I asked the voltage question because I picked out caps from the same family of Nichicons that you suggested for C1 and C2. They didn't have any lower than 160v.
So I'm replacing 10 caps and 4 diodes. Sounds like a reasonable project for an evening.
Any opinions on replacing the ceramics in the tone circuits? I’ve read a couple of threads where this was suggested. I don’t want to go on a search and destroy of all ceramic caps, but if there are some strategic caps to replace, I’m game. (Yep, I know that sq questions are subjective, but since I’m already going to be under the hood...)
I think 160v is too high for replacing caps that are rated at 50v (C4a+b) and 25v (C5,C6). I'm not a cap expert but I've always been under the impression that you shouldn't more than double the rating of the original and usually I don't even go that high. I think caps are designed to operate optimally at voltages that are closer to their max rating not way under it. The same or slightly higher voltage rating (one step up) is OK for an added bit of safety - like using 450v to replace 350v or 160v to replace 100v. The other issue is size. It's much easier to fit the modern lower voltage caps into tight spaces. I don't worry about caps being from the same series (family) or even from the same manufacturer. The main thing I look for is a high hour rating at 105 degrees. Sometimes I'll look at some of the other specs like ripple but the high hour, high temp rated caps are all high quality.
I've never messed with the tone circuits on the VMs. In general, ceramic caps hold up pretty well over time and most people will advise you not to replace them unless you identify a specific problem with that part of the circuit. Others will point out the many technical flaws of ceramics and say replace them all. I don't doubt their flaws but I don't have the equipment it would take to measure the difference and I wonder whether most people could hear the difference. I've never really experimented with replacing them in vintage gear. When I build something, though, I never use them except as snubbers in hybrid rectifier circuits.
I know some people remove or rebuild the Tone-O-Matic circuit on these. I drew out a schematic for a guy a while back who rebuilt his on a separate board. The circuit is shown on the schematic enclosed inside a dotted line but it's sealed inside a PEC so you can't check the individual parts. My advice is to listen to it as is first.
I've learned to approach the tone controls on the VMs differently than on other amps. On most amps people start with controls at noon, which they assume to be flat (although they rarely are), and they tend to turn them up from there. On the VM, you should always start with the controls at minimum (8am) and - with your eyes closed - turn them up until you like the sound. Forget about the knob position and expectations about what the tone controls should look like based on prior experience. Do it totally by ear. That's probably good advice in general but especially important on the VMs.
The exception to the advice about not replacing ceramic caps is when they are used as coupling caps to the output tubes, such as in the VM (C14,C15,C24,C25). Conventional wisdom is that they are especially poor performers in that role. They are often leaky - allowing
significant DC voltage on the grid of the output tube - so you should measure for that. (Even new quality caps will often allow a bit of DC voltage to pass) I will usually just replace them regardless but sometimes they are (I know I'm going to get flamed for saying this) part of the magic of a particular amp, if they're not leaky. So, check the voltages and listen. If you like the sound you can leave them alone.
If you want to replace them . . . well, the debate on which coupling caps to use is endless, very subjective and not a one type fits all circuits issue. I rebuilt a VM 1448 years ago and used vintage .033uf PIOs that sounded fantastic. I don't remember what I put in the 1428 I'm currently using, probably just decent quality film caps, and it sounds great too. I've also listened to some of these amps in completely original shape and been blown away too. And I replaced the original ceramic coupling caps in an Eico HF-81 a while back and wished I hadn't. I should go back into it sometime and put some ceramics back in, or maybe some silver micas to see if that was indeed the issue. So . . . it's a slippery slope and YMMV.