Switch setting for 6L6 in VH4 and bias question.

[Ailean]

Hi all

My lovely new VH4 has unfortunately blown a tube (or maybe 2) That is being sorted.

In the mean time I'd like to try a set of NOS Phillips 7581A (6L6GC/ STR387) that I have. These are of the 6L6 type but run at 35w each rather than the usual 25w, so 140w total for the quad

Ok, so I need to know what setting to use on the internal switch in the VH4, the one that says EL34/6550 on mine. Now I believe it used to say EL34/6L6 before the use of 6550's so as I'm assuming that I need to leave that switch in the current 6550 position?

I'm a little nervous as these tubes cost £180 per quad and I don't want to blow them up!

Also can anyone explain how to calculate the cathode current for biasing? As in what it should be... becuase my calculations don't add up therefore I'm doing it wrong!

My calculations were:

35w per tube
500v plate voltage

35w/500v = 0.07A or 70mA

bias to about 90% power and you get 63mA each.

But the 6550's are only supposed to be bias to 40mA each, so I'm doing it wrong, what have I done wrong and why?

Edit: Also, it the VH4 class A or AB? I'm assuming A?

Cheers!

 

[studiojig]

Curious.... how did the tube blow? were they really old? I mean, I know tubes blow but just wondering

 

[Ailean]

Just a faulty tube I suspect. It's a new head, 2009 build, had it just over a month. It would appear that power tube manufacturers have decided their profit margins look better if we keep having to buy new ones!

 

[bronz79]

My vh4 is from 2008, the switch is between el34 and 6l6 you must use the 6550 position i think.
Vh4 is class A/B.

Out of curiosity..the red tube fault in the rear are lighting on?

 

[Ailean]

yeah, fuses 1 and 2 blew. I replaced and fuse 1 blew again quickly, and tube 2 was glowing bright blue which can't be good

 

[studiojig]

Just a faulty tube I suspect. It's a new head, 2009 build, had it just over a month. It would appear that power tube manufacturers have decided their profit margins look better if we keep having to buy new ones!

So, then this head came with SED 6550's as recently recommended by Peter? If so, it would seem even those have QC issues. Anyway, sorry to hear that and hope you get things running properly soon.

 

[Ailean]

If I were Peter this would be driving me crazy. It's not Diezels fault, these tubes come tested from the factory, for all I know I got 1 dud in 1000. It's being sorted, and it gives me the excuse to try out the NOS Phillips

However form now one I'll keep on spare set of power tubes around.

The Phillips are in now, I've biased to 30mA to be safe which is down on what they can handle anyway, and they are within 2-3 mA of each other. Lovely

 

[Hey_bert_whtcha_doin_bert]

It's class A/B. ALSO DON'T BIAS TO 90%!!!!!!!!!!!!!! THEY WILL BLOW.

The 35w/plate voltage is maximum do not exceed obviously. Then you want to multiply that by .7, which in your equation is 49mA. Diezel's aren't 500v but that still isn't far off and the mA difference isn't going to matter much.

Should be 35w/485v=.0721.... x .7 = (50.5mA) 50mA or 70% plate dissipation.

so from 60% to 70% plate dissipation about 40mA(43mA is 60%) to 50 mA

ALSO IF YOU USE THE TRANSFORMER SHUNT METHOD MAKE SURE YOU DMM IS LOW INTERNAL RESISTANCE OR THE READING WILL BE WRONG!!!!!!!!! YOU WILL BLOW TUBES/FUSES! Get a very good DMM or use a bias probe.

 

[steve_k]

Don't confuse a tube's current draw with output. 6L6's @ 35 watts, don't make it a 140 watt power section.

Flip the switch to EL34. This drops the output voltage of the transformer to enable you to be able bias tubes that draw big spreads in current (ie. 34's vs. 6550's). Without it, you would be messing around with resistors. On the 34 side, the V will be about 472V. For 6L6GC's bias them around 40. Biasing higher won't do anything audible but will shorten the life of the tubes.

Invest in a Weber Bias Rite or similar. These will measure bias per tube. If you do it the at the transformer, it will be per pair, although there is only one bias adjustment for the quad.


Steve

 

[Ailean]

It's class A/B. ALSO DON'T BIAS TO 90%!!!!!!!!!!!!!! THEY WILL BLOW.

The 35w/plate voltage is maximum do not exceed obviously. Then you want to multiply that by .7, which in your equation is 49mA. Diezel's aren't 500v but that still isn't far off and the mA difference isn't going to matter much.

Should be 35w/485v=.0721.... x .7 = (50.5mA) 50mA or 70% plate dissipation.

so from 60% to 70% plate dissipation about 40mA(43mA is 60%) to 50 mA

ALSO IF YOU USE THE TRANSFORMER SHUNT METHOD MAKE SURE YOU DMM IS LOW INTERNAL RESISTANCE OR THE READING WILL BE WRONG!!!!!!!!! YOU WILL BLOW TUBES/FUSES! Get a very good DMM or use a bias probe.

Cheers for that. I'm using a Weber Baisrite, and it's measuring the voltage at 500v, so that's what I'm basing my calculations on, and I realise that 35w is the max, and to bias to 90% for class A and 70% for class A/B.

I've turned the bias right down to 30mA just so I could get it up and running, I'll tweak the bias up a bit tomorrow maybe.

35w/500v * .7 = 49mA

I guess I'll bias it to around 45mA.

Don't confuse a tube's current draw with output. 6L6's @ 35 watts, don't make it a 140 watt power section.

Invest in a Weber Bias Rite or similar. These will measure bias per tube. If you do it the at the transformer, it will be per pair, although there is only one bias adjustment for the quad.

Steve

According to the site I bought the tubes from the plate disipation is 35w, which I thought was the output? Here is a link to the blerb:

http://www.watfordvalves.com/product_detail.asp?id=1317

I'm using a biasrite, brilliant bit of kit!

 

[Duolos]

This is why we have techs

 

[steve_k]

This is why we have techs

Try looking for one in the Middle East!

You guys are all a PM away at least . The next best thing to owning a Diezel amp in all its German glory is the customer service over here. Don't see this on any of the other manufacturers websites.

Now, if I could just get Peter S to kick me out a new cab, I would be good to go.

Steve

 

[Ailean]

This is why we have techs

Totally. BUT sending it to a tech doesn't teach you anything except the cost of shipping and how much you miss your amp. Doing it myself is a rewarding learning experience. Plus I'm a geek by nature and trade