EL34s and 6L6s working together in perfect harmony

[Ramsay]

The day before yesterday I was testing a pair of 6L6 in a tube amp that works with both four EL34 valves, as with four 6L6 (or either).
To this end, I changed the switch on the back of the amp that changes the bias of EL34 to 6L6, took off the two EL34 and put in place two 6L6. Then flick the switch back off to reduce the power of 100W to 50W and played for some time, only with the 6L6s on, and then suddenly came to my head a "crazy idea "
Why not test in conjunction 2xEL34 + 2x6L6?
I decides to take the test, because as the 6L6 work with a more negative voltage on the control grid that EL34, then these additional negative voltage on the control grid of the EL34 theoretically would not harm them.

So I decided to do a controlled test, only in the clean channel and at 1 / 3 of the volume with a 6L6 and a EL34 in parallel on one side of the push-pull and two other correspondents on the other side.

I called first only with the 6L6s and after some time, flick the switch that connected then with the EL34s and immediately there was an increase in loudness at all four valves were now working.

And the result was that while the bass remained unchanged but maintaining that "high bottom-end" typical of the 6L6s, the mid-range grew up and were in the style "British" characteristic of the EL34s.

Another thing that surprised me was that the clean channel remained clear, without causing any distortion.

So, I dimed the ambient light and played guitar for about 20 minutes, always checking to see if the card was a red valve, which fortunately did not happen.

So, with this configuration, I got the best of both worlds, ie, the strong bass of 6L6s and the british style of EL34s.

Now, technically speaking, I think this only works in some reasonable way in which the amplifier bias voltage of the EL34 and 6L6 is not much different.

Some time ago, I measured the voltage bias of the amplifier tested, with EL34 tubes and then with 6L6. With EL34 the bias voltage marked -46 VDC , whereas with 6L6 marked the bias voltage marked -54 VDC, or a difference of only -8 VDC, which means that the EL34 should be working on a curve that preserves the linearity of the amplified signal, but may reduce the effective power to 25W/15W in each EL34.

If a colleague of the forum want to test something like this, test at your own risk, but always with the bias switch (if any) placed in the position 6L6, or who have a tube amp with 4 four 6L6s only , It can test directly , taking out two 6L6s and putting two EL34 instead.

But who has an amp with four EL34 tubes that don't have a bias switch can't replace two tubes for 6L6, because the tubes will burn in 5 minutes and can potentially burn the output transformer as well.

Below, two photos showing the valves and EL34s 6L6s lit and running.
http://i223.photobucket.com/albums/d...ayDSC00741.jpg
http://i223.photobucket.com/albums/d...ayDSC00739.jpg

Yesterday (07/29) after being in playing guitar amplifier test since 5 o'clock in the afternoon, I decided to test the background.
I thought, if it burn something, it does not matter and at 19:32 and flick the switch that connects the two EL34 to work together with the two 6L6.
And the sound was already good with my pedalboard on this amp with two 6L6 only triggered when I called together the EL34, that's where the sound was extremely good, organic, a sound I had never gotten this amp before!

I then used the amplifier in this state until 21:44 at night, or more than 2 hours of testing and the amp worked perfectly.

I just still can not believe this thing worked so well.

Forgive me for my english.

[MWJB]

"I decides to take the test, because as the 6L6 work with a more negative voltage on the control grid that EL34, then these additional negative voltage on the control grid of the EL34 theoretically would not harm them."

Well some EL34 bias up the same as some 6L6. 2 different brands of 6L6 can bias up vastly differently. The additional negative voltage on the grids of the EL34, should you have any, mean the EL34 should be biased cooler than they are expecting and will last longer. But confirm the plate currents by measuring plate current in mA and try to keep the current draw on either side of the OT primary resonably matched.

"I just still can not believe this thing worked so well." If you prefer the sound of the mix, keep doing it. I would have been more suprised if it hadn't "worked".

[Enzo]

In fact, Bob at Eurotubes has been selling integrated sets of EL34 and 6L6, specially selected for their biasing needs to do exactly that - work together in the same amp at the same bias voltage. He has sold these sets for years and years.

[Ramsay]

In fact, Bob at Eurotubes has been selling integrated sets of EL34 and 6L6, specially selected for their biasing needs to do exactly that - work together in the same amp at the same bias voltage. He has sold these sets for years and years.

Enzo

My tube amp is a Carvin Legacy VL100 and it has a switch for 6L6s or EL34s.
The current bias voltage is switched to 6L6s and for this reason, the EL34s are underbiased.
So, instead of 50W the EL34s provides 20/25W decreasing the effective power of the amp to 70/75W.
But, the sound achieved is so so good, that I will keep it!!

[defaced]

Do some searching on dual bias supplies. You can gain the ability to properly bias any common set of power tubes with any other common set of power tubes provided their operating conditions are similar enough (load impedance, plate/screen voltages are the biggest). Enzo and PaulP (?) Worked up a schem a couple of years ago that works well.

[tedmich]

Bob also told me I could swap out EL34s and 6L6s in my Ultra without any of that silly variable bias supply or switching found in the JSX...haven't tried it cause I imagine I'd need to buy some more power tubes soon... win/win for Bob?

[Ramsay]

Last saturday, after playing guitar for 3+1 hours with this configuration my amp fails, not fully, but with a strange noise and with a extreme overheating (by luck I don't burn my hand at the back plate of the Head).

And this thing happened about 1 hour after I changed the serial resistor of 60 ohms to 90 ohms in my power attenuator.
I think that one or maybe the 2 6L6 break up and maybe they are with the plate in red.

I realy don't know why this happened.
A friend of mine said: you are most corageous in testing this configuration.

Would it be due to the power attenuator??
Forgive me for my english, because I am not american.

I appreciate if you can answer!!

[MWJB]

"Last saturday, after playing guitar for 3+1 hours with this configuration my amp fails, not fully, but with a strange noise and with a extreme overheating (by luck I don't burn my hand at the back plate of the Head).

And this thing happened about 1 hour after I changed the serial resistor of 60 ohms to 90 ohms in my power attenuator.
I think that one or maybe the 2 6L6 break up and maybe they are with the plate in red."

Power attenuator? Surely if you are looking for reduced power you wouldn't be using 4 power tubes to start with?

"90ohms in my power attenuator" Do you have a schem? What does thios 90ohm resistor do?

You have made no mention of the bias currents the amp ws running before it failed? As long as the hotter idling tubes were within reasonable perameters you shouldn't have had a problem.

[Ramsay]

"Last saturday, after playing guitar for 3+1 hours with this configuration my amp fails, not fully, but with a strange noise and with a extreme overheating (by luck I don't burn my hand at the back plate of the Head).

And this thing happened about 1 hour after I changed the serial resistor of 60 ohms to 90 ohms in my power attenuator.
I think that one or maybe the 2 6L6 break up and maybe they are with the plate in red."

Power attenuator? Surely if you are looking for reduced power you wouldn't be using 4 power tubes to start with?

"90ohms in my power attenuator" Do you have a schem? What does thios 90ohm resistor do?

You have made no mention of the bias currents the amp ws running before it failed? As long as the hotter idling tubes were within reasonable perameters you shouldn't have had a problem.

I use a Dr.Z Airbrake clone with several jacks P10 instead of a switch.

Yesterday I turn on the amp on and looking at the back I saw the anode of one 6L6 in red and that strange noise like a zumm not usual remains.

So I took off the EL34s and the plate (anode) of that 6L6 returned to normal.
So I thought this problem was solved.

But, today, when I turned the amp on, thirthy seconds after, came a high noise like a Zuuummm and so the fuse of the line filter breaked.

Unfortunately for me this 6L6 GE is the top of line of Grove Tubes and it is made in California.
But, the most important for me is the output transformer. If it is safe, all is good.

Soon I will put back the four EL34 tubes and I will do a rebiasing in the amp!!

So, don't try this at home (mixing EL34 + 6L6) because without the appropriate circuit this thing is like a suicide for the tubes of your amp!!

[MWJB]

"So, don't try this at home (mixing EL34 + 6L6) because without the appropriate circuit this thing is like a suicide for the tubes of your amp!!" Piffle. If you have followed the steps outlined it is as easy as falling off a log...I just wouldn't try it at YOUR home.

The 6L6 tubes are redplating because they are drawinng too much current, you still have not submitted any details on this front. The GE6L6 were not made in the US, they were made overseas using US manufactured parts.

[Ramsay]

But only one of the 6L6's is redplating.
Moreover this amp is underbiased. So I think that 6L6 died and burned the line filter fuse.

[jmaf]

Ramsay, I attempted to help you last year when you were building your attenuator, which if I remember correctly is this: http://i223.photobucket.com/albums/d...adeesquema.jpg

I warned you back then that this attenuator had flaws. Fast forward to 2011.

Your attenuator may have been the cause, but more likely it's a combination of things. You've been driving these mismatched tubes to the extreme, and they have probably worn themselves out with time.

The natural mismatch between the pentodes and tetrodes, plus the impedance mismatch of your attenuator, plus you cranking this thing for 90 minutes may have been enough to drain that tube down this time IMO.

Your best bet now is to just substitute the 6L6's for a new pair. AND unless someone has matched these EL34's and 6L6's to less than 10% quiescent current(DC plate resistance?) I wouldn't be attempting the mixed tube idea. So maybe you oughta get a matched quartet of the same tubes.

[MWJB]

"But confirm the plate currents by measuring plate current in mA and try to keep the current draw on either side of the OT primary resonably matched." I wrote this in the first response to your original post. Now you write..."Moreover this amp is underbiased. So I think that 6L6 died and burned the line filter fuse." Why is the amp underbiased?

JMaf - +/- 10% is a pretty tight margin, 3mA @ 30mA mean? Many vendors won't guarantee matching better than +/- 5mA. There are 6l6 & EL34 that will bias up within these perameters anyhow....even if they don't quite, keep one of each on either side of the primary, match current draw within reason on each side of the primary, take the hottest biased tube as the limiting factor...I can't help feeling I've said all this before..;-)

[Ramsay]

Ramsay, I attempted to help you last year when you were building your attenuator, which if I remember correctly is this: http://i223.photobucket.com/albums/d...adeesquema.jpg

I warned you back then that this attenuator had flaws. Fast forward to 2011.

Your attenuator may have been the cause, but more likely it's a combination of things. You've been driving these mismatched tubes to the extreme, and they have probably worn themselves out with time.

The natural mismatch between the pentodes and tetrodes, plus the impedance mismatch of your attenuator, plus you cranking this thing for 90 minutes may have been enough to drain that tube down this time IMO.

Your best bet now is to just substitute the 6L6's for a new pair. AND unless someone has matched these EL34's and 6L6's to less than 10% quiescent current(DC plate resistance?) I wouldn't be attempting the mixed tube idea. So maybe you oughta get a matched quartet of the same tubes.

jmaf, the attenuator schematic is correct and it works about 2 years without issues.

The 25 ohms resistor in parallel, in the attenuator input works like a dummy load. The output transformer is setted to 16 ohms to feed a 16 ohms Vintage 30 loudspeaker.

Perhaps like you said the combination of the mismatched tubes with the attenuator it caused the issue, but why only after 8 hours of use?

"But confirm the plate currents by measuring plate current in mA and try to keep the current draw on either side of the OT primary resonably matched." I wrote this in the first response to your original post. Now you write..."Moreover this amp is underbiased. So I think that 6L6 died and burned the line filter fuse." Why is the amp underbiased?

MWJB, the amp in fact is not underbiased, but biased with the quiescent current at the standby switch of 100 mA as recommended at the Carvin VL100 schematic.

People says that the ideal is 70% of the plate dissipation which would result in a quiescent current of 185 mA for 4 EL34 tubes.

I thank you both for the answers. Soon at my vacation work I will open the amp to verify the voltages, current, diodes and power supply capacitors. After this I will change the tubes.

[MWJB]

"MWJB, the amp in fact is not underbiased, but biased with the quiescent current at the standby switch of 100 mA as recommended at the Carvin VL100 schematic. People says that the ideal is 70% of the plate dissipation which would result in a quiescent current of 185 mA for 4 EL34 tubes."

Neither of these perameters tells us what each tube is drawing independently of the others. Therefore we do not know the idle current dawn by each tube...the amp is not corrctly biased. 2 tubes could be drawing 5mA the other two 45mA. I wouldn't want to see more than 160mA drawn in total...more likely 130-140mA if all tubes were drawing equal current.

70% of 25 is 17.5W, 17.5W/450v = 39mA...17.5/500v = 35mA...so I don't understand where the figure of 185mA total comes from?

If you have biased the amp as experts at Carvin suggest and your tubes have failed ask Carvin to send you some more ;-)