Resistors between the preamp grid and anode?

MikeM

What do resistors do between the preamp grid anode do? I'm going over some Fender designs, and specifically this one

http://www.schematicheaven.com/fenderamps/bassman_5f6_schem.pdf

And compared to a more simple layout (A 5F1 which is my reference), the bassman has a 100 k between the anode and the grid(Or would this be seen as the anode of stage 1 in that preamp and the second anode?)

Very interested in all these variables..

Gearhead

Look again. The resistors you speak of are between high voltage and the anode. The signal is taken from between anode and resistor. Sounds familiar? They are just plate resistors, nothing more. Drawn funnily, but that's just Guru to make it harder to read. If you have simple secrets, guard them better. Another way of drawing and it would have been more obvious that they waste the second half of the first 12AY7 on the second input, I wouldn't copy that unless you have 4 arms and 20 fingers...

Edit: that second half of the second preamp tube is better spent: COPY THIS. It makes the amp a lot faster by being able to push more current into the tonestack and long tail.

Edit 2: If I were to build such an amp, I would give it one input running into a stage built like that 12AX7 of the 5f6 schematic you refer to (one voltage, one current gain triode). Then, tone stack. Then another voltage/current double triode and then phase splitter. No high gain, but attack when you push the pick and subtlety when you go easy.

MikeM

DOH!

Thanks man

However for example my amp has loads of gain, would there be disadvantages to controlling each of my gain stages (The second phase in each 12ax7?)

Getting closer and closer each day!! Time to have a choke wound ?

Gearhead

You mean putting a volume control after each stage? This is exactly how apparently high gain amps were 'discovered'. I would say it is better not to put in gain stages that you do not need, except if you are looking for high gain sound.

MikeM

I see. Perhaps then I should consider running 2 channels, using one triode for each channel and from there have 2 completely different preamps - that way I don't have to worry about losing too much gain from insertion loss. Trying to figure out some Vox ac15 schematics. Feels like I am getting there ? Sooo many options it's scary.

I think I need to visit you and chat about some preamps a bit more as there are some things that still baffle me. At the moment it is the resistor between the second stage cathode and ground - Plate resistor?

Gearhead

Dunno which resistor you're talking about? 5F6 schematic? Which half of which valve? Value?

TomCat

@ Mike

Trust me,If you add two cascaded triode stages between pre 1 & 2 the least of your problems is insertion loss....you'll be purposely adding lots of insertion loss to tame the beast.... ? ?

BTW worked a little on that very thing on my amp and the OD tone is sounding really lekker.

Karel-Mars

MikeM wrote the resistor between the second stage cathode and ground - Plate resistor?

Here is a guide to terminology: the type of basic circuit used in guitar amps is called a resistance coupled triode. There are a few other types of topologies.

Rc = Grid leak resistor (it determines the appropiate load for the previous anode to work into - usually 1M for a 12ax7)
Rk = Cathode resistor (determines how much current flows through the tube - bias usually about 1.5V in pre-amps - 12ax7 likes a 1500ohm).
Rb = Plate (Anode) resistor (determines HT voltage drop and gain - about 100Volt drop - usually about 100Kohm)
Ck = Cathode bypass capacitor (large value increases overall gain)
Cc = Coupling Capacitor (critical cap - your sound passes through it)
Rcf= Next tube grid leak resistor (acts in AC parallel to the Plate Load resistor)

Main thing you do:
1. see that the tube does not run too hot and self destructs ☹
2. see that max ratings in tube data are not exceeded http://tdsl.duncanamps.com/show.php?des=12AX7
3. see that you have enough gain/drive for your needs
4. must sound good

TomCat

Just a question @ Karel.

If the Rc is lower, the current draw through the previous stage's anode will go up. The max anode current for a 12AX7 is about 1ma. If one were to exceed that by a small margin, what would happen?

Gearhead

I reckon same thing that always happens with tubes: nothing. They are hard buggers to kill.

TomCat wrote The max anode current for a 12AX7 is about 1ma. If one were to exceed that by a small margin

Have you measured this by measuring resistance of and voltage over the plate resistor and bias resistor, or are you taking this from hand calc / simulation?

TomCat

@ Gearhead

My question was more related to tone,gain etc rather than relating to tube life etc, although that might come into play.....maybe not, hence my question.

Regarding the 1mA Anode current, i believe that that is the max anode current a 12AX7 can normally deliver. So no measuring, calculating etc as the question was a general one and not relating to anything specific..I was just curious..... :-\

Karel-Mars

Hi Tom

You can make Rc any value from 0 to the max spec (1Mohm - I think - can't be bothered right now to check :-[).

With tubes you have DC conditions and AC conditions. DC conditions are about tube life etc. mostly (talking pre-amp tubes - generalising) and AC is about tone.

The fact that the Coupling Cap blocks DC, means the tube current draw will be unaffected by anything happening on the other side of it. Fit a 1M pot (Linear if you have) in place of Rc and turn it up/down while you play. Lower values should give more distortion/less gain. Ken Fisher uses this resistor (after third pre-tube) to create his signature tone in Trainwreck amps.

In Marshall Amps they drop Rc after the phase splitter (both sides) to 68K to drop gain and get more distortion (SF Fenders also have this feature)

TomCat

Ahhhh!!!!! Got it. My question came because I found that dropping the Rc on the second tube of my cascaded stage had a dramatic effect on the overdrive tone......for the better.

I went from the usual 1Meg down to 100k with dramatic results......Sounds absolutely fantastic but I was wondering about the anode current. Nice to know that that is not an issue.

Now that you mention it...a pot is a good idea..... :-\ :-\

Gearhead

Do you think it is the stage before the low resistor or the one after it (being hit less hard) that makes it sound better?

TomCat

Hard to say......I don't know enough about tube amplifier theory yet to make a call.....Karel would probably know.

Karel-Mars

The Rc resistor has an effect on the stage pre-ceding it. It is called down loading the anode. Look at the AC (sine wave) action around the pre-ceding stage. The cathode is at AC ground (decoupling cap). So the Anode is oscillating at a amplitude of mu (amplification factor) times the input signal. This oscillation drains away through the anode load resistor (into the power supply - should be a AC ground ideally), the tube itself (the plate resistance is like a resistor to ground) and the rest goes through the coupling cap, draining to ground through the grid leak resistor (all of these are in parallel - so the total resistance the anode sees is lower than the lowest resistance in the arrangement).

Gearhead

I had understood that theory, I was just suggesting to Tom that his lowering the gain might be beneficial because he now does not hit the next stage so hard. I guess it would be easy to find out with two pots: a lower value one for loading in parallel with a larger value one for volume.

TomCat

When I did the resistor change, the gain did not change that much, or at least not as much as the tone change was. As Karel mentioned, the tube started breaking up in a different/nicer way for the overdrive. Got grittier and more musical...almost like the 2nd harmonics came more to the fore.

Karel-Mars

You could also put a large resistor as a grid stopper for the next stage. As long as you do not exceed the max resistance to ground for the grid. I have seen a 470K coming from the centre lug of the volume pot, going to the next tube grid. It increases the Miller capacitance and rolls of tops.

nicovlogg

While there's this interesting discussion going I thought I'd ask my question - what is the tonal effect of changing the value of Rb? Been modding my Marshall JTM60 and there's a few different values I found on the internet. Tried quite a low one (33k) which gives me a vey nice crunchy distortion, but my amp is now a LOT darker. Is this change in high-end a result of the plate voltage changing, or is it something else?

Could I also use a variable resistor here to see what change it makes to the sound, or is there some kind of danger from running it with too much or too little voltage?

I changed quite a few components around so now it's difficult to pinpoint the change of each. :-\