MkIV Series A Volume Drop when Fx loop is engaged

[JAMz]

I’m having a slight volume drop and loss of top end when the switchable effects loop on the MkIV is engaged. I’m running a delay and a chorus pedals en the loop. Both are really good and true by-pass. The first thing I did was to take out the pedals and run a 6” patch cable between the send and return jacks. That way, the signal will pass thru a short cable eliminating the possibilities of having one of the pedals sucking out the tone/volume. The problem still occurs.

Could the problem be caused by a bad pre-amp tube? Is it V2?

Any pointers is greatly appreciated.

 

[Boogiebabies]

One of your pedals does not have enough output to drive the loop return.
Your looking to get unity gain with your effects so the loop has the same output as the preamp signal. If the effects do not have some type of output control, you may just have to live with it. If a preamp tube drops enough gain on one of the triodes it usually just dies and no signal gets passed, but some linger on and go microphonic or act weird. If you have old pre's, it would not hurt to change them. Preamp tubes generally come with the triodes close enough to prevent large variances in gain.
Most normal production tube come with gain factors of 95 on one and 90 on the other. Some pre's with better QC are now actually running 105-110 per triode. Still, not a life altering difference to the ears, but good to know they are being produced with the actual gain factor the 12AX7 was designed for.

If you do not have output/volume controls on your effecs, you can run a Boss GE-7 at the end of the effects string. Set the faders to flat and use it as a booster. It's pretty transparent and should get your effects signal to equal your normal output volume.

 

[JAMz]

Thanks for your response. So far I have ruled out the pedals that I'm running on the Fx loop as a posibble cause of the problem. Since when connecting a 6" cable between the snd snd rtn jacks I get what should be the most "transparent" scenario. I have repalced the pre-amp tubes with no avail. So I' guessing it must be a probelm with the amp's loop. I also cleaned the jacks with de-oxit. Maybe a cold solder joint...