Barcade
New Member
I made a little more progress tonight replacing some of the resistors
and added some rubber feet to the chassis to give it some clearance on the bottom
Dynakit ST-70 restore..... is it possible to DIY?
- Thread starter Barcade
- Start date
and added some rubber feet to the chassis to give it some clearance on the bottom
ferninando
Lunatic Member
the JJ should do just fine IMO.
the 5ways make spkr connection so much easier.. a good addition.
the 5ways make spkr connection so much easier.. a good addition.
honestly I'd go solid state rectifier and an appropriate drop resistor. The 5AR4 has the ever living snot worked out of it in the ST-70 and the modern ones don't seem to take the torture as well as the increasingly expensive and rare old ones did. Couple bucks gets you a pair of diodes and a resistor that will never give you trouble.
Did you ever determine the year of manufacture of your ST-70?
Many indications that it is an early version. Red wire, paper e-caps, single hole for 270k resistors on driver board
What are the numbers on the back of the bias pots?
Look at the bottom of the power transformer carefully for a stamped date
Many indications that it is an early version. Red wire, paper e-caps, single hole for 270k resistors on driver board
What are the numbers on the back of the bias pots?
Look at the bottom of the power transformer carefully for a stamped date
Barcade
New Member
How can i find out the year of manufacture? i see the numbers 137914 on the back of the bias pots
The 137914 is another indicator of early version. Mine have 137908. So early the pot numbering system is not in place yet
It could be the 14 week of 1959 or the 9th week of 1914
Look at where the wires come out of the power transformer. Look if there is a stamp. Mine has "59" on it
It could be the 14 week of 1959 or the 9th week of 1914
Look at where the wires come out of the power transformer. Look if there is a stamp. Mine has "59" on it
Barcade
New Member
How would i go about going with a solid state rectifier? is it plug and play?
Sorry if this is a dumb question i am pretty new to all this
Thanks
ferninando
Lunatic Member
AES has 2 plug ins 1 for ~$10 and ~ $15 for replace 6GZ34/5AR4
this is the one I will be using in mine.....no affiliation. There are a number of them available out there
https://www.tubesandmore.com/products/solid-state-tube-rectifier-yellow-jackets-yjr
https://www.tubesandmore.com/products/solid-state-tube-rectifier-yellow-jackets-yjr
Barcade
New Member
What does this mean?
"Note: Because there is no warm up of a rectifier tube when using this solid state device, it is critical that a standby switch is used that will allow the power tube filaments to warm up before high voltage is applied. This will prevent 'cathode stripping' in the power tubes."
shelly_d
Not An Audiophool
There is a coating on the cathode that emits electrons when heated. One of the failure modes of tubes is the exhaustion of this coating. It was thought that applying high voltage to a tube before both the bias and filaments were up to operating voltage and temperature would lead to premature failure of that coating (and the tube) by "stripping" the electrons from it. There is some evidence that suggests that does not really happen but I do not know the definitive verdict on the phenomenon.
Shelly_D
Shelly_D
Only thing with the plugins is that I don't know if they have the dropping resistors.
Some further technical reading on the power supply of the ST-70
Dynaco ST-70: Base Line Testing
the relevant bit is this though:
"To do a good job of mimicking the voltage drop of a GZ34 tube, there should be a 50 ohm 10 watt resistor installed BETWEEN the output of the SS rectifiers, and the FIRST filter cap."
1N4007's would do the job for the rectifiers. You can make that into a plug-in if you'd like, or if someone already makes it you're good. I know that some of those plugin devices aren't physically large enough to contain the drop resistor. Basically if its just the dimensions of a tube base, its a pretty good bet it has no dropper and its nothing more than a couple of 10 cent diodes in an octal plug.
me being me, I'd probably make my own plug-in to allow zero modification to the amp if someone didn't already make it for a price I can cope with.
Some further technical reading on the power supply of the ST-70
Dynaco ST-70: Base Line Testing
the relevant bit is this though:
"To do a good job of mimicking the voltage drop of a GZ34 tube, there should be a 50 ohm 10 watt resistor installed BETWEEN the output of the SS rectifiers, and the FIRST filter cap."
1N4007's would do the job for the rectifiers. You can make that into a plug-in if you'd like, or if someone already makes it you're good. I know that some of those plugin devices aren't physically large enough to contain the drop resistor. Basically if its just the dimensions of a tube base, its a pretty good bet it has no dropper and its nothing more than a couple of 10 cent diodes in an octal plug.
me being me, I'd probably make my own plug-in to allow zero modification to the amp if someone didn't already make it for a price I can cope with.
I too am sorting all this out but am told by folks in the know that a 50 Ohm 10 watt resistor underneath is all that is needed with the SSR.
In this discipline (tube amp restoration) and specifically the ST 70, there is so much info to wade through and determining what is fact vs. what is opinion/marketing/bravado can be a challenge.
Gadget73's link to Dave Gillespie's thread above is a tremendous resource for this endeavor.
The cathode emits electrons from across its entire surface. When the cathode is not fully hot some areas are hotter than others and emit more than others. So any current pulled through the tube heats those emitting areas more. Because the current flow is designed to use the entire cathode, not just portions of it, too much current is pulled from point sources and the cathode overheats in those regions. That damages the cathode by breaking down the alkali metal oxides, which also liberates gas, fractures the surface, and causes other damage. Cathodes are robust and can take a fair but of abuse before failing, but pulling that current gradually damages the tube and eventually kills it.
The internet claims cathode damage does not occur in signal tubes because transmitting tubes are rapidly destroyed from pulling current before the cathode is at operating temperature, but signal tubes take longer to degrade. The issue is temperature, but the degradation does occur, just at a slower rate.
ST70 power tubes are limited in their emission by a fixed bias. This will limit the current drawn from the
cathodes , the net effect is that the current density is constant over the cathode. There is no
overcurrent in points but rather is the current slowly increasing until the whole cathode emits.
When the cathode is not fully emitting (i.e. when it has not completely warmed up) and is conducting to the plate the electrons are emitted from points, not the entire surface. Those point emissions cause spot heating. This is not a function of bias. That's just the way cathodes work.
I think what you're talking about is grid control of the cathode. That's not what I'm describing. I am explaining the early stages of cathode heating, and how the islands of charge start and then coalesce into a contiguous surface. Once that surface is formed there is no spot heating (taking into account irregularities in the surface) because the charge is evenly emitted.
The current per area does not change as the bias actually depress emission. When more part
of the cathode starts emission the current will increase. But current per area stays constant.
So there is no heated spots. The situation you describe applies on "auto-bias" where the voltage drop across
a cathode resistor controls the emission.
