Analog Sauce: The Audioromy M828-A

AKA: Tube amp part II

Audioromy M828A on the bench
Audioromy M828A on the bench

I have been fooling around with this amplifier for a month now and I have to say, it is rather fun.  There are a few hazards when purchasing Chinese HiFi (ChiFi) equipment.

The first thing to note; several places such as Ebay and Amazon list this as a single ended class A amp. That is not true, it is a double ended class AB amp.  I confirmed this by measuring across the two sections of the output transformer.

Second thing to note; this amp came wired with a fuse on the hot side of the AC mains and the power switch on the neutral.  Switched neutral (AKA earth, return or ground) wires are a hazard, so I rewired it, putting the switch after the fuse on the hot side after the fuse.  Another safety thing, the edge of the metal chassis was not de-burred.  I took a flat file to it and removed the burr, thus avoiding any future lacerations.

Finally; there is no manual provided with this unit.  There are a few sets of instructions on how to re-bias after tube replacement which are technically correct but not the best way to go about it. Those instructions direct the user to solder a low value resistor from cathode to ground then measure the voltage drop on that resistor to calculate plate current. While this is a valid way to deduce plate current, the power output tube has two tubes in one envelope and the cathodes are tied together. The plate current can be calculated for both sides, but there is nothing indicating that the two sides are balanced and one of the tubes can red plate. This was also noted in those instructions found on line.

That being said, I thought I could type up a set of directions that are more suited for this amplifier.  But first, read this dire warning about working with High Voltage:

This amplifier has lethal voltages present during operation. It is possible that lethal voltages can be stored in certain components for days after the amplifier has been turned off and disconnected. By removing the protective covers, those components will be exposed and you may come in contact with them if you are not careful.

If you are planning to service this amplifier, it is vital that you have basic electronics and electrical knowledge. This includes all applicable safety procedures for working on high voltage components.

If you do not have this knowledge, please bring this amplifier to a qualified electronics technician or repair shop for service.

I am not responsible for any injuries or damage suffered to yourself or others if you decide to undertake repairs of this equipment.

I acquired a few of these Ulyanovsk GU-29 tubes and decided to try them out.  The maximum plate dissipation for this tube is 40 watts with bulb temperature of 175°C and ambient temperature of 20°C.  I measured the bulb temperature at 142°C and the temperature in my living room ranges from 20ºC to 32ºC (68ºF to 90ºF).  I could, in theory, bias these tubes for a higher plate dissipation, if I wanted to.

Ulyanovsk GU29 tube, made in the USSR
Ulyanovsk GU29 tube, made in the USSR, circa May 1964

I asked my Russian friend what the assembly line person or factory manager might think if he or she knew that the tube made in their factory would end up in being used in a home audio amplifier owned by a guy in New York.  She said “They would have a stroke.”  Ulyanovsk had and still has a heavy military presence, thus they likely assumed that all their products would be used by the Soviet Navy or Army.

Being that this particular tube sat around in a warehouse for 55 years, it was slightly gassy.  When I first turned the amp on, there was a distinctive pink glow and a couple of small internal arcs.  It probably would have been a smart idea to light up the filaments for several hours before applying plate voltage.  Unfortunately, I had that idea after I’d already energized the amplifier.  In any case,  I increased the bias and reduced the plate current.  After a while, things settled down and I got to work re-biasing the amplifier.

To re-bias the amplifier after new tubes have been installed, some initial data needs to be gathered.  Basically, this procedure involves measuring the resistance of the plate circuit, then measuring the voltage at the output of the plate voltage supply and the voltage at each of the plate terminals on the power amp tube.  The plate voltage on this amplifier is +460 DC or so voltage above ground potential.  Obviously, this is a dangerous voltage and if you are not familiar with working on high voltages, do not attempt this procedure.  The best way to measure these test points is to use clip leads; turn the amp off, let the capacitors discharge, place the clip leads on the appropriate test points, turn the amp on, make the measurement, then turn the amp off, repeat as necessary.

After replacement of the power tubes (V-5 and V-6), the bias for those tubes should be checked and adjusted as follows:

A. To measure plate dissipation as set by the factory, perform the following steps:

1. With amp completely turned off and disconnected from the AC mains, remove the bottom cover. Ensure that the large power supply capacitors are discharged to ground. With an accurate ohm meter, measure from the exposed lead on L-1 (TP-1) on the power supply board to the input to the anode resistor (R-21 or R-22 in the schematic diagram (TP-2, TP-3, TP-4, TP-5)) for each tube (four measurements total). Make a note of those measurements. For reference, my amp measured between 163 to 165 ohms.

2. Reconnect the amp to the AC mains and turn on power (be sure to read the dire warning about high voltage above). With an accurate DVM, set to DC volts scale, carefully measure the voltage on the exposed lead of L-1 on the power supply board to ground, make a note of it. This is the B+ voltage for the amplifier. Carefully make another measurement between the input of the anode resistor (R-21 or R-22) and ground (four total measurements, likely to be the same), this is the plate voltage for the power tubes. Make a note of that as well.

3. Subtract the plate voltage from the B+ voltage. For my amp, this was 462 VDC 458 VDC = 4 volts. This can also be measured between TP-1 and TP2 through TP-5 See charts 1 and 2 below. This is the voltage drop. Using ohms law, calculate the plate current for each section of the amp:

Voltage drop ÷ resistance = plate current or 4.17 VDC ÷ 163.2 ohms = 0.0255 amps (25.5 ma) plate current.

Using ohms law, calculate the plate dissipation for ½ of the power tube:

Plate voltage × Plate current = Plate Dissipation or 458 V × 0.0255 amps = 11.7 watts.

Add both sides of the tube together for the total plate dissipation.

Chart 1: Left power tube, V-5

Test points Resistance Plate voltage (TP-2/3 to gnd) B+ (L1 or TP-1 to gnd) Voltage drop Plate current Dissipation
TP-1 to TP-2 163.2 ohms 458 VDC 462 VDC 4.17 VDC 25.5 ma 11.7 watts
TP-1 to TP-3 163.9 ohms 458 VDC 462 VDC 4.16 VDC 25.3 ma 11.6 watts

Total power dissipation for V-5 is 23.2 watts or 77% of maximum for the stock FU29 tube. That is slightly above the commonly recommended safe range of 70% of maximum, but it is tolerable.

Chart 2: Right power tube, V-6

Test points Resistance Plate voltage (TP-4/5 to gnd) B+ (L1 or TP-1 to gnd) Voltage drop Plate current Dissipation
TP-1 to TP-4 164.4 ohms 458 VDC 462 VDC 4.23 VDC 25.3 ma 11.6 watts
TP-1 to TP-5 164.8 ohms 458 VDC 462 VDC 4.14 VDC 25.1 ma 11.5 watts

Total power dissipation for V-6 is 23.1 watts or 77% of maximum for the stock tube.

Test points for tube bias adjustment
Test points for tube bias adjustment
Pin out, 829B tube
Pin out, 829B tube

B. When replacing the power tubes, it is recommended that they be replaced in kind in pairs.

Step 1: Increase the tube bias (measured on pin 2 or 6 of the power tube) to -25 VDC and check the plate voltage drop on both tubes. Increasing the bias will reduce the plate current and thus the plate dissipation.  This will be noted as a decrease in the voltage drop.  A good starting set point would be 50-60% of the normal factory plate current (Vd ÷ Plate R) setting. The voltage drop can be measured directly by connecting the positive lead to TP-1 and then measure TP-2 to TP-5 with the negative lead. Use clip leads, placing them on the test points with the amplifier turned off.  Be extremely careful; these test points are +460 VDC above ground when the amp is energized. Read dire voltage warning above.

Step 2: Turn off amp, discharge power supply capacitors, replace tubes.

Step 3: Allow the new tubes to burn in for approximately 3-5 hours with reduced plate dissipation, make sure that the amplifier is connected to a suitable load on the speaker output terminals.

Step 4: With the DVM connected to TP-1 and TP-2, slowly bring the bias down until the plate circuit voltage drop approaches the values for the old tube.  Repeat procedure for each plate circuit (TP-1 to TP-3, TP-1 to TP-4 and TP-1 to TP-5). Recalculate plate dissipation. Be sure not to exceed plate dissipation of the tube! It is best if the tube is biased to run at about 70-75% of the maximum plate dissipation.

Step 5: With the amp fully warmed up, turn out all lights and observe the plates of both tubes for any signs of red plating.

Step 6: Carefully measure the balance between the two plate outputs of each tube by placing the DVM leads on TP2 and TP3 for V5 and TP4 and TP5 for V6. Alternatively, the test leads can be placed directly on Pu-1 and Pu-2 of the power tube under test. Between these test point pairs, the DC voltages should be zero or close to it. Note; there will be some fluctuations in the hundredths or thousandths volt ranges. Very, very carefully, adjust the bias control pots until the voltmeter reads zero or as close as you can get to zero.

Step 7: Recheck the plate dissipation for both sides of the tube, make sure that they are closely matched and not exceeding the maximum plate dissipation for the power tube in use.

I discovered several things during this process; it is very easy to red plate one side of the tube while adjusting the bias controls.  Fortunately, I noticed this right away and was able to stop the red plating quickly.  The Ulyanovsk tubes seem none the worse for wear.  As Alex Ovechkin says “Russian machine never breaks.”

Next, the schematic diagram I posted previously is not correct for this version amplifier.  There are two bias voltage controls, one for each grid.  There is no balance control, the tubes are balanced by making very careful adjustments to one or the other of the bias controls.  Updated schematic diagram:

Audioromy M-828A schematic diagram
Audioromy M-828A schematic diagram

When the amplifier is properly biased and balanced, the distortion figures should be very low, less than 0.5 to 1% THD at full power.  It makes a big difference.

The point of all this is to 1) have fun, 2) perhaps learn something about tube (or valve) circuits and 3) listen to really clean, good sounding audio.

Consulting work

I have been doing some non-broadcast related consulting work lately. It is actually sort of fun and pays well.  One thing that I have become involved in is solar installations, or more precisely data communications from solar installations.

It seems that a critical part of any solar installation are the production numbers.  Owners/investors like to see a return on investment.  They like to know that their system is working properly.  Getting hard data on electricity production is an important part of the customer service aspect for a solar installation company.  Being able to remotely monitor the system and be alerted of any faults or failures helps keep those production numbers where they should be.

Solar installation on a large fuel storage tank:

Thin film solar panels installed on a fuel storage tank
87.5 KW Thin film solar panels installed on a fuel storage tank

It turns out that those fuel storage tank facilities use a lot of electricity. Not just for the fuel transfer pumps, some of their product is heavy oil; #4, #6, Resid or bunker oil is very thick (or viscus). Tanks, pipes and pumps for those distillates must be heated to certain temperatures in order to move them. That is all done with electric resistance heating.

There is a very good book about oil and how it is extracted, transported, refined and used called Oil 101 by Morgan Downey.  It is an eye opening read, to be sure.

Looking at the tops of those tanks; there is a lot of unused area.  It is a novel idea to use that area to generate power for the tank farm.  The thin film solar panels come in rolls. They have adhesive backing and are peel and stick. The nice part about this type of installation; the steel tanks help keep the panels slightly cooler, which boosts their production on hot summer days.

Three phase solar inverters installed on fuel storage tank
Three phase solar inverters installed on fuel storage tank

In this installation, each inverter reports to a web site that logs all of the output data, as well area temperature and percentage of sun light. This system helps the installation company and tank owner know if there are any problems with the array.  In order for that to work, the LAN needs to be set up and a communication device used to connect to the public network.

All in all, that was a fun project.

By the way, if anyone needs a solar system installed, I know a company that can do it.

Emergency transmitter replacement

Bad weather or other disasters can strike any time of year.  Around these parts, the most dangerous weather events occur from early spring through late summer.  In the past twenty years or so, we have had tornadoes, hurricanes, micro bursts, flooding events and so on.  All of that got me thinking about what would happen if a tower came down, or a transmitter building was destroyed by fire, wind, water, etc.

If past events can predict future performance, there would ensue a mad scramble to replace damaged equipment and or get some type of temporary antenna into service.  That is what happened in great City of North Adams, Massachusetts when the tower that held the cell carriers, the 911 dispatch, and the local FM radio station came down in an ice storm.  Fortunately, we had a single bay Shively antenna at the shop that we trimmed up and installed on a temporary pole with 200 watts TPO.

That will cover the city of license, provided there is electricity…

What if there where an event that was so devastating that the electrical power would not be restored for months?  Think about hurricane Maria in Puerto Rico.   After that event, the infrastructure was so devastated that there was not even the possibility of getting a fuel truck to deliver diesel for the emergency generators at the hospital in San Juan.  It can happen.

With that in mind, I began poking around and thinking about how I would get something back on the air.  In the face of massive disasters, AM and FM radio is still the most effective way to communicate with the general public.  Radios are still ubiquitous in homes, cars and businesses.

Bext 30 watt exciter
Bext 30 Watt FM exciter

In a short period of time I came up with a couple of solutions.  First, the frequency agile Bext exciter uses a single solid state rectifier feeding 24 volts to the power supply board.  The audio input includes a mono balanced line level input which can be fed by a computer sound card or some other simple source.

Bext 30 Watt FM exciter power supply
Bext 30 Watt FM exciter power supply

From there +12, +15 and +20 VDC are created to run various circuits.  The heat sink cooling fan is the only thing that runs on 120 VAC, which is old and I might replace with a 24 VDC unit.

Bext 30 Watt exciter power supply voltage
Bext 30 Watt exciter power supply voltage

The power output is about 22 watts, which is not bad.  That will certainly get out well enough from a high spot and provide good coverage when the power is out because all the other in band RF noise generators will be off.

6 volt, 435 Ah batteries
6 volt, 435 Ah batteries

Then I though about the deep cycle batteries in my barn.  These 6 volt, 435 Ah units have been around for a couple of years, but last I checked, they still held a charge.  Other deep cycle batteries from things like golf carts, fork lifts, campers, boats etc could also be pressed into service.  The point is, 24 VDC should not be impossible to create.

To keep a charge on the batteries, this solar panel will work:

225 Watt, 36 volt solar panel
225 Watt, 36 volt solar panel

This setup would require some sort of 24 volt DC charge controller, which I found on Amazon for less than $15.00 US.  This charge controller has selectable 24/12 VDC output and also has two USB ports which would be handy for charging hand held devices.

I measured the power draw while the exciter was running 20 watts into a dummy load, it draws 120 Watts.

The final part would be some sort of antenna with transmission line.  For this situation, a simple wire center fed dipole hung vertically would work well.  This can be fabricated with two pieces of copper wire and a few insulators.

Simple dipole antenna
Simple dipole antenna

The lengths of each wire can be calculated as follows:

Approximate length in feet: 234/f (MHz)

Approximate length in inches: 2808/ f (MHz)

Approximate length in cm: 7132/f (MHz)

For the FM band, maximum length of wires needed will be 32 inches (81 cm).  Insulators can be made of anything that does not conduct RF; PVC, ABS, dry wood, dry poly rope, etc.

Emergency FM band dipole
Emergency FM band dipole, cut to 88 MHz, lowest FM frequency

I recommend to cut the wires slightly long, then trim little bits off of each end while watching the reflected power meter on the exciter.  To keep RF from coming back down the shield of the transmission line, make 8-10 turns, 6-8 inches in diameter of coax as close to the antenna as possible and secure with a wire tie.  This will create a balun of sorts.

My emergency FM kit consists of:

  • Bext Frequency agile exciter
  • 30 feet, RG-8 coax with N male connector on one end
  • 4 ten foot RG-58 BNC male jumpers
  • 1 four foot LMR-400 N male jumper
  • Dipole antenna, cut long
  • Solar charge controller
  • Small basic tool kit; hand tools, plus DVM and soldering iron
  • Power cords, extension cords
  • 300 watt 12VDC to 120VAC inverter (pure sine wave)
  • 20 feet audio wire
  • Various audio connectors; spade lugs, XLR male and female, RCA, 1/4 TRS, etc
  • Various RF connectors; PL-259, N, BNC, etc
  • Bag of 12 inch wire ties
  • 3 rolls of 3M Scotch 88 electrical tape
  • 100 feet of 3/8 inch poly rope

This is all kept in a sturdy plastic storage bin from the Home Depot.  If needed, the batteries and solar panel are stored in the barn along with an assortment of other goodies.

Will it ever be needed?  Well,  I hope not.  However, it is much better to be prepared to restore services than wait for somebody to show up and help.  Sitting around complaining about the government does not relieve those people in need during and after a disaster.

Tube Amp

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 cable, 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:

FU-29 Chinese equivalent to RCA 829B dual pentode tube
FU-29 equivalent to RCA 829B dual pentode tube

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 more rare are the 3E29 tubes, which were designed for VHF pulsed radar.  These are dual pentode tubes which 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.

Reflector factory, 6N3P-E dual triode tube
Reflector factory, 6N3P-E dual triode tube

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.

Audioromy M828A push pull tube amp
Audioromy M828A, power transformer and output transformers

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.

Audioromy M828A underside
Audioromy M828A underside

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 nor 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 where also several modifications suggested.

  1. 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.
  2. 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.
  3. Install a bias regulating circuit using an LM317 voltage regulator between the output tube cathode and ground.  This seems like a good idea.
  4. 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:

Audioromy M-828A schematic diagram
Audioromy M-828A schematic diagram

Full schematic here: http://www.engineeringradio.us/blog/wp-content/uploads/2019/03/M828A_full.pdf

After all my investigations where finished, I put the amp back together and plugged it in.  I then ran my known CD’s though 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 though my homemade speakers, which are mid range deluxe, stringed instruments sound 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 on 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 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 every 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.