5902 design problems

[Disabled_shredder]

i am building a micro amp using 6112s 6111wa's and 5902s. i am using a 269ax transformer and a 125c ot. the issue im having is that the datasheets say that it has a plate resistance of 15000ohms but also said it has a plate load resistance of 3000ohms. now that messes with me. either way 6k or 7.5k seems to be a decent ot primary impedance. well when i do my calculations im seeing a plate current of about 25 to 30 milliamps my b+ is about 190 and my voltage to the plates is about 180 or so which give me a plate dissapation of about 5 watts. which is wayy to high. ive read over UR12s 2 watt trainwreck amp and i cant get much from it other than that he set his grid bias at about 31-32 -vdc. i did that and got wayy to high an output. should i raise the ot primary and lower the bias or? im really becoming stumped and frustrated. any insight would be great this is the datasheet im working off of. http://www.mif.pg.gda.pl/homepages/fran ... 5/5902.pdf
i will try and get a schematic drawn up and posted so that way you all can see what my ps is looking like but for now im running a pair of 100/350s with 220k resistors to the standby then through a 5 henry at 90ma 103-5 ohm choke. thats my plate and my screens.

[Disabled_shredder]

here is the schematic of the psu

[Thomas_H]

Have you seen the Dwarf amp?
http://ampgarage.com/forum/download/file.php?id=18850

or the jj:

http://jjs.at/electronic/class_ab_subminiature.html

There you will find some information.

As to find the ideal OT I always use this:
http://www.dmitrynizh.com/tubeparams_image.htm

It's an java application, where you enter the tube load lines to make pspice models. The 5902 is a common subminiature tube, so you will find a pspice lyb for it, just enter the values and click on PP and it will calculate the ideal OT.

Cheers,
Thomas

[Disabled_shredder]

thanks a bunch. i had not seen the dwarf amp. i have studied the jj amp and the 2 watt trainwreck for what i can get out of them. also im running on a mac so i dont know if i can open the dymitrynizh software. i forgot about spice models. im gonna try and download that and see what i can do. so i did on one hand realize that i had the plates b+ hooked up to cathode! dumb late night mistake. guess those tubes are done. im gonna throw another set in there and hook them up to the function generator and the scope. but running the 6.8k primary and a neg bias of 30 or so vdc im getting 20-25 ma plate current. to be exact. in standby my b+ is 190 with just plates screens and pi hooked up and nothing else my voltage from the rectifier is 175 tube one from ct to plate has a resistance of 153.6 i have a voltage 172 and the 2nd tube has a resistance from ct to plate of 118.7 and a b+ of 172.5 i feel that the transformer is not balanced well but i dont do %'s very well but the calculations thus far puts me at about 2 and 3/4 watts plate dissapation. again. im gonna go put another set of tubes in and see if i can get a clean waveform on the scope!

[katopan]

Here' what I ended up with:
https://sites.google.com/site/chosenaud ... otscreamer

About 170V B+ and it worked great with an 8K OT. I've got a load line on that page as well showing 5.7K was optimal. Ear said that 4K sucked.

I got 21.5mA -21.7V bias (mine's cathode biased with a shared 470R resistor). I'm dissipating something like 3W out of a datasheet max of 3.7W.

[martin manning]

DS and Katopan, what voltage are you running on the 5902 screens?

[katopan]

Had to go back and look. Got 157V marked on my schematic for the 5902 screens with 21.7V on the cathodes.

Here's the load line that I drew up at the time.

[img:850:520]https://sites.google.com/site/chosenaud ... adLine.gif[/img]

[Disabled_shredder]

I think ive made all the mistakes i can and will make so far but i was running the screens at about 176 volts and the plates at 177-178. im pretty sure it was a grounding issue for the pi. but ive torn everything away and breadboarded the phase inverter with all pots so i can dial it in perfectly because i was getting a decent signal on one side but not on the other. but again i didnt tie my grounds to actual ground. dumb midnight mistake again! but i havent blown any tubes as far as i can tell. so thats a plus. ive been running my ot at 6.8k. i put the build down for the past few days to try and clear my circuit brain ailment. ill post results as i get them. but im pretty sure running plate resistors on the pi at 82 and 100k were to high gonna lower to 33 to 47k and 22 and 10k for the cathode tail was off to. gonna work at the pi and move on to the output.

[Disabled_shredder]

katopan. thank you for the info you posted! that is actually the missing link. your pi setup is what i was calculating for mine using loadlines approach! im re inspired! ill be posting info very soon. thanks everyone you have all given me inspiration

[martin manning]

For the 5902 with 178V on the plates and 6k8 load I'd lower the screen to 120-125 and bias at 12-15mA.

[Disabled_shredder]

i cannot lower the screens any more im using a choke. or am i missing something? and i thought the negative bias would be more around negative 30vdc. or my calculations are off.

[martin manning]

To make sure I'm understanding your voltages, are you running fixed or cathode bias on the 5902's?

[Disabled_shredder]

Fixed bias supply

[UR12]

For what it's worth I'm using a 6.6k output transformer and my B+ voltages average between 170-180 volts on the plates. I set my bias using a scope to adjust it for no zero crossing distortion in the output and after setting it, it came out to -30 to -32 volts of bias. After building over 100 of these sub miniature amps I have never had a problem with the 5902s running in this range. They are very robust little tubes. Measured output from 2 5902s in a PP arrangement is 2 watts clean power and about 3.5 to 4 watts when pushed to the max with the 6.6k OT in my amps. I should also note that this is the same setup I use on my Lil Devil and also my Microplex amps. I am in the design stages of another lil amp that uses the AC30 as a starting point with a pentode front end using a 6205 or 5840.

The problem I had with the Hammond 125c OT was its frequency response which is advertised at 150 - 15khz and was the reason I designed my own.

[Disabled_shredder]

dana i actually emailed you about the transformers you had built. i modeled my first pi and power tube section off the schematic you said you used the trainwreck schematic. i found it to not work for me. but i might be making newbie mistakes. i am aiming for a 2 channel build with a fender clean and a mesa style lead. i actually worked my bias off yours, where you said you set your bias to 30 31vdc-. i am going to rebuild the pi tonight and see what i come up with again. i have been making careless mistakes in my past tries ex. not grounding my pi and output section properly, measuring my bias off the pot and not at the tubes which was about 10 volts or so more. im very glad that you guys have chimed in i was starting to think i would be sailing alone on this thread. will post some updates as soon as i get the time to work on this one. i have some mixers that have been eating up my bench time. one thing im curious about is how can i lower my screen voltage more. i am using a 5 henry choke at 90 ma with a 103 ohm resistance. i figured that would be all i could do unless i took out the choke or got another one with higher resistance.