A Stereo 70 Success - Finally

[shelly_d]

I have finally gotten my Stereo 70 back, beat the red plateing and the hum and have some new (for me anyway) insights in to the stock driver board.

Some time ago, I lost the rectifier and power transformer when the filter caps when south. This was after going through several rectifier tubes. I made the decision to go with the SDS cap board and solid state diodes to increase reliability. Before I finished the great layoff came and the amp sat for want of a good power tranny. Over Christmas, my wife gifted me with enough money to finish the restoration provided I was judicious with it. There was not enough to get the replacement PA60 that various places sell, so I went with an Antec transformer:

http://www.antekinc.com/details.php?p=72

With solid state diodes you will get the B+ that you need to run the amp. It has twice the filament capacity that the amp needs, the only thing missing is the bias voltage. Playing around in Multisim I came up with a viable replacement circuit for it. The amp sounds great. I'll post more on this in subsequent posts to not bore you to sleep in on long one.

Shelly_D

 

[shelly_d]

Here is the schematic for the power tranny I used and how I achieved the right bias voltage. Pretty straight froward really, just a matter of getting the values right.

Tired now so more tomorrow, after I get some sleep.

Shelly_D

 

[mech986]

Nice! will enjoy waiting for the finished amp pics.

 

[GordonW]

Well done. I, along with Jay, am a big fan of choke-filtered SS power supplies. In fact, I'm about to build a 6L6 Magnavox-circuit integrated amp, using a SS bridge-rectified supply and choke. 300VAC secondary should give about 400v DC at idle...

BTW: What did B+ voltage to the output transformers wind up as, at idle? I'm guessing about 440v?

Regards,
Gordon.

 

[shelly_d]

Well done. I, along with Jay, am a big fan of choke-filtered SS power supplies. In fact, I'm about to build a 6L6 Magnavox-circuit integrated amp, using a SS bridge-rectified supply and choke. 300VAC secondary should give about 400v DC at idle...

BTW: What did B+ voltage to the output transformers wind up as, at idle? I'm guessing about 440v?

Regards,
Gordon.

Thank you for the kind words. The B+ supply is actually the supply as modified by the old SDS cap board. The choke is the original one from the original power supply. It is in poor shape and really should be replaced for long term reliability, but money being the way it is right now, I'm keeping the original for now.

I get a bit more B+ out of it then my sim indicated. I've got 427 V on the plates and 429 V on the screens. Have not measured on the plates of the two section of the 7199.

Pics will follow shortly, when I get them out of the camera and sized to put up.

Shelly_D

 

[Dandy]

... Pics will follow shortly, when I get them out of the camera and sized to put up.

Shelly_D

I find that the AK software resizes pics automatically. Should I not be doing this?

 

[shelly_d]

Here are a few pics:

A few notes:

I will be posting more on some of the things you see here soon. For now, the perf board is a B+ delay circuit, the terminal strip on the left is the filament supply and the terminal strip on the right is the new bias supply. Keen eyed observers will see a few changes at the input jacks as well.

More later. I want to make the posts readable.

Shelly_D

 

[shelly_d]

I did change the circuit on the board slightly and the results of those changes surprise me.

I replaced the coupling caps with .47 uF film caps.

I also changed the stereo / mono switch to a DPDT and built up an input circuit that puts a .033 uF mylar cap in series with the input. Something I feel the amp should have had for some time now and part of the concept of the Van Alstein bandwidth filter mod. The switch is to be able to switch that in and out of circuit. I expected the sound of the amp to change with the position of the input switch. I.e., I expected to hear the muddy midrange and treble the Stereo 70 is well know for without the input caps and it to diminish or go away with the caps in place. It did not happen. Sounds the same.

Now, except for the coupling caps and the input caps, this is exactly the same input circuit that I was running before the rebuild. Before the rebuild, the bandwidth limiters made a real improvement in the clarity of the midrange and treble. Now, either with the caps in place, or bypassed or bypassed with the filters, the amp sounds the same.

If you look at the Van Alstein article, he talks about the first section of the 7199 being driven into saturation by the low bass missing in the output and therefore the feedback signal. Perhaps, the size of the coupling caps was an serious portion of that deficiency and the increase in value, removed enough of that component to pull the tube back out of saturation for my installation. Unfortunately, my test bench is not good enough for me to investigate this in enough detail to report on it properly.

Shelly_D

 

[bktheking]

Are you going to put a cover over that toroid? I just got my choke btw from dynakitparts, $13.50 IIRC, $26 shipped to my door.

 

[shelly_d]

The Delay Board

While I was running sims to see if the bias circuit would work the way I wanted I noticed that the time frame for it come up to voltage was very much more then the time it took to bring the B+ up. The B+ takes just a few milliseconds. The bias however was taking a few hundred milliseconds to stabilize. I was concerned enough about that to want to do something about that. Looking on Mouser, I found a relay that could handle the voltage that the transformer put out and proceeded to put together a delay circuit. I make no pretension that this is a good circuit. I am absolutely convinced that there are better circuits out there but this one does what I want. If someone wants to post a better one for others to use in place of this one, please, I urge you to do so.

Shelly_D

 

[shelly_d]

Are you going to put a cover over that toroid? I just got my choke btw from dynakitparts, $13.50 IIRC, $26 shipped to my door.

No. I use the full cover that came with the amp.

I do know the choke is not expensive but I've already spent quite a bit on this amp and I'm still looking for a new job. So the choke will wait.

Shelly_D

 

[bktheking]

Ah got it, I wasn't aware of the job sitch. I ran mine with the cover on and found it got WAY too hot. I do however like the look with the cover on so i'm going to have to figure out some way of cooling it down.

 

[shelly_d]

Ah got it, I wasn't aware of the job sitch. I ran mine with the cover on and found it got WAY too hot. I do however like the look with the cover on so i'm going to have to figure out some way of cooling it down.

Mine doesn't. Have you looked at the voltage on the heaters? It is possible, that if the heater voltage is too high, say 6.5 ~ 7 volts, the tubes are getting too hot. Just a thought. I do keep mine on a top shelf, open on top and all sides. That may be part of it as well.

Shelly_D

 

[Wharfcreek]

Shelly,

Greetings my friend!! This is a wonderful thread you're building! Been a long time since I've seen anything 'creative' and 'frugal' done on the old ST-70. I have to admit, that while I'm a HUGE fan of Dynaco amps in general, it's been a while since I've been in to any of them. I have several SCA-35 projects to get back to.....and a complete build-up on an ST-70 that you may have inspired me to get into yet tonight. But, I'm curious where the inspiration came from on the bias circuit you created? I've done a couple similar circuits that I kind of 'Ganked' from Marshall guitar amps. They used a similar scheme in some of their units.....and a friend of mine who was my early 'mentor' in all this stuff referenced to 'trick' to me when I got stuck on a guitar amp I was building about 15 years ago......had everything mounted up and about half way soldered together when I realized I didn't have a bias tap on the PT. I was in shock at the time.....but using something similar to what you did, it all worked out in the end. Anyway.....hope you're doing well! I share your 'job' woes! I lost my position in November of '08 after 32 years with my company. I won't bore you with my story....just wishing you luck on that account.....and again, my compliments on the ST-70 resurrection!! Tom D.

 

[shelly_d]

Shelly,

Greetings my friend!! This is a wonderful thread you're building! Been a long time since I've seen anything 'creative' and 'frugal' done on the old ST-70. I have to admit, that while I'm a HUGE fan of Dynaco amps in general, it's been a while since I've been in to any of them. I have several SCA-35 projects to get back to.....and a complete build-up on an ST-70 that you may have inspired me to get into yet tonight. But, I'm curious where the inspiration came from on the bias circuit you created? I've done a couple similar circuits that I kind of 'Ganked' from Marshall guitar amps. They used a similar scheme in some of their units.....and a friend of mine who was my early 'mentor' in all this stuff referenced to 'trick' to me when I got stuck on a guitar amp I was building about 15 years ago......had everything mounted up and about half way soldered together when I realized I didn't have a bias tap on the PT. I was in shock at the time.....but using something similar to what you did, it all worked out in the end. Anyway.....hope you're doing well! I share your 'job' woes! I lost my position in November of '08 after 32 years with my company. I won't bore you with my story....just wishing you luck on that account.....and again, my compliments on the ST-70 resurrection!! Tom D.

Thank you for the kind words and good wishes. I too, wish you good luck with your job search.

For the bias, My inspiration started with a careful look at the original bias circuit and a few other amps with similar circuits. Since I own a copy of multisim it was easy to create the simulation circuit and see what I get.

My first attempt was using the two secondary sections in parallel because I felt the current draw for the B+ would be a lower load on the tranny. It would not give me a negative voltage out of that configuration however. So I then tried the series connected configuration with the center connection (like a center tap) grounded with a "full wave rectifier" configuration. That did provide the negative voltage. After that, it was a matter of getting the values of the resistors right then trying different values for the caps. I wanted less then 4 mV of ripple in the sim. With the two small caps in the circuit it sims out to less then 2 mV peak to peak. IT was only after I had built it that I realized that much of the hum that the ripple would produce would cancel out because of the push pull nature of the output.

Thanks for your comments. I really appreciate them

Shelly_D

 

[shelly_d]

Another tidbit. To cover the hole in the chassis from the PA60, I picked up the cover for a metal, 2 outlet sized, electrical box. Two of the 4 screw slots lined up with the existing PA60 mounting holes. Had to drill out 2 more to secure it properly. That was worth about a $1.75 I think.

Shelly_D

 

[Wharfcreek]

Sometimes there's just no end to 're-purposing'!!

 

[BinaryMike]

It's wise to add screen stopper resistors on the output tubes any time you upgrade the power supply section. 100R is usually a good choice. Dave Gillespie has written extensively on this topic.

 

[shelly_d]

It's wise to add screen stopper resistors on the output tubes any time you upgrade the power supply section. 100R is usually a good choice. Dave Gillespie has written extensively on this topic.

Thanks for that tip. Would 1/2 watt be robust enough? I'm assuming that the screen current is low enough that.

Thanks

Shelly_D

 

[BinaryMike]

1/2W should be plenty for amps in this power class. Snuggle the stopper resistors right up close to the screen pin of each socket.