Hi Everyone,
I am looking to tinker with a new project. I recently swapped out a KT88 amp to run 6CB5A into the 4.3K output transformers in triode mode at 330VDC B+ and 65mA bias. They will take much more before red plating, but that point works well and the output tubes should last forever. 🙂
Anyway, I am looking for a good set of sweeps with highish plate dissipation to run into a pair of 1.5K output transformers I have. I found a resource from 1996 where the author ran the 6LW6 into a 2.2K anode load in "modified UL?" for 90W on a relatively modest bias. Nothing looked too special other than the output transformer configuration. My outputs are incapable of the "modified UL" setup he ran, so I am looking either to run the 6LW6 in triode or Pentode. Preferably Pentode with a little feedback. I have seen decent reviews on the sound of the 6LW6, so I would like to give it a shot. I did notice in the article that these are much harder to drive than some other sweeps so a beefy driver is necessary.
Will the 6LW6 do OK into 1.5K?
Thanks for any information!
I am looking to tinker with a new project. I recently swapped out a KT88 amp to run 6CB5A into the 4.3K output transformers in triode mode at 330VDC B+ and 65mA bias. They will take much more before red plating, but that point works well and the output tubes should last forever. 🙂
Anyway, I am looking for a good set of sweeps with highish plate dissipation to run into a pair of 1.5K output transformers I have. I found a resource from 1996 where the author ran the 6LW6 into a 2.2K anode load in "modified UL?" for 90W on a relatively modest bias. Nothing looked too special other than the output transformer configuration. My outputs are incapable of the "modified UL" setup he ran, so I am looking either to run the 6LW6 in triode or Pentode. Preferably Pentode with a little feedback. I have seen decent reviews on the sound of the 6LW6, so I would like to give it a shot. I did notice in the article that these are much harder to drive than some other sweeps so a beefy driver is necessary.
Will the 6LW6 do OK into 1.5K?
Thanks for any information!
I believe I have the same article. It was found in Glass Audio magazine March 1999 issue. That amp used a specialized OPT wound like those found in McIntosh amps. It can't be compared with the typical P-P OPT. The OPT configuration is responsible for the extreme drive requirements needed for that particular amp.
I am building a 6LW6 amp with a 1250 ohm OPT using 4 tubes per channel, but I am aiming for 500 WPC from about 700 volts! A single pair of 6LW6's should work on 1.5 K if kept to a reasonable power output and B+ voltage to avoid running into the 1.4 Amp peak cathode current limitation.
Avoid triode operation. I blew up 4 tubes before figuring this out. The screen grid voltage spec is 275 volts, and it doesn't like much higher even in triode mode. I built an SE amp running the 6LW6's on just over 400 volts in triode. It sounded great and worked for several months before a tube went into runaway red plate death. After another blown tube I swapped out the power transformer to get somewhere in the 375 volt range (don't remember exactly) but two more tubes would eventually fail before I tore down the amp for parts. They all failed at idle, no music playing. I used this amp (a hacked TSE) on my PC so it did see a lot of idle time, but the stock SSE that went back in its place ran EH KT88's on 450 volts at 100 mA and it still works with the same tubes 6 years on.
Run the 6LW6 in pentode mode with 150 to 200 volts on the screen. My experiments so far have used 150 volts. You should be able to go up to about 500 volts on the plates before reaching peak cathode current capability, which should produce gobs of power. I tend to favor a circuit similar to Pete Millett's Engineers Amp for sweep tubes, but I haven't decided on my final design yet.
I am building a 6LW6 amp with a 1250 ohm OPT using 4 tubes per channel, but I am aiming for 500 WPC from about 700 volts! A single pair of 6LW6's should work on 1.5 K if kept to a reasonable power output and B+ voltage to avoid running into the 1.4 Amp peak cathode current limitation.
Avoid triode operation. I blew up 4 tubes before figuring this out. The screen grid voltage spec is 275 volts, and it doesn't like much higher even in triode mode. I built an SE amp running the 6LW6's on just over 400 volts in triode. It sounded great and worked for several months before a tube went into runaway red plate death. After another blown tube I swapped out the power transformer to get somewhere in the 375 volt range (don't remember exactly) but two more tubes would eventually fail before I tore down the amp for parts. They all failed at idle, no music playing. I used this amp (a hacked TSE) on my PC so it did see a lot of idle time, but the stock SSE that went back in its place ran EH KT88's on 450 volts at 100 mA and it still works with the same tubes 6 years on.
Run the 6LW6 in pentode mode with 150 to 200 volts on the screen. My experiments so far have used 150 volts. You should be able to go up to about 500 volts on the plates before reaching peak cathode current capability, which should produce gobs of power. I tend to favor a circuit similar to Pete Millett's Engineers Amp for sweep tubes, but I haven't decided on my final design yet.
I have no idea what nonsense this is. If you run the 6LW6s as pentodes, they're not at all hard to drive. As audio finals, all pents are pretty easy loads. It's the audio triodes, with their high reverse transfer capacitance and large input voltage swings that become a problem. Class C, RF can also be a handful, but not relevant here anyway.
The A Number One problem is acquiring 6LW6s, as these are as rare as seal feathers these days. Forch, the type was made with odd heater voltages to operate in color TVs that used daisy chained heaters across the AC mains without a PTX. I have a bunch of the 36LW6s myself. These can still be had and without costing a bundle.
As for the 1K5 OPTs, if that's 1K5 P-2-P, they should be good to go. I drew up some preliminary loadlines. Here's one (attached) Even though this is for a 337R phase load, the 375R load from a 1K5 OPT should still work just fine, maybe a little less output, but also less distortion as well. Still, we're getting better than 90W, and, really, how much do you really need anyway?
I'd run 'em as pents. These TV HD finals all have some very limited screen voltages, and so are unlikely to make good pseudotriodes. A few types can, but not always, and busting that screen voltage spec is inviting red plate poofage.
Besides, these TV HD finals as audio finals sound really good, with mostly h3 and relatively little h5 and above. All you need is just enough gNFB to take the edge off. You don't get all the pentode nastiness that some other types produce.
Attachments
I think the drive circuit in the article I saw was due to the tertiary winding as Tubelab pointed out.
I just said 6LW6 because it is the 6V version and I thought it would be more recognizable. I plan to use the 36LW6.
Just to make sure I am reading that correctly, that is 300V B+ with 110V on the screens and idling at 100mA per tube? What type of plate current will each tube see at maximum signal? I want to make sure I get an adequate power transformer.
Thanks!
I just said 6LW6 because it is the 6V version and I thought it would be more recognizable. I plan to use the 36LW6.
Just to make sure I am reading that correctly, that is 300V B+ with 110V on the screens and idling at 100mA per tube? What type of plate current will each tube see at maximum signal? I want to make sure I get an adequate power transformer.
Thanks!
Indeed that's the Mangini's (nice) amp:
http://www.tiffe.de/roehren/ga399ma.pdf
IMHO it's not a toy to play with but a "finished product" to build and use as is.
That is a practical application of the Crowhurst Modified Ultralinear that he described in principle back in 1956 in one of his articles. It is titled "triode vs pentode" (ref. n3 in the Mangini's article).
The reason why it needs a lot of drive is 57%-43% ratio anode/cathode ratio of primary turns. It's almost like a Unity Coupled amp but this is not necessary because it makes the rest of the amp too complicated without real benefit, especially in high power stuff. On top of that he didn't bootstrap the voltage amp like in the Mac to make things more complicated.....
IME, 75%-25% ratio or so is already more than enough with basically equivalent performance and much simpler front-end. Also, to further level things out overall feedback can be used, just in case! In fact Mangini used it anyway. It is also important the way the ouput transformer is done, especially for "deep" class AB operation.
I think Mangini just took the example in that article and built it although Crowhurst himself wrote that, as for the standard UL, the ratio is not fixed.
Thank you for the background! Looking at the amp, it was in no way something I was going to try to copy. Just too much going on for a fun DIY project.
The primary turns are splitted between anode and cathode in ratio 53%-47%. So that 2.2K is the total anode+cathode load end-to-end.
The screen grid is directly connected to the PSU. However it is called ultralinear because the G2 is not referred directly to the cathode or in other words G2 is referred to ground while the cathode sees the ground thorugh the transformer.
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Yes, yes, and yes.
Like it says in the loadline: Ip= 750mA
I= 0.75 / sqrt(2)= 0.53ARMS
PO= 0.532 X 337= 94.78W
Iave= (2 X 0.75)/pi= 477.47mA
Itotal= sqrt(0.22 + 0.477472)= 517.67mA
PDC= 300 X 517.67E-3= 155.3W
PD= (155.3 - 94.78)/2= 30.26W / final
You'd need a PTX that could do 155.3W. You can get away with a smaller PTX since audio amplification is far from a "brick of the key" mode such as FM or packet. The exception would be a guitar amp since guitar players overdirve their amps to such an extent that it comes close to "brick on the key". Music reproduction amps, not so much.
For music reproduction a PTX that could supply the 200mA of no-signal bias current plus what the rest of the circuit pulls would be enough. Though a beefier PTX has the added benefit of not running so hot.
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It's the same as the Mac except that the ratio is not 50%-50% and G2 is not connected to the OT. I don't call it "tertiary" because I use that for an additional G2 winding usually to run at lower voltage than the anode or for cathode feedback in much lower percentage were you can leave them out without troubles and only use the main primary windings. In the case of the article it would not be usfeul because if you left the cathode turns out the primary impedance would only be 630 ohms. So one should connect them all in series with the right phase to use it as a standard OT. However it's just a name....
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Perfect! I have a Bogen MO-200 power transformer that had a doubler to run 8 - 8417 tubes in cathode bias, so I'm sure it's good for well over 1A. Gives around 330V B+.
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