Scratch built 45/2A3 SET amp

Scratch built 45 SET amp

Got started on my second DIY amplifier – a 45 SET using a single stage 417A triode driver. I selected this circuit because I have heard it before and liked it in 45 and 2A3 versions. The 2A3 switch is pretty easy and just entails changing the cathode resistor and output transformer tap. Since I’m not sure that this amp’s <2w output will cut it full range with my 100db eff. speakers, I may end up using it in a bi-amp configuration on the Altec horns so I’ve added shunt bypassed level pots to each channel for horn attenuation. I used CAD to develop a chassis layout I liked and to arrive at a drawing I could use to produce numeric control tool punching code for a CNC turret punch press to process the steel.

These pictures were taken when I test-fit the large parts before starting on the wiring. No surprises, everything lined up and fit well and I guess this is where spending a bunch of time in CAD messing with the details really pays off.

Chassis

- The layout keeps all AC wiring in a confined space and only filament leads go anywhere near the amp stage. The central power supply section is further isolated by 2 steel stiffening channels that run front-to-back in the chassis.
- Metal work is all 16ga. (.062) stainless steel and the wood is South American mahogany with Watco natural Danish oil and Satin finish wax – all wet sanded in up through #800 grit paper.
- A Grayhill rotary on/off switch is mounted up front through a stainless plate mitered into the back of the front panel and capped with an AudioNote gold plated brass knob.
- Corcom EMI filter IEC power cord socket in back.
- Tube sockets are Azuma gold & ceramic – I thought about Yamamoto for about a second till I saw the price.
- All tubes are mounted in stainless steel sub assemblies and I have a few ideas I want to try for shock mounting these – I hear the 417A can be pretty microphonic.
- The hum and level pots are topped with more AudioNote gold plated knobs.
- All hardware is anodized socket head machine screws accept plate to wood mounting where I used stainless screws.
- Not sure about feet yet, I might build something out of Ebony or just buy some off BrianB, maybe Vibrapods…


Power Supply

- Power transformer is a potted James Audio piece.
- 5U4 rectifier; so far have a NOS 5U4GB CBS black-plate and a National Union 5U4G to try.
- First stage filter will be 4.4uF foil & paper in oil, NOS Russian military caps.
- James Audio 5H/250ma potted choke.
- Second stage filter is 100uF Elna Cerafine, channels separate from here. I considered all PIO but was advised by someone who has built this circuit that I would get a better balanced sound combining a PIO first stage with a Cerafine or Blackgate second stage. Reading Reichert’s (AudioNote) design philosophy in Sound Practices also confirms the Blackgate/Cerafine choice and the single element availability and price lead me to choose Cerafines.
- Last filter stage to driver tube is again Cerafine.
- I’m using precision 10 turn Bourns pots for humpots on the 45 (Caddock resistor and a Blackgate cap). A humpot may not be necessary but I’ve read they’re needed with some tubes (like the TJ’s) and I want the flexibility.
- I’m adding a chassis top switch on the HV centertap lead to allow delaying the turn on of the HV till after heaters have been warmed – should be a little easier on these stupid expensive tubes.
- I’ll do a balanced center ground configuration on the driver heaters as I found this reduced hum a touch in my last amp.

Amp Section

- Vampire gold plated copper input jacks.
- Cardas shielded copper signal wire.
- Alps blue velvet level pot in a “shunt” configuration using Riken resistors. I was originally going to use a single grid stopper resistor in the signal path but I want to try bi-amping and I believe this is the best place to attenuate level on my Altec horns. It’s real easy to bypass the pot if I want but adding it in later would be a major bitch. A stepped attenuator would be better but way more bucks and I like the fine adjustment of a pot with the horns.
- I chose the 9 pin 417A as a driver based on recommendation and since I’ve heard it in this circuit and liked it. I have a few tubes to try here – Raytheon 5842, Northern Electric 417A and Western Electric 417A. Since I’m using a sub-assembly type socket mounting technique, I can switch to octals down the road if I feel adventurous; I made a set of octal plates just in case.
- I’m using Mills, Caddock and Tantalum resistors, Blackgate caps.
- For the coupling cap I’m going to try the Jupiter beeswax-paper-foil. I was going to use my current favorite AudioNote copper foil-in-oil caps but was impressed with the sound of this amp circuit using the Jupiter so I’ll start there.
- For 45’s, I have a matched pair of Raytheon STs and RCA UX-245 globes, down the road I’ll try some other oldies (a buddy has a bunch) and maybe the EML or TJ’s.
- Output trannies are again potted James Audio with taps available for variuos impedances - I’ll hard wire for 8ohm speakers but like the option of 16ohm and other power tube loading if I want to try them.
- Binding posts are un-plated Cardas copper – a little expensive but I like the idea of copper instead of brass.
- Wiring will be NOS Western Electric cloth/silk covered copper.

I’ve put together 2 of these chassis as my tube mentor buddy wants to build another copy of this amp for himself. This is a great help for me as I can see how things like grounding and wire routing are supposed to be done and I’ll again do initial power up on his workbench using a variac. Of course, this is AFTER he gives ‘my’ attempt at wiring a thorough inspection – it’s pretty humbling to have your stupid mistakes pointed out by an expert but I can think of a lot worse ways for a rookie to find them… .

Wow, that's a real beauty you're putting together Billfort:thmbsp:

Carl

:thmbsp: Damn! Excellent work! :thmbsp:

Dude, YOU ROCK ! :ntwrthy:

Seriously, your attention to detail makes my stuff pale in comparison. I'm jealous of your CAD abilities and execution.

Your humble apprentice

Just amazing!! I'd love to read your thoughts on sonics once everything is in order. Beautiful!

Unbelieveable !!

Billfort,
Although I've always lived in the cold cruel solid state world and always will, Your workmanship is out of this world, what a fantastic job !!!!It sounds like You have tooling capabilites beyond what "Nam" has in his New Yankee workshop, but that takes nothing away from your expertise and design skills. I know ---- The Mechanical layout and assembly aspects of putting together an amp chassis alone, are extremely difficult - Yours is just absolutely eye-candy. Please take the utmost of compliments from a metal can/plastic tab dude --- great job !!!

Wow, thanks for the compliments guys! Actually sanyo, the wood here was done with a decent table saw, a good router and some purpose built templates, nothing really special. I’ve been on the Manufacturing Engineering and now IT sides of a sheet metal manufacturing business for about 25 years so the CAD-to-steel transition thing is comfortable territory. It is kinda nice though to have access to a stainless supply and an NC controlled shear, turret punch press and brake.

Wards, you gotta be crazy, I’ve been admiring your attention to detail since I joined AK a couple years ago. I just don’t know where you find the time and energy to take on all that you do; I think I’ve been kicking this 45 thing around in CAD for about a year now!

You should build another amp....

and maybe put it up for sale on AK here, no doubt it would be a work of art any audiophile would love to have in their system.

Ron

Maybe not, Ron may take a second mortgage to get ahold of one of those...

Maybe working on a 3rd mortgage...

But it sure is pretty!!!
Ron

Billfort, Each of your projects turn out as near perfect executions of audio art. How in the world do you (still) improve on them each time? :scratch2:


nice work!

Billfort what do you think about laser cutting of plates? I have access to a laser cutter but not CNC punching.

rigma

Laser cutting should do a great job, maybe even better than punching as you can get potentially cleaner hole edge finish and there is no tool stepping or nibbling - nice smooth cuts for complex shapes like the IEC socket hole.

I haven't paid too much attention to the latest generation CNC lasers as they are not suited to our product at work but I remember in the past that they were not really usable on stainless and better suited to mild steel & aluminum - this certainly may have changed however.

Circuit.

What's your take on the sound of those James output transformers?

A local friend has the James universal types in a 300B amp converted to 2A3.

I like them, silky and smooth, and can be found at good prices.

Hi Mike and welcome to AK. This is my second DIY amp and the first to use the James stuff so I haven’t heard it at home yet but have heard several SET’s built with it in a system very similar to mine. I’ve heard 211, 45, 50, 2A3 and 300B SET’s with James iron and really liked what I heard. The one that impressed me the most and convinced me to try James was the 211 IT coupled amp that included James iron in the interstage transformer area – from what I’ve been told, a very critical location for transformer quality.

I’m sure there are other transformer brands that are as good (better?) and I really like the ElectraPrint iron I used for outputs in the 300B amp I built. Actually, after hearing 2 300B amps – 1 with EP OT’s and 1 with the same James OT’s I’m using in the 45, I’d say the EP was a little better in the bass. Of coarse there is always Tamura or Tango which may be the best available but WAY out of my price league. The way it was put to me – if you can’t afford Tamura or Tango, James is a nice alternative.

I’ve heard 211, 45, 50, 2A3 and 300B SET’s with James iron and really liked what I heard. The one that impressed me the most and convinced me to try James was the 211 IT coupled amp that included James iron in the interstage transformer area – from what I’ve been told, a very critical location for transformer quality.

***Haven't heard the 211 or 50 tube yet, just the 2A3, 45 and 300B with James OPT's.
The interstage transformer pretty much has to pass full bandwidth, from what I can recall.
The 211 IT coupled amp does sound interesting.....

I’m sure there are other transformer brands that are as good (better?) and I really like the ElectraPrint iron I used for outputs in the 300B amp I built. Actually, after hearing 2 300B amps – 1 with EP OT’s and 1 with the same James OT’s I’m using in the 45, I’d say the EP was a little better in the bass. Of coarse there is always Tamura or Tango which may be the best available but WAY out of my price league. The way it was put to me – if you can’t afford Tamura or Tango, James is a nice alternative.

Maybe the ElectraPrint OPT was just a better impedance match with the 300B, I dunno...
The James OPT's, look nice, sound nice, and are affordable to the budget constrained.

I'm using a homebrew SET amp using TJ solid plate 2.5 volt 300B globes. It's a 6EA7 bridged IT(cheapie Allied IT) driver circuit.
It use's two 6AU4 half-wave dampers for FW rectification, CLC PS, a 36 mic oil, a 4 henrie choke, a 100 mic 'lytic bypassed with another 36 mic oil for the ouput cap. The driver circuit plates are each further decoupled with 100 mic caps.

The OPT's are Scholl, a little overkill, rated at 30 watts at 10 Hz.

I'd post a pic, but this amp looks like a third grade science project compared to the craftsmanship likes of you and Wardsweb.

Reliable as hell amp, and sounds pretty good too!

Thanks for the welcome Bill!

Your amp sounds like an interesting design Mike, I'd love to see pics, why not start a thread? My first amp (300B) started as a take on the Angela91 and was 'designed by committee' to a certain extent as it was a group project. I've learned a lot since and it evolved into something quite different over time - it really gave me a lot of respect for builders who can go their own way and design something unique like you have.

I tried the 2.5v TJ meshplate globe 300B's for a while (loner from a friend) in my amps and really liked them, especially the reduced hum with AC heaters. Ultimately, I preferred the WE300B, primarily in the midrange, and since I already owned the WE's, decided to pass on the TJ's. I sometimes second guess the decision though as the TJ's sounded so nice and I ended up having to go with DC heaters to finally kill the hum...