OK…so I’m on wi’ summat

[Cressy Snr]

I had the pleasure of listening to Steve's amp through his Mission speakers today. Very nice, extremely easy to listen too. I would call it a win!

Cheers Ray. Glad you liked it. I’m certainly pleased with the amp.

[simon]

It all sounded really nice I must say

[Mike H]

All sounds fab!

[Cressy Snr]

OK,
Quite a while ago I split the power supplies into two channels after the Hammond 10H choke, by using a 2 x Sowter CB25, 20H, 50mA chokes to feed the left and right channels. This meant fitting an extra 30uF motor run cap.

I know I said there would be no more schematics, but it is a bit difficult to explain without a power supply drawing.

Had to replace a dodgy input selector switch, so thought I’d post an interior view. Apart from the selector switch replacement, this is the format the amp was in when Ray and Simon visited.

6V6amp.jpeg (162.7 KiB) Viewed 2153 times

6V6PS.png (75.35 KiB) Viewed 2153 times

[Cressy Snr]

The thing is, this superb RTZ DAC does not put out the usual 2V. Ray mentioned this when he brought the DAC up to my place to dem it.

Now although the DAC sounded absolutely wonderful through the 6V6 SE amp and Mission speakers, the sound did lack a certain dynamism, which all smacks of insufficient gain from the front end 6SN7 when presented with the lower output of the RTZ DAC.

Cutting to the chase, I redesigned the amplifier to take a 6SL7 front end and fed this into a pair of Genalex reissue KT77s. Interestingly, when using the same anode voltage and cathode bias resistor value as the 6V6, the KT77s bias up exactly the same as a 6V6, with identical 12W anode dissipation, but a couple of Watts more power for the same settings. Basically, the KT77 might be better regarded as a souped-up 6V6 rather than as a posh member of the EL34 family.

This all tallies with my own experience, as I've never had a KT77 bias up anything like an EL34 when dropped into an EL34 circuit. What I didn't know until now however, is how similar, nay virtually identical to the characteristics of a 6V6, is the KT77. You learn something new every day.

So, back to the DAC and the increased front end gain, gives me a far better sense of dynamic power, light and shade to the presentation, making me even more pleased with the performance of the RTZ DAC.

[Ray P]

Good result Steve. I know I'm conflating two of your threads here but just to remind that, IIRC, the filter stage of your RTZ has NE5532 op-amps and it would be good to get you a listen to one equipped with the OPAs, which we've all found to be an improvement

viewtopic.php?p=209559#p209559

along with getting you a listen to the filter with the final op-amps by-passed. Once I have the use of a reflow oven again I will be assembling some more RTZ boards and I'm sure we can work something out if it's of interest.

[Cressy Snr]

Hi Ray,
Copied this reply from Tony to Leon from the thread you pointed to.

Just to say that I suspect most people did build with the OPA1678 and the only reason I built with NE5532 at the time was due to supply issues and huge lead times. (which is why Marcel specified them in the drawing/BOM)

I did buy some OPA1678 from Ebay at the time but didn't trust them so when Mouser finally had them in stock I used them for the subsequent builds.

Sorry you had to change them out Leon, I'd completely forgotten. Luckily I found those 4 in another box I'd squirreled away. All gone now though.

Edit: ps Steve's unit has OPA1678 fitted.

I’ll get the magnifying glass out later…ulp!

[Ray P]

Yeah, it'll be good to confirms Steve, my grey matter isn't so effective nowadays.

[Cressy Snr]

Yeah, it'll be good to confirm Steve, my grey matter isn't so effective nowadays.

Now then! After squinting through the top plate mesh, with the use of a strong light and magnifying glass, I can confirm that the filter op-amps are indeed OPA1678s.

[pre65]

my grey matter isn't so effective nowadays.

At least you have some.

[Ray P]

Now then! After squinting through the top plate mesh, with the use of a strong light and magnifying glass, I can confirm that the filter op-amps are indeed OPA1678s.

That's good then. Maybe just try the simplified filter board some time down the line...

[Cressy Snr]

OK, so the KT77s have been taken out and I’ve reverted to the NOS 6V6 in the power stages of the amp. Seduced by the impressiveness of the reproduction, I should have known better. I mean, I’ve been at it long enough to be sceptical about things sounding merely different as opposed to genuinely better.

The 6SL7s do their job of boosting the input gain to help the RTZ DAC, without the addition of KT77s to complicate matters. Yes, they bias the same as 6V6 and give a little more power but finesse goes out the window with them. In comparison to the NOS 6V6 (Sylvania or Mazda) the new production KT77s sound scooped in the mids and boosted top and bottom, which impresses for a few days but ultimately becomes nothing to do with music, merely tiring.

I have Sylvania round plate 6SL7 into Sylvania grey glass 6V6 and it’s all very nice again. Three times less heater power too.

[Dave the bass]

.......Seduced by the impressiveness of the reproduction....

Filth!

I like it

[Cressy Snr]

Inspired by Paul's adventure with the sixty-five-fifty SE amplifier, I've decided that great though the 6V6 amplifier is; it's not really dynamic (powerful) enough for the relatively inefficient 87dB/W Mission 773 speakers. Loads of finesse, detail and sweetness from the amp but a little lacking in punch.

I had solved the gain problem with the RTZ DAC by boosting the gain of the input stage by the use of a 6SL7, so that was half the problem dealt with. Next was the output stage. I was constrained by the pair of Sowter CB25 chokes in the power supply, they are only rated for 50mA; great for 45, EL84 and 6V6, but not a lot of use for a more powerful output valve.

PSUD to the rescue and I came up with a nice, quiet, CLCRCRC power supply using a GZ34 rectifier:

Ampinterior.jpeg (160.45 KiB) Viewed 1048 times

My old grey glass KT66 were pressed into service and we now have a 6W ish amplifier

KT66SE.jpeg (91.99 KiB) Viewed 1048 times

I'm well pleased with the results. The dynamism of the Russian reissue KT77s is there but without the scooped out sound. Instead It's bigger, richer, smoother, but with just as much detail as the KT77. A win I think.

[Cressy Snr]

For the sake of completion:

KT66SE.png (52.17 KiB) Viewed 956 times

KT66SEPSU.png (91.06 KiB) Viewed 956 times