rebuilt my 5e3> hum issue

[black_labb]

i've got the amp rebuilt, and i cant hear any hum, but the dropping resistors arent dropping much voltage at all for some reason, and voltages were too high to give a very clean sound. i didnt go any further as it was 6.30 am and time to go to bed. now im up and ready to finish it off. i'll try to figure out why the resistors arent dropping, and i think i'll change the voltage taps with the bias voltage (making the bias input 70vac) and then i'll just use a resistor to drop the bias. i took out the dc elevation from the heaters to save time. not sure if i'll put it back in.

im not sure if its as silent as it seems, as the extra voltage has dropped the gain a fair bit i think, but im pretty certain the hum has been reduced alot, but i'll see when its all together properly.

[black_labb]

now somehow i have 525vdc coming out of the bridge rectifier with 320vac in... with all tubes pulled (yes i measured both of those values, not just reading off the transformers specs). if i figure out why i can name a new rectifier type after me.
edit: was some interaction with the bias voltage.

[black_labb]

anyone have any explanation for why the -ve voltage supply for the bias was acting up? i was using the transformer shown here https://taweber.powweb.com/store/PTGPsch.jpg

i had the 70 and 410V going into the bridge rectifier (supposedly 340v, but came out as 320) and i had the 0v going to the bias supply. so what i was using was 70-0-340 (320)

the bias supply is a reversed diode into a filter cap. that went to a 22k resistor, a 10k trim pot with the wiper as the bias supply. the other side of the trim pot was connected to ground through a 12k resistor.

the only thing i can think of is that the cap opened up (i just realised that there was ~100v across the 65v cap, forgot to put it after the 22k resistor) so i may get a new one of those and wire it correctly.

[black_labb]

this is annoying. the documentation for this transformer is useless. i plugged the red/yellow tap into the red and got 380vac, so i assumed i got the wrong red tap. switched to the other and get around the same, as opposed to 320v. should of measured them all myself before i clipped and isolated each lead.

[MWJB]

"now somehow i have 525vdc coming out of the bridge rectifier with 320vac in... with all tubes pulled (yes i measured both of those values, not just reading off the transformers specs)." Measure voltages with all tubes installed, with no tubes fitted there is no current load on the PT voltage will read way high.

[black_labb]

"now somehow i have 525vdc coming out of the bridge rectifier with 320vac in... with all tubes pulled (yes i measured both of those values, not just reading off the transformers specs)." Measure voltages with all tubes installed, with no tubes fitted there is no current load on the PT voltage will read way high.

i was measuring the ac voltage in as well, so it wasnt a due to an unloaded value. when i disconnected the bias it went back to 450ish volts.

i think i may use a zener for the fixed bias mode

[loudthud]

How much B+ are you trying for? You really have to be carefull with tap used for bias. You can easily end up with several hundred volts. When the main B+ is inloaded, the bias supply will act as the low side of the bridge and push the B+ higher than it would normally go. If there is a standby switch, you probably won't have good bias before you turn the B+ on.

[black_labb]

How much B+ are you trying for? You really have to be carefull with tap used for bias. You can easily end up with several hundred volts. When the main B+ is inloaded, the bias supply will act as the low side of the bridge and push the B+ higher than it would normally go. If there is a standby switch, you probably won't have good bias before you turn the B+ on.

sounds like what i was getting. im going for around 420v or so after the rectifier, and then drop it down 25-30v via a resistor, or less by parralleling another.

would a bias supply like this be bad on the tubes, or would they not heat up enough in the time that the bias is developed? i've disconnected the bias supply so that i can get it all running well before messing around with the fixed bias option.

the ac voltage going in i used was 320v when disconnected, so it should be somewhere like 300vac loaded, so should be good enough for rock and roll. i'm not going for a copy as you can probably tell, but the 5e3 flavour is nice, and a bit of versatility cant hurt.

i'm not quite sure why the amp sounds so quiet, but i'm hoping the voltages will give me a good idea.

thanks for the help everyone

[black_labb]

there is 0v on pin 8 of the PI tube, so i'd say i'm pretty much running single ended, which explains the low volume. just need to figure out why now.



edit: all looks fine, i'm not sure why it is at 0v, but i'm thinking i might install a paul c mod if a tube change doesnt fix it (i'm pretty sure i've already tried different tubes, but i may as well try once more).

edit2: installed paulc mod. there is 50v on pin 7, but still 0 on pin 8. i measure 56k to ground from pin 7, so there isnt a short or an incorrect value there.

[loudthud]

I think you want to use the taps outline in the attached schematic. The B+ will float up to over 500V with no load (because of the bias supply). Not the ideal situation.

[tubeswell]

there is 0v on pin 8 of the PI tube, so i'd say i'm pretty much running single ended, which explains the low volume. just need to figure out why now.



edit: all looks fine, i'm not sure why it is at 0v, but i'm thinking i might install a paul c mod if a tube change doesnt fix it (i'm pretty sure i've already tried different tubes, but i may as well try once more).

edit2: installed paulc mod. there is 50v on pin 7, but still 0 on pin 8. i measure 56k to ground from pin 7, so there isnt a short or an incorrect value there.

Hi there black_labb

for a cathodyne PI, the cathode voltage should be 1/4 of HT and the plate voltage should be 3/4 of HT. (i.e.; the cathode voltage should be 1/3 of what the plate voltage is).

If your cathode is at zero, it is at ground potential, therefore there must be a short to the ground return path somewhere between pin 8 and the ground. (What's more the plate will be swinging even harder, because the cathode isn't goin' anywhere). So check your wiring again.

[black_labb]

Hi there black_labb

for a cathodyne PI, the cathode voltage should be 1/4 of HT and the plate voltage should be 3/4 of HT. (i.e.; the cathode voltage should be 1/3 of what the plate voltage is).

If your cathode is at zero, it is at ground potential, therefore there must be a short to the ground return path somewhere between pin 8 and the ground. (What's more the plate will be swinging even harder, because the cathode isn't goin' anywhere). So check your wiring again.

that was my thought, but when i measure the resistance to ground it measures 56k, as it should. im stumped on this. i assumed there was some wiring fault, so i thought rewiring it to a paul c mod would undo my mistake (which i couldnt find) and fix it. i feel like an idiot posting this, as there is no reason it shouldnt work


thanks for the diagram loudthud. thas just how i had it wired before when it would float to >500v. i'm thinking i may just switch to zener diodes.
would a 27v one seem about right for a 6v6 amp on 390-410v on the plates and 340v on the screens?

[tubeswell]

I think you need to look at it again, because something is shorting. Is there a wire shorting to the chassis off the back of your boogie board?

[black_labb]

could be something like that. maybe the highish voltages form an arc, where the multimeter doesnt go high enough. will check. thanks

[tubeswell]

Or maybe one of your spacer nuts on your board is touching the cathode eyelet or something