New 18W Build

[paniagua]

Not sure if the dynamic interaction between the V drop across the zeners and the effect on the bias affects the efficiency and/or current across the zener and/or EZ81 drop? Replaced one of the 20V zeners with a 5.1V. Right now, getting 12V drop (5.1V + 20V zeners), showing approx 50% drop. Initial B+ was 365V before I placed the zeners this time. B+ now sitting at 345V and EL84 cathodes at 12.6V. All other voltages look perfect. Also, is the EZ81 just glorified resistor since the protection diodes are doing the actual rectification - would this affect the V drop across the EZ81? Everything sounds and looks to be working fine. Happy.

I would call this a quiet build. Self-noise on par with my Boss Katana 50 on 25W output setting. I do have filthy mains though. Our refirgerator pumps junk and hum. The microwave is on a separate dedicated breaker and I can hear it through the amp. Pedal board adds some hiss with all pedals off but it's really only noticable when the volume is above 9 o'clock. With 103dB sensitive speakers, that's really loud in my house.

Not sure how much value there is in an FX loop in the 18W design but I wanted to find out. I went ahead and installed and uninstalled the passive loop today. Major ground loop hum, with and without pedals. I tried a couple different groundings and couldn't get rid of it. Might want to revisit it later, but I'm happy with my delay and reverb in front right now - I just want to play guitar. I was surprised by how different the different modes on each pedal sound so different from one another. Before the differrence between Tape and Warm delays was subtle. Now I have to turn the effect level/mix down.

I have a freind bringing over a 100dB cab this weekend, hoping to reduce headroom just a bit. Had more time to play with my dirt pedals today. They sound much better through tubes.

This amp sounds super dynamic and very responsive to picking strength. I was hoping it would be this good.

[JMPGuitars]

Sounds like you had a gremlin, and got rid of it when you changed your Zener situation. Nice.

Play around with the touch sensitivity while you're having fun. Finger pick gently with the gain up half way, then smash a chord.

[colossal]

Very nice build.

I noticed in your photo that your power tube cathodes are not linked, or if they are, then the connection is made under the board and not visible.

I would rewire, like this:

18W.jpg

[Bieworm]

Very nice build.

I noticed in your photo that your power tube cathodes are not linked, or if they are, then the connection is made under the board and not visible.

I would rewire, like this:

18W.jpg

Good point , but there are more underboard wires ..so it’s likely the cathodes are connected there. The voltage chart doesn’t show the obvious discrepancy neither imho

[paniagua]

Thanks. Yes, mix of above and below board connections. EL84 cathodes are linked.

A friend brought over a smaller cab loaded with a 10" Eminence rated at 100dB. We were both surprised by the difference in volume level and the earlier break-up achieved compared to the 103dB Cannabis Rex. I found a well broken in cheap Celestion Seventy 80/8ohm 98db locally for $35. If I don't like it, it can serve duty as a bench test speaker and probably just buy a Greenback if I want more earlier break-up.

When we cranked the 10", I blew the replacement Reflector 6N14P. Was working fine before. It appeared to be red plating and developed high level of hum. I decided to cut my losses with these tubes, got a refund, and picked up a set of TAD EL84. No complaints. I like them a little better than the Sovteks. Took some measurements and appear to be well matched and quiet so far. I found an old Sovtek 12ax7LPS in the tube bin I might try in the PI. I also have some JAN 12ax7WA I haven't rolled yet.

I ordered the head cab and should hopefully be here in a couple of weeks. I'm holding off on the faceplate, thinking I might want to add something more than volume and tone, but I'm struggling to justify more. I'd like a little more edge of break-up at sane volumes but a speaker change could fix that. I have 120K resistors I could swap on the plates. I could just build an attenuator as well. But there's already a world of tones in just the amp vol/tone knobs, and especially the guitar vol/tone knobs. I was surprised how well the amp responds to the guitar controls. I haven't even bothered to put my Boss EQ-200 on the pedalboard.

My boy has been spending more time playing guitar since building an 18watt. He plays in the dark sometimes, just to stare at the tube glow. He's impressed. We spent a lot of time together talking about and building the amp. It's been cool to hear him experimenting more musically, trying new songs, and messing around with pedals. He nailed a Ritchie Blackmore tone the other night. Cool. Priceless proud dad moment.

[JMPGuitars]

You're a lucky man, enjoy!

I had a 70/80 once. It's a fairly sterile speaker IIRC. Definitely good for a bench speaker. If you're using a standard 18W OT, you'll want a 16 ohm speaker.

[Bieworm]

Rule of thumb when it comes to celestions: stay away from the natural metal colored ones, except the alnicos

[paniagua]

The poor maligned Seventy 80 I think my boy will like it. His Metal Zone pedal might be a little too much though

My guitar buddy is a cabinet builder and runs a CNC machine. He has some scrap maple veneered baltic birch ply and offered to put together a cab for me. Going close to the 1974 dimensions, 24x20x10.5. Back will have optional closed vs open panels.

I would have preferred a 16ohm/full winding. If I remember correctly, ohming out the taps, they are all on a single winding. I'm not positive. The Mojo TX schematic shows a single winding though.