Hello,
I plan on using the iron from this amp to build a similar amp (less preamp and eq circuits) on a new chassis. Anyway, I've got the original amp running on the bench (after replacing some power supply caps) so I can take measurements, study the design and take a few notes prior to disassembly...
Looking at the schematics I found on the web, I notice the 4ohm opt connection to chassis:
Hoping someone can explain this to me (novice), I would have expected the common lead to connect to chassis. AND to make things worse, the actual amp has the BLACK lead as common and the YELLOW lead as 4ohm and the YELLOW lead is wired to chassis.
Lastly, the layout of the terminal strip on the back of the amp is:
16 8 <blank> 4
So if on the schematic you just switch the location of the word COMMON with 4ohm, that would make sense to me but then my amp is wired wrong with the 4ohm lead attached to ground?
There IS music coming from the speakers.
I hope this doesn't win me stupid post of the year...
Ahhhh... I've probably already been nominated anyway.
Cheers, Ed
I plan on using the iron from this amp to build a similar amp (less preamp and eq circuits) on a new chassis. Anyway, I've got the original amp running on the bench (after replacing some power supply caps) so I can take measurements, study the design and take a few notes prior to disassembly...
Looking at the schematics I found on the web, I notice the 4ohm opt connection to chassis:
An externally hosted image should be here but it was not working when we last tested it.
Hoping someone can explain this to me (novice), I would have expected the common lead to connect to chassis. AND to make things worse, the actual amp has the BLACK lead as common and the YELLOW lead as 4ohm and the YELLOW lead is wired to chassis.
Lastly, the layout of the terminal strip on the back of the amp is:
16 8 <blank> 4
So if on the schematic you just switch the location of the word COMMON with 4ohm, that would make sense to me but then my amp is wired wrong with the 4ohm lead attached to ground?
There IS music coming from the speakers.
I hope this doesn't win me stupid post of the year...
Ahhhh... I've probably already been nominated anyway.
Cheers, Ed
Yup. I have 3 of these and enjoy them. When you build the new amp, wire the outputs normally. That funky wiring is there for the strange phase adjustment switch and the "center" channel (L-R) Outputs.
Yup?
Which Yup?
The schematic is right, my amp is wrong; the schematic is wrong, my amp is right; both wrong; both right - or just forget it and use the black lead as common and attach it to the chassis for the new build?
Say, have you done any mods to the original tube circuits?
If so, Please share!
Thanks, Ed
Which Yup?
The schematic is right, my amp is wrong; the schematic is wrong, my amp is right; both wrong; both right - or just forget it and use the black lead as common and attach it to the chassis for the new build?
Say, have you done any mods to the original tube circuits?
If so, Please share!
Thanks, Ed
Hi Ed,
Every Eico I've restored sounds great. I went back to factory original in every case.
4 ohm to ground is normal. Beware that some of these were kits and not factory wired. My current fav is an HF-87 and I have another ST-70 waiting for me to get into.
-Chris
Every Eico I've restored sounds great. I went back to factory original in every case.
4 ohm to ground is normal. Beware that some of these were kits and not factory wired. My current fav is an HF-87 and I have another ST-70 waiting for me to get into.
-Chris
ahhh, yeah, funny thing is with this one, just replacing one of the 4 way multisection caps and a 30/400, a pot cleaning, tubes and some adjustement and I'm rather surprised how good it sounds with all the original pyramids!
If it wasn't for the rust spots on the top chassis surface, I might have gone with a restoration - front isn't bad, she's got all her knobs and the cage is pretty good...
The iron is nice 'eh?
If it wasn't for the rust spots on the top chassis surface, I might have gone with a restoration - front isn't bad, she's got all her knobs and the cage is pretty good...
The iron is nice 'eh?
I have 3 ST-70 and one ST-40. I did the following small mods to all of them:
- Removed the funcky center channel
- Removed the stupid balance check
- Wire the common to ground instead of the 4 ohm
- Fixed the loudness control.
- Replaced all caps and resistors.
It sounds very clean and more than it's rated power.
Recently I rewired one of the ST-70's as power amp only, but it was not quite the Eico HF-87. So, I am looking into some projects that I can use the parts. There is a Eico user group a Yahoo!Groups. There is a lot of info there.
- Removed the funcky center channel
- Removed the stupid balance check
- Wire the common to ground instead of the 4 ohm
- Fixed the loudness control.
- Replaced all caps and resistors.
It sounds very clean and more than it's rated power.
Recently I rewired one of the ST-70's as power amp only, but it was not quite the Eico HF-87. So, I am looking into some projects that I can use the parts. There is a Eico user group a Yahoo!Groups. There is a lot of info there.
Thanks Fred, what choice of coupling cap did you make for the power amp? And you just used one (half per channel) or two 12ax7's in front of the 6sn7's?
Hi Ed,
Only one 12AX7 precedes the 6SN7. Each half of the 12AX7 is the voltage amplifer for one channel. The pair of the 12DW7's are for the preamp, and the other pair of the 12AX7 are for the phono. The phono section is suprisingly good, it has both 47k and 100K MM inputs. The ST-70 is not hard on the tubes, one set of tubes can last a long time!!!
I forgot to mention, I did try wiring the OT as the factory circuitry and the way I mentioned in previous post. I didn't hear much of a different in sound quality. All the simple mods can be easily reverted. You can try each one of them and decide which sounds the best to you. The concenses from Eico user groups - the center channel, the balance check, and loudness mods are a must.
edit: if you want the original factory manual, email/pm me your email address. I will send you the pdf copy.
Only one 12AX7 precedes the 6SN7. Each half of the 12AX7 is the voltage amplifer for one channel. The pair of the 12DW7's are for the preamp, and the other pair of the 12AX7 are for the phono. The phono section is suprisingly good, it has both 47k and 100K MM inputs. The ST-70 is not hard on the tubes, one set of tubes can last a long time!!!
I forgot to mention, I did try wiring the OT as the factory circuitry and the way I mentioned in previous post. I didn't hear much of a different in sound quality. All the simple mods can be easily reverted. You can try each one of them and decide which sounds the best to you. The concenses from Eico user groups - the center channel, the balance check, and loudness mods are a must.
edit: if you want the original factory manual, email/pm me your email address. I will send you the pdf copy.
Yes, I've done the loudness mod already because the balance didn't seem quite right and the switch acted more like a mute than loudness...
Was also wondering about the strange center channel setup and the balance switch that connects the feeback circuits - thanks.
For the new power amp build, I was thinking about using separate 12ax7's up front, initially using only one triode section from each, this would give me the option of using the other halves later if the need arises. Not going to bother with the pre-amp / phono stuff, just volume pots...
I'd be interested to hear what you decide on for the new project and will send you a note for the pdf.
Thanks alot, Ed
Was also wondering about the strange center channel setup and the balance switch that connects the feeback circuits - thanks.
For the new power amp build, I was thinking about using separate 12ax7's up front, initially using only one triode section from each, this would give me the option of using the other halves later if the need arises. Not going to bother with the pre-amp / phono stuff, just volume pots...
I'd be interested to hear what you decide on for the new project and will send you a note for the pdf.
Thanks alot, Ed
Hi Ed,
Sent you the pdf file with subject: Fwd: New Order - Eico ST-70 manual,
don't treat that as a spam 🙂. It is approx 5M - both the user and construction manuals. You can take the whole thing apart and follow the instructions to put it back 🙂
BTW, the Sams schematic is wrong in the OT and terminal board
wiring. Your amp is wired according to the factory design, and you
will see from the Eico manual.
Have fun!!
Sent you the pdf file with subject: Fwd: New Order - Eico ST-70 manual,
don't treat that as a spam 🙂. It is approx 5M - both the user and construction manuals. You can take the whole thing apart and follow the instructions to put it back 🙂
BTW, the Sams schematic is wrong in the OT and terminal board
wiring. Your amp is wired according to the factory design, and you
will see from the Eico manual.
Have fun!!
Fred,
Do you still have the Eico ST-70 manual available in pdf format? Would it be possible to purchase a copy?
thank you,
bob k.
Do you still have the Eico ST-70 manual available in pdf format? Would it be possible to purchase a copy?
thank you,
bob k.
There are approximately equivalent Henries from 0 to 4 as from
4 to 16. End taps sometimes stand-in for cathode feedbacks. If
so, then GNDing a 4 ohm tap in the middle would be completely
normal. Not to be blindly "corrected" without consequences.
4 to 16. End taps sometimes stand-in for cathode feedbacks. If
so, then GNDing a 4 ohm tap in the middle would be completely
normal. Not to be blindly "corrected" without consequences.
Yup, yup, you are right, Ken.kenpeter said:
The reason of ST-70 grounds the 4 ohms was to allow the use of the center channel speaker which, trust me, is no fidelity of any kind 🙂 I don't know anyone use this "feature" thus far!!
When the ground is moved to zero ohm, the NFB is changed which in turn will reduce the gain. This amp has a lot of gain, so options are:
1. if you don't need a common ground, leave it as is, i.e. ground the 4ohm tap.
2. accept the fact that the gain is reduced, change to common ground and be happy.
3. a quick and easy change to restore the gain after #2 is to move the feedback from 16 ohm tap to the 4 ohm tap.
4. Instead of #3, add a 100R to parallel R56 and R57.
I don't have any scope or any test equipments that are still on my X'mas wishing list 🙂 I tried all of the above. Besides the gain, I didn't hear any sonic difference of any of the above mods. All of them are relatively easy to change and restore. None of the above will cause any firework 🙂 The st-70 has very good irons and is a good platform to modify to be an excellent full feature integrated amp.
BTW, one of my ST-70's is totally dismantled and is planned to convert to Miles 807 or 6BG6. Really, I shouldn't do that, but I couldn't justify to spend new money to buy new parts either ......
Center channel speaker??? Not exactly what I meant...
I was more referring toward a cathode NFB technique:
http://www.arcdb.ws/ST70C3/ST70C3.html
I don't know if this ST70 mod has any relevance to your Eico.
But certainly gives a better reason for GNDing the 4ohm tap.
I was more referring toward a cathode NFB technique:
http://www.arcdb.ws/ST70C3/ST70C3.html
I don't know if this ST70 mod has any relevance to your Eico.
But certainly gives a better reason for GNDing the 4ohm tap.
Hi Ken,
I am not so smart to figure this out. Below is from another Eico enthusiast who explained to me why Eico used the 4 ohm tap in the
ST-70 and the rationale behind the mod:
So, the center speaker was the prime reason of using the 4 ohm tap. Quite frankly, I didn't see that another vintage from those years had a center channel like that, maybe a broken marketing gimmicks, or I simply haven't seen enough 🙂 I think Eico was on the low end market a in those years, but the irons they used were of very high quality. One of the favorites of Chris, HF-87, is indeed very very good sounding, much better than any Dynaco ST-70 that I have listened to.
I am not so smart to figure this out. Below is from another Eico enthusiast who explained to me why Eico used the 4 ohm tap in the
ST-70 and the rationale behind the mod:
So, the center speaker was the prime reason of using the 4 ohm tap. Quite frankly, I didn't see that another vintage from those years had a center channel like that, maybe a broken marketing gimmicks, or I simply haven't seen enough 🙂 I think Eico was on the low end market a in those years, but the irons they used were of very high quality. One of the favorites of Chris, HF-87, is indeed very very good sounding, much better than any Dynaco ST-70 that I have listened to.
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