Remember These?

[Cressy Snr]

Well the next bit of fiddling (sorry Mike) will be ferrites on the 13E1 grid tags. Nick (Jack) gave me a few yesterday, so they’ll end up replacing the grid stoppers at some point.

[Greg]

A very honest and transparent sound. Very typical in my experience of OTL. I like it that there is one valve per channel. That cleans things up and further exposes the transparency. This was the same take away I had a couple of years ago after Ray demoing his 300B OTL. Speakers were not my cup of tea, but so what? They looked fecking great, were a very tidy professional design and apparently sound wonderful at home. Well done Steve. I know where you are. Finally getting it right at home makes everything else superfluous, and so it should do.

[Mike H]

Well the next bit of fiddling (sorry Mike) will be ferrites on the 13E1 grid tags. Nick (Jack) gave me a few yesterday, so they’ll end up replacing the grid stoppers at some point.

[Cressy Snr]

Tried the ferrites on the 13E1 grid leads instead of grid-stoppers. Cue a swarm of angry wasps.
Remove ferrites and put back 15K carbon comp stoppers; silence once more.

Looks like ferrites are not the answer in this case. Stoppers it is then, but thanks anyway Nick (Jack).

Definitely done fiddling now.

[Cressy Snr]

A very honest and transparent sound. Very typical in my experience of OTL. I like it that there is one valve per channel. That cleans things up and further exposes the transparency. This was the same take away I had a couple of years ago after Ray demoing his 300B OTL. Speakers were not my cup of tea, but so what? They looked fecking great, were a very tidy professional design and apparently sound wonderful at home. Well done Steve. I know where you are. Finally getting it right at home makes everything else superfluous, and so it should do.

Yep, it’s a great sound. I was talking to Ray on Saturday about these particular OTLs and we both agreed that neither of us would willingly use a valve amp with an OPT any more. Hopelessly inefficient and completely impractical on 99.999% of existing speakers, the single ended, single tube OTL is nevertheless, for me, Wolfgang, and probably Ray (once he finishes his) the ultimate valve amp.

The “Fane-Tastic” speakers are by no means perfect, but they do enable me to enjoy a single tube OTL and so, experience its musical transparency and excellence. Power sapping crossover components and impedance flattening Zobel networks are sadly not an option with a single tube OTL. A single wideband driver with a flat impedance characteristic is really the only practical option. 15 Ohms would be ideal, but as we heard 8 is possible, with a 13E1 given a following wind.

[pre65]

Definitely done fiddling now.

Oh yeah ?

I've got a bet with myself when the next "fiddle" will occur.

[Mike H]

Tried the ferrites on the 13E1 grid leads instead of grid-stoppers. Cue a swarm of angry wasps.
Remove ferrites and put back 15K carbon comp stoppers; silence once more.

Looks like ferrites are not the answer in this case. Stoppers it is then, but thanks anyway Nick (Jack).

Definitely done fiddling now.

Can't see why you couldn't have both. The resistors and the ferrites will be "grid stopping" in different frequency bands. So I'm not surprised if it wants the resistors to still be in there. The ferrites will only work in the RF range, but if you take the resistors out you might end up with very small value tuned grid coils and make an RF oscillator, which sounds like what happened.

Personally I think I'd want to put the ferrite bead furthest away from the grid, on the opposite lead of the resistor. i'm assuming the grid stopper resistors are right close up to the solder tags?

[Cressy Snr]

Tried the ferrites on the 13E1 grid leads instead of grid-stoppers. Cue a swarm of angry wasps.
Remove ferrites and put back 15K carbon comp stoppers; silence once more.

Looks like ferrites are not the answer in this case. Stoppers it is then, but thanks anyway Nick (Jack).

Definitely done fiddling now.

Can't see why you couldn't have both. The resistors and the ferrites will be "grid stopping" in different frequency bands. So I'm not surprised if it wants the resistors to still be in there. The ferrites will only work in the RF range, but if you take the resistors out you might end up with very small value tuned grid coils and make an RF oscillator, which sounds like what happened.

Personally I think I'd want to put the ferrite bead furthest away from the grid, on the opposite lead of the resistor. i'm assuming the grid stopper resistors are right close up to the solder tags?

Yes they are. Now you’re gonna have me fiddling again.

[Mike H]

Worth a sperrymint, if only to tick that off as having been tried. Only 3 things can happen, A. nothing, B. it makes it worse, C. it makes it better. You might need an oscilloscope to detect what's going on though, if not audibly obvious.

[Cressy Snr]

Worth a sperrymint, if only to tick that off as having been tried. Only 3 things can happen, A. nothing, B. it makes it worse, C. it makes it better. You might need an oscilloscope to detect what's going on though, if not audibly obvious.

OK
So what I did is this:
Grid stopper up against tag as normal
ferrite placed on grid feed wire
feed wire soldered to other end of resistor as close to resistor body as possible
any protruding wire cut off with flush cutting pliers.
Ferrite slid up wire until it is as close to resistor body as possible with a kink to keep it from sliding back along wire.
Resistor and ferrite end up at 90 degrees to each other

Result!
13E1 that was vibrating internally is now not doing it.
Treble clarity improved, imaging improved, soundscape larger, depth goes further back. I thought it couldn’t get any better but it clearly has.

So with the 13E1, we need highish value carbon comp grid stoppers, with a ferrite bead placed before each stopper, as close to resistor body as possible, and low value anode stoppers soldered up against anode tags.

The 13E1 does take a bit of taming, but it sounds fab once whipped into line.

Wolfgang is correct; small component changes are extremely audible with these single tube OTLs

[jack]

Glad there's a result. I'll look up the Farnell p/n for them ferrites...

[Cressy Snr]

Oh and RF is a bloody insidious little pest. I’ve discovered, albeit a bit late in the day, that getting a clean signal throughput is far from straightforward and the detrimental effect on sound quality from subtly intrusive hash is cumulative.

I’ve learned something really useful from this little outing and would argue that all the expensive audiophile components in the the world, come to nought if the signal path itself is full of unwanted shite. Not that there are any expensive bits in my amp, but if there had been any, they would almost certainly have been wasted.

[Ray P]

Glad there's a result. I'll look up the Farnell p/n for them ferrites...

Yes please Nick - it looks as though I'll need to get hold of some for my project.

[jack]

The ferrite beads are https://uk.farnell.com/wurth-elektronik ... dp/1635667

These are the same ferrites that I use on my uTracer (and other valve projects) to suppress RF. Always found them to be good (but not cheap).

HTH

EDIT: When I say "not cheap", in small quantities, they're ok, but the uTracer used nearly 100.

[Cressy Snr]

Glad there's a result. I'll look up the Farnell p/n for them ferrites...

Yes please Nick - it looks as though I'll need to get hold of some for my project.

Aye, the ferrites finish off very nicely, the overall performance of this single tube OTL.
It’s taken me nearly 15 years to understand that RF isn’t just about rectifier switching spikes
Anyway I’ve enjoyed being the guinea pig for SEOTLs using this particular tube. Can’t really complain about how it’s turned out.