Upgrade capacitors in tube amplifier?

I have upgraded crossovers with very good results.  I now have a "very good" IMO chinese tube amplifier that can probably be improved with a couple of capacitor upgrades.  

I don't have a schematic, but I have a picture and I expect some of you knowledgeable folks can point me to/confirm the ones to change.  My thought is to change the two large silver ones first and then maybe the four smaller silver ones (on the circuit boards). 

Would probably replace with something like Jantzen Superior Z-Cap or comparable and go with a higher voltage if space permits.  



I'd be careful about anything that is paper and oil in a tube circuit. Over time its not that unusual for them to develop a bit of electrical leakage; this can throw off the bias and operating points in the circuit.

FWIW we've had good luck with the V-Cap ODAM series in amps like this. That part also appears to have an oil component, but it doesn't appear to have a paper component. At any rate they sound pretty nice and we've not had any problems with them.
I really like the internal wiring on that tube amp. Very neat and organized. 80% point-to-point except with the few pcb boards and thicker wire than a lot of more expensive domestic amps.

My thoughts on the cap upgrades is to not do it unless you hear something you think is deficient and you have already upgraded the tubes and interconnect cables.

If you do upgrade just do the coupling caps connected to the tube sockets and realize a speaker crossover environment is very different from a tube amp. Make sure the capacitors are rated for higher temperatures as well as higher voltage.

What make is that tube amp?
The amp is a Willsenton 35i.  Around $88o including air shipping from china hi-fi.  I'm really impressed by it.
After looking at the Willsenton web site I see that they use generic EL34 tubes stamped with their Willsenton name. (I'm sure Willsenton is not manufacturing tubes.) At that price point they are likely not higher quality tubes. Even expensive tube amp manufacturers often use "budget" tubes expecting the buyer will roll them to higher quality tubes.

So if you haven't already done so I would recommend two matched pairs of Tung Sol EL34's and also upgrade the 12AX7 and 12AU7 vacuum tubes.
If you do upgrade just do the coupling caps connected to the tube sockets and realize a speaker crossover environment is very different from a tube amp.
1extreme119, I realize the circuitry is very different. Generally, do you think there is less potential for a coupling cap upgrade to be as effective as a crossover cap upgrade?
Ok i can see the pairs of caps on the circuit boards,, 
Q: are they in power supply, or audio circuit? 
Got to know, Power circuit caps will not result in superior sonics, 
caps in audio circuit, like the 2 on the tube sockets,,will alter sonics. 
If you can geta  solid tech geek to advise, they I suggest go with Munorf Supreme EVO SGO, or EVO SGO.

Really there's not much to work with,,I'd say keep this amp, and later on look at a Cayin, which offers lots of caps/resistors you can swap out. 
On resistors  go with Takman Rey's.
hope that helps

So if you haven’t already done so I would recommend two matched pairs of Tung Sol EL34’s and also upgrade the 12AX7 and 12AU7 vacuum tubes.

I have another thread on the tube angle. Have replaced the 12AU7 and have 12AX7 on order. I’ll study the EL34....these can get into real money.  Edit:  check reveals the Tung-Sol EL-34 are not too bad......but I'll still study the possibilities.

" 1extreme, I realize the circuitry is very different. Generally, do you think there is less potential for a coupling cap upgrade to be as effective as a crossover cap upgrade?"

The answer to that really depends on the other components in your tube amp and the other audio components in your end-to-end audio chain. Do you believe those two coupling caps are really the weakest link in your end-to-end chain? If not then it is unlikely that any improvement would be "revealed" by your system. BUT....this is also about having fun so if getting in there with your soldering iron and swapping components and trying to hear the difference will bring you fun then go for it. Been there and done that, so I’m the last one to poop a fellow audiophiles party.
There are three coupling caps. The first is at the top of the photo, one in each channel. To the far left and right beside the power tubes is a small circuit board; the coupling caps for the output tubes are mounted on that circuit board; they should be matched for best results.
Yup I see 6 decouping caps also. They look like they are directional. But they look to be good caps.. IF there is NOT a problem, leave it alone. 
I don't think it's broke in yet, is it? 1000 hours or so...
Your one of those guys.. LOL Because it's there.....Right...

Is something missing, a valve roll won't cure? Why do you want to mess
with a new unit? Highs not high enough, lows not low enough...?
What do you want to change? Sound wise.

100.00 will buy some real nice sounding Green EH EL34 for under 100.00. Rubys, ReIssue, Mullard, 100.00 or so..

12ax7 or 12au7s, 100.00 no more.. don't get crazy.. Tellies, PVANE, GEs, Mullards, Volvo, Mazda, RCAs. BE CAREFUL with all the NOS stuff, a LOT of rip offs... I test EVERYTHING...MATCH the valves it will make or break the SQ, in both small and large valves..


In a tube circuit the higher the voltage the better 600v is optimum 
the vh Odam is a excellent cap ,the VH Cu Teflon caps even better 
if it will fit and can afford.
I know this not a tube rolling thread, but I would suggest swapping the 12ax7 and 12au7 tubes first. I recommend trying some 1950's vintage Raytheon black plates. Try to stay around $70 for each tube. For the power tubes you could get some 807 tubes with an adapter.
My advice is, if you’re going to change coupling caps, stick with the various film and foil types, rather than a metallized film. They’re generally more expensive (but not always, depending on mfgs), but are of better construction, especially in regards to lead attachment. In a film & foil cap, the foil protrudes at each end of the rolled cap, and the lead is soldered or welded directly to the foil . Metallized caps have the dielectric protruding at each end, so a conductive paste is applied, which makes contact with the plate. The lead is then attached to this paste (a less reliable form of construction).

 Be aware that, for equivalent voltage ratings, film & foil types are generally larger in overall size.
Another vote for tube rolling first and caps maybe after 750-1000 hours of listening.

All the best.

By the time you install Nichicon caps, V-Caps and some Audio Note resistors you will probably spend half of what the amp costs new.  IMO the Chinese amps are below average in sound and if it were me, I would buy something else - I know that you don't want to hear that sorry. The caps and resistors will open up the sound and probably proved improved bass and dynamics if you want to travel down that path.

Happy Listening.
@chinook9, There are 7 film caps in there. The two on either side of the chassis appear to be the coupling caps. The two towards the front of the chassis appear to be doing additional filtering for the input tubes, they are tied to what appears to be a ground buss bar across the front of the amp.
The 7th cap appears to be on the power supply. The biggest improvement will be on the 4 coupling caps. Next would be the two caps acting as filters for the input tubes. Also agree with bigkidz though. If you add really good caps, they are likely to be 30-60 bucks each, and will quickly approach a significant portion of the amp cost. The amp appears to be made by same factory that makes the RFTLYS amp. good build quality; only a small notch down from the factory that builds the Primaluna and Cayin amps.

Have you tried any different output tubes? give the shuguang Nature sound series black glass a try. they sound quite good.

Both tube rolling and cap and resistor upgrades will improve sound quality. Only question is cost effectiveness. I'd say go for it, while you may spend as much or more than amp originally cost, its a learning experience. Better to start learning on cheaper equipment.

I started learning about parts substitution on cheaper equipment, such that I now trust myself with more expensive equipment.

I recently changed out caps, resistors and all tubes on PrimaLuna Dialogue Four. Miflex, Mundorf caps, Takman resistors, NOS miniature tubes, SED and Svetlana EL34. Sound quality improved to the extent I now consider this amp a real contender to totally stock Coincident Turbo 845. Prior to mods, sound quality just ok.
Thank you all for your input. 

I will do as recommended by many of you: roll tubes first and then consider the caps.  I took the bottom off the amp and found that the caps mentioned by most are two large 0.33UFD 5% 630VDC and four 0.022UF 5% 630VDC.  I realize you can't tell much by looks but they did look pretty good.  They are oval.

I have some ECC82 (Tung-Sol black glass) and 5751 (GE black plate) on the way and now I'm trying to decide on the EL34/6CA7.

Thanks again.
For your coupling capacitors (particularly the 0.33mfd), and importantly, if you physically have the room, consider either Jupiter Copper Foil Paper & Wax or Miflex KPCU-01. The Jupiters are a bit more expensive but you won’t go wrong with either.

I’ll second (or third) the suggestion for Takman REX Carbon Film resistors. Very musical.

While you’re at it, consider the output transformers. Look into Lundahl.

And look at the power supply rectifiers. I don’t see a tube, so it must be silicon. If it has Schottky rectifiers, you could consider replacing with (third generation) silicon carbide (SiC) rectifiers.

And has been pointed out above, once you start down this road, you’ll probably end up spending as much on component upgrades as the initial cost of the amplifier. Be sure your comfortable with that.
RAM tubes, best computer matching on planet

thanks to the genius of Roger Modjeski ( RIP )
I agree - don't mess with the circuit if it sounds good.  Their branded tubes are decent and I did not hear any significant difference with other tubes but the tube construction is a little better with Tung Sol or some others. 

The biggest upgrade you can do is buy the R-300B model. It is amazing! I also have the R8 with KT88 tubes and the R-300B is significantly more organic sounding. I have not owned the model you have but likely is similar in tone to the R8. I am running the R-300B thru Klipsch Forte III....simply amazing match.
The biggest upgrade you can do is buy the R-300B model. It is amazing! I also have the R8 with KT88 tubes and the R-300B is significantly more organic sounding.

calieng3, thank you for this information. Yong Lee recommended the R300B as an upgrade and I have been looking for anything on the comparison of the other Willsenton amplifiers and the R-300B.

I am playing with the R-35i now, but I know, with Covid19 limiting my other hobbies, I will be trying to improve upon it before too long.

My hobby, at this time, is to work at getting the best possible sound (IMO) for the least possible cost. 
Hi chinook9. Think about Mundorf capacitors for the audio caps. Get the best ones you can afford. I've used them in at least 8 stereo amps and they have made a positively remarkable difference. I get them from Canadian outfit that ships very quickly. Getting the right physical fit for the big electrolytics will be the biggest problem. Happy replacing. There's nothing like the smell of a hot soldering iron in the morning!

@johnss I just wanted to thank you for your comments on the RFTLYS tube amplifiers.  I've owned a RFTLYS A5 for two years now and I have never had any problems with it and after replacing the stock tubes, I'm very happy with how it sounds.  Your the first person I've come across that had anything good to say about RFTLYS. Most people have never even heard of RFTLYS much less ever seen or heard one.  The A5 is very well built with three large transformers that is reflected in it's weight of 51 lbs.  What a horrible name though, I think RFTLYS would sell a lot more amps if they could come up with a catcher name than RFTLYS.

I don't think upgrading the capacitors is going to help much, as most of the music is not going through them.  But it will be interesting to find out how it ends up.  I would upgrade the tubes first.