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Bose 901 Series 1 Equalizer #29827 Repaired!
I recently got to fix up another Bose 901 Series 1 equalizer which I received for repair. These are some of my favorite electronics to work on – they’re easy to work on and each one has its own history. Every one of these I’ve seen has been slightly different and this one was no exception.
This one in particular has 4 separate repairs. One is especially interesting.
The last one is somewhat clever. A 10K resistor, probably 5W, across those terminals is the modification to run this equalizer on 240V in Europe or similar. It’s been jumped with a solid piece of copper bus wire taking it out of the circuit but still leaving it in the equalizer if conversion ever needs to happen again.
Top-off testing was next. The neon indicator lamp in the power switch was flickering badly – it had likely been losing neon through the metal-glass interface very slowly over the past 40 years. It’s a neon lamp attached directly across the AC mains with a voltage dropper/current limiting resistor in series. The total power consumption is a few mA at line voltage.
Here it is removed from the circuit. The lamp/resistor combination is actually a single component – they’re welded together. I replaced it with an NE-2A/150K resistor combination, I believe the resistor is 1/8W the draw is so small. The envelope size of the new bulb is about half that of the old one, but it fits in well from the bottom to let wire tension keep it in place better.
After burn-in testing, the equalizer checked out perfectly! It has incredibly clean switches. The others I’ve serviced are much improved after cycling but can hang up the first time they’re used and these didn’t even need cleaning.
This one is going to be a great performer for a long time, and these are a lot of fun to work on.
(If you need yours repaired, I can help. If you’re looking for a parts kit to fix your own, I can offer that too!)
Klipschorn Meets Altec
I love old hi-fi stereo equipment, especially the larger stuff which really had a lot of personality. While doing some research on Klipschorns, I discovered a striking reinterpretation of the classic Klipsch design:
An Altec Lansing multicell horn in place of the integral horn on the classic design. That looks incredible, really commands the room!
Check it out: retro vintage modern hi-fi
Bose 901 Series 1 Equalizer Repair (Deux)
A client recently engaged me to repair his Bose 901 Series 1 equalizer. This second one was in a bit worse shape electrically than the previous one I’ve written about, with one channel entirely dead and scratchy switches. I received the equalizer and got right to opening it up to check out the internal components. It shows evidence of having been serviced a couple of times in the past, with several of the electrolytic and output capacitors replaced as well as some of the .015uF signal capacitors (the white rectangles near the center and center-right of the image.)
Right away, one problem is immediately visible – one of the output capacitors (C12, near the top right) has split down the side and shows visible leakage from the positive lead. The originally fitted capacitors are the silver cans; the black cans were fitted some time later. They must have been lower quality replacements, as it’s one of the previously replaced units that’s failed most obviously.
My client requested I replace all resistors outright, so I didn’t spend extra time testing the carbon composition resistors before getting straight down to replacement. In the past example I discussed, many of the resistors had drifted far beyond their printed tolerance markings, and as all carbon resistors are going to behave similarly, it would have been a purely academic exercise to see drift figures on these resistors which were going to be replaced anyway.
The circuit board is very easy to remove. All connections, except one set, are along the far side of the board. I de-soldered and cleaned the terminals of the output connections to make it easy to remove.
The underside of the board. Not visible in the photo, but visible when closely looking, I could see where flux had flowed on the board during previous rework. This is normal during rework, and if you know what you’re looking for it can give away previous service if you’re not sure if a board has been worked on before or not.
I started off with the electrolytic capacitors. The two primary filter capacitors were replaced with Sprague Atoms capacitors. Sprague has been around for many, many years and these were manufactured in the USA. I frequently find Sprague capacitors in the vintage equipment I service, it’s great to see they’re still being produced right here at home. The rail filter capacitors I replaced with high quality USA-made CDE capacitors.
Then on to the resistors and signal capacitors. All resistors were switched with metal film replacements which are temperature and time-stable with precision tolerances of 2% or better, with the overwhelming majority being replaced with 1% or better. My supplier sent some with a different body style, so several are physically larger but specified identically. The signal capacitors were replaced with polymer film models at 400V or 630V tolerances, the up-rating ensuring they’re being run well below their tolerance and so should be stable for longer than the life of the speakers they’re attached to. Finally, the output capacitors were replaced with Japanese production Nichicon capacitors which are frequently found in high-end audio equipment.
Fortunately, all 10 transistors (2N5088) were good and didn’t require replacement, but I do stock new replacements just in case.
Ceramic disc capacitors such as the 10pF model in the center of this shot aren’t subject to failure the same way the other models were, and so the originals were retained.
I used control cleaner followed by repeatedly cycling the switch positions to clean their contacts. The Tape Monitor switch remained a bit touchy, but the others cleaned up nicely. My client preferred to preserve the originality of the appearance, and the switch works fine with just a couple seconds of extra touching to make it connect, and so it was left original.
With these repairs, the equalizer should last nearly forever barring some unfortunate outside circumstances. Only the highest quality parts were used to ensure perfect sound reproduction and long life.
(If you need yours repaired, I can help. If you’re looking for a parts kit to fix your own, I can offer that too!)
New Project: Jamo MPA-101 Amplifier Repair
My friend brought me this Jamp MPA-101 amplifier he picked up at a garage sale, complaining in only puts out some rumbling and humming. It looks like it’s suffered a failure in the power supply at some point. I’ll be trying to bring it back to life, as it’s a nice compact desk amplifier. There’s no published service information, so this should be an interesting challenge – the boards are very repairable if I can track down where the faults are.
A resistor in the power supply section, the largest blue one near the center of the photo, appears to have been overheated as it’s visibly discolored. This likely means something has faulted in the power supply, most likely a capacitor. I’ll be taking the board apart to test the capacitors, and go from there.
Bose 901 Series 1 Active Equalizer Repair [Vintage Hi-Fi]
(See other Bose 901 Series 1 Active Equalizers I’ve fixed, and get info about your own repair: Bose 901 Active Equalizer Repair Page)
While some other projects have been pending information and parts, I’ve had a chance to work on the Bose 901 Series 1 equalizer that’s been on my bench for a couple of weeks.
The equalizer is a necessary component of the Bose 901 speaker system, Bose’s highest-end hi-fi equipment. The speakers are specially designed in a way that requires the signal to be equalized and pre-amplified, and if you don’t use the Active Equalizer they’ll really sound pretty bad. It’s surprising how many people have forgotten this fact about the 901-series speakers over the years, but the lack of using an equalizer might be where derogatory slogan “Bose: No Highs, No Lows” came from.
This particular model came to me from a client complaining about distortion and eventually signal loss in one channel, and general sub-par sound. He had the equalizer for about a year after purchasing the set from a collector, and it never sounded quite like it should and rapidly degraded from there. The capacitors in the unit had probably been going bad for a while but only crossed the threshold to completely dead after some time in use.
The Active Equalizer offers 30-some combinations of curves to select and enough pre-amp gain to maximize the speaker’s output.
The equalizer is fairly simple construction, using a single-sided PCB, ten low-noise 2N5088 transistors (five per channel) and an assortment of capacitors and resistors and a few inductors to do the work of changing the frequency curve of the audio signal.
With the top cover off, you can see inside clearly. This particular equalizer came to me with reports of distortion and low gain. Obvious immediately are the large orange and red capacitors. The red models especially have visible discoloration at the very top – exactly like I saw in the 1951 Farnsworth K-262P last month. This is both good and bad: it means the problem was easy to diagnose, but unfortunately, it also means that even 1960s-70s era metalized film capacitors can be nearing the end of their service life and that doesn’t bode well for a lot of other equipment from similar and slightly later years. I also noticed that many of the resistors are the original carbon composition type, which is known to absorb moisture and change values. As the resistors in the signal path, any drift can change the equalizer’s effectiveness. I spot-checked a sample of the resistors, and found that 30% of them had drifted past their stated tolerance and were also going to need replacement.
The filter capacitors. 2x500uF and 7x100uF @ 25V.
The EQ is easy to service. The board is secured with four screws and lifts up.
The back of the board is the business end. This is a simple, single-sided etched PCB common to the era. Very easy to work on with my Hakko desoldering tool. Since both capacitors and resistors have begun to fail, it’s only inevitable the rest of them won’t be far behind so I elected to replace most everything on the board, filters included.
On to the metal film signal capacitors, I’ve replaced them with identical capacitors with a higher voltage rating, between 400V and 630V. These should be more durable and last longer before requiring service again. Miniaturization means these beefier parts occupy the same footprint as their lower-rated vintage predecessors.
Next up, resistor replacement. I made a checklist to follow as I replaced each channel pair of resistors. I’m using high-precision metal film resistors instead of carbon composition resistors, all rated at 1W (versus 1/2W for the originals) and a maximum of 5% tolerance with many coming in at 1% to ensure long-term precision and stability. The original resistors were rated at Metal film resistors are less subject to drift in the first place, and selecting precise values ensures a long life and stability.
Some of the wiring had become brittle with age and needed to be re-soldered to the board after being flexed a couple of times. No big deal.
This project required about 5 hours of touch-time for the re-work and a couple more hours of research as it was my first 901 equalizer on my bench.
Quite a few replacement parts:
With brand new capacitors and high-precision resistors, this Bose 901 Series 1 Active Equalizer should be good to go for many years.
It’s likely that all the Series 1 and Series 2 equalizers could benefit from a rebuild at this point, and possibly even the Series 3 equalizers. (If you need yours repaired, I can help. If you’re looking for a parts kit to fix your own, I can offer that too!)
Identifying Your Bose 901 Series 1 Equalizer [Vintage Hi-Fi] [Production Runs]
I’m working on a Bose 901 Series 1 equalizer for a friend. The 1970s-era film capacitors have failed (like I’ve seen before) and the equalizer has begun to introduce distortion instead of remove it like it’s intended. As expensive, top of the line audiophile speakers they definitely shouldn’t sound like that, but fortunately it’s a pretty easy fix.
The Bose 901 Series 1 equalizer was offered in two production runs, which can be interchanged and either will work fine with a pair of Series 1 speakers, but they have slightly different components. I spent some time with the circuit schematics and identified how to tell the two apart – it’s extremely simple!
Along the back wall you can see the row of silver capacitors used in the power supply filtering. These are all 100uF 25V electrolytic capacitors, which help ensure the preamp transistors have clean power. On the Second Production of the Series 1 equalizer, there are 7 of these 100uF capacitors. On the First Production, there are only 3 of these capacitors. See schematic snips, and click through for full sized images:
Both models of equalizer also use different signal capacitors. The First Production run uses a total of 18: (.1 x 2, .047 x 2, .015 x 6, .01 x 4, .001 x 2) while the Second Production run uses only 12: (.1 x 4, .047 x 2, .015 x 6).
The equalizer is a necessary component of the Bose 901 speaker system, Bose’s highest-end hi-fi equipment. The speakers are specially designed in a way that requires the signal to be equalized and pre-amplified, and if you don’t use the Active Equalizer they’ll really sound pretty bad. It’s surprising how many people have forgotten this fact about the 901-series speakers over the years, but the lack of using an equalizer might be where derogatory slogan “Bose: No Highs, No Lows” came from.
With these very obvious circuit differences, it should be easy to tell which model you’re servicing and which parts to replace. Keep in mind, though, this article only applies to the Series I Active Equalizer. The Series II equalizer is compatible with the Series I speakers, but has a different circuit schematic.
Build Log and Review: Mable Audio VT-86PP Kit Hi-Fi Amp Project [Chinese Gear]
About a year ago, someone left a comment that really looked like spam on one of my pages advertising a web store selling tube audio gear and parts for guitar and audio amplifiers. Against everything I’ve ever learned about behaving on the Internet, and my better judgement, I ended up actually buying a kit from that vendor Mable Audio located in Shenzhen China. They have several kits available with a few options: 6V6 or EL84 for output, 6SL7 or 12AX7 for the input stage and also a beefier EL34/12AX7 amplifier for about double the price. They have a ton of components and guitar amplifier parts and kits as well.
I selected the 6V6/6SL7 kit because I like the larger tubes, and had a set of 1937-issue 6V6G tubes that look more interesting than the small GT-style tubes it came with.
The kit arrived as a box of miscellaneous parts and the chassis. There was no particular documentation included, but the schematic was sent to me via e-mail. Also, the silver decorative plate (in the center of the photo) is no longer included in these kits despite being pictured, as I was told after I purchased. Not the end of the world, though. The kit included a large power transformer, two output transformers (23% Ultralinear), two huge 470uF 450V filter capacitors, all the needed tubes, signal capacitors, connectors, controls, high quality ceramic sockets and lengths of 600V-rated wire to assemble it all with.
The amplifier itself has decent schematics, on paper anyway, although the output impedance is inaccurate on the datasheet: the datasheet, schematic, and transformer color code documents all specify multiple taps but the provided hardware only supports an 8-ohm speaker. This is acceptable, though, as most home theatre speakers are 8 ohm anyway.
INPUT IMPEDANCE : 100K ohm(RCA)
OUTPUT IMPEDANCE:4ohm – 16 ohm8 Ohm
OUTPUT POWER: 12w x2/(ultralinear)class AB P-P
DAMPING FACTOR : >3
FREQUENCY RESPONCE: 20Hz-20KHz(REF.OUPUT)
TOTAL GAIN: 28dB
INPUT SENSITIVITY: 300mV-600mV
S/N: >89dB (HUM NOISE <3mV
CHANNEL BALANCE: <1dB 20Hz-20KHz (MAX.VOLUME)
CHANNEL SERPRATION:>65dB 20Hz-20KHz
TUBE COMPLEMENT: 6P6P(6V6) X4
6N9P(6SL7)X2
POWER REQUIREMENT: AC220V OR 110V OR 240V + -5% 50~60Hz
Assembly wasn’t terrible, but the build quality of the chassis was marginal at best. The top surface of the chassis bolts to the chassis pan itself to hide mounting rails and dividers, and the holes on the top panel didn’t perfectly line up with the socket locations on the bottom panel. This meant that after mounting the sockets to the bottom where they belong the top cover wouldn’t slide over them because it was interfering by about 1mm on one of the sockets. It took significant pressure to force it on and made a very loud snapping noise when it did so, but the socket and cover were all intact. It’s definitely not coming off again, though. Additionally, the cover over the transformers is supposed to mount using several screws at the corners, but only two screw holes lined up.
The sockets are ceramic and the sockets gold-plated, and the transformers look appropriately sized, though, so it’s on to building past mounting up the parts. This project was a long time in the making, as I mounted the sockets and chassis up on July 12th 2011 and then did no further work until March 2012. I worked my way through the schematic, roughly right to left. This amplifier is a power supply, and two identical amplifier channels. I’d highlight on the schematic as I completed both mirror halves, and this made it easy to keep track of where I was going.
Some photos showing filling in the components:
After assembly, I had a few left-over parts:
A really weird assortment of leftover parts at that. I declined to connect the headphone output (on advice that it was probably not safe for any headphones that I might actually like due to the connection – not unique to this amp, many other circuits have similar issues) which accounts for 2 of those resistors, but there’s still 13 more + a small capacitor that weren’t called for anywhere. And there are two extra panel-mount RCA input jacks. And a ton of extra wire. But for all the extra they did include, there were 4 resistor types (8 resistors total) that were not included, so it was a trip down to Vetco Electronics in Bellevue to pick up replacements. The replacements are the Red- and Tan- bodied resistors. Not quite sure what to make of that.
I elected to replace the LED power indicator with an NE-2A neon bulb instead, so it would be the same color as the glow of the tube heaters.
There is a minor issue with this one, though. There is an anti-arcing capacitor across the switch. I placed the NE-2A between the switched side of the switch and the AC neutral (it runs directly from AC mains through a 150K 1/4W resistor). With the switch turned off, the capacitor allows AC leakage of a few mA to pass to the cold side of the switch. The current leakage is less than the excitation current of the transformer, so the transformer appears “open” to the low signal as it’s entirely eaten because it’s not strong enough to set up a magnetic field. This causes the current through the 150K resistor and neon….so the bulb lights up when the power is off, now. With the power on, the capacitor is out of the circuit and the transformer is an extremely low impedance, so all the current goes through the transformer primary and none into the 150K resistor and the NE-2A bulb. The result? The power switch indicator is backwards. The light comes on when the device is turned off, and goes out when you turn it on. I intend to fix this at some point, but am not entirely sure what the best approach might be. The volume control itself is abysmally low quality, probably the only part in the kit I actually think is just plain “bad”. The return spring is weak and the switch touchy and it binds up easily.
There was also another issue: I wired the volume control backwards. “Right” is lower, now, instead of “Left” as is the convention. I evaluated fixing this problem, but while poking around one of the volume control’s pins started coming out of its molding and I didn’t want to risk destroying the control, so that’s a problem that won’t be fixed in this iteration.
I did a quick visual for obvious shorts and powered it up the first time with the 6P6P (Chinese 6V6GT) tubes which were included, as I didn’t want to risk my classic tubes. If there’s going to be a problem it’d be on first power up, for any amplifier from anywhere. The transformers all made a great *THUNK * noise when they energized for the first time, but quickly de-energized again. I poked around some more and found I’d missed a solder connection on the bridge rectifier. I soldered it back on and tried again and everything was fine! First power-up successful, with no re-work needed.
Then again on the test bench with the 6V6G tubes:
Still good! And for some post-production glamour shots:
Now, I’ve moved the amplifier onto my desk and will use it for personal stereo when not using the Surround Sound receiver.
In conclusion: Mable Audio is a reputable supplier. Their product sounds great, and was of acceptable build quality for an entry-level amplifier. The chassis and platform itself will be a great starting point for more modification projects in the future. I rate this kit as a 3.5/5 due to the mechanical issues with the chassis fit and the power switch, but the electronics quality and circuit design seem to be solid. I would recommend this kit to anyone who wants to build an inexpensive tube amp from parts – but, given the complete lack of instructions or documentation beyond a schematic and some color code diagrams, make sure you go slowly and check your work and are somewhat familiar with how to read electrical schematics.
- Pro: Inexpensive for what it is.
- Pro: Good quality components, for the most part.
- Pro: Attractive styling.
- Pro: Fairly straightforward build.
- Con: Terrible quality power switch. Just terrible.
- Con: Mechanical fit on mine wasn’t the best.
- Con: Only 99% of the parts in my kit were the right ones.
For $175 + shipping from China, it’s not a bad deal. Similar kits from more well-known sources, or U.S./European makers, run at least double the price. Once assembled, it’s very attractive and has a lot of potential for future upgrades. This was a fun and rewarding project and a nice change of pace from vintage gear while still keeping the tube connection.
Huge and Powerful: The 1937 GE F-135 Repair Complete
I’ve finally had the time to finish one of my long-running projects repairing GE’s most powerful radio offering from 1937, the GE F-135. I picked it up from Craigslist back in November but other responsibilities kept me from getting much done on it until the 1st of this year. Finally, after a few months waiting, it’s finished and playing!
The radio came to me complete and in decent shape for the age. It’s missing the glass for the dial, but is otherwise completely intact and the finish isn’t in bad shape despite a few scratches here and there.
This radio is the current king of my collection. The best radio GE sold in 1937, it has a lot of innovative features – early APEX hi-fi reception, dual 6L6 output tubes putting out 20W of audio to a large and rich-sounding 12″ speaker; a total of 13 tubes including a tuned RF stage, dual IF stages, separate oscillator and “station seeking” automatic frequency correction.
Under the chassis it’s in decent shape too. It looks like it has been worked on before a few times – there are some ’40s, a ’50s and a couple of ’90s capacitors installed. There’s a sticker from a Seattle-area Jukebox Repair store on the back which probably explains the more-recent repairs; I looked up the address and they don’t seem to be around anymore.
First thing first after doing the complete set of intake checks on the radio, I gave the cabinet a thorough scrubbing and hit it with Howard Restore-a-Finish and it came out beautifully. The intake checks were uneventful so I didn’t take any photos, but the power transformer, all the IF transformers, oscillator and antenna and RF coils and the speaker transformer and coils were just fine. After applying the Howard’s, it really cleaned up the fading and covered over a couple of small scratches and really brought out the natural shine on the wood.
Then I tested all the tubes and found that most were in good shape (especially the 6L6G tubes installed) there were a few 6J5 and 6K7 tubes that needed replaced. I had these in stock and so it was easy to solve that problem.
I worked on this one under my kitchen’s vent hood as it’s currently too cold to vent soldering fumes outside. The chassis stand is the right width though so that’s perfect.
Every paper and filter capacitor needed replaced, as well as nearly every single one of the resistors which had drifted as much as +100% in value in some cases. Fortunately the coils are all intact or this could’ve been a much messier repair.
I also found a couple of places where the previous repairman who worked on the set may have been dyslexic, as there were a couple places were numbered resistors were reversed – i.e. R23 being in R32′s place and so forth. I imagine that couldn’t have made it work any better, anyway. I tested and replaced going along from the bottom up as needed. These are in-progress shots, so you might see a couple places where leads aren’t trimmed or components aren’t yet soldered. All of those issues were taken care of but might not have made it into the photo series.
Small caps dealt with, it was time to replace the multi-section caps. There is a 4-section can, insulated from the chassis, mounted up top with a set of 2 filter caps and 2 cathode-bypass caps. These all ran to a hole in the chassis where they went above. I snipped the long cross-chassis leads and moved the components close to their intended locations. In this photo, I’ve replaced the 10uF cathode bypass capacitor with its replacement. I like using bipolar caps for the large-value cathode bypasses but that’s just my preference and what I keep in stock (I keep those values around for repairing crossovers in old speakers) but you could use a standard polarized capacitor there.
Here’s the totally-complete underside shot.
There’s still a matter to deal with above the chassis, though. This is an AFC radio which uses a special and complicated transformer heading into the diode which recovers the audio. And it has a small resistor which is reading double it’s value and needs replaced as well, or it won’t align right.
Then I sealed the can back up:
I hooked the speaker and pushbutton assembly up on the bench and gave it a test run – it fired up immediately and started pulling in a few stations even on the Shortwave bands. The dial was off alignment a bit though, so it was time for that.
For the alignment, I pulled up the signal generator and started with an IF alignment before going back to the RF stages. This radio has a special IF arrangement with a procedure, so I aligned the 3rd IF primary, second IF secondary and primary, first IF secondary and primary, then went back and aligned the 3rd IF secondary that feeds into the diode. Aligning that discriminator was a maddening 10 minutes spent trying to nudge the adjustment ever so slightly. My goal was to get 0V between two segments, but it approached that point at an incredibly steep slope. I managed to get it there, though.
The original 0V spec was made with a primitive meter; I’ll take 0.01 on a more sensitive modern instrument. That’s perfect IF alignment. It was definitely worth it though. Now onto the RF, which involved tweaking something like 16 trimmers in a precise order with an RF signal generator at various frequencies.
Finally, it was all set! Time to reassemble.
At this point, the radio plays beautifully and pulls in stations from all over, and I’ve added a line input to let me hook up an audio source. The hassle of the AFC calibration was definitely worth it, it’s nearly like magic to watch it work. With the switch off, the radio tunes sharply and a station comes in over just a few degrees of rotation. With the switch activated, it’s like an entirely different radio – the same station will come in across about a quarter-turn of the knob, 2 divisions in either direction from the center frequency and it will block quieter stations from interfering.
The radio sounds great with a pretty good frequency response and more volume than I know what to do with, too. The relay for the motor is burnt out, though. I missed that on the initial checks so when I went to test the pushbutton function…I got a whole lot of nothing. I’ll make another post here when I do get the motor resolved but for now I’m going to hang this one up and start playing it. This was a very fun and enjoyable project and I have a beautiful radio with a commanding presence to enjoy for many years to come.
New Project Idea: Oddball Tube Hi-Fi Amplifier
I’m looking for a new summer project, and think I’m going to branch off from repairing radios to building some of my own gear. I’m interested in making a tube hi-fi stereo amplifier to go with my old speakers – and it’s something I can build. Audiophiles, guitar enthusiasts, and other people into tube audio have driven the price of most of the good audio tubes higher than I’m interested in paying, though.
The 6V6 audio tube is known and loved by enthusiasts everywhere, and came in a variety of other versions with different ratings, different glass, different power supply requirements but pretty much all produce the same output. I’m interested in making a miniature-tube amplifier, so I’ll be starting with the 6AQ5A tube – but these are pretty expensive. An alternately powered version, the 5AQ5 goes for only about $2/each so that looks like a winner.
I’ll be basing my amplifier on the Silvertone 69A, a dual-channel push-pull stereo amplifier manufcatured in 1960. This amplifier used a 5U4 rectifier tube to supply the high voltage. By replacing the tube rectifier (right, above) with a pair of 1N4007 silicon diodes (left, above) I’ll free up the 5V power supply. A set of four 5AQ5s draws 2.4A of current; a 5U4 draws 3A of current so there’s more power to spare and a guarantee of being able to find cheap hi-fi tubes for years to come without a lot of competition.
The schematic – with some bias and output mods – not showing the power supply or output tube modifications – that I’m basing the design is here (click for larger version!):
This should be a good sounding little amp if I get around to actually completing construction. I have most of the iron [transformers] in stock already, it’s just a question of buying some hardware and spending the time required to wire it. We’ll see.
