Look what Santa brought!

[korneluk]

Originally posted by Rob11966
I have also added ... digital bias meters for both channels...

Excellent work! Where did you buy the digital meters?

-- josé k.

[Rob11966]

Hi Jose,

Bought them off ebay - coldfusionx is the seller. Bought a back up yesterday, so they are still for sale. Search for 20v panel meter, if you have problems, let me know and I will send a link. they come in blue, red and green and sell for about USD 25. Coldfusionx is a great seller BTW, very quick and efficient.

Cheers,

Rob

[jackinnj]

Originally posted by Rob11966
Hi all,

I have just completed my second amp after about 200 hrs of work!

Now you can settle in "for a long winter's nap" as the fable goes.

Looks like one of your offspring (backup units) is pleased with your work as well.

Jack

[zigzagflux]

Beautiful, just beautiful.

What was your process for gold plating the aluminum? Copper first?

[Salas]

Originally posted by Rob11966
Hey thanks Victor,

Swapped the 40 year old motorola tubes for a couple of RCA tubes, hey presto, crackle gone! Bad tube I guess. The sound is better too. Thank you RCA.

Rob

You had grid leak current in one of those. That is why I was talking about a blocking cap. Having healthy RCAs is the best solution obviously. Nice.
You said something about less bass than with the EL34 amp. KT88 is normally superb in that. What is the configuration of your PSU? Same capacitors as in the schematic? I see many elcos in your pics though.

[Brion55]

A work of art! Very nice!

[Wavebourn]

Rob;

I put the answer to the wrong thread by mistake. Here it is:

Originally posted by Wavebourn
[B]Try 33 pF from 1'st grid to ground where cathode resistor is grounded. Try to increase a feedback cap that goes from anode of an output tube to cathode of the 1'st one twice, if the phenomenon stops decrease it gradually. And check if you've soldered it to the proper tube's anode.

[Rob11966]

Originally posted by zigzagflux
Beautiful, just beautiful.

Thanks Zigzagflux, I am very pleased with the overall result - if only my wife could see the beauty.....

What was your process for gold plating the aluminum? Copper first?

To be honest, I am not sure. I had the plating done by a plating company who do fantastic job - also surprisingly cheap for the amount of work involved. I had to drive half way across Sydney to get to them however!

Originally posted by salas



You said something about less bass than with the EL34 amp. KT88 is normally superb in that. What is the configuration of your PSU? Same capacitors as in the schematic? I see many elcos in your pics though.

I might have been a little premature with that critique. Having listened to it last night, the bass is fine. Also, my initial opinion was based on the sound with the faulty motorola tube.

The configuration is very similar to the schematic but I have used the SDS caps board instead of the original electrolytics. Each board uses 8 x 100uf 450v caps, with the caps joined in pairs (with a resistor network). This gives you essentially 4 x 50uf with an appropriately high voltage rating. I was planning to use individual sprague electolytics but had trouble sourcing 30uf caps with the appropriate voltage rating.

Otherwise the circuit is very similar except that -

1. I added a pot on the input side for volume control

2. I added a small transformer (which powers up when the amp is switched on through a mains isolator switch at the back) with solid state rectification to drive the LED panel meter, the LED in the switch and also to supply power for a couple of solid state relays (so I could use 12v through the on/off switch at the front and not 250v)

[Salas]

Originally posted by Rob11966


The configuration is very similar to the schematic but I have used the SDS caps board instead of the original electrolytics. Each board uses 8 x 100uf 450v caps, with the caps joined in pairs (with a resistor network). This gives you essentially 4 x 50uf with an appropriately high voltage rating. I was planning to use individual sprague electolytics but had trouble sourcing 30uf caps with the appropriate voltage rating.

200uF per channel is not poor. I believe you use no more than 50uF after each GZ34, then a choke, and then the 200uf combo, right? Because that tube must not be taxed with more than 50uF cap input filter. If like that, plus the bass sounds adequate by now, you are done. I must comment on the aesthetic and hardwiring quality too... Very nice indeed.

[Francois G]

Originally posted by Rob11966
... to be totally honest maybe not quite as good as my first project (with EL34 output tubes), seems t be a bit less punchy with the bass ..

Hi Rob,

With a skin so beautiful and workmanship so grand your Dyna clone deserves something better than the original penny-thrift 6AN8 design! I'm not sure what transformers you used, but the originals are capable of excellent performance. With you beefed up power supply you should get excellent bass with better VA/driver circuit.

I have both Dyna ST70 and Mk III's and replaced both front-end circuits with much overall improvement and no negatives that I could detect or measure. A lot had been written about the compromises made in the original designs and improvements suggested - there are several very good sounding PC boards/kits for sale. I will be happy to provide leads if you find yourself wondering about those.

Anyway, wonderful work! Thanks also for the tips on your panel meter source. I must have some!