Hi everyone!
I am currently in a process of building a EL84 SE amp with a 6N1-P preamp tube that needs to use a 220v to 220v PT that i don't have, but I know for sure that I have a PT from an old radio that puts out around 540v for EZ81 tube, but the interesting thing is that it has two phase wires and a center tap, when measuring between phase wires you get 540v but when measuring between center tap and only one phase wire you get around 270-275v so I was wondering can a man connect a SS diode rectifier between a phase wire and a center tap to get a similar V-out as 1:1 ratio 220v transformer I mentioned earlier, but keeping in mind that the secondary is now not 220v but it's actually 270v. Could that be done? Could you people please modify my schematic?
Problem no.2...
See how 6N1-P is connected like it's in parallel, I would like to remove that parallel connection and make it look like a champ preamp for more gain-ish sounds, but I need to mind the bias of that tube, but I don't really know how to calculate it, so if you could be kind enough to help me i would be really grateful.
Thanks for replying in forward,
FireBottle!
I am currently in a process of building a EL84 SE amp with a 6N1-P preamp tube that needs to use a 220v to 220v PT that i don't have, but I know for sure that I have a PT from an old radio that puts out around 540v for EZ81 tube, but the interesting thing is that it has two phase wires and a center tap, when measuring between phase wires you get 540v but when measuring between center tap and only one phase wire you get around 270-275v so I was wondering can a man connect a SS diode rectifier between a phase wire and a center tap to get a similar V-out as 1:1 ratio 220v transformer I mentioned earlier, but keeping in mind that the secondary is now not 220v but it's actually 270v. Could that be done? Could you people please modify my schematic?
Problem no.2...
See how 6N1-P is connected like it's in parallel, I would like to remove that parallel connection and make it look like a champ preamp for more gain-ish sounds, but I need to mind the bias of that tube, but I don't really know how to calculate it, so if you could be kind enough to help me i would be really grateful.
Thanks for replying in forward,
FireBottle!
Another thing that came to my mind... center tap is also used as a second wire of 6.3v filament supply, now that rectifier is connected to CT, can filaments also be connected?
A radio transformer with 540v, are you sure?
Looks to me it should be used like a two-phase rectifier, were the center tap goes to ground, resulting in 270v, which is an acceptable voltage for El84's.
Here some info>
The Valve Wizard
Are you sure it is a CT? Normally it would'nt be connected to the heaters. There should be another winding just for the heaters. If you know the name of the previous radio we could check the schematic to identify how it should be connected.
As for the preamp stages, have you seen the thrifty croaker or the TMC1? There is a version with the 6N2-P. I'm not sure how different they are, other than the 6n1-P is a replacement for the ECC85, which has a mu of 57, and the 6N2-P is similar to the ECC83, with mu=100.
Looks to me it should be used like a two-phase rectifier, were the center tap goes to ground, resulting in 270v, which is an acceptable voltage for El84's.
Here some info>
The Valve Wizard
Are you sure it is a CT? Normally it would'nt be connected to the heaters. There should be another winding just for the heaters. If you know the name of the previous radio we could check the schematic to identify how it should be connected.
As for the preamp stages, have you seen the thrifty croaker or the TMC1? There is a version with the 6N2-P. I'm not sure how different they are, other than the 6n1-P is a replacement for the ECC85, which has a mu of 57, and the 6N2-P is similar to the ECC83, with mu=100.
This is exactly the type of transformer needed to use the two-diode circuit called a full wave rectifier ( Full wave rectifier - Center tapped full wave rectifier )
Probably the transformer you have was originally used with a "tube rectifier", which is actually two vacuum diodes packed into one glass bottle.
That web page doesn't show the filter capacitor - you should add one as usual. It goes across Rl, with the positive end of the capacitor to the right end of Rl. (Rl is the load that the power supply is powering - in your case, your entire tube amp is Rl.)
If you use silicon diodes, the DC voltage you get will be nearly equal to the peak value of the sinewave voltage from the transformer secondary. I'm not sure if the 270V you mentioned is peak or RMS - if RMS, then you can expect about 380 V DC under light load.
You have two identical triodes in parallel, so each one is carrying half of the total current. In other words, the 10k anode resistor, and 580 ohm cathode resistor, are carrying double the current of one triode, and producing the correct bias voltages.
So, if you separate the two triodes (no longer in parallel), the anode and cathode currents will halve. To keep the bias voltages the same, you have to double the anode and cathode resistors.
So use 1.2k for the cathode resistor, and 22k for the anode resistor, on each triode. The 1 meg grid resistor can stay at 1 meg, since there is almost no grid current flow.
Each triode will need its own anode coupling capacitor, and its own cathode bypass capacitor. For guitar, 4.7 uF is big enough for (each) cathode bypass.
he anode coupling cap value depends on the load you are driving, but if you use the same 500k pot, then 4.7 nF (same as 4700 pF or 0.0047 uF) should be a good starting point for guitar.
Note that R8 should stay at 10k, since it is supplying current for both triodes whether or not they are in parallel.
Also, I believe you are missing a filter cap from the junction of R3 and R8 to ground - you must have this in place, otherwise your two non-paralleled triodes will interact in a very unpleasant way (and will probably oscillate.)
-Gnobuddy
If the high-voltage CT is *also* _one_ side of the 6V heater winding, then it will "work" but with more hum than we usually like for a guitar amp.
You find this plan in radios (lower audio gain than a guitar amp) and in the *oldest* guitar amps (when gain was low and speakers were bass-weak).
You find this plan in radios (lower audio gain than a guitar amp) and in the *oldest* guitar amps (when gain was low and speakers were bass-weak).
You have three choices:
1) Use the transformer as I suggested. You may encounter some hum because of the unbalanced heater power (0V on one end of the heater, 6.3V AC on the other end, rather than +/- 3.15V AC at the two ends.)
2) Use the transformer only for heater power, and ignore the high voltage winding. (This is a silly choice, I mention it only for completeness.)
3) Use the transformer only for B+. Use a cheap second-hand switch mode DC power supply for the heaters.
Choice #3 is the route I would take. Around here, I can often find a suitable second hand SMPS for $2 at a thrift store. You won't find a 6.3V DC power supply, but you can find one that is a bit more than 6.3V and drop the excess voltage with a series resistor.
One of my preamps uses an 8.4 V DC Sony power supply in this way, with a small power resistor in series to drop the voltage to 6.3V DC at the valve heaters.
Think about this for a second: it makes no sense at all to run wires carrying large AC currents (at a frequency in the audio band) through a sensitive guitar amp. Any electronics engineer or informed hobbyist knows that wires carrying AC produce local AC magnetic and electric fields that are picked up by nearby circuitry and turned into audible hum. So AC powered heaters make no engineering sense at all - they are a recipe for creating massive amounts of hum.
But turning AC into clean DC power for heaters was almost impossible in 1920 or 1930 when early mains-powered valve circuits were first beginning to be available. Back then, the only way to get clean DC at low voltage and large amperage was to use a battery. (There were in fact battery-powered valve radios before the mains-powered versions arrived.)
So, with no alternative, engineers and technicians made the best of a bad job, and learned to use AC for heater power, developing all sorts of complicated tricks in wiring and layout to try to manage the hum. Twist the heater wires, run them along the chassis walls, no, move them into the edges and corners, pray to the gods, offer incense and light candles, beg the universe to stop the hum, etc.
Today, things are very different. Clean, cheap, DC at the sort of currents and voltages suitable for valve heaters is readily available. DC power is lighter, cheaper, more efficient, and produces zero hum. It's far superior to AC heater power in every way.
So here in 2018, a hundred years after desperate engineers were forced to make do with AC power for valve heaters, it's about time we stopped looking in the rear view mirror, and started to use the most appropriate engineering solution. And the most appropriate engineering solution is to use DC from a small switch-mode power supply.
-Gnobuddy
The amp is finally done, and the outside looks "weirdly-beautifully" awful, so i called it The Franken Amp.
Everithing "works" for now, but sometimes it starts to squeal and it can be heard on the chassis, not only the speaker... Is it because OT is not grounded?
Everithing "works" for now, but sometimes it starts to squeal and it can be heard on the chassis, not only the speaker... Is it because OT is not grounded?
Congratulations! No hum problems, I take it?
The squealing (oscillation) is probably causing transformer core laminations to vibrate, and that in turn is vibrating the chassis.
Fixing oscillation problems may be easy or very hard, depending on what is causing the problem in the first place. You might just need to add suitable small capacitors to one or more of your valve stages, to roll off the high frequency response above 10 kHz. There is no guitar signal up there, so doing this won't change the sound of the amplifier, but it will usually stop high-frequency oscillation headaches.
The OT must be grounded for safety reasons, whether or not it fixes the oscillation.
Even with "only" 270V B+, the transformer can have up to maybe 500 volt peaks at one end, and 1000 volt peaks between the two ends. And it's an old transformer, probably with paper insulation that was weak to start with, and is even weaker now due to age. It should be grounded to the chassis, and the chassis properly grounded through a 3-pin plug and AC outlet.
I usually use insulating washers to mount the transformer to the chassis (which avoids a possible shorted-turn problem due to the transformer housing), and then solder a wire from the transformers metal housing to the chassis ground.
-Gnobuddy
It has no hum😎, DC on heaters, around 295V on the plate of EL84, groundeed the OT, fixed messy wiring and, works like a charm!
Thanks everyone for helping... especially Gnobuddy and PRR!
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