What do these numbers on my new tubes mean? How do I bias them?


Hello folks, me again with another question...

Today I got my new tubes from Watford Valves (highly recommended place btw) for my Herbert. I got some EL34s and some 6550Cs. (I'm gonna try running 4xEL34s with a pair of 6550s)

Obviously the website says:
60-70mA for a pair of EL34s
80mA for 6550, KT66, KT88.

Easy! But...

Watford Valves tubes come with 2 numbers on them e.g. my EL34s say
34/5.4 on them (always the same values for a matched pair/quad).

As far as I know I think the 5.4 is the required grid voltage. I assume the 34 is then the Plate current (in mA)? If it is, then for this particular pair of EL34s the current would be 34x2 = 68mA. This fits with Peter saying 60-70mA for a pair.

However, my 6550Cs say 66/6 on them. This would mean for a pair the plate current would be 66x2 = 132mA! Peter says 80mA though!?! :?

Can anyone shed any light on these numbers? (I know a few of you get valves from Watford Valves here)

a) Do they actually mean something completely different and I'm barking up the wrong tree entirely.
b) should it actually be biased to 132mA for this particular pair.
c) is what I said actually right but for some reason they shouldn't be biased to the same % current or something I don't get. Or using them at full power melt my Herbert's output tranny or something!?!

I guess I could ask Watford but they're probably closed for the weekend now, and I kinda would like a more Diezel specific answer if it's possible!

Thanks for any help. :)
Any ideas on this yet???

BTW: my Herbie is back with a full 6 tubes again (all EL34s atm) and all fuses not popped! Sounds fat in my room! Time will tell if it blows another pair when the volume gets up to 11:30-12:00! It's unfair to expect the people in the flats next door and upstairs to let me try the Herbie at 12o'clock on the volume knob! After blowing a pair of Ruby EL34s a week after getting it and the first time I cranked it up, then blowing the replacement pair I was sent the day after I installed them (again the first time I cranked it up after retubing) has put me off Ruby tubes. :x I'm sticking with Watford Valves fully drive tested ones from now. (...unless there is actually something wrong with my Herbie and it blows these too!!!) I'm sure stuff that shatters fuses can't do my Herbie any good no matter how well it's made. :?
What's going on here? I just took 2 EL34s out again and put a pair of these Svetlana 6550Cs in and it wont bais anymore!?! The svetlanas suddenly have a bias current of about 300mA. I can turn it down, but then the EL34s only have a bias current of about 2mA, which I can only make go as far as about 20mA!

What's going on? It should be able to do 4xEL34s and a pair of 6550Cs shouldn't it? Is it the C? Is C my enemy? I thought they were the same as 6550A or KT88s in general! I hope so as they cost me enough! Help!!!!!! To be specific they're Svetlana SV6550C or Svetlana / Winged-C / 6550 C.

Back to the EL34s then...
The numbers on the tubes has nothing to do
with the bias.
It´s a measurement done by the distributor
with a fixed plate and grid voltage.

If You use the same tube and number,
You don´t have to rebias.
Ah, ok. Cheers for clearing that up..

Any idea why I can bias 6x EL34s, but not 2x 6550Cs with 4x EL34s? does that not work?


I´m a little bit confused. :?

The suggested bias in the Herbert Manual for the el34´s is 30-35mA.
On the Diezel HP it´s suggested 60-70mA.


What is right?

In the manual is a 6550 at 40-60mA.

BTW, which 6550 do you put in the amp´s, if it is ordered with them? Can KT90 Ei be used too? I saw on some tubepages, that they are replacements for the 6550, KT88.

Cheers, have replied to you and CC'ed the shop on it too, just so they can keep track of it for warranty issues, just in case of tube refunds or replacements etc.

So it's definitely broken is it? :cry:

Ah that old chestnut....

Manual states current for an EL34.
Website states current required for a pair of EL34s (i.e. 2x the manual) since they're biased in pairs this is what you measure if you have the newest version Herbert or an Einstein where each 4mm banana-plug jack is for a pair of tubes.

I'm not so sure if you have an old Herbie or VH4 whether you're biasing per tube or pair. You'd have to work that out for yourself.