6L6->EL34 conversion, biasing question

[phyrexia]

Hi. I have a set of nice NOS EL34s and I want to try them out with the Mark III. A guy from Eurotubes told me that if I mod the amp for adjustable bias, then I can use them right out of the box with just a bias adjustment.

Then I found this http://www.schematicheaven.com/mods/6l6toel34mod.htm after looking around on the internet. It seems to contradict what Eurotubes told me.

Also I found www.nj7p.org which is a really cool site with tube specs.

Does anybody have any information that could be useful to me? I already tried the tubes, and nothing has blown up. I don't want to hurt the tubes or my amp though.

Thanks
Vic

 

[212Mavguy]

If you can gain access to some tube data sheets that display pinout for 6l6 and el34 you will notice that there is a slight difference. If you can navigate to www.tubeworld.com there are links to a huge collection of tube data sheets found on the home page there. Tubeworld's site was closed down for site maintenance but by now they should be back up and running. Sounds like you have already seen the pinouts for those tube types, but wanted to make sure you had access to a very cool site anyway...

Not knowing off the top of my head what I did, I do remember making that pinout change to an amp of mine, when I was finished, I could run 6l6 or el34 interchangeably. It was a single ended cathode biased amp, a heavily modded Fender silverface Champ, and after I installed a fixed bias pot, fixed/cathode bias switch, and bias test jack from Area 51 audio I set the fixed bias pot for el34, then when I wanted to run 6l6. kt66, 6f6, 6v6, 5881 I used the cathode bias setting, works perfectly that way. I run 6550, el34, and 807 with an adaptor socket using the same fixed bias setting for those three types. Close enough for rock and roll, as they say.

Some amps already come with the power tube sockets wired to be interchangeable 6l6/el34. Consider the services of a well experienced tech to get the work done for the mod to yours, and also consider what might happen should you ever wish to send the amp in to Mesa for repair after the mod was made...

I love the el34 sound and think that you will find that the vintage old stock el34 to sound like it was worth the trouble to make sure you could run them...

 

[stokes]

You need to connect pin 1 to pin 8 to make the socket usable for both tubes.To do the adjustable bias mod you need to replace one of the resistors in the bias circuit with a 100k pot.I like the multi turn cermet type.

 

[phyrexia]

Define usable? They work as is, am I hurting the EL34s? Thanks for your reply.

 

[stokes]

If you look at the pinouts for both tubes you will see that one of the grids in the 6L6 is internally connected to the cathode.The EL34 is not so you should connect it externally,by connecting pins 1 & 8.Make sure there is nothing else connected to pin 1 first.Sometimes amp builders will use an unused pin for some other component.