Output Transformer

[DeluxeMan]

Hi Folks. I am just completing my 2 stroke build, now down to wiring the transformers which I have bought from a company in the UK who have been making transformers to MOD spec since the 1940’s. I already have a 5E3 with these which sounds great.

Wiring label reads

Input red/black
com white
4o orange 8o yellow 16o black

The two thin red/black I gather are the input and will map to brown and blue but am unsure which way after reading supplementary notes.

Main confusion is that I will only need 8 ohm output for my speaker but am reading that I will need both 8 ohm and 4 ohm for this depending wether I am running 6V6/6K6 or 6L6/EL34/KT66.. I didn’t order an impedence switch but was looking forward to swapping output valves. Can I run all at 8 ohms to match my speakers or do I need to install a 4 ohm speaker jack for some?

Thanks!

[Robin]

ok, let’s see, the output transformer is there to match the high impedance/low current output of the power tube to the low impedance/high current load of the speaker. Note that the larger leads are on the secondary side of the transformer where the higher current is. In matching the power tube to the speaker load, you need to know what that primary impedance of the transformer is OR what the speaker load is.
You know the speaker load is 8 ohms and you know a single 6V6 (or 6L6)
in a class A circuit (like the Two Stroke) likes around 5000 ohms, so to find the “turns ratio” you use Ohm’s law: I=E/R. As an example if you apply a AC voltage to the primary side of the transformer, say 5v AC. Now measure the AC voltage of the secondary, say we get .21v AC. The ratio is 5 / .21 = 23.8, so that a turns radio of 24:1. The impedance ratio is the square of the turns ratio so: 24 x 24 = 576, Now multiply the impedance ratio (576)by the load (speaker) impedance (8 ohms): 8×576 and you get 4608. That’s about 4.5K which is close enough to 5K to work with a 6V6.

The bottom line is, the 4,8 and 16ohm taps of the output transformer are only accurate if you know the primary impedance of the OT. The transformer supplier should be able to provide it. Once you confirm that the primary for your OT is 4500-5500, you should be good to go with the 8 ohm tap. Clear?

[DeluxeMan]

Thanks Robin. I got the transformers specced through Barry at Vyse Amps in the UK who just went on a weeks holiday yesterday so helpline closed for the time being… (I could contact the manufacturer if I need to but i’ll trust Barry).

I’ll take it that the impedance is as you suggest for the time being. The two thin wires are the primarys (red black), these correspond to the brown and blue in the diagrams? I was going to run the red to the positive of the far left filter cap where it joins with the 10K resistor and the black to pin 3 of V2, then the white to speaker jack negative and the yellow to positive. Is this right?

Thanks again!

[DeluxeMan]

Wow.. Got it fired up and there is noise coming out of speaker, no smoke fire or brimstone and I had it on for a good 10 mins but some odd voltages including input jack 180v -+ DC…. (ow)

Voltages are at idle with valves in and speaker connected (obviously). Could somebody take a look and offer an opinion please?

Tubes are 12ax7, 6l6 and 5ar4, will change for 6v6 and 5y3.

V1
233
140
193
0
0
175
0
1
0

V2
0
0
361
338
0
0
0
27

V3
0
378
0
311
0
311
0
379

I feel like I’m nearly there, probably something stoopid!

[beelzebum]

If you are listing the voltages from top to bottom starting with pin 1, then you are saying that you have 140v on pin 2 of V1. Which schematic are you following, and do you have your input jack connected to pin 2 or 7 on V1?

[DeluxeMan]

Thanks beelzebum. Starting from the gap/notch clockwise looking from the back pin 1-8 (for octal) is right yes?.

My input jack is connected to pin 2 V1 with the 68k resistor and a shielded ground cable connected one end as per instructions. V1 is not earthed though. I am following layout diagram for two stroke here http://www.tonecentral.net/gearpics/homebrew/twostroke/Hunter_Two_Stroke_Layout-180.JPG and following the book. As well as occasionally checking with the weber Maggie layout too.

[beelzebum]

In that layout, even though it doesn’t look like it, if you follow the wire from V1 pin 3. It splits to a 47uf, 25uf, and a 1.5k. The base of the 1.5k should be connected to ground along with the center pin of the boost switch. If it isn’t, then you will get the voltages that you are showing.

[DeluxeMan]

There is no ground there, thanks for the pointer. When you say the base of the 1.5k which end do you mean? It looks like it’s the left and centre lug of input and centre pin for boost to be grounded (which it isn’t).

Thanks.

[beelzebum]

When I say the base, I mean the bottom side according to that layout. On that layout, the bottom of the 1.5k shows a line going down to the lowest pin on the input jack. They need to connect there. Then it shows a line going from there to ground. Also it shows the center pin on the input connected to the center pin of the boost switch, which are both connected to the bottom pin of the input. All of these should be connected to ground.

[DeluxeMan]

You’re a star! Not only had I left the ground lead out, also I had wired the 1.5k and the 68k both to the right input lug and left the left lug with only the 1 meg resistor connected to the right one. Rewired correctly now and wayward voltages gone.

Touching the input tip causes buzzing even with the vol turned down only turning tone down as well silences it. Will check all the pot wiring again now.

Thanks for your help man! Lifesaver (literally..)

[beelzebum]

No problem, just post if you have any other issues.

[Robin]

Yes, literally, good job beelzebum.

[DeluxeMan]

Well it sounds very nice with a 6K6 in it.. Bloody great in fact!! Everything works well and it’s quiet to boot. The boost switch is most useful!

Then it becomes complicated….

With a 6V6 a faint high pitched squeal is apparent from 0 on the volume dial to 9 oclock, then it works perfect till 4 oclock then squeal comes back (so it’s basically both ends of the dial). Diming the treble or using boost makes it worse.

Furthermore this is with a Weber sig12 speaker. With a Celestion Blue it’s worse with more of the dial affected but with a gorilla (Eminence or similar oem?) 30w I have for testing it’s virtually OK. (All 8ohm..)

With a 6L6 It was pretty bad all over the dials and it came back out again rapidly.

I am not sure I have the OT primarys the correct way, could this be the cause or does my impedance need adjusting somehow?

I’ve read about reversing OT secondarys too but why would it function perfectly with a 6K6? That was referring more to a constant whine anyway.

Having said this I am really happy with the NOS 6K6 tone and the volume level is exactly what I hoped for.. Have got my deluxe for more! Still the option for a few more watts would be handy.

Edit. Should I mention that the atoms were 40/16/16 instead of 40/16/8?

[beelzebum]

Just out of curiosity what kind of coupling capacitors are you using, Polypropylene, silver mica, PIO?

[DeluxeMan]

Mallory 150’s.

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