Hi all,
I've finished designing a solid state rectified tube amp, and I'm purchasing parts for it shortly. Before I do, I'd like to double check my choice of power transformer with those of you who are more experienced. Here are the details:
The current demands for the HT winding are:
- 2 x 6L6GC @ 52mA = 104mA
- 2 x 6L6GC screens @ 10mA = 20mA
- 1 x 6GH8A @ 12mA per side = 24mA
- 1 x 6GH8A screen @ 5mA = 5mA
- 4 x 12AX7 @ 1mA per side = 8mA
Total: 161mA
The current demands for the heaters are:
- 2 x 6L6GC @ 0.9A
- 1 x 6GH8A @ 0.45A
- 4 x 12AX7 @ 0.3A
Total: 3.45A
The HT is 400V, so a rough estimation of power demands would be:
Power = 400*0.161 + 6.3*3.45 = 86VA
I've selected the 300-0-300 Hammond 272FX transformer for this amp. Here are its ratings:
146 VA
115/125V Primary
60 Hz
600V C.T. @ 172ma.
5V C.T. @ 3A
6.3V C.T. @ 5A
It's got a way higher VA rating than I need, but I'm just slightly under the 172mA the transformer is rated for. I'm going to guess that this means I'll probably have a little bit of sag when things break up, which is okay because this amp is meant to sit on the border between clean and slightly overdriven.
For a full wave, capacitive input load, the Hammond Design Guide shows the peak will be 0.71*Vsec, or 0.71*600 = 426V. I can drop some of that if need be.
Can anyone confirm that this transformer should work for this amp, or offer any advice if it won't? I'm checking here to confirm because my PSUD2 simulations were ringing in around 350V once I accounted for winding resistances, 50V lower than I wanted to see.
I've finished designing a solid state rectified tube amp, and I'm purchasing parts for it shortly. Before I do, I'd like to double check my choice of power transformer with those of you who are more experienced. Here are the details:
The current demands for the HT winding are:
- 2 x 6L6GC @ 52mA = 104mA
- 2 x 6L6GC screens @ 10mA = 20mA
- 1 x 6GH8A @ 12mA per side = 24mA
- 1 x 6GH8A screen @ 5mA = 5mA
- 4 x 12AX7 @ 1mA per side = 8mA
Total: 161mA
The current demands for the heaters are:
- 2 x 6L6GC @ 0.9A
- 1 x 6GH8A @ 0.45A
- 4 x 12AX7 @ 0.3A
Total: 3.45A
The HT is 400V, so a rough estimation of power demands would be:
Power = 400*0.161 + 6.3*3.45 = 86VA
I've selected the 300-0-300 Hammond 272FX transformer for this amp. Here are its ratings:
146 VA
115/125V Primary
60 Hz
600V C.T. @ 172ma.
5V C.T. @ 3A
6.3V C.T. @ 5A
It's got a way higher VA rating than I need, but I'm just slightly under the 172mA the transformer is rated for. I'm going to guess that this means I'll probably have a little bit of sag when things break up, which is okay because this amp is meant to sit on the border between clean and slightly overdriven.
For a full wave, capacitive input load, the Hammond Design Guide shows the peak will be 0.71*Vsec, or 0.71*600 = 426V. I can drop some of that if need be.
Can anyone confirm that this transformer should work for this amp, or offer any advice if it won't? I'm checking here to confirm because my PSUD2 simulations were ringing in around 350V once I accounted for winding resistances, 50V lower than I wanted to see.
You are basically looking for Fender Bandmaster/Vibrolux/Pro Reverb transformers. Most have two voltages for either SS or tube rectifiers. Also check Classic Tone and Weber transformers. They are cheaper.
Fender Power Transformer, 8087, 125P5D, 125P5DX, 022798, 022806, Super Reverb, Bandmaster Reverb, Pro, Pro Reverb, Tweed Bassman, 120/240V
Weber is the cheapest, I use them myself and I use SS rectifier.
http://www.tedweber.com/w022798int
If you interested in this Weber, email to them to make sure they fixed the HT. I bought it almost a year ago, I discovered the voltage of the two side of the CT secondary is not balance. I reported to them and they acknowledged it and gave me another model as replacement. they likely have fixed the problem by now, but do check with them. For $52, it's worth your trouble.
Fender Power Transformer, 8087, 125P5D, 125P5DX, 022798, 022806, Super Reverb, Bandmaster Reverb, Pro, Pro Reverb, Tweed Bassman, 120/240V
Weber is the cheapest, I use them myself and I use SS rectifier.
http://www.tedweber.com/w022798int
If you interested in this Weber, email to them to make sure they fixed the HT. I bought it almost a year ago, I discovered the voltage of the two side of the CT secondary is not balance. I reported to them and they acknowledged it and gave me another model as replacement. they likely have fixed the problem by now, but do check with them. For $52, it's worth your trouble.
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You can't equate DC current draw with transformer AC current delivery. You will get lots of sag, and probably transformer overheating. Read up on capacitor input PSUs.
You need a high voltage secondary VA rating about three times the constant DC power draw. So 400V x 161mA = 65W, x3 = 195VA. Plus heater 22W=22VA (assuming AC heaters) so you need 220VA minimum. With good ventilation you might get away with 200VA. If you want reliability go for 300VA.
You need a high voltage secondary VA rating about three times the constant DC power draw. So 400V x 161mA = 65W, x3 = 195VA. Plus heater 22W=22VA (assuming AC heaters) so you need 220VA minimum. With good ventilation you might get away with 200VA. If you want reliability go for 300VA.
You're correct, it's the idle current. I assumed the ratings given on the transformer would be for idle.
Unfortunately, I live in Canada - shipping on that transformer costs an extra $60.
I've been trying to design with what A1 Parts carries, since they're pretty close by. Although, if as DF96 says, I need to aim for a much higher VA rating, A1 starts becoming more expensive than the Weber transformer plus shipping.
I'll look for more literature when I'm home from work, but in the mean time, what design guides or manuals for selecting power transformers have been most helpful to you?
How come? You check with multiple stores like Small Bear etc.?
What you want is a typical Fender Vibrolux, Pro, Bandmaster and Super Reverb. It is as common as it gets. Look at your local brands. 200mA is all you need. Typical Vibrolux and Pro Reverb has more tubes, so I am pretty sure any PT designed for these amps are good enough for you. PT is not critical, I would not get the expensive brands.
Try go to some music store that has service dept. Ask them whether they sell replacement PT for those amps.
Side note, I use Classic Tone output transformer for Vibrolux in both my amps, I talked to people in guitar amp forum and they all said Classic Tone is very good. I really like it. If I design another guitar amp using 2 EL34 or 6L6, I'll get the same one again. 40W is all people need, in fact, it's too loud. Both my amp have power scaling designed in that I can get down to 1W when fully cranked. They really sound good.
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Agreed, my amp is around 40W. Is your power scaling just a potentiometer in series with the PI cathode, to put it closer to cut-off?
I'd never heard of Small Bear. Checking them out now.
If you decided to buy from Weber, check out their high voltage filter caps, choke and others. I bought a lot from them, They have filter caps that are rated 600V and cheap. I use them, no hum, my amps are amount the quietest. I use their shinghan tubes. It's cheap and works. I am making a statement that most of the carbon comp resistors, F/T caps, expensive transformers are snake oil. I use a lot of Weber parts, all metal film resistors, all CERAMIC caps unless I cannot get the value and voltage rating. My two amps sound very good. It's all about the design. Check out if the shipping is the same if you order a lot of stuffs. That might make it worth your while.
Don't they have distributor in Canada? They are big name, their speakers have absolute raving reviews in guitar circle.
As I said before, shop all the stuffs you need and make one big order and see whether that make it worth the shipping. Look into their guitar speakers, I was on two of the biggest forum of guitar and I yet to hear a negative review yet. It's all raving reviews. I like WGS for speakers though, so I never have real experience with their speaker. I can only speak from reading all the posts.
I am not relate in any way with Weber, in fact I have two of the transformers that I cannot use because of the imbalance problem. They treated me nice, they sent me replacement. That's the reason I warned if OP decide to order that transformer, confirm with them the problem has been resolved.
As I said before, shop all the stuffs you need and make one big order and see whether that make it worth the shipping. Look into their guitar speakers, I was on two of the biggest forum of guitar and I yet to hear a negative review yet. It's all raving reviews. I like WGS for speakers though, so I never have real experience with their speaker. I can only speak from reading all the posts.
I am not relate in any way with Weber, in fact I have two of the transformers that I cannot use because of the imbalance problem. They treated me nice, they sent me replacement. That's the reason I warned if OP decide to order that transformer, confirm with them the problem has been resolved.
Unfortunately, there's only a handful of places to buy components in Ontario - and it pretty much jumps from cheap overstock to expensive hi-fi, without much middle ground. Everything else has to be shipped from the States. For example, I can't find reverb tanks near me.
I found a Hammond 290EX at A1 Parts in Etobicoke for $80, so assuming that transformer passes a double-check then I'll drive over there to pick one up. PSUDII is giving me a nice 400 in my simulations.
I found a Hammond 290EX at A1 Parts in Etobicoke for $80, so assuming that transformer passes a double-check then I'll drive over there to pick one up. PSUDII is giving me a nice 400 in my simulations.
A tube guru friend of mine told me that the current draw of a multisection RC filter in the power supply will approximately double the load seen by the power transformer. I don't know if that's exactly true, but it did make me aware that I had forgotten to consider that at all. In use, those caps are constantly needing to be recharged.
I also like to choose a secondary hi-V voltage that gives me at least 50 volts more than I need, so I can drop that 50 volts across the power resistors feeding the filter caps, so the filtering will be much better, and I won't need a choke. It also makes it easy to adjust the B+ to right where you want it, based on the Pd spec of the output tubes. Many amps push powertubes significantly beyond their max Pd rating, some claiming it sounds better. I don't. I prefer reliability, and longer tube life. My amps sound as good as anything I've heard.
I also like to choose a secondary hi-V voltage that gives me at least 50 volts more than I need, so I can drop that 50 volts across the power resistors feeding the filter caps, so the filtering will be much better, and I won't need a choke. It also makes it easy to adjust the B+ to right where you want it, based on the Pd spec of the output tubes. Many amps push powertubes significantly beyond their max Pd rating, some claiming it sounds better. I don't. I prefer reliability, and longer tube life. My amps sound as good as anything I've heard.
The 6.5V filament at 3.15A is too low. Plenty of HV current that is not necessary as guitar doesn't not produce continuous signal. But filament burns continuously. Also, 6.5V??!!
Or else, it's good price for no shipping. Maybe look for a small 6.3V transformer to supplement it.
I don't think it's true RC filter at supply doubles the load on the transformer. Surge current might be higher during charging part of the cycle, but the duration is shorter and average out.
Most of the Vibrolux/Pro/bandmaster replacement transformers have both SS taps and tube rectifier taps that give about 30V higher, that's exactly what you are looking for.
Very few guitar amps "need" a choke. Explore a bunch of present day fender, marshall, peavey etc amps and see how few include a choke.
The string of filter caps only need to be recharged to compensate for the small amount of leakage, any bleeders, plus the charge that is USED by the circuit.
Instead of just accepting a rule of thumb from your guru, get out a calculator. The voltage drop along the B+ string is due to the current drawn by the tubes. A reasonable rule of thumb is a milliamp per triode, and so on, as posted in the first post. Those resistors are not using power, they are simply transporting it to the tubes. If all your tubes draw 250ma, I can't imagine how the B+ resistors would draw an additional 250ma.
The string of filter caps only need to be recharged to compensate for the small amount of leakage, any bleeders, plus the charge that is USED by the circuit.
Instead of just accepting a rule of thumb from your guru, get out a calculator. The voltage drop along the B+ string is due to the current drawn by the tubes. A reasonable rule of thumb is a milliamp per triode, and so on, as posted in the first post. Those resistors are not using power, they are simply transporting it to the tubes. If all your tubes draw 250ma, I can't imagine how the B+ resistors would draw an additional 250ma.
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