Peavey Alphabass round two-troubleshooting

[catalin gramada]

Hey,
An other problem founded at this amp is the power transformer who did not have a proper rated curent ability. I can live with the fact it will sag-otherwise a characteristic of its voicing.But even the filament winding is not usable.For instance. Have 230V on the wall with amp power selector set at 220V .Without power tubes in the voltage on the rail is 6.66V. Then, installing power tubes one by one I get:6.4V with one tube, 6.2V with two tubes...and so on 6.1/5.9/5.6 and finally 5.4V with all power tubes installed. Is clear I cannot use this voltage to supply the heaters and the amp need at least a separate tranny for the heaters. Opinion please ? Thanks.

[catalin gramada]

I have a 10A heaters tranny at hand I can use, but to install... where ? ...Outside the case...? some ideas please ? Thanks.

[catalin gramada]

Well checked again the circuit and guess what ? The voltage in the point where tranny leads are connected to the board is 6.4V but the voltage at the end of the rail is 5.4V . So it seems 5.4W are dissipated to the pcb trace...no tranny problem at all

[catalin gramada]

Schematic and layout

AlphaBass - Bass Head Amp.pdf

0.185 ohm pcb track resistance at 5.4A is exactly 1V dropping between the points.
I may admit it still be possible to supply 1.2A for preamp through the thin pcb traces but for power tubes is clear it needs a hefty 6A cabling circuit

[catalin gramada]

And no, the pcb tracks didn't working even for 1.2A preamp tubes 5.9v here vs 6.4 board input, there are delicate thin traces.
At least the tranny is ok, the heater cabling is something I can do to make it right as should be...
It could be also a imperfect soldering somewhere, everything need carefully check

[catalin gramada]

The heaters wiring was proper done and now the voltage is consistent at board input and end of the rail at 6.5v.

[catalin gramada]

Heaters supply solved as it was just a matter of wiring. Also all capacitors in power supply was replaced with new ones.
Anything works as it should, voltages seems in range, still the amp in not completly quiet, it have a slight hum. Any thoughts about a good start to track it please ?
AlphaBass - Bass Head Amp.pdf

[Enzo]

Same thing I said in the pother thread about this amp: there are endless sources of hum in an amp. You have to track down the source, then we can determine what to do about it.

[catalin gramada]

Thanks Enzo.This hum was here from beginning, changing filter caps didn't do nothing.The hum balance do almost nothing. Circuit below. May I try a "classic" elevation in respect with the heaters directly from plate ? A 220/10k divider may do so...?

[catalin gramada]

Removed 4,7k link and elevated by 220/22k from the plate B+ at 50V around. Is the same thing, the light hum is still there...☹

[Helmholtz]

Why do you entertain two threads on the same topic?
Confusing.

[catalin gramada]

Why do you entertain two threads on the same topic?
Confusing.

Something happen with the thread posted in "Lobby" it shows for me as 'redirect whatever it means", was not sure if is still visible for all or not? But one of it should be deleted of course.

[Enzo]

May I try a "classic" elevation in respect with the heaters directly from plate ?

Removed 4,7k link and elevated by 220/22k from the plate B+ at 50V around. Is the same thing, the light hum is still there..

These are just guesses. Find the source. Is the hum 50Hz or 100Hz? Do ANY of the controls affect the hum in ANY way?

Is there good B+ on BOTH pins 3 AND 4 on EVERY power tube? Missing on one tube would leave the amp unbalanced.

Does the graphic adjust the tone of the hum?

Is there ripple on either 15v rail?

IN STANDBY measure ripple on pin 5 of the power tubes.

Does removing V1 stop the hum? V2? V3? V4?

V4 is the driver tube, and it is a 12 AT7, NOT 12AX7. Is your V4 the proper type?

Push a shorted plug into the FX return. Does that stop the hum?

Tests like these will narrow down the source much more efficiently than maybe this or try that.

[catalin gramada]

I will check , thank you but is 100 cps and seems to come from first or second stage.. Is not so big, the new fan cooler instaled do the same amount of noise.

[catalin gramada]

The hum is pretty consistent and not dependable by settings. Think the power tubes are the problem as I detect also at startup a harsh noise which disappear in few seconds. But still works. I recorded 100w rms onset clipping with a 6p3s-e sextet (5881 equivalent) biased 50 percent, with 465v on plate at throttle. 4 ohm load.