bassman questions

[fredouille]

Hi all, several questions in the same post, hope it won't be messing

1) on a blond, 5881 tubes new biased correctly, i was sending signal keeping checking the bias V on the 1 ohm resistors.. while increasing till full power (25v on output which is a lot by the way), the current increased in the tube till 100 ma instead of the 35 ma without signup.. of course no guitar plays constantly a sinus but looks dangerous for the tube..
looks like the baby is too power, 25v = 150 watts, looks crazy. by the way i added grid stoppers on 5881 1,5k as it was going in oscillation with a signal at high volume, now it is ok

2) the same blond, with most of components new, high quality, when no volume I hear some small crackling, I tought power tubes were out, but with new, same.. I guess remains only power tube sockets and or bad solder on power tubes..

3) on a blackface, rebuilt also, it is not so powerfull, initially over biased it was , now not.. i was wondering which resistor to play with to change the power, which question could apply on the blond which looks like having too much.. phase invertor ? or the 2 resistors coming from each channel ? 500K on the blond and 220K on the blackface

[TUBEDUDE]

How's the bias voltage look on the control.grid? Sounds like the bias voltage losing control of a gassy tube. But since a new tube does the same I'd see if the bias voltage on the tube is changing. Maybe touch up the joints in the path until the bias is stable then put the tubes back in and check that the voltage remains stable.

[fredouille]

How's the bias voltage look on the control.grid? Sounds like the bias voltage losing control of a gassy tube. But since a new tube does the same I'd see if the bias voltage on the tube is changing. Maybe touch up the joints in the path until the bias is stable then put the tubes back in and check that the voltage remains stable.

look the channel 1 vas modified, it is double or more too load from channel 2. i will put back to stock to see
with channel 2 at fun power the tube current was also increasing but less.

I will in did check the bias voltage on control grid, I didn't check it.

but look logically, when the tube is really used, it is logical to have more current draining to cathode 1 ohm resistor or I'w wrong here ?

as I said the amp was fully rebuilt , all tubes new, all voltage at 10% ok , sounding great.. just too powerfull which would damage the power tubes

[LOUDthud]

In a Class AB power amplifier, bias current is set with no signal. It's not valid under any other condition.

[pdf64]

1/ In a AB amp, an increase in cathode / anode current with signal, ie higher than the idle level, is normal.
Much of the additional current is transferred to the load, rather than dissipated by the valve.
Hence the load gets hot etc.

To measure 150W, surely the amp is being overdriven?
The sine wave input gets clipped at the output valve control grids, lots of harmonics added, and resulting output is probably more akin to a square wave.
A square wave of the same peak voltage as a sine wave has twice the power.
So your blond bassman may be more like 70W at the onset of clipping.

2/ Maybe a 9 pin valve is a bit noisy, or the circuit board isn't a perfect insulator?

3/ To reduce the power output of the output stage, the high voltage supply to the anodes / screen grids could be reduced somehow.

What heater voltage do you measure on these amps, eg 6.3V or perhaps higher than that?

[fredouille]

1/ In a AB amp, an increase in cathode / anode current with signal, ie higher than the idle level, is normal.
Much of the additional current is transferred to the load, rather than dissipated by the valve.
Hence the load gets hot etc.

To measure 150W, surely the amp is being overdriven?
The sine wave input gets clipped at the output valve control grids, lots of harmonics added, and resulting output is probably more akin to a square wave.
A square wave of the same peak voltage as a sine wave has twice the power.
So your blond bassman may be more like 70W at the onset of clipping.

2/ Maybe a 9 pin valve is a bit noisy, or the circuit board isn't a perfect insulator?

3/ To reduce the power output of the output stage, the high voltage supply to the anodes / screen grids could be reduced somehow.

What heater voltage do you measure on these amps, eg 6.3V or perhaps higher than that?

1) ok clear so increase of current in cathode at full power is normal
yes the amp is overdriven on 1 channel.. it is not stock version, I will revert, i got it like this, bass pot and some resistors around are not right. it was even a 1M resistor from this channel to phase inverter, instead of 500K, I reverted to stock but now it is too powerfull

2) old board.. with all new components.. I changed resistors on power tubes also , I will check all solders

3) screen grid on power tubes ? I already added 1,5K which prevents oscillation. I think it is the 2 resistors coming from each preamp to phase invertor.. heaters are ok, I checked all voltages

[Stevem]

What is the resistive load value you’re testing with and what is the power supply voltage?

I assume your testing into close to a 4 ohm load.

Of the top of my head I got to say that getting 150 peak watts will take darn close to 500 volts.

[fredouille]

What is the resistive load value you’re testing with and what is the power supply voltage?

I assume your testing into close to a 4 ohm load.

Of the top of my head I got to say that getting 150 peak watts will take darn close to 500 volts.

220v.. on 4ohm yes.
I reviewed the channel 1 back to stock now I have 20V pic on output 100 watts which looks better
the tubes are making much noise at full volume

[pdf64]

1/ In a AB amp, an increase in cathode / anode current with signal, ie higher than the idle level, is normal.
Much of the additional current is transferred to the load, rather than dissipated by the valve.
Hence the load gets hot etc.

To measure 150W, surely the amp is being overdriven?
The sine wave input gets clipped at the output valve control grids, lots of harmonics added, and resulting output is probably more akin to a square wave.
A square wave of the same peak voltage as a sine wave has twice the power.
So your blond bassman may be more like 70W at the onset of clipping.

2/ Maybe a 9 pin valve is a bit noisy, or the circuit board isn't a perfect insulator?

3/ To reduce the power output of the output stage, the high voltage supply to the anodes / screen grids could be reduced somehow.

What heater voltage do you measure on these amps, eg 6.3V or perhaps higher than that?

1) ok clear so increase of current in cathode at full power is normal

Yes, its documented in the typical operating conditions in info from manufacturer, compare 'zero signal plate current' and 'maximum signal plate current' data (maximum means at the onset of clipping, note the distortion data)

3) screen grid on power tubes ?

Yes.

.. heaters are ok, I checked all voltages

What is the heater voltage?

A full voltage survey may be beneficial; V DC or V AC as appropriate at every lug of every valve socket, no signal, all front panel controls at CCW.