Matt J wrote: ↑Sun Jun 22, 2025 3:52 pm
Everything has been ok so far, except for the output jacks. I keep getting a connection to ground at the positive lug of the main and external jacks where the 4 ohm tap from the OT connects and the NFB line feeds back to the preamp board.
No problem here. That's perfectly normal. Your meter is reading the very low resistance of the OT secondary winding which is just a fairly short length of fairly heavy wire. Put your Switchcraft 12A jack back in and wire the switch lug to the sleeve(ground) lug.
Gotcha!
I'm going to work on wiring up the power supply board later today and post updates from there.
Thanks!
Matt J.
Update:
Wired up the power supply, etc, and gave it a smoke test with my current limiter.
Everything came out ok, no pops or bangs. Voltages are look good.
Martin, your biasing mod works great. Dead center noon on the bias pot puts both 6L6GCs at ~55% max plate dissipation.
The only issue I am having is with the FET. That was my greatest concern going into this since I normally omit it all together. Switching it on works fine, but instead of a boost there is a noticeable volume drop and the tone is very thin and almost nasally. I'm wondering if it is an issue with the FET bias, so I've been checking the voltages. I may have missed something when using the biasing procedure. I'm getting a ~10.5 VDC drop across the 10k resistor on the board with ~2.6VDC going to the FET's drain. I still have Ervin's PCB FET board with the parts to make it, so I'm going to put it together as a back up, install it, and compare the two.
Matt J.
Tried to get a picture of the FET board to see if there are any wiring errors, but it is in an odd spot in the chassis so this was the best I could get.
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Matt J wrote: ↑Wed Jun 25, 2025 3:29 pm
Martin, your biasing mod works great. Dead center noon on the bias pot puts both 6L6GCs at ~55% max plate dissipation. 😎
Fantastic! The chassis wiring looks great, too.
Matt J wrote: ↑Wed Jun 25, 2025 3:29 pm
The only issue I am having is with the FET. That was my greatest concern going into this since I normally omit it all together. Switching it on works fine, but instead of a boost there is a noticeable volume drop and the tone is very thin and almost nasally. I'm wondering if it is an issue with the FET bias, so I've been checking the voltages. I may have missed something when using the biasing procedure. I'm getting a ~10.5 VDC drop across the 10k resistor on the board with ~2.6VDC going to the FET's drain. I still have Ervin's PCB FET board with the parts to make it, so I'm going to put it together as a back up, install it, and compare the two.
Matt J wrote: ↑Wed Jun 25, 2025 3:29 pm
Martin, your biasing mod works great. Dead center noon on the bias pot puts both 6L6GCs at ~55% max plate dissipation.
Fantastic! The chassis wiring looks great, too.
Matt J wrote: ↑Wed Jun 25, 2025 3:29 pm
The only issue I am having is with the FET. That was my greatest concern going into this since I normally omit it all together. Switching it on works fine, but instead of a boost there is a noticeable volume drop and the tone is very thin and almost nasally. I'm wondering if it is an issue with the FET bias, so I've been checking the voltages. I may have missed something when using the biasing procedure. I'm getting a ~10.5 VDC drop across the 10k resistor on the board with ~2.6VDC going to the FET's drain. I still have Ervin's PCB FET board with the parts to make it, so I'm going to put it together as a back up, install it, and compare the two.
I think you're on it... Vd is way too low.
Gotcha! What Vd should I am for when I retest?
Being a huge David Lindley fan, I wanted to build this in honor of him since those El Rayo-X tones he gets are my favorite ODS tones. I know he ran into the FET input, but don't know how he has his set. I did change the treble pot value from 500kl to a 250k j taper pot, plus omitted the 22nf pot across the bass pot wiper and CCW. I read a lot that omiting them makes the eq controls a little more active.
I plan to keep this amp set for overdrive tones with the FET kicked in for a lead boost.
Matt J.
You want to get the voltage at the drain to be 1/3-1/2 of the voltage at the top of the drain resistor. 1/2 is ~center biased, 1/3 will give you a bit more FET distortion. It seems like that's where HAD liked it based on measurements posted by CW and shown in traced schematics (i.e. SSS 002). Running On Empty tones are killer. Saw him play that live back in the day ;^)
martin manning wrote: ↑Wed Jun 25, 2025 4:47 pm
You want to get the voltage at the drain to be 1/3-1/2 of the voltage at the top of the drain resistor. 1/2 is ~center biased, 1/3 will give you a bit more FET distortion. It seems like that's where HAD liked it based on measurements posted by CW and shown in traced schematics (i.e. SSS 002). Running On Empty tones are killer. Saw him play that live back in the day ;^)
Gotcha, just with ~13.45 vdc at the base of the 10k resistor I need to adjust the 10k to get about ~4.33 vdc at the drain?
Yes. But maybe a better way to say it is make the ratio of the voltage at the top of the drain resistor about 2.5 times the voltage at the bottom (electrically speaking). The actual voltages will change depending with the bias setting since the source impedance of the supply is high, basically the150k dropping resistor.
Matt J wrote: ↑Wed Jun 25, 2025 6:48 pm
The FET is 2N3823 from Mouser if that makes any difference with biasing.
No difference. It's very similar to biasing a triode stage where you can center bias for max headroom 2:1 Vdd/Vd, or off-center for more distortion, ~2.5
Attempted the biasing method again with two 9V batteries wired for 18V on the new PCB board.
I get ~18.23 VDC at the base. Adjusting a 10k pot instead of a fixed resistor gets me to the 2.5x voltage of about 7.292 VDC when the pot is at 5.6k.
I do not have a 5.6K pot at the moment, but do have some 5.1Ks. Do you think that would suffice?
My worry is the actual B+ it will be receiving from the B+5 node might throw things off.
martin manning wrote: ↑Fri Jun 27, 2025 4:57 pm
Yes, it will move when you switch to B+ power. I would connect it to B+ and bias with that.
Connected the B+ up and tested the voltages.
The closest I got the 2.5x ratio was ~8.75 VDC on the bottom and ~8.45 VDC on the top with the pot measuring ~4K resistance. I do not have a 4K resistor in stock now, but could try to get close with paralleling some 8.2k resistors
I tried it with the 5.1K measured on the pot since that's the closest resistor I have in stock. That gave ~9.69 VDC on the bottom with ~3.28 VDC on the top with a 2.954x ratio.
Matt J wrote: ↑Sat Jun 28, 2025 12:28 am
Connected the B+ up and tested the voltages.
The closest I got the 2.5x ratio was ~8.75 VDC on the bottom and ~8.45 VDC on the top with the pot measuring ~4K resistance
I don't understand these numbers, but 3.9 out of 4k is only 2.5% off, so pretty close. Recall a ratio of 2 is ~center biased, and 3 is extremely off-center. I would say 2 to 2.5 is where you want to be depending on how much distortion you want to introduce.
