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Old 25th February 2007, 05:36 PM   #1
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Default Eico ST70 Rebuild / Upgrade

Hi Vacuum People,
After enjoying reading this section for a year or two I got the bug to get a Eico St-70 from ebay. It came as a factory assembled unit that has not been modified or updated till today!
It has all GE tubes in it that tested OK.
Sy and Kevin seems to have a respect for the amplifier configuration and my past I always liked the VTL driver section that this kinda models.
Now Hear we go.
I replaced the bias , coupling caps , cathod bypass on power amps front end and checked the PS caps. I listened to it at this point and worked nice , however I dont need a preamp so I seperated pre and amp and found the poweramp has a ton of gain on its own. What is a good way to reduce gain without changing the amps basic sound. Dont want to change the feedback loop if posible that leaves me with a resister ladder at the input. Is there a Other option?
Also as far as 7591 go. What is the best new production tube out there that lets this amp function as designed and if I rewire the sockets do I have options other than the 6l6/5881 . ( Heater current limitations)
With that said I see Sovtek has the 7591XYZ. Would the amp make power spec with this tube?
I am open to any thoughts on upgrading this peice of audio history. Being a old Dynaco fan ( I was from NJ and did a report on Dynaco in Hi School) The Eico sounds much better in stock form than it has a right to.

Thank you for any input and have a great day.

PS You Guys withis all the tube gear How do you stay cool?
Ceiling fan messes up my Image and I dont listen in the nude.





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Old 25th February 2007, 05:49 PM   #2
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"PS You Guys withis all the tube gear How do you stay cool?
Ceiling fan messes up my Image and I dont listen in the nude."

Somehow that's both funny and disturbing.

However, I have an Eico ST-40 that I re-tubed with all JJ tubes. Bpught it off ebay and ran it up gradually with a variac. It also sounds darn good.

So the same question applies to me as well. I ordered a Ming-Da for regular use, and plan to restore the Eico. I figure this is as good a place as any to chime in since they use the same outputs.

Marc
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Old 25th February 2007, 07:10 PM   #3
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The 7591XYZ was a stopgap measure. Forget about it.

JJ and EH offer current production true 7591s. JJ has been having problems in Octal based tubes, of late. So, EH is the primary recommendation. However, the EH tube comes in a "bottle" that is larger than NOS and doesn't always fit. "Look before you leap."

Back in the days of yore, Fisher, Scott, etc. used grid leak resistors that exceeded the documented max. value. With tubes of that time, they got away with the "chicanery". Current production 7591s meet the spec's, but are not forgiving of overvalue grid leak resistors. So, minor changes to vintage units are in order.

7591 Data Sheet
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Old 25th February 2007, 11:44 PM   #4
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Eli,

I am rather new to this, so please bear with me....

If I understand you correctly, the ST-40 is self biased and I should actually measure the current and change resistors as needed.
Yes?

It is playing fine now, but I had planned to rebuild it anyway. I did measure caps and resistors before I flipped the switch, and surprisingly everything seems to be within tolerances.

Marc
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Old 26th February 2007, 01:24 AM   #5
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When I brought my St 70 up on the variac my bias was OK then in about a 1/2 hour it started to rise to .8 of a volt across the 10 ohm resisters. It was time to go to work.
For you with the cathode bias you need simple Ohms law. v/r=i.
Yes you are correct to change the resister to a higher value if you need less current. As you do this you will decrease max. power.
As Sy has pointed out before its worth your while to change the output stage to fixed bias. In the st40 you will need to make you own negative supply but you will gain in output power, and damping factor.

Have fun.
Fred
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Old 26th February 2007, 01:29 AM   #6
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Marc, If you dont already have this link its worth reading...
http://home.netcarrier.com/~rstevens/st-40.html
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Old 26th February 2007, 02:49 AM   #7
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I have indeed read that link, thanks. The loudness switch mod is on my list, although I may end up bypassing most of this anyway. In fact, someone has already bypassed the balance control and re-wired the speaker terminals to be more logical.

Marc
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