5902 pentode triode mode

[Disabled_shredder]

i have been fiddling with a custom build using all sub mini tubes. i had a friend that lives on the road and he cannot find an amp anywhere that can get down to the volume he wants with the tone he desires. the data sheet for the 5902 shows graphs for connection to triode mode but i have had no such luck. i am running about 166v on the plates and 161 on the screens i am running a fixed negative bias at about -22-25vdc depending on the tube to keep it under 3.5watts plate dissipation and some where around 21 29 ma cathode current. i have 1.5k grid resistors and 4.61k 1% screen resistors because i didnt have any 5.6k resistors. i realize that having the power tubes in triode mode might not reduce the volume level as much as a ppimv but im curious and like to try everything. all my voltages held when i switched to triode mode but i got no waveform on the outputs. could it be the screen resistor values or the fixed bias? ill post a schem and diagram when i get the chance

[tubeswell]

Can you post a schematic of your amp (including B+ and HT voltage information)?

[Disabled_shredder]

be glad to the only thing i have found on cpu to draw schems is diylc i usually do things by hand. is there any freeware for mac that would suite me so i can be precise and thorough?

[katopan]

Switching from pentode to triode mode should leave the screen resistor connected at the screen leg of the 5902s, and switch the other end between the screen supply and the respective 5902 plate. Is this how you've got it set up?

Often triode mode needs a cooler bias (more negative voltage / less plate current) for the valve to stay happy.

[tubeswell]

be glad to the only thing i have found on cpu to draw schems is diylc i usually do things by hand. is there any freeware for mac that would suite me so i can be precise and thorough?

If you have a mid-2012 or later mac this works http://dhost.info/jschem/ if you download and install Legacy Java SE 6 for mac. (see the following link for the Java version)
http://thegothicparty.com/dev/article/h ... mac-10-10/

[Disabled_shredder]

i have it set up exactly as in this photo. i also read somewhere that switching to triode mode could cause redplating to some degree soo i upped the neg bias before hand but not to an extreme amt. i think im gonna give it another shot. also, are you saying that when i switch to triode mode i eliminate the screen resistors? ill also take a picture of the breadboard so maybe you can get a better representation of my insanity! haa

[tubeswell]

I'd switch the screens over to the plates for triode mode myself. Use a 1M resistor in parallel with the switch (in series between the screen and the screen supply node) to avoid arcing from switch bounce.

[Disabled_shredder]

i tried to take the best photos i could of the schems that i drew. more or less i ripped off the power between a fender and a trainwreck. the power amp started as a trainwreck i believe but at this point its something Frankenstein'd and the preamp which i have not fully made functional was taken from my single favorite versatile amp a laney gh50-l but i guarantee it will not be much of a laney when i get done modifying! hope these photos help

[Disabled_shredder]

some more photos of the schematics

[katopan]

You can switch to triode mode including the resistors or not, it doesn't really matter. I used to have it in my 18 Watt and had it as you've shown with the Lord Valve diagram. I never switched live though, always shutdown the amp to toggle that switch.

Given that you're breadboarding you could try hardwiring as triode instead of switching just to test it out.

[Disabled_shredder]

i have tried both hard wired and toggle. resistors or not and if i run in triode mode i get zero waveform. well ill get a little actions around a mvpd or so. but i think thats more ripple than wave. another thing im curious about is running the 1 meg in line with the screen supply so i run the 1 meg between screen supply and screens or screens and plate?

[SilverFox]

In looking at the schematic, is your grid connection being made to a screen instead? I could be reading the output tube diagram incorrectly but it appears the grid connections are made to a screen.

silverfox.

[Disabled_shredder]

no as far as i can tell all connections are correct. but again i do things different and wrong to most so it might be wrong but right to me but connections are grid pin 1 cathode 2,4,8 plate pin 5 and screen pin 7. i just dont understand how the tube can what i assume is going into complete cutoff when i connect it in triode mode with the bias lifted to almost -27vdc. but i guess thats what you get when you decide to design an amp using 50s technology that was never designed for musicians. if your gonna cut a different path then you better have a sharp machete!

[tubeswell]

For triode mode, tie the screen and plate together.

[Disabled_shredder]

yes i have tied the screen and plate together and i get no signal on the output. i dont know where its a primary impedance issue a voltage issue a bias issue that is running it into cutoff or what at this point.