audio-talk

Cant really do much to the input/driver stage, as it’s all on PCBs, including the 9 pin sockets, so there’s not much chance of breaking anything. I’m done messing about now, honest.

Mike H wrote: ↑Wed May 15, 2019 5:23 pm Just don't fiddle with it 'til you break it.

I messed around with mine and broke it. Thought I might make it a bit better. Haven't been back in the room since last Friday when it broke. When I can do it, I'll try and put it back to how it was, I hope.

Cheers,
Stephen

Cressy Snr wrote: ↑Wed May 15, 2019 11:36 am I’d say to Ray that it’s worth rolling 6H30pi, 6N1P and 6CG7 to see which version of the music you prefer, as the one you choose will of course, be system dependent.

I plan to experiment Steve but first I need to give myself a kick up the backside to get it finished!

Was playing the amp earlier today and my phone was charging near it. “Bit bi dit, bi dit, bi dah dzzzzzzzzzzzzz” went the speakers, seconds later a text arrived on the phone. “Oh, that’s interesting” I mused. Blasted amp was picking up RF.

To cut a long story short, I replaced the 1K grid stoppers on the 13E1s. No 10K left, so 15K carbon comps had to do. Got the missus to ring my phone with it next to the amp and silence reigned, until said silence was broken by my phone ringing. Much better! Later on my daughter texted me to pick her up after work whilst phone was still charging next to amp, and this time, no noises prior to the text arriving.

Now I’d said that treble had been better with the low value grid stoppers, yet now it was better with the 15K stoppers, than either the 10K or 1K had been, and better yet, the separation between instruments was improved as was stage width and depth...go figure. Not that I’m complaining, but 13E1s are fussy bleeders in power cathode follower mode. They’re are not as cantankerous as 6C33Cs by any stretch of the imagination, but nevertheless, parasitics are something to keep a weather eye on with them.

I remember Steve giving me a pair of top-cap 12E1 years ago and they used to take off into the stratosphere without much provocation. I never did tame those and ended up using the ubiquitous 6080 and 6336A as regulators pass tubes instead.

So were the 1K's carbon composition or something inductive ?

Sure you have it sorted, but AFAIK, that sequence is when a phone is talking to a cell with GSM (2G). If it finds one it can switch to latter protocols so you may not be comparing like with like.

It seems to be OK at the moment.
Having built the Transcendent 300B and the Mini-Beast OTLs, I know that Bruce uses 10K grid stoppers on each 300B/EL509.

On the push pull EL509 output stage of the beast he uses 100K stoppers. Some commentators think grid stoppers are the work of the devil and push the ferrite bead solution. But I’ve found with these valves -13E1/6C33C (which let’s face it were never designed for audio frequencies) that they have to be held down with a veritable forest of restraining straps.

Trouble is with these, is that it then becomes a balancing act between high value of stopper and value of grid resistor, because if the combination of the two goes one way, the bias gets unstable; the other way and the thing oscillates. The cathode CCS goes some way to avoiding the former but it can’t help the latter.

Trouble is with these, is that it then becomes a balancing act between high value of stopper and value of grid resistor, because if the combination of the two goes one way, the bias gets unstable; the other way and the thing oscillates.

That generally is the point of inductive solutions, they don't affect the DC conditions.

Nick wrote: ↑Thu May 23, 2019 10:55 am

Trouble is with these, is that it then becomes a balancing act between high value of stopper and value of grid resistor, because if the combination of the two goes one way, the bias gets unstable; the other way and the thing oscillates.

That generally is the point of inductive solutions, they don't affect the DC conditions.

I’ll see if I can find some ferrite beads before the weekend. If not, it’s something to think about for the future.
Though the amp seems to be working fine ATM, ferrite beads ought to be more elegant solution than forcibly slowing down the output stage.
It’s all a useful learning curve.

Cressy Snr wrote: ↑Wed May 22, 2019 11:12 pm Was playing the amp earlier today and my phone was charging near it. “Bit bi dit, bi dit, bi dah dzzzzzzzzzzzzz” went the speakers, seconds later a text arrived on the phone. “Oh, that’s interesting” I mused. Blasted amp was picking up RF.

I've frequently had that. And on different amps, ever since I've had a smart phone. I don't worry about it and I don't try to fix it.

Mike H wrote: ↑Thu May 23, 2019 4:20 pm I've frequently had that. And on different amps, ever since I've had a smart phone. I don't worry about it and I don't try to fix it.

Aye,
It’s high time I took a leaf out of your book.
Bruce Rozenblit himself says amp design is full of compromises and perfection is impossible because “optimizing one aspect of the design will alway cause another to deteriorate”

It's called "engineering" - it's what differentiates us from scientists and theoreticians.

I have some suitable ferrets (sic) that I can bring along if you want...

Yep, but just to be contentious its also called marketing. I use as an example how it wasn’t possible to DC heat that OTL until Andrew did it.

Just fed and cleaned out the Ferrets, its my morning job.

jack wrote: ↑Fri May 24, 2019 7:31 am
I have some suitable ferrets (sic) that I can bring along if you want...

Thanks Nick, that’d be good!

I once tried putting a small value coil in series with the output of each OPT, between where the NFB is taken off and the speaker terminals. It's purpose was to prevent the amp (an EL34 p-p type) going into oscillation if a large (-ish) value capacitor was put across the terminals. It worked like a charm. Unfortunately it also killed off the upper treble