Hello everyone, just getting some opinions. I’ve had an idea in my head for a while that everyone seems to think I’m crazy for, I’m looking to build(or likely have built; this may be out of my experience range) what I consider the ideal gigging tube bass amp. Power is 300w, ideally clean. I know that the Svt is the go to for this sort of thing but both of mine have been unreliable tube eating behemoths. Having had mine apart so many times they’re quite complex pieces of design, and I may be of the persuasion that they could be simpler.
My sketch out was to use Svt transformers as they can be acquired used for reasonable money, anything else is probably $800 or more in just iron and I think they’ll do what I need. That gives me ~670 vdc loaded and around 1900 ohms for a plate load if I remember right. I’m unsure of tubes, but I have considered a quad or sextet of 6kg6s, 6lw6s, or 6146s. If I have to kill lots of tubes in the process I’d rather it not be $100 6550s. There have been posts of many successful builds with sweep tubes but I have absolutely no experience with them so I’m open to advice.
The abject goal here isn’t cost, though like most of you I’m sure, I’d like to keep it reasonable. My main goal is reliability and simplicity or as close as I can get. I hate having to undo 10 plugs, many pots and jacks, and remove two boards any time I have to do something.
Essentially, if presented the above design criteria, what would y’all do? I’m still a puppy in tube electronics, and I’m sure there are fellas on here that have done this longer than I’ve been alive. I’d love to hear suggestions.
Before someone says try solid state, I have and it didn’t last one gig without fire and fixing that one almost took an act of Congress to get parts, so I’d like to stay tube if possible.
My sketch out was to use Svt transformers as they can be acquired used for reasonable money, anything else is probably $800 or more in just iron and I think they’ll do what I need. That gives me ~670 vdc loaded and around 1900 ohms for a plate load if I remember right. I’m unsure of tubes, but I have considered a quad or sextet of 6kg6s, 6lw6s, or 6146s. If I have to kill lots of tubes in the process I’d rather it not be $100 6550s. There have been posts of many successful builds with sweep tubes but I have absolutely no experience with them so I’m open to advice.
The abject goal here isn’t cost, though like most of you I’m sure, I’d like to keep it reasonable. My main goal is reliability and simplicity or as close as I can get. I hate having to undo 10 plugs, many pots and jacks, and remove two boards any time I have to do something.
Essentially, if presented the above design criteria, what would y’all do? I’m still a puppy in tube electronics, and I’m sure there are fellas on here that have done this longer than I’ve been alive. I’d love to hear suggestions.
Before someone says try solid state, I have and it didn’t last one gig without fire and fixing that one almost took an act of Congress to get parts, so I’d like to stay tube if possible.
You seem to have reliability issues with your bass amps, so you want to build something better, more reliable, but simpler. Maybe you should investigate why you are having issues with your current hardware. Is the bias set correctly when you retube the SVT? Are you using the correct speaker impedance setting for your speakers? Getting either of these wrong shortens tube life.
There have been lots of things called SVT over the years, some of which are not even all tube amps. For that matter Ampeg itself has gone through several reissues. The schematic I have is from a typical early 70's 6550 version. For its intended use I do not find it overly complex. It does have two different preamp channels, and you could easily do away with the channel you don't use. The power amp does not seem overly complex for a 300 watt tube amp. If I were doing this today I would replace the follower stages with mosfets to slightly simplify things.
The original first generation Ampeg SVT tube head used 6146 tubes. Many went up in flames. The 6146 is a decent tube for mobile radio equipment, but the screen grids can't deal with the current surges seen when used for audio in an amplifier that gets driven into clipping a lot. Ampeg switched to the 6550 for that reason. Later versions of the SVT seemed reasonably reliable. I saw exactly one go up in flames since the 70's and I got to fix that one. The speakers being used could not eat what the SVT fed them so they failed one by one until the SVT had no load. At this point it spit fire! Autopsy revealed a lot of damage including the power and output transformers and even a tube socket.
You don't state your experience level, but state that you have never worked with TV sweep tubes. I would not call a 300 watt bass guitar amp a good project for a relatively inexperienced amp builder. The plate voltage seen in a typical tube amp that is driven to near clipping is about twice the B+ voltage. I have measured peaks of about 2500 volts in a guitar amp that ran on 430 volts but was driven "way past 11" with an ADA MP1 using a preset that I called "Jimi." With your expected 670 volts of B+ you could easily see NORMAL voltage peaks in the 4000 volt range. The build and parts quality must be excellent at this level.
You mention the 6KG6 and 6LW6 TV sweep tubes. These are the big boys in the tube amp world and either would be a good choice for a high powered tube amp. I have seen 250 watts flow from a single pair of 6LW6's. The issue with these tubes is that there are lots of different versions of each that have been made over the years, especially the 6LW6. You could get by with 4 6LW6's but you would need 4 that are of exactly the same construction. I have seen 6 different kinds of 6LW6's in two different glass envelopes. For this reason, I have decided to use the 26HU5 in my 1 killowatt tube amp build. There are only two different kinds, and most seem to be identical GE's.
As I have stated before many times. If you have never build a high powered tube amp before, going straight into a monster amp build is a proposition with a low probability of success. I learned this the hard way with racing engines, but my first guitar amps only made 5 watts and I worked my way up slowly.
There have been lots of things called SVT over the years, some of which are not even all tube amps. For that matter Ampeg itself has gone through several reissues. The schematic I have is from a typical early 70's 6550 version. For its intended use I do not find it overly complex. It does have two different preamp channels, and you could easily do away with the channel you don't use. The power amp does not seem overly complex for a 300 watt tube amp. If I were doing this today I would replace the follower stages with mosfets to slightly simplify things.
The original first generation Ampeg SVT tube head used 6146 tubes. Many went up in flames. The 6146 is a decent tube for mobile radio equipment, but the screen grids can't deal with the current surges seen when used for audio in an amplifier that gets driven into clipping a lot. Ampeg switched to the 6550 for that reason. Later versions of the SVT seemed reasonably reliable. I saw exactly one go up in flames since the 70's and I got to fix that one. The speakers being used could not eat what the SVT fed them so they failed one by one until the SVT had no load. At this point it spit fire! Autopsy revealed a lot of damage including the power and output transformers and even a tube socket.
You don't state your experience level, but state that you have never worked with TV sweep tubes. I would not call a 300 watt bass guitar amp a good project for a relatively inexperienced amp builder. The plate voltage seen in a typical tube amp that is driven to near clipping is about twice the B+ voltage. I have measured peaks of about 2500 volts in a guitar amp that ran on 430 volts but was driven "way past 11" with an ADA MP1 using a preset that I called "Jimi." With your expected 670 volts of B+ you could easily see NORMAL voltage peaks in the 4000 volt range. The build and parts quality must be excellent at this level.
You mention the 6KG6 and 6LW6 TV sweep tubes. These are the big boys in the tube amp world and either would be a good choice for a high powered tube amp. I have seen 250 watts flow from a single pair of 6LW6's. The issue with these tubes is that there are lots of different versions of each that have been made over the years, especially the 6LW6. You could get by with 4 6LW6's but you would need 4 that are of exactly the same construction. I have seen 6 different kinds of 6LW6's in two different glass envelopes. For this reason, I have decided to use the 26HU5 in my 1 killowatt tube amp build. There are only two different kinds, and most seem to be identical GE's.
As I have stated before many times. If you have never build a high powered tube amp before, going straight into a monster amp build is a proposition with a low probability of success. I learned this the hard way with racing engines, but my first guitar amps only made 5 watts and I worked my way up slowly.
Yes, I will freely admit inexperience. The issue is I’ve not been able to find anyone with enough experience running audio through sweep tubes to tell me if it’s crazy or not. I would likely try to find someone to put this together for me, as while I am capable of fixing high powered amps clean sheet designing one is a different matter.
As for my current unreliability, I’m of the opinion that current production tubes are a not insignificant issue. I’ve killed a shoebox full of kt88s and 6550s over the years I’ve used these as my main amps, usually with the added benefit of plate resistors/screen resistors/grid resistors/capacitors/burnt traces etc. I’ve started using my classic more as the protection circuit usually keeps the amp from catastrophic failure when the tubes go. My main problem is the cost of repairs are starting to become untenable, non withstanding the fact I do the repairs myself. A good 6550/kt88 is about $100 these days, and buying them from a place that weeds out bad ones is usually even more. I’ve started using nos tubes in it in the hopes that something that was made to some kind of appreciable quality standard lasts longer but I have no evidence to support that, mostly hope and prayer.
My hope is in that using something like a sweep tube would not only cut down on retube costs, but also give me access to tubes that were truly made for taking a beating, not a new 6550 that doesn’t even meet the specs that the tube was originally designed to have.
As per the obvious things to check, I’ve always set both amps to spec on bias and always ran them into the correct load. I’m not necessarily voting out the fact I’m an idiot but I think I’m doing it right based on what I’ve read and learned about these huge amps.
As for my current unreliability, I’m of the opinion that current production tubes are a not insignificant issue. I’ve killed a shoebox full of kt88s and 6550s over the years I’ve used these as my main amps, usually with the added benefit of plate resistors/screen resistors/grid resistors/capacitors/burnt traces etc. I’ve started using my classic more as the protection circuit usually keeps the amp from catastrophic failure when the tubes go. My main problem is the cost of repairs are starting to become untenable, non withstanding the fact I do the repairs myself. A good 6550/kt88 is about $100 these days, and buying them from a place that weeds out bad ones is usually even more. I’ve started using nos tubes in it in the hopes that something that was made to some kind of appreciable quality standard lasts longer but I have no evidence to support that, mostly hope and prayer.
My hope is in that using something like a sweep tube would not only cut down on retube costs, but also give me access to tubes that were truly made for taking a beating, not a new 6550 that doesn’t even meet the specs that the tube was originally designed to have.
As per the obvious things to check, I’ve always set both amps to spec on bias and always ran them into the correct load. I’m not necessarily voting out the fact I’m an idiot but I think I’m doing it right based on what I’ve read and learned about these huge amps.
I was in the PA business for 40 years, have lifted or help lift many SVTs as well as micing and taking direct outputs of all sorts of tube amps.
I love the sound of tube amps, but not the weight and reliability issues that come with the tube eating behemoths.
Solid state power amps are a reliable commodity now, you can find class D amps for less than $800 with far more than 300watts of clean power that weigh under ten pounds.
I'd use a reliable low power tube amp/speaker with a simple resistor speaker to line level pad to drive the power amp, power amp to your big speakers of choice.
But if you want to haul the heavy iron, follow Tubelab_com's advice!
Art
I put together a rig in a panic with a peavey tube preamp and a Behringer qsc knockoff power amp. It’s 2500 Behringer watts so about 1200 in the real world, but it’s a heavy massive beast. The transformer in that amp is an easy 20 pounds, and it sounded fine but wasn’t enough power through my small cab. I was underwhelmed by the tonal quality but I may need to try an ampeg style preamp to be sure. There just wasn’t any character to the sound in my opinion.
I wanted to stay tube and preferably through hole or point to point bc I think they’re easier to work on and less pain in the *** than dealing with surface mount stuff when they fail. I had a much easier time finding 6550s than I did a same batch set of t03 lateral mosfets, for the grand irony in that.
I have a dean Costello amp coming that’s turret board and based on the hiwatt dr405 power amp so I’m hoping that’s my saving grace but I wanted to get opinions on what I had listed above. I got two so far, thanks for the input fellas.
I wanted to stay tube and preferably through hole or point to point bc I think they’re easier to work on and less pain in the *** than dealing with surface mount stuff when they fail. I had a much easier time finding 6550s than I did a same batch set of t03 lateral mosfets, for the grand irony in that.
I have a dean Costello amp coming that’s turret board and based on the hiwatt dr405 power amp so I’m hoping that’s my saving grace but I wanted to get opinions on what I had listed above. I got two so far, thanks for the input fellas.
To clarify, I did indeed think about a low wattage and load resistor setup but that’s a lot of gear to haul
I don't think the Peavey pre-amp voicing really compares to the Ampeg sound, especially the sound of output tubes and transformer saturation.
The Behringer NX-6000 is 2x3000 watts (peak) at 4 ohms and only 13 pounds.
Class D amps are far more efficient than the old heavy QSC knockoff, drawing about half the power from the wall for a given power output, so won't brown out the stage power on peaks.
With something like an Ampeg PF20T or PF50-T, and a NX-6000 you could have the Ampeg voicing and tube sound in a lighter package than the SVT, and considerably more clean power.
That said, less power into a big cab will beat more power into a small cab- 300 to 3000 watts is +10 dB, but on a single speaker more than half that power is lost due to power compression, the voice coil heating and it's impedance rising to double or more than it's resting impedance- 4 ohms becomes 8 or more.
The impedance doubling as the speaker heats is also probably not good for the SVT when pushing it to the limit- as the speaker heats more voltage is required for the same level.
The difference between 1 speaker and 8 is 9dB of sensitivity- in the real world 30 watts into 8 speakers would be louder and cleaner than 300 (or 3000) watts into one.
Art
The Behringer NX-6000 is 2x3000 watts (peak) at 4 ohms and only 13 pounds.
Class D amps are far more efficient than the old heavy QSC knockoff, drawing about half the power from the wall for a given power output, so won't brown out the stage power on peaks.
With something like an Ampeg PF20T or PF50-T, and a NX-6000 you could have the Ampeg voicing and tube sound in a lighter package than the SVT, and considerably more clean power.
That said, less power into a big cab will beat more power into a small cab- 300 to 3000 watts is +10 dB, but on a single speaker more than half that power is lost due to power compression, the voice coil heating and it's impedance rising to double or more than it's resting impedance- 4 ohms becomes 8 or more.
The impedance doubling as the speaker heats is also probably not good for the SVT when pushing it to the limit- as the speaker heats more voltage is required for the same level.
The difference between 1 speaker and 8 is 9dB of sensitivity- in the real world 30 watts into 8 speakers would be louder and cleaner than 300 (or 3000) watts into one.
Art
That has recently changed. I’ve started gigging with a fridge, and ostensibly it would be the cab to use with the new rig. I don’t have an issue with a 40 pound power amp actually, considering I’m used to carrying a 100 pound Svt, it’s a featherweight by comparison. I do agree the fender style peavey preamp may not be the greatest way to achieve that tone, but I want to wait for the right ampeg preamp to come available before throwing more money at bass amps.
The original SVT cabinet used eight 32 ohm speakers all wired in parallel. When one blew the average "amp tech" replaced the bad one with a budget Radio Shack quality 16 ohm (or less) speaker. Hey, I worked at Olson Electronics where our house branded speakers were Utah's which would live for a while and didn't sound too bad. Either way the replacement would fry rather quickly since it got more power than all the others due to its lower impedance. Somewhere down the line of speaker swaps and outdoor shows in the Florida heat, a chain reaction of speaker blowouts would occur blowing all the speakers, leaving the SVT unloaded at full crank which set the OPT on fire. This usually started an arc - over to ground in the OPT, a tube socket or somewhere else. If a heavy handed amp tech had ever bypassed the internal fuse that was present in some SVT's then the power transformer could get fried as well.
The amp that I repaired had a good internal fuse and the user fuse was correct, but the arc-over occurred at the tube socket between pin 3 (plate) and pin 2 (heater) this fried a bunch of stuff including the power transformer and a 12AX7!
Just to show what some TV sweep tubes can do here are some pictures of a little "this one goes beyond 11" test I did. I had been experimenting with some HiFi test boards so the setup shows two of my Universal Driver boards (foreground) with 6CG7 tubes installed. Each was used to feed a push pull UNSET output board with a pair of 26HU5's in each board. The blue OPT's are 3300 ohm Edcor's rated for 100 watts each. I had run a bunch of tests up to about 175 watts per channel which was as far as I wanted to push those transformers. The gold colored load resistors seen between the OPT's were only rated for 120 watts each.
I then decided to use only the driver board on the right to feed both sets of output boards. I got out the MOAT (Mother of all Transformers) which is the 25 pound round can sitting on the aluminum plate on top of the wimpy Edcors. It is rated for "400 watts @ 20 Hz." I fired up the biggest power supply I have and turned it up all the way, which was a bit over 620 volts at up to 1.7 amps. The big spiral wound brown thing is an 8 ohm 500 watt resistor used as my test load.
After some tweaking I decided to crank it. I started turning things up and the power kept coming. In the clean power department, how's 352.5 watts at 0.817% THD. In the lean on it till it clips mode I get 512 watts at 3.79% THD. I decided not to push my luck any further at 525 watts. Not bad for 4 power tubes that can often be found for under $20 each. Yes 4 tubes can do well over 500 watts, but I doubt they would be too reliable while doing it. I plan to use 6 tubes per channel in my 1 KW tube amp with a microprocessor controlled brain to keep an eye on things so that it lives long and prospers.
The amp that I repaired had a good internal fuse and the user fuse was correct, but the arc-over occurred at the tube socket between pin 3 (plate) and pin 2 (heater) this fried a bunch of stuff including the power transformer and a 12AX7!
Just to show what some TV sweep tubes can do here are some pictures of a little "this one goes beyond 11" test I did. I had been experimenting with some HiFi test boards so the setup shows two of my Universal Driver boards (foreground) with 6CG7 tubes installed. Each was used to feed a push pull UNSET output board with a pair of 26HU5's in each board. The blue OPT's are 3300 ohm Edcor's rated for 100 watts each. I had run a bunch of tests up to about 175 watts per channel which was as far as I wanted to push those transformers. The gold colored load resistors seen between the OPT's were only rated for 120 watts each.
I then decided to use only the driver board on the right to feed both sets of output boards. I got out the MOAT (Mother of all Transformers) which is the 25 pound round can sitting on the aluminum plate on top of the wimpy Edcors. It is rated for "400 watts @ 20 Hz." I fired up the biggest power supply I have and turned it up all the way, which was a bit over 620 volts at up to 1.7 amps. The big spiral wound brown thing is an 8 ohm 500 watt resistor used as my test load.
After some tweaking I decided to crank it. I started turning things up and the power kept coming. In the clean power department, how's 352.5 watts at 0.817% THD. In the lean on it till it clips mode I get 512 watts at 3.79% THD. I decided not to push my luck any further at 525 watts. Not bad for 4 power tubes that can often be found for under $20 each. Yes 4 tubes can do well over 500 watts, but I doubt they would be too reliable while doing it. I plan to use 6 tubes per channel in my 1 KW tube amp with a microprocessor controlled brain to keep an eye on things so that it lives long and prospers.
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I can only concur what has been advised so far: excess 300W tube amp is not an ideal beginner project, and overall SVTs should be fairly reliable so hard to imagine how to build a better mousetrap.
And why tubes? You are not going to overdrive a 300W amp, are you, so what is the advantage to, say, MOSFET output with, say, tube drive? Or a class-D module with such? (Not that excess 300W solid-state amp would be an ideal beginner project either).
And why tubes? You are not going to overdrive a 300W amp, are you, so what is the advantage to, say, MOSFET output with, say, tube drive? Or a class-D module with such? (Not that excess 300W solid-state amp would be an ideal beginner project either).
I generally dislike class d amps for bass, it may be a bit of musician bias but I just don’t care for the feel of the amp, I think there’s value in how a giant power supply reacts to transients in traditional a/b.
I purchased both of these amps thinking that they’d be reliable but neither goes more than 2 months without some sort of problem. I’d be willing to try an ampeg style preamp with a mosfet power amp but finding either is a bit of a problem these days.
I generally prefer tubes as they’re easy to replace when problems arise, and usually makes for an easier amp to repair.
I purchased both of these amps thinking that they’d be reliable but neither goes more than 2 months without some sort of problem. I’d be willing to try an ampeg style preamp with a mosfet power amp but finding either is a bit of a problem these days.
I generally prefer tubes as they’re easy to replace when problems arise, and usually makes for an easier amp to repair.
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Wow! This is exactly the sort of thing I was looking for, that would make for one intense MI amp at 512 watts. I may not even need the b+ as high as I thought if they can pass that much current.
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Try a sextet of 26DQ5. With 125 on the screen, they act like 6550’s with 330 on theirs. You may want to run it up to about 150 to get the full 300W with the 1900 ohm OPTs. 21LG6 is similar and CAN pass more current if you ask it to. I built something similar to the 135W Bassman out of 4. This “next class down“ 24W sweep tube can be had under ten bucks a pop.
Working slowly and carefully, there’s no reason for blowing very many. My first “real” tube amp - that was actually taken to completion and actively used - started out something like that SVT. A bit under voltage (560 at load, using Antek power iron and the Hammond OPTs), but delivered 200 beautiful watts. Too good for a dabbling bass player, so build two and call it hi-fi. They sound like a goddam Crown Macrotech 600 to me. Building them TODAY I’d be using those 26DQ5’s instead of the 6550, and likely would sound exactly the same. I later tried using the 475V power trafos to get the fullB+, but truth be told I liked the sound better the way it was so it went back. Not one single mishap worth mentioning, but some non-fatal issues which were ultimate solved.
An alternative if you are using really high current tubes like the LW6 is to run them “low and hot”. Drop B+ and run a low Ra-a. That makes buying real tube OPTs a pain and expensive, but if you consider something like an Antek 8T800 in reverse, it gives 1400 ohm R a-a with all the power handling you’d ever need. That will get you there with a 500V B+.
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I hadn’t actually thought about the high current capabilities of the lw6, but lowering b+ might do the trick with a suitable ot. Would be one massive chunk of iron, I wouldn’t know where to begin with sourcing that. Are you running series heaters on yours? The 26 volts has to be an interesting transformer to source.
Speaking of, I had thought about taking a large high current industrial step down transformer and running it in reverse to get my hv, though I don’t know how well it’d work. It would give me ~480v which the high current tubes could probably eat but that still leaves the question of the ot.
This forum is great, I should have just asked here first. I knew someone had to have tried it. Thanks guys!
Speaking of, I had thought about taking a large high current industrial step down transformer and running it in reverse to get my hv, though I don’t know how well it’d work. It would give me ~480v which the high current tubes could probably eat but that still leaves the question of the ot.
This forum is great, I should have just asked here first. I knew someone had to have tried it. Thanks guys!
Also, what sort of secondary impedance and bandwidth were you getting with a reversed power transformer? I’ve never heard of doing it that way, but I’m very intrigued
I custom wound the power trafo in that 26DQ5 amp. It has plate and screen voltage taps, 6.3 and 26.5V heater windings. When using off-voltage sweeps, you can always use a separate toroid to just run those heaters. Much cheaper than getting something custom. I make a lot of my own trafos.
Reversed power trafos don’t always work as OPTs. Most toroids do, but the typical useage of putting the output primary on the 120/240 and stepping down for the speaker results in limited range of use. To about 240 volts RMS plate to plate (ie, about a 250-300 volt B+, tops). Within those limitations they work better than they have a right to. Bandwidth goes up past my hearing. But they have ones meant for tube RF finals - up to 950-0-950. You can put a lot more than 240 volts RMS into that SAFELY, and extract a nice 8 ohm output from the 120V windings. At a few hundred watts. My early experiments with the 800 and 1500 VA units are encouraging, although the kilowatter is back burnered till I get done building my new house. I’ve built a nice little 14 watt per channel PP class A out of one of the small ones, and it sounds terrific. And something that pushed about 50 watts through one of the same size, but was really just an experiment.
120:480 industrial control trafos are spot on for making big B+ at high power. I have several, up to 900 VA just lying around waiting for me to get back to the big stuff. Only issue is mechanical hum. They tend to run the flux density on the high side because in the application they don’t need to run silent. A multi-hundred watt tube amp with a mechanical buzz does have its appeal. Bear in mind a 480V output will generate almost 700V of B+ And regulation will only be about 88-90% to full load so you’re back to the full B+ design. Mine have taps down to 380V, which could be operated with that 8T800 making a 350W amp. That was the plan. On a 480V tap, you’d just use that Hammond 1650W. They are expensive, but GOOD.
Reversed power trafos don’t always work as OPTs. Most toroids do, but the typical useage of putting the output primary on the 120/240 and stepping down for the speaker results in limited range of use. To about 240 volts RMS plate to plate (ie, about a 250-300 volt B+, tops). Within those limitations they work better than they have a right to. Bandwidth goes up past my hearing. But they have ones meant for tube RF finals - up to 950-0-950. You can put a lot more than 240 volts RMS into that SAFELY, and extract a nice 8 ohm output from the 120V windings. At a few hundred watts. My early experiments with the 800 and 1500 VA units are encouraging, although the kilowatter is back burnered till I get done building my new house. I’ve built a nice little 14 watt per channel PP class A out of one of the small ones, and it sounds terrific. And something that pushed about 50 watts through one of the same size, but was really just an experiment.
120:480 industrial control trafos are spot on for making big B+ at high power. I have several, up to 900 VA just lying around waiting for me to get back to the big stuff. Only issue is mechanical hum. They tend to run the flux density on the high side because in the application they don’t need to run silent. A multi-hundred watt tube amp with a mechanical buzz does have its appeal. Bear in mind a 480V output will generate almost 700V of B+ And regulation will only be about 88-90% to full load so you’re back to the full B+ design. Mine have taps down to 380V, which could be operated with that 8T800 making a 350W amp. That was the plan. On a 480V tap, you’d just use that Hammond 1650W. They are expensive, but GOOD.
The 1650w was something I did have in mind, the cost offset from using an industrial control transformer could cover it. I don’t mind a bit of noise, at the sort of sound level that this thing would output I don’t think anyone would notice. I’m very interested in the high current low voltage approach, if an lw6 could maintain that current level reliably. The toroid would handle it, and if it’s good from 30-5k it’s good enough for what I’m doing, the cabs I use don’t have tweeters anyway. What did you use for drive? I’ve heard sweeps don’t like screen drive.
I was getting 525 watts at around 600 volts, but I have a 1250 ohm OPT.
The big sweep tubes like the 6LW6 tend to have a peak cathode current rating of 1.4 amps and an average cathode current rating of 400 mA. The 26HU5 also has the same ratings. The 6 / 26DQ5 is rated for 1.1 amps peak and 315 mA average. With tubes this big The plate dissipation rating tends to be the limiting factor in making big power. Sweep tubes tend to be very conservative in the dissipation rating since they run at full power anytime the TV set is on. Turning down the volume on the sweep circuit shrinks the picture size since that energy is being used to "sweep" the electron beam across the face of the picture tube.
TV sweep tubes tend to have a larger cathode than an equally rated audio tube. This draws more heater power but makes the tube more efficient so less of your plate current is turned into heat. I tend toward odd voltage sweep tubes because they are cheaper than the equivalent 6 volt tube. Powering the heater is easy, use a 19 volt laptop brick for the small 17 and 18 volt tubes and a cheap SMPS from Amazon for the 26 and 36 volt tubes. Most 24 volt SMPSs can be turned up to 26 volts or close enough to work and there are 36 volt SMPSs for the 36 volt tubes. Just get a supply that's good for 2 to 3 times the current needed for all your tubes since some of them don't like to start up into a cold heater.
I have used reverse wired industrial control transformers to power big tube amps. As mentioned, they might buzz a bit and they do get pretty warm.
It sounds like I’m in the right ballpark as far as parts, might either of y’all have a schematic for me to wrap my head around? I know you had mentioned mosfet driver stages Tubelab, I’d be fine with that but could also use tubes. I would imagine that a large current output tube wants an equally large drive current, I’ve got a bunch of 12at7s and 12bh7s for Svt spares that might would do if they would fit the bill. I don’t have many mosfets around but I think I have a few 20n20s.
The big monoblocks were a Williamson-ish design, with individual K-followers for each power tube. The idea was to be able to independently bias them and still DC couple them to a low Z source. Whole thing is overkill, could probably eke out the same power out of 2 pair instead of three if I raised the screen voltage, but I wanted to keep it not too far from 300, since I play music loud, and have been known to run 10 dB into clip. It actually still sounds good running that hard - and so do those boat anchor Crowns. At the full 300 watts, I’d run 3 pair regardless.
The bass amp dispenses with any fancy driver, and used a Mullard topology to ensure no low frequency issues. Doesn’t clip quite as clean, but hell I designed it to be a toy. Turned out a nice one. It’s got a built in preamp section with a Baxandall bass/treble and a couple gain stages. The only “special” part is the asymmetrical sweepable midrange based on a gyrator. I have the preamp schematic…. somewhere.
The bass amp dispenses with any fancy driver, and used a Mullard topology to ensure no low frequency issues. Doesn’t clip quite as clean, but hell I designed it to be a toy. Turned out a nice one. It’s got a built in preamp section with a Baxandall bass/treble and a couple gain stages. The only “special” part is the asymmetrical sweepable midrange based on a gyrator. I have the preamp schematic…. somewhere.
