6B4G - The Resurrection Shuffle

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IslandPink
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#301 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

Probably best to put this on another thread to avoid antagonising Steve any further !
I will shift some comments over tomorrow morning.
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#302 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

What's all this "antagonise" stuff?
I am not antagonised just perplexed because I can't see what the hell brought this lot on.
I've made perfectly valid design choices, that I can't see anything wrong with.

If we look at the sequence of events we get (Names changed to protect the innocent):

Bill - I made a 3 stage amp it sounds really great even with two sets of caps in the signal path
The power driver really kicks up a storm and shows the 2A3 who's boss.

Ted- Yeah My 3 stage Transcendent OTL sounds fantastic and it too uses two sets of caps
I agree with the power driver, much better than those farty little things that 2A3s usually get driving them.

Fred - you dont wanna be doing that. 2 stages is plenty. You've got extra components introducing uneccessary rolloffs all over the place.

Bill - But it sounds great and I've separated the poles by factors of ten as some clever people say you should.

Fred- you've fooked up the bass with that 15Hz second order rolloff on the output stage - and what they say about separating poles applies to power supplies not audio circuits - you stupid, half witted ignoramus!

Bill - No I haven't fooked up the bass and no I'm not, it sounds great and here's why.

Fred- well I don't buy your explanation you're still an idiot, you know the thing about a little bit of knowledge? well you're a prime example of that proverb :wink:

Bill - Well that's me fooking told then!
Best put the amp on a scope, it's probably a right bloody mess.
Hmmmm.....No oscillations, no instability, no ringing, no RF.
Let's try making a mobile phone call from right next to the amp to check that pesky GHz band.
Nope, no buzz, no dit dit dit.....nothing....WOW not had that before.

Clean as a whistle then....Oh and by the way, did I tell you, it sounds gr..RIGHT THAT'S ENOUGH. :lol: :lol: :lol:

(just a bit of fun, just a bit of fun)
Last edited by Cressy Snr on Mon Nov 20, 2017 6:16 pm, edited 1 time in total.
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IslandPink
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#303 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

...please....stop
My mind is going ....
HAL%20interior.jpg
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#304 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

I'll get me coat. :love5:
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#305 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Upped the dissipation of the 2A3-40 output stage to 25W, with a little bit of tweaking component values and an increase in the HT voltage. Took the opportunity to do this whilst I was repairing a minor fault with the amp.

TV damper diode soft start has been replaced by a Mullard GZ34 with both plates tied together and the 75 ohms of series resistor, after the soft start, has been removed. HT still comes up nicely, with no overshoot and warmup time is now reduced to 12 seconds from turn-on.

Coupling caps to output stages had to be changed after the amp had gone dead on one channel. The fault was easily identified, as on the offending channel, the capacitor lead out had sheared off right where it exited the cap body. I managed to find a couple of unused 220nF, Maplin, polyprop caps at the bottom of a box, so all was not lost. Was glad that's all the fault was.

In terms of sound quality, a bit more muck has been cleaned off the window into the studio.
I love this rose pink creation.

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#306 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

For the sake of completeness here's is the power supply schematic:
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#307 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Image

And yet another version of the power supply. The definitive.....honest :angel5:
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#308 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Using this hybrid bridge configuration, with a 1970s Russian harp filament 5U4G, The Groovy Pink S.E.T. makes its best music.
The difference between this and previous configs is not a subtle one......interesting.
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#309 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

Good work !
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#310 Re: 6B4G - The Resurrection Shuffle

Post by izzy wizzy »

Have you ever tried TV dampers for rectification? Like 6AX4.

cheers,

Stephen
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#311 Re: 6B4G - The Resurrection Shuffle

Post by pre65 »

I use 6D22s in several of my builds, mainly in hybrid bridges.

Love the slow warm up and MUCHcheaper than GZ type rectifiers.
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#312 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

izzy wizzy wrote: Tue Dec 19, 2017 11:00 pm Have you ever tried TV dampers for rectification? Like 6AX4.

cheers,

Stephen
Yes I've used EY500s and 6D22S in hybrid bridges as Phil has mentioned above. They are OK. I used them in the very first iteration of the 6B4G monoblocks from 2010. Nothing against them.

On this amp however, I got to thinking, which is always a dangerous thing where I'm concerned.
I figured that because I'm using DHTs in the output stage of my SET, then a DH rectifier, would be perfect, especially a DH with a harp filament like the Russian 5U4G has. I reckon, though I've no evidence, that these harp filament rectifiers put less harmonic distortion onto the power rail, giving a cleaner supply. (tinfoil hat clamped firmly onto my head)

Of course, there's always the danger when using a DH rectifier, that the HT will come up too fast and overshoot, causing over voltage on the power supply caps before it settles back. And with a choke input supply the voltage variation on power-up can be very wide indeed, but I figured that the DHR rectifier would come up in sync with the DHT output stages and thus the 2A3s would start to draw their 75mA each, pretty damn quick, preventing the HT from overshooting from lack of critical choke current.

I tested the startup with a meter and that was exactly what happened - no overshoot. The full power-up takes seven seconds with the DH rectifier and the LEDs biasing the IDH input stage rise to full brightness within 6 seconds, so these miniature 7 pin tubes are also pretty quick to heat up.

Ear against the speaker and the residual hum is audibly smoother with the Russian 5U4G than with either a GZ34/37 or the common or garden inverted "V" ribbon filament on a conventional 5U4G.

Yes I'm a sad bastard who listens to the quality of the hum from an AC heated SET amp, but the resulting sound with the harp filament DH rect is better than with a "normal" NOS 5U4G and an IDH GZ37 , which incidentally drops exactly the same voltage as the first two, and also better than the GZ34, which drops less voltage. As I said, it's interesting. :)
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Nick
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#313 Re: 6B4G - The Resurrection Shuffle

Post by Nick »

I would say if you have hum the sound of it will matter, it will be a indication of the harmonics that are being added, the closer to a sine the better IMHO.

I would worry about your caps though for the case of a failed 2a3, would the voltage stay low enough to save the cap?
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#314 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Aye,
The caps are only 500V rated, but I built the thing with what I had left after getting rid of most of my stock of parts.
Could put a shunt resistor across the PSU so it draws critical current all the time, or rework the power supply so that it becomes one instead of being split, then we'd have half the capacitance but 1000V capability, then again, do we really need all that capacitance in the first place? I mean really? For a low power, Class A SET?

Then what about the valves the themselves under gross over-voltage conditions?
These are the questions I asked myself when planning.

The whole thing had me head-scratching at the start because of the lack of parts suitable for fail-safe operation, hence the fitting of delay relays, slow-starts and all sorts of gubbins to protect the PSU, from over voltage at start-up, but not thinking about what if a 2A3 filament went open.....then what?

I'm loath to mono and series the power supply as the amp sounds so well separated out and all of a piece as it is.
Shunt resistor is the simplest solution. Everybody else does it with LC supplies don't they?
I'll work it so that the resistor causes the PSU to be able to make up for one failed 2A3. Statistically speaking, two failing at the same time or both filament transformers packing up at once is nigh on impossible.

Alternatively, I could start it up with a 2A3 unplugged and a long pole to the on/off switch :lol:
Last edited by Cressy Snr on Wed Dec 20, 2017 11:55 am, edited 1 time in total.
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#315 Re: 6B4G - The Resurrection Shuffle

Post by pre65 »

When I built the 2A3 that Nick designed (long time ago) I did monoblocs.

The PSU was choke input with a shunt resistor to keep it in choke input mode. From memory it was a 100W Arcol attached to the aluminium chassis.

The shunt resistor value (in k ohms) was something like choke value (in H) X 0.9 from memory, I had it written down somewhere but can't find it at the moment. :(
The only thing necessary for the triumph of evil is for good men to do nothing.

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