Super Rocky

What people are working on at the moment
Cressy Snr
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#16 Re: Super Rocky

Post by Cressy Snr »

Had the Hammond 1627SE output transformer on the bench today to check the suitability of the 4R winding as a source of feedback. Put the mains on the primary and a couple of meters on the two secondaries. This is what I got:
24FF454A-AF39-45CB-A184-F865A78A623B.jpeg
Should work reasonably well.
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Mike H
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#17 Re: Super Rocky

Post by Mike H »

Something's a bit off, I get different powers if actual 8R and 4R loads were applied.

For 8R (17.5:1), 30W; for 4R (26:1), 21W.

Working backwards from 30W into 8R and 4R, the first becomes 15.5:1, the second 22:1

Bu this is probably splitting hairs, it will work. :D

Be warned tho that when I tried this, the freq response was crap because of the basically poor coupling of a very low impedance winding (few number of turns). I found that in every case I need to get the NFB direct off the same secondary that's connected to the speaker for best results.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
Cressy Snr
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#18 Re: Super Rocky

Post by Cressy Snr »

In the system and playing:
89F5C50A-7473-4CA2-BC2D-E2CBE8D2AA8B.jpeg
Sounds sweet, detailed and more importantly it lets the music through very nicely.
It’s different from the NVA clone of course, but there would have been little point building it if it had sounded the same.

There was one glitch in the process of putting it into service. The input stage choke was snubbed with a traditional 10n/10K cap/resistor combo, but all it did was cause a buzz in the speakers, so it was removed toot sweet, which quietened things considerably.
I had found myself with only 1 pair of 220nF 1KV caps in my boxes, so they ended up on the output stage PSU choke, with the trad snubber on the other choke. Anyway, I’ll have to (horrors) buy a pair of suitable caps for the input PSU choke snubber. The EZ81 switches off pretty cleanly anyhow, according to Morgan Jones.

There is some hum, near to the speakers but I can live with that. I’ve built far worse hummers in my time than this one; it’s just that I’ve been spoiled by the silence of the OTL and the NVA clone.

The amp has been built on the OTL’s wood frame. I made a conscious decision to make the OTL modular in design, so that by simply disconnecting the electrical supply and the inputs to the driver boards, the whole top plate could be lifted out, complete with all the circuitry, leaving behind all the input cabling, volume controls, selector switch, speaker outlets and electrical input and fusing. All I had to do then, was to build the Super Rocky on the correct size top plate, drop it into the frame, connect up the inputs from the selector switch, the speaker outlets and the electric supply and we were good to go.

In the end I couldn’t be arsed with ultralinear and faffing with feedback, so the KT77s are triode wired and sound none the worse for it.
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Mike H
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#19 Re: Super Rocky

Post by Mike H »

:thumbleft:
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
steve s
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#20 Re: Super Rocky

Post by steve s »

Looks great Steve
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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IslandPink
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#21 Re: Super Rocky

Post by IslandPink »

Good effort ! - glad you are back in the game and OK :)
"Once you find out ... the Circumstances ; then you can go out"
Cressy Snr
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#22 Re: Super Rocky

Post by Cressy Snr »

Thanks chaps. :)

Have been working on it this morning to further reduce the hum. I changed out the choke in the input stage supply for a Hammond 156C, 8mA 150H choke. Critical current for that is very low, due to the high inductance. It only needs a 6mA constant current draw resistor to hold the supply in choke mode. When added to the 4mA combined draw of the 6SL7s, we get 10mA, which is slightly over the 8mA spec, but a couple of milliamps is hardly going to cause a serious problem.

I installed a pair of 100nF caps, as a spike snubber arrangement across the Hammond choke, and these seemed to work very well. I figured that the higher DC resistance of the 150H choke would mean I could easily get away with halving the cap values for the snubber.

Also I soldered a wire to each of the metal cases of the Allen Bradley pots and took the other ends of those to the earth busbar, thereby shielding the pot internals like they do with guitar amps.

Taken together, these few steps have virtually silenced the amp. There is a bit of low level hum with an ear to the drivers, but as these are 100dB efficient, I don’t think I’m going to get it any better unless I go DC on the input stage heaters. I’m not going to do that, the amp is now quiet enough.
Last edited by Cressy Snr on Tue Mar 31, 2020 2:40 pm, edited 1 time in total.
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Ray P
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#23 Re: Super Rocky

Post by Ray P »

Good to see you in such high spirits Steve :D

Nice little project too!
Sorry, I couldn't resist!
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Ray P
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#24 Re: Super Rocky

Post by Ray P »

Ray P wrote: Tue Mar 31, 2020 1:50 pm Nice little project too!
'Little' might not have been the right word, but you know what I mean.
Sorry, I couldn't resist!
Cressy Snr
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#25 Re: Super Rocky

Post by Cressy Snr »

Ray P wrote: Tue Mar 31, 2020 1:51 pm
Ray P wrote: Tue Mar 31, 2020 1:50 pm Nice little project too!
'Little' might not have been the right word, but you know what I mean.
Indeed! :)
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Max N
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#26 Re: Super Rocky

Post by Max N »

Good to see you back on form Steve :D
Cressy Snr
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#27 Re: Super Rocky

Post by Cressy Snr »

Been ironing out a few small teething problems over the past couple of days and have also been swearing like a trooper; more of which later.

Anyway, here is how the Super Rocky looks now:
Image

Apart from changing the volume knobs to chicken heads and the selector switch to a Marshall type knob, plus the port and starboard indicator lights either side of the mains switch, the eagle-eyed among us may have spotted a couple more things different. Just behind the rear power supply cap can be seen a 9 pin valve socket...grr :x and there is a toggle switch next to the larger of the rectifier valves.

The power stage supply choke has also been changed out. OK so it’s not blue, but I think the natural metal finish, breaks up the wall of blue at the rear and evens up the number of blue components on the top plate, giving a more balanced look. That’s my excuse and I’m sticking to it. I should, perhaps have thought about the colour scheme a bit more. If I had, then the transformers would have been painted a nice maroon colour to match the speakers’ Rexine front panels, but that can be changed further down the line.

So, to business,
It eventually turned out that the HT from the 450V winding on the Danbury mains TX had come in slightly too high with the GZ37 rectifier valve.I had been under the impression that everything was spot on, but it turned out that I’d misread the GEC datasheet (not difficult given my state of bewilderment) and was slightly over-dissipating the valves. These being modern Russian Genalex reissues of the originals, I was not convinced the valves would last long, being thrashed.

The only way to get the HT down further, was to use one of my collection of direct heated rectifiers. That was not possible with the choke input supply as I had 500V caps, and the fast startup would have sent the HT on the output stage supply through the roof at switch-on. I could have used a shunt resistor in that supply, the same as with the input stages, in order to prevent this, but the heat issue, such a resistor would have caused was something I wasn’t prepared to put up with.

Instead I had been intending to use my sole Amperite 6N045T thermal time delay relay, and delay the 5V rectifier heater feed by 45 seconds, after which the power valves would have been warmed up. This would have eliminated HT overshoot with the proposed direct heated rectifier and protected the supply caps from over voltage. The hole had been punched, the socket fitted and the pins wired. I went to pick up the relay tube and promptly knocked it off the table where dutifully, it smashed itself to pieces on the floor tiles! Let’s just say the air was blue and leave it at that!

So we had a valve socket installed in the top plate that was now neither use nor fecking ornament. After a few hours to calm down and the red mist to dissipate, the only remaining viable solution to the direct heated rectifier problem, was to make another goddamned hole in the top plate and put a manual switch in the 5V rectifier feed hence the switch next to the rectifier valve, in the picture above.

So that was OK; throw the main switch on the front panel, wait 20 seconds then throw the switch next to the rectifier and yep, no HT overshoot, and the HT was right with a 5U4G.

The choke got changed because it was buzzy as feck. The replacement wasn’t as bad, but as Jimmy Cricket says, ‘wait, there’s more.’ I removed the centre-tapped cap snubber and put in a parallel RC network across the choke, to work with the choke inductance and stick a whacking great impedance peak in there, right on 100Hz. I found it on Patrick Turners website.

It is certainly not the trad 10n/10K RC network. http://www.turneraudio.com.au/audiofilt ... page2.html. According to him: with a suitable parallel RC network across the choke, it is possible to fool the supply into thinking it has a choke of twice the Henries, for the cost of a resistor and cap. Nothing new perhaps, but it’s certainly a great bit of info.

As Turner himself found with his amp work, the effect of this network on my amp (as was his) was quite remarkable. The choke mechanical buzz was greatly reduced and the background hum, suddenly became on a par with the OTL and the NVA clone. There is none! A bloody superb result. I must have got it right. Maths is dead useful in electrical engineering you know. But FFS don’t ask me to write out what I did. I’m still traumatised!

As far as this circuit is concerned, it can only use the KT77. EL34s would be over-dissipated with this HT level and only a 288R cathode resistor. The KT77s in this circuit dissipate around 24W - 6W below their absolute max and 1W below their design max; so EL34 is not a drop-in replacement. It’s true that KT77s can often be used in EL34 circuits; fine, but not the other way round; at least not in my circuit. That restriction however, really does not matter in this amp, as I won’t be using EL34s. The KT77 is one heck of a good valve if your circuit is tailored to it, and it alone. Having had the experience of using KT77 in an EL34 circuit, believe me, you’ve not heard what it is capable of, if that’s all you’ve done with it.

The result of all this additional work is the best sounding valve amp I have ever made. Not difficult of couse, given the rubbish (apart from the OTL) I have the habit of building, :lol: Still I’ve made a small step forward in my own understanding of that pesky AC stuff.
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Cressy Snr
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#28 Re: Super Rocky

Post by Cressy Snr »

Took delivery of a nice pair of black glass, USA Tung Sol 6SL7 first stage valves earlier this morning.
41F89772-AA7B-4317-899F-A9C796B9A8AC.jpeg
The venerable, short-bottle Hitachis had been retired from my mate Clive’s Rocky amp, after 10 years fo service because they had become a bit down on emission, one more than the other, unbalancing his channels. I was able to compensate for this because I have separate L & R volume controls. Nevertheless, I felt I ought to get some new ones in. The Tung Sols should last a few years.
Last edited by Cressy Snr on Wed Apr 08, 2020 11:13 am, edited 3 times in total.
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Ray P
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#29 Re: Super Rocky

Post by Ray P »

Nice addition Steve.

When you describe your Rocky as the best amp you've ever built does that relegate the 13E1 OTL?
Sorry, I couldn't resist!
Cressy Snr
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#30 Re: Super Rocky

Post by Cressy Snr »

Ray P wrote: Wed Apr 08, 2020 11:11 am Nice addition Steve.

When you describe your Rocky as the best amp you've ever built does that relegate the 13E1 OTL?
Not exactly Ray. The OTL, like others of its ilk has a unique way with music. The power consumption is the thing that bothers me about it. I’m glad I built it, as it was/is a major test bed for developing the multiple power supply approach I’ve used in the Super Rocky.

As a result of the learning gained from the OTL’s power supplies, the Super Rocky is certainly the best sounding and most musical non-OTL valve amp I’ve built to date.

Compared to my 2006 amplifier, the Super Rocky incorporates a completely different power supply setup: choke input rather than cap, one power supply per stage and a little tweak to the output stage choke to make it seem twice the inductance. Hell, it can’t even use EL34s. So though, superficially similar, it isn’t the same thing at all. Not really. I need a different name for it. :lol:
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