Valve amp enthusiasts?

[Jonny69]

The transformer turned up earlier this week and I fitted it this evening, like-for-like. Doesn't seem to be working though, which is annoying. The valves aren't glowing so I think I've either mis-wired the heaters or there's a problem. I need to get a multi-meter before I proceed because my old one has gone walkies too.

[Jonny69]

I finally got my multimeter back yesterday evening so I had a quick test this morning before I went to work. The fuse inside had blown so I wasn't sure what was up.

Just over 6V on the low tension - good. A shade over 500V AC from the transformer, so it's a bit higher than rated, but this is open load so it will read a fair bit higher than the loaded voltage. Well over 750V DC once rectified though - which was a bit scary watching the multimeter needle fly off the scale but, again, no problems there.

I plugged the valves in one amp with a fresh fuse and fired it up. There was a high-tension flash inside one of the output valves which popped the fuse, so I'm assuming that was a dead valve, so I swapped it out. Basically, I don't seem to be getting any heater glow from them, so I think I need to do some diagnostic on them to check the voltage is getting to the heaters and that the heaters haven't actually died. I haven't actually tinkered with anything else inside the amp, so hopefully it's just a few of the valves that need replacing and it'll be good to go.

However... if it is a couple of popped valves, it might mean that I didn't need to replace the transformer after all... Ah well, a spare will come in handy for something

[Jonny69]

Ok so yesterday I wasn't getting any sound output and the heaters didn't seem to be glowing. I looked up all the valve datasheets for the pin-outs so I could check for continuity on all the heater coils. All were fine. I then checked for voltage at the valve bases and only one at the far end was showing voltage. So I had to open the amp up again.

On valve amps you've got to be EXTREMELY careful because of the high voltage, so I unplugged it completely and put the multimeter across the HT and waited for the voltage to drop off. Once safe I could start pulling wires. I found the heater wire was loose in the connector, so the connector was showing 6V but the wire going off to the valves wasn't connected. Once reattached I connected it all back up and flicked on the power. This time it all lit up properly and started kicking out heat like it should do. I forgot to polish the valves before I turned it on so it quickly started smelling of burning fingerprints, but it officially works again! I just need to get hold of one new EL34 and I'll have the stereo pair working again. I'm planning to incorporate a switch-box so I can run multiple inputs, which means I'll be able to hook up the DVD player, TV and an aux input.

To put a strange perspective on things, the last time I ran this was before I had MP3s. They sound pretty horrible through a valve amp...

[Nutcase]

Nice one

[Fayshun]

Originally Posted by Jonny69

To put a strange perspective on things, the last time I ran this was before I had MP3s. They sound pretty horrible through a valve amp...

Digital meets analogue, in reverse!

[Jonny69]

Scored 4 matched EL34 output valves. Old meets new:

[Jonny69]

Some improvements to the EL34 amp and a sneak preview into the new portable AA-powered EL84 amp project

There are quite a few things on the EL34 amp that I never sorted out when I first built it and I decided to sort them all out. You might be able to see in the first pictures that it's built in three ally boxes that are screwed together, but it has a sag in the middle. This is because it's only got 4 feet on the outside edges and there was never a proper front and rear panel available to hold it all straight. One day I might make a new panel, but in the meantime it just needs some more feet:



You'll see the outside ones aren't straight. I occasionally used to move them back into place but the whole amp gets so hot the adhesive goes soft and they move around!

Next up is the volume control. I'd originally fitted a cheap Maplin dual gang potentiometer as the volume control. I can't remember if it went crackly or caused the amp to hum but I disconnected it some years ago and used to use a preamp for the volume control. My setup is different these days and it needs a decent volume control so I can hook it up to the CD player, DVD, TV etc. I didn't want another cheap pot because they don't last long so I splashed out on a pair of good quality guitar pots, which are bigger and have metal shafts. Old pot in place:



The replacements:



Fitting them was a matter of drilling a new hole in the left hand amp case. I was really hoping the existing wiring would reach, because cutting, stripping and soldering coax cable is an utter chore and I hate doing it. No such luck:



Time to cut up a Cambridge Audio phono lead. Expensive, but I have a box of these that I've 'acquired' so this hasn't cost me £20...



And fitted:



This took hours, but it's done now and I don't have to worry about it any more. It's made the amp a lot more useable and all I need to do now is make up a switch box for multiple inputs. What it looks like from the front:



I just need a pair of matching volume knobs now

[Jonny69]

The secret project is an AA battery powered EL84 amp. The EL84 is the little brother of the EL34 and puts out about 3-5W in class A or 10-15W as a push-pull pair in class AB. I'm planning to use a single EL84 in class A driven by an EF86 valve as in the old Mullard amps, but powering the lot by 4 AA batteries. I'll be using a 555 timer chip to make a 50hz astable circuit, run that through a pair of NPN and PNP power transistors which will be able to deal out some current. This gives me a strong 6V AC source. That goes backwards through a 4.5V mains transformer which should give me around 300V AC. I'll bridge rectify that with 4 diodes and I should have a nice smooth 300V DC to power the amp from my rechargeable AA batteries

I've bought some goodies, namely a hammerite-finish box from Maplin, some valves, an RS output transformer and a small 4.5V mains transformer.

Box:



The planned layout is to have two valves at the front and the output transformer at the back:



I've tested the power supply circuit already. I used the same setup to make the high voltage 100hz spark to light my pulsejets and I can tell you it's quite effective because I've taken a few shocks from it! This is my original prototype from a few years ago that I'm using for sizing up. This is basically what's got to go inside the box:



Plus the battery box, plus the amplifier circuit and it's looking like a bit of a squeeze! I think it'll just about fit:



Original valve amps were built on tag boards and I have no way of making circuit boards any more because I sold my etching tank on eBay to free up a bit of space. I'm going to make this amp on Veroboard because it's much like tag boards but a bit more versatile and I should be able to make the circuits a bit more compact. Inside the box I need two boards: one for the PSU and one for the amp:



For now, that's as far as I've got. I need to get the PSU done first, which is almost identical to the circuit board a few pictures above, except I'll be using smaller transistors. I'm *hoping* I'll be able to squeeze it on because it's VERY tight for space. I also have to allow for some space to pass leads through to a switch on the front panel and a volume control pot.

[volospian]

That looks interesting! I really need to get my backside into gear and start work on a guitar amp. I just seem to have so many things to do at the moment, and so little time to do them in.

[volospian]

Originally Posted by Jonny69

I just need a pair of matching volume knobs now

I have a bag of assorted coloured knobs like these if you want a couple. PM me an address and I'll chuck a couple in a bag. I think I have a few of each colour, so you even get a choice

I *may* even have a couple of Marshal style knobs left, but I'm not sure about that. I used 6 on a fuzz pedal I made for a mate, but I can't remember how many I bought in the first place. I'll have a look when I'm back at home...