Stumped on build with no sound

[cdemike]

Just wrapped up a series of modifications on a 50w Hiwatt SAP clone I built and I'm getting no sounds from it after the modifications. I've checked the usual candidates, and I'm getting some weird signs that I can't put together. Notably, the amp worked fine before the modifications and most of the amp is unchanged. The old schematic was very similar to the SAP schematic on Mark Huss' site (https://hiwatt.org/Schematics/DR_Pre2InputSAP.gif) though the preamp power supply was modeled after the Jimmy Page version with the PI and screens sharing a node and with V3 having a dedicated supply parallel with the V2 and V1 branch (https://hiwatt.org/Schematics/DR_Pre2InputJP.gif). The mods were as follows:
- Add a choke to replace the 100R resistor found in most Hiwatts between the reservoir node and the screens node;
- Eliminate the can capacitor filtering V1, V2, and V3 and replace with axial caps. This also involved reassignment of the cathode follower formerly proceeding the input gain stage to share a B+ supply with V2;
- Correspondingly rework the grounding layout such that the preamp filter cap cathodes connect to their respective stages' cathode resistors (an effort to reduce noise by making the grounding scheme closer to a star grounding scheme);
- Reassign V1 and V2 to move the normally-unused triode to pins 6, 7 and 8 of V1;
- With V1b free, add a cascading gain stage to the front of the amp using values borrowed from an EP-2 Echoplex preamp; there is still an input allowing guitar signal to bypass the EP-2 stage, i.e., which should be like a normal SAP Hiwatt.
Here's an updated schematic that shows the grounding scheme:

IMG_1900.jpg

As it stands, I'm not able to get any guitar signal to reach the speaker. I've determined the amp is indeed amplifying signal in portion of the amp where the signal path was unimpacted (i.e., from the volume control onward), though unsurprisingly this is very quiet given the loss of a gain stage over the typical Hiwatt topology. With the volume controls dimed, there is quite a bit of hum that is significantly worse than prior to the modifications. I think some of this is to be expected given that there is now an additional gain stage, but it seems out of proportion to what I would have predicted. That prompted me to check whether there is oscillation occurring, but I'm not seeing AC voltage at the output section plates even with the volume control dimed. I will need to make some lead dress changes because oscillation does start when I have the boost control up, but I don't think that's the issue since there's still no signal using the "direct" input bypassing the EP-2 boost stage. Voltages check out (though will likely require revision -- plate voltage at the first gain stage following the boost section is now too low, IMO, but that's easy to fix and a bridge I'll cross when I get it working again), and I'm not able to find an issue where signal would be getting dumped at the first post-boost stage with continuity reads looking good and consistent with the schematic as far as I can tell.

Here's a gut shot of the whole amp:

IMG_1899.jpg

Close-up of the board relating to boost and "input" gain stage:

IMG_1897.jpg

Another close-up on the boost and "input" gain stage:

IMG_1898.jpg

Any sense of what might bet happening?

[R.G.]

Hmmm. My first guess was voltages off - but you've checked those.

Second guess is some funny business in the boost pot/jack setup. Dirty jack contacts, high resistance wiper, etc.

After that, I'd start opening the signal path after each plate and seeing if the signal suddenly got bigger when isolated. Following stages always load plates a little, and tone/volume setups lose you about 20db pretty often, but a dramatic loss more than expected could expose a solder thread, solder ball, bum-new capacitor, etc.

[LOUDthud]

Just checking... No guitar signal from the Hi, Lo or Direct inputs, correct ? How about if you plug the guitar into the FTSW jack ?

DC Voltages look workable, must be a fault in the signal routing or a short to ground somewhere.

[Phil_S]

...a short to ground somewhere.

FWIW, this is my first thought. You've got a whisker touching the chassis or something like that or possibly a missed or cold solder. Get the magnifying glass.

[cdemike]

Thanks everyone for the help with this! I wanted to post an update since I was able to get back to this project yesterday. I suspect I had a whisker from one of the shielded wires because after cleaning up the input jack area with some rubbing alcohol and some keyboard duster it started making output again. I flowed some more solder over the shield leads to be sure that wouldn't happen again. Unfortunately the amp now has a bad 120hz hum and what I think is an oscillation. Since these are new issues and a broader layout rework seems like the best next step, I created a new thread in the Vox/Hiwatt section: https://ampgarage.com/forum/viewtopic.p ... 80#p472280

Thanks again!