Hiwatt hi-gain 50 head biasing review

[carpediemgr]

Hi to all HIWATT friends i have done a complete review on HIWATT HI-GAIN tube amp biasing because the advices i receive was poor and i have to spend a whole day measuring by myself to be sure at least in THEORY !!!

When i receive the head i realise that it was not so loud on the tubes stickers just was writtens 35,5 on the 1st tube and 35 on the second nothing more. Placing the Cathode current probes i saw on the meter 24-25 mA on each tube this means from what i have heard that the tubes were working very cold and really in theory the amp was not so loud. I sent an email to HIWATT and adviced me as this :

"The reason why you set the bias voltage and not current is the bias voltage is common to all valves whereas you will only be measuring the current of one valve which is not necessarily going to be correct for the other valves,(you would have to adjust a preset for each valve which after a week or so would be out of balance again due to uneven wear)....please follow my advice or take to a tech. These amps are pretty much bullet proof so set the bias voltge to -32v and the amplifier will be fine."

There is one true in this advice that the tube due to uneven wear the cathode current could not be fixed for a long time.

So i try at first the The negative grid voltage method they advice.

I measure on PIN 3 first the Plate voltage and was 450 VOLTS. BE CAREFULL DONT DO IT AT HOME THIS VOLTAGE KILLS.

After i measure at PIN 5 NEGATIVE GRID VOLTAGE of -32 VOLT

i check the cathode current "my method" and show 46 mA !!!!!!!!

this mean that the HIWATT advice push me make the amp work on
450x0,046 = 20,7 WATT each tube this mean 80% + of the maximum plate dissipation. Most technicians recomend not to exceed the 70% of the maximum plate dissipation. So the The negative grid voltage method is highly inaccurate because the same grid voltage can produce drastically different plate currents in different tubes of the same type. This method should be avoided.

2nd trial : Plate/cathode current method

EL-34 MAX 25 watt 70% of 25 WATTS = 17,5 WATTS each tube
17,5 watt / 450 V (plate v) = 0,0388 mA
using the cathode current probes i place 39 mA on each tube ( V1 39 ma ,V2 38,8 ) and the amp sounds loud and great !!
I measure PIN 5 Grid voltage show -34 VOLTS so this 2 volts make a big deference they advice -32 ...
I am not sure that this current will stay for a long time i mean after some hours of playing the amp but at this time sounds great.

P.S when i receive the head the power was .... 450 v x 0,024 = 10,8 watt means 2 x 10,8 watt means i receive a 50 watt head producing final 21,6 watt with the false biasing !!!

My question never answered is if we bias an amp over 70% but we dont push it to play with the volume pot full right at MAX ,maybe play with half power of 50% OF VOLUME what will happen except the reduce of TUBES life hours ???

Many thank hope this helps HIWATT and HI-GAIN new series AMP OWNERS

Michael Greece

[Steve Conner]

P.S when i receive the head the power was .... 450 v x 0,024 = 10,8 watt means 2 x 10,8 watt means i receive a 50 watt head producing final 21,6 watt with the false biasing !!!

This is wrong, it's an old myth that refuses to die. The idle dissipation of the tubes hardly affects the power output, you'll get 50 watts regardless of the setting. All the other information you posted seems good and useful, though.

My question never answered is if we bias an amp over 70% but we dont push it to play with the volume pot full right at MAX ,maybe play with half power of 50% OF VOLUME what will happen except the reduce of TUBES life hours ???

The bias setting has a small effect on the amp's tone. If you don't use enough idle current, it can sound a bit thin and weak when you play at low volume. You can turn the idle current down as far as it'll go to hear the effect for yourself. But 32mA seems to be enough to fix this with EL34s.

[g1]

One more thing: you need to recheck the plate voltage every time you change the current and adjust your calculations accordingly.

[carpediemgr]

thanks for the advises dear friends
i recheck the Plate Voltage and the changes were very very small between 452 maybe at 454 max so these deferencies dont make sence to this amplifier and no sence to calculate for just 2 or 3 volts of increase , maybe in deferent amps the changes are biger and need recalculate .