Super Rocky

[ed]

This may be a bit presumptious but...

If you really want to compare and you're interested in this version of rocky then maybe Steve Shiels will accommodate a comparison between your Rocky and his mofo. I can bring an F2 or my new planned mofo with jfet front end. That would be a more telling and even an interesting comparison.

This week I've swapped out my srpp 2A3 for the F2 which is precisely what we're talking about. I did it because I'm becoming neurotic about valve wear and I don't know if I can detect it or not. Steve last tested the valves on the 2A3 before Christmas, as I was selling it. There is a mismatch between the two 6sl7s and also the 2A3s...so I put them in the channels that gave the best balance...I now only have 1 spare set of 2A3s and no 6sl7s...so it kind of highlights the maintenance aspect.....

when playing through my alpair 10Ps I can't tell the difference between the 2 amps...well I can, but one day gives a different result to another, so it's psycho babble.

personally I think I favour the mosfet, but source follower. I'm pretty sure I'm conscious of the sound of the output impedance.

[Cressy Snr]

This may be a bit presumptious but...

If you really want to compare and you're interested in this version of rocky then maybe Steve Shiels will accommodate a comparison between your Rocky and his mofo. I can bring an F2 or my new planned mofo with jfet front end. That would be a more telling and even an interesting comparison.

This week I've swapped out my srpp 2A3 for the F2 which is precisely what we're talking about. I did it because I'm becoming neurotic about valve wear and I don't know if I can detect it or not. Steve last tested the valves on the 2A3 before Christmas, as I was selling it. There is a mismatch between the two 6sl7s and also the 2A3s...so I put them in the channels that gave the best balance...I now only have 1 spare set of 2A3s and no 6sl7s...so it kind of highlights the maintenance aspect.....

when playing through my alpair 10Ps I can't tell the difference between the 2 amps...well I can, but one day gives a different result to another, so it's psycho babble.

personally I think I favour the mosfet, but source follower. I'm pretty sure I'm conscious of the sound of the output impedance.

Yep, the comparison will be educational. I look fwd to getting together when this lot is over.

[Ant]

It's interesting to me because I've heard similar, but the other way around. When I got the f5 I tried everything to trip it up, comparing with the el34se, and with a Samson servo 260.
The el34 was maybe 4w with a following wind, choke input psu nothing special at all.
The Samson I think is class a/b, about 100w. The f5, 25w with sod all gain compared to the other two.
With these big fanes, I tried all 3, the el34 got dismantled, the Samson is only still here as a spare.
The 34 was........ Just crap compared with the f5, the Samson wasn't bad as a stand in if the f5 blew up.
I still haven't got bored with the f5 or wished it did whatever better. It seems to match with the fanes very well. The Samson seems far too powerful, or has too much gain because its impossible to get it off the stops on the volume control without it deafening me, whereas the f5 seems to grab the fanes by the scruff and work.
I read that Nelson pass designed it with his high sensitivity speakers in mind so maybe that's why as the fanes are about 100db efficient, it doesn't need any more gain with these.
I haven't a clue as to what the topology of the f5 or the Samson are, but my el34 just sounded crude and coloured (in a bad way) and mushy.
I don't think the f5 is very comparable to the Samson or the NVA clone, but I think the Samson is fairly comparable with the NVA clone. It was grey and uninvolving with the fanes, but not with the 90db metronomes. It was fine with them and the little 6" drivers, as if it was designed to be used with smaller driver monitor speakers or something rather than high sensitivity drivers.
Which is why I gave up on amps. I can't get f5 style performance out of valves, or anything remotely like it.
That amp just does something I like with these speakers

[Cressy Snr]

....but my el34 just sounded crude and coloured (in a bad way) and mushy....

Half a sentence and that’s me rumbled.

[pre65]

....but my el34 just sounded crude and coloured (in a bad way) and mushy....

Half a sentence and that’s me rumbled.

He needs a masterclass in EL34 amp building from Dad.

[Cressy Snr]

....but my el34 just sounded crude and coloured (in a bad way) and mushy....

Half a sentence and that’s me rumbled.

He needs a masterclass in EL34 amp building from Dad.

Masterclass? You jest!

[Ant]

I knew sod all then and even less now. The el34 I built was just rubbish. No other ones I've heard are rubbish

[pre65]

I knew sod all then and even less now. The el34 I built was just rubbish. No other ones I've heard are rubbish

Don't worry Ant, you have skills galore in the turntable department, something to be proud of.

[Mike H]

....but my el34 just sounded crude and coloured (in a bad way) and mushy....

Half a sentence and that’s me rumbled.

He needs a masterclass in EL34 amp building from Dad.

Could be just naff valves. Never much liked the EL34's I got, probably Chelmer Valve 'Premium'. My KT66's kicked the sh!t out of them in terms of nice sounding.

[steve s]

This may be a bit presumptious but...

If you really want to compare and you're interested in this version of rocky then maybe Steve Shiels will accommodate a comparison between your Rocky and his mofo. I can bring an F2 or my new planned mofo with jfet front end. That would be a more telling and even an interesting comparison.

This week I've swapped out my srpp 2A3 for the F2 which is precisely what we're talking about. I did it because I'm becoming neurotic about valve wear and I don't know if I can detect it or not. Steve last tested the valves on the 2A3 before Christmas, as I was selling it. There is a mismatch between the two 6sl7s and also the 2A3s...so I put them in the channels that gave the best balance...I now only have 1 spare set of 2A3s and no 6sl7s...so it kind of highlights the maintenance aspect.....

when playing through my alpair 10Ps I can't tell the difference between the 2 amps...well I can, but one day gives a different result to another, so it's psycho babble.

personally I think I favour the mosfet, but source follower. I'm pretty sure I'm conscious of the sound of the output impedance.

Not presumptuous at all ed, it would be great to have a day comparing all our handiwork..
Give it a month or so ? and I'm sure the rules will be more relaxed. We can then see how we feel about it...

I've been quiet these last few days..
Been doing a bit of work on the new mofo, but it's been slow,
I've converted a couple of transformers to give me 20 volts on a couple of dc power supply boards I've built. built a couple of mini mofo boards and mounted them on the cpu coolers. Got most things to finish it off but there's quite a bit to do still.
I've just ordered a couple of dc to dc converters I'll give those a try too to see how / if they effect the sound.

The breadboard mofo is still going strong. I've had no more issues with the biasing. Sounds good.

[Cressy Snr]

I’ve had a very interesting day with a few resistors and some clip leads.
With the Hammond 1627SE output transformers, it has been an interesting juggling act to find the best compromise between output power and distortion.

I’ve ended up at a nice, clean, extended sounding 3W, running 69mA at 300V. At higher currents there was a treble roll off and blurred bass, whilst lower currents gave the characteristic, un-dynamic, weak sound, exhibited by many of my previous builds. Interestingly, the 69mA standing current I arrived at, with the parallel 223R combination of 270R with 1K5 in the cathodes, is exactly half the maximum 138mA standing DC spec of the Hammond output transformers.

Wonder if gapped, single ended output transformers have a sweet spot. I don’t know enough about them, but it seems the music snaps into focus at both ends of the spectrum, when these transformers are biased half way along their operating curve or am I talking crap and it’s just coincidence and it’s the power valves that have the sweet spot. Maybe it’s a bit of both.

I’m rambling again.

[Cressy Snr]

More rambling:
Power supply setup is how I want it now. I wasn’t happy with the 4V rectifier setup on the input stage, even though the Mazda UU5 looks beautiful. The bodging with dropping resistors bothered me and the previous business of having the EZ81 on the same winding as the input stage heaters had bothered me even more - but I had no spare octal sockets. I phoned our Ant to see if he had any the other day, but no luck there. He’d thrown a load of valve sockets away, years ago. So, Yorkshireman that I am - I came up with a satisfactory solution: one that didn’t involve spending any money.

I dug out a pair of 6CL3 TV damper diodes and a pair of novar sockets, punched another hole in the top plate and wired them into a hybrid bridge, to feed the power stage. The octal socket recovered from that position, then went into the hole where the British B5 socket for the front end power supply had been.

The heater to cathode voltage limit of these 6CL3 is only 300V positive average, so the heaters were tied to the cathodes, to avoid any potential problems. This meant that I could bring the power stage HT out from the centre tap of the spare 6.3V heater winding I had on the big transformer. This winding had previously fed an indicator light, and of course I couldn’t have 350V worth of rectified current sitting on the tags of the indicator bulb fitting so the bulb wiring was removed and discarded. I still have one red light on the front.

It all works beautifully together sound-wise, and all the valves have a consistent look, which finally brings the whole design together visually.

I had a casual ebay search for 6CL3 damper diodes and they have gone up in price considerably since I bought mine 10 years ago - up from a fiver each, to £40. 6D22S and EY500A are considerably cheaper; probably because they are top cap valves. I have a few of those anyway.

Why would anyone want to lavish so many resources, so much effort and so much iron on a modern reproduction of a rarely used beam tetrode from the 1950s. Damned if I know, but somebody has to be daft enough.

[izzy wizzy]

Was trying to follow what you've done with the 6CL3. It sounds as though you powered them from a 6.3V winding, tied that winding to the cathodes at the centre tap of the 6.3 winding and took the HT off there. Is that right? If so, all good. Somehow I imagined you'd tied the 6.3V winding at one end and then took the HT off the centre tap which seemed odd.

Anyhoo, I just went through a long process of trying to get dampers like the 6CL3 at sensible prices to future proof the GM70 amp in the absence of MVs. Only silly money off ebay as you've found but I went and got 20 from the USA at $5 ea. Sure, $40 postage and £25 duty but I thought that not bad for less than £7 ea landed.

[Cressy Snr]

Was trying to follow what you've done with the 6CL3. It sounds as though you powered them from a 6.3V winding, tied that winding to the cathodes at the centre tap of the 6.3 winding and took the HT off there. Is that right? If so, all good....

Yep..that’s right.

...Anyhoo, I just went through a long process of trying to get dampers like the 6CL3 at sensible prices to future proof the GM70 amp in the absence of MVs. Only silly money off ebay as you've found but I went and got 20 from the USA at $5 ea. Sure, $40 postage and £25 duty but I thought that not bad for less than £7 ea landed.

Sounds like a good deal. American prices of these things look still to be reasonable. However the prices of the no-top-cap dampers over here, are ludicrous these days.

[Paul Barker]

Wonder if gapped, single ended output transformers have a sweet spot. I don’t know enough about them, but it seems the music snaps into focus at both ends of the spectrum, when these transformers are biased half way along their operating curve or am I talking crap and it’s just coincidence and it’s the power valves that have the sweet spot. Maybe it’s a bit of both.

I’m rambling again.

Don’t think so. Most noticeable change in transfomer performance comes when its saturated, that distortion you won’t miss. Transformers go from sweet to ... unimaginable high distortion.