Mosfet fx-loop

[ciambom]

Hello,

I'm designing a mosfet-based fx-loop to put in my diy tube amp.
I used high voltage mosfets IRF820 because the can be supplied by the same voltage of the tubes.
The purpouse is to make an fx-loop that can work well with ALL stompbox effects, not only choruses and delays but also overdrives and boosters.
The fx-loop must be serial and must work with all amp types, no-master volume amps too.

This is a drawing of it:


I tested my fx-loop on an Orange TT clone putting an overdrive and a graphic eq with booster in it.

Problem 1:
With the overdrive off, the booster increases the volume, with the overdrive on the booster only increases the saturation, that mekes me think to a possible mosfet saturation.

Problem 2:
With both the overdrive and the booster on, the gain at 3/4 and the master at 3/4 the amp starts to oscillate (larsen)

The same pedals work well in the loop of a JCM900 Dual Reverb and a BlackStar BH100H.

If anyone have an idea of possible circuit improvements I'll be grateful.

[defaced]

As a source follower, the IRF820 will be fine, so using it in the send is going to work. In the return, you have this setup to amplify and send signal off of the drain. The large input capacitance has the possibility of killing your high end. You may want to consider other devices for this. ZVN0545 and the LND150 are two devices that would do this one well.

Dumb question, why are you putting dirt boxes in the loop? Most people use those to overdrive the front end of an amp.

Where is R10 (15K) soldered? If it's purpose is to be a gate stopper, it should be soldered as close to the gate pin as possible. This could be part of your oscillation problem.

And as always, grounding and lead dress apply here and can cause oscillation problems.

If you haven't read RG Keen's stuff on HV MOSFTES, MOSFET Follies, over at GEO FX, take the time to read it. Good stuff. Also put MOSFET Follies into the search here to get some good discussion on the topic.

[pdf64]

If you want the loop to work well with all amps/pedals, then 0.091 potential divider at the input may put too high a signal level out to the loop, eg 60Vrms may be available, sending 5.45V out.
1/ Did you patch the booster before or after the overdrive?
2/ What (roughly) is the oscillation frequency - low bass motorboating or trebly screech?
Pete.

[ciambom]

As a source follower, the IRF820 will be fine, so using it in the send is going to work. In the return, you have this setup to amplify and send signal off of the drain. The large input capacitance has the possibility of killing your high end. You may want to consider other devices for this. ZVN0545 and the LND150 are two devices that would do this one well.

Dumb question, why are you putting dirt boxes in the loop? Most people use those to overdrive the front end of an amp.

Where is R10 (15K) soldered? If it's purpose is to be a gate stopper, it should be soldered as close to the gate pin as possible. This could be part of your oscillation problem.

And as always, grounding and lead dress apply here and can cause oscillation problems.

If you haven't read RG Keen's stuff on HV MOSFTES, MOSFET Follies, over at GEO FX, take the time to read it. Good stuff. Also put MOSFET Follies into the search here to get some good discussion on the topic.

I answer to your questions:

1) I heard about these components (ZVN0545 and LND150 ) but it seems to be impossible to find them in Europe (or at least in Spain).

2) by putting the OD box in the fx-loop I can get a tone which is impossible to reach putting it in front of the amp. The graphic eq/booster must be put in S/R to get an increment of volume for the solos.

3) the 15K grid stopper is very close to the gate, this is not a problem.

[ciambom]

I think this is the problem too.
Hello Pete,
I supposed a signal from the preamp of 30V peak-to peak. But I don't have idea about how to calculate it.
I don't have an oscilloscope at home and I can't measure it.

I answer to your questions:
1) the booster is after the OD
2) the oscillation is treebly

Now I have two questions:
1) how big should the potential divider be? I see the Plexi reissue 1987x has a potential divider of 0.066, do you think it works?
2) do you think I should increase some caps values?

[pdf64]

Signal level - an overdriven 12AX7 on a 300V supply may have around 60Vrms on the plate, whereas after the tone stack, the signal level will reduce to under 20V (depending on the tone stack values). So a 'one design for all applications' won't give the best results for all applications. Also pedals may react very differently to different signal levels, eg compressors, overdrives. Therefore it's best to make the 'send' level variable or adjustable somehow.
Oscillation - may be positive feedback via the power supply. Good practice is to feed the loop from it's own node (dropper resistor / cap) and to bypass the electrolytic cap with a film / ceramic cap, around 0.01 to 0.1uF. If still a problem then feed the 'send' and 'return' circuits from their own individual nodes.
Also, are you powering the pedals from batteries or a power supply? If power supply then try the overdrive from a battery.
Pete.

[defaced]

I answer to your questions:

1) I heard about these components (ZVN0545 and LND150 ) but it seems to be impossible to find them in Europe (or at least in Spain).

Try the Spanish version of Mouser: Mouser Electronics - Electronic Component Distributor

[J M Fahey]

Fully agree with above postings and add: stompboxes are designed to be used with straight guitar signal levels, say 50 to 100mV RMS.
Peaks and transients may reach 1000 mV if strumming hard.
1) The *first* triode , typically 1/2 12AX7, raises this 50X, so we have 2.5 to 5 V RMS, with 50V peaks.
*No* battery powered pedal can handle that. Period. Not even an 18V powered one.
2) Of course, you can pad that signal 50X , let the Pedal do its magic, and re-amplify it 50X again with a MosFet.
Which brings up the question: why involve that 12AX7 at all?
Why not just feed the guitar straight into the pedal and then into a high voltage Fet preamp?
Because that's what you are essentially doing.
Going through a clean triode, very far from oits overload point, is practically "transparent" , meaning it does almost nothing to the signal.
Maybe it introduces a *very* subtle nonlinearity or distortion, 1% or less at those levels.
That's why adding working loops into clean tube preamps does not pay up too much, at least in my book.
3) In *distorting* tube preamps, such as Soldanos, Mesas, Laneys , modern Fenders (HRD, etc.) yes, the loop lets you place "clean" effects after that tasty distortion and are useful.
4) When you say "my pedal sounds very different there than plugged up front" yes, you are right, it sounds as different as it will by hitting it with 5X the original signal, even with your 10X attenuation.
The 15X reamplification might introduce its own quirks too.
Might even sound good or better, although I guess many won't take it gracefully.
5)

the booster only increases the saturation, that mekes me think to a possible mosfet saturation.

With both the overdrive and the booster on, the gain at 3/4 and the master at 3/4 the amp starts to oscillate (larsen)

A 9V powered booster can put out 3V RMS *tops*, clipping beyond that.
If you slam it with that signal, or even a somewhat higher one, it won't boost level (it can't) but will add its own gain; that may cause your Larsen effect (European name for acoustic feedback).

The same pedals work well in the loop of a JCM900 Dual Reverb and a BlackStar BH100H.

I find the SS JCM900 preamp a "big, +/-15V powered pedal"; don't remember the Blackstar schematic but they will certainly drop and reamplify signal level properly
6) Where did you put it on yout Tiny Terror? The real distortion there (and part of its charm) is generated by overdriving the Phase Inverter, at the end of the signal chain, we are talking on the order of 100V RMS there and unfortunately, *after* any practical Loop.
7)

If anyone have an idea of possible circuit improvements I'll be grateful.

Yes, clone the JCM900 !!!!!!!
No, I'm kidding, but consider cloning the Blackstar Preamp, or at least a simpler JCM800 type preamp, where you can drop what the Master Volume sends to pedal levels, and then reamp to drive the output stage (which might still be a 12AX7 and a couple EL84 if you want to keep it small) ; you would still keep the Tube magic.
Good luck.

[ciambom]

Thank you Pete.
The Tiny Terror doesn't have tonestack, so I think I have the whole 60Vrms.
For the oscillation I don't think it is due to the power supply because I put the dropper resistor and the cap but it's the same. I could add the "small" cap, I'll try..

[ciambom]

I know that the TT has the PI cranked all the time, but also in a master volume amp, a well designed fx-loop should work with the master at max too.

[ciambom]

Thank you Mike, Mouser has got it!!

[GainFreak]

Hi,

I recently built the MOSFET loop from Marshall 2466:



I ran into couple of problems.
1/ I'm using BS108 instead of TN2404/BS107. Also instead of J202 I have BF245B (I also have by hand some J201, MPF102, 2N5457 and 2N5484). I'm running the loop from the screen supply (410V) with a 56k resistor at 175V.
2/ The return stage wasn't passing signal. I changed the 220k resistor (R31) to 100k and it passes signal now. I have around 63V at FET2 drain and around 2.8V at FET1 source and 26.5V at FET1/2 D/S connection.
3/ I have a loss of low end when switching the loop on and off. It's not much but it's noticeable. The loop is between TS Treble taper and the Master
The 27k/2n2 combo (R52/C68) adds some brightness which made me disconnect it from the circuit. Everything else is per schematic.
I'm not very much into transistors and stuff so I would appreciate your help concerning this problem. I'm not sure whether the voltages are OK or if other types of FETs from the listed above would be more appropriate in this schematic.

[Steve Conner]

The 63V is a bit low. I suggest trying a JFET with a lower Idss, like the J201, 2N5457 or even the BF245A.

The fastest RC time constant I can see in the circuit is 12.2k/1uF, which is -3dB at 13Hz, so I don't see any cause for a loss of bass. What value was your master volume pot? If it wasn't 1M the loading on the tone stack will be different, as the input impedance of this circuit is a little under 1M.

You can boost the bass by making C21 smaller, since it provides negative feedback around FET1/FET2

[GainFreak]

The 63V is a bit low. I suggest trying a JFET with a lower Idss, like the J201, 2N5457 or even the BF245A.

The fastest RC time constant I can see in the circuit is 12.2k/1uF, which is -3dB at 13Hz, so I don't see any cause for a loss of bass. What value was your master volume pot? If it wasn't 1M the loading on the tone stack will be different, as the input impedance of this circuit is a little under 1M.

You can boost the bass by making C21 smaller, since it provides negative feedback around FET1/FET2

The Master is 1M. Actually I tried to increase the C21 while thinking about what might be the cause but no change. Will have to try decreasing it.
Last thing I did was to feed the return stage sine wave signal and see what happens with the wave form (no real scope, only software using the sound card).
I had to tweak R32 value (BF245B was still there) to around 3k until I got nearly perfect sine wave. Before the bottom was not quite symmetric to the top. After that I measured 94V at FET2 drain (plate resistor 100k).