Hi,
I am new in the forum, and I would like to ask some advice about my amplifier.
I just built a 8Wrms guitar amplifier. The circuit is quite straight forward, two 12AX7 for the pre-amplifier and two EL84 for the power amplifier.
I use a 220VAC to 200V toroidal transformer for the power with 0.1Amax, so I expect to have 280VDC max at the output.
I tested the power supply and without any load it delivers 280VDC as expected. However, as soon as I connect the full circuit the voltage drops to 180VDC. As a consequence my amp only delivers 4Wrms.
The circuit for the power supply is also quite straight forward. I used a diode rectifier an a set of capacitor and resistors. I am not using any choke.
I calculated that with an 8ohm load my circuit will need about 80mA maximum. So, my toroidal should be OK.
So any idea about why I get this voltage drop? How do I solve this issue<
Thanks everyone
I am new in the forum, and I would like to ask some advice about my amplifier.
I just built a 8Wrms guitar amplifier. The circuit is quite straight forward, two 12AX7 for the pre-amplifier and two EL84 for the power amplifier.
I use a 220VAC to 200V toroidal transformer for the power with 0.1Amax, so I expect to have 280VDC max at the output.
I tested the power supply and without any load it delivers 280VDC as expected. However, as soon as I connect the full circuit the voltage drops to 180VDC. As a consequence my amp only delivers 4Wrms.
The circuit for the power supply is also quite straight forward. I used a diode rectifier an a set of capacitor and resistors. I am not using any choke.
I calculated that with an 8ohm load my circuit will need about 80mA maximum. So, my toroidal should be OK.
So any idea about why I get this voltage drop? How do I solve this issue<
Thanks everyone
Doesn't sound like it?, he mentions 'diode rectifier' (singular), plus 'resistors' and no choke - doesn't mention the values of the capacitors (or anything else), but it looks like the worst possible choice in every department.
As already mentioned, he needs to post the exact schematic.
Voltage-drop is quite normal. The amount depends on the format of rectification, the value of the capacitance plus any resistance in the supply and the load current presented.
Without seeing a schematic, no-one can tell why the voltage is apparently low.
Sounds to me, you have a single diode half-wave rec, which is probably the least efficient format....
Without seeing a schematic, no-one can tell why the voltage is apparently low.
Sounds to me, you have a single diode half-wave rec, which is probably the least efficient format....
Here is the circuit for the power supply. I have the amp circuit draw by hand, but I will upload it asap. I attach the PP transformer to B1, the pentode screens to B2 and the pre-amp to B3. I have a USB scope, but I need a x100 probe. So I cannot measure the ripple, at least for the moment.
Attachments
Have you measured the actual current that your amp draws? Perhaps there is an error causing excessive current, dropping the voltage.
Why are you stacking filter caps? You'll never exceed the 350V rating with a 200V secondary. Put them parallel for a much stiffer supply.
You may want to check in PSUD how your setup should perform.
Why are you stacking filter caps? You'll never exceed the 350V rating with a 200V secondary. Put them parallel for a much stiffer supply.
You may want to check in PSUD how your setup should perform.
I measured the current with no load no input: 4.8mA at the 4x12AX7, 6.30mA at 2xEL84 screen and 43.9mA at the push-pull transformer.
In any case, I removed all the load and the power supply returns 220VDC. So, I think the toroidal is broken :-/. Before it was returning 280VDC. I will order a new one.
Thanks everyone for your replay
In any case, I removed all the load and the power supply returns 220VDC. So, I think the toroidal is broken :-/. Before it was returning 280VDC. I will order a new one.
Thanks everyone for your replay
1) all raw (that means unregulasted) power supplies drop.
Normal is 20%
Now yours drops more, so the transformer is clearly underrated, use a twice as large transformer (meaning 220VAC secondary, 200mA)
Why that much?
Tube amps are horribly inefficient, a 100W Marshall pulls 375W from the wall outlet, the equation is worse for small amps because no matter whatb they put out, you still have to feed filaments, screens, etc.
To boot ,fashion among amateur builders is to bias tubes to 70% or some similar nonsense, which is a heavy load to the supply even without playing a single note.
2) Don't know about European suppliers, but you need a transformer "rated for a VOX AC15 or Marshall 20W " type.
Search under these terms, you'll see they are far larger than what you imagine.
Knowing the final use, winders take all details in account when designing one.
3) your supply is excellent, but way over rated.
If that's fine with you, it's fine with me also, now yopu need to ever frate that transformer
4) there's something very wrong there: your unloaded voltage MUST return to 280V if you disconnect all loads.
Don't buy "more of the same" , get the proper one, specific for Guitar amps.
How are you feeding your filaments?
Normal is 20%
Now yours drops more, so the transformer is clearly underrated, use a twice as large transformer (meaning 220VAC secondary, 200mA)
Why that much?
Tube amps are horribly inefficient, a 100W Marshall pulls 375W from the wall outlet, the equation is worse for small amps because no matter whatb they put out, you still have to feed filaments, screens, etc.
To boot ,fashion among amateur builders is to bias tubes to 70% or some similar nonsense, which is a heavy load to the supply even without playing a single note.
2) Don't know about European suppliers, but you need a transformer "rated for a VOX AC15 or Marshall 20W " type.
Search under these terms, you'll see they are far larger than what you imagine.
Knowing the final use, winders take all details in account when designing one.
3) your supply is excellent, but way over rated.
If that's fine with you, it's fine with me also, now yopu need to ever frate that transformer
4) there's something very wrong there: your unloaded voltage MUST return to 280V if you disconnect all loads.
Don't buy "more of the same" , get the proper one, specific for Guitar amps.
How are you feeding your filaments?
The transformer spec are:
- Pri: 0 - 220 - 230 - 240 V
- Sec 1: 200 V @ 0,1 A
- Sec 2: 6,3 V @ 1,5 A
Enough current to head the four tubes.
It is to close to maximum.
Heater current for every EL84 is 0.76, total 1.52 Amper, and the two 12AX7 are 0.3Amp each, so that sums up to 2.12AMPs. This is overloading so the high voltage pays the price.
You have 80mA just for the power tubes and 5mA for the preamp. The overload on the heater windings is 0.62*6.3=3.9 watts. Divide that by 280 volts and see how much it takes from the high voltage windings
Also, measure the DC resistance on the secondary high voltage wingdings. If it's few hundred Ohm it will drop the voltage by DC OHMS times the CURRENT.
You need a bigger TRANS...
Heater current for every EL84 is 0.76, total 1.52 Amper, and the two 12AX7 are 0.3Amp each, so that sums up to 2.12AMPs. This is overloading so the high voltage pays the price.
You have 80mA just for the power tubes and 5mA for the preamp. The overload on the heater windings is 0.62*6.3=3.9 watts. Divide that by 280 volts and see how much it takes from the high voltage windings
Also, measure the DC resistance on the secondary high voltage wingdings. If it's few hundred Ohm it will drop the voltage by DC OHMS times the CURRENT.
You need a bigger TRANS...
Ups, I messed up the design. I will change the transformer for this one
Pri: 0-220-230-240 V
Sec 1: 250 V @ 0,12 A
Sec 2: 6,3 V @ 3 A
That should do the trick
How about biasing it to class AB? It will drop some current but you still need this current when a signal is applied.
I have another idea: loose one power tube but leave the bias current the same, that is, put a power resistor that will keep the same 40ma flowing through the OT. Make it a Single Ended amp. You need this current to avoid putting your OT in magnetic saturation. This will save you the 0.76 amp on the filament.
But I really like the 6v6 idea better. Replacing the PT will be best.
I have another idea: loose one power tube but leave the bias current the same, that is, put a power resistor that will keep the same 40ma flowing through the OT. Make it a Single Ended amp. You need this current to avoid putting your OT in magnetic saturation. This will save you the 0.76 amp on the filament.
But I really like the 6v6 idea better. Replacing the PT will be best.
Measure the voltage drop across the R's feeding the large caps in the power supply to see what the actual current being drawn is. V/R = current. This current measurement will include the amp circuit AND current drawn for keeping the large caps charged (not just the amp circuit). You can reduce the size of these R's feeding the big caps to a point, at the expense of filtering. That's what I would do, and did. When choosing the quiescent current for the EL84/6BQ5, set it so the power dissipation spec of the tube is not exceeded. A little less than that might be wise if the tube is mounted upside down. 6BQ5's run hot due to the heater current being almost twice that of a 6V6. Volts across the tube, cathode to plate, times plate current (same as cathode current minus screen grid current) = PD. I built a similar circuit with a 6V6, and ended up with more like 5 watts, before too much PD in the 6V6.
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