I got busy last Friday. As I mentioned in my last post, Gleeson Mechanical was scheduled to replace our air conditioner/electric heater with a Day and Night heat pump. Things started off slowly as they didn’t show until nearly 10am, then they had to return to the shop because they forgot some necessary item. They didn’t really get started until nearly 11am.

The two guys they sent out were efficient and the installation was completed by 2:30pm. They did a good job and hauled the old unit, including all of the underfloor ducting away. We have a new heat pump, new ducting, new return box and a new thermostat. It works really well. The only gripe I have is this – I told them to be aware of the property line. Our lot is narrow – all of the Viewpoint lots are narrow. They placed the unit carefully, but they ran the evaporator drain line over the property line, creating a trip hazard for my neighbor when they walk the path to their back deck.

Our neighbors, Steve and Deena said they weren’t concerned about the drain line.

Meanwhile I received a delivery. A few weeks ago, I ordered a set of hand wound pickups for my Stratocaster guitar from Adam Asmus (Tone Hatch Pickups). His site showed a two-week wait for the pickups to be wound and he shipped them exactly two weeks later from Norfolk, Nebraska. However, it took the USPS a week to get them past Omaha, Nebraska.

I went to work on it right away. I removed the pickguard and old pickups, then installed the new ones and soldered it all together.

I’m still tweaking the pickup heights, but these are very smooth sounding pickups with what I can only describe as a richer tone. I thought I would lose some output with these, but I don’t really notice a change in volume.

If you know me or have followed this blog, you know I like to build and play vacuum tube guitar amplifiers. The warm tone of vintage-type guitar amplifiers and the feel when you drive them hard can’t be duplicated in my opinion. Recently I started looking at vacuum tube stereo amplification.

Stereo amplification is much different than instrument amplification. In a guitar amplifier, a certain amount of distortion is desirable – you want the ability to increase the sound level of the guitar and create everything from a clean sound to an over-driven distorted, fat tone.

In stereo hifi amplification, you’re aiming for a transparent gain in sound levels while faithfully reproducing the original tones. I became intrigued by the idea of low-power, single-ended, class A amplification. I learned a lot from Steve Deckert’s website for his Decware amplifiers. I would love to have one of his Decware Zen Triode amps, but the cost of an entry-level Decware is about $1,000 and there’s a long waiting list to buy one – it could take eight months or more before I could have one built. He hand builds everything. This is only a 2.3 watt amplifier. If you wonder how 2.3 watts could drive a speaker, check out his website.

I did some looking around and found affordable alternatives made in China. These obviously don’t have the same build quality, but there are people who have documented the shortcomings and how to fix them and come up with a reasonably good sounding amplifier. After researching a bit, I decided to try an amp made in China by Douk Audio and sold in the US under the brand name, Nobsound.

There are two reasons I decided to try this amp – first, it’s affordable at $315 on Amazon. Then, most of the issues on this amp are documented and fixes are easy. It’s a hand-wired, point-to-point (PTP) chassis that’s easy to modify. I figured this would get me in the door of stereo hifi amplification without breaking the bank.

They list this amp as a 6.8 watt amplifier, but I think that’s wishful thinking. It may be able to generate 6.8 watts of power, but the distortion level would be unbearable way before you got there. It’s really a 2 or 3 watt usable power amp.

Amazon delivered the amp on Friday and also delivered another product I needed for this amp. Douk Audio made a few questionable choices in components for this amp. Many of the early adopters of this amplifier complained of tubes burning out and if the rectifier tube burned out, they didn’t know how to diagnose and replace it – they just wrote it off as a cheap Chinese throwaway amplifier.

There are a couple of reasons this happened. First of all, Douk Audio sends these amps to North America for Nobsound with the same power transformer used in China. The thing is, China household voltage is nominally 110 volts. In North America 120 volts is the norm and it’s not unusual to see 122 or 124 volts at the wall outlet. The power transformer at 120 to 124 volts was supplying higher than specified voltages throughout the amplifier, killing vacuum tubes.

To counter this, I ordered an APC Line-R 1200-watt regulated power supply. It has three settings for the output voltages and can handle up to four devices connected to the output totaling 1200 watts. One of the settings is 110 volts. I measured an actual 108 volts with the 110 preset on mine. This should be fine. This unit costs about $60, so now I’m $375 into the project.

I connected the amplifier through the APC unit and connected my Celestion Bookshelf 8-ohm speakers. These aren’t the highest efficiency speakers and I hoped I would get enough volume out of them. Speaker efficiency is rated with a specification called sensitivity. A one watt, 1kHz signal is applied and the sound pressure level (SPL) is measured one meter from the center of the speaker. My speakers are rated at 89db – the minimum sensitivity recommended by Steve Deckert for low-watt use.

I was surprised! The amp sounded decent and had no problem driving the speakers. But, there was more to do. The vacuum tubes supplied with the amp were mostly good quality Russian military grade surplus tubes. This included the rectifier tube – I think Douk Audio figured they could address the rectifier failures by upgrading to the Russian tube instead of the cheap Chinese tube originally used and I hear is still in use in the China market. This would get them past the warranty period.

The power tubes are very cheap Chinese tubes. The Chinese tube is designated 6P1 and is a copy of the Russian 6P1P (6П1П in cyrillic). The tubes supplied with the amp are poorly made with a dirt-like contaminate visible inside the glass tube envelope. It’s like looking through a very dirty window. I tried scrubbing the glass with steel wool to see if it was on the outside – it’s the inside of the tube! I ordered two matched pairs of 6P1P – EV (6П1П-EB in cyrillic). Although this is a single-ended tetrode amp, it’s unusual in that two tubes are wired in parallel for the output of each channel – thus the need for matched pairs. This cost $60, so now I’m $435 into it.

I bought these tubes from an outfit called Riverstone in California. They are new old stock Russian military surplus made in 1985 and 1987 at the Svetlana factory in Saint Petersburg, Russia. This plant was built by RCA in 1937. The EV or EB suffix indicates an extended-life, ruggedized tube and these have the OTK quality control codes.

No major tube manufacturer makes these tubes anymore, just a couple of small Chinese companies do. Luckily, there are large stockpiles of the NOS Russian high quality tubes still available. The change in output tubes made a huge difference in sound quality. The frequency range was extended, reproducing higher highs and lower lows, the difference was dramatic.

But wait, there’s more. Douk Audio has another design flaw. In the power supply, they used two 150uF electrolytic capacitors – one after the rectifier tube and one following the choke. For the rectifier tube, this is bad news. I’m guessing they did this to simplify inventory and cut costs by stocking a boatload of capacitors of the same value. The thing is that larger, higher capacitance filter capacitors to smooth ripple in DC current can be a good thing. But, there is a limit and too much isn’t necessarily better. The rectifier tube is a Russian 5U4C tube which is equivalent to the RCA 5Z4. RCA’s datasheet specifies no more than 40uF – the Brimar datasheet for this tube is more conservative and specifies 33uF maximum. Well, 150uF is too much and places undue stress on the rectifier tube.

So, I ordered a Vishay Beyschlag 22uF 450 volt electrolytic capacitor made in Austria from Digi-key to replace the 150uF Chinese one. I also ordered four high-quality Solen 0.22uF (220nF) 650 volt capacitors made in France to replace the no-name brand Chinese coupling capacitors. With tax and shipping, it was another $30 – making this project cost a total of $465.

The wiring and solder connections inside the amp are tidy and impressive. I expected to reflow bad solder joints, but I didn’t find any bad ones. The cap replacements took me a lot longer than I anticipated. I told Donna this was because, in my usual fashion, I spend extra time looking for a part or tool I had in my hand a few minutes before. This can add 10-15 minutes to the job. Then I spend time looking for a small fastener or item I dropped and spend another five or 10 minutes on my hands and knees looking for it.

At the end of the day, I have a very delightful little single-ended class A vaccum tube amp for under $500. Can’t beat that! Last night I was listening to an acoustic guitar piece by Ronnie Earl and the sound reproduction was so clear, I realized I could hear his fingers sliding along the strings on chord changes! It was like he was sitting in front of me playing his guitar. Now that I know this amp is a keeper, I’ve ordered back-up vacuum tubes to have on hand, although these Russian military grade tubes should be long-life items.

On Sunday, our friend Sini flew in from San Diego. Donna picked her up at the airport and they had fun plans for the next few days. They made it into a girls retreat by renting a room at the Westgate Painted Mountain Resort and planned hiking and horseback riding activities. Last night, we all went to Baja Joe’s for a Mexican seafood dinner – Sini’s treat. Thanks, Sini!

Donna took Sini back to the airport this morning. I’ll add some photos of the hike and horseback ride in my next post as this is getting too long already.

Speaking of seafood – as I stated above, Friday was a busy day, but Donna topped it off with a dinner of walnut crusted tilapia with broccoli on the side.

The weather has remained pleasant with daily highs around 70 degrees and overnight lows in the upper 40s to 50 degrees. The mornings have had some high clouds that burn off before noon, but yesterday, we had some sprinkles of rain. It wasn’t much – I was on the pickleball court giving my coaching clinic and we managed to play through the short, light shower. The week ahead doesn’t show any significant changes.

*Just so you know, if you use this link to shop on Amazon and decide to purchase anything, you pay the same price as usual and I’ll earn a few pennies for the referral. It’ll go into the beer fund. Thanks