low output on scratch build

[dixiepig]

Hi all,

I just completed an AC 30 clone. it was a true scratch build in that i used a power and output tranny from an old Bogen AP-250 stereo amp that had been laying around mom and dads basement for about 25 years. the voltages on the power tranny fit with what the schematic reflected although i couldn't find any info, like a spec sheet on it anywhere. Anyway got the thing together and it works fine, dead quiet, could be more pleased in general. the issue is the volume is very low. i can crank it all the way up, as well as whatever guitar is close, and it has really low low low output. My question is could maybe the current that the power tranny provides be too little? I looked at a Mojotone for a VOX 30 and it indicated 270mA, seems like the thing would give at least that much it was feeding 4 7355s on the Bogen and i am using 4 EL84s on the build. Just looking for some advice before i go any further.

[m3moser]

how about the voltage readings to all the tubes?  Are they consistent with the schematic your using to build the amp??

[archaos]

it indicated 270mA

It could be as low as 230mA as well.

Yet, if the PT fed 4 x 7355, then it should a fortiori feed 4 x EL84 (see attached).

How about the OT specs, the speaker(s) used & your tube voltages ?

[HotBluePlates]

Yes, post tube voltages.

Do you have more than 1 speaker jack? If so, how did you wire them up?

[dixiepig]

i will  get some voltage readings ASAP. as far as the hookup for the output jacks, they are standard RCA type jacks 4 per strip. i am only using 3 of the jacks ( 4-8-16 Ohm), the common wire from the secondary OT is attached to the ground lug on the strip, all ground lugs soldered together. each tip has the corresponding lead from the OT soldered to it. there is another wire attached to ground lug that goes to the main ground strip connected at one of the PT Lugs.

Also the OT has a primary inpedance of about 3.7k.

[HotBluePlates]

Hmm... strip of 4 RCA jacks for the speaker outs. Did you happen to recycle that part from a donor amp? I'm wondering if the plugs are making poor contact to the jacks, or if the jacks are oxidized.

My original thought was with regards to the dual speaker jacks common in Fender amps. The intended main speaker output jack is wired with a shorting plug; if you copy that wiring, but plug into the external speaker jack (with nothing in the "main jack"), then you get extremely low, distorted output. That's because most of the output signal is being shorted to ground.

But since you didn't use 1/4" jacks, it sounds like that isn't your problem.

It would be very handy if you could inject a test signal to the phase inverter input, to verify the amp functions properly from that point to the speaker. That would help you divide-and-conquer, and not spend time chasing problems where they aren't.

[6bq5]

don't know if the last post went through or not so here goes again just in case. i am getting 395V at the cathode (pin 9) on each power tube. When i tested the PT it showed 265-0-265. also to answer another post just listed. I used a new jack strip and there are just outputs for the different impedance's, one jack out for each, and one not used. I am beginning to wonder about something else. when I first completed the thing and powered it up i was getting nothing but hum. i went back through and found when i installed the "Cut" pot into the chassis and tightened it down, a lug was forced back making contact with the pot body. i moved it back and away and the thing worked fine except for low volume. i checked the volume pot as well visually and with a continuity tester and found no such grounding there also the volume pot does function from nothing to very low output. Also i have tested it with both my Standard Les Paul and SG just to eliminate any unknown guitar issues even went as far as to run the guitars through my Bassman amp just to make sure my luck wasn't REALLY bad! Cut Pot, Bright switch, volume, it all works just not much output.


pin 9 is not the cathode on el84's----it's the screen.     and given the hv sec value quoted that's too high for the tubes to be drawing any current.  check the voltage on the pin 3 ---the cathodes----and also make sure you have them hooked up to ground through your cathode resistor/cap combo.  it sounds like to me you your're getting no sound because the tubes aren't drawing any current.   rh

[dixiepig]

OK lets try this on for size-we wont go into why this has changed (duh)-   voltage reading for pin #9 is 315V, voltage reading at pin# 3 is 9.75V.

[6bq5]

yep that sounds about right.    NOW if it's still no/not much sound at least you know the tubes are on and drawing proper current and the screens are in the right range, etc.     so it must be something else.   you could benefit from a little listening amp rig that doug shows how to build elswhere on this site----one of the handiest things ever.    rh

[dixiepig]

I built one last year (not from this site) the results have been good/bad on solid state amps. since I have had so much luck on this site in the past i will do just that as I am beginning to suspect something along the signal route more than anything else.

[archaos]

+ 1, you'll find the culprit for sure thanks to your stetho so.

[dixiepig]

just as a wrap up. built the listening probe etc. worked great. As it turned out that %$^& volume pot was still grounding out. Reattached the pot working good now. Thanks for all the advice.

[archaos]

Excellent.