FU-19 (FM30, 829B) Single-Ended Tube Amp!

Jon L · « **Reply #80 on:** 1 Mar 2025, 05:36 am »

I ordered two RCA 3E29 tubes from two sources and realized one had a top mica and the other didn't, among other differences. Doing some research, it seems like only very early (1940's-early 1950's) 3E29 tubes had top micas. Being OCD, I found one more of each and ordered them to match, although I doubt they sound much different..or do they?

0228251828_HDR_1740804618449 by drjlo2, on Flickr

FullRangeMan · « **Reply #81 on:** 1 Mar 2025, 09:12 am »

Dont know about the sound but the mica plate increase heat dissipation and isolation.

Jon L · « **Reply #82 on:** 1 Mar 2025, 05:16 pm »

Quote from: FullRangeMan on 1 Mar 2025, 09:12 am

Dont know about the sound but the mica plate increase heat dissipation and isolation.

Yeah, playing two different tubes on L and R, it's difficult to tell if they sound different, but so far, I would say every 829B, 3E29, and FM30 tubes sound at least similar enough that I don't think I could tell them apart in blind tests, which is great since they ALL sound fantastic

The smaller tubes, FU19, 5894, do sound different in a more "modern" way.

mick wolfe · « **Reply #83 on:** 1 Mar 2025, 06:31 pm »

FWIW, the stock supplied tubes in the FU29 sound pretty good. The exception being a slight hum from one of the 6N2's. These soon to be replaced with Voskhod 6N2P-EV's. Have some NOS 829B's on the way as well. I'll report back if I discover anything earthshaking.

Jon L · « **Reply #84 on:** 1 Mar 2025, 09:52 pm »

Quote from: mick wolfe on 1 Mar 2025, 06:31 pm

The exception being a slight hum from one of the 6N2's. These soon to be replaced with Voskhod 6N2P-EV's.

Let us know how the 6N2P-EV sounds compared to the Chinese 6N2, which sounds unexpectedly good to my ears. In the past, whenever I "upgrade" stock Chinese tubes with Russian tubes, I seem to gain detail and bass at the expense of cooler tone..

mick wolfe · « **Reply #85 on:** 3 Mar 2025, 11:29 pm »

Quote from: Jon L on 1 Mar 2025, 09:52 pm

Let us know how the 6N2P-EV sounds compared to the Chinese 6N2, which sounds unexpectedly good to my ears. In the past, whenever I "upgrade" stock Chinese tubes with Russian tubes, I seem to gain detail and bass at the expense of cooler tone..

I'll take the heat on this one. I should have done further research.

The Russian 6N2P's that were sent do not work in this circuit.(FU29) Apparently Chinese 6N2's are the only option. I just ordered a pair of Shuguang 6N2's for back up. Hopefully the RCA 829B's I bought ARE a true drop in.

mick wolfe · « **Reply #86 on:** 3 Mar 2025, 11:46 pm »

General AI overview I found too late...."A Russian 6N2P generally cannot directly replace a Chinese 6N2 with the 6N2P needing a lower filament voltage than the Chinese 6N2." This all of course after I'd put the 6N2P's in the FU29 only to hear low level moaning as opposed to music.

Jon L · « **Reply #87 on:** 4 Mar 2025, 12:06 am »

That's odd. I asked multiple AI's (ChatGPT, Gemini, and Perplexity), and I get the answer 6N2 and 6N2P are interchangeable. If asked specifically about filament voltage to Gemini:

'A "6N2P" tube has a filament voltage of 6.3 volts, while a "6N2" tube also typically operates at 6.3 volts; essentially, they are considered the same in terms of filament voltage as they are very similar tubes with only minor variations, with the "P" signifying a slightly different manufacturing process in the Russian designation system.'

If not compatible, that's a bummer because I just paid for a bunch of 6N2P tubes..

======

Upon further research, there are people who have successfully replaced Chinese 6N2 with Russian 6N2P:

"I replaced the Chinese 6N2 tubes in my Nobsound MS-10D hybrid amp with Russian 6N2P-EV tubes for a very noticeable increase in SQ. Likewise I've used them in place of 12AX7 using socket adapters off eBay. There should be adapters let you run 12AX7 instead I should think"

https://audiokarma.org/forums/index.php?threads/6n2-tube-replacement.101311/

Your FU29 amp is different from my FU19 (FM30) amp. FU29 amp uses two 6N2 tubes and claim 10 watts whereas FU19 amp uses only one 6N2 and claim 4-6 watts. Perhaps the 6N2 parameters are different. I would also check to see if that 6N2P tube that didn't work is defective..

FullRangeMan · « **Reply #88 on:** 4 Mar 2025, 01:16 am »

GSTube have 6N2P-EB at $1.67 each.
https://www.gstube.com/catalog/?key=6n2p&submit=Find
ЕВ (EV) higher reliability, longer life and mechanical ruggedness.

6N1P have the warmest tone, warmest of all likely alternates, slightly lower resolution
6N2P more dinamic than 6N1P (more gain).

mick wolfe · « **Reply #89 on:** 4 Mar 2025, 04:27 pm »

Jon L.... I'm hoping for your sake that the FU29 is the exception that can't use the Russian variant. That said, I'll have the tubes tested to get to the bottom of this. As it is, I could easily live with the Chinese 6N2's. (maybe because I'll have to) The earlier hum I mentioned seems to have dissipated....at least for now. Now to pop in the RCA 829B's with my fingers crossed

mick wolfe · « **Reply #90 on:** 4 Mar 2025, 09:16 pm »

First off, mine are stenciled 6H2pi(pi symbol) not 6N2P. However, from all I can gather, the specs are identical on all these Russian 6N2/6H2 varieties. They were sold to me as 6N2P-EV/EB. Bottom line, both test fine even under operating conditions. Now here's where it gets interesting. My tube tech looked at both the Russian 6H2 and Chinese 6N2 and said "these aren't the same tubes just by looking the two pin bases." .... pin base wiring is completely different. Chinese 6N2 filament wiring at pins 1 and 9. Shield at pin 5. Russian 6H2 filament wiring at pins 4 and 5. Shield at 9." He said the only true equivalent of the Chinese 6N2 he could find is just another Chinese 6N2.

opnly bafld · « **Reply #91 on:** 4 Mar 2025, 09:58 pm »

You may need a new tube tech.

FullRangeMan · « **Reply #92 on:** 4 Mar 2025, 10:02 pm »

Quote from: mick wolfe on 4 Mar 2025, 09:16 pm

First off, mine are stenciled 6H2pi(pi symbol) not 6N2P. However, from all I can gather, the specs are identical on all these Russian 6N2/6H2 varieties. They were sold to me as 6N2P-EV/EB. Bottom line, both test fine even under operating conditions. Now here's where it gets interesting. My tube tech looked at both the Russian 6H2 and Chinese 6N2 and said "these aren't the same tubes just by looking the two pin bases." .... pin base wiring is completely different. Chinese 6N2 filament wiring at pins 1 and 9. Shield at pin 5. Russian 6H2 filament wiring at pins 4 and 5. Shield at 9." He said the only true equivalent of the Chinese 6N2 he could find is just another Chinese 6N2.

(I or i) are tubes for hi freq impulse use,
They are not intended for audio range
hence the low price.

mick wolfe · « **Reply #93 on:** 5 Mar 2025, 12:14 am »

Quote from: opnly bafld on 4 Mar 2025, 09:58 pm

You may need a new tube tech.

If I find a new tube tech, do you think that will magically make the Russian 6N2P(6H2pi) work in the FU29

The Russian tubes I bought won't work in the FU29 regardless of how many tube techs are brought in for consultation. Maybe another Russian variant would work with my situation being an exception to the norm, but it is what it is. Bottom line, I'm perfectly happy with the Chinese 6N2's anyway.

Jon L · « **Reply #94 on:** 5 Mar 2025, 01:08 am »

Quote from: mick wolfe on 4 Mar 2025, 09:16 pm

Chinese 6N2 filament wiring at pins 1 and 9. Shield at pin 5. Russian 6H2 filament wiring at pins 4 and 5. Shield at 9." He said the only true equivalent of the Chinese 6N2 he could find is just another Chinese 6N2.

That's very strange. From diagrams I found on Chinese 6N2 tubes, 4 and 5 are filament and 9 shield

Jon L · « **Reply #95 on:** 5 Mar 2025, 01:30 am »

And the photo of inside of our FU19/FM30 amp shows 6N2 pin 4 and 5 (yellow wires) are filaments. I guess I will find out when my Russian 6N2P tubes show up. If bored, you might want to look inside your FU29 amp and see if pin 4 and 5 are wired as filaments?

mick wolfe · « **Reply #96 on:** 5 Mar 2025, 02:31 am »

Well, if that's the case I would think the Russian variants would work fine for you in the 19. As that fits my tech's observation of the Russian tube. (pins 4 and 5 filament)
This leads me to believe the 29 is a different animal as my Chinese 6N2 works perfectly with pins 1 and 9 filament, pin 5 shield.
This whole thread was never meant to rain on anyone's parade. It was just cautionary tale warning of the issue I encountered. At this point really a non issue as I'm happy just sticking with the tube that works. If I'm tempted to roll tubes, it'll be with the 829B's. The NOS RCA 829B's I just popped in work great BTW.

Jon L · « **Reply #97 on:** 5 Mar 2025, 03:16 am »

Quote from: mick wolfe on 5 Mar 2025, 02:31 am

The NOS RCA 829B's I just popped in work great BTW.

Love that tube

The stock 6N2 tube really sounds darn musical and neutral. I'm listening to chinese 6N2->RCA 829B as I type. Fabulous sound!

FullRangeMan · « **Reply #98 on:** 5 Mar 2025, 03:21 am »

Quote from: Jon L on 5 Mar 2025, 01:30 am

Wow, a real audiophile amp, very simple circuit.

mick wolfe · « **Reply #99 on:** 5 Mar 2025, 04:34 pm »

Problem clarified....maybe. Under intense light and magnification I was able make out the stenciling. The tube called out as a 6N2 in the amp specs is actually stenciled 6H3pi. Which I think is in no way a 6N2.