audio-talk

[little eddy]

Thanks to all the hospitality, advice and encouragement received at Owston, and this has enthused me sufficiently enough to consider my first self-build project. Given my limited experience of upgrades to existing kit, Steve the organiser suggested that I might consider a Rocky EL34/6L6 power amp based on The Shadow’s original designs. I know The Shadow/Cressy got up to a Version IV but not knowing whether there was a Rocky V film, I’ve decided to call my amp “Rocky - The Quatroquel”. I don’t have a classical upbringing but it’s supposed to mean something like ‘sequel No. 4’.

As I said in my introductory note, I am/was a professional electrical engineer and my main philosophies are to keep thinks as simple as possible but to do them as well as I can. I think this project is well suited in that it is simple but I intend to be meticulous in terms of the circuit design and choice of components, (as well as the care taken with the final build).

I will first start with the PSU so I have tried to insert my first draft schematic and invite any comments and/or advice.

My thoughts and resultant questions are as follows:

1. I haven’t got into the audio stage design just yet but is there any benefit in using a TX secondary voltage of greater than 275-0-275 with the Rocky 6SN7/EL34 design?
2. Do the connections to the GZ34 have to reflect the phase/direction (I forget the correct technical term) of the transformer secondary 275 and 5V windings?
3. On some transformers, the 5V winding is centre tapped (e.g. Hammond 370HX and 372JX) and from one internet design I’ve seen the schematic would be somewhat different with the output line connected to the centre tap. Are there benefits of this design and therefore is it worthwhile hunting down a transformer with a 5Vct winding?
4. Is a CLC Pi filter preferred to a simpler capacitor reservoir? With one of the benefits of the GZ34 being its soft start, why shouldn’y a large reservoir capacitor bank be used and if so, how large, (obviously this would invalidate my discharge calculations below).
5. If using a CLC filter, I have read that C1 should probably be between 5 and 10uF in this instance. In terms of the choke, some say make it as big as you can afford and most people seem to have settled on a 10H Hammond 193J. In terms of C2, I have read two separate articles, one saying that C2 should be 3 or 4 times C1, the other saying go as large as possible up to 250uF. I have some 150uF caps so would suggest these unless anyone advises otherwise.
6. I recall reading somewhere a recommendation to use a small high quality capacitor in parallel to large smoothing capacitors. Is C11, say a 0.47uF polyprop cap OK (Maplin N35CN), or would someone have a different recommendation or is this pointless?
7. I intend to add a discharge resistor to the output of the Pi filter. If the voltage is circa 370V and I use an 80K resistor with the aforesaid total of 160uF, by my calculations the voltage would fall to 35V after 30 seconds and 3.5V after 1 min. Is this an acceptable rate of voltage fall?
8. I have read that an amp such as this can be prone to hum due to the use of an AC filament supply. Switching to dc might be the ultimate way to go but I have read that by applying a bias of circa 50Vdc reduces this likelihood so in the attached design I propose to combine this with the dissipation circuit by replacing the simple resistor with a potential divider and connect a leg to the 6.3V centre tap. I propose to make R1 69K (47K+22K) and R14 10K which if Vout is 370, the filament bias will be 47V. The biggest power dissipation will be via the 47K @ 1W so propose to use 2W metal films for all of the dissipation resistors.
9. I have seen one design on the internet with two parallel rectifiers (GZ32s). I initially thought was this might be to permit an increase of Pi filter capacitance levels but in that particular instance, the total capacitance used was 68uF. I then asked myself whether it might be worth having a separate rectifier, Pi filter and discharge circuit for each channel. It wouldn’t cost a lot and is quite easy to incorporate if designed in from the start. Although I have no experience on this subject, something tells me that this should be beneficial over the currently shown shared HT power supply. Has anyone any thoughts or experiences on this?

Hopefully some of these questions are challenging enough to provide some stimulation of the old grey matter and prove tempting enough to warrant a response. I’ve been told that with valve amps, the key items are the output transformers and power supply so I am particularly looking forwards to replies regarding the dual rectifier scenario.

Thanks in advance,

Little Eddy.

[cressy]

i havent had any hum issues with mine at all, all i get is a mechanical hum due to the mains tx being in the potting box without compound around it, with regards to the gz34, i tried a 5u4g which lowered the voltage slightly and was a little better with the 6l6. other than that there hasnt been any changes to the amp at all, i think al newall tried a slightly different driver arrangement but i cant remember what it was, it sounded faster at owston this time. watch out for microphonic drivers though as i has an intermittent noise problem with the rca's i was using that i havent had at all with the golden dragons in it now. i think ed tried a dual rectifier in his iirc so best ask him if that was the case. best of luck with it its a good one and will i think benefit from being taken a step further

[little eddy]

Here I have posted an updated power supply schematic as suggested by Al.

I was looking at ways to improve upon what I believe is an excellent stating point.

My first question relates to the TX/HT voltages. The valve datasheets suggest that they can handle voltages larger than those used on some of the designs I've seen with TX secondary of say 275-0-275. How far is it recommended to go and what would be reasonable?

I was considering various stages from the 'standard' fully shared supply, through to a dual mono configuration. The attached suggests separate CLC filter for each channel. It also suggest doubling up of the recifier so I can increase the capacitance levels of th Pi filter.

The final stage would be to have two TX's and a totally split design.

Anyone any experiences? Can you suggest a reasonable starting point without adding too much unnecessary cost?

Thanks,

Little Eddy

[little eddy]

Channel Schematic

Find attached 1st draft of schematic based on Al's SRPP front end and EL34/6l6 output.

Two questions at the moment:

Should I be taking the SRPP output from above or below the top valve cathode resistor, (I've read that below is preferable so long as the input Z of the next stage is at least 200k)?

Should I be starting with a grid stopper going into the EL34/6l6?

[ed]

Hi Eddy

most srpp configs I have seen have the coupling above the resistor, but there a many ways to skin the cat(psst don't tell graeme)......As usual John Broskie is eloquent on the subject:

http://www.tubecad.com/articles_2002/SRPP_Deconstructed/index.html

hope its of some use

btw circuit is looking good

Ed

[Nick]

If you want to use the cap in the amp b+, it might be worth adding a R before it to provide a degree of decoupling. Also I would tie a 1M resistor between the vol pot grid and ground to prevent the grid floating with a noisy pot wiper.

[Nick]

Are those 150R cathode resistors right in the first stage?

What current is it running at ?

[SteveTheShadow]

Hi,

I agree with Nick on the SRPP 150R cathode resistors. There'd be a hell of a current running through those 6SN7s with those in there.

I would suggest that with the HT you have, a pair of 430R resistors would ease things down a bit and put the op points of the 6SN7s more in line with their current handling capabilities.

Also you might like to consider upping the cathode resistor on the EL34s to around 600R. I found that it sounded a bit hard at that current. 600R eased that down also. Could have been my speakers at the time but our Ant's version you heard at Owston has 600Rs in and sounds better than my original did. I also built one with 600R cathode resistors for another friend. He loves it to bits.

Steve

[Nick]

I was thinking more like 1k in the 6sn7 cathodes, that would put it a 6ma or so, a bit low, but higher current would move a bit close to the 0v line IMHO.

[al newall]

http://www.geocities.com/MyDesign_99/Srpp6sn7eng.html

Thats the 6sn7 version of the schematic i used.

[al newall]

I think i've got a 22K resistor in mine. I'll have to check.

[Nick]

But that SRPP runs at 200v b+, we started with a circuit at 400v. I just think you will be getting near grid current with that low a cathode resistor on a 6sn7.

Sorry to sound a tad huffy, but just cos its on the internet don't mean its right...

[al newall]

[Nick]

[al newall]

None taken, whatsoever.

Just wanted to make it clear that i'm only passing on info and not attempting to design amps for people. God forbid.

If you have any thoughts on optimising this amp in whatever form it ends up, i'll be watching with interest.