Tube Amp Archive

It's fine. And depending on the length of the original lead that's left in place, sometimes it's better to straighten it so it stands straight up off the board like a fence post. Form eyelets in the new component leads that then slip down over the fence posts and solder that. Better than "J hooks" I...

Yep that's the one, apparently it's a semi-common issue w the amp if Rheinhardt himself tested it and found it to be suitable. As I understand , this decision was made in some point in production to lighten the entire amp, as originally there was just a 12V winding on the PT, which made the amp hea...

The one you linked to is a 120-watt, 10A supply. Is that what the Shiva needs? That's a bit of an overkill to be used in a guitar amp... EDIT: Is the video you found? I guess the original unit is indeed quite powerful haha. Other amps I've seen that use 12V SMPS are more like 20W and maybe 1.8A at m...

I've heard good things about the enclosed supplies from Recom and CUI. https://www.mouser.com/ProductDetail/CUI-Inc/PSK-S25B-12?qs=OlC7AqGiEDlspMPOZq0jhg%3D%3D&countryCode=US¤cyCode=USD https://www.mouser.com/ProductDetail/RECOM-Power/RACM30-12SK-277?qs=amGC7iS6iy%2F8EgA8HmojdQ%3D%3D https://w...

The typical arrangement in these amps is two 50/50 caps wired in series-parallel for a total of 50uF filtering. how does one series/parallel 2 caps? Two dual-section radial caps. The two sections of each cap are paralleled with themselves, then each cap run in series. 50uF + 50uF paralleled = 100uF...

Roe wrote: ↑Thu Sep 04, 2025 11:39 am

Helmholtz wrote: ↑Wed Sep 03, 2025 9:04 pm See corresponding thread on the marshallforum.
I experimented with increasing the reservoir cap value of my '68 1959. Didn't like it.

it sounds better on low-voltage amps than on high-voltage ones normally

Interesting. My personal amp is a '70 Super Trem modded for 5 gain stages. Vp is 510v.

UPDATE

Heard back from Hiwatt after sending the amp in under warranty. They say they found the problem—it was a bad AC/DC SMPS. Hopefully when the amp returns to me all will be well.

Not sure about the answer to number 1, but I think if your HT fuse is spec’d appropriately your PT should be safe. To know the time constant, I think we’d minimally need to know what specific capacitor(s) you plan to use (or their ESR) and the HT secondary resistance of your PT. That’d be a simplis...

I'm considering raising the two main reservoir caps to 100uF/100uF in a modded 1959 or 2203 circuit. The typical arrangement in these amps is two 50/50 caps wired in series-parallel for a total of 50uF filtering. But I want to try raising this to 100uF total. Depending on the 100W Marshall schematic...

Hmm. I believe that you already made sure that the PT CT went only to the first filter cap negative. Otherwise that would be the next place to look. I will move it again and double check. In your schematic, I see that the PT CT is where the HV fuse lives. That means that there is a wire from the PT...

Isn’t the PI V4 and the CF V3? No, not in Hiwatts. The phase inverter is the 3rd tube and V4 is a cathode follower. It's not used for signal amplification. I'm pretty sure it sets the DC bias voltage for the PI and helps provide a low impedance signal to the power tubes. Anyway, if the hum stops wh...

Something else I noticed, regarding the design of most 50W Hiwatts, is where the OT CT is referenced. In a typical 50W Marshall, OT CT connects to one side of the choke, along with a 50uF+50uF filter cap (100uF total), to the standby switch, and then to rectified DC right at the output of the rectif...

So here's where I'm at. 1. Appears to be 120Hz hum? 2. Hum is not affected by any of the amp's controls (volume, gain, EQ, etc.). 3. Adjusting bias trimmer does affect volume of hum. 4. Hum disappears when pulling PI tube (V3). 5. Hum disappears when pulling cathode follower tube (V4). 6. I tried pa...

The ONLY thing that affects the volume of the hum is the bias trimmer. Lower bias = lower hum. Higher bias = louder hum. The preamp is relatively noise free. Maxing out the amp's volume control only adds a bit of normal hiss. No hum. Mmmmmm. Sounds suspiciously like ripple on the bias voltage. Is i...

I have had this same issue in a few amps and this situation might be the cause in yours . In particular the Univox U45B model comes to mind as having this issue and its very very prominent in that model amp Due to how close in the U45B the power transformer is to the output transformer. ( a little ...