70s ODS with footswitchable FET

[Matt J]

My apologies Martin, I mistyped!
I should be that I got 8.75 VDC at the bottom of the 10K pot and 3.44 VDC at the top. So, a ~2.54x difference with 4k measured at that point on the pot.

I'm going to try the 3.9k, wire everything up, and see how it goes.

Thanks again for all your assistance, Martin!

Matt J.

[Matt J]

Update: I got the FET up and working. It sounded ok, but nothing that really jumped out to me. I preferred the amp on its own. I may look at a separate FET pedal in the future.

Otherwise, the amp is fantastic! I've removed the FET board and switching components and converting it to a single footswitch for the OD. I'm tweaking things right now, playing with different bright cap values and such.

Martin, your biasing system still works great. I've got both tubes running ~55% max plate dissipation. Once I've got things dialed in tone wise I'll work on balancing the PI. I balanced it earlier after getting it first fired up, but as it gets some playing time I'm see what may have drifted.

Next up will be putting the cab together. It's a natural finish pine cabinet I'm going to give a slight stain too. This will be my first time attaching grill cloth to a speaker baffle though so I'm a bit anxious about that and watching as many tutorials as possible.

Pictures and clips as I finish it up!
Matt J.

[martin manning]

Good to hear you got it working. You might have tried the higher voltage option, with 20V or so on the FET drain, but if you had it biased in that 2 to 2.5 Vdd/Vd ratio you probably got a decent idea of what it has to offer.

[Matt J]

I'm really enjoying tweaking the OD entrance trimmer and playing around with different settings.

I have notice that the Accent switch doesn't have as much impact as I thought it would versus a full presence control. Should I try a 2.2uf cap instead and does it have to be an electolytic or could I use a poly cap instead? I have a couple of those on hand.

Matt J.

[martin manning]

Poly cap is fine. The larger it is, the more the presence boost will move down into the mids.

[Matt J]

Finally had the time to finish it up last week. Been putting it through it's paces, testing out different settings and breaking it in.

• Removed the FET Board and Relay, left it just with the OD footswitch like the original was.

• Took the 50K OD Trimmer off the board and added a control along the back. Marked the 33K point on the control. I tend to leave it right around there.

• Picked up a head cab off of Reverb. I liked how it came out, but it was made for a Ceriatone chassis so it is slightly wide for this chassis.

• Currently running it with some Tung Sol 6L6GCs, EH 12AX7s in V1 and V2, and a TAD 12AT7 in V3. Got it biased around 55% max plate wattage dissipation.

• Right now it is running into my WGS ET65 shop cab, and 8 ohm speaker so I am shaving off a little headroom and getting a bit more compression running out of the 4 ohm OT tap.

It absolutely growls. No need to run it in clean mode. I just keep the OD on and fine tune it to that.
Matt J.

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[martin manning]

Looks great, Matt! You might consider putting th FET boost back in without the relay switching. It can produce some very useful tones.

[Matt J]

Thank you, Martin! I appreciate the kind words and all your helpful advice! (I love how that updated bias circuit works).

I didn't get any pics, but I did put the FET back in for a bit, on its own with the special input jack like on the originals. It was ok, but I wasn't really that impressed with it. I've tried including the FET in the past, but it just never clicked for me. I've tried some FET pedals that I liked a lot more because they were easier to fine tune. I may pick up another one for this amp, perhaps one of the VHT models. I did have a Cause and Effect FET Dream pedals that I loved and really regretted selling.

Matt J.

[TheC-Man]

This looks fantastic Matt! The red switches look really good. Thank you for sharing!

[dbharris]

Looks great! Are you liking the way it plays?

-Dan

[Matt J]

Looks great! Are you liking the way it plays?

-Dan

Thanks, Dan!
Here are my observations after about two weeks of playing it. I don't have any single coil guitar right now, just two Gibson Explorers with AlNiCo humbucking pickups, so it may be overdriving a lot earlier than you would with a Strat or Tele:

• The Overdrive is really dependent on how hard you are pushing the preamp compared to 102, 124, 183 or similar Overdrive Specials with a Ratio control. Leaving the preamp too clean causes the OD to be very anemic. I wasn't really looking for clean tones with this amp anyway and usually keep the OD engaged at all times. I just pull it back with my guitar's volume control. I wouldn't recommend this amp if you're looking for a clean/OD volume balance unless you added in a separate master volume like on a blues master. I can see why HAD started updating this circuit with the ratio control so there would be a better balance between the two.

• Having the OD trim on the back is a nice addition. It basically sets the floor for how minimal the distortion can get, but I tend to leave it around the stock 33K to ground mark on the pot's sweep.

• The B+ voltages are a little lower with the Pacific Audio transformers than what I normally would see with the Heyboer, Hammond, Mercury, or Classictone PTs I have used in the past. Right now at 120VAC I'm getting about 438 VDC on the power tube plates, about ~35 volts lower than what I usually saw when I built my 102, 124, and 183 ODS versions. You can feel it in the power amp when you push it, there's a certain compression and squish that comes along with those lower voltages.

• I've never had an ODS amp feedback so easily than this one.

• The amp, by nature, is very bright, so you have to learn how to tame it by playing with the treble control. The Accent switch adds more brightness than anything else. When it is engaged it almost feels like a preamp boost has been turned on. I ended up sticking with the .01uf original value. I discovered I had it wired wrong originally and that is what caused it to be so weak originally. If I go in and change anything it would be to play with that cap value to volume and treble jump is not so prominent when the accent is engaged.

• The preamp controls are VERY dependent on the preamp volume. You are not going to get much out of the bright or deep switch with the preamp volume down and, speaking of the deep switch, it is more of a mid shift when engaged and thankfully it does not leave the bass frequencies flabby or tubby.

Overall, I am very pleased with the amp. Setting out I knew I wanted to build something just for Overdrive and gravitated more and more towards the 70s circuit. I've tried building this amp a few times before but ended up over complicating it with too many added options that just caused issues down the road. Being a huge fan of David Lindley's El-Rayo X tone this is the perfect amp for that, it's just a pity I don't play slide guitar!

Matt J.

[dbharris]

Looks great! Are you liking the way it plays?

-Dan

Thanks, Dan!
Here are my observations after about two weeks of playing it. I don't have any single coil guitar right now, just two Gibson Explorers with AlNiCo humbucking pickups, so it may be overdriving a lot earlier than you would with a Strat or Tele:

• The Overdrive is really dependent on how hard you are pushing the preamp compared to 102, 124, 183 or similar Overdrive Specials with a Ratio control. Leaving the preamp too clean causes the OD to be very anemic. I wasn't really looking for clean tones with this amp anyway and usually keep the OD engaged at all times. I just pull it back with my guitar's volume control. I wouldn't recommend this amp if you're looking for a clean/OD volume balance unless you added in a separate master volume like on a blues master. I can see why HAD started updating this circuit with the ratio control so there would be a better balance between the two.

• Having the OD trim on the back is a nice addition. It basically sets the floor for how minimal the distortion can get, but I tend to leave it around the stock 33K to ground mark on the pot's sweep.

• The B+ voltages are a little lower with the Pacific Audio transformers than what I normally would see with the Heyboer, Hammond, Mercury, or Classictone PTs I have used in the past. Right now at 120VAC I'm getting about 438 VDC on the power tube plates, about ~35 volts lower than what I usually saw when I built my 102, 124, and 183 ODS versions. You can feel it in the power amp when you push it, there's a certain compression and squish that comes along with those lower voltages.

• I've never had an ODS amp feedback so easily than this one.

• The amp, by nature, is very bright, so you have to learn how to tame it by playing with the treble control. The Accent switch adds more brightness than anything else. When it is engaged it almost feels like a preamp boost has been turned on. I ended up sticking with the .01uf original value. I discovered I had it wired wrong originally and that is what caused it to be so weak originally. If I go in and change anything it would be to play with that cap value to volume and treble jump is not so prominent when the accent is engaged.

• The preamp controls are VERY dependent on the preamp volume. You are not going to get much out of the bright or deep switch with the preamp volume down and, speaking of the deep switch, it is more of a mid shift when engaged and thankfully it does not leave the bass frequencies flabby or tubby.

Overall, I am very pleased with the amp. Setting out I knew I wanted to build something just for Overdrive and gravitated more and more towards the 70s circuit. I've tried building this amp a few times before but ended up over complicating it with too many added options that just caused issues down the road. Being a huge fan of David Lindley's El-Rayo X tone this is the perfect amp for that, it's just a pity I don't play slide guitar!

Matt J.

Hi Matt,

Thanks for sharing your opinions. It's never too late to get a lap steel and learn! You'd be surprised how cheap you can pick up a decent vintage unit. To do the Lindley thing, the FET is necessary in my humble opinion. But I can dial in great tone with both inputs.

I think a lot of what you are hearing with the preamp being pushed earlier is the lack of the LNFB network at V1. Could also be the different PI. I really like the deep switch with strats. I leave it off for everything else as my amp is noticeably more lively without the deep activated.

As to the amp being bright, a Dumbleator can really help smooth it out. Though this is a bit of a diversion from the 2nd gen sound.

I'm glad you are liking it and hope it brings you lots of inspiration when you play.

-Dan