Bizarre Electronic Conundrum !

[rreiser]

I am not a tech or engineer. This one stumps me. Running two amps stereo with effects unit. One amp EF-86 front end. One 12ax7 front end. Equal volume. Cornish buffer is 1st pedal. 5 OD pedals after buffer.. EF-86 amp hooked up but not turned on. Pedals into 12ax7 amp sound fine , unity gain. Turn on EF-86 amp but keep it in standby mode and 3 of the 5 pedals are 1/3 less volume in 12ax7 amp. You can actually hear the volume go down in the 12ax7 amp (with one of the 3 pedals on) as the ef-86 amp warms up. Remove ef-86 tube and put amp on standby and there is no volume drop in the 3 pedals into the 12ax7 amp. Pedals into EF-86 amp sound fine , no volume drop when both amps are on.. I decide to take the 3 pedals that drop volume in the 12ax7 amp (when the EF86 amp is on) and move them before the Cornish buffer. Problem solved. No volume drop in 12ax7 amp when EF-86 amp is powered up. . Anyone want to venture WHY ? The amps both spec out perfectly no stray voltage ect.

[stratman_el84]

What you're experiencing is the loading of the signal as the tube warms up. A tube acts as a variable resistor and will act just like placing a resistor to ground on your input when it warms up. The only practical way around this outside of modifying the inputs of the amps to add buffering is to add a buffer unit between each amps' input and the pedals to isolate the two amps' inputs from each other. You might try using a switch to switch between the pedals and use that buffer to sum the pedals and isolate the two amps' inputs from each other. Hope that helps.

Cheers!

Strat

[michael_smurphy]

yeap +1

[Blue Strat]

It's always a good idea to have a buffer directly connected to multiple loads. Some use a the two outputs of a stereo chorus to do this.

[RedMan]

What you're experiencing is the loading of the signal as the tube warms up. A tube acts as a variable resistor and will act just like placing a resistor to ground on your input when it warms up. The only practical way around this outside of modifying the inputs of the amps to add buffering is to add a buffer unit between each amps' input and the pedals to isolate the two amps' inputs from each other. You might try using a switch to switch between the pedals and use that buffer to sum the pedals and isolate the two amps' inputs from each other. Hope that helps.

Cheers!

Strat

This almost makes sense but he says he gets the volume drop when the EF86 amp is in standby too.

[mark norwine]

depends on the type of standby. I had an amp once that accomplished "standby" by lifting the power tube's cathodes.....

I've seen others that simply ground the power tube's grids.

I'm not saying "that's a good way to do it" (it's not, IMO)...just saying that such amps do exist. Under such circumstances, the preamp is "alive & cooking"...

[donnyjaguar]

Hmmm....I'm thinking the only way that the impedance going into the EF86 amplifier should change between being turned OFF then ON is if degeneration is applied to the input (control grid) of the EF86 itself. Guessing this isn't too likely. If you really want to know, lay the schematic on me. :)

[rreiser]

I also did the test with two 12ax7 amps and the volume drop did not occur with either amp. Only when the Ef-86 mp is connected to one of the stereo outputs of the Eventide and powered up (even in standby) does the volume drop occur in the other amp when one of the three pedals is engaged. If I put the 3 pedals before the buffer there is no problem so thats what I'll do.

[RedMan]

I'm thinking theres a slightly shorted element inside the EF86 tube.

[David Robrecht]

I was working with Russ last night on this odd problem. We did try a different EF-86 and that didn't change anything. The input to the amp in question is nothing out of the ordinary. It does have something to do with the EF-86. If you power up the amp as you play through the other one with an OD pedal the volume slowly drops as the Ef-86 amp warms up. The amp that contributes to the "problem" is not affected itself by this phenomenon only any other amp that is connected to the stereo out put. We also tried different stereo pedals to feed the amps. I guess I can accept that the EF-86 changes the impedance or loads it down, but it's strange that it only does it on some pedals. At least we identified what was causing the issue. :AOK
Dave

[ClinchFX]

It sounds to me as if you are running dual mono and not stereo. Stereo implies two separate signals, not one signal split.

Mike is correct that it's usual to have the signal to each amp buffered.

It seems obvious that the EF86 amp is loading the signal down as the tube heats up. The symptom indicates that the EF86 preamp has a lower input impedance than is normal for a guitar preamp, but that shouldn't change as the tube warms up. I guess one possibility is that there is a fault making the bias or maybe screen voltage so hot that the tube is drawing grid current, but you'd surely hear that there is something wrong when you use that amp.

The 3 pedals that are being dragged down by this amp probably have higher output impedance than the other pedals.

Without schematics, we're all just guessing.

Peter.

[rreiser]

Been trying to contact the builder of the EF-86 amp for a few days to get his input. Hopefully tomorrow.