Max clean headroom/12AX7?

[polifemo]

I´m about to changed the anode resistors - from 100k to 150k -on the triods of a 12AX7.

I would like to know which value on the chatode resistor that will give me max swing/clean headroom with this new/increased anode resistor?
I´ve heard that 1k - 1,5k should do it with a 100k resistor, but what about a 150k?

Also, the "bass roll off point" (set by the combination of chatode resistor/cap of the 12AX7) is 205Hz on the first triode with my 100k anode resistor (1,5k/1uF)

What will happen if I change the anode resistor to 150k?
Will the frequency move up or down?

[Blue Strat]

There are calculations for all this stuff. I'm sure you can find them online.

In general, you want the ration of Rp to Rc to be smaller than usual. 100K/1.5K = 67 (approximate gain of the circuit). Something closer to 50 will generally run cleaner.

This is just a very general concept though. You really need to figure out the "load line" to get the answer you're looking for.

[polifemo]

There are calculations for all this stuff. I'm sure you can find them online.

In general, you want the ration of Rp to Rc to be smaller than usual. 100K/1.5K = 67 (approximate gain of the circuit). Something closer to 50 will generally run cleaner.

This is just a very general concept though. You really need to figure out the "load line" to get the answer you're looking for.

I was hoping to get the answer right here :BEER

[Blue Strat]

I'm sure you were;)

I'd try something close to 3K. In terms of low end response, you'll have to experiment with different caps. The lower the value, the cleaner the stage will run.

[Old Tele man]

...also have to remember that as you increase the plate load resistor, you're simultaneously decreasing the plate current due to the increased voltage drop subtracting from the supplied voltage and thus decreasing the effective plate voltage.

...the usual "Rule-of-Thumb" is about 1mA plate current at about 100-250 Vp.

[electroid]

The answer also depends on how much signal you plan to pass through this gain stage. If you limit the current too much, high level signals will be clipped. You have more room to play with values in the first gain stage than you do in later stages where the signal levels will be higher.

[polifemo]

The amp I´m referring to is a Epiphone VJr head and I just did some measuring* in it:
(*Black lead attached to ground with crocodile clip.
Red lead in right hand. Left hand in pocket.)


- B+ 342

- R3. TOP 277
- R3. BTM 162

- R4. TOP 278
- R4. BTM 160

EL84:
- Pin3. 10
- Pin5. 6
- Pin7. 308
- Pin9. 293

12AX7:
- Pin1. 162
- Pin3. 1-2
- Pin4. 6
- Pin5. 6
- Pin6. 160
- Pin8. 1-2

Is this good or bad :? :? :?

How/where does one measure: B1, B2, B3?

My mods are as follows:

R1 1M (moved to input jack)
R2 10k
R3 150k
R4 150k
R6 470R bypassed with a 500pF cap
R7 470R
R8 1,5k
C4 1uF
R9 1,5k
C3 removed
R10 470R 5w
R14 233R (2x470R in parallel)
C5 1000 uF
OT01 from Ampmaker
I´ve also added an extra BIG bottle electrolyt cap/68uF on first filter.

Stock schematic VJr
http://i26.photobucket.com/albums/c109/prof2915/Epi_VJH_Schem_Stock.jpg

[John Phillips]

If you're trying to increase clean headroom you're going the wrong way with some of this. In particular, you don't want to increase the plate resistors from 100K to 150K, and you definitely don't want to increase R10 - if anything, decreasing it will help. Decreasing R12 (maybe to 1K) might help too. You need to keep the gain down in the early stages and maximise the power stage voltages.

Unless your new R14 value is from a scope check, you may want to experiment with that too - or even fit a trimmer (eg a 50-ohm 1W in series with a 220-ohm resistor). If it's not centering the waveform you will get earlier clipping.

[polifemo]

and you definitely don't want to increase R10 - if anything, decreasing it will help. Decreasing R12 (maybe to 1K) might help too. You need to keep the gain down in the early stages and maximise the power stage voltages.
Unless your new R14 value is from a scope check, you may want to experiment with that too - or even fit a trimmer (eg a 50-ohm 1W in series with a 220-ohm resistor). If it's not centering the waveform you will get earlier clipping.

Increasing the values for R10 and R14 were made in order to prevent the powertube from running to hot.

R10 was adjusted to get the voltage slightly above 300V and then R14 was adjusted to get the dissipation set at about 12W. (I think I´m at about 12.1W, with the screen grid current subtracted)

[polifemo]

Maybe I have to clearify some things.

My mods are´nt made in order to get the max clean headroom out of the amp.

My goal is to keep the 1:st triode - more or less - clean, and then to have the 2:nd triode overdriven by the first stage BUT with a good dynamic range!

I asked this question in order to have an idea of which way to go. (I´m new at ampmoddin´)

This far my question* has received replys ranging fro 1k-3k :confused:

(*I´ve asked this question at several places)