Remember These?

[Ray P]

Sounds an interesting concept.
Would make switch on's a lot less hairy.

I'll let you know how it goes...

[RhythMick]

I've ended up listening to the OTL breadboard for most of today. Loads of Christmas classical orchestral and choral music.
I can't listen to any of my other amps, which could be a slight problem when I have to dismantle the breadboard to box it up.

Excellent. It's painful having to dismantle a prototype which sounds that good.

[Cressy Snr]

Breadboard is all trimmed up for Christmas.

[Ray P]

Those baubles will undoubtedly exert some sort of, as yet unknown, micromolecularorthomorphicgravitas that will have improved the sound quality by 157.67%. Just Imagine if you had red ones available.

[Cressy Snr]

Two and a half grand a set to you Sir. Cheap at half the price!

[Ray P]

Done!

[Cressy Snr]

I've now set about removing that dreadful voltage doubler from the driver stage supply.
I may be some time.

[Wolfgang]

What are your concerns with the voltage doubler and what will you use instead in this position? What kind of psu do you use now for the 13E1?

This might be also interesting for using the 13E1: I finally added a socket/made the necessary connections for testing the 6C33 with my TS tube analyzer. It's very hard to keep the 6C33 stable enough for a clear measurement and the mu/rp is quite a bit off compared to what the data sheet says. Also when the amp is turned on one tube starts conducting about 10 sec earlier than the other tube because of the very slow warming up of the heaters. I never have encountered this with other tubes before, at least not to this degree.

I have ordered a whole bunch of new 6C33 and will find out how they behave and if there is some kind of a "general tendency" in order ot get a better feeling for what's going on.

I don't know how this applies to the 13E1 when used in triode mode but it's a known fact that beam tetrodes show a very fast rise in anode current when B+ is applied. I think these kind of tubes should have a B+ soft start that accompanies the slow warming up of the heaters (about 20sec for the 6C33) when used in OTLs.

[Cressy Snr]

What are your concerns with the voltage doubler and what will you use instead in this position? What kind of psu do you use now for the 13E1?

This might be also interesting for using the 13E1: I finally added a socket/made the necessary connections for testing the 6C33 with my TS tube analyzer. It's very hard to keep the 6C33 stable enough for a clear measurement and the mu/rp is quite a bit off compared to what the data sheet says. Also when the amp is turned on one tube starts conducting about 10 sec earlier than the other tube because of the very slow warming up of the heaters. I never have encountered this with other tubes before, at least not to this degree.

I have ordered a whole bunch of new 6C33 and will find out how they behave and if there is some kind of a "general tendency" in order ot get a better feeling for what's going on.

I don't know how this applies to the 13E1 when used in triode mode but it's a known fact that beam tetrodes show a very fast rise in anode current when B+ is applied. I think these kind of tubes should have a B+ soft start that accompanies the slow warming up of the heaters (about 20sec for the 6C33) when used in OTLs.

Hi Wolfgang,

High ripple, needing generous capacitance, noise and poor regulation are my main concerns with voltage doublers. My input stage draws 60mA (30mA per channel) as it uses 5687 tubes, which run at twice as much idle current as the 6SN7s that yourself and Ray are using in the driver stage.

The supply I have now installed, is a normal CLC supply, fed from a 0-280V HT winding. This outputs too high a voltage for the input stage, but it is the lowest voltage transformer I have. Handily however, a 75C1 glow tube in series with each 5687 drops the HT to 324V,which is bang on target.

[Cressy Snr]

Here is the breadboard, running the new driver stage power supply:



This is quieter than the voltage doubler supplied input-driver stage. I can't comment on the sound yet, as I've only had time to check voltages. That'll have to be later on.

My switch to energise the HT to the output stage seems to work beautifully. Although it's not all that sophisticated and obviously has no automation, it is only the same thing that Bruce Rozenblit used on the 300B OTL with the 25Hz filament supplies: switch filaments on, wait 30 seconds then energise the HT.

I wait one minute whilst the big cathodes of the 13E1s fully heat up, then throw the HT switch.

[ed]

Steve

I've just noticed this thread. If you are still looking for a 30 second delay then the one I was using for the 572 amp has been sitting on the corner of my desk for the last 5 or 6 years. It's the one that came out of the chassis that I brought to the last Owston for you. It used to switch the 500v tranny for the 572s. The board also has 2 x 5v voltmeters. The whole thing displays the seconds countdown and the voltages on a 2 x 16 character lcd display.

if it's any good to you just shout and I'll put it in the post.

[Cressy Snr]

Thanks, Ed.
You're a gent. I've emailed you.

[Cressy Snr]

OK,
I've now got a chance to listen to what gives with this improved power supply to the driver stage:

The 115V windings on the giant toroid are, for the moment, parallelled up to feed the pair of output valves. The output stage is drawing around one fifth of the 1000W rating of the transformer itself, giving a steady anode voltage of 160V and about 147V a-k.

The toroid feeding the input and driver stages is rated at 600mA and is drawing 60mA so is ten times derated. Complete overkill of course, but there it is. 400V is at the output of the 100uF-10H-100uF, CLC supply, and is dropped to 324V by a single 75C1 neon VR tube in the feed to each 5687. These glow tubes are handy as they have a generous maximum current of 60mA, so with 30mA through each one, they are just cruising along. The amp is more quiet than with the voltage doubler. The Transcendent Sound Minibeast is quieter, but I think that being on a breadboard with wires flying everywhere, this thing is about as quiet as it'll ever get.

So, to the sound with a proper power supply to the input/driver stage:

It's taken another step forward in terms of realism and musical presentation. The performers hang in space between, above, in front, behind and to the sides of the speakers and the studio is brought further out into the room. It's doing more of that OTL thing with vocals and treble detail IOW.

HF instrumentation is presented at a higher level in the mix, but never becomes strident or overbearing, just a bit more easy to hear and seperate out. It's not one of those "oh my God, I've not heard that before" situations; simply that the background is easier to hear into and relates better to the overall musical picture.

Vocals are really where this amp shines. It's eerie. Not a lot else to say really.
At the bottom end, I'm mostly satisfied with it. Feedback needs increasing very slightly, just to tighten things up a smidge.

Overall a nice afternoon's work.

Next job is to create another identical power supply so that each 13E1 gets its own CLC setup. This is feasible now, because having now discarded the Voltage doubler, I have both 115V windings to play with.

[steve s]

This is one amplifier that I can't wait to hear Steve... to me it's got all the best ingredients for its type.

[Ray P]

Great work Steve, it's good to see you're wringing more performance out of the breadboard build.

I imagine the amp will work really well with Mr Shiels' speakers.

I've achieved zero progress on my own project so far this break as we've been entertaining every day - last guests today so hopefully I'll get to the bench over the weekend.

I've been wondering whether there might be any gains to be had from using a discrete component current sink instead of the TL783 - something like the K&K Audio kit. about a third of the way down this page;

http://www.kandkaudio.com/other-kits/

Switching from LM317s to TL783s resulted in a definite sound quality improvement so there would seem to be scope for further experimentation.