audio-talk

[Darren]

Would you mind bringing me up to speed Dave?

This 6B4G amp ( My second build too ) How are you driving it. Off the 6EM7?

[Nick]

Maybe its my fault, Dave asked if he could bolt another stage after the 6em7, and I suggested that maybe its not the best thing to do. The 6b4g/2a3 would not be passing much more current than the 6em7 so it seemed better (and maybe more fun) to start again.

I sugested using a pentode (c3g) to drive the output, as they are nice and cheap, and its a circuit I am using at the moment, so if Dave had problems it would be easy to debug remotly.

Also using the pentode, it seems worth using a VR tube on the g2 to help things.

Its almost the 2a34 circuit that James D posted on the WD forum some time ago under (I think) the low noise supply thread.

[Dave the bass]

^ Wot he said

I've got the 6B4G's on the way, 0A2's and C3g's I'm persuing.

Valve bases will come next pay day as I've spent my sweetie money this month on 2x 150mA 10H chokes that just said 'buy me' (£20 the pair) on them plus the 6B4g's.

It'll be monikered 'Komrade DHTski' BTW.

As ever thanks go to Nick for all the guidance and help and everyone else for the encouragment.

DTB

[Darren]

I must admit I've never used a pentode. Well not unless you count the 813 but that's a different kettle of fish.

Umm, I asked cos I intend to hang some valves off the end of my PX4 amps sometime. I was going to just use a cap take-off for now, but ultimately I would like to try interstage transformers. Luckily I have all the bits.

I must try to find the time to put something together before I stagnate.

[Dave the bass]

Dunno if anyones seen thi site yet but I was searching info on 6C4C's and found this site http://nostubes.tripod.com/index.html

Might be of use to the big Valve boys, GM70's for sale.

I've been supplied 6C4C's and thought I'd been dun but it looks like they're 6B4G equivalents. I take it thats OK still play with?

DTB

[Nick]

Just a bot more on this, its not Daves amp, but I spent some time last night working with Simon on his c3g driven 2a3 amp. This is almost the same as the suggested circuit for Dave.

(I will post the circuit for discussion later, its based on James D's one)

Anyway, we had a couple of problems, first it kept blowing fuses, but after staring up with a variac, we tink this was down to the rectifier not coping. It produced a firework display in the valve unlike anything I have seen before.

The big problem we had on the bench was the pentode was prone to RF instability, so Dave, make sure you have some grid stoppers (I used 150R carbon 0.25W) right up against the g1 and g2 pins.

After that (and a different rectifier) we only hade one remaining problem, there seemed to be a 1hz (ish) tick on the scope (only a few mv on the grid, but not right). So I declaired "Ha, motorboating", so a couple of things were tried to isolate the two stages (ok I know its not meant to do that with two, but there was a choke between the stages, so I talked myself into thinking it could be the problem).

No improvement, so just before thinking of pulling it all apart, I realised what it was, with I suspect a look of "doh" on my face, I turned my temperature controled soldering iron off, and the "problem" was fixed. It was caused by the heater cycling on and off.

So we transfered it to the floor. Connected to speakers, and turned on.

Now we got real motorboating, in that the VR tube was flashing on and off. But I had seen this before, and it was fixed by applying a bigger cap after the choke to the first stage. Not a perfect solution, but good enough for the moment. So we had music .

But there was still a bit of hum, but we fixed this by connecting the center tap of the c3g heater to 0v, it was floating before. After that it was nice and quiet. Ok a bit of hum, but the input was connected to the CD player via clipleads, and it was all hanging out. But still quiet enough given that it was sonnected to 94dB/W speakers.

And it sounded realy nice. I think we were both impressed how nice it sounded.

So the upshot of this Dave, is yes the idea works, and it also sounds very good. So now we just have to wait for your version.

I think Simon is hoping to bring it to the comming Eggfest, so hopefully more people can comment on it there.

[simon]

A big thanks to Nick for solving the case of the poorly amp last night!

Nick's summed it up very well. The roman candle display from the GZ34 was quite entertaining, though not very good bang-for-the-buck. Just glad it was my rectifier and not Nicks... Turning the solder station off to cure the motorboating is something that will be recounted over many a beer I suspect.

Initial impressions of the amp were encouraging - I'll try to get some hours on it before EggFest/OwstFest. Hopefully James will be there to hear his design. I may even pluck up the courage to try my RCAs (only dualplates) now it seems to be behaving.

Dave, I think you'll like the amp. With Nick's nice OBs it got right to the heart of the music. I have a few 0A2s as I stocked up wondering if I was killing them when the amp wasn't well. I've more than I'll probably ever use so you're welcome to a couple.

Simon.

[Dave the bass]

[Darren]

And what's wrong with valve fondling?

Tut, some people just don't understand do they.........

[Andrew]

[simon]

Set the C3g-2A3 amp up tonight when I got home and ran it for a few hours. It sounds very nice, plenty of headroom into my 97db Fostex. It's doing something in the bass that the WE91 with KRs doesn't, very nice.

The missus was a bit shocked when she got home though to find a breadboard in the sitting room - her view is that all valve amps are dangerous, let alone one that look like this:



Simon.

[Clive]

Are we sharing the same wife?

When you said she was shocked, was she shocked? Just a bit shocked...ok, it wasn't the HT then.

[simon]

Shocked along the lines of "What the ***** that doing there!!!"

[Dave the bass]

[Dave the bass]