[chi0001]

I recently purchased this amplifier and I'm considering upgrading the coupling capacitors. Has anyone here replaced them? Is it worth the upgrade in terms of sound quality or performance?

[Jon L]

I recently purchased this amplifier and I'm considering upgrading the coupling capacitors. Has anyone here replaced them? Is it worth the upgrade in terms of sound quality or performance?

Your post reminds me to be proud of myself for sticking to "Don't fix it if it ain't broke" principal.  It's amazing I have not tried to change coupling capacitors in my two FU-19 amps.  I have finally learned to leave well enough alone 

Perhaps you can try and let us know..

[FullRangeMan]

+1. Dont fix what is not broken.
Instead buy alternative tubes.

[chi0001]

Thanks, Jon and Rullrangeman, for the advice! I read through all the posts and decided to order RCA 829B tubes. They're on the way, and I'm excited to try them out!

[chi0001]

I tried both the Beijing FU19 tube and the RCA 829B, and they both sound beautiful. The FU19 has more punch, but if you can find an RCA 829B, I highly recommend giving it a try!

[Jon L]

Yup.  This is also one of the rare tube amps that sound "good enough" with mere stock tubes 

[Jon L]

This reminded me to fire up the amp, and yup, just sumptuous and naturally resolved music.  Love it!

[chi0001]

I've noticed something fascinating: no matter which tube I use—12AX7, WE420, 6H2N-EB, or even the Beijing 6N2-Q that came with the amplifier—they all sound different but absolutely beautiful in their own way. This makes me think the Beijing 829B might be a great option, especially at just US$8.00. I asked the seller which tube pairs best with it, and they recommended the 6N2 with the 832A. Has anyone tried this combination? Any thoughts or experiences to share?

[Jon L]

Beijing 829B might be a great option, especially at just US$8.00. I asked the seller which tube pairs best with it, and they recommended the 6N2 with the 832A. Has anyone tried this combination? Any thoughts or experiences to share?

It's odd the seller would recommend 832A since it has much lower maximum plate dissipation compared to 829B.  I suppose if this amp's plate and screen voltages were very low at baseline, it might be fine, but I would double check with them. 

As far as Beijing 829B (Chinese), I am using them in one of my systems where it sounds great.  This system uses soft dome tweeters and the somewhat literal presentation of the Chinese tube matches well.  In another system using extremely-detailed compression drivers, I do use NOS American 829B which sound a bit more refined.   

[Jon L]

Wow, I can't believe I left this amplifer in stock form for this long, which speaks to how beautiful the stock amp sounds, but the inevitable curiosity won out, so I started with cutting the global feedback loop, which is simple cutting of the two wires (red arrows).  There has been reports of improved sound quality by doing this mod, and in the past, I have owned tube amps with variable feedback and have preferred the zero-feedback mode.

Snip snip, and voila!  There is more bloom top to bottom, more harmonic overtones and "air" with presentation more forward and pretty.  This is the sound many would associate with the vaunted single-ended tube signature, and I can definitely see people preferring this sound, especially if the speakers/systems leaned in the cool, analytical side. 

However, for my system and preference, image sizes just bloomed a bit too diffuse, sacrificing imaging focus and specificity.  Although the air-suffused sound was sweet and pleasant, the level of resolution and iron grip on driver control took a half step back.  This will be less of an issue with smaller speakers, but with the 15" woofers in Altec A7-500, the decreased driver control was noticeable enough to be bothersome with fast-paced, bassy music. 

In the end, I did reverse this mod, but it's worth trying for different systems, especially since it's free 

[FullRangeMan]

However, for my system and preference, image sizes just bloomed a bit too diffuse, sacrificing imaging focus and specificity.  Although the air-suffused sound was sweet and pleasant, the level of resolution and iron grip on driver control took a half step back.  This will be less of an issue with smaller speakers, but with the 15" woofers in Altec A7-500, the decreased driver control was noticeable enough to be bothersome with fast-paced, bassy music.

To a better image and sound-stage you need a better and big OPT as Hi B or Nanocristaline. Always remembering that the EI OPT delivery the traditional warm and musical sound of tubes and the C core lamination offers a more neutral sound.

[Jon L]

To a better image and sound-stage you need a better and big OPT as Hi B or Nanocristaline. Always remembering that the EI OPT delivery the traditional warm and musical sound of tubes and the C core lamination offers a more neutral sound.

I really can't complain about parts quality considering the $130 I paid (pre-shipping, pre-tariff).  It's a minor miracle how good this thing sounds with what it has 

[FullRangeMan]

Anyone knows how much is the Primary Impedance  in these output Transformers?

[shadowlight]

Any recent links to purchase the amp from, especially the one with choke on the top. Also, which voltage ones are folks ordering 110v/115v/120v? Is there higher power option?

[FullRangeMan]

Any recent links to purchase the amp from, especially the one with choke on the top. Also, which voltage ones are folks ordering 110v/115v/120v? Is there higher power option?

There may be 220V as China use 220V 60Hz.

[Jon L]

Any recent links to purchase the amp from, especially the one with choke on the top. Also, which voltage ones are folks ordering 110v/115v/120v? Is there higher power option?

Prices are significantly higher post-tariff, but this is the store I bought from before.  You can order the 110V and choose the deep chassis model. You will also want to order FM30 tubes as this store ships with FU19 tube (perhaps you can message them to change tubes before shipping with additional charge?)
https://www.aliexpress.us/item/3256806188732044.html?spm=a2g0o.order_list.order_list_main.159.6fd61802m1ql8b&gatewayAdapt=glo2usa

[Jon L]

Next logical modification is coupling capacitor upgrade.  The stock cap is a metalized poly cap 0.334 uF/400V.  This is going to be an uncommon value when it comes to boutique caps, so I used Jupiter copper foil 0.22uF cap I already have in rotation.

After a few hours of re-settling, the expected changes were present, e.g. smoother yet more detailed presentation, better bass definition and punch, richer yet cleaner tonality, etc.  On the other hand, the stock cap is not a poor-sounding cap.  It mainly commits sins of omission as far as slightly lower level of resolution from top to bottom but sounds balanced, and the amp already sounds great with the stock cap.  In fact, a little extra top-end "zing" of the stock cap is a pleasant effect with the stock tube configuration. 



 

[shadowlight]

There may be 220V as China use 220V 60Hz.

Sorry, meant something more than 4w.

[shadowlight]

Prices are significantly higher post-tariff, but this is the store I bought from before.  You can order the 110V and choose the deep chassis model. You will also want to order FM30 tubes as this store ships with FU19 tube (perhaps you can message them to change tubes before shipping with additional charge?)
https://www.aliexpress.us/item/3256806188732044.html?spm=a2g0o.order_list.order_list_main.159.6fd61802m1ql8b&gatewayAdapt=glo2usa

Thank you.

[FullRangeMan]

Sorry, meant something more than 4w.

You need a more efficient speaker.
4W is the max that this tube delivery in Single Ended.
It will delivery high SPL with a hi efficiency plus 100dB speaker.
UltraLinear or PushPull will not have this good sound.