Re: epi vj tubes bias etc

Jim wrote:
Chief_Billy@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.

.

Relevant Pages

  • Re: Actual amp modding question (Electar Tube 30)
    ... given that the bias is ... R5, the cathode resistor of the first stage, is way too low. ... available on the negative side (the tube is conducting more than it ... so the voltage on the plate will be lower). ...
    (alt.guitar.amps)
  • Re: epi vj tubes bias etc
    ... which means changing bias resistor. ... resistor in series with the primary, ... 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. ... Line input will reduce heater voltage (mine went to 6.25 ...
    (alt.guitar.amps)
  • Re: epi vj tubes bias etc
    ... which means changing bias resistor. ... resistor in series with the primary, ... Line input will reduce heater voltage (mine went to 6.25 ... Why take chances with 7V on the heaters and high plate voltage and bias on the power tube? ...
    (alt.guitar.amps)
  • Re: photodiode dark current at zero bias
    ... photodiode is connected to input of a CMOS op-amp and the bias voltage is less than +-0.3mV. ... Disconnecting the photodiode showed the circuit stray leakage current to be less than 5pA, ... I assume you're using the usual op amp TIA, with the + input and the PD anode grounded, the - input connected to the PD cathode, and a big feedback resistor connected between the - input and output of the op amp. ...
    (sci.electronics.design)
  • Re: photodiode dark current at zero bias
    ... The photodiode is connected to input of a CMOS op-amp and the bias voltage is less than +-0.3mV. ... I assume you're using the usual op amp TIA, with the + input and the PD anode grounded, the - input connected to the PD cathode, and a big feedback resistor connected between the - input and output of the op amp. ... After attaching a CRO to the output of the op-amp I noticed an amazingly high frequency 20mVpk-pk signal, ...
    (sci.electronics.design)