The Amp Garage

I agree, the switch on the speaker jack is not needed and adds complexity.

I like sluckey's diagram, but I'd add a 220ohm 5w resistor across the output jack as insurance against contact corrosion or bad cables.

The most immortal amp I know of is the Peavey CS-800. We used a pair of them back in the day for our PA. I remember replacing a lot of output transistors (which were fortunately socketed). To be fair, unless you have a whole matched set of outputs, mismatched Betas etc., will cause more failures th...

The 'stiffness' of a supply is controlled by the size of the filter caps, and to a lesser extent, the impedance of the AC source. This is probably why the Marshall style uses a Pi filter (C/R/C) to compensate for the high source impedance. If you have a dedicated bias winding that can supply enough ...

I may be oscillation, these sometime pop up during note decay on multi-stage high gain preamps. Do you have a scope?

Wouldn't hurt to check the filter power supply caps. The amp is likely 30 years old at this point.

I remember that paper magazine article (from way back in the day). The writer was electrically competent, and this was way before the internet. This was a hobbyist magazine, though, not an industry journal. A couple of points. Most (all?) tubes were designed for AC heater power, so probably little t...

Something like that is easier to afford now that you can get $1-2 digital meters from AliExpress.

All the above are certainly good reasons, however in this specific case I think the first stage is to control the frequency response via the split cathode bypassing, and the second two were where non-stock values were needed, e.g., .0147uF.

I did one of these years ago, but made it powered so I could use relays. I used high power DPDT relays to switch the amp input and low power relays to switch the guitar input. (I also included an attenuator in the box.) Fully double-insulated to avoid ground loops with one amp ground chosen arbitrar...

The build will affect the frequency response. I don't know by actual measurement, but I'll bet any OT meant for a guitar amp will have less bandwidth then, say, a MacIntosh OT. Extending the HF range means minimizing the inductance and inter-winding capacitance - this is why some high quality OTs wi...

The 400 PS (and 300 PS, I actually own one of these - anyone want to buy one, lol?) No thanks, I already have one of each. I bought the 300PS new when they first came out. After the second set of overly expensive 6550's blew along with all the collateral damage, I shelved the thing. I've been runni...

The 400 PS (and 300 PS, I actually own one of these - anyone want to buy one, lol?) use a voltage doubler scheme to get 700 VDC out of a 245 VAC PT winding. So its actually one step 'worse' than a normal half-wave rectifier circuit. They make up for this by using massive "soup can" 450 or 650uF stac...

The other issue not mentioned yet is voltage rating. Resistor voltage rating is often related to size, and the inter-lead gaps tiny SMD resistors require means not a lot of high voltage tolerance. They could be (and probably are) used for input resistors, cathode resistors, etc., but could not in ma...

If it's a linear pot, then yes. If it's a log taper, then the response->rotation will be wonky.

An easy way to test for a linear pot is to center it and measure the resistance from center to each end, it should be approximately equal.