentiometer switch respectively a gain switch for the clean section.

I use the volume control to adjust betweeen single coils and humbuckers.

'4' for HB (treble 6½, bass 4)
'6½' for Single coils (Treble 4, bass 6)

I think the OD gets more or less the same signal that way.

[butwhatif]

there are so many variables to the -voltage- question that make it hard to answer....most judge by ear, myself included, but i also use a scope to see what waveform follows the sound, it is easier to judge that way i think, at least there is a solid reference point---

[stiltamp]

Thanks again!

@butwhatif
What for your value range four good sounding did you measure using the scope at last ?

[Tonegeek]

Its true that 100k/1.5k on V1 delivers more output into the OD trim, than a High Plate version.... realised that once I recorded the amp with first high plates and then later with low paltes. The difference was a staggering 4 dB on the meter in Cubase 5.

I would think it should be just the opposite. what am I missing?

[butwhatif]

100kusually puts more dc on the plate, lower cath=lower bias

[bluesfendermanblues]

exactly

220K/150K plates have lower output, but relative to 100k, more low end, which is why you often use the 4.7nf high pass on OD input, with high plates.

Try for yourself. If you want to get real scientific, put pink noise through each (100k vs 220k/150k) preamp and monitor the result on a spectrum analyzer

[dogears]

Higher plate loads have more gain. Someone made a mistake in their testing I believe. Lower plates have less output, but clip sooner and softer with more %THD.

From Aiken:

Gain (fully bypassed cathode)

Some typical 12AX7 numbers:
transconductance: gm = 1600 micromhos
plate resistance: ra = 62.5K
amplification factor: mu = 100
The equation for the midband voltage gain for a fully-bypassed cathode (ignoring the effects of the following load resistance, Rl) is:

Av = (mu*Ra)/(Ra+ra)
= (100*100K)/(100K+62.5K)
= 61.5
= 35.8dB
Where:
Ra = the total load resistance, which is Rp in parallel with the input resistance of the next stage, Rl. If there is no Rl, as in this case, Ra = Rp.
ra = the internal plate resistance (62.5K for a typical 12AX7)
mu = the mu of the tube (100 for a typical 12AX7)
dB = voltage gain in decibels = 20*log(V1/V2) = 20*log(61.5/1) = 35.8dB
Note that the above calculations are for a single stage alone. They do not take into account the loading effect of the following stage. For instance, if you have the above stage driving a 1Meg volume pot, the effective midband AC load resistance is the parallel combination of the plate resistor and the input resistance of the following stage, in this case, 1Meg. The effective load resistance, Rl , is then 100K in parallel with 1Meg, or 90.9K. Therefore the midband gain would be:
Av = (mu*Ra)/(Ra+ra)
= (100*90.9K)/(90.9K+62.5K)
= 59.3
= 35.5dB
As you can see, the stage gain has dropped from 61.5 to 59.3 due to the loading effect of the 1Meg pot. Smaller input resistances on the following stage will load the first stage even more, further reducing gain. If the load is at least ten times the value of the plate load resistor, it can usually be ignored for the purposes of gain calculations.

exactly

220K/150K plates have lower output, but relative to 100k, more low end, which is why you often use the 4.7nf high pass on OD input, with high plates.

Try for yourself. If you want to get real scientific, put pink noise through each (100k vs 220k/150k) preamp and monitor the result on a spectrum analyzer

[butwhatif]

That is correct, for a given single stage, the gain will be less w/ 100k plate as opposed to higher plate resistors .