ODS #004 FET

[Bombacaototal]

I recently got very curious about the FET on those old ODS amps. I think the one on #004 may be somewhat similar to Lindley #008 (which the man himself would not let HAD update it at all).

The topology is very similar to what we are used to, from later interations, but the two main differences (besides some values) are the 68K (on a switch) and the 9.2K, interestingly enough I do not see a FET switch on the amp.

I know this schematic was full of mistakes, but wondering if anyone has attempted this. I am not a big fan of the FET in general and I am hoping I will find the magic with Lindley's set up, as he famously only used his amp in this mode.

Has anyone attempted this? If not I may be the guinea pig on this one...

[erwin_ve]

I didn't try it yet, maybe on a similar manner there is a very interesting thread from Charlie Wilson:
https://ampgarage.com/forum/viewtopic.php?f=4&t=34108

Later added by Martin Manning's bias procedure with added values and fet response to the values used in Charlie Wilson's thread.
This is very interesting because the way Dumble used it is way to create harmonic distortion. In the Ods this is done with tubes with high plates creating also harmonic distortion. They differ sound wise but Dumble was definitely going for it with the Fet voltage and bias.
You can see Martin's simulations here:
https://ampgarage.com/forum/viewtopic.php?f=5&t=18389

[Bombacaototal]

I didn't try it yet, maybe on a similar manner there is a very interesting thread from Charlie Wilson:
https://ampgarage.com/forum/viewtopic.php?f=4&t=34108

Later added by Martin Manning's bias procedure with added values and fet response to the values used in Charlie Wilson's thread.
This is very interesting because the way Dumble used it is way to create harmonic distortion. In the Ods this is done with tubes with high plates creating also harmonic distortion. They differ sound wise but Dumble was definitely going for it with the Fet voltage and bias.
You can see Martin's simulations here:
https://ampgarage.com/forum/viewtopic.php?f=5&t=18389

Many thanks Erwin, interesting read, will go in more depth during this week. I am honestly tempted on trying that early ODS arrangement. My last two attempts at more obscure circuits (Borderline and SSS#001 temolo) didn't yeild in results, as they didn't work, hope this one will be worth the extra effort. Should I make the 68K Switch external or internal? I am not sure how much of a difference adding the series resistor will make.

On a separate note, SSS #002 FET used a F1175, would the 2N3823 be an equivalent replacement? I have a FET board set with #002 values (per Jap schematic), but with the 2N3823 JFET. Will try the battery bias method to see if it works

EDIT: Just tried, I get 12.42V measuring both sides of 8.2K and 9.13V measuring both sides of 10K (schematic attached for reference)

[erwin_ve]

I don't know exactly how the 68k is switched, looks like the voltage to the fet is grounded when not in use?
For the sss002 you can use various kinds of Fet, but many are unobtainable.
The ones I use are 2N4416A and 2SK117. Both sound good.
The Sss002 handdrawn schematic has a error: 3k3 on the entrance has to be 3M3.

Edit you measured the voltage from reference points to ground I hope?

[Bombacaototal]

I don't know exactly how the 68k is switched, looks like the voltage to the fet is grounded when not in use?
For the sss002 you can use various kinds of Fet, but many are unobtainable.
The ones I use are 2N4416A and 2SK117. Both sound good.
The Sss002 handdrawn schematic has a error: 3k3 on the entrance has to be 3M3.

Edit you measured the voltage from reference points to ground I hope?

Thanks Erwin, yes I was wondering about that ground point as well.

Thanks for the note on the 3.3M, I indeed used the correct value. I think the 2N3824 worked just fine. A picture of my bias voltage using the values of 002

EDIT: 8.2K was indeed measured to ground, but the 10K I did exaclty as per Martin picture, find mine below

[Bombacaototal]

One other thing I have been thinking about is polarity, an extra uneven gain stage added by the FET will make the input and output of the amp out of phase. I presume users of FET will use the speakers in reverse polarity, which is probably why HAD didn't have the FET footswitchable?

I am wondering if this may be part of why I don't like the FET circuit...

[erwin_ve]

One other thing I have been thinking about is polarity, an extra uneven gain stage added by the FET will make the input and output of the amp out of phase. I presume users of FET will use the speakers in reverse polarity, which is probably why HAD didn't have the FET footswitchable?

I am wondering if this may be part of why I don't like the FET circuit...

First: get the bias correct and then worry about speaker phase

When adjusting Rs you can aim for around 6,2v across Rd (half of 12,4) which should be more in line with the voltage ratio's Martin posted and the SSS002 schematic voltages.

[Bombacaototal]

One other thing I have been thinking about is polarity, an extra uneven gain stage added by the FET will make the input and output of the amp out of phase. I presume users of FET will use the speakers in reverse polarity, which is probably why HAD didn't have the FET footswitchable?

I am wondering if this may be part of why I don't like the FET circuit...

First: get the bias correct and then worry about speaker phase

When adjusting Rs you can aim for around 6,2v across Rd (half of 12,4) which should be more in line with the voltage ratio's Martin posted and the SSS002 schematic voltages.

Thanks Erwin, I am doing the measurements again, one of the batteries was measuring 4.6V only, probably why my measurements were all over the place. Now with 2 new batteries

My voltage divider R1 is 8.2K (reduced headroom and increased distortion) and is measuring 18.76V (which is basically the batteries together). My 2.7k is Rs (from Martin diagram) and is measuring 3.839V. I need to get this to 6.2V then. I am getting 14V across the 10K. Because I am going for the low VD maybe I need to wait until amp is done to do the B+ method

Do you think this discrepancy to the original is due to my choice of FET?

[erwin_ve]

Rafael: the Rs is controlling the current and therefore the voltage across Rd.
A Fet has differences in specs even when it is the same model. Just like a output tube.

If you replace the Rs with a 10k pot, you can successfully bias the fet. Then measure the setting of the pot and use a resistor closest to the pot setting

[Bombacaototal]

Rafael: the Rs is controlling the current and therefore the voltage across Rd.
A Fet has differences in specs even when it is the same model. Just like a output tube.

If you replace the Rs with a 10k pot, you can successfully bias the fet. Then measure the setting of the pot and use a resistor closest to the pot setting

Thanks Erwin, makes sense now! I just ordered a 10kb pot, didn't have any left. Will let you know of the outcome once I have done the exercise

[Bombacaototal]

Finally got some time to go through the bias. I decided to hardwire a 10kb trimmer to make life easier. I get a bit over 9V across the 10K resistor, but the interesting thing is that my trimmer is a bit over 9K (wired in), seems a bit high, no!?

[martin manning]

I get a bit over 9V across the 10K resistor, but the interesting thing is that my trimmer is a bit over 9K (wired in), seems a bit high, no!?

You are mixing things up a bit there. Using 18V (2x 9V batteries) to set the bias and dropping half of Vdd across Rd is for the max clean boost set-up, assuming a 150k/18k divider from ~300V. Here I see 220k/8k2 divider, which will produce a lower Vdd. Per the FET bias write-up, I would connect the FET board to the amp's high voltage and then adjust the Rs trimmer to set the bias.

[Bombacaototal]

I get a bit over 9V across the 10K resistor, but the interesting thing is that my trimmer is a bit over 9K (wired in), seems a bit high, no!?

You are mixing things up a bit there. Using 18V (2x 9V batteries) to set the bias and dropping half of Vdd across Rd is for the max clean boost set-up, assuming a 150k/18k divider from ~300V. Here I see 220k/8k2 divider, which will produce a lower Vdd. Per the FET bias write-up, I would connect the FET board to the amp's high voltage and then adjust the Rs trimmer to set the bias.

Thanks for the help Martin and for clarifying, makes sense indeed. With the 220k/8.2K the amp had 2.7K cathode resistor, so I imagine it will be somewhat around this. With the trimmer there it will be no problem to adjust it once in the amp, and with the B+ connected.

[Richard1001]

In the posted schematic above, i think the switch is in the jack as indicated by the dotted line. The 68k is probably the resistor to V1, but likely drawn by someone who didn't know much about electronics...

For my own amp i added a resistor after the Fet's coupling cap and a small 10p cap to eliminate RF. Maybe for some this could be helpfull getting rid of noise / radio interference.

[Bombacaototal]

In the posted schematic above, i think the switch is in the jack as indicated by the dotted line. The 68k is probably the resistor to V1, but likely drawn by someone who didn't know much about electronics...

For my own amp i added a resistor after the Fet's coupling cap and a small 10p cap to eliminate RF. Maybe for some this could be helpfull getting rid of noise / radio interference.

Great point on the 68K Richard! I think you are correct indeed.

Also thanks for sharing your mods. I personally never had RF issues with the stick design thou