weird biasing problem ? "high" cathode voltage
Moderators: pompeiisneaks, Colossal
weird biasing problem ? "high" cathode voltage
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.
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.
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Re: weird biasing problem ? "high" cathode voltage
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.
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!
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!
Re: weird biasing problem ? "high" cathode voltage
15 volts on pin 8 through 1 ohm?
Re: weird biasing problem ? "high" cathode voltage
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 ?
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 ?
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- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: weird biasing problem ? "high" cathode voltage
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!
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!
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!
Re: weird biasing problem ? "high" cathode voltage
Is this a clone you've built, or a commercially made amp? Either way, the schematic doesn't show 1 ohm cathode resistors?
Maybe the whole issue is bad connections to 0V.
So is the LTP conducting, eg what's the voltage across R54, 60 and 61?
Maybe the whole issue is bad connections to 0V.
My band:- http://www.youtube.com/user/RedwingBand
Re: weird biasing problem ? "high" cathode voltage
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)
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)
Re: weird biasing problem ? "high" cathode voltage
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 ?
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 ?
Re: weird biasing problem ? "high" cathode voltage
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 ?
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 ?
- pompeiisneaks
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Re: weird biasing problem ? "high" cathode voltage
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
~Phil
tUber Nerd!
Re: weird biasing problem ? "high" cathode voltage
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. (
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. (
- pompeiisneaks
- Site Admin
- Posts: 4222
- Joined: Sat Jan 14, 2017 4:36 pm
- Location: Washington State, USA
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Re: weird biasing problem ? "high" cathode voltage
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
~Phil
tUber Nerd!
Re: weird biasing problem ? "high" cathode voltage
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.
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.
Re: weird biasing problem ? "high" cathode voltage
Did it hum before? There may be a ground loop with the PCs sound card (probably wouldn't happen if using a laptop).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. (
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.
My band:- http://www.youtube.com/user/RedwingBand