So, kenneth, looking at the AX84 schematic: http://www.ax84.com/p1/AX84_P1_091125.pdf, you would recommend hooking up the heater power circuit exactly how it is shown there.
How would I modify this if I have a center tap?
How would I modify this if I have a center tap?
Ah, this actually often results in a poorer sound in a guitar amp. The particular kind of compression that results from a saturated transformer is one of the things that makes it worth having a tube amp for guitar as opposed to solid state. You're not building a hi-fi.
Sorry, I don't just mean to raise a string of objections... good luck with the amp, it's not all about what you build, more that you built it at all.
w
I meant better for guitar amps, my mistake! Both my PT and OT are mercury magnetics and are considered very good transformers for guitar amps. They are also a friendly company, I highly recommend them!
You connect the center tap to the cathode of the output tube.
This method works, and saves three parts. The cheapskate in me likes it, but the engineer in me usually forces me to add the three parts, and adjust the voltage up a bit higher (50 volts of so).
You want the heater to be positive compared to the cathode. The heater is hotter than the cathode and if it is negative compared to the cathode current can flow from the heater to the cathode allowing some AC signal to get to the cathode causing hum.
In the AX 84 circuit shown both cathodes of the 12AX7 are at 1 or 2 volts above ground. Connecting the heater to the output tubes cathode will elevate the heater circuit to about +7 volts making it more positive than the 12AX7 cathodes. That circuit will work as shown.
Your amp uses one section of a 12AX7 as a cathode follower and its cathode voltage is shown as 156 volts. This means that grounding the heater circuit as shown is applying 160 to 180 volts of DC plus signal voltage to the heater to cathode insulation, which is only rated to 180 volts on some 12AX7's. I would use a resistor divider to set the heater voltage to about 100 volts in this circuit.
So, because I have a cathode follower, I cannot use the EL84's cathode as the ground for the heaters? This makes sense I suppose because the cathode follower's cathode is at a higher DC voltage than the EL84's, and thus setting the ground of the heater to the EL84 will not mitigate any hum issues introduced into the cathode follower... if I'm not mistaken.
So raising the heater to 100 volts should be pretty simple, should I use the HT+ at the first filter cap, or the HT+ as far away from the power transformer as possible (in my schematic, C or D), and do a simple voltage divider (~300 volts, so 66k and 33k., attach center tap to the top of the 33k.
Would that be okay?
Thanks everyone for all the help.
So raising the heater to 100 volts should be pretty simple, should I use the HT+ at the first filter cap, or the HT+ as far away from the power transformer as possible (in my schematic, C or D), and do a simple voltage divider (~300 volts, so 66k and 33k., attach center tap to the top of the 33k.
Would that be okay?
Thanks everyone for all the help.
If "V1" is both triodes in parallel, that's a good bunch of gain there. Remember that each triode is seeing the equivalent of a 440K plate resistor. If you wanted to replicate the gain of a typical Fender stage (not saying you do...), only doubled, you'd want to use something like a 47K there.
But that's something you can play with once it's built. Tweaking with circuits is fun! :grin:
- Scott
But that's something you can play with once it's built. Tweaking with circuits is fun! :grin:
- Scott
Hum issues are more of a problem as you get closer to the input jack. I don't think that too much hum will get into the cathode follower. The HK voltage is very close to the maximum spec and may be violated under signal and normal part to part variations. That is why I recomment the voltage divider.
It doesn't matter much and there are opinions for both ways. The resistor divider is fine, but add a capacitor to ground to insure against coupling from stage to stage through the heater circuit. The value isn't critical, something around 1 uF.
So if I put a 66k from a high voltage tap and a 33k in parallel with a cap to ground, and then hook the center tap of the heater transformer, I will be all set? (so the heater is at ~100V DC).
Maybe not. The 66 K resistor will have 200 volts across it. That will result in about .6 watts of heat. I would use a 2 watt resistor for this one. 66K is not a standard value, use a 68K. The 33K resistor could be 1 watt. My amp designs use 1 and 2 watt resistors with a .33 uF 200 volt cap.
Using Ohm's Law, you are getting about 3mA of current through this divider. So, using Ohm again, you are going to dissipate about 0.6W through the 68K and about 0.3W through the 33K. I think the normal recommended safety factor for resistor dissipation is a factor of three or four, so I would not do this...
If you have already ordered these resistors, you could put two 33K in series and two 68K in series to make a 66K/136K voltage divider. This will halve the current draw to about 1.5mA and your resistors will be well within the dissipation limits.
Here is a link to a great spreadsheet for working out all of these standard calculations, I use it all the time now, look for the link 'tubeformula.xls' joeltunnah.com
Resistors are so darn cheap, it is always worth ordering extras in case you make a mistake 😉
Almost as bad as my typical mistake - can't find the right part, assume I forgot to order it with main parts order, re-order, then while waiting for delivery, find I had it all along... For my current project, I thought I had ordered every part before I began construction. Have had three deliveries since 😉
Good luck!
Almost as bad as my typical mistake - can't find the right part, assume I forgot to order it with main parts order, re-order, then while waiting for delivery, find I had it all along... For my current project, I thought I had ordered every part before I began construction. Have had three deliveries since 😉
Good luck!
Most people recommend using a resistor with at least twice the wattage rating than what the resistor will actually dissipate. That rule comes from the old carbon comp resistor days. The carbon comp resistors were (and still are) notorious for changing value and becoming noisy when subjected to repeated hot - cold cycles. The modern stuff holds up pretty well, and these are about the mose non critical components in the entire amp. Just mount them so that there is room for air flow on all sides (not touching the chassis, PC board, or other components) and they will be fine.
The voltage rating for most 1/2 watt resistors is too low for being wired across the supply.
- Status
- Not open for further replies.