Biasing a Blackstar HT Venue amp

[ecc83]

Thanks Dave, will give the 6l6's a try. I'll keep you posted.

Tony

Further thoughts Tone.. The 6L6 has ony about 1/2 the gm of a '34 (the 6550 seems comparable AFAICT) and this means you will have to drive it harder for a given power output*. Fortunately the PI stage in the HT-100, and indeed all the HT/Series One range, is a very sophisticated design capable of large voltage swings and from a very low source R compared to the bread and butter k.coupled, ECC83 version.

*I strongly suspect that a lot of the "improvement" peeps claim to hear from replacing 34's and especially '84's with L6's or V6's is due to the fact that the rest of the amplifier has to work about twice as hard for the same subjective loudness.

Dave.

[Shedder]

Now take the 2 signals from that wee socket and feed them into your multimeter as per above and bias to 100mV using PR2.

Hi, Do you happen to know what is the bias value for a stage 60?

I just replace all the valves in the amp, and was getting a reading of 93mV, which I thought was way too hot, So I brought it down to 63mV which might be too cold, as the amp now has a lot of background noise when I play certain notes ( notably the A note, sounds like someone whispering a loud "R" behind the note)

I used a matched set of Harma retro cryo EL34's in the power stage.

Any ideas?

Thank you

[ecc83]

Hi, Do you happen to know what is the bias value for a stage 60?

I just replace all the valves in the amp, and was getting a reading of 93mV, which I thought was way too hot, So I brought it down to 63mV which might be too cold, as the amp now has a lot of background noise when I play certain notes ( notably the A note, sounds like someone whispering a loud "R" behind the note)

I used a matched set of Harma retro cryo EL34's in the power stage.

Any ideas?

Thank you

All EL34 Blackstar amps* are biased at 25mA (=25mV) per valve and therefore the Stage 60 should be set for 50mV. Also check the op stage balance on a 1mVfsd AC millivoltmeter...Don't have one?!! Lug against speaker, adjust for minimum residiual hum (all controls at zero).

As for the noise can't really help you., miccy new valve? It is certainly nothing to do with biasing.

*Except the HT-20 which is set for 11.5 volts across one of the 220Ohm "cement" cathode resistors.

Dave.

[Shedder]

All EL34 Blackstar amps* are biased at 25mA (=25mV) per valve and therefore the Stage 60 should be set for 50mV. Also check the op stage balance on a 1mVfsd AC millivoltmeter...Don't have one?!! Lug against speaker, adjust for minimum residiual hum (all controls at zero).

As for the noise can't really help you., miccy new valve? It is certainly nothing to do with biasing.

*Except the HT-20 which is set for 11.5 volts across one of the 220Ohm "cement" cathode resistors.

Dave.

Thanks Dave.

I never thought I was running them too hot....ah well, back to the biasing.
Cheers

[ICBM]

That does sound suspiciously like a microphonic power valve. Put the old ones back one at a time and see if it goes away when either (or both) of the new ones are out of the amp - don't worry about biasing just for this test.

[Shedder]

I have now adjusted the bias to 50mV.....but the background noise is as bad as ever. I will try to replace the output valves one at a time and see if this helps.

If it is a case of a microphonic valve, will Watford valves replace it? The valves are new, only bought them in september.

[Shedder]

I swapped out the power valves....one at a time. After the first one swapped, the background noise was still there but not half as bad. After I swapped the second, it disappeared altogether.... So it would appear that both EL34's were microphonic. I will have to send them back and just bias the old ones again until the new ones arrive.

[Shedder]

Thanks for your help ICBM and ecc83!

and sorry for hijacking this thread.