120w valve amplifier.

[BusyBee]

Very nice.

I like to see this traditional approach. I find it hard work, and have little success!

[Diabolical Artificer]

Ta. This is the first valve amp I've built and it has been hard work, frustrating at times, it's been good craic too.

What is it your trying to build and what problems have you encountered?

Andy.

[BusyBee]

I have nothing specific at the moment, but have tried small items of valve, chassis based, equipment in the past. I tend to find I get on better with more modern approaches than traditional point to point wiring. It seems to get very messy even though I have been instructed in those ways. I think I have far more experience in stripboard and similar so think in terms of that. I am tending to work toward modular approaches. Using those, I find I can easier isolate problem areas. With a metal chassis it often seems like a total rebuild is needed. I can see that a combined approach is perhaps ideal, especially if I ever work on higher power valve RF equipment wherin the PA output would need good screening.

[Diabolical Artificer]

Had a bit of a snafu with the tfmr I wound. After wiring up the LH CH PSU I found the heater DC supply doing odd things. The OP would drop to 1.1v. I found the cause, the crowbar protection circuit that should fire an SCR at 13v was on. It's exactly the same as it's twin apart from the source of C106D SCR's. Farnell didn't have any so I got 5 off ebay. Needs more investigation.

As mentioned I screwed up when I wound the tfmr, I wound the winding for 11v, that should have given me 15.4v DC prior to the LM317 minus a volt for diode drop. I'm actually getting 13.7v DC. This isn't enough headroom; with a load, IE valves, this drops to 10v ish. Bummer.

After some head scratching I found a little tfmr with secs of 16.3V (19v measured) which I just might be able to squeeze in. However I am going to see if I can pinch a bit of current off the 18.9v AC EL34 heater windings. This should be ok, though not ideal, as they are massively over wound.

To add to the fun and games the phase splitter isn't working. Still it's nice to be doing some electronics after fettling the case and metal bashing for a few weeks.

Lastly, as I have no OPT, could I use two big arse resistors as loads for the EL34 OP stage? The OPT pri Z should be 1900 ohms, could I use to girt big 1k resistors as temporary loads?

Andy.

[Murphyv310]

These things are sent to try us Andy.
Without bumming my little stereo 3-3 has been performing beautifully now for months with hardly a complaint, my deaf ears though could do with a little treble lift, hardly an issue with the amp.
It'll be well used over the festivities.

[Refugee]

Have you tried a bigger smoothing capacitor on the unregulated side to make the drops between half cycles smaller.

[Crackle]

As mentioned I screwed up when I wound the tfmr, I wound the winding for 11v, that should have given me 15.4v DC prior to the LM317 minus a volt for diode drop. I'm actually getting 13.7v DC. This isn't enough headroom; with a load, IE valves, this drops to 10v ish. Bummer.
Andy.

hi
Is it imposible to add a few turns overwound on the transformer?

Mike

[Crackle]

As mentioned I screwed up when I wound the tfmr, I wound the winding for 11v, that should have given me 15.4v DC prior to the LM317 minus a volt for diode drop. I'm actually getting 13.7v DC. This isn't enough headroom; with a load, IE valves, this drops to 10v ish. Bummer.
Andy.

hi
Is it imposible to add a few turns overwound on the transformer? Use PVC insuated wire if you can get in through the remaining gap.

Mike

[pwdrive]

Andy wrote: "Lastly, as I have no OPT, could I use two big arse resistors as loads for the EL34 OP stage? The OPT pri Z should be 1900 ohms, could I use to girt big 1k resistors as temporary loads?"

Try not to confuse impedance with resistance, the primary impedance of the transformer is governed by the turns ratio and the secondary load impedance, the primary's resistance is in effect constant no matter what and will be considerably lower than the primarys impedance.

If you a used a resistor(s) to the value of the transformers primary impedance then that would mean that at maximum anode current the anode voltage will probably be considerably lower than the screen voltage which could lead to excessive screen grid dissipation.

If you just want to connect something in the absence of the transformer so you can do a few basic voltage checks then guestimate what the primary DC resistance would be and use a resistor of that value, failing that connect the anode direct to HT.

But really you need an output transformer, if power measurement or full power test is your goal.

Lawrence.

[Diabolical Artificer]

Thanks for the suggestions all. I've sorted the problem of the 12.6v PSU by making it 6.3v and rewiring the chassis htr winding. I couldn't add turns as the centre of the toroid is plugged with plastic padding. It'd mean drilling this out etc etc.

Thanks Lawrence, I know the difference but thought the OP stage would need to see an impedance similar to the OPT, which as you say can't really be done with resistors if the HT is connected at the centre of two 1k R's in series.

Andy.