Dynaco 70 restore

[Blackdog61]

but you did fix what was not broke , im just going to run the tube with no yellow sheet , no added diodes , just like dynaco does in original form , how is this over thinking

No problem as far as I can see. The Yellow Sheet mod has been discussed, and discussed on AK; with many weighing in that it’s not necessary at all. My point had more to do with which diodes to use and whether to double them up, then whether to pay $25 or $50 or $100 or more for the tube, or…………..
I’m not sure the quality of rectifier tubes is up to the old Mullards or whatever and then with higher line voltage there could be a problem. I use a bucking transformer and have had good luck with the basic Sovtek 5AR4 but then, I might be overthinking it.

 

[analogadict]

but you did fix what was not broke , im just going to run the tube with no yellow sheet , no added diodes , just like dynaco does in original form , how is this over thinking

This topic has been recommended in a previous post.
https://www.audiokarma.org/forums/index.php?threads/dynaco-st-70-base-line-testing.578485/

I would suggest reading it, especially the section about rectifier behavior under full load, before deciding what is overthinking and what is not.

Regards,

 

[Eddie hy fi]

No problem as far as I can see. The Yellow Sheet mod has been discussed, and discussed on AK; with many weighing in that it’s not necessary at all. My point had more to do with which diodes to use and whether to double them up, then whether to pay $25 or $50 or $100 or more for the tube, or…………..
I’m not sure the quality of rectifier tubes is up to the old Mullards or whatever and then with higher line voltage there could be a problem. I use a bucking transformer and have had good luck with the basic Sovtek 5AR4 but then, I might be overthinking it.

i did look it over again and it looks easy enough for me to do , but the increased current worries me because a lot of these mods require other mods , i just installed a new can cap but its only stock specs and 525 volt , that voltage is not enough to be going to solid state rectifier and who know if its even enough voltage or capacitance for the yellow sheet mod , and i have no bucking transformer { what ever that is } i just want to be moor sure of any mod that i do , maybe i do have a bucking transformer if you mean a AC line power supply type transformer that can supply many tube amps at the same time

 

[Eddie hy fi]

This topic has been recommended in a previous post.
https://www.audiokarma.org/forums/index.php?threads/dynaco-st-70-base-line-testing.578485/

I would suggest reading it, especially the section about rectifier behavior under full load, before deciding what is overthinking and what is not.

Regards,

That is a great post for sure ! so much info there , and im not a dynaco expert , i do have and love my pass 3 x preamp , but this is my first 70 amp , Since im not going to be asking this amp to play any bass , it will be electronic crossed over with a sony TAD-888 , and asked to play from 500 HZ to 8 k hz in a four way system where a pair of mcintosh 30s play the lower mids from 250 hz to 500 --i feel like in stock form it should do this fine , ITS A DYNACO 70 !

 

[gadget73]

doing the yellow sheet mod requires no other changes. voltages do not change, and there is no need to change capacitance in the filter.

 

[Blackdog61]

i did look it over again and it looks easy enough for me to do , but the increased current worries me because a lot of these mods require other mods , i just installed a new can cap but its only stock specs and 525 volt , that voltage is not enough to be going to solid state rectifier and who know if its even enough voltage or capacitance for the yellow sheet mod , and i have no bucking transformer { what ever that is } i just want to be moor sure of any mod that i do , maybe i do have a bucking transformer if you mean a AC line power supply type transformer that can supply many tube amps at the same time

I purchased a bucking transformer on the ESlay auction site which was already wired up. It has one outlet and a power switch. Inside is a transformer which drops the line voltage by 7 volts. The increase in voltage on the power grid since these amps were first introduced is a problem because it increases voltages in the entire amp circuit. 6.3v on the heaters may be 7v and so on. I noticed that my amp runs cooler as well.
As gadget73 mentioned, the yellow sheet mod doesn’t affect voltages in any way. It simply provides some insurance to protect the rectifier. I did it while my amp was opened up and while replacing the SE rectifier in the bias circuit, I wired two to the rectifier. Since then, I saw that some recommend putting two in series, but see no reason to open the amp up again to do it.
FWIW, my ST-70 is using the stock circuit but has a new board, larger filter capacitor, individual bias circuit all purchased from Dynakit Parts. I’m running 6GHA tubes with adapters. It is quiet and the PT only gets slightly warm after a few hours of use.
I see no reason to switch to SS rectification or make any other changes. You are on the right track as the ST-70 is a solid design which can perform very well.

 

[Eddie hy fi]

Thank you guys ill consider your words , i still wonder why to change the wires to different pin #s on rectify socket, while doing the yellow sheet , but there must be a reason ---i do have a nice power supply transformer set up , i inherited it from uncle after he passed , he put it together and was running seven mcintosh amps and a solid state for his bass off of it , in a surround sound set up ------its just a heavy transformer 50 or 60 pounds , in a metal box , one switch on the front and one or two out lets on the back and has a heavy power cord and i use a power switch strip from this , until now i never thought of it as a voltage drop but i think it does drop a few . had to sell off his macs for the estate but i bought two of them my self and got the power supply and crossover , then i bought his JBL speakers too and started to put his original stereo back to the way he had it in the 80s ---------one switch on the box turns on the whole system

 

[Lavane]

It's to stabilize the diodes so they aren't just hanging out in the wind when soldered in.

 

[Blackdog61]

I believe the yellow sheet mod actually has the diodes doing the rectification and the tube retains the soft start. ?.?.?

Eddie, that transformer box looks like an isolation transformer. Whatever voltage it sees on the input is what it puts out, but it should leave any noise, including clicks or surges from things like household appliances on the other side of it. I added one to my system and it improved the sound significantly. A bucking transformer drops the voltage by a set amount.
125 in = 118 out. 117 in =110 out.

 

[zackthedog]

Thank you guys ill consider your words , i still wonder why to change the wires to different pin #s on rectify socket, while doing the yellow sheet , but there must be a reason ---i do have a nice power supply transformer set up , i inherited it from uncle after he passed , he put it together and was running seven mcintosh amps and a solid state for his bass off of it , in a surround sound set up ------its just a heavy transformer 50 or 60 pounds , in a metal box , one switch on the front and one or two out lets on the back and has a heavy power cord and i use a power switch strip from this , until now i never thought of it as a voltage drop but i think it does drop a few . had to sell off his macs for the estate but i bought two of them my self and got the power supply and crossover , then i bought his JBL speakers too and started to put his original stereo back to the way he had it in the 80s ---------one switch on the box turns on the whole system

I definitely hear you about not wanting to change the sound of the amp, and I'm not fan of SS diodes unless absolutely necessary. But I think the two small mods being suggested here (the rectifier diodes and the screen resistors on the output tubes) are merely insurance against the fact that modern-production tubes are, in general, just not as well-made as the original Mullards and GEs. They're just common-sense safety features to insure against failure. Dynaco didn't include these features because tubes back then were more reliable than they are today. They won't alter the sound of the amp.

 

[Eddie hy fi]

Ok thanks , i still thinking about it , for now ive just ordered a sovtek 5ar4 tube , trying to get quad of 34s too but i keep getting out bid , JJ s are the only ones that seem easy to get for cheap

 

[Pio1980]

Ok thanks , i still thinking about it , for now ive just ordered a sovtek 5ar4 tube , trying to get quad of 34s too but i keep getting out bid , JJ s are the only ones that seem easy to get for cheap

Because they are in production and being shipped.

 

[Eddie hy fi]

I believe the yellow sheet mod actually has the diodes doing the rectification and the tube retains the soft start. ?.?.?

Eddie, that transformer box looks like an isolation transformer. Whatever voltage it sees on the input is what it puts out, but it should leave any noise, including clicks or surges from things like household appliances on the other side of it. I added one to my system and it improved the sound significantly. A bucking transformer drops the voltage by a set amount.
125 in = 118 out. 117 in =110 out.

Ok thanks , yes ive all ways considered it a isolation trans too , multi tube amps on one circuit , what would i do with out it ? - helps with any ground loop issues too - i monitor the line voltage coming out of it with this large , vintage , heathkit line voltage monitor that i found when clearing out uncles closet , i figured he must have been thinking about doing the same thing

 

[gadget73]

My usual suggestion for buying tubes is to get them from a reliable seller rather than randomly on ebay where you may not really know how they were tested or if they actually match like claimed. I've seen people claim "matched" just because the tubes visually look the same and thats not what matters. Also if you get them from a real vendor, you have someone to contact if it turns out you got a dud.

 

[zackthedog]

My usual suggestion for buying tubes is to get them from a reliable seller rather than randomly on ebay where you may not really know how they were tested or if they actually match like claimed. I've seen people claim "matched" just because the tubes visually look the same and thats not what matters. Also if you get them from a real vendor, you have someone to contact if it turns out you got a dud.

Ditto times 10! Don't buy new tubes on eBay. Tubedepot and The Tube Store are two reliable dealers.

 

[gadget73]

I suppose I should come clean on this before someone calls me on my BS, I did buy my set of JJ's used from somewhere here on AK. They weren't tested as I recall, just sold extremely cheaply with claimed low usage. They test fine and the price was low to the point that I was willing to take the risk.

 

[RS Steve]

Just so happens I started a ST-70 restoration, this will be my third over that few years. Maybe this one will stay, but who knows. Decided to order a new driver board that gets rid of the 7199's, balance controls, also upgraded speaker binding posts and RCA inputs. Already installed the rectifier tube diode mod, new can cap, and screen stability resistors, just waiting on the new board to arrive. Meanwhile I will rummage through my stash of tubes and pick out some good ones. Anxious to see how the OP's amp turns out.

 

[Eddie hy fi]

what is the value of screen resistors and what pin numbers , on out put tube socket , do they go across

 

[Pio1980]

what is the value of screen resistors and what pin numbers , on out put tube socket , do they go across

They are 100 ohm 1/4 watt.
Disconnect the 1k 1/4 watt resistors between tabs 5 and 6 from tabs 6 and connect the now free ends directly to the board with wire extensions. Move the output transformer tap leads from tabs 4 to tabs 6 and connect the 100 ohm 1/watt resistors between them.
Move the feedback wires from the board from tabs 4 to tabs 6.

 

[Pio1980]

Fwiw, I also recommend replacing the two 1.5 meg 1/4 W resistors on the board with 1.2 meg 1/4 W to give the splitter triode a bit more linear margin.