18Watt.com

Honey dripping off a hot buttered biscuit.
1kHz into the TMB channel, controls at 1/2 ("noon") and MV cranked. Measured at the FX loop send.

Here is a summary of my build process
https://m.youtube.com/watch?feature=you ... y7GnsaEYJY

Here are the final voltages annotated on the schematic

Just for information in case it is useful. The Orange TH30 I have been repairing also switches the output impedance from 16 ohm to 8 ohm when it switches the cathode bypass to two valves from four.

Dd

Updated process video with some final build pix:
https://m.youtube.com/watch?feature=you ... 62g#dialog

Craig.. how is the sound on 18 watt mode? Does it differ from an 18W or is it very good?
I would like to adapt this on a tremolo TMB ...but 36w and 18w switchable

To my ear, in half power mode it's identical tone-wise but a few dB quieter. I gave it to its owner before doing any speaker A/B testing or demo recordings but I'm hoping to have him record some demos for me.

I did use a matched quad and the bias resistors are 75+75 Ohm so I would expect it to sound similar in 18 and 36 watt mode, as it does. Ceriatone uses 68+62 Ohm bias resistors so there might be a touch different sound if you built one that way.

Did you notice any more headroom and apparent punch/grunt, girth, and/or depth (e.g. more 3D, broader, more reverby) in 36W mode?

Nope. It's pretty identical. I do get more headroom when I turn on the NFB switch though. I voiced my Normal channel like a Tweed Deluxe and my TMB channel like a Plexi, so the cleanest setting is plug into the normal channel and switch NFB ON. Dirtiest setting is plug into TMB channel, dime volume and switch NFB OFF ( & set MV to desired loudness). Punchiest/girthiest is plug into TMB Hi gain jack, jumper TMB Lo gain jack to Normal Hi Gain jack, leave NFB OFF and use all 36 Watts.

Thx craig for the info!

My plan is to mostly play on 18w mode. I'm thinking about making both EL84 pair separately on-off switchable. That way I can alternately use one pair so they wear off the same. It's going to be a single channel amp anyway. For clean mode I'm thinking about a switch to lift the cathode caps from the 2nd preamp stage..

crgfrench wrote: ↑

Sat 09/12/20 11:04 pm

Nope. It's pretty identical. I do get more headroom when I turn on the NFB switch though. I voiced my Normal channel like a Tweed Deluxe and my TMB channel like a Plexi, so the cleanest setting is plug into the normal channel and switch NFB ON. Dirtiest setting is plug into TMB channel, dime volume and switch NFB OFF ( & set MV to desired loudness). Punchiest/girthiest is plug into TMB Hi gain jack, jumper TMB Lo gain jack to Normal Hi Gain jack, leave NFB OFF and use all 36 Watts.

Thanks for the tone report, Craig. That's interesting. Reason I ask, is that a lot of guys report a sense of more apparent weight, if you will, with the same preamp, but into a 4xEL84 power amp versus 2xEL84. An AC15, for example, does not sound or feel the same as an AC30. Of course there is the volume difference, larger output transformer, and pushing a second speaker will spread the sound out a bit more so that's not quite an apples to apples comparison.

colossal wrote: ↑

Sun 09/13/20 8:08 am

...An AC15, for example, does not sound or feel the same as an AC30. Of course there is the volume difference, larger output transformer, and pushing a second speaker will spread the sound out a bit more so that's not quite an apples to apples comparison.

True. Plus, the Vox circuits are different. e.g., the AC30 Top Boost uses a shared 1k5 + 22/25 on the first tube's cathodes whereas the AC15 has a 1k5 + 22/50 on one triode and a 6k8 with no cap on the other triode. plus quite a few other differences. So I would expect an AC15 to sound different from an AC30. On the other hand, switching to half power in my 36W is the exact same circuit and components, so I would expect it to sound very similar yet quieter when going to 18W.

Is the bigger Iron (PTx and OTx) not giving a different sound on 18 watt mode than the regular 18w Iron gear?

Bieworm wrote: ↑

Sun 09/13/20 11:36 am

Is the bigger Iron (PTx and OTx) not giving a different sound on 18 watt mode than the regular 18w Iron gear?

That is a good question! Unfortunately I could only speculate as I have no way to make a direct comparison. The 36W OT is Heyboer HTS7717 from GDS, while the 18W OTs I have used have all been Heyboer HT6135 from Mojotone. PT also from GDS here, vs the 6179 from Mojo.

The way to test this would be to swap in the 18W iron and run the amp at 1/2 power. That would be an interesting test. Can't happen here with this particular build, since it's already delivered to its owner. However I have (almost) enough parts kicking around to actually do that with another amp. That won't be happening this year, for sure, as I'm back on an 18W Fullerwell build now (using a Drake 784-103 OT repro from Chris Merren). But perhaps after that? ...

geoff 1965 wrote: ↑

Tue 07/14/20 4:22 pm

give us some feedback on that effects loop when it's finished

Quick note on the FX loop: The original board worked great (switching FX in & out), but was causing strange randomly intermittent volume spikes and drops as well as nasty noise that sounded like "radio station hiss between stations". I removed it from the amp and examined it closely, and found that the cathode leg of C5 was not soldered into the board, just sitting there dry in the via. I contacted Saul, who was awesome and sent me a replacement board immediately. While waiting for the new board, I soldered the cap leg myself and reinstalled the first board, but unfortunately there must have been some other issue too as the noise issues persisted. I pulled the board again, and because the owner had been waiting 3 months at that point offered to deliver him the amp sans FX loop (the 3 months was caused partially by a 1 month delay of the pink tolex thanks to the USPS but that's another story); he wanted the amp so I delivered it with the promise to install the new FX loop when it arrived. Well it arrived quickly, but the guy can't stop playing the amp long enough to give it up for a day! So someday hopefully soon I'll get it back and install the new loop board for him. In the meantime he's plugging into the High gain British channel jack, sending to his pedalboard from the Lo gain British channel jack, and taking the return into the High Gain American channel jack. This gives him a wet/dry control via the volume controls on each channel. But it is an "all front-end" solution, vs the buffered loop between the preamp & PI.

Bieworm wrote: ↑

Sun 09/13/20 11:36 am

Is the bigger Iron (PTx and OTx) not giving a different sound on 18 watt mode than the regular 18w Iron gear?

One other thing I just remembered: The 36W will have slightly less sag than the 18W because it uses the GZ34 rectifier rather than the EZ81. The EZ81 has a higher sloped forward voltage drop curve than the GZ34. Not as much sag as a 5U4GB and, obviously less than a 5Y3, but more than the GZ34 which offers the least sag of all vacuum tube rectifiers.

This was how I designed the tonestack:
https://youtu.be/RHZD43EHzpI

2 years later, the owner brought it back for the FX loop installation. He didn't want to let go of that amp even for a weekend this whole time! Installed the replacement Saul's Tube FX Loop and Bob's your uncle. Happy owner. Also I replaced the 6' Power cord with a 25' cord at his request.