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How do I bias the output tubes?

Purchased off ebay, had no tubes. purchased new tubes.

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  • Music Master
  • 3,107 Answers

The adjustment method varies from amp maker to amp maker. if you're unfamiliar with the procedure, you're better off to take it to the shop and get it done right. If you really want to do it your self, VHT has a meter they sell for about $100 online so you can see where you're at and then you adjust the bias trim pot inside your amp to balance them out to where you want them.
But again, you're best off leaving it to a pro.

Posted on Nov 23, 2014

Testimonial: "I thought I had entered the amp's description. It is a line 6 Spider Valve HD 100 Mk1 Bogner. There are no bias pots, but Molex connectors without any jumpers. Locating a schematic for this is proving to be impossible."

  • Richard A Lutz
    Richard A Lutz Nov 24, 2014

    I am sorry, I thought I had entered the make and model. This is a Line 6, Spider Valve HD100 MK1. No bias pots, Molex jumpers... And they are missing. Finding a schematic for this, or anything else Line 6, is proving to be exceedingly difficult. Hence, my query here. Thank you.

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Could be a few things, most simple the Tubes, and or biasing, I have had a few amps that certain notes or chords would cause harmonic distortion within the tubes, and the tubes where good? Another possibility is the output transformer? Sometimes when they are failing the volume varies... hope this helps?

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The bias voltage goes to the output tubes only. These go into the larger tube sockets.

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When replacing the two output tubes you should check the voltage at TP26 or either cathode pin of the power output tubes to ground. Set the bias by adjusting pot R82 to 60mv which is 60 ma across the 1 ohm metering resistor. This coresponds to 30 ma per tube. Changing other tubes requires NO adjustment.

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Check the bias on the tubes! Some brands can run with bias set wrong. Also some amps have 4 output tubes and people don't know that MATCHED pairs have to be seperated to each side of center of the group of four tubes.
You probably only have a single EL84 tube in your unit.
I could not find the schematic for this unit... IF the plates of the EL84 are coloring, the bias is set wrong. The plates should remain BLACK and not coloring to dull red or orange. Telling if the tubes are "burned up" can only be done with a tube checker or electronic test equipment or by seeing a bright purple glow between the elements.
Now this is supposedly only a 5 Watt unit so that means it is for LOW volume practice only... If you overdrive this small amp you can PROBABLY burn almost any tubes you put in it. The sound you can expect out of this will not be louder than most of the consumer boom boxes at full volume.
If you overdrive and "flattop" the unit the power tube will overheat and fail... this amp runs the tube in "Class A" which makes failure under overdrive more likely than in Class AB or B. I can't go into the engineering details other than to say at volumes above the design the output transformer will saturate and the tube will have to dissipate way above its ratings.

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Fender vibrolux. Very (loooud) loud hum/buzz All tubes but the 6l6's removed. History: bought this amp as a repair project. replaced all caps. originaly the amp was quiet but but had very little volume...


Here is the schematic:

http://elektrotanya.com/fender_vibrolux_6g11_schem.pdf/download.html

Check the quality of the DC bias as being free of ripple.across teh 25 mfd/50v cap filtering the bias.
If much ripple there, replace the cap. Also the .05 bypass on the swinger of the intensity control.

This should be an easy fix, but you might need an oscilloscope to diagnose it quickly... or just parallel caps across these to see if it kills the hum.

The main high voltage filter caps are also suspect as well as the bypass of power to the ealier inverter stage a 16mfd/450v

There are lethal voltages in this so use great caution when troubleshooting... the caps hold a nasty charge.

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First thing is that if you bought MATCHED pairs of the EL34's a COMMON mistake is to install the pairs wrong. Correctely install a pair as the OUTSIDE tubes of the line and then the remaining pair as the middle two tubes of the line of tubes before biasing.

Funny thing, I just ran across this problem you have with a Bedrock brand amp that uses the same configuration as your V3 tube does. This is really a stupid configuration engineering wise as they tie a plate at high voltage to the grid of the second stage in the tube as a cathode follower, This means that the second section has the cathode at a reasonably high voltage relative to the heater with wide audio swings on it as well... Tubes are NOT designed to work that way and result in noisy behavior, One would have to select tubes that have very low heater to cathode leakage in the second section of the tube... What you are seeing is the result of very poor engineering practice. Marshall is probably testing tubes for the leakage and branding them as such...

I think that designers of this circuit have NOT had experience with tubes and have applied transistor type designing to this use... it is plain stupid and myself I would re-engineer the circuit if it was mine or build a tube tester capable of measuring the heater cathode properties to select a suitable tube to use.

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1 Answer

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One, You may Have a bad 1/4" jack on the Input, or Two you may have unballenced Bias on the Output Tubes. This is a technical adjustment in the Amplifier.

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I was playing through my Jvm410 it was sounding great then the volume got quieter then no sound all lights work whats the problem power tubes maybe?


Yes, you MAY have destroyed the output tubes...

When replacing, make sure your MATCHED pairs go into V1 and V4 and the other into V2 and V3 respectively. Many don't realize the proper placement of matched pairs.

Open unit and if you find the tubes with the plates glowing red OR purple haze down between the elements, the tubes are gonzo.

Check the bias at connector CON1, Between pin 1 and 2 is a 1 ohm resistor in the cathode of the V1 and V2 side. Adjust the Bias1 for the number of milivolts matching the required bias current for two of the output tubes. Do the same for pins 2 and 3 of CON1 for the second side using the BIAS2 control. Fir your amp, you will want each of these set to 70millivolts.

It is possible (hopefully) that one of the smaller tubes burned out. These are much less expensive. EACH of these will have TWO points of light visible when they are operating. If you only see one, then 1/2 may be burned out. When turbes burn out, they cool down and sound will fade out.

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Well whatever you do do not fire it up. If you have a VARIAC or know someone wh has a variac use that to bring the supply voltage up gradually in case a output cap is dried out. What kind of amp is it? The tubes are stuck due to oxidation of the tube socket and pin on the tube. If you have already powered it up and it didn't smoke your input transformer and you didn't hear any humming or buzzing I would say the next step would be to check the tubes out. If you have access to a tube tester obviously use that however you can check the bias after they warm up. You will also want to do the usual maintenance stuff like cleaning the potentiometers with contact cleaner and also check your 1/4 input and output jacks.

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Start easy. You could have a bad power switch. Do you get any kind of indication of any life at all, like tube filiments or front panel indicators? If the fuse is blown you have a fault that is drawing excess current, in which case you can check the rectifiers that create your bias voltages, yank the output tubes to see if they have an internal short, less likely would be a shorted filter capacitor (although I have seen it). You could also have a bias problem on the output tubes that causes them to draw excess current and likely makes the tube(s) glow bright red before popping the fuse. If this is the case, look for shorted or leaky coupling capacitors and burned bias resistors, or maybe just a tube with internal shorts, and you will likely have junk tubes as a bonus (they don't take kindly to glowing bright red- meaning a glow that is far more intense than the soft glow of the filiments). If the fix gets more involved than all of this, give me your email addy and we can get hard core.

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