Min's World of Tube Audio

My New Website

2008-08-19T21:34:00.000-07:00

I have a new website dedicated to marketing my tube amps. If you have been following my blog, you might find it interesting. Please check out: www.tubeaudiolab.com

WE91A Monoblocks

2008-08-11T21:43:00.001-07:00

Western Electric 91A (WE91A) Monoblocks, construction in progress

Based on the article by Joe Roberts in Sound Practices, summer 1992

Pure Class A , singled ended directly heated triode, 9 wpc

JE Labs 2A3dx II

2008-07-30T22:39:00.000-07:00

JE Labs 2A3 Deluxe Single Ended Triode Amplifier

Completed on 7/30/08

3.5 wpc of pure class A DHT SE sound

Tubes: 2x 2A3s, 2x 76s, 2x 6SN7s, and 5U4

JEL 2A3dx

2008-07-11T10:56:00.000-07:00

JE Labs 2A3 Deluxe Single Ended Triode Amplifier

Completed on 7/10/08

3.5 wpc of pure class A DHT SE sound

Tubes: 2x 2A3s, 2x 76s, 2x 6SN7s, and 5Z3

Excellent match for those mini back loaded horns

A pure delight for bottleheads...

Those OPTs are trusty Transcendar custom wound in Hollister, CA. They really are fine sounding irons. This is 3rd amp that I built using those irons. It won't be the last for sure.

Completely handwired.

Dynaco ST-70 Type with KT-88s in Triode Mode

2008-06-01T21:47:00.001-07:00

ST-35 and VTA SP-6

2008-06-01T21:25:00.000-07:00

Dynaclone ST-35 with VTA SP-6 Preamp

Custom order

Completed on 5/31/08

Back Loaded Horn Speakers with Fostex FE-87

2008-05-24T22:38:00.000-07:00

Scott LK-48B Restoration Project

2008-05-04T22:26:00.000-07:00

Scott LK-48B Restoration Project

17 watts per channel integrated tube amp

4x Telefunken EL-84 (6BQ5/7189)

4x Telefunken 12AX7

2x 6U8A

I often hear fairy tales about how people unexpectedly find something really neat for peanuts in the world of audio. Such tales usually begins with a visit to a local thrift store or estate sale. Well, that sorta happened to me, except I didn't visit no store. I got it on eBay. Got a near mint Scott LK-48B (a kit version of 222C) for $304 by bidding with 30 sec to go in the auction (aka sniping). Yes, like they say on eBay, you don't just buy things - you win things. I was very happy with my purchase and was paying a reasonable price for an amp of such caliber.

When I got the amp I was mostly excited about hooking it up and listening to the amp for the first time. And I was not dissappointed! It sounded soooo good. There was a little glitch with one of 6U8As that failed to light up once in a while and caused one of the channel to go dead. The tube seems to be doing ok now, but I already placed an order for a replacement.

So, here is the fun part - since I had that minor problem with the 6U8A, other tubes caught my attention. I thought it might me a good idea to see if retubing the whole amp was in order. So, I started investigating other ones. To my surprise, all my other tubes, save the good 6U8, looked like they were original Telekunkens! After doing some research on the web, it was confirm that these were indeed genuine Telefunkens. So, basically I had an amp that had tubes that cost twice as much as the amp they are plugged in. Wow, I felt like one of those people on Antique Road Show!

Anyway, I am going to refurbish this beauty up. The chassis will be polished. All pots will be cleaned and lubed. I already placed an order to recap the whole thing, with Auricaps and all. More pictures to follow...

DC Double Darling

2008-04-09T22:31:00.000-07:00

Dynaco A-25 Speakers Restoration

2008-03-19T22:24:00.000-07:00

WE91 clone Project

2008-03-05T18:30:00.000-08:00

WE91 (Western Electric 91) 300B Clone Amp

JJ 300B (4oW heat dissipation) driven by 5693 pentodes

Conservatively around 9 wpc

Completed Feb 2008

So, I have been building loads of SET amps in pursuit of sonic perfection (more like psychological warfare) and I have come to a conclusion that such a thing is unattainable until I built this. I guess what really happen was I built something that I can be content with for a long time to come. I have built several DHT single ended triode amps with big bottles such as 300Bs, 6B4Gs, 2A3s, etc. in various combinations of designs, and I kept finding myelf comparing them not upto par with some of the Darling amps that I built. What was really disappointing was that Darling amps cost only 1/3 of these amps to build!

I did have high hopes in the WE91A. I have been studying this amp for a while on the web, and, after talking with Raymond Koonce who built many of WE91A clones, I decided to embark on building one for myself. It wasn’t an easy decision because parts cost alone was going to be well over a grand. I also wanted to custom paint the chassis, so it was going to be double the work.

I really didn't want to compromise on parts used for this amp. I wanted make sure that this amps will last for a long time to come. Every part used well exceeded in their respective power handling ratings. A good example of this is the Hammond's OPTs that are rated 30W. The amps only puts out 9 wpc. There are in all 9 irons, three of which weigh 35 lbs together. My estimate of the finished amp is around 55 lbs.

I detest tube amps with PCBs, mostly because I like to do things in old school fashion. Everything is point-to-point wired. I tried using as few electrolytics as possible. The only places that I used electrolytics were bypass and filament bias caps. The bypass caps are Black Gates. I used Solens in many places. The coupling caps are Hovlands. The filter caps (large silvers one sticking out on top) are oil filled ASC polypropylene caps. I could have saved a lot of money using alternate parts, but I didn't want to. The power supply section is very robust with a split rail PS with separate chokes.

I was originally going to have the chassis and iron bells powdercoated, but it didn't work out due high cost and lead time. So, I decided to just spray paint the whole thing using automotive paint and clearcoat. It was a lot of work. I put on at least 5 coats of cherry red paint + buffing with rubbing compound in between coats. I then put on about 8 layers of clearcoats with buffing in between coats. I then baked the whole thing at 145'F overnight. I guess it came out alright. I also have to mention that I learnt an important lesson. I neglected to wear a respirator while spraying and really compromised my immune system. I came down with the flu the next day, from which I was bedridden for 3 days. The flu lasted about a full week.

I have never punched or drilled this many holes before. I still marvel at how I fitted all that stuff in a 17x10x3 steel chassis.

Ok, so how did it sound? When I heard it play music for the first time, I said to myself, "Finally, this is what I'm talking about!" I think I have finally found a 300B amp that I could fall in love with. JE Labs 300B was alright, so was GSG 300B hybrid, but they weren't anything that I would get excited about.

The WE91A is so SMOOTH. This amp makes listening so effortless. Even with some of tube amps I have, which I think are decent sounding by anyone's standard, I would get hearing fatigue after a few hours. This one could sing the whole day for I care.
I don't know what it is - 5693 pentode drivers, the design, voodoo- whatever it is it's working for me and I am loving it. Was it worth the flu, money, and time? Absolutely! All that was forgotten when I heard it play my favorite records for the first time.

1626 Darling Single Ended Triode SET Tube Amplifier V

2008-02-09T13:05:00.000-08:00

1626 Darling SET Tube Amplifier

0.75 wpc of pure SET sound

5th Darling on sale on eBay

This amp is headed to Singapore. The guy loves the amp. I do hope that he's got a pair of efficient speakers for this to drive. I never get tired of building Darling amps, especially when they bring in good profits to perpetuate my hobby.

JE Labs 300B SET Amps

2008-02-09T13:04:00.001-08:00

JE Labs 300B SET Amplifier with James 6123HS OPTs

Custom ordered

Completed on 2/25/08

This was by far the simplest amp that I've wired so far. It helped to have all the big components mounted on top and to have all that space underneath. The only thing that I would have done differently is to have the OPT where the choke is. Steve Chu, my customer, wanted it this way for looks and I just had to make it work. Well, customers are always right. When I first fired the amp up, I noticed that there was clicking noises coming from the bypass cap for 6SN7. Apparently, the voltage handling on the cap was a bit low and was working to its limit. I replaced them with 47uF 160VDC and the clicking went away.

Under the hood with everything mounted on the top plate. Notice the octopus of wires coming out of those holes off of power tranny.

Voila! The finished amps. I thought my Dynaco MKIIIs were big. They are puny compared to those 300B monoblocks!

Yep, all those irons for glorious 9 watts of SET audio.

These monoblocks weigh at least 40 lbs each.

Size comparison of James 6137HS iron to 6113HS. The 6137HSs are SE 30W 7K primary and 4, 8, 16 ohms secondary taps. I am putting 8 ohms loads to 16 ohms taps to lower the primary Z to 3.5 k for 300Bs. It's all about ratio when it comes to trannies.

How does it sound? I am very happy with these amps. They really groove to the music when listening to rock or pop music. Great textural rendition and rhythmic integrity when listening to complex ensemble. 9 wpc is plenty for my listening level and the efficiency of my speakers, but it's still just 9 watts. They do seem struggle a tiny bit when I try to push the full power of orchestra through them. All in all, their dynamic performance is amazing for most music I have CDs, mp3, and records for.

Will I build these for myself? Probably not, mostly because of its size and the cost of construction. I think the same caliber of sound can be achieved using other designs in stereo at 3/4 of cost or less.

1626 Darling Single Ended Triode SET Tube Amplifier IV

2008-02-09T13:03:00.001-08:00

1626 Darling SET Tube Amplifier

0.75 wpc of pure SET sound

4th Darling sold on eBay

Tidy Transmission Line DIY Speakers

2008-02-09T13:02:00.000-08:00

Transmission Line Speaker Design by SDS Labs

My first DIY speaker

Completed on 2/10/08

This is my first attempt to build DIY speakers. I have always been curious about how transmission line speakers sound for a long time. It seems to me like there are as many people who are interested in the TL design as there are people who are into tube amps. Being a experimentalist at heart, I had t give this one a try. I used Sheldon Stokes' Tidy TL design. What attracted me to the design is its compactness: most of TL type of designs that I read about are either really complicated folded horns or 6' tall pipes like Voigt Pipes, neither of which had much appeal to me for the lack of WAF (wife acceptance factor) and cost of construction.

I used some 2' x 4' 1/2" MDF boards that I had purchased to build a dog house for Sophie, but I never got around it. The biggest challenge that I faced in making these speakers was cutting. I so wish I had a table saw for this. I just didn't get the most accurate cuts using circular and jig saws. I had to do lots of sanding and patching up with wood filler to hide my mistakes. But it worked out ok in the end. The speakers are veneered in mytrle burl (purchased on eBay for 100 sq/$20) and finished in 3 coats of natural stain, followed by 3 coats of satin polyurethane.

How do they sound? They seem to perform best with vocals and instrumentals. They definitely do lack bass extension (below 80 Hz), but this was to be expected with this design. I have a subwoofer picking up the slack on it, so I don't have any problem with that. One thing that I do want to try next is to use different drivers. The ones in there are 5.5" Dayton PA drivers that I purchased for $22/pr at Partsexpress. They tend to sound more like PA speakers than hi-fi speakers. I am leaning towards any of Fostex 6" 160 series, and I will post the result of that upgrade when it happens.

1626 Darling Single Ended Triode SET Tube Amplifier III

2008-02-09T13:01:00.000-08:00

1626 Darling SET Amp

This is my 4th Darling I made to Sell

Sold on eBay in 30 minute!

VTA SP6 Preamp

2008-02-09T13:00:00.000-08:00

Roy Mottram's VTA SP6 Preamp

Completed on 1/16/08

PCB available at www.tubes4hifi.com

I guess I got the point where all the tweaking with power amps has been done and it's about time that I looked into preamps and speakers. So far, I have gone through several solid state preamps (NAD, Adcom, etc.). I also tried a passive preamp with a homebrew phono preamp. All these gave me performances a lot to be desired. I have always wanted to try a tube preamp or two, but I was somewhat put off by off-the-wall prices of preamps that were worth considering. I guess the Dynaco PAS-2/3 would have been an option that can be had for under 200 bucks, but some tweaking (PS upgrade, recapping, and tone control removal) would have to be done to make it listenable. I figured that if I were to spend more than $200 on a decent preamp, I might as well built something new. After having done some research, I narrowed it down to Aikido preamp or Roy Mottram VTA SP-6. I settled for the latter, mostly because I had to have a phono stage.

The harware (knobs and RCA inputs) on the chassis is salvaged from my passive preamp. Built on an oak base stained in dark cherry, and finished in high gloss polyurethane. I ended up using a 12x12 lexan polycarbonate as a top plate to showcase all the electronics and glowing tubes inside. The preamp uses 2 NOS RCA 12au7s for the linestage and 2 TAD 12ax7s for the phono stage. This probably is the best sounding preamp that I owned so far. It sounds very neutral and lets all the subtleties in recording pass without any coloration. It's fast, articulate and versatile. I would recommend it to anyone who's serious about their audion stuff.

JE Labs 2A3 SET Amp

2008-02-09T12:55:00.001-08:00

JE Labs 2A3 Single Ended Triode

3.5 wpc DHT SET Sound

Finished on 1/5/08

This is my first attempt to handwire a hi-cost SE DHT amp. I have built several Get*Set*Go amps that are SE DHTs, but they were all built using PCBs. As much as I have liked how the GSG amp sounded, I felt like I would never fully appreciate amp building unless I handwired a SE DHT. Since building a hum-free and low-noise SET is often difficult, I welcomed the challenge of building one that was not only hum-free, but one that sounded good.

Since this was going to be a keeper amp, I didn't want to compromise on parts used. I wanted to make sure that the amp was overbuilt with resistors, caps, and irons that were well over the required power rating. I went with James 6123HSs, which BTW was compared quite favorably with Hashimoto OPTs by many fellow amp builders. The schematic I used was JE Labs Simple 45/2A3, with a minor change: instead of 1 k for the 2A3 cathode resistors, I used 900 ohms. I got more out of those Sovtek 2A3s this way. I never really liked the electrolytics in the PS, which I think have a tendency to produce mechanical sound. So I went with all Solens PP caps in the PS section.

Gotta love those gold plated binding posts. Thet are oversized and heavy duty.

Square waves at 40 hz

At 1 kHz

At 10 kHz

At 20 kHz

Wiring was quite easy, but building the chassis was not. I am not going into details of how the chassis was made, but it suffices to say that I could use some good set of tools, including a table saw. So, how does it sound? Simple awesome! The amp is very musical. It has all the excellence and frailties of SE DHT. Very well balanced with tight bass. Mids and highs are very fluid. I love listening to classical music through this amp. I really think that the James OPTs do a great job at frequency response. Will I keep it? 100% yes. I finally found an amp worth of my collection. I hope this will be a family heirloom.

1626 Darling Single Ended Triode SET Tube Amplifier II

2008-02-09T12:54:00.001-08:00

1626 Darling SET Amp

Tung-Sol 1626 Outputs Driven by RCA 12SL7

0.75 wpc of pure SET magic

Completed on 12/13/07

Sold on eBay

6AS7 6080 Lilliput Single Ended Triode SET Tube Amp II

2008-02-09T12:53:00.000-08:00

6AS7 6080 Lilliput Single Ended Triode Amplifier

6AS7/6080 Output driven by 12AX7

1.5 wpc

Completed on 12/2/07

Sold on eBay!

DIYTUBE GSG 300B Amp

2008-02-09T12:52:00.000-08:00

Directly-heated Triode (DHT) GSG 300B

4.0 wpc of Pure Single Ended Power

Completed on 11/16/07

Sold on eBay!

DIYTUBE 6B4G GetSetGo GSG SET amp

2008-02-09T12:51:00.000-08:00

GET*SET*GO Single Ended Triode Amp

2x 6B4G with 6SL7 Driver, 6AX5-GT Rectified

3 wpc of pre SET power

Completed on 10/17/07

My friend Steve Chu whom I made the original GSG SET with 6B4G for wanted have the 300B mod done on his. So here you have it. Those big bottles are JJ 300Bs. I honestly can't tell if they are any better than EH 300Bs that cost 40% less. They look pretty imposing though. The sound got cleaned up quite a bit. I really do like the solid state rectifier with the amp. I personally prefer sharper sound. I tend to get a warmer sound and less definition. Some folks like that. It's merely a matter what you like the best.

Shure M-65 Clone Phono Record Tube Preamp Preamplifier

2008-02-09T12:50:00.000-08:00

Shure M-65 Clone Phono Preamp

RIAA equalization using a pair of 12AX7

Powered with a pair of filament transformers

Completed on 9/26/07

Inspired by the fact that my new Adcom GTP-600 preamp didn't have a phono stage...

Yep, I could have used a bigger chassis, but I worked with what I had on hand. I Can't believe I fit all the parts in a box this small. It looks cool messy like this with an open top, so the cover's off. It sounds pretty natural and brings outs the vintage vinyl sound. The only modifications that I made on the original M-65 schematic is rectification on the filament power supply and the increase in capacitance for the PS. A cool little project under 60 bucks...(I did use parts pulled from my other guitar amp that I gave up on).

Dynaco Mark III MKIII Monoblocks - Refurbished

2008-02-09T12:49:00.000-08:00

Vintage Dynaco Mark III (MKIII) Monoblocks

60 watts per block

1x 6AN8 drives 2x KT88 per block

Restored and converted using a pair of MKIIs

completed on 9/13/07

6L6GC SET "Simpleton" amp

2008-02-09T12:48:00.000-08:00

"Simpleton" 6L6GC Single Ended Triode

1.5 watt per block

1x 6SL7 drives 2x 6L6GC

Designed by me

Sold on eBay!

Built on 9/2/07

I happened to have a aluminum base painted in cashmere beige for another project the never got finished. It was a perfect fit for the top plate. The 6L6GCs are brand new Svetlanas that I orignally bought for my ST-70, but I wasn't impressed with how it sounded with those tubes. So, I am using a matched pair for this amp and the other for a guitar amp. The 6SL7 is nearly NOS Tung-Sol. They all look neat with the brown base. The rectifier tube (6X5) is Sylvania.

This amp sounds pretty good. I was a bit concerned with the 7.5k primary on those Valve Juinor OPTs, but multiple secondaries allowed for experimentation and I liked 4 ohms tap to 8 ohms load with my speakers the best. The bias is very similar to 6V6 tubes, so I tried those on the amp and it sounded great with a plenty of SET power. All in all, this project was a success. I built it to sale on eBay, I like it enough that I might just keep it...

13EM7/15EA7 (6EM7) "Miniblok" SET amp

2008-02-09T12:47:00.000-08:00

"Miniblok" 13EM7/15EA7 (6EM7) Single Ended Triode

1 watt per block

15EA7 - Dual Triode (Driver/Amp) Silicon Rectifiers

Sold on eBay

Inspired by late Fred Nachbaur

Built on metal-on-wood chassis.

Finished product! 13EM7s are dual triodes that have one section capable of high gain but low power, thus ideal for a preamp stage, and the other capable of low voltage gain and high current, thus ideal for a power stage.

Under the hood. Again, like the 6080 SET, it's a bit crammed, but the layout is well thought of before hand and minimal wiring is employed. The power trannies are two 12.6 VAC filament transformers in parallel, powering the two 12.5 V 450 mA filaments and a reverse connected 12.6 VAC > 240 VAC transformer for the plate voltage. There is a bit of hum (hardly audible), which could be due to AC filament supply. Fred used -3o VDC for fixed grid bias. The man was genius. He knew how to use cheap (including tubes) parts to make amazing sound tube gears. The schematic for "Miniblok" is available here.

These amps sound real nice. Don't let 1 wps fool anyone - these probably are one of loudest amps I have heard save the ST-70. The sound is open and spacious with strong mids and a plenty of tight bass. I have a tube audio buddy who built one of these in stereo and think that it sounds better than his 300B that cost him a fortune to build! 13EM7s do make some nice SETs. It would be interesting to hear a Push Pull version.

6AS7 6080 Lilliput Single Ended Triode SET Tube Amp

2008-02-09T12:46:00.000-08:00

"Lilliput" 6AS7/6080 Single Ended Triode

RCA 6SL7GT Driver - 6AS7/6080 outputs
Rectified with 6AX5GT

Completed on June 2007

Built on compact 8x5x2 Hammond chassis

6AS7s can be substituted with 6080s. They are twin triodes with incredibly low plate resistance, often used in OTL designs or head phone amps.

Under the hood . A bit crammed, but got everything in there including those tiny output trannies that fit comfortably in one hand. In spite of their small size, they pump out good volume with efficient speakers.

The amp was originally designed to be driven by a 6SN7, but I had some trouble biasing the tube with parts that I had on hand. So, the 6SL7 came into rescue and it worked out great. This project gave me an idea that I might want to experiment with later - run 6as7 in parallel per channel and hook up bigger OPT with lower primary impedance (1.25 k to 8 ohms). That ought to yield around 7 wpc. Hmmm...

DIYTUBE Dynaco ST-70 - Refurbished

2008-02-09T12:44:00.000-08:00

Vintage Dynaco ST-70 Modified with DIYTUBE ST-70 Driver Board

35 wpc in stereo and 70 w in mono
Completed in June 2007

The ST-70 is an amp that no tube aubiohobbyists should live without. This was an eBay find that was in pretty good working + cosmetic condition. However, I wan't terribly impressed with its sonic characteristics, particularly it's tubby bass and lingering muddiness in mids+highs. So, it was was time for a makeover.

...and after!!!!

The chassis was completely gutted and cleaned first with Fast Orange to get rid of years of organic deposits. The chassis was then polished several times with Turtle Wax's chrome polish, followed by Blue Magic metal polish, all of which are cheaply avaiable at local discount autopart stores. All tube sockets were replaced with ceramic octals.

and after...

Shannon Park's DIYTUBE ST-70 driver boards are a standard upgrade of the 7199 boards. The tube complement is 1x 12ax7 and 2x 12au7. The design is pretty much a time tested modified Mullard design dating back 50 years. The board allows individual biasing of output tubes for push pull perfection.

Under the hood...everthying was rewired and neated tucked away.

The only thing that's left to be done on the amp is to retube. 3 Svetlana and 1 Mullard EL34s (Russian productions) are on the power duty for now, and I would like to experiment with 6L6GCs or KT77s. All in all, the restoration was a great success wiht much improved sonics. It pumps out some serious wattages of pure listening enjoyment - deep and accurate bass and very forward mids. Great sounding amp!

DIYTUBE Dynaco ST-35

2008-02-09T12:43:00.000-08:00

DIYTube Dynaco ST-35

17 watts per channel
DIYTube ST-35 rev. D 12AX7/12AU7 - EL-84 Outputs - SS Rectifier
Completed on May 2007
Sold locally

Built on a wood base finished in golden oak. Output and power irons are 1650F and 270HX respectively. JJ Electronics' EL84 output tubes are driven by RCA's 12AX7A and 12AU7A in push pull configuration, putting out around 17 wpc.

These gold-plated binding posts connect to 8 ohms speakers. I didn't find the need for making connections for 4 ohms speakers, as I neither own a pair nor have any plans to use one.

Shannon Park's DIYTube ST-35 is available at DIYTUBE. I am very impressed with the sound production using this board. IMHO, it probably is the best Dynaclone 35 product available.

So, how does it sound? When I first turned the amp on outfitted with brand spanking new JJ EL84s, I really didn't like this amp at all. It sounded harsh and totally lacked "warmth" that tube listeners crave for. My disappointment was big especially when it wasn't cheap to build it. But about a month into listening, the EL84s are finally broke in and sounds amazing now. Its sound production is unembellished, pure, vibrant, and warmly rich. Bottom's up, got a winner here!