Was: The Oscillator. Now -the Green Machine Lives

[SteveH]

Gents,
So after much discussion and help from you all; my amp is now fully functional Yellow_Light_Colorz_PDT_01 Yellow_Light_Colorz_PDT_02 Yellow_Light_Colorz_PDT_03
I just got done playing some Glenn Miller through my Fostex with the amp in Triode mode....Simply fantastic !
The amp does have some hum though; and right now the troubling thing for the Power Transformer is that the lowest I can get my bias on the KT88's is about 85mA. I'd be happier if it were down around 70mA - So I may have to swap that resistor on the SDS board.
Regardless, minus those issues; it really does fill the room with sound through my Choruses. Once I fix my growing pains I'll report back more.

Pics of the guts can be found back on this thread:
http://www.diytube.com/phpBB2/viewtopic.php?t=2344





Thanks again to everyone for their relentless help !
-Steve

[mesherm]

I think that if you repace R3 on the SDS board with a 4.7k or 5k resistor you should be able to get the KT88s down to a lower idle current. 85ma each is pushing the power tranny. Another mod you might try is to solder a UF4007 diode in series with each 100 ohm "triode mode" resistor. It can go before or after the resistor but should be orientated so that the banded end lead goes toward the tube. This should give you a bit more power in triode mode as it allows the DC to the screen grids but blocks any AC.
Nice build by the way. I have an amp very similar except it uses Hammond 1650G (no screen taps). It has the upgrade PT also and with KT88s I was able to run as high as 75 each without the PT overheating although the weak link was the single C-354. I used the CopperCap that simulates double 5AR4s though to handle the extra current.

[SteveH]

Thanks again. I will really have to get this bias down; the power transformer gets seriously hot after 20mins or so. We'll see what I can do tomorrow.

Edit - Well this has been bugging me; so I haven't gone to sleep yet Yellow_Light_Colorz_PDT_09

I had a 4.7K resistor lying around, so I subbed it into the SDS board. Now my bias is down to 61.5mA per tube all the way around...The PA060 thanks me !
However, my HUM is still as it was before. Just to verify; but I do need to run a ground from the - binding post to the star point - correct ?
What is the best method for trying to track this down ? I have a scope and function generator- just not sure how to use it in this application.

One thing to note - When I use a screw driver to adjust the bias pots, I get inductance into it from the power tranny....creepy kinda....So I took some measurements, and think that this may be the cause of some of my hum issues:
Im getting like 481v B+ right out of the rectifier (with a single rectifier in place). I forget what my VAC reading was - but it surely was over 400v - no where near the 366 that it is supposed to be. Any idea why this is ?? My line voltage is about 124vac - so that's making my heater voltages elevated as well.

[Ty_Bower]

Very nice looking amp... but where is the "Green" part?

[TerrySmith]

481v B+ is perfect, the Triode tranny you have is rated @ 300ma, it ain't gonna break a sweat. I noticed the choke frames are not grounded, ground them.

That wooden base gave me an idea, with a taller base, you can use longer caps on the SDS cap board!

What is the part # of those Edcors?

[SteveH]

Very nice looking amp... but where is the "Green" part?

Ty,
If ya look at the underside of the chassis - you can see that at first the wood dye came out eagles green.....It is a powdered 'mix to your liking' ratio dye; and well...I goofed ! Thankfully the button shellac fixed it....But my girlfriend still makes fun of me about it Yellow_Light_Colorz_PDT_09

[SteveH]

481v B+ is perfect, the Triode tranny you have is rated @ 300ma, it ain't gonna break a sweat. I noticed the choke frames are not grounded, ground them.

That wooden base gave me an idea, with a taller base, you can use longer caps on the SDS cap board!

What is the part # of those Edcors?

Terry,
Thanks I'll try it ! The only other voltage question I have is on the board where J20 should have 350v on it; it has like 375v. J19 is much closer to what it should be at (specs say 300 - I measure 303).
The Edcors are CXPP60-MS-4.2K
http://www.edcorusa.com/products/transformers/cxpp/cxpp60-ms-4_2k.html

The whole reason I went with the wood base was due to me wanting more current than the C354 would allow...So the Hammond 159v came to mind, as it is 1.5H; but only 27 ohm. Deciding to put two in series so that I'd have double the inductance of the C354 at the same resistance; I needed a way to tuck 'em.

[SteveH]

I noticed the choke frames are not grounded, ground them.

Unfortunately that didn't cure my hum.....

[TomMcNally]

Hi Steve -

That's a monster amp - nice job.

Track the hum down systematically ...

See if it's there with the inputs shorted at the RCA jacks.

The hum could be radiating from the chokes to the
inputs ... try different lead dress, and maybe different
angle mounting of the chokes.

You'll find it.

... tom

[SteveH]

Tom,
Are you on the air today ? I have the FM3 all warmed up :D
My biggest concern is that the hum is coming from the chokes interacting. Im going to play around and see what I find...

Thanks again,
Steve

[TomMcNally]

Hi Steve ...

Yep - 100,000 watts of ERP !

I wonder how many people are listening on Dynaco
equipment today .... hmmm

I guess it would be easy enough to shield the chokes
with steel if you found that they were radiating.
Hum can be hard to track sometime, but you'll get it.

... tom

Proud to be New Jersey ... New Jersey 101.5

[TerrySmith]

Hey Tom, the internet feed sounds pretty good, I'm listening now on a Diy-35! Yellow_Light_Colorz_PDT_06

[kheper]

Correct-a-mundo! That live feed is superior to
most of crapo stations I try to listen to.

I did not know Tom was a D.J.

Tom --> USB sound device --> DAC --> equalizer --> ST70 --> Klipsch Hersey

[TomMcNally]

We feed the streaming company with higher bandwidth than
most stations, with a well processed signal. It does sound
better than most.

Yeah, I've been doing this since I was a kid ... hard to get
it out of my blood, so I'm a Radio Engineer during the week,
and play DJ on Sundays. I also sneak in $ome voiceover$
into the mix "during lunch" for some of the Atlantic City
casinos.

[SteveH]

Tom,
Thanks again for the help. The best I was able to do mount-wise with the chokes was to put them 180' out of phase from each other, and to mount then at a -45' angle, so at least they will not be on the same exact plane as the power tranny....We'll see how that works out tonight.
I have to admit that the feed out of my squeezebox during your show was better than what came out of the FM3 Yellow_Light_Colorz_PDT_09