New 5F6A Build - Super Low Volume

[rvaughan]

I just finished a 5F6A build, Mojotone kit.  I added adjustable bias and a master volume.  Set it up with some 10" Weber speakers.  Went through the Robinette startup and everything checked out.  Preamp and rectifier tubes are new.  Power amp tubes I pulled from another.  When I hooked up the SG to try it with a guitar I get ultra low volume with knobs maxed out.  The tone is good but sounds like its on 1 instead 12.  Any tips would be appreciated.  Thanks

{edit - fixed title typo -PRR}

[sluckey]

Check for wiring errors.

[Bieworm]

A missing coupling cap would do that.. been there😉

[bmccowan]

"ultra-low" had that once, was a bad standby switch.

[labb]

I would build me R.G. Keen’s audio probe and use it to inject a signal starting at the power tubes and determine where the amp is loosing the signal. Then you know where to look

[rvaughan]

This is my first build and I've learned a lot in a short amount of time but obviously have a ton more to learn. Thanks  everyone for their input.

[Williamblake]

The answer is simple, the question is the hard part. Any pictures?

[rvaughan]

Any idea why the 68K resistors would measure 33K.  I checked all resistor values before they were installed. 

[tdvt]

They are in parallel in that configuration & are reading correctly.

Check out Sluckey's amp scrapbook, input jacks on page 12

https://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[labb]

If you used shielded cable check to make sure that you don’t have a whisker grounding it out.

[rvaughan]

Found the problem.  Bad wiring at the input jacks.  The amp sounds great.  Appreciate all that can do this and the knowledge you freely share.  Now that I've done this once, I actually need to learn what I did.  Thanks all.

[rvaughan]

The amp sounds great but.... the low end seems muddy, not very round.  Also, the presence control doesn't seem to affect the tone.  Is there anything that can be done in the circuit to improve the muddy low end?  I've played a telecaster and gibson SG through the amp.  Low end tone is not great for either.  I'm using Weber speakers.  Weber's recommendation for the Bassman was a combination of two 10A125 (8 ohm, 30w, light dope) and two 10F150 (8 ohm, 25w, light dope).

[PRR]

5F6a is not muddy, and the presence does something.

You found one "bad wiring". They run in packs. Look for more bad wiring.

[Esquirefreak]

What type of master volume did you put in your Bassman? Some of them can make the amp muddy when turned down low. If you have a LM type master volume, then turning it down low affects the presence knob as well.

/Max

[rvaughan]

Type 2.  Delete grid leak resistors and run shielded cable from coupling caps to 250K dual pot with 2meg resistors.