Having finish the big amp, that started as an 807 amp, but morphed into a EL34 amp, I wasn't happy. The amp weighed over 60kg! Also the case just wasn't right, so I started pretty much from scratch, this time opting to make two monoblocks.
I kept the amplifier chassis's but redid the PSU's housing them in wooden boxes. One is pretty near finished, see pics, the other is about half way through. I've yet to apply a finish on the wood, which is sycamore and at the moment the PSU is wired up temporarally.
The fan is there to cool the cathode follower resistors and once I've drilled some holes around the EL34 OP valves, jets of cool air will cool them too.
It's meant a lot more work, but I'm a lot happier with the end result and the monoblocks don't need a crane to lift them.
TFL, Andy.
That's quite a beast! You've put a hige amount of work into it, I hope the results make it all worthwhile.
It is Jeffrey and I have. Remember there's two. I've gotten really fed up sometimes as the project just seems to keep dragging on, especially when I've made a few daft mistakes. However after some tinkering it has pretty good specs - frequency response flat from 15hz to 25khz, 0.2% THD @ 100w. There is one issue it has that needs looking at, namely it starts to self oscillate with a speaker attached just before switch off, making me jumps with a loud squeek. An RC LPF on the OP stage should sort that though.
Andy.
What a beautiful job Andy, so professional. I bet it performs as good as it looks.
To cure the switch off oscillation you will need to look at the smaller transformer in the PSU as one of the power rails generated there is decaying first when it is powered down causing one stage of the amplifier to briefly change from inverting to non inverting so that the NFB line goes positive.
You could briefly power down the small transformers to see what happens or add extra capacitors to the rails they create.
I am assuming that the second item that looks like a transformer is a choke.
Thanks Trevor. I haven't had a chance to listen to them properly to be honest, even as MK1. I did run them briefly with my workshop speakers and they did sound cracking but I've yet to sit in a comfy chair, bung on an LP and crank em up. TBH I lose interest once I build amplifiers, it's more fun mostly building them especially when things don't work properly, gets the old noggin fizzing.
Simon, the small tfmr powers a 50v PSU and RC timer to SW in the big relay which SW's out two big resistors, one on each half of a 180v secondary (to give us 500v unoladed, 450v loaded) which sit under the relay, after 10 seconds; a soft start circuit in other words. The 12v winding powers a +/- 15v PSU which runs the VU meter driver, clipping indicator and fault circuit. There are 10 ohm sense R's on each EL34 cathode to ground , if a EL34 draws more than 100mA , a thyristor latches switching in a relay which cuts the neutral mains to the primary of the big toroid tfmr. So I don't think that's the issue, but will bear it in mind.
This oscillation on SW off problem started after I changed the front end from a bog standard triode gain stage to one with local NFB (which improved the frequency response no end), like this one here -
http://www.valvewizard.co.uk/localfeedback.html This is AC coupled to a LTP. I'll draw the circuit later and post.
Cheers, Andy.
With that gain stage you would need another resistor in series with the anode resistor and a big enough capacitor to hold the HT up until the output stage has powered down.
Hi.
A diode in the HT line to the LFB stages with a normal 32uf decoupler will work just as well as the large HT drop off on the following stages won't be reflected back without reducing available HT with an extra high value resistor, works for me!
Thanks Simon and Trevor, nice tips. The stage is de-coupled from HT with a dropper and 10u cap.
Had a busy week on this amp, everything went wrong, it worked one day, the next oscillated like a demon on amphetamines., oh and the input stage started acting up. After much testing and investigation i found the LM317 regulated PSU that power the frontend and LTP was crashing to 1.2v ish, and found this was down to said reg going into current limiting foldback. bunged a TIP3055 series pass Q on, job's a good un.
Re oscillation, i ended up using a few RC step network's, I believe they are called to kick the oscillation in the head. So that 's a 2k2 and 2200p from each "anode" to ground, a 4k7 and 1000p across the front end anode R, and finally a 470r and 100n across the load. This is from memory, I was aiming for anything over 30khz attenuation , but think this is a tad low. I managed to pop a F counter on one oscillation briefly, it started about 50khz and ended at 100khz as the caps drained after SW off. Clocking HF like these is tricky as some are over 100v P-P in amplitude, one has to be quick.
Amp now steady as a rock, but I still have to test it on other speakers and alter the FB cap. briefly squarewave tested it yesterday = very good at 400hz and 1khz but iffy at 10khz, it's very rounded. Once I have a circuit that stay's the same for longer than a day, I'll draw it up if anyone's interested.
Andy.
Go for it Andy.
I've found most valve amps begin to dislike a square wave around 10KHz, of course that's why they sound nicer to the ear than those with three legged fuses!!!!