Voltage on Preamp Tubes?

[Old Tele man]

Or you could get yourself one of these :

http://cgi.ebay.com/Heathkit-Heath-Z...QQcmdZViewItem

Paul P

...uh, "end-of-life" for 6L6 would 70% of 6000 µ-mhos, or 4200 µ-mhos...not 1300 µ-mhos!

[Joe Bee]

Re. the safety of using toggle switches, look at the data sheet, they all have a voltage rating. If you don't exceed that, and make sure the current stays reasonable as well (i.e. don't charge up filter caps through them), you should be safe.
If that does not make you feel comfortable, why don't you use a relay for switching? This way your toggle switch only ses 12V, which can e.g. be derived from the heater winding.

[Steve Conner]

I used those little toggle switches that fit in a 1/4" hole as power and standby on a 50w amp for years. I was fully expecting the standby switch to explode, but nothing happened.

With DC, the voltage rating of a switch is limited by arcing on break. Too much voltage and current, and the arc will stay lit, which will actually cause the switch to melt or explode. This has happened to me in the past, but it took 600V DC at 5A to do it to a cheap plastic rocker switch rated for 240V AC.

If you have a double-pole switch, you can connect both sets of contacts in series to increase the voltage it can break.

[chocopower]

Wich amperage rating would be fine?

10A? or 8A would be ok?

[Enzo]

On even a 100 watt amp, the whole B+ is less than an amp, so it wouldn't much care whether 8A or 10A was used.

[Manic]

I agree that they rarely show the voltage at the plate. But, they almost all use either 100K plate resistors or 200K plate resistors (or 200K in one stage and 100K in the next). So, regardless of what the specific values are, some are being run quite "hot" and some quite "cold". That's got to have some impact on sound. They must be doing it on purpose.
Chip

at AX84 they have scems showing bigger plate resistors early.
The Blues preamp has 220k on both stages
Lead II Preamp 100k on the first and 47k on the next 4 stages


I have also seen on some fenders , 100k on the first stage and 120 on the later stage like an AB763....hmm. I dont know what to make of this either...
now a 6g3 is 220k first then 100k later.
I happen to be looking for a warmer tone in My 5e3x2...so from what I have gathered 220k will warm up the stage?....sry Im tryin to get the hang of all the this great info.

[Joey Voltage]

Hi All,
I'm continuing to think about building a stand-alone pre-amp for exploring high-gain ideas. In my last post we discussed power supplies. One important design choice is how high in voltage I want to go. Looking at examples out on the web, there is a huge range of voltages applied to the pre-amp tubes. Here are some example plate voltages used on 12AX7 pre-amp tubes:

260V Fender 65 Deluxe Reverb Reissue (my current amp) schematic

300V First stages on Marshall JCM-800. pre-amp, power-supply

357V First stages on Mesa Boogie MK II. schemtic

359V First stages of Soldano SLO-100 schematic

402V First stages of Mesa Dual Rectifier schematic

So, my question is...what is the effect of the voltage supplied to the pre-amp tubes? How does it affect the sound? Also, why do these amps almost always seem to ratchet up the voltage for the later pre-amp tubes? How does it affect the qualities of the overdrive?

Thanks,

Chip

Speaking generally, higher voltage stages are cleaner, punchier, and have more headroom (ability swing more signal before the point of clipping), while lower voltage stages tend to be flatter in response, smoother, and distort earlier.

Keep in mind the Supply voltage and plate load is only one factor. You also have to take into consideration the cathode resistor. The two work together to set the operating point of the tube, which effects the way it performs, which effects the way it sounds. The cathode resistor has a direct effect on how much voltage appears at the plate of the tube as well. so you really have to analyze the system as a whole to get a better idea of how to achieve your design goals.


For example, if you wanted to design an amp for clean playing, you may want to have a higher supply voltage, use a plate resistor at about twice the size of your tubes internal plate resistance, draw a load line to pick a bias point that will give the maximum amount of signal swing. and keep the stages to a minimum. High gain preamps are a lot more complicated, and really tricky to get sounding good.

when experimenting with larger plate loads, it isn't fair to just choose a larger plate load and throw it in place of the old one, and then base your judgments just on changing that one variable, you need to consider how you are effecting the operating point of the tube, and may need to adjust the cathode resistor, and Cathode bypass cap to get a fair comparison. Also, larger plate loads will produce more voltage gain, so you may need to attenuate the signal more than usual.

[mooreamps]

Mesa Boogie stage

The only thing that I can see of concern would be the cathode voltage on stage V3B at 216 volts. The data sheet for max heater to cathode voltage for a 12ax7 is 100 volts. For the ECC-83 shows it at 180 volts. I would put the pre-amp heaters on an elevated DC bias to offset this condition.


-g

[MWJB]

Not an issue, cathode follower stages often run in excess of 350v on a 12AX7 cathode. There is some anecdotal evidence to suggest that the EH12AX7LPS is not best suited to these applications, dunno about the RI Tung Sol (similar filament design?) - but most other 12AX7/ECC83 will be just fine. Tweed Fenders & JTM45/Plexis never needed DC biased preamp tubes.

[mooreamps]

If it's 350 volts on the "cathode", your saying the tube is fully saturated ??

-g

[MWJB]

Sorry, my mind was obviously "elsewhere"! It's usuual to haqve 350-400v on the PLATE of a cathode follower stage. However, <200v is typical on the cathode.

Apologies, Mark.

[mooreamps]

No worries.....

-g

[booj]

Q. So, my question is...what is the effect of the voltage supplied to the pre-amp tubes? How does it affect the sound?
A. 100 volts - harmonica amp (at the plate)
150 volts - guitar amp
200+ volts - bright
Also, why do these amps almost always seem to ratchet up the voltage for the later pre-amp tubes? How does it affect the qualities of the overdrive?

That may be due to the power supply architecture. The power tubes get the highest voltage, and the preamp supplies are taken from that in stages, the last stage being the one that supplies voltage to the first stage preamp.

[Joey Voltage]

Q. So, my question is...what is the effect of the voltage supplied to the pre-amp tubes? How does it affect the sound?
A. 100 volts - harmonica amp (at the plate)
150 volts - guitar amp
200+ volts - bright .

It really isn't that simple. There are many factors that will influence how gain stages will sound, so I can only give you a very general answer at best without specifics (see posts above). In general, higher voltage stages will sound brighter, punchier, cleaner, and somewhat tighter. Stages supplied from lower voltages tend to sound rounder, spongier, browner, and will have less headroom, so they will distort sooner (a stage can only possibly swing as much as it's supply voltage). Again, this is VERY general.

Also, why do these amps almost always seem to ratchet up the voltage for the later pre-amp tubes? How does it affect the qualities of the overdrive?

There can be a couple of different reasons for this too. It can be a byproduct of successive decoupling (in series), which is a little better in terms of smoothing out ripple to the most critical stages, although some manufacturers do use parallel decoupling....I think bad cat or soldano comes to mind. It can also be very engineered that way. Assuming you are talking about higher gain amps, you may want to have the stages later in the signal chain (they are likely the ones that most likely will clip the hardest) to have more headroom. You may not. Again, there are alot of things that will affect the overdrive qualities, so it's not really an easy/simple question to answer. I stress these are very general answers. I hope this was at least somewhat helpful.

[daz]

i haven't read the thread aside from a few of the first posts. And i also know little about any of this. But what i can offer here is what i was amazed to find with my high gain 2204 style build. I spent months trying everything imaginable to remove a hard and harsh sounding hi end that was not normal for any amp, especially a 2204 which when in good shape with good tubes is a very smooth sounding amp. I had i think around 200v on the lowest stage. I had tried raising the last node's R to see what happened at one time and it seemed to help a bit, tho not much. i never even checked the voltage tho. I later lowered it more and did the same to the node before it and the PI node, tho not as much there. the voltage at the 1st stage was down to just over 100 v ! i think it was 108 or thereabouts. The amp's problems were gone and at this point it sounds truly awesome. High end sustains like mad and is smooth as silk and dynamic as can be. I suppose i could ask for more, but that would be asking a lot because it beats every marshall i've owned, no, make that every AMP i've owned hands down....all with 100V on the first stage and not a lot more on the second. Why? F if i know. Like i said, i know little to nothing about this. But i think the point is that if you go by the book you may never reach your goal. Sometimes it's good not to know too much !