ST70 Modifications

My father picked up an old ST70 and before even checking to see if it worked, he sprung into changing things out. I can understand changing out the old caps and resistors for new ones, but some of these mods seem excessive.

On the GZ34 Rectifier tube he's placed two diodes in line with the input from the transformer. From what I can gather, the entire point of the GZ34 tube is to convert 360vAC to DC voltage. But the two diodes he's installed do just that, making the tube useless.

We attempted to bias this amp last night and we turned the 10K pots down all the way to the left, and when starting up we watched as the voltage soared way past 1.56vDC and exceeded 2.5v before we switched it off. The GZ34 is getting very bright, very quickly. My immediate thought is that those two diodes are to blame. When I ask why the diodes are there I'm met with "it's a hybrid circuit". I've seen some examples of hybrid circuits using GZ34, this isn't quite the same, it's closes to a GZ34 emulator.

any opinions here?

The diode mod is well-known and quite all right- often called the "yellow sheet mod" which you can find in this forum if you look it up.
As for bias, you turned it UP ALL THE WAY. That's right- start with the pots all the way to the right and try again.

TheCaddyMan99 wrote:My father picked up an old ST70 and before even checking to see if it worked, he sprung into changing things out. I can understand changing out the old caps and resistors for new ones, but some of these mods seem excessive.

On the GZ34 Rectifier tube he's placed two diodes in line with the input from the transformer. From what I can gather, the entire point of the GZ34 tube is to convert 360vAC to DC voltage. But the two diodes he's installed do just that, making the tube useless.

We attempted to bias this amp last night and we turned the 10K pots down all the way to the left, and when starting up we watched as the voltage soared way past 1.56vDC and exceeded 2.5v before we switched it off. The GZ34 is getting very bright, very quickly. My immediate thought is that those two diodes are to blame. When I ask why the diodes are there I'm met with "it's a hybrid circuit". I've seen some examples of hybrid circuits using GZ34, this isn't quite the same, it's closes to a GZ34 emulator.

any opinions here?

You might have problems with the bias circuit(s).

Remove all EL34 and GZ34 , turn on and measure between pin 5 and chassies. A negative
voltage between 16-35V should occur adjustable with the bias pot. If not
start fixing the bias circuit.

Actually, powered on with no tubes, measuring AC voltage from main transformer to the rectifier, I'm getting 360vAC on one, and 172vAc on the other. Current Theory is that power transformer or the multicap is bad.

I put the Rectifier back in and took voltage measurements on the multicap, they are all over the place.

A is reading 194vDC instead of 305, B reads 420v instead of 375, C reads 491 as opposed to 415, and D reads 488v instead of 435.

TheCaddyMan99 wrote:Actually, powered on with no tubes, measuring AC voltage from main transformer to the rectifier, I'm getting 360vAC on one, and 172vAc on the other. Current Theory is that power transformer or the multicap is bad.

I put the Rectifier back in and took voltage measurements on the multicap, they are all over the place.

A is reading 194vDC instead of 305, B reads 420v instead of 375, C reads 491 as opposed to 415, and D reads 488v instead of 435.

A asked for you to measure the pin5 of EL34 sockets, the BIAS voltage. What is that ?
( do this with the rectifier and power tubes removed)

Also measure the AC voltage between 4 and 6 in the gz34 socket.

nope, just replaced the main transformer, everything is stable and working as should be.

Before replacing the main transformer, lug 4 of the bias circuit was reading -73v DC and lug 7 was reading -0.001.

Just biased with all tubes in, and both sides are at solid 1.56vDC

Before everything was all over the place. Popped down to Paramus on my lunch break and had Kevin from Dynakit Parts test the transformer and confirmed my suspicions. Got new transformer installed, didn't measure any voltages inside, don't really care to anymore, all I know is that instead of bias voltage soaring past 2.5v DC within a minute, it's staying at a rock solid 1.56v.

TheCaddyMan99 wrote:nope, just replaced the main transformer, everything is stable and working as should be.

Before replacing the main transformer, lug 4 of the bias circuit was reading -73v DC and lug 7 was reading -0.001.

Just biased with all tubes in, and both sides are at solid 1.56vDC

Before everything was all over the place. Popped down to Paramus on my lunch break and had Kevin from Dynakit Parts test the transformer and confirmed my suspicions. Got new transformer installed, didn't measure any voltages inside, don't really care to anymore, all I know is that instead of bias voltage soaring past 2.5v DC within a minute, it's staying at a rock solid 1.56v.

Fine you solved it. Bad mains transformers are not that common that's why i asked for
detailed measurments. But that's history now.
Yes, Kevin is a serious person that knows his job!