Mojotone BF Bassman AA165 - No Sound

[Briac]

Could use a little help... First build. Completed Assembly, in testing phase and getting no sound, no hum, silence. If photos will help, i'll upload when approved. Line voltage 120AC, speaker is good, all tubes glowing, power indicator lights, no blown fuses, no sparks smells etc. I used a dim-bulb current limiter, is not in use now. I have checked the under-board runs for continuity, re-flowed some solder joints. I tied the two top pins together and the two bottom pins together on the bright and deep switches.

Filament = 6.5vac across Pwr and Pre tubes

Rectifier GZ34
#2 x #8 - 5.21vac (yellow)
#4 x #6 - 728vac (red)

Power Tubes
#1 - 54vdc
#3 - 0.91vdc
#5 - 53.8vdc
#6 - 0.89vdc

Pre Tubes
#1 - 0.92vdc
#2 - 0.65vdc
#3 - 0.03vdc
#6 - 0.94vdc
#7 - varies w/ volume

Bias Pot
#1 - 41.8vdc (resistor)
#2 - 54.5vdc (Middle)
#3 - 63.2vdc

Thank you


(AA165 Schematc) http://www.mojotone.com/Amp%20Kit%20...31761&gc=clear

[Chuck H]

First... Congratulations on a perfect first post with voltage readings, photo, schematic links, no personal ranting about frustration and a decent synopsis

A couple of things that could be better though... Your use of dashes to separate the pin# from the voltage figures makes it impossible to determine if your bias voltage is positive or negative, the photo quality makes it hard to see some details and I wonder if the schematic is for your actual amp since it doesn't include a rectifier tube and yours has one?

Still, by far one of the very best opening posts I've ever seen and welcome.

What is the DC voltage on pin #8 of the rectifier tube?

[Dave H]

It has bias voltage but no B+ voltage. The rectifier tube has AC but what is the DC voltage on pin 8? Could the standby switch be open circuit? Measure the voltage on both its terminals to check.

Edit
The rectifier AC red wires look like they are connected to pins 5 and 7 not pins 4 and 6.

[Enzo]

Agree with Dave, your rectifier tube looks like pins 5 and 7 instead of 4 and 6 for the HV AC. And that would explain the lack of B+ voltage in the circuit.

[Briac]

Thank you for the reply, with standby on pin #8 is -0.37vdc. with standby off it creeps positive.

[Briac]

Double checked, and you are right. I errored with the connection. I'll let tubes cool, drain the filter caps, again. and give it a go with the dim-bulb, etc. I stared at that for hours, never saw it.

Thank you for the reply

[Briac]

Corrected rectifier AC red wires. No smoke. Pin 4 & 6 = 738vac, pin 8 = 490vdc, "Progress!" I've got hum. Too much hum, so I turned the bias pot and the hum lowers. with guitar plugged in, a lot less volume than I was expecting. Normal channel is low volume, volume adjustment is clean linear. Bass (gain) channel is low volume, volume adjustment is scratchy non-linear, low tone noise added, cleans up at 8 or 9 on volume knob, still low volume from the speaker.

Thank you all, for the replies... onward.

[Chuck H]

You shouldn't attempt to play through the amp anymore for now. Your symptoms combined probably indicate grounding and wiring errors. Can you provide higher res pics? A few to cover all the circuits and wiring. It's a small PITA to trace circuits from pictures and higher res shots help a lot.

Avoid the tedium of just staring at the circuit trying to spot an error. I, you, anyone can overlook a mistake many times that way. A better way is to draw the tube sockets and control layout and then, using your amp as a guide, start drawing what's actually happening in your amp and comparing it to a proper reference along the way. It may take two or three hours but that's less time than it'll take if you don't do it this way. There may still be errors when you're done, but fewer. Giving you and us a chance to isolate for troubleshooting.

How did your amp get a rectifier tube when the posted schematic has a diode rectifier? What reference did you build from?

On a strictly opinionated note... There are companies that will strip a few feet of several colors of lead wire and charge by the foot. It may have cost a little more but at this, perhaps most difficult stage you would have coded traces instead of a sea of yellow spaghetti

[pdf64]

How did your amp get a rectifier tube when the posted schematic has a diode rectifier? What reference did you build from?

See the mojotone link in post #1 (it shows an option for a tube rectifier).

[Chuck H]

See the mojotone link in post #1 (it shows an option for a tube rectifier).

Ha! Looked at the schem so fast I never checked the wee box. Doh!

In that case Briac could sketch a hand drawn schematic based on his actual build instead of a layout, if it suits him better. Either way is good for finding errors.

[Briac]

Hi Chuck H
Noted: no more guitar. yes, wouldn't want to hurt the amp or myself. attached more pics and links to diagrams (2). Mojo doesn't have diagram for rectifier tube circuit online. They sent me a printout. no walk-through, procedures or best practices. Trying to follow schematic, diagrams and adapting procedures from a tubedepot Walk-through and also, what I could find online. Yes in hindsight, I realize this was ambicous for a first build. Rectifier tube is included on previous schematic link.

I'll get more voltage readings, since the AC connection has been corrected.

Mainboard wiring Diag1 (AA165) note the white wire which comes from between positive side (8uf cap and 27k resistor) on filter cap board.
http://www.mojotone.com/Amp%20Kit%20...31776&gc=clear

Filter Cap Diag2 : Note this diag doesn't show the white wire which starts between positive side (8uf cap and 27k resistor) and goes to mainboard.
yes, I did connect this. It is my understanding that this is a drain for the filter caps. (?)
http://www.mojotone.com/Amp%20Kit%20...31776&gc=clear

more Circuit Pics, better resolution.


Let me know if pics aren't right. these are in 2M, the other VGA.

Thank you

[g1]

Filter Cap Diag2 : Note this diag doesn't show the white wire which starts between positive side (8uf cap and 27k resistor) and goes to mainboard.
yes, I did connect this. It is my understanding that this is a drain for the filter caps. (?)

The drains (& balancing resistors) for the filter caps are the 220K's across the 100uf caps.
Where the white wire connects to the 8uf is node E of the power supply. It feeds only tube V1.

[Briac]

Thank you, I put in my notes.

[Briac]

After AC connection corrected.
Symptom; Low output both channels, bass channel non-linear volume & noise. when bass channel volume increased to (2+), volume and low tone hum appears. continue increasing volume channel to (7+), volume and low tone hum drops off. acts like square wave with low tone hum.

Voltage Reading:
Rectifier (VAC)
Pins 2 & 8) 5.23vac
Pins 4 & 6) 740vac

Power Tubes (VDC)
V5 V6
Pin 1 -61.2 -61.2
Pin 3 492 493
Pin 4 491 491
Pin 5 -48 -48
Pin 6 491 492

PreAmp Tubes (VDC)
Order : V1 / V2 / V3 / V4(PI)
Pin 1 239 / 0 / 302 / 280
Pin 2 0 / 0 / 0 / 60*
Pin 3 2 /-0 / 2.2 / 98
Pin 6 240 / 290 / 288 / 273
Pin 7 0 / 0.078** / 0 / 64
Pin 8 2 / 2.4 / 2.3 / 98

* nice click from speaker ** Hum present when probed

Bias Pot (VDC)
Pin 1 ( - 41.5) resistor
Pin 2 ( - 62.8) middle
Pin 3 ( - 63.0)

Thank you for the replies

[Justin Thomas]

Briac,

Pin 8 of V3 has no voltage? That section of the tube is not conducting (working). Remeasure to confirm, & check connections to that cathode, ground... also parts values, etc. Since it's a build, new doesn't always mean good - open resistors, shorted parts, etc. Probably not the source of all your troubles, but a source nonetheless.

One question: you mention a "gain" channel. I checked Mojo's description and they mention no such thing? To get a gain channel out of an AA165 would need mods. Are you intending on the amp being built as-is, or have you incorporated "hybridization" mods? I know these amps get quite crunchy as I've owned several, but they're not "gainy" in the normal sense unless modded. If you are building the amp vintage-like, you may want to call the "gain" channel "Bass" or sonething else that keeps your intention clear - someone else may think you've modded yhe amp with a Marshall pre, different design, circuit, and possible source of problems if so.

Just check that V3 cathode (pin 8) first...

Justin

Edit: try a different 12AX7 in V3 first.