Powering an Effect Loop

[SolidStateCircuitReject]

Hi Folks.

Adding a LND150 based effects loop to a tube amp. The power requirements are 250V to 400V so I ran off a 12ax7 plate that was putting out about 300V. However once I turned the amp on it was so clean sounding. So as I check the voltages I see a drop then from 300VDC to 175VDC.

So the question is should this happen? I have only installed tube effects loops in the past so this is my first ss effects loop install. I guess I am really wondering if there is an issue with the effects loop circuit itself or if there is a better way to supply it?

Thanks!!

[sluckey]

You can't use the plate of a 12AX7 as a source of power for your project.

[John]

He can just tap into the B+ supply somewhere though, right? Assuming it's under 400 v.

[pompeiisneaks]

It would depend on the current requirements of the effects loop and what the amp power supply can handle actually.  Say if the power supply delivers 450mA and the current tube complement requires 450mA of current, it will definitely cause problems with the power supply and transformer.

IF it can handle the current requirements, then you'd want to pull it off a separate point in the power supply B+ rail and filter it even more for that section, or it will cause sag in other areas.  Like sluckey indicated, the power supply filtering was setup for the current amp, taking it off the preamp tube means you're creating a big voltage drop there with nothing to provide the backfill.  It's a pretty complicated change that requires understanding the entire power supply requirements of the amp as it exists as well as the add-on board.   Best suggestion would be to, at a minimum, pull the power off near the exit of the rectification, and if it's too high of voltage, put in a dropper resistor to get it down where you need it (Or a zener or two etc?)  Then you may need to add in another filter capacitor.

~Phil

[John]

Good point! Although... I'm wondering if he's hooking it right onto the socket pin. I think that would interfere with the audio signal anyway. I wouldn't think that a SS effects loop would suck much current. Not that I know a darn thing about 'em.


Peeked at the data sheet; looks like they draw a max of 30 mA.

[sluckey]

You can't use the plate of a 12AX7 as a source of power for your project.

My point is you can't use the plate of a 12AX7 as a source of power for your project. Because... the plate is in the signal chain (unless maybe it's a cathode follower. However, you can use filtered B+ that's available on the other side of the plate load resistor. There's a difference! Perhaps that's what the OP meant to say???

 

[2deaf]

300V on a 12AX7 plate seems a little high for typical amps but really typical for the HT node that is supplying that 12AX7.

If this is a kit, they rarely draw any more than 4mA (unless you have the very poorly designed one by some guy named Soldano).  Metropoulis draws 1.3 to 1.7mA depending on the HT voltage you give it.

They typically recommend tapping into the amp HT at the PI or the screen node, although Mojotone recommends the final filter cap after the last dropping resistor.  A 10K dropping resistor and a 10uF filter cap are sometimes included on the kit board.

[SolidStateCircuitReject]

Thank you guys for the answers... so I was pulling the power from the wrong side of the resistor  - I assumed something was wrong, hence the question. Usually for tube loops I pull power from the B+ that feeds the PI in marshall based amps but I was a bit confused with this amp in particular since it is something of a mutt design... some dual rec/SLO values in the pre, power mostly based on a JCM800/JMP 100 watt design... including the Power tranny, so values at B+ do not reach under 400 until dropping resistors for tube feeds. Hence I stupidly tried to pull it from a tube plate.. smack in the middle of the signal path 

At the 3rd B+ node I am getting a reading of 415VDC. So I will tap in there, put a 100K/2W resistor to take it down to hopefully in the ballpark of 300VDC +/- for the effects loop feed. Does anyone foresee an issue with that scheme?

Thank you for the informative answers!!!

[sluckey]

In addition to the dropping resistor you will need a filter cap after the resistor. 22 to 47µF should be sufficient.

[Ed_Chambley]

If this is the Mojotone loop, certainly break into the B+.  Metro loop is similar.  This loop will change the tone of the amp, either one. The LND150 is not really made for audio, but does work.  It is more with switching duties.


There is good discussion here:


http://ampgarage.com/forum/viewtopic.php?f=6&t=29741


The guys take about the different ones and how to tweak them for the voltages and how to get the most transparent tone from them.  Worth the tie to read the thread IMO.

[2deaf]

At the 3rd B+ node I am getting a reading of 415VDC. So I will tap in there, put a 100K/2W resistor to take it down to hopefully in the ballpark of 300VDC +/- for the effects loop feed. Does anyone foresee an issue with that scheme?

It would probably help if you told us what kit you have.  Tube Town is the only one I see that specifically calls out 250V to 400V for the supply voltage.  I'm not familiar with the TT loop, but if it is a source-follower type, 100KOhms is too large for a dropping resistor.  At 250V, a source-follower loop is likely to draw 3mA, so a dropping resistor of 55K/1W would be indicated.  If aiming for the middle of the supply range, 27K might be a good first try.   

[purpletele]

If this is the Mojotone loop, certainly break into the B+.  Metro loop is similar.  This loop will change the tone of the amp, either one. The LND150 is not really made for audio, but does work.  It is more with switching duties.

There is good discussion here:

http://ampgarage.com/forum/viewtopic.php?f=6&t=29741

Good timing Ed,

I bought a Mojotone FX Loop for my Gies Audio Dual Watt, which has a dry AB763 with EL84's.

I need to determine if I should use trim pots, chassis pots, no pots but jumpers. 

I'll check out the link

The guys take about the different ones and how to tweak them for the voltages and how to get the most transparent tone from them.  Worth the tie to read the thread IMO.

(Untangled quote; Willabe)

[SolidStateCircuitReject]

At the 3rd B+ node I am getting a reading of 415VDC. So I will tap in there, put a 100K/2W resistor to take it down to hopefully in the ballpark of 300VDC +/- for the effects loop feed. Does anyone foresee an issue with that scheme?

It would probably help if you told us what kit you have.  Tube Town is the only one I see that specifically calls out 250V to 400V for the supply voltage.  I'm not familiar with the TT loop, but if it is a source-follower type, 100KOhms is too large for a dropping resistor.  At 250V, a source-follower loop is likely to draw 3mA, so a dropping resistor of 55K/1W would be indicated.  If aiming for the middle of the supply range, 27K might be a good first try.

Yes, it is the tube town one - and yes 100k was way to much. 56k put me right at around 300VDC. I have no however added a 22-47µF cap... yet.

[2deaf]

Yes, it is the tube town one - and yes 100k was way to much. 56k put me right at around 300VDC. I have no however added a 22-47µF cap... yet.

I don't think you need a 22-47uF capacitor because there is already a 10uF/450V capacitor on the board that most likely already serves that purpose.  Track it down to see if connects to the B+ input.

[2deaf]

I bought a Mojotone FX Loop for my Gies Audio Dual Watt, which has a dry AB763 with EL84's.
I need to determine if I should use trim pots, chassis pots, no pots but jumpers. 

If you use a 1M send trimmer, your input impedance will range from 333K to 1M as you adjust the setting.  This may or may not be a consideration for you.

The return trimmer involves a long story, but the skinny is "don't do it".

The Mojotone loop has evolved (a little).  It used to have a 470K/1M voltage divider in front of the Gate on the Return LND150.  This is good to knock the signal down to a size that can be better amplified by the LND150 as it is configured.  But it is bad for the frequency response with the highs starting to roll-off at 700Hz.  The LND150 claims a reverse transfer capacitance of 0.5pF typical and 1.0pF max.  But look at the conditions -- V_GS=-10V, V_DS=25V, f=1MHz.  At the conditions we use these things at, the reverse transfer capacitance is way higher and the Miller Effect is a serious problem.  So Mojotone replaced the 470K resistor in the voltage divider with a 0.1uF cap. and the frequency response  straightened right up.  However, it now takes only a small signal to get a grossly distorted and clipped signal out of the Return LND150.  If you look at the Output Characteristics chart for the LND150, it is apparent that you cannot get enough degenerative feedback from the Source resistor to clean-up the signal before the negative side (positive side at the Drain) clips.  Anyway, what you would want to do is to always turn the output on your effects device down low and take the maximum signal off of the LND150 Drain.  If you had a trimmer on the output of the loop, it would always be set to max. to get the best performance out of the loop.       

[smudge]

Hi guys - I'm resurrecting an old thread and wondering if anyone could tell me whether this loop could be powered with a B+ of only 220v? I'm using the Tube Town version and I realise it asks for a B+ of 250-400vdc. My project idea is to create an external charge pump from 12v to 220v and I'm wanting to power the loop externally from this.

I realise the LND150 has a max of 500v but I can't work out what the absolute minimum is. What happens if I try and power with 220v at about 15ma?

Could anyone help me?

Thank you

[PRR]

Has anyone posted a link to this Tube Town version?