Posted: Mon Jul 23, 2007 11:40 pm Post subject: Biasing Problem
I'm having trouble setting the biasing on my VL-503. I can get Tube 2 to illuminate the "In" (green) light, but Tube 1 is stuck in the "Over" (red light) position.
A few observations:
1. Manipulating the Tube 1 adjustment screw has no effect on Tube 1's bias reading, but it does affect the bias reading for Tube 2.
2. The tubes seem to be lighting off normally (dim orange burn at top and bottom), and the amp sounds fine when played.
3. Switching the tubes around (that is, moving Tube 1 to the Tube 2 socket, and vice versa), had no effect.
Jay h,, my guess would be that the 2 output tubes are not matched close enough for correct bias using the trim pots on the back of the amp,,,as I think those trim pots are like fine tuners,,,for when el34's are replaced with 6550's and vise versa,,,,the true bias control is inside the chassis,,, and is set with a scope,,,so as I said ,, my guess would be the tubes are out of spec with each other ,,,If you know an old tv or radio repair guy,,,or as my case here at the local Radio Shack,,, take both output tubes and get them checked,,, my guy charges me $.50 cents a tube,,,
That makes sense. Considering that the amp is 12 years old, it's quite possible that they're out of balance.
Assuming I have to replace them, can I set the bias on the new tubes using the fine-tuning adjusters on the back of the amp? (I'd be buying a "matched" pair, so presumably they'd be pretty close to begin with.) I'd prefer not to have resort to the scope method, if possible.
Jay H,,Yes,,asuming that everything in the signal chain from input jack to output mains is still within specs,,,a matched set of tubes may be all you need ,,,
Be aware that it takes at least 15-20 minutes for the big bottle tubes like 6550's to heat up,, to set the bias,,, thats in the on postion,,, not standby,,
Also,,at my past job working for Ampeg as a booth tester QC guy,,,I have seen many instances of so called matched tubes ,, not being matched close enough,,even after the unit has passed all the scope tests,,mainly because the tubes never reached break in or the max output untill,, I get them in the booth ,, and play them,, the last thing i did was check the bias lights on the back before the unit went on to be packed and shipped,,,
[quote]Any recommendations for a replacement brand/model?[/quote]
Not really ,, Price being the main point of intrest,
I called St.Louis Music,, my former Employer,,now Loud Technologies,,el34's are less expensive than 6550's,,buy almost half,,,also el34's make that amp 50 watts ,,versus the 6550's 75 watt rating,,,,,personally ,I've heard both in a VL-502 ,, and I prefer the 6550's ,,but your prolly looking at least $100 for a pair,,,
Jay,,glad to hear the amp sounds better than ever,,,wooot,,,
As for the bias lights in the back,,,I take it you still have a solid red on one side,,,even if you turn both sides down(counterclockwise)all the way with the amp controls at zero(counterclockwise) with the amp in the "on" mode ,,,not standby??
There is a chance that the LED lights are giving you incorrect readings ,,I have seen it before,,,,,The control pot may actually be working ,,just the circuit that controls the lights may not be ,,even on one side,,,and one side will have an effect on the other ,,normally,,,,,,
Having said that,,,You could try this,,
With all the controls on the front on zero,,,the bias controls on the back on zero,, the amp plugged into speaker cab,,,,the ohms set correctly with switch on the back to match the total ohms of the speaker cab,,warmed up for 15-20 mins in the on position,,,
With a small flat screw driver(I use a guitar pick) bring the working bias pot up till it just goes from red to green,,,With the good side showing a flickering green,,just barely on green,,it should take less than half a turn,,,??
Now adjust the other bias pot ,,going from zero(counterclockwise) to 10(clockwise)in a smooth fluid motion,,, take notice of the green light from the good bias pot,,,, if it goes out or turns red ,,,flickers,,,,you know the pot with the red light is working ,,,I hope this is the case,,,
If this is the case,,,all you can do is visually set the red side to match the same angle as the head of slot of the green side,,,then just back both pots down just a smidgion,,,,simdgion= as in feel the pot move just a bit,,,take notice of the color of the output tubes ,,,they should both have an even tempered glow at top and bottom,,,
Presuming everything is still working correctly in this amp,,,this setting is biased just a little on the cold side,,to allow for tube break in and aging,,
this is the way I set up new amps to be shipped,,,,
I would still suggest to take this amp to a qualified tube guitar amp tech ,,,,if still in doubt,,,,
When you say to set the controls on the front of the amp to zero, are you referring to gain, master volume, tone controls, etc.? I haven't been doing that up till now, as I didn't realize it could make a difference in the biasing procedure. If it can, though, I'll definitely give it shot.
Do all of the knobs need to be set on zero, or just the ones that apply to the channel the amp is on while I'm biasing it?
Jay,,as long as you unplug the guitar cord from the input jack on the front of the amp ,,when setting the bias on the back of the amp,,the controls,,master,,tone knobs are a non issue,,
As a saftey precaution,, I always zero all the knobs on a new amp ,, part of my training,,,
Normally on a new amp ,, the bias is set with a scope and signal generator pluged into the input jack of the amp ,, with the master on 10(max),, the clean gain at about 50%(half),,, takes about 30 seconds after the amp is warmed up,,At that point it was passed on to me in the test booth,,,a 6x8 room,,you can imagine the sound level with a 300 watt guitar or bass head powering a 4x12 or 8x10 cab,,,any how,, most amps intial bias setup will change ,,as 2 miunutes on a test bench isn't long enough for the tubes to reach break-in ,, thats is what my job was ,, to give the amp the final last inspection,,for apperance and sound check ,,, and check the bias,,,usually I had to turn the bias down a little ,, ,, because the longer new tubes run ,, the hotter the bias gets ,,,mind you in the manufactoring environment,, I still might only spend another 2 minutes playing with the amp,,, speed is of the essence,,,,,,so I usually sent them out with the bias set slightly on the cold side,,, to allow for aging,,over time the bias will change aswell,,
Just so you know ,,,Any amp designed or modded by Lee Jackson will eat power tubes and spit them out for lunch if the bias it set to high(hot),,as with any amp,,setting the bias Hot is great for sound,, if money isn't a issue,,,,,
Long story short,,the bias controls on the back of the amp are only used when there is no input from the front of the amp or all controls are set to zero(without a scope and a signal generator),,,,normaly you should be able to get both sides to show green when the amp is on,, with a load(speaker cab) plugged in,, with the amp in the on postion,,, they should turn red when you plug in the guitar and play sending input to to the power amp section,,, as you would when normaly playing through the amp,, in either channel,,,
Using the method you suggested, I've managed to set the bias so that one light is illuminated green (bias correct) and the other is unlit (bias low), slightly colder than green. For now, that seems to be the best combination I can dial in without eliciting a red light.
While the amp still sounds great to my untrained ears, I'll probably have it checked out by a tech in the near future. At least now I won't be frying tubes in the meantime.
I know of instances where either the board or the bias pots themselves are not grounded fully. Tighten all the board screws and check out the bias pots.
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