weird biasing problem ? "high" cathode voltage

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

Post Reply
Ang3lus
Posts: 243
Joined: Sat Oct 29, 2011 8:53 am
Location: Israel

weird biasing problem ? "high" cathode voltage

Post by Ang3lus »

Hey all,

an amp i've had for a while, which gave me hell since day 1, I managed to fix all the problem, but more keep showing up.

this time, it started blowning fuses, had a loud hum or loud peak sound (nothing plugged in) and blown a fuse.

opened it up and checked the bias, i get (fixed bias), -39v on one side, and -41v on the other side (pin 5), HV (pin3) gives me a steady 520v on both sides, pin 8 thru the 1ohm method gives me 15v on one side, and 0v on the other.

the only mod i did to the amp schematic wise is using 15k swampers instead of 4.7k.

edit: I checked the coupling caps on the EL34s, getting 241v on one leg and -41v on the other, same reading on the 2nd side only with -38v.
I stupidly conneceted the cathodes (all of them) to a common ground on the chassis, disconnecting that gave me negative voltage on one side of the tubes (Cathode across 1ohm) but still none on the other side.
only other thing that seems weird to me is C51, it is giving me 15v on both sides (capacitor shorting ?)


schematic is attached, would love some input, at a total loss.
You do not have the required permissions to view the files attached to this post.
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: weird biasing problem ? "high" cathode voltage

Post by Stevem »

Blowing fuses and a hum that does not come blasting right on is most likely the sign of two things, 1 ) atleast one bad output and you may have a shorted output transformer.

View the output tubes in a dark room any of them that start glowing red when the hum starts is either bad or has too low a bias voltage, and that flash are bad also.
I would install them one at a time in a socket that reads good in regards to having plate voltage (pin 3) screen voltage ( pin 4) and atleast -39 volts on pin 5.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Firestorm
Posts: 3033
Joined: Fri Jan 25, 2008 7:34 pm
Location: Connecticut

Re: weird biasing problem ? "high" cathode voltage

Post by Firestorm »

15 volts on pin 8 through 1 ohm? :shock:
labb
Posts: 223
Joined: Sat Aug 02, 2008 2:03 am

Re: weird biasing problem ? "high" cathode voltage

Post by labb »

deleted
Ang3lus
Posts: 243
Joined: Sat Oct 29, 2011 8:53 am
Location: Israel

Re: weird biasing problem ? "high" cathode voltage

Post by Ang3lus »

I managed to do alot of tests yesterday, i did not find a short between the primaries and 2nderies, however, checking resistance between 2nderies and chassis ground gives me 4ohm with all windings, i dont know if its ok or not

Tried another set of tubes, that are fully wirking on a 2nd amp, getting same results, one side giving no bias and the other gives me a few hundred volts(thru 1 ohm), im guessing its the transformer then ?

I used fx loop send to a SS poweramp and preamp is fine, sounds good.

The amp always had a problem with cracklings and whistling teapot, could it be that the OT was bad from the begging and just now died on me ?
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: weird biasing problem ? "high" cathode voltage

Post by Stevem »

Are you sure that's a 1 ohm 1 % resistor?
There's no way you could be pulling 15 volts on a output tube Cathode without the tube being so red hot that you would blow main fuses or melt the glass case of the tube!

Those tubes should produce a D.C. Voltage reading across those 1 ohm resistors of no more than .045 volts at idle!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
pdf64
Posts: 2688
Joined: Sat Mar 12, 2011 9:23 pm
Location: Staffordshire, UK

Re: weird biasing problem ? "high" cathode voltage

Post by pdf64 »

Is this a clone you've built, or a commercially made amp? Either way, the schematic doesn't show 1 ohm cathode resistors?
Ang3lus wrote: Sun Sep 17, 2017 8:11 am... only other thing that seems weird to me is C51, it is giving me 15v on both sides (capacitor shorting ?)...
So is the LTP conducting, eg what's the voltage across R54, 60 and 61?
Maybe the whole issue is bad connections to 0V.
Ang3lus
Posts: 243
Joined: Sat Oct 29, 2011 8:53 am
Location: Israel

Re: weird biasing problem ? "high" cathode voltage

Post by Ang3lus »

Its a clone i've built, added the 1 ohm resistors.

i'm out for vacation till saturday night so i'll only be checking the voltage then.

I do suspect i the OT is dead, maybe a bad tube when it was in the practice room, tried another set of good tubes and getting the same readings, like one side is not pulling any current (the one that reads 0 when i check for bias)
Ang3lus
Posts: 243
Joined: Sat Oct 29, 2011 8:53 am
Location: Israel

Re: weird biasing problem ? "high" cathode voltage

Post by Ang3lus »

the preamp side of things is working fine, connected from fx send to an external poweramp and it is fine.

my question is this, since this amp has no useable clean channel anyway, and the OT is (probably) dead now, can i disconnect the OT completely and use FX send as a PRE-out and use an external power amp ?

is this essientialy how preamps work ?

no need for the phase splitter and such ?
Ang3lus
Posts: 243
Joined: Sat Oct 29, 2011 8:53 am
Location: Israel

Re: weird biasing problem ? "high" cathode voltage

Post by Ang3lus »

the preamp side of things is working fine, connected from fx send to an external poweramp and it is fine.

my question is this, since this amp has no useable clean channel anyway, and the OT is (probably) dead now, can i disconnect the OT completely and use FX send as a PRE-out and use an external power amp ?

is this essientialy how preamps work ?

no need for the phase splitter and such ?
User avatar
pompeiisneaks
Site Admin
Posts: 4222
Joined: Sat Jan 14, 2017 4:36 pm
Location: Washington State, USA
Contact:

Re: weird biasing problem ? "high" cathode voltage

Post by pompeiisneaks »

In theory yes, sometimes the output from the preamp can be a bit either hot or low depending for the power amp, but it should work. the phase inverter is only to allow the push pull output stage to work correctly. If you send it to another power amp, part of what's required for any push pull power amp is a phase inverter.

~Phil
tUber Nerd!
Ang3lus
Posts: 243
Joined: Sat Oct 29, 2011 8:53 am
Location: Israel

Re: weird biasing problem ? "high" cathode voltage

Post by Ang3lus »

so before fitting in a new OT i decided to just test the preamp with a free scope meter hooked to my sound card.

playing at some settings with the volume control, both clean and dirty channels.

1. I see 50hz and 100hz hum
2. when i hit the guitar hard and then mute the strings, i get like a drop in signal (no signal at all actually) and then it rises up back again, like the caps are discharging and charging but not fast enough. (
User avatar
pompeiisneaks
Site Admin
Posts: 4222
Joined: Sat Jan 14, 2017 4:36 pm
Location: Washington State, USA
Contact:

Re: weird biasing problem ? "high" cathode voltage

Post by pompeiisneaks »

I'd be very careful connecting that to anything until you've validated the output voltage is AC and not DC and in a lower voltage range, I think typical preamp output voltages should be in the sub 20v range, but can't recall. Do you have a coupling capacitor on the output of the last preamp stage to block DC?

~Phil
tUber Nerd!
Ang3lus
Posts: 243
Joined: Sat Oct 29, 2011 8:53 am
Location: Israel

Re: weird biasing problem ? "high" cathode voltage

Post by Ang3lus »

i'm using the send connection on the preamp, it's made as -10db so i doubt there's anything harmful in connecting that to anything.

it looks like heater hum rather than a ground loop, looks like something is bad in the power supply section because the preamp tubes are fed as DC heaters, maybe a bad lead dress or a bad cap, after some bad experience with JJ products (suspecting the caps here and had 2 bad batches of tubes) I will use F&T in my next build, i decided to dismantle the amp completely and build something else inside.
pdf64
Posts: 2688
Joined: Sat Mar 12, 2011 9:23 pm
Location: Staffordshire, UK

Re: weird biasing problem ? "high" cathode voltage

Post by pdf64 »

Ang3lus wrote: Sun Oct 01, 2017 7:43 am so before fitting in a new OT i decided to just test the preamp with a free scope meter hooked to my sound card.

playing at some settings with the volume control, both clean and dirty channels.

1. I see 50hz and 100hz hum
2. when i hit the guitar hard and then mute the strings, i get like a drop in signal (no signal at all actually) and then it rises up back again, like the caps are discharging and charging but not fast enough. (
Did it hum before? There may be a ground loop with the PCs sound card (probably wouldn't happen if using a laptop).
The drop in signal may be bias shift, so extreme it's causing blocking distortion.
Perhaps there's a bad part somewhere, way off value?
It may help to undertake a voltage survey, both at idle and with signal, to see if the operating conditions at any of the stages change a lot.
Post Reply