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ST-70 / 7591 / AA-100 Clone Finished !

PostPosted: Fri Dec 02, 2005 11:07 pm
by TomMcNally
My Mouser parts arrived today, and the ST-70 board went together in about 45 minutes. The amp is working and sounding great. One strange thing I'm trying to figure out is the bias. I put 10 ohm resistors from the cathodes of the 7591's to ground, but I only get 1/10 the voltage I should at the 4 test points. 50 mv instead of 500 mv - verified on 2 meters. The balance pots on the driver board work as expected, and the takeoff points on the board read -35 volts. The 7591's are running very cool to the touch.

What am I missing here ?

By the way, I went with a 40 mfd 500v off the 5AR4, then a Hammond 156R choke, 100 mfd 450v (Transformers connected here) then a 1.5 K resistor and 40 mfd (Screens here), 5.6K resistor, 40 mfd (driver 350 v), 22k resistor and 40 mfd to the driver 300 v... No hum at all, with no filament balancing resistors. The B+ is pretty high - 500 volts either side of the choke. Nothing running hot or exploding yet, but I need to do something about it. I tried one of those big Chatham 5R4WGY's which are supposed to have like a 60 volt drop, but I still got 500 volts.

Sorry for the long winded post, I'm just at a loss here - the amp sounds good and is nice and loud. I guess the next step is to take some measurements. Yes, they are 10 ohm resistors, not 1 ohm ! haha

Thanks for all of the help along the way.

New JJ 7591's NOS Raytheon 12AU7's NOS RCA 12AX7 Used Heathkit/Mullard GZ-34/5AR4 Allman Brothers - Eat at Peach CD

... tom

Re: ST-70 / 7591 / AA-100 Clone Finished !

PostPosted: Sat Dec 03, 2005 5:02 am
by Shannon Parks
TomMcNally wrote:The B+ is pretty high - 500 volts either side of the choke.

Hi Tom,

So whats the voltage drop across the choke? If there really isn't a voltage drop, you still need to lower adjust your bias to get more draw (maybe -32V to -30V?). 'Course with 7591's I'm not sure what a good target voltage will be (eg whatever is 75% of their max dissipation). Maybe shoot for 40mA or .4V at the bias points then see where the HT is at. Then with the increasing loading of the power tranny the HT will drop.

TomMcNally wrote:I guess the next step is to take some measurements. Yes, they are 10 ohm resistors, not 1 ohm ! haha

That was my first thought! Yellow_Light_Colorz_PDT_05

Hey if those 7591's sound good now (surprised it works!), boy howdy, they'll sound great pretty soon!


PostPosted: Sat Dec 03, 2005 6:28 am
by TomMcNally
Hi Shannon !

With the bias pots fully cranked, I can get .185 at the test jacks. I just adjusted for .150 and I can hear it getting louder.

There was basically no drop across the choke last night, and it was stone cold also. I'll measure it again when we get this figured out.

Since I'm out of range on the 10k pots, what would you suggest I change to get more range ? I'm using the values shown on your schematic. The Heathkit schematic shows a raw -57 volts, brought down to -16 with precision resistors (it wasn't adjustable) and then two pairs of 470k where the your ST-70 board uses 150K.

Their target value seems to be -16 volts at the tube. googled sources seem to be looking for 70 ma cathode current per channel, one guy likes to run his lightly at 22 ma per tube.

One other question - when I get the bias right, how close should I tweak the dropping resistors for the 350 v and 300 v for the driver board ?

It's been playing for about 8 hours and sounding sweet !

Thanks !

10k to 4.7k

PostPosted: Sat Dec 03, 2005 7:18 am
by TomMcNally
I changed the 10K resistor from the bias pots to ground to 4.7K ... now I can get a good range. At .4 volts at the test jacks I could smell the tubes ! I backed it down to .3 and will listen awhile, and later on balance everything out and recheck the high voltages. There is definitely more bass now, still sounds great.

PostPosted: Sat Dec 03, 2005 9:50 am
My goodness!

Isn't this stuff fun!?!?!?!?

Congrats on getting it running. I wish I had more time (and space) to build about 15 more different amps! I'd try some 7591's too.

I do have an Eico ST-70 that I have not cranked up yet. After I get some more work stuff done maybe I'll fire that one up.

Enjoy the new amp!!!


PostPosted: Sat Dec 03, 2005 11:17 am
by erichayes
Hi All,

Back in the bad old days when there were only NOS 7591s and 7868s, we'd rebias the Fisher, Scott, Sherwood etc. receivers and amps that used them so they had -17~ -19VDC on the control grids (the manufacturers usually ran them at -15VDC). That cooled them down enough to let them last more than a year but still had them running comfortably in class AB land. Anything over around -21 volts is excessive and will cause crossover distortion if the amp is pushed at all.

PostPosted: Sat Dec 03, 2005 11:21 am
by TomMcNally
Thanks Ed ... yeah I have 2 or 3 amps for every room in the house now !

There are some pictures on my web page of the mostly finished amp.


PostPosted: Sat Dec 03, 2005 4:51 pm

Even MORE awesome! Nice Web site and great looking amps. I like the 95 watt amp you've got. I've got to try some higher powered stuff. I'm waiting with baited breath for Shannon to release details on the ikeZilla!

Have a great weekend all.


PostPosted: Sat Dec 03, 2005 8:25 pm
by TomMcNally
Eric - OK ! Now I know what crossover distortion sounds like. I backed down from 30 to 25 ma and the sound cleaned up. Shannon - The power supply is now loaded better, the choke is working and the voltages are more in line. I'm not expecting any exploding caps. The music sounds great - that's what it's all about. Ed - That Velleman K-4000 KICKS, my daughter has shaken the house with it many times. It's in the living room on a pair of old Cerwin Vega! speakers.