Revalve Advice!

timmysoft

im going to do a revalve in my Genz El Diablo 100 this weekend so I thought I'd just get a bit of advice before doing it! 

Im a pretty competent person, I worked for 8 years doing subsea oil and gas survey cable terminations so I'm not shy with my multimeter and the amp had external bias point and trimmer.

im looking at putting 6L6gc valves in, Genz recommend a figure of 0.450 volts but I tend to prefer my amps a bit colder, is there any reason why I couldn't drop that down to say .400v? 

Another quick question, which run hotter under usual operating conditions? 6L6 or EL34 ?

jpfamps

Biasing your amp a bit cooler as you suggest will be fine.

6L6s have a higher anode dissipation rating, 30W vs 25W, cf EL34s, so you could bias 6L6s a bit hotter.

In practice El34s and 6L6s are biased pretty much the same.

timmysoft

Cheers @jpfamps ;

would lowering the voltage have a significant effect on the temperature?

are there any cons to biasing colder? I'm a preamp gain guy! 

jpfamps

timmysoft said:

Cheers @jpfamps ;

would lowering the voltage have a significant effect on the temperature?

are there any cons to biasing colder? I'm a preamp gain guy! 

The voltage is an indirect measurement of current through the valves.

Reducing the current at idle will reduce the power dissipation in the valve, so they will run cooler and hence should last longer.

If you bias too cold, then in a push pull amp then you get a situation where for part of the signal neither valve is conducting, which does not sound good. This phenomenon is referred to cross over distortion.

ecc83

I s that 0.4V across a one Ohm sense resistor for all the valves?

Dave.

timmysoft

ecc83 said:

I s that 0.4V across a one Ohm sense resistor for all the valves?

Dave.

I don't know what this means! It just gives me a guide figure of .450v


thanks again @jpfamps ;

crunchman

A lot of amp manufacturers factory settings are quite conservative.  You might find the 0.45V is already not all that hot.

Do it and then have a listen.  There may not be a noticeable difference between the two settings.  If there is, then use your ears.

ICBM

Whether ".450V" is hot or not depends what the resistor value is and how many valves it applies to.

For example if it's 10 ohms and applies to one valve then the idle current is 45mA - which is a bit too hot for a 6L6 at conventional plate voltage in my opinion, although it's pretty much dead on 70% of rated dissipation.

If it applies to all four valves then it will likely *not* be 2.5 ohms - which is a non-standard resistor value - so the actual current may be lower, if for example it's 2.7 ohms.

In any case, you can lower it safely as you don't go into crossover distortion - how to tell is simply to set the amp up for a full clean sound and play gently - if you hear odd distortion, increase the current setting until it goes away.

DJH83004

Can you  just confirm, is there just one bias test point on the amp or two (1 per pair)? the reasoning being I suspect 0.45v across a 10 ohm cathode sense resistor equates to 45mA / 2 which would apply to a pair of valves. In which case depending upon the anode volts they are running quite cold at the factory default setting anyway. Unfortunately had a quick trawl and there are no schematics out there.   

timmysoft

DJH83004 said:

Can you  just confirm, is there just one bias test point on the amp or two (1 per pair)? the reasoning being I suspect 0.45v across a 1ohm cathode sense resistor equates to 45mA / 2 which would apply to a pair of valves. In which case depending upon the anode volts they are running quite cold at the factory default setting anyway. Unfortunately had a quick trawl and there are no schematics out there.   

Just the one set of test points I think. The amp does run quite hot as stock, well it feels hot! 

DJH83004

When you say a 'set' of test points, is that one for each pair of valves or one test point for the whole amp. BTW even when we say they are running quite cold, the valves themselves are still too hot to touch 

timmysoft

DJH83004 said:

When you say a 'set' of test points, is that one for each pair of valves or one test point for the whole amp. BTW even when we say they are running quite cold, the valves themselves are still too hot to touch 

Yeah it's just a black and red terminal and a trimmer for the whole quad.

ecc83

I have found a schematic for a Diablo 60 with 6L6 and it has  a 10 Ohm in each cathode and a 10k 'build out' resistor to the test points. I cannot find a bias value.  And! This thing is weird! It uses a TL072 as the OP valve driver via a centre tapped transformer.  Why? That must cost way more than an ECC8x? Maybe they found 10,000 traffs for bugger all?

Dave.

ICBM

Ah, so if it has a 10-ohm resistor for each valve and averages the result, then .45V does equal 45mA per valve.

Treat that as an upper limit and bias anywhere between .35 and .45V and it should be fine.

timmysoft

ICBM said:

Ah, so if it has a 10-ohm resistor for each valve and averages the result, then .45V does equal 45mA per valve.

Treat that as an upper limit and bias anywhere between .35 and .45V and it should be fine.

i'm going to try in the middle at about .37/38v

DJH83004

I guess if the anode volts are typically around 460v and taking off the screen grid current, when measuring off the cathode, 35 - 38mA would seem sensible, personally I wouldn't push it to 45mA, there just doesn't seem a lot of point as why risk pushing it to the limit and a valve arcing over mid gig = your worst nightmare