Re: epi vj tubes bias etc

Chief_Billy@xxxxxxxxxxx wrote:

On Apr 25, 7:02 pm, Jim <a...@xxxxxxxxxxxxxxxxx> wrote:

Jim wrote:

Chief_Bi...@xxxxxxxxxxx wrote:

On Apr 23, 1:05 pm, Jim <a...@xxxxxxxxxxxxxxxxx> wrote:

RichL wrote:

"Twang" <coleman_patr...@xxxxxxxxxxx> wrote in message
news:1177335985.956652.256490@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

so..
I was reading groove tubes blurb about it's ratings system..

but I also googled claims it was bs..

and I'm saying to myself..

do I want the el84s in my vj head to be biased cold med or hot?

it's not just about tube life. it's about tone..and clean headroom..

anyone used different brands.. different bias.. and what's yer
konk!lusion?

probably going with jj/teslas.. tung sol in the pre..
trying out a 12at7 when it gets here..
so consider that as well..

thanks hey.

TWANG

I'm considering installing 12AT7 also to increase clean headroom a
little
bit, also standby and bright switches.

I'm not sure how much headroom you'll be able to squeeze out of that
amp. You're really limited by the single EL84. Cooling down the bias
may help, which means changing bias resistor. Some guys put a power
resistor in series with the primary, but that lowers headroom.

To the contrary...

The only reason you have more headroom now is that it was biased too hot
to begin with, and you changed the bias to a better point.

The VJ has a 115VAC input transformer, yet MOST USA wall outlets are
117-122VAC (day and time dependent). This results in ALL voltages in
the amp being greater than they should be..the amp is over-biased,
heater voltage is 6.7 - 6.9. , and headroom is not optimized .
Reducing the MAINS input, using a 20-ohm/20-25W resistor in the MAINS
Line input (Brown wire) will reduce heater voltage (mine went to 6.25
Vdc) and all DC supply voltages (my EL84 plate voltage dropped from
322 to 305Vdc).
Bias cathode current with stock 220-ohm cathode resistor (R14) dropped
to 42mA with 9.2 Vdc across R14. Allowing for 4 mA screen-grid
current, the EL-84 is now biased at (305-9) x .038 = 11.25W whereas
stock it was at 13+ watts..way to hot and beyond dissipation specs.
Headroom is much better now with the tube properly biased with lower
DC voltages throughout due to a simple insertion of a resistor in
series with the primary.

Lowering plate voltage alone WILL lower the headroom, UNLESS you change
bias.

And yes, I know that the resistor stayed the same. But bias changed
because voltage changed. Some guys change the cathode resistor to a 270
ohm. I plan to experiment with both values.






The higher the plate voltage, the higher the potential output power.

In other words, you could bias the amp for MORE headroom at 322V on the
plates than you now have with your new 305V.- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -


I know there are several ways to 'skin this cat'..I prefer to stay
within the design center limits of the specs for the EL84, i.e., Vp <=
300 Vdc, and the Vk and Ip came in right on target simply by reducing
MAINS input..no other components needed changing.

I plan to insert the 20 ohm 20W resistor in the primary. I'm using two prized Mullards in this amp. Why take chances with 7V on the heaters and high plate voltage and bias on the power tube?



With the input resistor network changed (22K/1M properly configured),

I think I went with the standard 68K/1M

there is plenty of signal drive though the pre-amp to mitigate the
need for those R6/R7 mods,

Well, I put in a Fender tone stack! I'm tweaking the padding so I get to the point where the bright cap does the right thing at the right levels (warms up just as the power tube starts distorting).


particularly with humbuckers. Changing R6
to 470K would 'equalize' the divider network formed by the two,
taking into consideration the effect of the volume control in parallel
with R7, but the amp will break up at high-noon on the dial.

....and that's a bit early for my tastes. I don't especially like the tone of this amp when dimed. I like in with a more subtle distortion. I've already got a Marshall and a Mesa. I want more of Fender cleans with an EL84 distortion (even though I also have a Super Reverb).
.

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