Hi,
I'm looking to build a small low watt practice amp with 6aq5 PP for my nephew and I'm having a hard time finding a small PT that doesn't cost more than the rest of the parts. The only thing I can find in low voltage is a stand alone reverb PT with 260V, not center tapped. Would that be sufficient? The only other thing I can find less than $50 is 550v. I could pay $70 for a 380v but that's too much money. The other constraint is size. It's a small little case that has about 3-4 inches square to put the PT.
Thanks,
Daniel
I'm looking to build a small low watt practice amp with 6aq5 PP for my nephew and I'm having a hard time finding a small PT that doesn't cost more than the rest of the parts. The only thing I can find in low voltage is a stand alone reverb PT with 260V, not center tapped. Would that be sufficient? The only other thing I can find less than $50 is 550v. I could pay $70 for a 380v but that's too much money. The other constraint is size. It's a small little case that has about 3-4 inches square to put the PT.
Thanks,
Daniel
I think that Triad Transformers N-68X would be a great thing to make that amp with, they are cheap here in Europe and cost only about 15$, they are a 1:1 rato transformer with 1x primary and 2x secondary in one to one ratio output each, so I think that there maybe is a way of connecting that transformer in series to get a 1:2 ratio PT that would put out about 230V.
I used a Hammond 186D120 transformer, along the same lines as the Triad one. The Hammond is a 120:120/240V isolation transformer. No 6V tap, I used 12AQ5's and got a 12V switching wall wart to feed the heaters. The Hammond, I used the secondary winding as the primary and used a bridge rectifier off the 240V (120 + 120) primary windings. It gives 280V with no load and I think 270V with tubes, I don't have them in right now. Here is the schematic, I used a Mosfet for the PI saving a tube. I have a circuit in it to go from a bass-treble BF style controls to a Tweed tone control. I don't know if I would go to the trouble again but maybe a switch to increase the midrange resistor.
AS-05T200 - 50VA 200V Transformer - AnTek Products Corp ?
50VA, 200V and 180V taps, 6.3VAC heaters, $28
A 100VA, 275V version is $34.50.
other voltages are available
Disclaimer: have used neither
50VA, 200V and 180V taps, 6.3VAC heaters, $28
A 100VA, 275V version is $34.50.
other voltages are available
Disclaimer: have used neither
Last edited:
You could wire the Triad normally with 120 / 240 volt primaries and a single 120 volt secondary. It will produce more than 120 volts, since it is rated to make 120 volts at a full resistive load with 120 volts on the primary. In one of my little guitar amps I get 134 volts on the secondary and 165 to 170 volts of B+ with a diode bridge rectifier. That's enough voltage for a few watts of audio with most small tubes. You would still need a 6.3 volt filament transformer to heat the tubes.
My go - to transformers for over 20 years of guitar amp building has been the 6K... series from Allied Electronics. These are made by Hammond, yet cost less than the same Hammond. Unfortunately the prices started catching up with the "vacuum tube stuff" price curve recently.
I liked the Allied 5K56VG for all small (under 25 watt) amps when it was $38, but now its over $50. With 540 VCT, 5 and 6.3 volt windings it will make about 320 volts of B+ and work well with 6AQ5's, 6V6's, EL84's and 6CW5's.
The 6K7VG was $42, now $59 will squeeze over 50 watts from a pair of KT88's on 440 volts with a 5AR4 rectifier, more with silicon.
The little 6K49VG is a bit low on current for a full on 10 watt 6AQ5 amp. Its rated for 50mA but is 50Hz capable. You can draw more on 60Hz. I have the 120 mA 6K56VG feeding my 300B stereo amp over 150 mA. It gets pretty warm but so do all Hammonds. It costs only $28. You will get almost 300 volts of B+ with a silicon rectiifier. 6AQ5's have no problem eating this, but you could use a 6X4 or 6X5 tube rectifier to lose a few volts.
There are several other ways to get there from here. Low watt means different things to different people. My ultra low budget low watt amp makes 4 watts at full crank and costs $50 to $60 including transformers, but doesn't use 6AQ5's. It uses series string radio tubes designed to run off line voltage. I power it with a Triad N-68X ($15 from Mouser) and use a $4 70 volt line transformer from parts express as the OPT. Output tubes are 32ET5's, driven by an 18FY6 (1/2 a 12AX7) and a 18FW6 (a small pentode like a 6AU6).
Schematic:
I like those. Good price and they seem like they would work. There's a 300V that I'm thinking would be good to get about 250V on the plates plus power a PI and preamp tube.
One thing I have not done is hooked up a transformer without a center tap already there. You all make it sound easy, so I'm sure I can find some diagrams.
Thanks!!
I have this one but have not used it yet.
AS-05T280 - 50VA 280V
Power transformer for tubes. Has 280V coil with 260V tap and 2 of the 6.3V 2A coil.
Electrical Characteristics
Outputs 3x Power 50VA Output Voltage Current 280V 0.09A 260V 0.09A 6.3V 2A 6.3V 2A
AS-05T280 - 50VA 280V Transformer - AnTek Products Corp
I haven't used it yet.
AS-05T280 - 50VA 280V
Power transformer for tubes. Has 280V coil with 260V tap and 2 of the 6.3V 2A coil.
- Product Brief
- Weight: 2.0lb
- Dimensions: 3.6" d x 1.6" h
Electrical Characteristics
Outputs 3x Power 50VA Output Voltage Current 280V 0.09A 260V 0.09A 6.3V 2A 6.3V 2A
AS-05T280 - 50VA 280V Transformer - AnTek Products Corp
I haven't used it yet.
Find one or more 30VAC transformer and use voltage multipliers. GCWills has trodden this path for his "Lamington" series of amplifiers.
The simpliest is the one used in the Solo (12ax7 + EL34 SE) - the thread at AGGH also includes discussion on where to find all those 100uf/400VDC caps cheaply.
See the power supply page on Valve Heaven for a bigger one.
The simpliest is the one used in the Solo (12ax7 + EL34 SE) - the thread at AGGH also includes discussion on where to find all those 100uf/400VDC caps cheaply.
See the power supply page on Valve Heaven for a bigger one.
Is this something that will work with a transformer stepping down voltage from a primary 120v to a secondary of 24v or 48v etc.? Just hook it up backwards and get a ratio of 5 or 3:1? I can see getting 600 or 360v doing this, but I'm curious about how safe it is.
It's not safe. You cannot exceed the voltage rating of the winding even if you do wire it backwards. That's why some of the previous suggestions were to use isolation transformers backwards. That way you put 120V on a winding rated for 120V.
No. The secondary on a 24-48V transformer has a low resistance and the real bad thing is low inductance. So when you hook it up to 120V onto the secondary there is not enough impedance to limit the current through the winding. You end up having the winding act as a resistor rather than an inductor and the winding will fry.
If you are in the ballpark as far as voltage goes then it is an option. In the case where I flipped the windings and used the secondary on 120V, the unloaded secondary puts out about 130V unloaded with the intended primaries hooked up to the line. Some people have used a 120V:12V and fed the tube heaters and then hooked up a 120:12 backwards so the 12V and 12V are connected and they take the higher voltage generated to drive the HV circuitry. Not ideal either but if your transformer on the line is bigger than the other one and can manage the heaters also you can get away with it. People have done that in Oz Land because electronic equipment and parts cost an arm and a leg.
I don't think I should blatantly reference one of my eBay listings here, but I can't find it anyway...apparently I deleted ended listings.
So perhaps PM me if 2-3/4" high, 2.33" wide, 2.7" deep (end bell-to-end bell), 285 VDC @ 80 mA DC (405 V no load) (with your rectifier & filter cap), works for you. oh, 2nd winding is 14.5 vac @ 1 a ac)
Apologies if inappropriate here.
Murray
So perhaps PM me if 2-3/4" high, 2.33" wide, 2.7" deep (end bell-to-end bell), 285 VDC @ 80 mA DC (405 V no load) (with your rectifier & filter cap), works for you. oh, 2nd winding is 14.5 vac @ 1 a ac)
Apologies if inappropriate here.
Murray
I was looking at the Antek 10 VA toroids as PP OPT's: dual 120 pri, AN-0105, -0106, -0107, -0109, (5,6,7,9 V sec.) but don't know how the inductance is...I would have assumed decent but apparently in general some people find toroid PT primary inductance on the low side.
Any field data on such misuse?
Any field data on such misuse?
This works _BUT_....
I once had a 120V:30V transformer. Unlabeled. Too stupid to look at the Ohms. I wired to 120V, the room-lights dimmed, and my 50V meter went SLAM! Yes, you really do get 480V, until the smoke comes out.
If you put "too much" voltage on an iron core winding, the iron core "saturates". At this point it is no better than air. The winding current gets VERY large, far past any normal use. If you don't pop a fuse, the transformer overheats and burns.
A "120V" winding has some allowance for high wall voltage. 130V may be fine. 140V may run hot. 160V is almost certain to smoke. Likewise and proportional for lower voltages. A "24V" winding may take 26V but 30+V is sure to be bad.
For low wattage valve amps I use two cheap standard transformers wired back to back.
I used 240VAC to 12VAC transformer to give heater volts. Then 12VAC into another transformer wrong way around to go back to 240VAC. Works very well.
I used 240VAC to 12VAC transformer to give heater volts. Then 12VAC into another transformer wrong way around to go back to 240VAC. Works very well.
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