Dynaco 70 restore

[Eddie hy fi]

Why limited choices for output tubes? There are lots of EL-34's, 6CA7's, and
KT-77's to choose from.

So just what is the deal here are kt77 the same tube as EL-34 and 6ca7 ? -- i can use any of these?----------This amp , if it comes to life , is to be used in four way stereo system to where it will be asked to play no base , only 500 HZ to 8K into large JBL horns , so with that in mind what would be a good midrange tube that wont brake the bank ?

 

[analogadict]

The so-called "yellow sheet" rectifier socket mod is highly recommended for improved tube and amp reliability, I use two 1N4007 diodes in series on each leg for increased piv margin, and jumper tab 4 with tab 6 to double current capacity of the tube now acting as a time delay ramping series resistance.
View attachment 2618392

Is this mod a better alternative to just replacing the GZ34 with diodes? And, is it okay to use UF4007 diodes instead of 1N4007 ?

Regards,

Anwesh

 

[Pio1980]

Is this mod a better alternative to just replacing the GZ34 with diodes? And, is it okay to use UF4007 diodes instead of 1N4007 ?

Regards,

Anwesh

UF4007s are fine as alternatives.
It's an alternative to just going solid state, the tube does provide B+ voltage warm-up ramp-up.
The alternative is the Gillespie solid state rectifier substitute that is basically similar, but use a 50 ohm 10 watt resistor in place of the tube. It does not provide warm-up ramping but the resistor does mitigate the charge-up current surge at the first filter elyticap.
Plug-in Gillespie 5AR4 rectifier substitute I made;

 

[Lavane]

So just what is the deal here are kt77 the same tube as EL-34 and 6ca7 ? -- i can use any of these?----------This amp , if it comes to life , is to be used in four way stereo system to where it will be asked to play no base , only 500 HZ to 8K into large JBL horns , so with that in mind what would be a good midrange tube that wont brake the bank ?

The KT-77 is a drop in alternative. I ran a set for awhile. I liked them.

 

[Pio1980]

This is the socket mod I used for the 5AR4;

 

[Pio1980]

Why , can you explain why , because im being told to watch out for most of these mods that dynaco would have done it , if it needed it ?

While there was definitely cost-cutting with the stereo 70, I think the use of the output screen stopper stabilization resistors wasn't universal. Marantz did include them in their tube amps of the same period. The reason is they help discourage screen current runaway, and provide protection for the output transformer by opening under excessive current like fuses when it does occur, usually from a defective tube when they are used.

 

[peterh]

Read https://www.audiokarma.org/forums/index.php?threads/dynaco-st-70-base-line-testing.578485/
for a well informed insight of ST70 including some areas of improvements.

 

[gadget73]

i do have three of the originals the 4th is clearly burned out and they look like mullards , but unit was shipped to me across the usa with the tubes in amp and the box and amp took a beating , one small tube even broke up , i wont be using ant of those tubes

fair enough, but some of them may still be serviceable. Would be worth having them tested before tossing them.

 

[Pio1980]

fair enough, but some of them may still be serviceable. Would be worth having them tested before tossing them.

If it produces a usable matched pair or triad, perhaps, with the 3rd one as a spare for a pair. P/P amps should really run MP outputs per channel for optimum results, IMO. That includes Mac gear, fwiw.

 

[Eddie hy fi]

UF4007s are fine as alternatives.
It's an alternative to just going solid state, the tube does provide B+ voltage warm-up ramp-up.
The alternative is the Gillespie solid state rectifier substitute that is basically similar, but use a 50 ohm 10 watt resistor in place of the tube. It does not provide warm-up ramping but the resistor does mitigate the charge-up current surge at the first filter elyticap.
Plug-in Gillespie 5AR4 rectifier substitute I made;
View attachment 2618951 View attachment 2618952 View attachment 2618953

Thanks but im just not going to do the solid state rectifier for now , i hear that it effects the sound in a bad way and hard on the tubes during warm up , now i could be wrong on this , but one time i tried a 5u4 plug in solid state on a nice sounding tube amp and it did work but did not sound as good as a 5u4 tube , i can not explain why ? cause i dont know

 

[Pio1980]

The Gillespie substitute behaves more like a tube under dynamic cdx than a simple drop-in silicon rectifier replacement without the 50 ohm resistor, as the B+ voltage will move around a bit more under load, but have greater current carrying capacity under load than any single tube rectifier. The resistor also helps with taking the peak charging stress off the power transformer which the simple silicon module can cause. My rebuilt OEM Stereo 70 with the socket mod can use either a rectifier tube or the substitute with no other changes required.

 

[Pio1980]

The only technical effect that rectifier tube rolling can have is varying current limiting and B+ voltage stability under a dynamic loading, which can affect SQ in varying ways. Engineering preference usually dictates the "stiffist" most stable power source practically achievable.
Some folk find soft "sponge-y" B+ source distortions preferable as part of "tube sound".

 

[Lavane]

I use SS rectifiers in my M-125 mono blocks. They have a thermistor and resistors to mimic a tube rectifier. You could use a GZ-37, and it would hold up better than a 5AR4, but would cost you a lot more. The 5AR4 should last a good while u less you like to play it loud a lot. Adding either the yellow sheet mod or expanding it to what pio1980 posted is helpful, along with the screen stoppers too.
I have rube amps with SS rectifiers and tube rectifiers. My favorite has SS.

 

[Eddie hy fi]

What about a tube convertor plug in, to then be able to run a 5u4G tube , i see them available on e-bay , has anyone tried this , then better rectifier tubes at cheaper cost could be used ? or is this a no no ? my mcintosh mc 30s use coke bottle sylvanias 5u4s and i would never convert them to solid state

 

[Blackdog61]

I did the yellow sheet mod as shown on post #16 and have had no problems with the inexpensive Sovtek rectifier. It’s only my experience but I don’t see the need to overthink it or fix what ain’t broke.

 

[Pio1980]

What about a tube convertor plug in, to then be able to run a 5u4G tube , i see them available on e-bay , has anyone tried this , then better rectifier tubes at cheaper cost could be used ? or is this a no no ? my mcintosh mc 30s use coke bottle sylvanias 5u4s and i would never convert them to solid state

Not sure of your meaning here, to use 5U4 in place of 5AR4? They have same pinout, the 5U4 has higher resistance and lower current carrying capacity. Hardly an upgrade under any definition.

 

[gadget73]

more importantly, larger voltage drop, and it has a fast warm-up so no advantage over solid state in that regard.

also its a 3 amp heater vs 2 amps, so there is some extra load on the transformer that it probably doesn't really want.

 

[Eddie hy fi]

I did the yellow sheet mod as shown on post #16 and have had no problems with the inexpensive Sovtek rectifier. It’s only my experience but I don’t see the need to overthink it or fix what ain’t broke.

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

 

[AlexHempel]

Yep, 5U4G warms up very fast. The amp I've got on the bench right now gets 460 volts of B+ for at least five seconds before the output tubes warm up and it drops to 360. Glad I did PSUDII sim and knew to get 500 v caps.

 

[Eddie hy fi]

Ok well it sounds like no 5ur here , so ill just use a new jj to get amp up and going then plan on buying a tug sol or tad for future use , then just keep the jj as a back up , allso have a used , old GE 5ar4 that came with the amp , maybe it would work too