6V6GT biasing questions

[waspclothes]

Hey guys,
I have a Deluxe Reverb (silverface) and I'm trying to bias it for a new set of power tubes (EH 6V6GT - "6V6EH").

It seems like every source I read says something different (and conflicting) about what exactly to set the bias at.

I'm using a Weber bias rite head, going to two multimeters.. one showing the plate voltage and the other showing the plate current.

The first thing I read was to set the bias based on the idle dissipation. The idle dissipation being a percentage of the max dissipation. This seems to get confusing fast, since some sources say 6V6GT's are 12W and some say they are 14W max dissipation. The other problem is that nobody can agree on what percentage of max dissipation should be the idle dissipation. I've read 50%, 50-60%, 60-75%, etc.

Then I crack open one of Gerald Weber's books (A desktop reference of hip vintage guitar amps) and this is what he says:

Biasing is critical in a Deluxe Reverb, because if the tubes are biased cold, the plate voltage will increase beyond the limits of the 6V6. 30 mA per tube is what I recommend. After biasing, I check the plate voltage and, if it is over 425 volts, I bias the output tubes for more current, until the plate voltage is no more than 425V.

Going by Weber's advice, I get all the way to 36 mA per tube to get the plate voltage down to 425V. But wouldn't that be an idle power dissipation of over 15W? (greater than 100% of the 14W max dissipation).


The second question I have is, how does setting the bias cold vs. hot affect headroom? If I'm looking for more clean headroom, what side of the scale should I be shooting for when I'm biasing this?

[pdf64]

Search on http://www.mif.pg.gda.pl/homepages/frank/vs.html for 6V6GT and check equivalent types, the 6V6GT has a 12 watt plate, the 6V6GTA has a 14 watt plate.
For clean headroom, aim towards 50-60% static dissipation but don't get too obsessed about it.
It's best to use GW for entertainment purposes, not as a technical reference.
For that, there's plenty at http://www.aikenamps.com/index.php/white-papers

Near in mind that when GW wrote that, very tough USA production, eg GE 6V6GTA, were still easily available and reasonably priced.
Current production tubes may not last long with such treatment, though the JJ may be an exception.
Pete

[Pedro Vecino]

Then I crack open one of Gerald Weber's books (A desktop reference of hip vintage guitar amps) and this is what he says:
"Biasing is critical in a Deluxe Reverb, because if the tubes are biased cold, the plate voltage will increase beyond the limits of the 6V6. 30 mA per tube is what I recommend. After biasing, I check the plate voltage and, if it is over 425 volts, I bias the output tubes for more current, until the plate voltage is no more than 425V."

That makes no sense.
Voltages in these amplifiers have a very wide margin due to various factors. Decrease the voltage at the expense of a strong bias setting is a bad idea. This is not his function.
Voltage itself (within a range) is not the real problem for power tubes. Itīs the voltage/current relationship.

My opinion is that in a Deluxe Reverb 10 wats dissipation is a good adjust (83% understood as 12 watts over modern 6V6 models) if the voltage is between 400 and 425.
With 425, 24mA measured across the plates is OK for a full round tone. If the voltage is 440, for example, 22 mA. is good.
From here, if you consider as good sound the obtained result decreasing 2, 4, 6... mA., better for the tubes. In my opinion Deluxe reverb is critical in this respect.
In fact itīs probably the Fender amp with a adjust more careful and precise from factory among all (I mean the reissue model).

[waspclothes]

Search on electron Tube Data sheets - Search for 6V6GT and check equivalent types, the 6V6GT has a 12 watt plate, the 6V6GTA has a 14 watt plate.
For clean headroom, aim towards 50-60% static dissipation but don't get too obsessed about it.
It's best to use GW for entertainment purposes, not as a technical reference.
For that, there's plenty at White Papers

Near in mind that when GW wrote that, very tough USA production, eg GE 6V6GTA, were still easily available and reasonably priced.
Current production tubes may not last long with such treatment, though the JJ may be an exception.
Pete

Thanks for the link Pete, it's nice to see the datasheets for the tubes themselves. I've biased the amp in the 50-60% region as you suggested and I think it makes sense to do it as a % like you talk about instead of GW's advice which seems a little simplistic.