el34`s

[washburn 1]

Hi guys
Can i ask in what way weak or failing el34`s will do in terms of power output and tonal quality? Also` when biasing an amp i presume the readings/adjustments should be done with the amp ticking over (quiet) rather than being played? Any thoughts would be most welcome.
Thanks. Kev.

 

[mickeydg5]

Hi and welcome

Weak power tubes may have less power than normal, loose bottom and high end or become less articulate than once was or as compared to others.

Biasing is done with amplifier ON and no signal; nothing plugged in and all drive/gain/volume knobs at zero.

Caution: high voltages present inside tube amplifiers.

 

[washburn 1]

Thanks mate,all as I expected! Time to splash the cash.bias previously done correctly. But can a dodgy tube loose its bias setting? I guess so?. . K

 

[MartyStrat54]

Any power tube can drift and affect the bias, but an old tube is more prone to do so.

 

[chee16]

...... and all drive/gain/volume knobs at zero.

I didn't know this. Interesting.....

Also, to OP, what amp are you using?

 

[washburn 1]

If I told you its an MA50h, through 2x1936 cabs you'll probably laugh. 1 is a vintage. Has sounded ok til now. Also distortion /gain sounded a bit ragged. Not normally like it. Must start exploring. . Cheers all. All comments welcome. kev.

 

[MartyStrat54]

I didn't know this. Interesting....

Yep. Better recheck your bias.

 

[mickeydg5]

You know I was thinking about weak sounding tubes and biasing asked within the same post but I did not write anything to that effect.

A good bias will sound good.
A bad bias will do one of two things;
1. roar, run hot and maybe burn something up (under biased)
2. run cold, sound weak and thin (over biased)

and like Marty pointed out, bias can drift so it is good to check once in a while.

I also notice the MA's getting mixed reviews, but maybe the bias is set cold or maybe a tube upgrade could help depending on what is in there.

 

[washburn 1]

Thanks for all your comments guys. So if you set the bias reading low that means its over biased yes? Currently using 1 svet and 1 tad which have worked well together until now but suspect one or both are getting past it. Have recently biased them at 30ma and liked the result as it seemed to tame the slight mid/treble harshness but they're both getting on a bit. Will have to shell out for some new ones. Any thoughts on brand? silly question, can the amp work with one or is the circuit dependent on both to work? What would actually happen if one was removed? Could it damage anything? Just thought I could try each valve independently.

 

[mickeydg5]

No do not try to operate with only one power tube. The amplifier is a push-pull design requiring equal tube(s) per side.

30mA may be good but could be low depending on the power tubes plate voltage. It is best to have a plate voltage reading, calculate a bias and then set rechecking as you finalize the bias setting.

Again as warning, plate voltages are very high. If in doubt bring it to a technician.

The TAD and Svet can work just fine together as long as they are running similar, mA of both within reason. A good matched set can be biased within about 1%. If matching is too far out it may cause hum/buzz/noise.

 

[washburn 1]

Cool. . Just for interest,the ma has bias adj for each valve (50 watt version only) so odd valves can be biased to match. Does biasing depend on what make the valves are ,or do all el34's bias to the same reading?

 

[mickeydg5]

Most EL34s will bias within a range but can be different from make to make or between valve to valve from the same make.

That is right the MA amplifiers have two bias adjust, one for each side of the push-pull.

The only way to see what is going on with tube matching is to set the control grid of all tubes at the same negative voltage and then read mA to see where they are falling. Other than that it would take a tech with a good tube tester and he would still do the amplifier control grid voltage versus tube current comparison.

Doing that means fooling inside the amplifier where there are high voltages and you do not want to short anything.

 

[washburn 1]

so, can an amp actually be biased effectively just using the bias pots with test leads on the test pins or should other measurements be taken also,.. as i dont really want to delve in too deeply without knowing exactly what i`m doing. dont want to spark out just yet! Kev.

 

[mickeydg5]

They were all previously mentioned.
1. plate voltage
2. control grid voltage
3. cathode current measured in mV

It is best to know and use all of these when biasing.