Guitar volume on new build

[Joe6v6]

I just finished a new build, its a cathode bias t-wreck express that uses el-84's , 18 watt transformer set. The amp works fine & sounds very nice, especially cranked the Bass opens up & it becomes very full & dynamic. The issue im having is the volume on the guitar has very little range, almost an on off switch. Going from 10 to 8 drops the volume dramatically. This is a known guitar that does not do this with other amps. I have not ran into this before and am wondering what in the amp, if anything, can be changed to help this situation.  .   .    Joe 

[glass54]

Hi Joe,

Can you possibly supply a schematic and is the Volume Control Pot a "Log" or "Linear" Pot (maybe a photo to prove it ie Pot Labels)

Regards
Mirek

[Latole]

OMO you may have some mistake in wiring. Look at your work

https://diagramas.diagramasde.com/audio/trainwreck_express.pdf

scroll on page 8

[pdf64]

That schematic has at least 3 errors -
Input grid leak resistor is on the jack’s switch contact. So with nothing plugged in, it will be noisy, and with something plugged in, if there’s no DC path to 0V common, the bias will drift off.

The fuse and switch are in different legs of the PT primary, it’s unspecified which is which. With the fuse is in the neutral, a problem between live and ground won’t blow its fuse.

But that might not be a problem, as there’s no mains safety ground connection shown.

If the input grid leak has been wired up properly, but the trimmer was turned down, especially all the way to 0ohms, then the input impedance would be 22k, and a guitar plugged straight in would behave as described.
The trimmer seems pointless; just use the instrument’s tone control.

[Joe6v6]

The schematic I used for this layout showed no input grid resistor but I made it 8k2 just to have something there, ive seen from 2k2 - 47k on different schematics. Should I try removing it or a higher value? That trimmer is something I would not have messed with.

[pdf64]

Sorry I assumed that the schematic Latole provided was the one used.
Yes, try taking the input grid stopper from 8k2 up to the typical 33k.

[sluckey]

I don't think there's anything wrong with your amp. Some guitars just behave differently with different amps. Try a different guitar with this amp and see if has the same behavior.

[PRR]

Power off. Stick a cord in the input jack. Measure the Ohms across Tip and Sleeve at the far end of the cord.

If it is not between 100K and 10Meg, it's wrong. (An exception could be if an input cap, but the plans here don't show one.).

[Joe6v6]

Power off. Stick a cord in the input jack. Measure the Ohms across Tip and Sleeve at the far end of the cord.If it is not between 100K and 10Meg, it's wrong.

I did this and got 5k , The input jack is in a tight spot , I pulled out and it read 1 meg. I unsolderd it freed up some space to make sure it would have clearance & all is as it should be. Needless to say the whole amp came alive & is now what I expected it would be, and the guitar volume control behaves properly. Im still not exactly sure what was shorting out but something was. Thanks & I will be sure to use this check on future endeavors.  .   .     Joe

{EDIT: quote-collision -- PRR}