6G15 Clone problem

[keithpop]

I'm not sure if I'm posting this in the correct place but....  I built a 6G15 Reverb Unit about 12 years ago.  It worked great until a couple weeks ago when I a filter capacitor shorted and blew the fuse.  I replaced the cap but now when I turn the mixer control up I lose volume and the tone seems to flatten as well.  I've already replaced all of the tubes and unplugged the reverb tank.  Still has the problem when I adjust the mixer control up.  Any ideas??  I also replaced the capacitor a second time to make sure that wasn't the issue.

thanks for any thoughts on this.

keith

[HotBluePlates]

... a filter capacitor shorted and blew the fuse.  I replaced the cap but now when I turn the mixer control up I lose volume and the tone seems to flatten as well.  I've ... unplugged the reverb tank.  Still has the problem when I adjust the mixer control up.

Which filter cap was shorted/replaced?

This happens when the tank is unplugged? If so, then only the dry signal path should be operating. Unsolder the middle lug of the Tone control. Does the volume loss still happen now?

[keithpop]

Thanks for the response!  The filter cap that shorted was the one right after the choke (40 @ 450VDC).  I unsoldered the center lug of the tone control and it acted the same.  Volume decreases and tone flattens.  Also.....the reverb no longer worked but I guess that was to be expected.

One thing I forgot to mention is the B+ has been running around 350-360 volts, which is high but it has been doing that for 12 years with no problem.  There weren't good options at that time for direct replacement power transformers.

Thanks agan for the response and for any other ideas you may come up with.

keith

oh....and I also swapped out the reverb tank...with the same results.   

[HotBluePlates]

The filter cap that shorted was the one right after the choke (40 @ 450VDC).

Well that's unfortunate, as I hoped it was the one after the 10kΩ resistor.

I unsoldered the center lug of the tone control and it acted the same.  Volume decreases and tone flattens.

Unsoldering that lug disconnected everything in the reverb path, so you can very likely rule all of that out as the problem.

The only thing left that can change while you rotate the Mix pot is the resistance of the Mix pot itself. So that tells me something (I hope).

Have you changed cables recently from the 6G15 to the amp? Are you using a regular guitar cable instead of a low-capacitance cable like George L's, Klotz, Mogami or Gotham Audio?

What I'm thinking is this:
-  With the entire reverb circuit disconnected, only the dry path remains. The sonic change only happens when the resistance between the Dry output and 6G15 output increases. For this to roll off highs, there must be a capacitance to ground, and the increasing resistance results in a low-pass filter that rolls out highs in conjunction with the capacitance to ground.

-  If the last filter cap was the culprit, the roll-off should have stopped with the Tone control disconnected. That's because the last stage of the reverb path has its plate connected to the filter cap through its plate resistor, but the dry path is isolated by virtue of it using a cathode follower. This only leaves the output of the cathode follower to the input of your guitar amp as the problem.

-  The most likely place for there to be significant capacitance to ground between that cathode follower and your amp is the interconnecting cable. Use a shorter length of cable, or use low-capacitance cable. For what it's worth, don't believe the hype on Monster Cable... that stuff is very much NOT low-capacitance. Hoffman used to sell Klotz cables from Kendrick, which were pricy but are low capacitance. But you can get the George L's cables and some ends and create your own.

-  Note that all tubes have input capacitance, and high-mu triodes like 12AX7's have a LOT of input capacitance. Some loss of highs may not be avoidable, but the interconnecting cable is your one good opportunity to reduce it. I think that's gonna turn out to be what you'll find will work.

[keithpop]

Thank you!  I believe you've done this before.....
I just picked an old cable out of my bag yesterday and plugged it in to test the capacitor fix.  I'll be removing the connectors and throwing the cable away.  That was it.

Do you use 6K6 or 6V6 tubes in your reverb unit?  I've been using 6V6 because my B+ is so high but I have several 6K6's.  I'm a little afraid they can't take the voltage.

Thank you again for sharing your knowledge.  I truly appreciate it.

keith

[HotBluePlates]

I haven't had a 6G15 in 6 years or so, but when I did, I generally used 6K6's. I did mainly because I could find them cheaper than buying 6V6's.

It's worth pointing out that I also occasionally used 6K6's in my 5E3 copy. So I wouldn't worry about voltage issues.

[keithpop]

I'm going to try a couple 6K6's for comparison.

Thanks again!

Have a great week.

keith