A15MK2 Deluxe Very low Volume on both channels

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    • A15MK2 Deluxe Very low Volume on both channels

      Hi to all,
      Just finished building the kit checked all voltages and everything was within spec everywere,except for the heaters at 7.2v and a line votage of 241v.set bias to 34.8mv on V6 and V5 was 29.8MV.
      Plugged guitar in and get a very low volume on both channels with everything maxed out,tried it in all speaker outs (4,8 and 16).plugged guitar into effects loop return and get no signal?.
      Going through checking everything right now but would like a little help please.

      Just checked the smooth switch and get 307v on all contacts so looks like that is broken.


    • Just checked the output transformer and i get the following results -
      Powertubes in no speaker connected
      pin 6/7 = 310.6v
      pin 12 = 310.4v
      pin 13 = 310.2v

      only the 2 powertubes in and no speaker connected
      Resistance between 6/7 and 12 is 88K Ohm

      Resistance between 6/7 and 13 is 83.8k oHM

      Put a new switch in for the smooth and that is now sorted.


    • Sounds like the signal goes to ground. Here's some suggestions.

      Shielded wire can be tricky if the wire gets too hot while soldering. This is the obvious place to start. Check for shorts (0ohm).
      Check your OT and wiring.
      Check that your output jacks/cabinet are grounded properly (i think they are switching, so check with a cable if you are uncertain).
      If this dosen't help, you have to go through the signalpath and check for shorts or deviations.
    • Thank you Acron for your reply..fairly new to amp building but will go through the circuit tonight with an audio probe and try to find out were the signal is going.would have thought though that if the signal was going to ground i wouldnt get any output signal at all?the volume is very low but audible maybe just above whisper quiet but audible enough to know that both channels and all controls work perfectly.

    • I have never used an audioprobe, even though it may ease the work. I usually just check that all the resistors to ground from the signalpath have the right value and that the caps do isolate.

      After you have checked the signalpath (and solderjoints), and still can't find any error. Take som closeups and post them here (a couple of overview, main board, potentiometers, input, fx-loop, output, ot, tubes, groundlugs).
    • Thank you gentlemen for giving a helping hand,the send and return are both working fine, have a good strength signal through the audio probe at v5 and v6 at pin 5 and pin 4 but then drops volume before speaker outputs.hope i am doing things right and need some guidance on were to look next.

    • You could try to take the preamp out/fxloop out just to be certain there are signal going through. It should be ok though.

      Check the nfb wire from 4ohm out (have to be wired correct on the jack) to R47. Measure from R47 to ground, and double check 4ohm output to ground. If somethings wrong: cut the shield from ground and test again.
      Check the outputjacks once more. If they are grounded wrong you will not have sound.
      Check the OT. If you have wired it wrong or turned it the wrong way (signal in on taps); trouble again.
    • Changed tubes around and get a very slight increase in volume,but not a lot at really.speaker outs checked again but plugged speaker into the 16 ohms instead of 8 ohms just to see if that was any better and main fuse blew straight away?new fuse in and tried it back into 8ohms,very low volume but didnt blow fuse,so turn off amp and back into 16 ohms and main fuse blew straight away.

      checked wiring and from output transformer to speaker outs and it is as layout .

    • Hi,
      let's try to figure out what is really going on:
      1) the PI (V4)
      The output is pin 1 & 6, best to measure after C23 / C24 => you should measure about -21V DC (the bias voltage of the output tubes) and an AC voltage, which is much higher than the input signal at pin 7 (the so called long tail PI is not only providing two output signals with 180 degrees phase shift, but also amplifies the signal).
      2) The output transformer
      If you are blowing the fuse when connecting a speaker at the 16 ohms outout, either your OT is broken or you have connected it incorrectly. Please measure the resistance between the primary leads (12 and 13 each against 6/7) and then measure the resistance between 12 and 13 each against each of the secondary leads (disconnect the amp from 230V and take out the tubes before measuring)
    • The shame of it :( connected r55 and r56 to blue plate voltage connection instead of together,which i thought should solve it if i connected them together as they should be, but now havnt got any bias voltage on pin 8.have got -23.08 on pin 5 0f V5 and V6 but no voltage on pin 8 of both tubes.How do i sort this one gents please.