Going to ReCap my V-M 1428

nmwhitneyjr

Active Member
I've never done this before, so I'm going to start slow... electrolytics only. According to the Sam's Photofacts, there aren't that many electrolytics to begin with. I plan on restuffing the cans, if possible (but I sent a query to Hayseed Hamfest just in case I lose my nerve). I've done several hours of reading on how to proceed, but I want to know from folks any advice that you would like to have known (but didn’t know) BEFORE you started recapping.

I'm also curious as to what brands of electrolytics seem to do well in older tube amps.

Peace,
Norm
 
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I’ve never recapped an amp, but I’ve built two cmoy headphone amps, a Pete Millet starving student headphone amp (12au7 version), rebuilt the ccu in my old Mercedes w123 diesel, and built crossovers for my Nova 8B speakers.

I’m not a fast solderer, nor a slow solderer... I’m a half-fast solderer...

I guess I should’ve made that clearer.;)
 
Lots of folks on here like to restuff can caps but I've always found it to be unnecessarily complicated what with having to remove the original can, cut it open, clean out the gunk and then stuff modern caps in there without shorting anything. Then put the can back together again and remount it.

I just leave the original can in place and disconnect it and mount the new caps underneath the chassis, usually in the same area as the original can. From the top, the chassis is visually the same, just as with a restuff. The VMs have quite a bit of space underneath, unlike some other integrated amps and receivers, which makes them relatively easy for a newbie to work on. On something with limited space that needs certain value caps that aren't easily found, the Hayseed ones are undoubtedly the way to go but they do cost more than putting them underchassis. I've never used their services but I know they have a great reputation. Not sure if they allow you to pick the brand of cap or not.

I usually use Nichicon or Panasonic caps that are rated for 105 degrees. Digi-Key is a good source and you can get free shipping if you order by snail mail and pay with a check or money order. I've posted about this before and it will save you quite a bit on small parts orders where the shipping charges make up a significant portion of the total order cost.
 
I'm out of town right now so I can't post pics but Raul just went through his VM. Check his posts to see if he has pics. Here is a description of the process I use that I sent him in an email:

It's really not that hard but there will be variations depending on the layout of the amp and the space available. What I generally do is to make a drawing that shows the original wiring including which color wire goes where. I do this even if I have a schematic. Then I remove all the wires or parts that are connected to the terminals of the can cap, except any connection that is going to the outer shell of the can since that's a ground. After I remove all those wires and resistors, I clip off the protruding tabs coming out of the can so that they aren't in the way. I cut off as much of the tab as I can. Be sure to cover them up with a small towel or something while you're cutting them because they will go flying and could injure you. Use eye protection. Then you just have to figure out how to arrange the new caps so that they fit and the resistors that go between them can fit. I typically connect the ground lead of the new cap to one of the mounting tabs of the original can since that is a ground. Of course, check with your meter to make sure it is actually grounded. I cut the other lead to the desired length and bend it so that it has an eyelet or loop and then attach the leads from the resistors and any wires through there. Make the loop large enough so that all the resistors and wires will fit through it.
Another method, if you have space and a convenient existing hole in the chassis, is to mount a terminal strip and attach the caps to the strip. Typically, that's not an option and I don't like to drill holes in the chassis. Of course I leave the original can in place for preserve the original look.

Please note that this refers to typical cans for which the neg is grounded. The VM uses a voltage doubler so for one of the caps in the doubler the neg is connected to ground and the other the neg is connected to the pos of the other cap. See the schematic. I forget without looking if they used a tubular cap underneath or an above chassis cap that is covered in cardboard to prevent you from touching the neg side of the cap, which has voltage on it. Some amps also have the positive grounded to create a neg voltage for a bias supply but the VM uses cathode bias so not an issue here.

Also, since this uses SS diodes and not a tube rectifier, you can increase the value of the caps in the doubler. I forget what the original values are but I would at least double them. Caps in series have half the value of the individual caps (assuming they're the same value) and double the voltage rating. So a pair of 100uf 250v caps in series equals a 50uf 500v rating. I'd probably go with at least 330uf 250v parts in the doubler. I know of one well respected tech who routinely goes with 820uf caps in doublers.

You could also go higher in the rest of the PS too, maybe 50% to 100% higher. The designers would have gone higher too but caps back then were not as small and were more expensive so they compromised. With a tube rectifier, your options for the value of the first cap are limited and different for each type of tube so you have to check the data sheet.
 
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My primary concern IS the power supply caps. The amp has a very low 60hz hum (that based on my reading, is likely the PS caps).

You've convinced me to ditch the idea of restuffing the cans... and I'm now thinking about increasing the cap values in the PS section as well. Not sure how to identify the doubler in the PS section though. Would that be another can cap? The 1428 has 3... two topside and one cardboard covered underneath.

I read through Raul's V-M posts. He wired in a cooling fan behind the power tubes to push air across the power tubes and out the back. Would that type of setup be helpful (assuming it didn't induce any noise)?

I've also considered replacing the ceramic caps in the signal path... but I'm thinking I need to walk before I run.
 
C1 is the "upper" in the voltage doubler. The negative on that will be at +150v. Just make sure that is insulated from chassis. I do not know the physical arrangement in here, but if its a top-side can with a sleeve, you'll need to retain that. If you go with individual caps, make sure it can't short. Sometimes the negative terminal is still bonded to the can of the capacitor. I found that out the hard way when I got across the middle of the voltage doubler on my big Bogen. That one was at some +350v, and it did not feel very good when I hit it.
 
C1 is under the chassis... looks like a paper wrapped can on the photofact. C2, C3 and C4 are in cans on top of the chassis.

As far as voltage goes... definitely building a cap bleeder and keeping my left hand in my pocket.
 
I’ve never recapped an amp, but I’ve built two cmoy headphone amps, a Pete Millet starving student headphone amp (12au7 version), rebuilt the ccu in my old Mercedes w123 diesel, and built crossovers for my Nova 8B speakers.

I’m not a fast solderer, nor a slow solderer... I’m a half-fast solderer...

I guess I should’ve made that clearer.;)

Sounds good, plus you have some very knowledgeable folk here at AK who can walk you through the process. Apropos of nothing I spot the exact same amp many years ago at a thrift store and for what ever reason I passed on it. I still think about that every now and then. Best of luck with yours
 
Here's the schematic of the PS section:

View attachment 1170406

Yeah, C1 and C2 are the doubler caps. I'd go with a pair of these: https://www.digikey.com/product-detail/en/nichicon/UCY2E331MHD/493-5463-ND/2597729

For C3a and C3b, which are 80uf and 50uf stock, I'd either go with 150uf and 100uf or 100uf for both is fine too. For C3c (30uf stock) I'd go up to 47uf and for C3d (20uf stock) up to 33uf. You could either stay with 350v (stock) or go up to 450v on all those. Sometimes they have a better stock of 450v parts than 350v and there won't be much difference in price and size.

I forgot that the 1428 also has DC on the phono tube (12AX7) heaters - that's C4a and C4b. Go with 150uf 50v and 270uf 50v for those.

You also need to replace the cathode bypass caps on the 6BQ5s - those are C5 and C6. Use 47uf 25v parts there. I'd also be tempted to replace the diodes in both the PS and the phono heater supply with UF4007s, which are cheap and quieter.

Nichicons will be fine for all of the above. Look for the ones with the highest number of hours at 105 degrees. Radial can versions will be cheaper than axial, which are usually harder to find too. Since you're new to this I'd wouldn't suggest using the snap in type cans since it's easier to work with the types that have lead wires (radial cans). Be aware of the minimum order for each cap, some are only available in large quantities but there are plenty of good choices with minimum of a single cap. Again, you can save a bit by ordering by snail mail to get free shipping.
 
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I've also used axial lead Illinois without complaints. Selection in that style is much more limited though.
 
Oh, and on the DC heater supply for the 12AX7s, note the orientation of the caps is positive to chassis ground which results in a negative voltage supply to the heaters.

Not sure why they did it that way. The only other times I've seen that arrangement is when the negative voltage was used as a bias supply for the output tubes but that's not the case here. Anyone know why??
 
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...)
 
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I'm going to derail my own thread briefly...

I'd like to build a bucking transformer to run this little amp and tuner off of. Our wall voltage gets as high as 123v. I keep finding schematics with a switch allowing for two levels of voltage drop. I only want one that gets me down to 115v-117v. I think this hammond will do the trick: https://www.mouser.com/ProductDetail/Hammond-Manufacturing/166N6?qs=dhCoiiGcT1IAXD2gHZeluA==

But I need to find a schematic for the simpler bucking circuit.
 
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.
 
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I'm pleased with the sound for now (and that includes the motley assortment of tubes) so I'll stick to the electrolytics for now. I haven't been under the chassis yet so there's every chance someone has already done some re-capping (hopefully not).
 
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