EL84 BIAS - how much ??

[Buhl]

Hello Forum
I have a Beard Integrated tubeamp, using 6 EL84's pr. side.
About 6 months ago, i had the output tubes replaced by new ones, and the guy who did it turned the bias down, I remember.
But, the amp dosent get all that hot anymore - in fact, I can touch the tubes during operation now - witch was a guaranteed burned finger before
I also think that the amp dosent sound as good as before.

I want to have my old, good sounding heater back !

I read 2,35 mA between the metal case, and the bias input on the PCB - right after the trimpotmeter that reads Bias.
How much can I push theese tubes - any suggestions would be most welcome.

Thnx in advance.
Hans Houmøller - Denmark

[fdegrove]

Hi Buhl,

This one is a rather tricky one.
So far I haven't seen any technician solving this problem.
Whatever anybody did to your amplifier:
As it is now, it is working the tubes pretty close to cut off (I don't have any exact figures here.)
In other words it is most likely to work fully in the low ranges of class AB1.(i.e. more B than A)
As you state,you can hear the difference.
As far as I can tell:
the problem with this particular design is that you need to match the output tubes and ,to boot, you better match them in the amplifier.
My observation is :the technician probably has tried to bias these tubes but was unsuccesfull.
He always saw one tube or the other glowing cherry red.
He therefore resorted to cutting them off nicely which you now hear as clipping,the tubes are getting overloaded by the previous stage.

Other then the above I have no cure really,

Best rgds,

[Buhl]

Thnx for the reply fdegrove !

As a matter of fact, the technician, a local hififreak, swapped a lot of tubes in and out of the socekts with the old bias setting, before turning the bias down - he ment that the tubes was beeing runned to close to the limits, and would die again quickley.

Despite the high bias setting, the old tubes ran for about 1½ year, and the amp was used when I got it - I can easily live with 1½-2 years of operating time pr. set of tubes.

So, the tubes are matched to sustain higher bias.
If I would try to up the bias a little, how smalle stemps should I proceed in ??

Is measuring mA between bias and gound the right way to measure the bias ??

again, thnx alot !

Hans Houmøller

[Buhl]

Hi Again
I just measured the bias on each of the 6 output tupes pr. side - and get 0.128 mA - 0.129 mA on each - I think to recal that the old setting was near 0.18 - could this be correct ?
Would it be to much you think if I turned the bias up again to 0.18mA / tube ??

Thnx

Buhl

[fdegrove]

Hans,

Despite all the efforts,I don't have a cut and dried answer here.

Admittedly:if you do not mind the expense of changing the tubes every 18 months,the fact remains that is likely to give you trouble in the long run.
If memory serves me correctly the bias is shared between all the tubes and all I've seen so far is it getting into thermal instability.
(runaway bias).
This,to my mind is a design flaw and the only simple cure I've seen so far is exactly what this techician did.
You can try to up the bias a little bit (say 5 mA at the time) but I can already predict trouble.
A design such as this one should come with individual tube bias and there's just no way I can predict it's behaviour.
It is however very likely to drift off into instability.

My solution at the time (although seemingly expensive) was to replace all tubes with E84L/7320's from Siemens (or other tightly matched tubes).
This presented a not unconsiderable bill to the customer and I guess most of the owners sold their amp later on.
Passing the hot potato as they say.

I'm sorry for you, but I don't have absolute answers for you here and short of redesigng the amplifier this is my best shot at it.
Luckily for you the output tubes used aren't frightfully expensive.
I hate to think what to do with 8417's here....

Rgds,

[HDTVman]

Hi Buhl,

I would like to get a look at the output configuration of this amp. It may be possible with a simple mod. to give each output tube it's own bias adjustment.

What class is the output suposed to be (A, AB, B)?

Later
Bruce

[fdegrove]

Buhl,

Think our replies overlapped.Sorry about that.
Can you give the anode voltage on these output tubes?

Thks,

[fdegrove]

HDTVman,

It was sold as a class A1 amp.
Real life showed it as mostly Class AB1 though.
Had the customer fiddled around?
I don't know,the customer is always right and so is the manufacturer...
Does that help you?

Rgds,

[Buhl]

Thnx Frank
I can se the problem with the shared bias and the "poorly" matched tubes - the 12 is matched roughley out of about 20.
I will try to up the bias to about 0.15 later this evening I thnik - 12 new tubes are only 50 euro, so I think its worth the try.

Thank you SO much for the qualified and fast response !!!


Hans

[fdegrove]

Buhl,

Before you do anything,can you give me say half an hour to look up the data for that tube?
I got the sneaking suspicion either one of us didn't put the comma in the right place.
Excuse pls.
I'll report back unless Bruce wants to do the hard work here?


Cheers,