Bias setting Ampeg V4BH

[breogan]

Hi there!

This is my first post, i hope this question is not so stupid! :D

I've been given an Ampeg V4BH for tube replacing and biasing. This a 4x6L6 amp. And I've used a 6L6GC matched quarted from TAD (Tube Amp Doctor from Germany). This is my first time adjusting the bias so I have a few questions. I hope you can help me.

This is the procedure I've follow. Values were measured after 3 hour of heating. Tubes are named 1-4 from left to right (front view)

1) I've set the bias voltage (Pin 5) to -48V (Tube1 and T2) -46V (T3 and T4)

2) Plate Votage (Pin 3) is 451V (for all tubes)

3) This amp has fuses on the cathode so I've removed each the fuse and measured the current for each tube. I've got:

Tube1 - 31.1ma
Tube2 - 31.4ma
Tube3 - 35.6ma
Tube4 - 37.1ma

4) Then I've measured the Screen Current (Pin 4) measuring the Voltage drop across the screen resistor (I've measured the screen resistor too). I've got:

Tube1 - 1.44ma
Tube2 - 2.03ma
Tube3 - 1.87ma
Tube4 - 1.74ma

5) I've calculated the static dissipation wattage as (Ic-Is)*Vplate. I've got:

13.38 0.51
13.24 0.51
15.21 0.59
15.95 0.61

Tube1 - 13.38W (51%)
Tube2 - 13.24W (51%)
Tube3 - 15.21W (59%)
Tube4 - 15.95W (61%)

Now my questions: :eek: :confused:

1- This values look right so far. Could someone confirm that I am not doing anything wrong?

2- I've noticed that tubes are matched by pairs. Is this normal?

3- I don't know if this bias is too cold/hot for this type of amp. I've been recommended to set the bias around 55% but, do you have any recommendation for this type of amp? Which would be a nice value? I am not a Bass player so I can't judge by ear!

Thank a lot for taking time to read this.

[J Martin]

Use the hottest tube to set the bias (tube 4). You can go a little bit higher in current. Divide the pairs so there is equal load on each side of the transformer primary, ie 1 and 3, and 2 and 4. I guess there is a reason you couldn't get a matched quad?

[breogan]

Well, this was supposed to be a "matched Quad" :(

I've tried to swap tubes 2 and 4 to have the same power dissipation both sides of the OT. Now I don't understand anythig... This are the values I get without changing the bias.

T1- 14.98W (58%)
T2- 14.51W (56%)
T3- 12.77W (49%)
T4- 12.97W (50%)

Is normal that tube behaviour changes this way?

[breogan]

I've just find out that I've made a mistake measuring the Cathode currents. I've measured it again and everything is correct. The set is perfectly matched. This is what I've got now.

VPlate=443V

Tube IPlate Watts
T1- 32.05ma 14.20Watts (55%)
T2- 31.23ma 13.83Watts (53%)
T3- 31.44ma 13.93Watts (54%)
T4- 31.20ma 13.82Watts (53%)

Do you think this is reasonable bias point?

Thanks!

[Bassman3]

I sent you an email with my contact info if you want the service packet for this thing.
Thanks

Dave Ryan
St Louis Music & Loud Technologies
Service Center Administrator for
Ampeg, Crate, Mackie Speakers
1400 Ferguson Ave
St Louis Mo 63133