Ideal spot for FX loop in SE plexi build

[peckjed]

Unfortunately I don’t have a schematic. I can try and draw up a layout sometime soon. However, the amp is a Fargen Mini Plex MKII. I removed the power scaling since it was pretty much useless and added in a mojotone buffered effects loop. It’s inserted between the treble and master volume pot. I know typically an effects loop can also be put at the PI tube but the SE amp doesn’t have that. Any other typical SE FX loop spots to try out?

[pompeiisneaks]

Wow those diodes floating in the air with B+ on them seems super dangerous... At a minimum I'd try to shrink tube over them all as much as possible so if they break free they have a lower chance of colliding with things and doing harm.
Also putting those cathode resistors right next to the cathode caps is probably a bit hot for them...

As for single ended, the only good place I'd think you can put it is either pre or post the tone stack to see which has the most/least impact. I don't know if it's got a cathode follower, if it does, probably post tone stack, because your effects loop might get loaded down by the tone stack itself?

~Phil

[alkuz1961]

If I see correctly, the hovering double diodes are a high-voltage circuit coming from a power transformer. But then it is not clear how the HV-fuse is connected to this circuit (red wire). Apparently, this protect power tube only, and the rest of the circuit remains unprotected.

[peckjed]

I was definitely going to move the diodes/shrink tube them (it was there as a placeholder to make sure everything was working).

The Cathode resistors was Bens handy work - he used to put the cap ON TOP of the resistor.

Any good way to identify a cathode follower?

[pompeiisneaks]

A couple things that you'll see on a cathode follower, the DC voltages at the cathode will be much higher than a normal tube, 70 to 100 vdc or more. Also there's no anode resistor, instead it comes straight off the B+.

They're quite commonly used to drive a tone stack so there's no loading down of the circuit by the stack. I found what I think was a schematic of the amp on hoffman amps, not sure it's the right one, but it does have a two tube setup, with a cathode follower into the tone stack that then would feed the SE output stage.

~Phil

[xtian]

I was definitely going to move the diodes/shrink tube them (it was there as a placeholder to make sure everything was working).

The Cathode resistors was Bens handy work - he used to put the cap ON TOP of the resistor.

Any good way to identify a cathode follower?

If this is a tubes up build, then putting the cathode cap on top of the resistor is actually a smart idea (heat rises).

Cathode followers do not have plate resistors--the plates are directly connected to B+.

[pompeiisneaks]

Here's the thread I found with what 'seems' to be the schematic for this amp:
https://el34world.com/Forum/index.php?topic=17518.0

I see what seems likely in the post by silvergun maybe 10 or so posts in. There's this image:



It seems to me to be the schematic that might be right. That's the cathode follower style circuit on the right.

~Phil

[peckjed]

Here's the thread I found with what 'seems' to be the schematic for this amp:
https://el34world.com/Forum/index.php?topic=17518.0

I see what seems likely in the post by silvergun maybe 10 or so posts in. There's this image:



It seems to me to be the schematic that might be right. That's the cathode follower style circuit on the right.

~Phil

That post is definitely about the same amp as mine. So if that were the case would it make it so the return of the FX loop either be at pin 8 of v2 (pre tone stack) or on the treble pot for post tone stack? (sorry if im confused)

[pompeiisneaks]

exactly right, I'd give both a listen, and see which sounds better, but I'm inclined to think after the wiper on the treble pot is going to be the best. Otherwise the job of the cathode follower is somewhat defeated before the tone stack and your effects loop is going to have to perform that job. It may not do it very well.

~Phil

[peckjed]

exactly right, I'd give both a listen, and see which sounds better, but I'm inclined to think after the wiper on the treble pot is going to be the best. Otherwise the job of the cathode follower is somewhat defeated before the tone stack and your effects loop is going to have to perform that job. It may not do it very well.

~Phil

I’ll give it a try. The issue I’m having is when I crank the volume (gain) knob the reverb/delay in the loop also gets a bit gritty. So I’d like to figure out a way to avoid that.

[pompeiisneaks]

exactly right, I'd give both a listen, and see which sounds better, but I'm inclined to think after the wiper on the treble pot is going to be the best. Otherwise the job of the cathode follower is somewhat defeated before the tone stack and your effects loop is going to have to perform that job. It may not do it very well.

~Phil

I’ll give it a try. The issue I’m having is when I crank the volume (gain) knob the reverb/delay in the loop also gets a bit gritty. So I’d like to figure out a way to avoid that.

That's likely due to the fact taht it's got a bit too much gain for the effects loop. You might want to see if there's any instructions that came with the effects loop to manage that, if not then someone smarter than me likely could tell you what size of pot you could use inline to dump some of the signal to ground to set the input gain level. I don't have enough experience to know more than that I'm pretty sure I don't know how you determine what size pot you'd need. I often see some cases where the gurus recommend 25k or 100k or 1M pots and I don't know how you really determine what's optimal.

I'd love to understand that better myself I 'think' it has to do with knowing what the impedance at that stage should be, and matching the full value of the pot to be equal to it... but not only am I not sure, but I don't know how you calculate that either

~Phil

[peckjed]

exactly right, I'd give both a listen, and see which sounds better, but I'm inclined to think after the wiper on the treble pot is going to be the best. Otherwise the job of the cathode follower is somewhat defeated before the tone stack and your effects loop is going to have to perform that job. It may not do it very well.

~Phil

I’ll give it a try. The issue I’m having is when I crank the volume (gain) knob the reverb/delay in the loop also gets a bit gritty. So I’d like to figure out a way to avoid that.

That's likely due to the fact taht it's got a bit too much gain for the effects loop. You might want to see if there's any instructions that came with the effects loop to manage that, if not then someone smarter than me likely could tell you what size of pot you could use inline to dump some of the signal to ground to set the input gain level. I don't have enough experience to know more than that I'm pretty sure I don't know how you determine what size pot you'd need. I often see some cases where the gurus recommend 25k or 100k or 1M pots and I don't know how you really determine what's optimal.

I'd love to understand that better myself I 'think' it has to do with knowing what the impedance at that stage should be, and matching the full value of the pot to be equal to it... but not only am I not sure, but I don't know how you calculate that either

~Phil

Thanks for the responses! This is the manual for the mojotone: https://www.mojotone.com/products/effec ... =overwrite

I added the return trimpot per the instructions with a 1 meg pot. It didn’t mention anything about a send pot but I’d like to believe it’s possible?

[pompeiisneaks]

If nobody else responds, I'm sure you can give that a try, usually it is a kind of 'set it and forget it' thing, once you have a balanced input/output that's at unity, you're golden. you could possibly even hard wire in a voltage divider network instead once you find the right values?

~Phil

[pompeiisneaks]

Oh they even talk about adjusting the return gain in step 5 Modification (optional)

If you need to adjust the return gain you can change the 470k return gain but similarly you could reduce the send input level by adjusting the 10k at the send side... possible mods that may help tame that loops open loop gain so it's not coming into the effects with too much gain.

[sluckey]

It’s inserted between the treble and master volume pot.

That's probably the best spot.