Amp bias range and running tubes hot...

Nightburst

Nightburst

New member
Been checking on my amp today and I ended up with more questions than answers...
Also found out there's a Bogner forum! So I signed up and posted this but love to hear your opinion about this as well:

Hi guys,
Didn't know a Bogner forum existed, so I'm new here. I hang out at the SD forum for a quite while though. Proud owner of a 1994 white cassis 101B model with all the extra options on it!
And now for the question:
Does the Bogner XTC have a different bias range as opposed to other brands out there?

Here's a little more background information:

About 2 years ago my Svetlana =C='s went bad and I looked around for new powertubes acros the web and I stumbled upon the dr. tube site. Now this site still has great tubes and information. Dr. tube himself is quite helpful and knowledgable so props go to out him. I live in Holland btw.
I asked him how to get more low end and growl, like a mesa. And he recommended me some JJ E34L's. When I got the set I couldn't set the bias right, wouldn't go lower than 60 mA or so, and they went red hot immediately!
Quickly switched off the amp and mailed dr. Tube again. I send them back and got a 'colder' set that fell within the bias range! Set it to 37 mA and got a sound out of my Bogner that I still can't believe! That really was the ticket to tone I was looking for! Very fat growly tone with a huge bottom end. Also more glass on the high end but never piercing, more piano like. Hope you can make sense out of these vague descriptions lol but i see these terms used more often when refering to sound in written words.
Anyway 2 years later these tubes started to pop so I checked and yep one of them was dead. Now I always have spare tubes around, so I looked in a drawer and found some EH EL34 made in russia 05 05 tubes brand new in the box.
Thing is, I can't get them biased lower than around 40 mA. I can't turn the pot down any lower. Also the readings fluctuate quite a bit, never really settles down to a constant number and there's a difference between them varying from
1 - 3 mA. Is this normal? I think 39/40 mA is a little too hot for these tubes but can't really harm them? Will it harm the amp or transformers?
I took a reading on the E34L's before taking them out and they read around 40mA as well. Strange because I always wait for the amp to settle before putting it all together and rock on.
I know I should just get another batch of JJ E34L's but I can't untill next month so I was wondering if it's ok to use the EH for now?

D. the M.
 
Re: Amp bias range and running tubes hot...

I can't find find the schematic for this exact Bogner so I can't tell you anything for sure.

Is that this baby?
http://www.bogneramplification.com/customshop/Ecstasy20thAInfo.php

The heat dissipation in a tube depends on a plate-cathode voltage.
So, as you see here (EL34) you can have a cold biased amp with as much as 42 mA.
On the other hand, if your plate voltage goes up to 500 V you probably should not go above 35mA.

http://www.webervst.com/tubes1/calcbias.htm

It seems to be a great amp, I wouldn't risk of frying it.
You should find out these voltages for your amp, and make a table for biasing (hot/avg/cold) and stick it somewhere inside.
Probably best to check it with amp repair man, and Bogner forum/tech support.


Maybe someone here on SD forum, already has some good info, let's wait and see :)
 
Re: Amp bias range and running tubes hot...

Thanks mr. duck!
You are correct, I am just too scared to measure the plate voltage and I'm also not quite shure where to measure it exactly.
I've read that Bogner xtc's go up to 500V plate voltage. Thats lethal dosis! So I'm not messing around. I can pull of a bias safely but anything else is scaring the heck out of me.I also found out that this amp is a real beach to put back together again as some of the tolex will unfold as I slide the Chassis back in!

I kind of asumed that if I was in the ballpark then I'm fine, from there go for what my ears tell me. I've been playing for a while now and don't smell anything burning yet, nor does the amp get red hot... yet. Actually I do smell something but thats new tubes burning/breaking in I hope!
I've been messing around before with tubes and I got away with it but I was really wondering if the amp can be done any harm by using my method.
 
Re: Amp bias range and running tubes hot...

I think you will be safe at 40mA with the EH tubes.

The JJ E34Ls are a high transconductance (more reactive) tube and can easily spike to 200mA at clipping while the EH EL34 is a bit lower in transconductance and generally will spike lower (150mA) at clipping.

In a nutshell, the lower transconductance tubes can handle a little more quiescent current than its high transconductance brethren...the trade off is that the lower transconductance tube will have less "punch", though it may not be noticable since you will be comparing them to old tubes which may have lost some of thier reactance.

Hope this helps.
 
Re: Amp bias range and running tubes hot...

Also forgot to mention:

When I just got the amp I took it to a tech and the svetlana's that were in then ran at
32 -34 mA. He even put tiny lables on the Tubes with that number.
So my asuming is not that far off, I think it's rather odd that a simple matched set EL34's of a well respected company like EH are no direct replacement? 40mA is quite hot I think.
Now I remember why i put those in a drawer in the first place lol.
 
Re: Amp bias range and running tubes hot...

glassman said:
I think you will be safe at 40mA with the EH tubes.

The JJ E34Ls are a high transconductance (more reactive) tube and can easily spike to 200mA at clipping while the EH EL34 is a bit lower in transconductance and generally will spike lower (150mA) at clipping.

In a nutshell, the lower transconductance tubes can handle a little more quiescent current than its high transconductance brethren...the trade off is that the lower transconductance tube will have less "punch", though it may not be noticable since you will be comparing them to old tubes which may have lost some of thier reactance.

Hope this helps.
Click to expand...

Thats a pretty high Amp peak your talking about! I'm not gigging at the moment so I'm not cranking it at home enough to make them power tubies work.
I've had my meters hooked up and messed around with pedals settings and played some heavy stuff turning it up loud but I've not seen those spikes in numbers yet.
Thanks for your input and I love that avatar! took me a while before I noticed it was a lamp lol.

And yeah your right, I dont really notice the difference now because those E34L's were pretty worn out and dull. I remember hating those EH's before and now they sound good...
 
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Re: Amp bias range and running tubes hot...

You're welcome Mr. V :)

This is from another webpage:

"Amps vary; two identical amps can have plate voltages which differ by as much as 20%.
Just because you have a schematic that specifies the plate voltage in your amp as being at 450VDC,
don't expect to see that voltage when you take a measurement.
TAKE the reading, don't assume the voltage will be as specified. Trust your meter."


That's why I would take it to an amp tech, let him measure that voltage, and feel safe once and for all.
Just write the voltage somewhere, so that you or next owner will know it.



p.s. Don't go inside the amp yourself.

"More specifically, peak voltage greater than 42.4V is hazardous;
voltage less than or equal to 42.4V, or SELV (safety extra-low voltage), is non-hazardous.
It is difficult to know when electricity can cause serious injury or be fatal.

Contact for only 1 to 3sec with currents of only 6 to 200mA can cause electrocution by disrupting the normal rhythm of heart muscles, resulting in fibrillation and leading to death"

@glassman
Disregarding the input signal and low transconductance, he has to make sure the amp does not go above 80% of heat dissipation in idle. Probably better to set it to 60%.
 
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Re: Amp bias range and running tubes hot...

cronnin said:
You're welcome Mr. V :)

This is from another webpage:

"Amps vary; two identical amps can have plate voltages which differ by as much as 20%.
Just because you have a schematic that specifies the plate voltage in your amp as being at 450VDC,
don't expect to see that voltage when you take a measurement.
TAKE the reading, don't assume the voltage will be as specified. Trust your meter."


That's why I would take it to an amp tech, let him measure that voltage, and feel safe once and for all.
Just write the voltage somewhere, so that you or next owner will know it.



p.s. Don't go inside the amp yourself.

"More specifically, peak voltage greater than 42.4V is hazardous;
voltage less than or equal to 42.4V, or SELV (safety extra-low voltage), is non-hazardous.
It is difficult to know when electricity can cause serious injury or be fatal.

Contact for only 1 to 3sec with currents of only 6 to 200mA can cause electrocution by disrupting the normal rhythm of heart muscles, resulting in fibrillation and leading to death"
Click to expand...

Yeah I know this stuff is extremely dangerous!
But I've done crazier things trust me ;), Have been zapped by 230V is no fun I can tell you! That's whats coming out of the walls here. And thankfully everything is grounded too.
I'm just chickensh!t when it comes to possibly ruining my amp! To bias this amp you need to slide the chassis out a little and you can reach the biaspot from underneath. I use a very long double isolated screwdriver to adjust the pot nice and safely with one hand in my pocket.

Now I'm willing to measure this voltage as long as I can find a good tutorial?
I can't find anyone I trust my amp with. The tech I used to go to had to close his shop and retired :(.
 
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Re: Amp bias range and running tubes hot...

You may feel chickenish but it's nothing to be ashamed about.
I mean, the amp is real beauty. It's sacrilegious just to open it up :)

Well, I would start with basic info like this:
http://www.diyguitaramp.com/bias.html

What bothers me is that the amp has class A/AB switch.
Easiest way to do this is to change the bias.
So, by flicking the switch from AB>A you are probably internally changing the bias voltage.

For example ( EL34 table) :
http://www.diyguitarist.com/Images/BiasChart-EL34.jpg
for 450V if you want to jump from AB into A
the bias current needs to jump from 31.7 to 52.8 mA

It's hard to guess how exactly Bogner solved this trick. You'll probably need someone experienced to help you will all the details.

I would have ask you now what is the current you measure when you send it to class A mode,
but since you already have 40mA that can easily flick your amp bias into 67mA and send it into flames :foot:

Maybe your amp works well these days just because you didn't have the time to try that out? :)

p.s. Please don't do it :)
 
Re: Amp bias range and running tubes hot...

You and I have the exact same amp, and I would suggest that you keep the bias at 35mA - 37mA max. By biasing mine hot for a few years, one day there was no sound, and the LED's on the front were very dim. I took it to Bogner and they said the power regulator needed to be replaced. It's basically 1 component.

Around that time, the other guy on this forum who owns the same amp, Metalking, described the same problem. I did him a favor and went over to Bogner and got that component. I mailed it to Jeff so he could solder it in himself. Problem solved.

Since all of our Bogner XTC's are 15 years old, I'm guessing yours is next. Because the Bogner is a voltage monster, the voltage regulator probably has a big job to do in that amp, also controlling all the LED's. Keeping the amp biased around 35mA is a good idea in such a fine tuned machine as the XTC. If you ever come across the problem I described....no sound and dim LED's, tell Bogner your power regulator went out and they'll send you that component.
 
Re: Amp bias range and running tubes hot...

Gearjoneser said:
You and I have the exact same amp, and I would suggest that you keep the bias at 35mA - 37mA max. By biasing mine hot for a few years, one day there was no sound, and the LED's on the front were very dim. I took it to Bogner and they said the power regulator needed to be replaced. It's basically 1 component.

Around that time, the other guy on this forum who owns the same amp, Metalking, described the same problem. I did him a favor and went over to Bogner and got that component. I mailed it to Jeff so he could solder it in himself. Problem solved.

Since all of our Bogner XTC's are 15 years old, I'm guessing yours is next. Because the Bogner is a voltage monster, the voltage regulator probably has a big job to do in that amp, also controlling all the LED's. Keeping the amp biased around 35mA is a good idea in such a fine tuned machine as the XTC. If you ever come across the problem I described....no sound and dim LED's, tell Bogner your power regulator went out and they'll send you that component.
Click to expand...

Jonesy...that power regulator is for the 12 volt supply for the channel switching. When it goes, the amp goes into "no channel" and there is no sound. Framus Cobras use the same setup and have the same issues. The bias point has no effect on this part of the circuit.
 
Re: Amp bias range and running tubes hot...

Curious, how are you measuring plate current when you set bias? A tool like a Webber Bias-Rite would let you check both plate voltage and current when installing a new set of tubes. With both measurements you'll be able to see just how hot or cold your tubes are being run and set them to a safe range.
 
Re: Amp bias range and running tubes hot...

Thanks for the input guys! I learned some more about tube amps, mission succesful.
I'll just get new tubes, it's not worth risking damage in a special amp and those E34L's sounded monstrous!
For the time being I'll put in those old svetlana's as I've kept them somewhere. They're old but will bias right.
 
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Re: Amp bias range and running tubes hot...

@Koreth:

I think it's a matter of taking a normal multi- or volt meter and measure the plate voltage over pin x and pin y.
Anyone knows what pin x and y are supposed to be? (1,2,3,4,5,6,7...)
 
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Re: Amp bias range and running tubes hot...

These are the pins on a tube, when you look directly at them.
If your looking at the tube socket from inside of the amp, with the tube sticked in, diagram is same.

http://www.audiodesignguide.com/my/el34_pse/EL34_Socket.gif
http://www.hometheaterhifi.com/volume_7_3/images/micrex-L1-el34-tube-diagram.gif

Measuring voltage between Plate und Katode would be pinds 3-8.
That's the voltage you need.
On lots of amps, cathode is grounded, so that Plate-Katode voltage is just Plate-ground voltage.

People usually don't measure directly on the tube socket, bacause it is hard to hold both probes on both pins, and not to make shortcut while the amp is on.

It better to find (trace) these spots somewhere else on the schematics, where it's easier to stick a probe on.


Usefull link with graphs and operation mods.
http://www.drtube.com/datasheets/el34-sed2002.pdf


Please someone confirm all this :)
 
Re: Amp bias range and running tubes hot...

That's why a tool like the Bias-rite is so handy. Rather than trying to get a measurement on both pins with a tube in the socket, you can see where the tube is running at without sticking your meter probes and/or hands into the chassis, keeping them safely the heck away from high voltage
 
Re: Amp bias range and running tubes hot...

glassman said:
Jonesy...that power regulator is for the 12 volt supply for the channel switching. When it goes, the amp goes into "no channel" and there is no sound. Framus Cobras use the same setup and have the same issues. The bias point has no effect on this part of the circuit.
Click to expand...

Thanks for clarifying that for us, because I wasn't sure if the bias effected that regulator. Before this, I'd never even heard of a power regulator, because most amps don't need one.....since they don't have complicated switching systems.

I always thought 35mA was kind of cold for EL-34's, but it's what Bogner sets them at.....maybe due to the high plate voltage? All I know is my amp sounded slightly better when it was around 38mA, so I may play around with the bias this weekend. I use a mA bias tool.
 
Re: Amp bias range and running tubes hot...

Koreth said:
That's why a tool like the Bias-rite is so handy. Rather than trying to get a measurement on both pins with a tube in the socket, you can see where the tube is running at without sticking your meter probes and/or hands into the chassis, keeping them safely the heck away from high voltage
Click to expand...


Agreed.
I was so delighted when I saw it in your post, cause I havent heard of it before.
For a pro amp tech this gadget is a must.
 
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