Finally my DIY tube amp project gets kick-started

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Hari Iyer

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Those who have been following my post will know that i am trying to build a tube amp for almost 2 years now but could never finalize any design. I had even thought of re-building my current 300B SET amplifier with better parts and a new chassis, but gave away the thought because it was getting pretty expensive and moreover having the same 3 stage capacitor coupled ckt with boutique components may not be a very high change from what i am currently hearing. I scanned many designs over the past two years and with tubes like 2A3, EL34, 45, 203D, 00B and the like but never could finalize anything. I later decided to go with the 'Directly Coupled' type of tube amplifier and came across the Loftin White schematic but did not pursue that further. Some time later FM @yogibear shared with me a EL34 SET DC design which had potential, but even that could not materalize.

Earlier this year, i thought of buying a readymade amplifier and came across a Japanese brand by the name "Softone" who made some 300B tube which was a hybrid design with direct coupling. This was a reasonably priced setup ($850/- + shipping + CD) but by the time i could decide the C-19 issue started and the rest is history. I also thought of getting their EL34 or KT88 class A push-pull design which was also very nice and hybrid design, but again there was a increase in CD and related c-19 shipping issue which put the plan in the back seat.

In my search, i came across a gentleman by the name of Jeff Medwin, who usually comment about various designs in forums and i got in touch with him through a PM and we started discussing about various possibilities. This was in around mid April and after many discussion i had finalized his design around last month.

What i was using till now - 300B, SET, DHT, DC heated filament, 3 stages (pre-driver + driver + output) with capacitor coupling stages.
Power supply - SS diodes with C-L-C filtering, output stage is EI audio transformer, power supply transformer is EI. If you have read my earlier amplifier modification post, i have made many modifications to this amplifier with changes in all resistors, capacitors, driver and input tubes, wires etc. The only components that are still probably retained are the power and output transformers, the volume control and the Chassis.

What i wanted in my new amplifier is - IDH ( indirectly heating) AC heating type, only 2 stage of amplification (driver + output), Directly coupled sections. Power supply uses the LES(low energy storage) type of design with L-C-L-C filtering. The LES is a proprietary design of Dennis Fraker (Jeff Medwin's mentor) of serious stereo fame who have pioneered this design almost 3 decades ago. If you search other international forums you might find this design very very controversial and gets all the bad name of being a very poor design. But Jeff handles all the bad press with wit and confidence. Most FMs who claim the poor design have neither used or heard his amplifier and come to conclusion which is quite normal in this hobby. That's when i decided to build this particular amplifier and discover what the "fuss" is about his power supply that gets trolled so much. The discussion in the forum gets so nasty to the extend that Jeff has been "Banned" in atleast 3 forums which i know of. But again who cares if you have many followers and admirers of your design. Anyways.

Jeff is also helping me source some critical components at a very attractive price which i probably could not have sourced at that price and i thank him for that for the effort. I had ordered some components directly from mouser.in which i have received early this week. If all my supplies arrive on time without the c-19 posted risk, almost 90% of the components should be received before end of this month, after which i shall plan to assemble. The output transformers will be procured from "Softone" which is a R-core type. The EMS Japan currently don't ship to India/ US and 83 other countries and this is not going to change soon. I am borrowing a Delta audio output transformer from FM @Beast_of_burden in the interim period for my testing. The power transformers are ordered from Silchar Transformers (a big thankyou to FM @sadik for helping me with this).

I have made a rough design of the chassis which i will be building with teak wood + brass sheets. Chassis parts have been ordered from theaudiocrafts.com. The entire amplifier will take around 4 to 6 months to complete and am expecting the mono-blocks to be ready by Jan-2021 to be fully ready and functional if all turns out well.

Thanks for looking.

This is what i received 30% of components from mouser.com earlier this week.
IMG-20200804-WA0010.jpeg
 

kbhuvi

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At last sir the game is on. Will follow your progress with a keen eye.

If possible, please enlighten us with the choice of caps and resistors and the reason behind it.
 

Hari Iyer

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If possible, please enlighten us with the choice of caps and resistors and the reason behind it.
The DCLink capacitors are capable of handling pulse quite well. These are meatalized polypropylene film type. The resistors are Arcol (Ohmite). Max dissipation around 8 watts. I have choosen 50 watts so that heat dissipation is even and distributed. Also these were Jeff's choice. Some more Mills and Roderstein resistors are expected by this month end. Other capacitors are GTO snubbers, Tan Hai audio tone and orange drops tone capacitors. Will update once received.
 
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Hari Iyer

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This new amp will be using 6FQ7 as driver tubes and ,KT88 as final tubes in SET configuration. The driver tube will be directly coupled to the final tubes with no interstage transformer or a coupling capacitor. I will not be using the ultra path connection from transformer to G2 but will connect the B+ after voltage drop. The power supply is dual shunt regulated for the driver tubes. The cathode bias current around 42mA will have the KT88 dissipation around 62% of its max capacity ( a golden ratio). This is will mean no tube replacement for my life time.

The 6FQ7 were mostly used in vintage B&w and colour tube television for H- stage and V-stage deflection section. This particular tube is ultra low nise, medium mu and not microphonic.

All components are boutique parts. As per JM recommendations I have opted for Softone R-core audio output transformers which would arrive by the end of the year (hopefully). In the mean time I may be receiving Delta Transformers for my testing ( thank you @Beast_of_burden.)
 
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Hiten

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All the best Hari.
From where did you purchased 9 pin valve sockets ?
Regards.
 

Hari Iyer

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There was some minor 10% build progress this week as per the attached images. I have used a 3mm aluminium base plate to mount the components. As you can see the amplifier is build not in 2D but in 3D. This is because the combined heat dissipation of all the 3 power resistors will be around 21 watts and the base plate could get pretty hot. I don't want the capacitors mounted on the base plate to get melted because of this. These capacitors will be mounded an inch above the base plate and the resistors will be below the capacitors to save space. Both the chokes are 270 deg out of phase mounted and are 90 deg out of phase w.r.t the power transformers to avoid mutual coupling. The spacing of the components are with consideration of low wiring lead losses. All components are mounted on a sandwitched layer of silicon washers to isolate the component vibration from reaching the chassis (especially the transformers and chokes).

Thanks for looking.
IMG-20200827-WA0020.jpeg IMG-20200827-WA0022.jpeg
 

Hari Iyer

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What brands have I used in this amp,

Resistors - Mills, Roderstein, Arcols
Capacitors - Wima DC Link, Orange Drops, Tai Hans, Audiophiler, GTO snubber
Tubes - GE, EH
Chokes - Hammond
Hash chokes - Vishay
CM chokes - NEC
Wires - Mill specs silver plated copper - various guage
Tube sockets, Banana terminal, RCA input, power switch - all Korean make
Power Transformer - Silchar technologies
Output transformer - Delta transformer ( later
will change to Softone)
IEC terminal - Indian make
Screws - SS type ( Indian make)
 

Hari Iyer

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Removed the sandwich silicon washers from the small transformer and chokes as they were not giving a rigid fit because of their size. I have gone back with conventional mounting on those and discover later if they need special mounting . The other components are still having the silicone washers for support to prevent vibrations from reaching them.

I am still working out options to support the tube sockets from vibrations and isolation from the transformer. I am not sure as of now if the silicone washers mounting trick will work there too. Tube sockets isolation is critical for reducing microphonics and THD reduction imo. Looking for ideas.
 

Hari Iyer

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I am facing some challenges with the AC input voltage post the lockdown era. My power transformers were designed for a primary voltage of 230V AC. But off-late i am measuring higher than normal AC input voltage in my area. In the morning around 10AM its around 235V, in the evening around 240V and after 10PM as high as 249V AC are measured in the past one week interval. This high input voltage in transformer primary is causing a higher secondary AC voltage too. For instance when its 249V AC input, my secondary voltage is as high as 465V AC rather than the normal 425V AC thats required.

My B+ voltage need to be around 510V DC, but with the increased AC voltage it would be around 575v DC which is quite higher than normal. Also my filament transformer which should be 6.3V AC because of the increased AC input voltage reads 7.6v AC again which is quite here than the tolerance for the tubes. For this i will be using a 0.47 ohms resistor in series of both the transformer leg to keep them around 6.5v AC.

So what options do i have now? One is return the transformer and get it remade for various primary taps of 220V, 230v, 240v, 250v and switch them as per your input AC voltage. Two use a high wattage WW resistor to drop the DC voltage which will generate more heat and increase the impedance of the power supply albeit with a much reduced AC ripple.

Alternatively what i am going to try is use an incandescent bulb in the power supply to drop the voltage which will help me with - A reduce the DC voltage from 575v to 525v. B - reduce the AC ripple from 4.5v to just 345mV. C - Work as a current limiter and help to reduce the initial inrush current so that the tube heater can be on before the B+ supply gets fully on. The incandescent bulb is non-linear compared to a resistor and this is where it helps in current limiting. Also the cost of this bulb is very basic and easily available at the local electric shop. Don't have to worry about mounting like a resistor as they can be mounted outside the chassis just like a tube. I have worked various bulbs of 15W to 100W that will help me to achieve the desired voltage drop. But again everything is just speculation as i will need to work on first without this and later adjust to the reality.

On the built front - one channel monoblock chassis was delivered early this week and i have mounted the components on them. Once i receive the balance parts will start the build again.

Thanks for looking.
 

Hari Iyer

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Why not use something like stabilizer.
Good option, again the question of increasing the impedance of the power supply line and space to keep the stabilizer. I am not sure if the stabilizer work to keep the voltage always to 230v or if they operate only beyond 250v. Will need to discover this.
 

yogibear

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Use an AC stabilizer but their output voltage is normally 220volts. I am going to face similar problems soon with my upcoming builds. I do have 230 / 240 volts primaries but that’s not a solution. A power conditioner seems to be must.
 
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