AMPEG V4BHU - need service manual or shematic

[mozwell]

Martin
these voltages will drop when you have power tubes installed as you are not drawing any current on the power tubes.
with power tubes out, you can inject a signal & trace it through all the way to the power tube gates, to make sure the preamp is all ok. The schematics list expected voltages throughout the circuit.
You can do this with bulb limiter in or out.....
Set the bias to the maximum negative voltage.

with bulb limiter in, install the power tubes & fire it up. as long as the bulb is not full brightness all the time, then power up without the bulb, adjust the bias & test it into a dummy load.
Note, always have a load attached (dummy load ok) when you have power tubes installed

[MHO]

Dear Mozwell,

thanks for your appraisal and the tip with the preamp.

That is also my view, that the voltages on measuring point A (anode voltage) will go down to 460 V with installed power tubes due to current and voltage drop at the tubes. All other voltages should follow.
Yesterday I checked also the anode voltage at both driver tubes (12AX7WA and 12AU7/ECC82). Voltages are ok. By the way I' ve ordered a full set of new tubes last week and changed also preamp and driver tubes before measuring.
The new matched power tubes (Sovtek 6L6WXT) are currently not installed.

Due to the fact that F4 was defect, the old V4 (5581WXT) should be the affected tube.

While I wait for delivery of some parts to build a dummy load which I ordered on monday, I will check the full preamp as you described (DC Voltages and signal voltages with 0,6V input signal). Will also set bias to maximum negativ voltage.
I think it's now the right time to build a dummy load for service puposes :-) To drive the 4 *12 speaker cabinat to my home ist not the right way :-)

When dummyload is ready, I will install the powertubes , measure the real anode voltage and will then calculate the maximum BIAS current. After then I will set the BIAS of 70% of maximum BIAS current (sould be 45mA max, for Anode Voltage of 460V and max.power dissipation of 30W)

When I'm ready I will give you a feedback with the results.

Many thanks.

Martin

[MHO]

Dear all,

after building a dummy load, checking some voltages at the preamp and setting the BIAS to the maximum negativ voltage, I put in a matched quad of new powertubes (Sovtek 6L6WXT).
I measured the anode voltage at all tubes (450V), dummyload connected to the amp and set the BIAS to 40 mA. I think that should be a good value for a first sound test next week. The BIAS curent varies between 40 mA and 37 mA along the differnt tubes.
For measureing the BIAS current I used a TAD BIAS Master.
To have an indication for the signal output of the amp, I connected an attenuator (Tube Towns T-POS 100 with signal input bulb and an additional dummy load ) to the amp. The bulb was blinking while playing a connected Fender Strat :-). Everything should be ok now. On Thursday I will test the AMP and his sound with a connectes 4*12 speaker cabinet in the rehearsalroom.

Many thanks for your great support.

All the best,

Martin