I guess this is probably a ping for Shoog but anyone may have knowledge. Considering trying toroidal PT as OPT for bass guitar amp, say 200W. Please don't say post in the I&A forum, the knowledge in this forum is most appropriate.
Any help appreciated. What VA rating required? What tubes (big bass amps usually use parallel 6550 or EL34, but are lower Rp tubes the only way to go for this)? Cap parafeed or SS CCS to keep DC out?
Any help appreciated. What VA rating required? What tubes (big bass amps usually use parallel 6550 or EL34, but are lower Rp tubes the only way to go for this)? Cap parafeed or SS CCS to keep DC out?
Using toroidal power transformers as OPT's is a crap shoot, especially as the power level goes up. Often times a smallish power toroid will have enough primary inductance to make a useful OPT at low frequencies and not a whole bunch of leakage inductance to kill the highs. As the transformer gets bigger, these requirements get tougher. You have to try a transformer to see if it will work.
The lowest note on a standard bass guitar is around 41Hz. There are plenty of people tuning lower than this. A bass guitar can have a higher harmonic content than a regular guitar, so you want the high frequency response to reach at least 5KHz, or as high as your speakers will go.
Maybe not the only way to go but often the easiest. Remember you are trying power transformers, the driving impedance of the wall outlet is less than 1 ohm! If your OPT is short on primary inductance you want the driving impedance of the output tubes as low as possible.
I have tinkered with power toroids for guitar amp OPT's with some success. You want a transformer rated for 50 Hz operation, and usually a little bigger than the expected power output. I have had the best luck with sweep tubes that have "Schade" type local feedback wrapped around them. A sweep tube has a high peak current capability and a lowish Rp as pentodes go. Schade feedback lowers the Rp further. See Pete's big red board for an excellent example.....it will make 200+ WPC, but that's stretching it. Several examples were made at 125WPC, parallel the channels and be done with it.
Best luck with big toroids.....find a 120/240 isolation transformer with 4 120 volt windings that does NOT have an epoxy center. Wire one primary and one secondary in series for each half "primary" and wind your own speaker secondary through the core, right over the existing tape. The primary and secondary windings are not always identical to make up for losses, use one of each in each side to maintain balance.
Let the DC flow through the windings, just keep the balance close. Note, perfect DC balance is not always what you want. Adjust the balance for minimum distortion at the lowest frequency.
The lowest note on a standard bass guitar is around 41Hz. There are plenty of people tuning lower than this. A bass guitar can have a higher harmonic content than a regular guitar, so you want the high frequency response to reach at least 5KHz, or as high as your speakers will go.
Maybe not the only way to go but often the easiest. Remember you are trying power transformers, the driving impedance of the wall outlet is less than 1 ohm! If your OPT is short on primary inductance you want the driving impedance of the output tubes as low as possible.
I have tinkered with power toroids for guitar amp OPT's with some success. You want a transformer rated for 50 Hz operation, and usually a little bigger than the expected power output. I have had the best luck with sweep tubes that have "Schade" type local feedback wrapped around them. A sweep tube has a high peak current capability and a lowish Rp as pentodes go. Schade feedback lowers the Rp further. See Pete's big red board for an excellent example.....it will make 200+ WPC, but that's stretching it. Several examples were made at 125WPC, parallel the channels and be done with it.
Best luck with big toroids.....find a 120/240 isolation transformer with 4 120 volt windings that does NOT have an epoxy center. Wire one primary and one secondary in series for each half "primary" and wind your own speaker secondary through the core, right over the existing tape. The primary and secondary windings are not always identical to make up for losses, use one of each in each side to maintain balance.
Let the DC flow through the windings, just keep the balance close. Note, perfect DC balance is not always what you want. Adjust the balance for minimum distortion at the lowest frequency.
Makes me wonder if anybody has experimented with using one of the OTL topologies to drive a power toroidal.
Tubelab said it all really.
I have had good success using Toroids and when I have used 150VA ones with an input power of about 7W I have measured good response down to 10hz. However these babies are designed for a lower limit of 50hz and they don't tend to throw extra into the mix if it costs more money and is not part of the design brief. If you look at the commercial toroidal OT's they are simply enormous and that is down to the fact that the core has to get bigger to take you down to the 10-50hz range. I have also found that they respond very differently depending on the orientation of the primary windings - I usually tie the outers together and apply B+ and use the inner ends of the windings to the plates. If you do it the wrong way the excess capacitance tends to resonate the transformer out past 40khz (2-3x the signal peak) which saturates the core and results in much poorer response below 100hz. Getting it right is down to using your ear or a signal generator and scope. I find I can hear the difference reasonably well, get it wrong and it sound fatiguing.
I have recently been using 30 and 50 VAtoroidals with success - but they are easier to saturate than the big ones.
I would say that you are pushing these transformers to their limits trying to use them for a bass amp duty, but if you have some or can get some they are worth a try.
I very much agree that they work much better for low rp and Schaded designs because its easier to drive the relatively low inductance and get the ratio's in the right ball park. They can sound really good with the right valve.
Shoog
I have had good success using Toroids and when I have used 150VA ones with an input power of about 7W I have measured good response down to 10hz. However these babies are designed for a lower limit of 50hz and they don't tend to throw extra into the mix if it costs more money and is not part of the design brief. If you look at the commercial toroidal OT's they are simply enormous and that is down to the fact that the core has to get bigger to take you down to the 10-50hz range. I have also found that they respond very differently depending on the orientation of the primary windings - I usually tie the outers together and apply B+ and use the inner ends of the windings to the plates. If you do it the wrong way the excess capacitance tends to resonate the transformer out past 40khz (2-3x the signal peak) which saturates the core and results in much poorer response below 100hz. Getting it right is down to using your ear or a signal generator and scope. I find I can hear the difference reasonably well, get it wrong and it sound fatiguing.
I have recently been using 30 and 50 VAtoroidals with success - but they are easier to saturate than the big ones.
I would say that you are pushing these transformers to their limits trying to use them for a bass amp duty, but if you have some or can get some they are worth a try.
I very much agree that they work much better for low rp and Schaded designs because its easier to drive the relatively low inductance and get the ratio's in the right ball park. They can sound really good with the right valve.
Shoog
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World Audio Design offered an amplifier kit that used a toroid as the output transformer.
They quickly redesigned it (as the mkII) to use an EI output that had sufficient inductance to effectively load the valves. They offered the new transformer to all the mkI buyers so that they could upgrade to the proper specification.
Look up WAD KEL80
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I suspect the real reason they changed out the transformers on the WAD KEL80 is because they had no DC self bias arrangement. In operation the bias would drift and apply DC to the toroidals - which would make them distort badly. This happens at relatively small DC levels of 5-10mA of imbalance. Bias adjustment would become an essential chore which most people would neglect.
Fixing the DC bias for both sides of the PP pair is absolutely essential for getting satisfactory performance from toroidals and is the main reason why most people will never use them.
Shoog
Fixing the DC bias for both sides of the PP pair is absolutely essential for getting satisfactory performance from toroidals and is the main reason why most people will never use them.
Shoog
I did the same thing. I used P-P UL84's and an Antek toroid, maybe 25VA, not sure. I was getting 15 clean watts out of that amp and about 20 watts cranked. I measured the frequency response of the output stage, feeding the signal generator in after the tone stack. I don't remember the result, but remember that it wasn't quite HiFi, but far better than I had expected, or needed.
I have moved twice since then, and I still haven't found that amp, or any of the stuff that was with it, like the 8 inch Alnico speaker.
Guido Tent sells a bias servo for a couple of hundred bucks that works really well. If you are doing it for only one amp, that may be the way to go.
I designed a servo for an amp I was making with Plitron transformers and had issues with the board I had made. I was going to have another version fab'd but decided to just go with Tent's and get the project finished.
I'm in no hurry to build the big amp. I have recently moved twice and need to finish building my new lab first.
When It's time I will probably go back to a project I started in 2008 and pick up where I left off with microprocessor control. A 1KW tube amp needs some constant monitoring to keep a big bang from happening. I plan to have a lot more than bias under the watchful eye of a microchip.....I even want it to test it's own tubes.
When It's time I will probably go back to a project I started in 2008 and pick up where I left off with microprocessor control. A 1KW tube amp needs some constant monitoring to keep a big bang from happening. I plan to have a lot more than bias under the watchful eye of a microchip.....I even want it to test it's own tubes.
That sounds cool. My last two amps had a microprocessor that did a power sequence to avoid dimming the lights at power-up. There were also comparators that tell the microprocessor if any of the power tubes draw too much current. If that happens, the microprocessor shuts the amp down.
The feature does work, I have inadvertently tested it.
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