I am new to this forum and am hoping for some advice.
I have recently aquired a 820vct @ 100ma power transformer with 2x 6.3v @ 1.25 and 2.5 A and 5v @ 2A taps.
I have been looking for a schematic for a power amp i could build using this transformer but have been having a bit of a hard time. This is my first tube project so I am hoping to keep it relativly simple, but am still looking for a good sounding amp. I was thinking of a single ended tube rectified circuit, but other than that I am totally open minded.
If anyone out there has an idea of a schematic that would work for this amp i would really appreciate it.
Thanks, Mark
I have recently aquired a 820vct @ 100ma power transformer with 2x 6.3v @ 1.25 and 2.5 A and 5v @ 2A taps.
I have been looking for a schematic for a power amp i could build using this transformer but have been having a bit of a hard time. This is my first tube project so I am hoping to keep it relativly simple, but am still looking for a good sounding amp. I was thinking of a single ended tube rectified circuit, but other than that I am totally open minded.
If anyone out there has an idea of a schematic that would work for this amp i would really appreciate it.
Thanks, Mark
The 100 mA. 410-0-410 VAC winding will power 2X 300Bs if you use a choke I/P filter power supply. The B+ rail will come in at about 330 V. The "typical" 300B operating point is 300 V./ 60 mA., but a little higher voltage and a little lower current will work nicely. An inexpensive 7Z4 rectifier (2 USD) will take care of 300B B+. The heater of the 7Z4 would be powered by 1.25 A./6.3 VAC winding.
The 2 A./5 VAC winding will power 1 300B filament. You need to acquire a 2 A./5 VAC filament trafo for the 2nd 300B filament.
The 2.5 A./6.3 VAC winding fed into a voltage doubler made from Schottky diodes and regulated by a 7812 3 terminal regulator will power the heater of the twin triode you use for the driver stage in each channel. I'm guessing that a 12AV7 (5965) will have enough gain to serve as your driver.
As there is no current to spare in the 300B B+ supply, you will need to build a separate B+ supply for the drivers. A Hammond 269GX power trafo will take care of that need. Rectify the driver B+ with a 2 USD 7Z4. The 269GX has a 6.3 VAC winding that will power a 7Z4's heater.
Plitron (http://www.plitron.com) model PAT 3035-SE trafos will take care of coupling the 300Bs to the speakers.
The 2 A./5 VAC winding will power 1 300B filament. You need to acquire a 2 A./5 VAC filament trafo for the 2nd 300B filament.
The 2.5 A./6.3 VAC winding fed into a voltage doubler made from Schottky diodes and regulated by a 7812 3 terminal regulator will power the heater of the twin triode you use for the driver stage in each channel. I'm guessing that a 12AV7 (5965) will have enough gain to serve as your driver.
As there is no current to spare in the 300B B+ supply, you will need to build a separate B+ supply for the drivers. A Hammond 269GX power trafo will take care of that need. Rectify the driver B+ with a 2 USD 7Z4. The 269GX has a 6.3 VAC winding that will power a 7Z4's heater.
Plitron (http://www.plitron.com) model PAT 3035-SE trafos will take care of coupling the 300Bs to the speakers.
With a 5Y3 tube rectifier, which would drop a few volts; you could try the loftin-white direct coupled 2A3 amplifier. It needs around 400V and 120mA IIRC, you could just about get by with your transformer. You could use a separate 6.3V 500mA transformer for the preamp valves.
Of course I would probably choose a SE KT88 with negative feedback but thats just me.
Of course I would probably choose a SE KT88 with negative feedback but thats just me.
Both the idea of a SE KT88 and A loftin-white direct coupled amp are appealing to me. Im not totally sure which i would be happier with, ive been looking around and it seems like the loftin white might be a good idea. Has anyone heard one first hand how do they sound, would I be able to get a reasonable amount of power out of it. Im not looking for alot of power just enough for a nice listening level. Would running 100ma as apposed to 120 cause a problem in the final design?
A 2A3 will yield about 3.5 W. There are stability issues with the Loftin-White topology. The so called monkey topology is also DC coupled and more stable.
The EXACT ratings of your power trafo have become critical. Is that 100 mA. rating for RMS AC current or is the rating for available DC current with a cap. I/P filter? You can safely draw only about 56 mA. of DC with a cap. I/P filter if that secondary is rated for 100 mA. RMS.
I suggested 2X 300Bs with a choke I/P filter with power trafo limits in mind. The available DC current when choke I/P filters are employed is slightly greater than the AC RMS.
Cap. I/P filters a voltage rich and current poor. Choke I/P filters are current rich and voltage poor. You need enough current to provide for 2 O/P tubes. With your power trafo, a choke I/P filter is clearly indicated.
At approx. 8 WPC, a 300B based amp mates reasonably well with medium efficiency speakers (94-95 dB.). 2A3 amps really need speakers that are 98+ dB. efficient.
The EXACT ratings of your power trafo have become critical. Is that 100 mA. rating for RMS AC current or is the rating for available DC current with a cap. I/P filter? You can safely draw only about 56 mA. of DC with a cap. I/P filter if that secondary is rated for 100 mA. RMS.
I suggested 2X 300Bs with a choke I/P filter with power trafo limits in mind. The available DC current when choke I/P filters are employed is slightly greater than the AC RMS.
Cap. I/P filters a voltage rich and current poor. Choke I/P filters are current rich and voltage poor. You need enough current to provide for 2 O/P tubes. With your power trafo, a choke I/P filter is clearly indicated.
At approx. 8 WPC, a 300B based amp mates reasonably well with medium efficiency speakers (94-95 dB.). 2A3 amps really need speakers that are 98+ dB. efficient.
>>The Transformer is 100ma ac rms<<
There you are. The only way you can get the DC current for 2 O/P tubes out of that trafo is with a choke I/P filter. That means you can't get the Volts needed for DC coupling.
Don't fret; a CCS loaded IDH triode driver cap. (NOS "bathtub" oil?) coupled to a 300B will sound GOOD.
Combination bias using 150 Ohm cathode resistors will give you 7.5 V. of cathode bias at the 50 mA. idle current. That's enough to stabilize the operating point and prevent runaway, even though most of the grid bias comes from a supply. BTW, the unbypassed cathode resistors generate a LITTLE local current NFB. Your amp is going to be linear.
The amp will be heavy. In addition to the power and O/P trafos, you are going to need a low DCR 10 H./100 mA. filter choke and 2X low DCR 10 H./50 mA. filter chokes.
There you are. The only way you can get the DC current for 2 O/P tubes out of that trafo is with a choke I/P filter. That means you can't get the Volts needed for DC coupling.
Don't fret; a CCS loaded IDH triode driver cap. (NOS "bathtub" oil?) coupled to a 300B will sound GOOD.
Combination bias using 150 Ohm cathode resistors will give you 7.5 V. of cathode bias at the 50 mA. idle current. That's enough to stabilize the operating point and prevent runaway, even though most of the grid bias comes from a supply. BTW, the unbypassed cathode resistors generate a LITTLE local current NFB. Your amp is going to be linear.
The amp will be heavy. In addition to the power and O/P trafos, you are going to need a low DCR 10 H./100 mA. filter choke and 2X low DCR 10 H./50 mA. filter chokes.
Hi,
Does the project have to be an amp? The power tx you have is marginal for service in an amplifier. Especially a stereo one. I would put that in a super linestage. Choke input of corse, but as much for the sonic effect as for the voltage requirement. Shunt regulate its PS with gas tubes and current sources. Current source load the amplifier tube too. Really Good Sound is started with a linestage.
If you already have such a piece, the HV winding can just do two SE 300B's. Keep the rest of the filament requrements on different trasnformers, and it will increase the capacity just a bit. Choke input is just as Eli described, and quite useful for this app. Here is where you should get the best you can afford, Fortunately the Hammond Iron is pretty good. Even the 500 mA 10 Hy piece is not too expensive.
A 320 volt/50 mA 300B is going to put out more like 5 watts, maybe 6. depends slightly on load...
Good luck, keep asking questions, and don't let somebody stop you, justby saying "that's not the way to do it", or "it won't work that way". There are more than a few 'right' ways, that's sure!
regards,
Douglas
Does the project have to be an amp? The power tx you have is marginal for service in an amplifier. Especially a stereo one. I would put that in a super linestage. Choke input of corse, but as much for the sonic effect as for the voltage requirement. Shunt regulate its PS with gas tubes and current sources. Current source load the amplifier tube too. Really Good Sound is started with a linestage.
If you already have such a piece, the HV winding can just do two SE 300B's. Keep the rest of the filament requrements on different trasnformers, and it will increase the capacity just a bit. Choke input is just as Eli described, and quite useful for this app. Here is where you should get the best you can afford, Fortunately the Hammond Iron is pretty good. Even the 500 mA 10 Hy piece is not too expensive.
A 320 volt/50 mA 300B is going to put out more like 5 watts, maybe 6. depends slightly on load...
Good luck, keep asking questions, and don't let somebody stop you, justby saying "that's not the way to do it", or "it won't work that way". There are more than a few 'right' ways, that's sure!
regards,
Douglas
Bandersnatch makes a good point about tieing the filament windings off to make every last VA available to the HT winding.
Also, since the higher the B+ rail voltage is the more power the amp will produce, SS rectifying the 300B B+ with Cree 1200 PIV Silicon Carbide (SiC) Schottky diodes seems indicated. Schottky diodes are as quiet as vacuum diodes. The instant on feature of SS diodes can be dealt with easily by independently switching the 300B B+ trafo's primary. With SS rectification, the B+ rail will be about 365 V., a significant improvement.
Combination bias has an added benefit in reducing the power robbing voltage drop across the cathode resistor. Squeezing an extra 10 mA. out of the HT winding seems safe enough, as long as filaments are not being energized. Set the bias adjustment pots. to give a drop of 8.25 V. across the 150 Ohm cathode resistors. An 8.25 V. drop equates to a 55 mA. idle current.
Let's take a look at the additional power trafo requirements. A trafo with 2X 5 V./2 A. windings is needed to energize the 300B filaments. Either a Hammond 369EX or 369GX (depending on driver B+ requirements) will take care of driver B+, 300B bias supply needs, and heater power for a 7Z4 rectifier and the driver twin triode. I think that either a 6N1P or a 12AV7/5965 with CCS loading will take care of the driver requirement.
Also, since the higher the B+ rail voltage is the more power the amp will produce, SS rectifying the 300B B+ with Cree 1200 PIV Silicon Carbide (SiC) Schottky diodes seems indicated. Schottky diodes are as quiet as vacuum diodes. The instant on feature of SS diodes can be dealt with easily by independently switching the 300B B+ trafo's primary. With SS rectification, the B+ rail will be about 365 V., a significant improvement.
Combination bias has an added benefit in reducing the power robbing voltage drop across the cathode resistor. Squeezing an extra 10 mA. out of the HT winding seems safe enough, as long as filaments are not being energized. Set the bias adjustment pots. to give a drop of 8.25 V. across the 150 Ohm cathode resistors. An 8.25 V. drop equates to a 55 mA. idle current.
Let's take a look at the additional power trafo requirements. A trafo with 2X 5 V./2 A. windings is needed to energize the 300B filaments. Either a Hammond 369EX or 369GX (depending on driver B+ requirements) will take care of driver B+, 300B bias supply needs, and heater power for a 7Z4 rectifier and the driver twin triode. I think that either a 6N1P or a 12AV7/5965 with CCS loading will take care of the driver requirement.
Thanks for the advice everyone
I have been doing a great deal of reading on my own and in regards to the posts made on this forum in the past week. I am kind of having second thoughts about using the transformer for a marginally powered power amp when i could use it for a good line stage pre.
As my goal is to learn as much as I possible can about tube audio a pre would be just as bennificial i belive. It does set me back to ground zero design wise but i think it will give me a bit more flexability.
Mark
I have been doing a great deal of reading on my own and in regards to the posts made on this forum in the past week. I am kind of having second thoughts about using the transformer for a marginally powered power amp when i could use it for a good line stage pre.
As my goal is to learn as much as I possible can about tube audio a pre would be just as bennificial i belive. It does set me back to ground zero design wise but i think it will give me a bit more flexability.
Mark
Mark,
While the line stage idea is good, don't abandon the power amp idea. There are a few more tricks we can pull out of the bag to get you a 300B amp that produces the "standard" 8 WPC.
We are looking for ways to increase the B+ rail voltage. You can use the idle 6.3 VAC/2.5 A. filament winding to buck the primary voltage up. You connect the filament winding in series with the primary winding. 1 arrangement increases the B+ secondary voltage, while the 2nd arrangement decreases the B+ secondary voltage. You have to measure with a multi-meter to find the voltage enhancing arrangement. A 2nd tweak for increasing the B+ rail voltage is to use a smallish cap. in front of the 1st inductor. "Normally" a 10 nF. ceramic part is used solely to protect against inductive kick back. If 300 nF. of ceramic capacitance is installed, we "steal" a few Volts without substantially altering choke I/P behavior. The cap. can't be much larger, as somewhere around 1 muF. behavior will change to cap. I/P.
While the line stage idea is good, don't abandon the power amp idea. There are a few more tricks we can pull out of the bag to get you a 300B amp that produces the "standard" 8 WPC.
We are looking for ways to increase the B+ rail voltage. You can use the idle 6.3 VAC/2.5 A. filament winding to buck the primary voltage up. You connect the filament winding in series with the primary winding. 1 arrangement increases the B+ secondary voltage, while the 2nd arrangement decreases the B+ secondary voltage. You have to measure with a multi-meter to find the voltage enhancing arrangement. A 2nd tweak for increasing the B+ rail voltage is to use a smallish cap. in front of the 1st inductor. "Normally" a 10 nF. ceramic part is used solely to protect against inductive kick back. If 300 nF. of ceramic capacitance is installed, we "steal" a few Volts without substantially altering choke I/P behavior. The cap. can't be much larger, as somewhere around 1 muF. behavior will change to cap. I/P.
Thanks for the advice everyone
I have been doing a great deal of reading on my own and in regards to the posts made on this forum in the past week. I am kind of having second thoughts about using the transformer for a marginally powered power amp when i could use it for a good line stage pre.
As my goal is to learn as much as I possible can about tube audio a pre would be just as bennificial i belive. It does set me back to ground zero design wise but i think it will give me a bit more flexability.
Mark
I have been doing a great deal of reading on my own and in regards to the posts made on this forum in the past week. I am kind of having second thoughts about using the transformer for a marginally powered power amp when i could use it for a good line stage pre.
As my goal is to learn as much as I possible can about tube audio a pre would be just as bennificial i belive. It does set me back to ground zero design wise but i think it will give me a bit more flexability.
Mark
I have been looking at simple line stage SRPP amp, as well as a Balanced out line stage pre. Lots more reading i think before I decide on an actual schematic. I suppose I am leaning more towards a preamp now as it seems a little more logical as i would have an abundance of power to play around with.
Douglas, do you have any recommendations of where I can read alittle more on a pre design similar to the one you were describing. Or a specific schematic I could have a look at.
Ive heard good things about black plate tubes any thoughts??
Douglas, do you have any recommendations of where I can read alittle more on a pre design similar to the one you were describing. Or a specific schematic I could have a look at.
Ive heard good things about black plate tubes any thoughts??
eli, true. looking at the table below, one can get 8wpc on a 300B using the highlighted operating point.
PlateVoltage_ Grid Bias_ Plate Current_ Load Resistance_ Power Output_ 2nd Harmonic_ 3rd Harmonic_
Volts_ Volts_ Milliamps_ Ohms_ Watts_ db_ db_
200 -42 30 2000 3 20 31
200 -39 40 2500 2.6 26 38
200 -37 50 2500 2.5 30 45
250 -55 30 2000 4.9 18 27
250 -55 30 4500 3.2 27 40
250 -52 40 3000 4 26 36
250 -50 50 2500 4.4 26 39
250 -48 60 2000 4.7 26 38
250 -48 60 2700 4.1 30 45
250 -45 80 1500 5 26 41
300 -65 40 2500 6.7 20 30
300 -63 50 2000 7.2 21 29
300 -63 50 3000 6.1 26 37
300 -61 60 2400 6.6 26 37
300 -61 60 3400 5.6 30 44
300 -58 80 1700 7.5 26 37
350 -76 50 3600 7.8 26 38
350 -76 50 5000 6.2 30 45
350 -74 60 2000 10.2 21 30
350 -74 60 3000 8.3 26 38
350 -74 60 4000 7 30 44
350 -71 80 2200 9.6 26 39
400 -91 40 5000 8.4 26 37
400 -89 50 3000 11.5 21 31
400 -89 50 4000 9.4 25 38
400 -87 60 3500 10.5 26 38
400 -87 60 5000 8.3 30 46
400 -84 80 2500 12.5 25 37
450 -104 40 6000 9.5 26 38
450 -102 50 5000 10.7 27 39
450 -102 50 6500 9 30 45
450 -100 60 4000 12.5 26 38
450 -100 60 5500 10.1 30 44
450 -97 80 2000 17.8 21 30
450 -97 80 3000 14.6 26 37
450 -97 80 4500 11.5 31 45
a Tamura F-475 will also be very cost effective at that operating point. i have a friend who made one using WE tubes and it sounds great.
cheers!
PlateVoltage_ Grid Bias_ Plate Current_ Load Resistance_ Power Output_ 2nd Harmonic_ 3rd Harmonic_
Volts_ Volts_ Milliamps_ Ohms_ Watts_ db_ db_
200 -42 30 2000 3 20 31
200 -39 40 2500 2.6 26 38
200 -37 50 2500 2.5 30 45
250 -55 30 2000 4.9 18 27
250 -55 30 4500 3.2 27 40
250 -52 40 3000 4 26 36
250 -50 50 2500 4.4 26 39
250 -48 60 2000 4.7 26 38
250 -48 60 2700 4.1 30 45
250 -45 80 1500 5 26 41
300 -65 40 2500 6.7 20 30
300 -63 50 2000 7.2 21 29
300 -63 50 3000 6.1 26 37
300 -61 60 2400 6.6 26 37
300 -61 60 3400 5.6 30 44
300 -58 80 1700 7.5 26 37
350 -76 50 3600 7.8 26 38
350 -76 50 5000 6.2 30 45
350 -74 60 2000 10.2 21 30
350 -74 60 3000 8.3 26 38
350 -74 60 4000 7 30 44
350 -71 80 2200 9.6 26 39
400 -91 40 5000 8.4 26 37
400 -89 50 3000 11.5 21 31
400 -89 50 4000 9.4 25 38
400 -87 60 3500 10.5 26 38
400 -87 60 5000 8.3 30 46
400 -84 80 2500 12.5 25 37
450 -104 40 6000 9.5 26 38
450 -102 50 5000 10.7 27 39
450 -102 50 6500 9 30 45
450 -100 60 4000 12.5 26 38
450 -100 60 5500 10.1 30 44
450 -97 80 2000 17.8 21 30
450 -97 80 3000 14.6 26 37
450 -97 80 4500 11.5 31 45
a Tamura F-475 will also be very cost effective at that operating point. i have a friend who made one using WE tubes and it sounds great.
cheers!
So the transfo arrived today and it was described poorly. It actually turns out to be 820vct @ 100ma DC. Doesnt specified if its for a cap or choke input though. I would assume choke but Im not really sure, any ideas out there? It is a old military piece made by ITC it is the tp-6827. So perhaps the dream of a standard single ended design isnt so far off after all.
Mark
Mark
Usually they're rated for cap input. Measure the winding resistance and calculate the loss - 0.1^2 * R. One rule of thumb I use is to allow 2 Watts per winding - a bit more on large pieces, less on smaller ones.
Hammond has a page on transformer ratings:
http://www.hammondmfg.com/pdf/5c007.pdf
Hammond has a page on transformer ratings:
http://www.hammondmfg.com/pdf/5c007.pdf
You can get 100 mA cap input or 150 mA choke input, at a lower voltage, of course. If you use choke input with a solid state rectifier, be sure to use a small input cap (0.1 - 0.5uF) or an MOV to protect the rectifiers from surges. Download the PSU simulator program from duncanamps.com and you can run through all the design possibilities quickly.
You can use that transformer with a L input to bring the voltage lover
for the Loftin White constructions or tru with a C input for higher voltages for VT4C tubes.I suggest L input.You have to have a minimum of 150-200 mA curent capability for the stereo.
Also You can use the 6.3V out for supplyng the two heater transformers for the output tubes (6.3 to required voltage center tapped)
for the Loftin White constructions or tru with a C input for higher voltages for VT4C tubes.I suggest L input.You have to have a minimum of 150-200 mA curent capability for the stereo.
Also You can use the 6.3V out for supplyng the two heater transformers for the output tubes (6.3 to required voltage center tapped)
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