Low output on Gibson Thor Bass Amp (voltages)

[theundeadelvis]

Hey guys! Hope everyone is well, and happy mom's day to all your mothers!


So, a while back I picked up a Gibson Thor Bass amp (50 watts). I've replaced the caps and the tubes, and it powers on and sounds great. No noise, and great tone. However, the output is way too low. Cranked all the way, it's not at all unpleasantly loud. You could easily carry on a conversation with someone while playing.


The schematic can be found here: http://i.imgur.com/cFJ4HAw.jpg

The voltages on pins 6 and 7 of both preamp tubes are a bit low, but I'm not sure what to look at, in order to fix that?

V1
6 - 132 v
7 - 89 v

V2
6 - 264 v
7 - 268 v

Both my power tubes read 450 v at pin 4.

Can anyone point me at what I should be focusing on? Does anything obviously stand out? Thanks in advance!

[sluckey]

The voltages on pins 6 and 7 of both preamp tubes are a bit low

Those voltages seem fine to me. Take voltage readings for all tubes and all pins. Also get readings for all 4 power supply nodes.

[eleventeen]

While you're at it, check bias-supply volts at the anode (non-bar) end of the "backwards" diode D3, and see what kind of bias you have on the pin 5's of the EL34's. If somehow that bias has drifted way negative (say, -45 to -60 volts) your output tubes could be darn near shut off. Another indication of this would be if the output tubes are physically "not very hot" to the touch after some operation time, even at idle. Ordinarily, EL34's get pretty hot and at -34 volts bias/450 volts plate, they should. Be careful!


You might consider throwing a simple voltage divider and pot network in there to allow you to vary the bias.



It also appears ("plug and socket connections") that the controls are located away from the main chassis(?)


You could as well have a dirty connection in those connections, probably fixable by unplugging-plugging in those connectors a few times. Follow up w/a spritz of Deoxit or equivalent (and a few more unplug-plug back in repetitions) if they are dirty.

[theundeadelvis]

Thanks guys for the quick and detailed responses! I've got the voltages that sluckey requested, and I'll get back to the amp and take the measurements eleventeen suggested. Thanks again guys!

[theundeadelvis]

One other thing, my DMM couldn't read pin 3 on both tubes. It would go into an error and shutdown.

[eleventeen]

Well, you're getting your bias (-38 v) no problem and it's well within tolerance.


I vote for the dirty connector. Although....on V2, the phase inverter, you have cathodes (pins 4 & 9) at +25 volts while the grids are at +15 volts....which means the grids are -10 volts relative to the cathodes. That seems like it would tend to cut off those triodes, but your other voltages seem pretty normal (plus, on that V2 tube, they are symmetrical, meaning normal) so that is definitely a secondary guess on my part.


Definitely a low parts-count amp!

[HotBluePlates]

The schematic can be found here: http://i.imgur.com/cFJ4HAw.jpg

Notice the link at the bottom of the page for the Hoffman Schematic Library? Why post to an external site when the schematic is already hosted here?

Although....on V2, the phase inverter, you have cathodes (pins 4 & 9) at +25 volts while the grids are at +15 volts....which means the grids are -10 volts relative to the cathodes. That seems like it would tend to cut off those triodes ...

It's a long-tail pair, and the grids ae bootstrapped. The circuit impedance at that point is probably 5-10MΩ so you need a meter with a couple-hundred-MΩ input impedance to get an accurate measurement directly probing the grid. If you want a true indication of the bias of the long-tail, measure the d.c. voltage at either end of the 820Ω self-bias resistor; the difference in the 2 voltages is the bias of the long-tail (because the 1MΩ resistors should pass no current).

[theundeadelvis]

The schematic can be found here: http://i.imgur.com/cFJ4HAw.jpg

Notice the link at the bottom of the page for the Hoffman Schematic Library? Why post to an external site when the schematic is already hosted here?

Sorry HBP, I already had that schematic pulled up in my imgur library, and didn't think to use the one here.

[sluckey]

V1-5 should be zero volts. V3-2 should be 6.7VAC. V4-2 should be 6.7VAC. Please recheck these.

What kind of meter do you have?

[theundeadelvis]

Sluckey, those are 6.7 VAC. I forgot to switch my DMM over to AC. My DMM is just a cheap Craftsman I picked up years ago.