Designed 2014, Steve Jones, VK2YLD.
Material Copyright 2007-2018, VL2YLD
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REAL radios glow without smoke.
As I read through different articles on the net, I realised that I had not built something from the ground up for a lot of years. Sure I fix a lot of gear for different people and do upgrades and restorations but... nothing new since the mid 70s. I have accumulated a fair tube collection over the years and decided to put some of these to good use. Scratching around on the 'net I found a description of a 17w amp using a pair of 6CW5s in P-P and some cheap output transformers sourced from here in 'oz. I ordered a pair of these Altronics M1115s and when they arrived I thought they looked a little small for the job, even though they are rated for 25W. I decided to have a look at some test figures before I made a decision. Primary inductance was low at 6H with the secondary open, but dropped to a respectable 20mH leakage inductance with the secondary shorted. Whilst the bass may be a bit soft, the top end should be good, but I like some rumble with my music so I decided to make this a 2.1 system instead. Why not make a third channel for driving a sub? Having said that, a 50w centre channel should do it because I don't really want to blast the furnature to bits, and so a couple of spare 6L6s seemed to levitate from the junkbox. I also have some EL34s, but I prefer the 6L6 because its a 'Beam' power tube, while the EL34 is a true pentode, and the heater current is way higher than the humle 6L6. Hmmm ok... That just about does it for why, or why not (?).... now for the how bit....
Working the layout for this was not going to be an easy task, especially when adding up the numbers, then another thought came into it too... A lot of amps I have seen and took a liking to had displays of some sort to hold the attraction apart from the lovely warm glow in the glass. Digging through the junkbox some more revealed a couple of EM84 style tubes with the light strip that closes toward the tube's centre... It looks a bit dull and it's been done... How about 6BR5s (EM80)? These are an old tuning indicator and the shadow opens like an umbrella... Ive only got 1, but have some Russian EM80s.. Look good, but movement can be confusing as it's a double shadow and not a very good sensitivity... More digging and I found a box with 5 EM85s in it. These are from old tape decks and were used as recording level indicators and they also contain a driver triode to improve sensitivity. These tubes are really rare now and these are the original German made 'Lorenz' tubes... Done, toss 3 on the list.. oops.. 17 tubes... ahhh, why not?
EM84 Style Russian tubes
I'll take 3 thanks!
The Russian EM80s - Shadow ok closed, but opens as petals rise...
Now that decision is done, layout can be sorted out. I decided to go with some convention and lay the power supply to the left, then the 3 amp blocks with the pre-amp to the front and the displays right up front. The left and right channels have 2 6CW5s as output, a 12AU7 phase splitter, and a 12AT7 pre-driver. The sub has 2 6L6s as output, a 6N2P phase splitter and a 6N2P as pre-driver and L+R mixer with active low-pass filter configured to start to roll off about 250Hz. There is another 2 12AT7s for the stereo RIAA phono pre-amp. I'm using solid state rectifiers in the power supply because I can't afford any more heater current. I decided to use some Russian signal tubes firstly in the sub channel as most of these tubes are in plentiful supply. The heater chain is wired to be easily converted to 12v and 12AU7s and 12AT7s can be swapped in if needed.
I have decided to go with the 6N2Ps against the 6N1P in the Sub channel mainly for the specs of the tubes but there is also a big jump in heater current from the 1P to the 2P plus the 2P has more gain. Starting with the mains transformer, I found one in an old linear power supply that is suitably large and is wound using a plastic bobbin. I started off by doing 1000v megger tests on the primary to make sure the windings and insulation are in good order. This done, I then squeezed a 10 turn winding around the outside of the other windings, and with the help of a variac, juiced it up to exactly 250v on the primary, then ACCURATELY measured the voltage on the 10 turn winding. Make a note of this figure for calculation of secondary turns ratio.
Next is to start pulling it down carefully to avoid damage to the laminations or the bobbin. With the laminations removed, next is to remove the old secondary windings but LEAVE the primary winding and primary to secondary insulation intact as this is a single section bobbin. This makes sure the original primary winding and over-temp safety fuse etc is all reusable. (Check local regulations for insulation specs pri-sec/pri-core) I added 2 layers of Kapton film over the existing primary insulation to bulk up the dielectric strength a bit. The Kapton film is rated at 6Kv/layer so that makes sure the insulation will hold up. Discard the old removed wire.. NEVER RE-USE OLD WIRE as the enamel may have been cracked or otherwise damaged and could cause a shorted turn when rewound, creating a beautiful light and sound show complete with smokescreen. Too bad it will only do it once!, and then someone has to clean up the mess...
I calculated the secondary turns for the voltages required, and came up with wire diameters for the required current. A few more calculations to make sure all this would fit on the bobbin and rewound it according to the winding layout sheet (attached later!). I wound the 6L6 heater winding first to act as a screen from mains bourne noise. Then came the main B+ secondary, the bias winding, then the other 2 heater windings at 6.3v, 300v, 70v and 6.3v respectively, with 2 layers of Kapton film between each winding. Careful winding technique and layering is required to make sure the whole thing fits together and runs as efficiently as possible. Lacquer and the final insulation applied and reassembled, time for the big test. All voltages are within expectations so this is then shelved while the other bits are worked on.
Now for the chassis... An old rack mount switch-mode power supply donated the chassis and the main filter capacitors. After boning it out, the layout was printed out and stuck to the top with masking tape. I carefully centre punched all the holes and removed the front panel to cut the racking wings off and make up the timber inlay. A few waves of the drill-press and swiss-cheese is the result!! It looks a bit daunting at this stage with chunks missing everywhere, but it didn't take long to fill the holes back in..
To Do.... The DC rectifier of the preamp heaters, the sub amp channel, the RIAA preamp, come up with a front panel timber insert and mount the controls, input jacks... and anything else I forgot!
TO BE CONTINUED.......
The heaters are wired in 3 strings. The 4 6CW5s, then the 2 6L6s, drivers and splitters on the second, and the RIAA Phono stage and the indicators on the third. This third string is lower consumption as it is intended to run them from a DC supply to reduce hum induction into the low-level signal path. The power supply is an elcheapo PCB I bought that was originally designed as a dual rail filter, but it adapted great for this one. I have yet to fit the filter chokes to the chassis, so I have fitted a 120ohm resistor instead of the second choke. This will end up as a choke input L-C-L-C filter to get the best regulation. Notice the 2 resistors by the input bridge, these are the bleeders that kill the capacitor charge on shut-off, as there are 4x 470uF 450v caps in there and that WILL hurt if it gets a hold of you.
With most of the initial wiring done, only left and right channels completed to a stage where I am ready for the 'smoke test'.. Basic circuitry around the tubes done, no sub or RIAA yet. The B+ secondary is 320vAC, tapped at 250vAC, so the B+ is on the 250v tap, juiced up with the variac and test power supply..
290v B+, -90v bias supply and no smoke. Tubes inserted and bias adjusted to a modest 35mA per tube. B+ now 251v on load and heaters spot on 6.3v RMS. No extraneous noises, so I rigged a volume 'pot into a hole on the front and plumbed it up. Sounds great, even without the negative feedback hooked in yet. A few tests with dummy loads and the CRO, shows it delivers 19w to a 7ohm resistive load before the wave-shape starts to flat-top, but I think this is due to the small O/P tranformers saturating the core. Indicators adjusted to just fully close the shadow at this level. They will have to be readjusted when I raise the B+ to final level, and that will also raise the output power a touch too..
The power supply is pictured here. Filter caps are the salvaged 4x 470uF/450vw and they are wired in 2 parallel sections of 2 caps, with the 120ohm coupler immitating the second choke. A 2k2 5W resistor is used as the dropper for the preamp B+ that ends up just short of 210v.
My Tube Amp Project