120w valve amplifier.

[Murphyv310]

Andy your last video comes up unavailable?

[radioman]

Video works OK for me.

Andy

[Murphyv310]

For some odd reason it wont play on my phone OK though on the PC.

[Diabolical Artificer]

No idea Trevor. I can just about get my head round valve amps, witchcraft, the Kabbalah and even some Zen Koan's, but the vagaries of modern communication tech is best left to the slightly deranged and the Gods.

I have the amp to heel again and stopped the "bouncing". Bit hard to describe in lieu of a video. I had 1u and 100n from HT to ground on the amp chassis where the HT comes in. Took these off, all well.

A.

[Diabolical Artificer]

Bit of an update video here - https://youtu.be/qHmO9qxaRNo I'm still slogging away at it, winding another mains transformer. I've found a cheap source of big toroid's on ebay, think it was £30 for a 700VA, which is a tad bigger than the other one I wound, IE 625VA. I went over the playing field yesterday to measure out the HT winding's, which are 2 x 243.3ft, but ran out of copper wire. I have some on order from Brocott's. Then it's a case of twisting the two together to form one bi-filar wire, bung it on a foot long "bobbin" then wind it on. With that done the other secs are a doodle.

I've also built a VU meter driver. It uses a TL074 opamp, one as precision rectifier, another as a voltage amp and another as buffer. I didn't use the last bit. The only modification I did was add a 100k preset on the IP, as the circuit as is, loaded the OP off the ECC83 too much. I've also had to add an attenuator SW because with an OP off the ECC83 of 6v RMS, this means the meter tops out , IE FSD, at about 35W. this is just a 6K8 R, with a SPST SW.

I also added a peak/clipping indicator, which turns an LED on at 5.9v RMS, glowing brightly at 6v RMS.

Lastly I've built a protection circuit that fires a triac at around 100mA cathode current. Each EL34 has a wire running off the cathode to the protection PCB. This is just a peak detector in essence. It took a lot of experimentation to get it right. As I said at 1v the triac fires, which supplies 12v to a relay, with cuts the negative mains to the mains tfmr. There is also an ultra bright diode on the front panel to indicate a fault.

These circuits are all on one PCB, with the PSU, which is a half wave rectified +/- 15v, stabilised by two 5W zeners.

If anyone's interested, I'll attach the circuits.

I forgot to say I have been using the amp to listen to music and it sounds well. However it's a bit bass heavy, which is easily solved. It's tricky to appreciate any difference though, to other amps, as it's a mono amp at present, so I stuck to old Beatle's stuff, a bit of Donovan before he went all trippy and Charlie Mingus sounds especially good as does Pentangle - double bass heaven.

Lastly for a while, I had to reduce NFB a tad, increasing the NFB R from 4 to 5k. This means NFB is about 1/50th. I'm still not completely au fait with the math's IE. closed loop gain etc and have no idea how to apply the result to reduce oscillation. I intend to build a simple circuit using an opamp to drive a single ended EL84 in an effort to nail the math's etc at some point soon.

Finally after all the changes THD is 0.04% @ 1/2W, 0.08% @ 12.5W, and 0.18 @ 50 and 112W using 1khz. F response is flat from 50hz to 6khz where it starts to roll off, down by 3dB @ 20khz. Square wave looks good at 1khz with a bit of rounding of the leading edges, but at 10khz it looks more like Grandad's blunt old #3 saw used for cutting breeze blocks. Meh, it'll d

The peripheral circuit's have been the most interesting and fun bit on this amp, I've learned a lot building them. I nicked most of them, but modified them somewhat, spending hours bread boarding circuits, good craic. I'll do a complete schematic when i get a mo and post.



Andy.

[Murphyv310]

Good stuff Andy.
Like to see the finished circuit when you've time.

[Diabolical Artificer]

Cheers Trev, will do when I get a mo.

Andy.

[Diabolical Artificer]

Schematics as promised. Here they are in Docx format if anyone wants them to study by the fireside of an evening. There's a few drawing mistakes but nothing that should'nt be too bamboozeling. The voltage at the tap on the cathode follower htr supply is 200v. Oh and there are 0.1u caps accross the 12.6v htr supply,bias supply bridge rectifiers to stop switching noise. Same as on the HT supply BR's. The "symbol is a conflation of a BR and N suppression caps.

A.

[Diabolical Artificer]

More progress pics. Slowly slowly catchee monkey.

[Diabolical Artificer]

Ran out of edit time. Pic one shows the finished face of the amp. 2 shows the chassis underneath, bluetak a temp measure. 3 show finshed metal work on RH channel with fan in place. 4 shows top of chassis/fan. I had to glue reinforcing plates around the fan as I ran out of the 2mm ali sheet and had to use 0.65mm. You can see the LH channel mains tfmr. This was a lot quicker to wind than the last one. I got curious stares from villages as I measured the HT winding out on the park it being 243.3ft. A woman came out of the community center and asked what I was doing, but scurried back in when I tried to explain - "I'm winding a transformer" .... What's a tfmr?" Not easy to answer and she wasn't interested.Last time I did it in our garden but it got tangled on bushes and got kinks in it.

I'll order some paint this week, British racing green, should look ok with the brass. I'ts taking a long time but getting there, slowly etc.

Andy.