Rocky - The Quatroquel

[Dave the bass]

Blimey! Thats low!

Do you listen to a lot of half speed dub through 48" Drivers in TL speakers per chance? If you don't I reckon you'd not hear any difference.

Just IMHO n all that.

DTB

[Mike H]

I was gonna say how low a freq. do you want to hear without apparently going off? Being as if the room can't accommodate the wavelength, 'sall a bit academic

Yes have seen 680k used for EL34. Tho most types' recommendations advise not larger than 470k.

Usually 220nF into 470k is quite enough.

Or can be 470nF into 220k.

[little eddy]

Yes I didn't suggest that the filter F-3dB frequency is audible in itself, but I was more interested whether:

1. maximising the grid resistor in my case to 470k from the current 220k will give an audible difference and/or lead to a useful increase in gain (I could use a little bit more gain when not using a pre-amp). It sounds as if this would not be detrimental so from a technical standpoint would be recommended.

2. Do smaller capacitors have a lower impact on the sound than larger ones as well as being less expensive? For some reason my gut feel is 'the smaller the ac-coupling cap the better' so when I'm looking to try alternatives to my current 0.68uF orange drops, I could go as low as 0.22uF or perhaps even 0.15uF, (F-3dB of 1.5 and 2.3 Hz respectively).

This logic also seems to have been followed on some of the clasic PP amps such as the Quad II @ 680k/0.1uF (although I would elect to use 470k due to wanting to try a number of valve types)

[Mike H]

Interestin' points all, yes a smaller cap value implies presumably smaller dielectric and foil areas. Does that matter? Only one way to find out

Also would agree about the 470k grid leak then

[little eddy]

On the schematic that I posted on the 18th incorporating MJ's Beta-follower, might it be worthwhile looking at 'boutique' alternatives to the 4.7uF cap or is this unlikely to yied any audible differences (I initially used bog standard Rapid electrolytics)?

[Mike H]

Well the way I understand it, read somewhere the larger the Voltage rating ergo the lower the ESR (equivalent series resistance). On the basis the ripple current is likely to be correspondingly larger. It said.

So could be an ornary one but say 100V.

Or could be Rifa PEG124 type, electrolyte in paper (RS)

Or you could be really fussy and maybe use audio polyproylene?

[little eddy]

The current 4.7uF Rapid electrolytic is only 10V rated as per the MJ text. Total grid bias across the MJ circuit measures 4V.

It just seems that functionally the cap might be doing something similar to a cathode resistor bypass cap, regarded by many as influencing amp sonics.

I was thinking more like Elna electrolytics from say Hi-Fi Collective, 25 or 35V depending upon the Cerafine or Simlic types.

[Mike H]

Okay then!

[little eddy]

Well at Owston I definitely got the message that there was some 'compression' attributable to my Rocky amp when you tried it on Will's OB on Sunday.

The only changes since its last outing were the use of LED bias, Morgan Jone's B-follower, and Russian green PIO dc de-coupling caps recommended by Nick due to some sibilance.

Nick kindly gave some advice so out have come the above three. The first two are replaced with the original SRPP running at around 7mA, and some new Russian 0.1uF K72P-6 Teflon caps. The first part was easy but the Russian Teflon caps were much bigger than I thought. I found room by standing them vertically with some cap clamps and quite pleased with the results from a physical perspective.

I know the Russian caps are an unknown quantity to me and guess I will need to give them quite some time to bed in.

[Mike H]

OK so it's back to more like cct's shown on pages 1 ,3 - 4 of this thread? I'm using 6SL7 SRPP, works OK AFAIK.

And what's the output valve now?

And not to alarm anybody but I read somewhere Russian Teflons may need "running in"

 

[ed]

apologies for the OT but I'll be quick....just as a matter of interest Mike, what is the load on your 6sl7 srpp???????

[little eddy]

And what's the output valve now?

The output valves are Osram KT66 running at about 60mA and a bias of 34V.

And not to alarm anybody but I read somewhere Russian Teflons may need "running in"

I've just read on the Bottlehead forum one guy saying they need 100 hours and suggested burning them in from the speaker output of an amp with dummy load (which I could have tried if I'd read his comments first). I'm not taking them out now. Will just have to listen to them for a few weeks.

[Mike H]

IIRC what I read was 100 hours (or something) of white noise

apologies for the OT but I'll be quick....just as a matter of interest Mike, what is the load on your 6sl7 srpp???????

470k plus KT66 grid. Not using cathode connection of upper triode but anode of lower.

The output valves are Osram KT66 running at about 60mA and a bias of 34V.

Awlrighty then

[pre65]

Looking at what's in the amp "graveyard" seems I have enough for a "Rocky" amp with EL34. Any new projects must use a 5K OPT so this would be ideal.

Mike H posted a drawing some while ago and I wondered if Mike had ever turned this into a Spice simulation ?

http://www.audio-talk.co.uk/phpBB2/file ... 1b_201.gif

The PSU is 350V frame transformer - hybrid bridge - 2uf - 27H - 235uf -HT

Should give me a HT of around 375V.Front end is 6SN7 (Russian).

This is the amp chassis (monoblocks) that were 829b PSE and more recently 826.

Here is a better copy of the drawn circuit.

[Mike H]

Well I did for mine anyway!

Anyhoo we're going for 6DN7 CF now? Cos can't do SRPP with just one 6DN7 as far as I know