I have dipped my toe into the world of tube (or valve) audio. The first thing that I learned was that in general, tube amps are expensive. It seems that the least expensive amps run about $1,000 US, and from there it seems the sky is the limit.
There are a number of less expensive Chinese versions floating around, most of the tube audio experts call them garbage. Myself; I am not so sure. There are also a lot of somewhat dubious claims made by the same experts about speaker cables, AC power conditioning, and so on.
I was going to build a single-ended tube amp based on the KT88 design found here:
That is a whole series of videos, eighteen in all I think, on the design and construction of a single-ended KT88 audio amp. If you have the time, well worth it to watch.
Then I decided that I really do not have a lot of time for that and I just wanted to try a tube amp and see if there is really that big of a difference. Thus, I purchased one of the Chinese designs based on the RCA 829B tube, which is kind of exotic looking:
That is the Chinese version FU-29, there is also a Russian радиолампа ГY-29. The good news is that there are lots of these tubes available for not too much money. New Old Stock (NOS) RCA 829Bs run about $25-30 each. A Ulyanovsk GU-29 (NOS) runs about $10.00 (made in the USSR). Somewhat rare are the 3E29 tubes, which were designed for VHF pulsed radar. These are dual pentode tubes that can be run ether parallel (single-ended) or push-pull. They were originally designed for VHF transmitters, but have been put into use in HF transmitters and audio amplifiers. The USSR versions are long-life militarized versions and designed for aircraft radar; flying upside down at Mach 2 in -50 C temperatures 18,000 meters AMSL… My Russian friend tells me I am joking. I am joking.
The driver tubes and phase inverters are 6N3P-E (6N3, 6N3P, 6N3P-EV, 5670, 2C51 or 396A can also be used) which is a double triode tube, made by Reflector in Sartov, Russia. These tubes are also militarized long-life versions.
The Audioromy M-828A amplifier seemed like a good compromise between price, power, and workmanship. I ordered the amp from Amazon and it took about a week to arrive. The first thing I did was take it apart and look at it. I was expecting poor workmanship and cheap components, etc. Overall, it seems to be pretty well-made. There are two printed circuit boards; one for the power supply, the other for the front end before the two power amp tubes. The power supply uses solid-state diodes, which some view as a compromise to a tube amp design. There are also several power supplies on one board; 460 VDC B+ for power tubes, 220 VDC screen supply, a -25 VDC grid bias supply, 12 VDC for the audio switching relay, +6 VDC for the driver/phase inverter filaments. I like the idea of DC filament voltage on the driver tubes.
This amp is configured for push-pull and rated at 30 watts per channel. I will test all of that plus measure THD, frequency response, and so on.
There is no manual, which I find a little bit annoying. Also, there is a lack of a schematic diagram or any instructions on biasing and balancing the tubes when they are replaced.
Being thus annoyed, I did some deep diving on the intertubes and found that some people had posted on how to re-bias and re-balance the thing after tube replacement. There were also several modifications suggested.
- Replace the input potentiometer with something a little more substantial. It does seem to be a little bit cheap and I do not like the notches in the volume adjustment. I will do this mod.
- Replace the coupling caps with oil-filled units. Not so sure about this one, but I might try it just to see if it makes a difference.
- Install a bias regulating circuit using an LM317 voltage regulator between the output tube cathode and ground. This seems like a good idea.
- Roll (replace) the input and power tubes with better versions of US-made or Russian-made tubes. The input tubes are 6N3P-E tubes from Reflector (Sartov, Russia) which are already pretty good tubes. I might replace the FU-29’s with a set of GU-29’s at some point.
There appear to be several schematic diagrams with slight variations based on the changes in design over the years. Several designs have different input and phase inverter tubes. Some have different power supplies, still, others show no anode resistors or a cathode resistor. This is the diagram for the amp that I own, which was produced circa 2018 or so:
Full schematic here: https://www.engineeringradio.us/blog/wp-content/uploads/2019/03/M828A_full.pdf
After all my investigations were finished, I put the amp back together and plugged it in. I then ran my known CDs through it and it sounded a bit rough. I was a little bit disappointed until someone said that it takes about 10 hours or so for a tube to break in. I connected it to my speaker test load (8 ohm, 50 watt resistors) and let it run for a day.
What a difference a day makes. The second listen to the same CD proved to be much, much better. There is definitely some coloration from the tubes. A side-to-side comparison between my solid-state Kenwood VR-309 amp and the Audioromy M-828A has the tube amp sounding much richer. There is no real way to say it; it sounds full while detailed and clean all at the same time. Playing through my homemade speakers, which are mid-range deluxe, stringed instruments sounds very detailed. You can hear the pick hit the strings on an acoustic guitar. You can hear the bow scrape across the strings on a cello. It is unlike any amp that I have ever owned.
I am enjoying very much listening to Dave Mathews and Tim Reynolds Live at Luther College CD as I am typing this.
Now, I don’t know what the difference between this amp and the $10,000.00 version of the same tube amp made in Canada, other than the $9,500.00 difference in price.
A few comments about this amp and the 829B push-pull amp design. First of all, since the screen grids are connected internally, there is no way to run this tube in ultra-linear mode. Usually, ultra-linear mode involves taking feedback from the output (anode) or the output transformer and feeding it into the screen of the power tube.
Secondly, it is widely commented that these amps are notoriously difficult to bias and balance. One or both sides of the output tube will red plate due to over current. I am hopeful the LM-317 bias regulator circuit will take some of the difficulty out of this. With an ordinary push-pull amplifier, the balancing issue is taken care of with matched tubes. Since both tubes in this push-pull circuit are in the same envelope, getting a matched pair is not likely. So, the tricky act of balancing the two outputs from the same tube will have to be carried out each time the tubes are replaced. That being said, hopefully, a set of those Soviet tubes will last for a long time.
One thing that I did do is make a bunch of voltage measurements and noted them on the schematic diagram. If there are ever any problems with this unit, having a set of base voltage measurements should go a long way toward troubleshooting and repairing it.
Finally, while the 829B is a rather exotic tube, it likely does not perform to the level of an EL86 or KT88 single-ended design. That being said, I have no problems with purchasing this amp and I am enjoying the toob audio sound very much.
14 thoughts on “Tube Amp”
Great post, Paul. Could you share details of the bias regulator circuit when you install it?
Barry, not problem. I’ll also post the THD and power test results, when I put the Russian tubes in.
Dear Paul !
Could you share details of the part whole changed
I have been looking fooling around with this thing for a while. The first thing I did was change the volume input control to a Audio Note 100K audio taper pot, ANVPOT 81914. This is much than the stock unit.
I also replaced the 0.22 uf coupling caps with Mundorf EVO Oil MUNDORF-76424. That made no noticeable difference in the audio therefore the stock coupling caps are good enough quality for this amp.
I looked at the LM-317 cathode current limiting circuit and decided not to try that. The reason is that this amp runs AB1, thus the fixed bias supply needs to be well regulated. If the tube temporarily draws too much current during a big audio peak, that cathode circuit will start to conduct and add distortion to the output. I think the tubes, particularly the Russian ones, can handle a little bit of extra current on large audio peaks, so long as the program material has a lot of dynamic range.
I did replace the tubes with newer Russian ones. The input tubes are Vinniza 6N3P-EV. The -EV tubes are ruggedized for military use, the seem to do a better job with the input audio.
If I where to make more modifications to this amp, I would look at the power supply. It seems a bit light in the screen supply area. I would also improve the filament circuit for the driver tubes.
Dear Paul !
Thanks for shared the information, Just I got two vacuum tube 829B RCA brand . I am interesting with Audioromy M-828A amplifier from China made. I have plan buy the Audioromy M-828A amplifier back , then modify some part like as you was did your amplifier. I think if you change the Pre-amp tube 6N3P-EB to 12AX7 so sound will better. And we can replace Power Supply circuit with rectifier tube ( 5U4G, GZ34…) . It’s my some ideas. Good luck and success !
The 12AX7 change out is a great idea, especially since there is already an existing 12 volt filament circuit. The other thing that I am going to change are the 33K pots on the bias control. The stock units are kind of cheap and it is difficult to get a good adjustment point. Bourns 3296W-1-303LF multi turn pot looks like it will do the trick.
I am continues waiting for you change other part to modify this amplifier. I saw the ebay have sale tube 829B with pricing around 25USD per each.
Dear Paul !
please share the part was you changed it , like as LM-317 control Bias , 0.22uf capacitor , VR….
Any idea on the specs of the output transformers?
Hello Audioromy 829b users and potential buyers!
I am using and modding this amp for some time now.
It is good as is from the factory but after some tweaks is another league. If you know what to do is definitely worth to make it better.
Not recommended for someone not familiar with soldering and high dc voltages!
This amp can arrive with wrong connected wires to main AC socked.
Resulting live voltage on the amp enclosure. Easy job just swap 2 wires. Input and driver tubes make huge impact on the sound.Chinese tube as are bad, bad, bad. Russians are better but 6750 or 2C51 instead 6N3 are much better than any Russian I tried.
Using GI30 instead of FU29 is another story. Much better bass, serious kick and more musical that you can have with GI30.
Genuine NOS 829 tubes are available and not expensive, not tried yet by myself.
Replacing all coupling caps is really recommended.
Sometimes Chinese use caps available for them at the moment.
Fake WIMA caps are possible so…
Each amp may have different caps and values.
I used MKP 4x 220nF instead 4x 47nF and 4x 1uF instead 4 x 0.33nF.
This was massive improvement to stock capacitors.
Caps voltage has to be this same or higher than original!
Upgrading or increasing 2 power supply electrolyte caps will provide more accurate bass and less distortion when played louder. I just soldered additional 2x 470uF 450V one each to the existing power caps.
Power tubes need to be properly balanced and biased to avoid red plating after change. If you have oscilloscope you may to fine tune
NFB by change tiny 47 pF ceramic caps and NFB resistor.
That depends on tubes used in the amp.
It could be extremely good amp after mods as it responds well to all changes. NOS tubes are cheap compared to other amps and sound quality after mods can be shocking for many people’s.
I did had a lot tube amps before. I do have 3 now.
This one is best value for money for me now. Damian
I can confirm that changing biasing and balancing pots for multi turn pots make whole procedure much easier.
Analog voltage meter may help with balancing power tubes as digital ones are to fast sometimes.
Please let me know of the primary impedance of OPT at 8 Ohm.
That is a good question. I have not investigated the OPTs, however, it I have a few minutes this weekend, I will.
Answer to your question: I have a turns ratio of 32:1 making the impedance ratio 1,024. For an 8 ohm load, the plate impedance is 8K ohms.