Remember These?

What people are working on at the moment
Cressy Snr
Social Sevices have been notified
Posts: 8461
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Wed Jun 12, 2019 10:25 pm

Ray P wrote:
Wed Jun 12, 2019 10:20 pm
Yes, space is always a limiting factor in a domestically acceptable project. I only ask because you mentioned it at Owston - as an experiment you could hang some wires out of the bottom of the chassis to a remote SMPS but that becomes a dangerous game if you like the result, sometimes ignorance is bliss!
Aye, I’ve done fiddling with this thing now I think.
I’m sitting here immersed in Nick Drake’s “Bryter Later” on vinyl as I write this and it is hard to imagine anything could be better.
It’s just a wonderful sound.
All the people on the left - wap-bam-boogie!
All the people on the right - boogaloo!

User avatar
IslandPink
No idea why I do this anymore
Posts: 6905
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

Re: Remember These?

Post by IslandPink » Thu Jun 13, 2019 8:16 am

Cressy Snr wrote:
Wed Jun 12, 2019 10:25 pm
I’m sitting here immersed in Nick Drake’s “Bryter Later” on vinyl as I write this and it is hard to imagine anything could be better.
Every track a cast-iron classic !
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

Cressy Snr
Social Sevices have been notified
Posts: 8461
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Sat Jun 15, 2019 11:53 pm

Once again, having a nice musical evening with this amp.

Since Owston I’ve decided to pay a bit more attention to speaker placement. I’ve rammed the Fane cabs right up to the rear wall, so that the rears of the cabs are only 1” from the wall surface. The skirting board prevents me getting them any nearer. They are then toed in by about 3 degrees. The result is a well balanced sound with great bass.

The Fanes are a bit like the old Linn Saras, inasmuch as they need to be hard up against the wall before they really start to sing, which should come as no surprise as I did after all, design the speakers to be placed up to a wall, but I’ve just found that this literally means right against it. A couple of inches forward and the bass extension is affected quite noticeably. I hadn’t given such detailed thought to the speaker positioning, so didn’t realise what I had been missing out on.

Re the 13E1, It is also worth noting that unlike most power valves, the 13E1 has a centre-tapped heater, just like the ECC83 etc, so no need to create a two resistor artificial centre tap from the 25V transformer winding. And when the centre tap terminal on the valve, is grounded back to the negative tag on the first power supply cap, the result is absolute silence with 100dB/W speakers; no hum whatsoever.
All the people on the left - wap-bam-boogie!
All the people on the right - boogaloo!

Cressy Snr
Social Sevices have been notified
Posts: 8461
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Fri Jul 05, 2019 11:16 pm

Hmmmm...
So this evening I reintroduced the system to the OTL and boy did I hear what I’d been missing over the past week. I’d tweaked up the NVA A60 clone so that its HF performance was close to the OTL, but putting the 13E1 OTL back in made my ears go “ahhhh”

There’s just something about a good single ended tube amplifier that tends to relax the old eardrums. Whatever it is, I love that gorgeous midrange tone and that liquid treble these single tube power cathode follower OTLs produce; it’s just a magic sound.

I’ll never build an inverted OTL as I simply don’t have the knowledge to execute a design or build the power sequencing circuitry. I’m going to have to be content with experimenting with output caps; maybe a nice 4700uF slit-foil SuperTech.

The NVA clone is a fantastic bit of solid state kit, but sadly, for now I prefer my beloved 13E1 SEOTL.
All the people on the left - wap-bam-boogie!
All the people on the right - boogaloo!

Cressy Snr
Social Sevices have been notified
Posts: 8461
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Sat Jul 06, 2019 8:48 am

Jesus, you get up in the morning and don’t half read some fooking drivel. “Sadly I now prefer my beloved 13E1 SEOTL”… FFS!
Both amps are good in different ways. Sometimes I like one, sometimes I like the other. The SEOTL is the current fave.
There…fixed that.

Tip...Don’t post without reading through what you’ve written. And definitely don’t post when you have had too many sherbets :mrgreen:
All the people on the left - wap-bam-boogie!
All the people on the right - boogaloo!

Wolfgang
User
Posts: 118
Joined: Mon May 07, 2018 3:08 pm

Re: Remember These?

Post by Wolfgang » Sat Jul 06, 2019 7:36 pm

I’ll never build an inverted OTL as I simply don’t have the knowledge to execute a design or build the power sequencing circuitry.
To build the amp itself is as easy or difficult as building any other tube amp. If you don't want to build the circuitry you can simply use an on/off switch for the heaters in order to turn them always on first and off last ( about 30s delay). I think you do the same for B+ with your 13E1 SEOTL.
I will soon find out how good the auto DC offset circuit will work. But also in this case you can adjust/readjust the DC offset in the same way as you would have to do this with any other amp without an output cap.

So all this shouldn't be the reason for not building the Inv OTL.

And, mind you, it's not just the output cap that defines the sound of the Inv OTL. The inv. tube operation contributes the biggest part. So even if you would use an output cap it would still sound like an Inv OTL and not like a typical SE OTL. It also doesn't use a CCS and has no addition to the sound from the voltage controller.

And just for the record: I still like my 6C33 SEOTL. But the Inv OTL simply sounds so completely different in my system and opens new ways into the music.

User avatar
Ray P
Social outcast
Posts: 3020
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

Re: Remember These?

Post by Ray P » Sat Jul 06, 2019 9:58 pm

Hi Wolfgang. Even if Steve were to entertain building an inverted OTL, I think the constraint he, and others, would face with the 6AS7 circuit you've used is owning 8ohm speakers. Although I have 15ohm Lowthers similar to your own, it was the prospect of having an amplifier with a bit more flexibility regarding its load that go me wondering about using 6N1Ps in the 5687 circuit.

Wolfgang
User
Posts: 118
Joined: Mon May 07, 2018 3:08 pm

Re: Remember These?

Post by Wolfgang » Sun Jul 07, 2019 12:57 am

The biggest advantage of tubes like the 5687 for the InvOTL is that the heaters need a less powerful transformer.
Considering the output impedance the 6AS7 is relatively close to the 5687: 3,4ohm for ten 5687 and 5,6 ohm for 10 6AS7.

If one measures the CDF for a single driver speaker one will end up probably in most cases around 2-3ohms ( I measured 3 different BLH speakers - but that means of course nothing statistically speaking). Without changing the perfect response curve too much this can be easily extended up to 7 or 9ohms. In the case of the DX4 in the C-horns the response curve for 9,5 ohm is still very close to the ideal at 2,5ohms. And it sounds actually a lot better and more relaxed at 9,5 ohm than overdamped at 0,8ohm like with the 300B SEOTL with the original NFB settings. So I use Zout of 9,5 ohm with all my amps for the C-horn.

And how it appears to me after weeks of listening /testing with one channel the magic sound of the InvOTL comes mainly from the tubes with their respective plate characteristics. The amount of idle current also has quite some influence on the quality of the sound and also whether the grids are choke loaded or resistor loaded.

In the light of this I would rather try to measure the CDF and adjust the speakers upwards than using tubes with different plate characteristics for driving the same speaker at lower impedance. But it would be of course very interesting to try different tubes and hear what the people have to say about the sound.

User avatar
Mike H
Amstrad Tower of Power
Posts: 18286
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

Re: Remember These?

Post by Mike H » Sun Jul 07, 2019 5:01 pm

Image

Cressy Snr wrote:
Sat Jul 06, 2019 8:48 am
Jesus, you get up in the morning and don’t half read some fooking drivel. “Sadly I now prefer my beloved 13E1 SEOTL”… FFS!
Both amps are good in different ways. Sometimes I like one, sometimes I like the other. The SEOTL is the current fave.
There…fixed that.

Tip...Don’t post without reading through what you’ve written. And definitely don’t post when you have had too many sherbets :mrgreen:
Image
 
I never make the same mistake twice. Instead I prefer to fill my day with a variety of different mistakes

User avatar
Ray P
Social outcast
Posts: 3020
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

Re: Remember These?

Post by Ray P » Sun Jul 07, 2019 5:15 pm

With other stuff on my plate an inverted OTL isn't going to be a project for me just yet but I'm very tempted to have a go at one sometime. Perhaps I can use the hiatus to try and assess the suitability of 6N1P valves by finding out about plotting the curves.

Cressy Snr
Social Sevices have been notified
Posts: 8461
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Sun Jul 07, 2019 11:09 pm

Well I’ve not had too many sherbets today. :)

I’d been using a pair of 10K TOCOS pots borrowed off our Ant as volume left and right controls. Nice pots, Anyway I found, while tidying up the hobby room, a 250K stereo TOCOS pot I bought years ago, when Nick Lucas first started stocking them; so this evening I fitted said 250K pot to the front end. I didn’t think this SEOTL could get much better, but it has improved again.

The top end has found more extension from somewhere and the amount of midrange magic has taken another step up in quality. Sometimes OTLs can be a little bit ethereal and this one is no exception, but the presentation with the 250K pot, is a lot more grounded and confident. I have a wider, finer range of volume adjustment too, which is what you’d expect, so I’m not confined to the first 90 degrees of turn from zero volume. Needless to say, I can now return the 10K pots their rightful owner.

I have a pair of 250K, Allen Bradley mono pots: those that I used on that lovely pair of 2A3 monoblocks I built around 10 years ago. These were something special if I remember rightly, but alas, the shafts are now too short to allow the knobs to be fitted; pity that. Never mind, the OTL is singing very nicely thank you, and is once again, out in front of my solid state NVA clone.
All the people on the left - wap-bam-boogie!
All the people on the right - boogaloo!

User avatar
IslandPink
No idea why I do this anymore
Posts: 6905
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

Re: Remember These?

Post by IslandPink » Sun Jul 07, 2019 11:14 pm

There's not much that can beat a good old-school carbon pot for volume control. I am very grateful for Steve S for providing me with one for the tweeter amp.
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

Cressy Snr
Social Sevices have been notified
Posts: 8461
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

Re: Remember These?

Post by Cressy Snr » Tue Jul 09, 2019 7:13 pm

IslandPink wrote:
Sun Jul 07, 2019 11:14 pm
There's not much that can beat a good old-school carbon pot for volume control. I am very grateful for Steve S for providing me with one for the tweeter amp.
Yep, those carbon pots are excellent. My TOCOS carbon pot has decent channel balance as it was one of the first batch that Nick Lucas imported. He only does mono TOCOS pots these days and sells the stereo ones cheap (£5.00) on the understanding that people only use them as mono pots.

I had a 100K “scratch suppressor” (Blencowe’s description) resistor on the first stage grid to ground, in parallel with the 10K TOCOS pot that used to be in there. However that value was no good with the 250K pot, so each PCB now has a 1Meg in parallel with the pot, as the scratch suppressor. I wanted to go higher but couldn’t find anything bigger than 1Meg in my parts boxes. Still, the 1Meg only takes 50K off the total R of the pot, which is not too bad.

The frequency response at HF is down a couple of dB, at 20KHz due to the RC filter formed by the pot and the 96pF of Miller capacitance of the 6H30pi, but that tames some of the HF peaking of the Fane speakers, and the result is delightful. The sound of the amp/speaker system with the new volume pot and 1Meg grid leak, is nothing like the sound it had before the pot change. It is a very useful step up. Excellent stuff.
All the people on the left - wap-bam-boogie!
All the people on the right - boogaloo!

User avatar
Mike H
Amstrad Tower of Power
Posts: 18286
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

Re: Remember These?

Post by Mike H » Tue Jul 09, 2019 8:30 pm

I'm a great fan of the Cosmo Tocos pots. :thumbleft:
 
I never make the same mistake twice. Instead I prefer to fill my day with a variety of different mistakes

User avatar
Ray P
Social outcast
Posts: 3020
Joined: Thu Nov 22, 2007 5:18 pm
Location: Somerset

Re: Remember These?

Post by Ray P » Fri Aug 23, 2019 4:17 pm

Hi Steve, a bit of a numpty question if I may.

I've got a couple of the ferrite beads for the 13E1 anodes as per your successful experiments a few posts back. Do I just thread the anode wire through them or is something more elaborate required?

Post Reply