A 60 volt AC winding would result in about 80 volts of raw negative voltage which is sufficient for KT88's. A bit more voltage might be a good idea if you want to experiment with other tubes in the future. A 70 volt winding will give you over 90 volts. That should cover anything.
I don't think it is necessary.
It's been about a year since I listened to it. It's rather punchy and dynamic, not like the typical SE amp. Of course the output tubes and transformer makes a big difference too.
Yes
This increases the local feedback around the output tubes. Not the same as GNFB. This type of feedback is quite effective in Petes Big Red Board. I used 200K on that board, but I used the rather wimpy 6GU5 hexode for a driver in that board because I needed the extra gain. I am using the 6HB6 here which is closely related to the EL84 (but costs $1). I still have plenty of gain, maybe too much.
Your curves stop at 250 volts....my driver board doesn't start working until 300 volts......So, are we ready for some smoke! Data to follow.
I have already triode wired the board and fired it up, but my Vista box locked up in the middle of my last post. I have learned to save a local copy every few minutes. So...
My little red board (at least with the parts currently installed) requires at least 300 volts to work. The output is distorted and unsymmetrical below 300 volts. Not wanting to blow the 13GB5's on power up, I used 300 volts. I don't want to use my big HP for the plate supply since it has been known to blow things up, and I can't shut it off quick enough. I hooked the board up to the old Fluke since it goes bonkers under overload and folds over, this can save a runaway tube. Triode wired sweep tubes sometimes have a tendency toard bias creep, and runaway.
As with other sweep tubes they act really different in triode. Previous testing in pentode required 25 to 30 volts of negative bias to get 35 ma of idle current. Now I need -68 volts on the grid to get 35 mA. This pair of tubes did not show signs of instability, or try to runaway.
The tubes did seem to work rather well in triode. The baseline distortion is 1.4% rising gently to 1.9% at 35 watts and hitting 3.66% at the onset of clipping at 40 watts. I let it run at 40 watts for about 10 minutes, no visible glow on plate or grids, and the bias returned to 35 ma when I shut off the drive.
My little red board (at least with the parts currently installed) requires at least 300 volts to work. The output is distorted and unsymmetrical below 300 volts. Not wanting to blow the 13GB5's on power up, I used 300 volts. I don't want to use my big HP for the plate supply since it has been known to blow things up, and I can't shut it off quick enough. I hooked the board up to the old Fluke since it goes bonkers under overload and folds over, this can save a runaway tube. Triode wired sweep tubes sometimes have a tendency toard bias creep, and runaway.
As with other sweep tubes they act really different in triode. Previous testing in pentode required 25 to 30 volts of negative bias to get 35 ma of idle current. Now I need -68 volts on the grid to get 35 mA. This pair of tubes did not show signs of instability, or try to runaway.
The tubes did seem to work rather well in triode. The baseline distortion is 1.4% rising gently to 1.9% at 35 watts and hitting 3.66% at the onset of clipping at 40 watts. I let it run at 40 watts for about 10 minutes, no visible glow on plate or grids, and the bias returned to 35 ma when I shut off the drive.
Nothing blew up yet, so I had to try 350 volts. Now you need -76 volts of bias. The distortion remains under 1% up to 25 watts and under 2% up to 35 watts. The 5% point and clipping is around 50 watts but power supply misbehavior makes accurate readings impossible. Drawing about 450 mA from a 300 mA supply will do that.
Today has been the first day this year that I have made it one whole day with no AC, I am sure that south Florida's energy consumption was below normal today, so I am doing my part to keep it up!
The 13GB5's still showed no signs of runaway at 375 volts in triode. That is a good sign. However, two tubes being tested for two hours is not a valid test. I built a triode wired amp using 6LW6's a few years ago running 400 volts. It would blow a tube randomly every few months ago, so I have been avoiding triode wired sweep tubes since then. Previous experiments with 6AV5's went the other way. I could abuse them every which way and they wouldn't blow.
These are still unknowns. If you go triode, stay at 300 volts....or less and put a fuse in your B+. When a sweep tube runs away, it goes fast. Its working one second, and a big ball of sparks the next.
I got some mystery toroids that I want to try as OPT's tomorrow, then it's off to some different output tubes.....and a 500 mile ride for some turkey on Thursday.
Wow... I've been out of touch on vacation for a week, and much has happened! Glad to see George is going down the sweep tube path!
I think I've decided to work up a build using PPP 807's (or 1625, 5933, or 829B... maybe 6146?) to do ~100W. I'm thinking B+ of 600V, G2 of 225-300V, 5k load impedance. I'm hoping to configure the driver so it can run down to 200V to allow the G2 supply to run the driver (829B is rated 225V max, 807 300V). Should only need around +/-35V of grid drive so hopefully it will work that low.
That will be step one. Then the rest... power supplies, etc. I think I'll ditch using any of my existing iron and spend the $$ for new Edcor transformers, and have them do a custom power xfmr. Contribute to the economy
Pete
I think I've decided to work up a build using PPP 807's (or 1625, 5933, or 829B... maybe 6146?) to do ~100W. I'm thinking B+ of 600V, G2 of 225-300V, 5k load impedance. I'm hoping to configure the driver so it can run down to 200V to allow the G2 supply to run the driver (829B is rated 225V max, 807 300V). Should only need around +/-35V of grid drive so hopefully it will work that low.
That will be step one. Then the rest... power supplies, etc. I think I'll ditch using any of my existing iron and spend the $$ for new Edcor transformers, and have them do a custom power xfmr. Contribute to the economy
Pete
This is exciting! Time to dust off the workbench. I hope I can contribute something. If nothing else, I'm great at designing the chassis(subjective, I know), and will certainly share my cad drawing when we get to that point.
I don't have any power iron, so will definitely join in getting what you have made.
I don't have any power iron, so will definitely join in getting what you have made.
HI Pete, look set to go with 100W or more, means that you leave the prototype triode-KT88? there has ben some progress? was happy with the sound?.
I have the tubes and I'm waiting a budget of EDCOR for the custom transformer.
750V (375-0-375) at 400mA, center tapped.
70V at 150mA.
6,3V at 10A.
5V at 3A.
As you see it? if you are thinking about ordering some custom transformer valid for the trioded-KT88, I'll wait.
I was waiting for to get some more information, and maybe, a list of parts.
I'm not an experienced builder, but I think I could do it.
I have the tubes and I'm waiting a budget of EDCOR for the custom transformer.
750V (375-0-375) at 400mA, center tapped.
70V at 150mA.
6,3V at 10A.
5V at 3A.
As you see it? if you are thinking about ordering some custom transformer valid for the trioded-KT88, I'll wait.
I was waiting for to get some more information, and maybe, a list of parts.
I'm not an experienced builder, but I think I could do it.
Pete mentioned transmitting tubes, so why not. It's just a test so why not go BIG. Well I have some 833A's, but I don't have the bench space nor a few KV, nor an OPT big enough to eat it......so think a little smaller.....Why not a pair of 813's. I know I could squeeze more juice out of some bigger sweep tubes, but a pair of brightly glowing DHP's would look COOL.
OK, I got tubes, sockets, and plate caps, so.....hook em up...light em up.....and....
I have 4 tubes and ALL of them are DEAD.....
One of them is so dead that I can measure 0 ohms between G2 and G3.
Well at least they look cool.
NEXT......something smaller.....way smaller
OK, I got tubes, sockets, and plate caps, so.....hook em up...light em up.....and....
I have 4 tubes and ALL of them are DEAD.....
One of them is so dead that I can measure 0 ohms between G2 and G3.
Well at least they look cool.
NEXT......something smaller.....way smaller
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That's one of my little load resistors...its only rated for 120 watts, which I have already exceeded this week. Really big amps need a really big resistor. This one is rated for 500 watts, and I have fed it 525 watts using one of Petes big red boards and 35LR6's with both channels paralleled.
Yes, there is. The specs state that for maximum fun AB2 is needed. This little board won't really go there until I make a mosfer buffer board. Then you will see some AB2 testing on the 4D32 and some more mundane tubes like the 6L6GC. The mosfet buffer board will allow for some screen drive experiments on some sweep tubes like the little 13GB5 too.
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