ax84 SEL with FX Loop

[adamG]

Hi Guys,

I built the amp itself approx.1,5 year ago. It's really cool one. It has the option of Lead/Rhythm.
But now, I struggle how to get married the amp with attached FX Loop?!
It doesn't work both after the Treble and MV. In A input the signal is too weak. In B is too strong. Anyway, in both inputs the sound is unpleasant, distorted in hights, in tight frequency.
Could you advice where the loop circuit combine within the amp?


Thank you in advance.

Regards,

Adam

[PRR]

> In A input the signal is too weak. In B is too strong.

There's hardly any difference, A or B. Maybe 2:1, which is not a large difference in musical volume. Wrong resistor value?

> in both inputs the sound is unpleasant

Voltages on MOSFETs, all three pins.

[2deaf]

Under the best of conditions, the driver MOSFET will clip at or around 25V_p due to the biasing which puts a limit of 50V_p on input A and 25V_p on input B.  Under the worst of conditions, the recovery will clip at or around 3V_p putting a limit of 6V_p on input A and 3V_p on input B.  So it would be very easy to overdrive this device no matter where you place it.

Since the driver can only swing 25V_p, placing the T-T Loop after the Treble control is out of the question and it would be better after the Master Volume.  The Master Volume will become the Effects Send control, the T-T Effects Send control should be permanently set to max., and the T-T Effects Return control will become the Master Volume.

Since the original 1M Master Volume will now be operated near its minimum, you might replace it with a 750K resistor in series with a 250K potentiometer that goes to ground.   

[adamG]

Voltages on MOSFETs, all three pins.

I didn't measure mosfet's voltages. It was late yesterday. Today, I'll do it and report the score.

Thank you.

Regards,

Adam

[adamG]

So it would be very easy to overdrive this device no matter where you place it.

Since the driver can only swing 25V_p, placing the T-T Loop after the Treble control is out of the question and it would be better after the Master Volume.  The Master Volume will become the Effects Send control, the T-T Effects Send control should be permanently set to max., and the T-T Effects Return control will become the Master Volume.

I'm afraid, you're right. First, I put the loop after MV with same tonal result as after Treble...

Since the original 1M Master Volume will now be operated near its minimum, you might replace it with a 750K resistor in series with a 250K potentiometer that goes to ground.

I forgot to mention, my version works with 6L6 as power stage. Do you think I should swap the MV from 1M to other value, also?
Do you mean MV circuit as first 750K resistor after Treble and then 250K pot going to ground?
I also applied dpdt switch to on/off the loop. So, I can easily hear tonal differences. If I provide above solution of resistor and pot, how the amp works with the loop off?

I decided to provide this loop, because it works excellent in previously built D_uble SE HRM. So, I hoped that in this circuit It would be working, as well;)
Any other ideas?

Thank you and regards,

Adam

[2deaf]

It's different if you are using a bypass switch, so disregard the 750K/250K pot idea.  There is still the problem with the Send driver clipping.  Attached is an idea that will prevent this.  You should confirm that there is 470K between Input A and Input B and that there is 470K from Input B to ground when nothing else is connected.  If you have enough B+ voltage and you cannot adjust the Return Level so that there is no distortion, then there is probably something wrong with the kit.

[adamG]

You should confirm that there is 470K between Input A and Input B and that there is 470K from Input B to ground when nothing else is connected. 

I confirm above.

If you have enough B+ voltage and you cannot adjust the Return Level so that there is no distortion, then there is probably something wrong with the kit.

I am afraid, I do not get this "enough B+":) Could you further explain this, please?
There was no kid. My friend was very excited the whole idea of mosfet loop. So, He searched for the T-T schematic and then made his own pcb project. Later on, I'll make few pisc of it within the amp. For now, you can see the amp here http://darius-amps.pl/en/plexi-se/

Thank you very much for Temp Schematic! Are you sure that A in is the high one? Or, as I understand, both inputs will be converted acc.to your schematic, right?

Regards,

adam

[2deaf]

I am afraid, I do not get this "enough B+":) Could you further explain this, please?
There was no kid. My friend was very excited the whole idea of mosfet loop. So, He searched for the T-T schematic and then made his own pcb project.

Thank you very much for Temp Schematic! Are you sure that A in is the high one? Or, as I understand, both inputs will be converted acc.to your schematic, right?

Tube-Town sets the minimum B+ at 250V and I have had trouble with LND150 recovery stages with an E_bb of 150V.  I just wanted to make sure that you weren't having clipping due to a low B+.

I suggested the external 220K resistor as a non-invasive fix for the actual kit.  There may be a language problem here, so I attached a schematic of what I meant.  You would connect the signal from your bypass switch to In A on the schematic.

 

[adamG]

I see. No, power of B+ voltage is quite enough. It's approx. 317VDC. On plate I have 325V. So, it seems ok.
Tomorrow, I'll test the solution.
Meanwhile, thank you very much!

Kind regards,

Adam

[adamG]

Hi,

I'm afraid, there's no possitive report.
The amp is quiet with signal put on In A, together with external res.220k to ground on In B.
Here's measured voltages:
Q1-D 313VDC, S 1,62VDC;
Q2-D 211 VDC, S 0,32VDC.
What can We do now?
I went through several circuits of cathode follower and notced other points loop or reverb connection.
For example, input from second stage and output on cathode follower grid.
What do you think?

Regards,

Adam

[2deaf]

Here's measured voltages:
Q1-D 313VDC, S 1,62VDC;

A 1.5K bias resistor on a LND150 usually results in a I_D of 0.42mA or so.  That would make your Source idle voltage around 43V_dc and you are showing 1.62V_dc.

[adamG]

You seem to be right, but there's another 100k in series with 1,5k.
Same small value in Q2 -S.
It's possible I soldered  both mosfets wrong. I mean changed over D's and S'. But, this loop is not the first one...
Anyway, I'll check them again.

Regards,

Adam

[2deaf]

Same small value in Q2 -S.
It's possible I soldered  both mosfets wrong.

A 390r bias resistor usually results in 0.89mA which would give 0.347V on the Source of Q2.  0.32V is pretty close and just a 5% deviation from 390r would account for the difference.  Variations in LND150's could also account for some or all of the difference.  In any event, the voltages on Q2 look normal.

[adamG]

I went through schematic and I'm rather sure I did not make any mistake with both mosfets soldering.
Anyway, today I'll double check resistors on Q1. Maybe, I mislead some of them

Another issue, idea. I've got spare hole on chassis for tube. What do you think of tube based fx loop, if I'm not successful with mosfet one?

Thank you very much and regards,

Adam

[2deaf]

I went through schematic and I'm rather sure I did not make any mistake with both mosfets soldering.
Anyway, today I'll double check resistors on Q1. Maybe, I mislead some of them

Another issue, idea. I've got spare hole on chassis for tube. What do you think of tube based fx loop, if I'm not successful with mosfet one?

Did you ever find out what was wrong with the MOSFET loop?

I designed a tube based fx loop for your consideration.  It can drive a home recording line level device that has an input impedance as low as 25K with 7V_rms at the Treble wiper.  It can also be used with stomp boxes.  The Send can drive 16.6m of cable with a cut-off frequency of 15KHz.

[adamG]

Hi,

I've just noticed you prepared the schematic for my need:)
Thank you very much!
I was silent for some time, because this amp is not my main build. Actually, I work on several others. Crazy me;)
Very soon, I'll get back to this one with "standard" Dumbleator circuit testing within.
I meant this circuit ,when I mentioned tube based fx loop.
Do you think D style fx will work fine within this amp? Otherwise, I'll have to get back to your schematic. Anyway, I'll take further look on your design, as well.

Thank you and regards,

Adam