Dynakit ST-70 high Voltage readings

[Andrew7dg]

Dynakit St-70

Short stack PA060
Yellow sheet mod (2 x 1N4007) on rectifier
Selenium rectifier has been replace with a 1N4007 also
5AR4 Mullard still in use
7199 still being used

Bias caps have been replaced
Upgraded Cap Can has been replaced

PCB board still has much of its stock circuit

In the manual I was checking the Voltage points after the amp was warmed up
All my voltages were high

I expected the increase because of the previous 117 it expected was now getting 122.5 AC V from modern increases

Some numbers

EH L34
Pin 3/4- 460dc volts

5ar4
Pin 2/8 - 473 volts
390AC it is seeing
But the 1N4007 diodes are cutting it down to
203 AC

Seems pretty high. Has anyone else read higher voltage on theirs?

 

[pwgtang]

That is normal for solid state rectify, My ST-70 EL34 pin 3/4 even higher it is about 485 to 488 Volt with 40ma bias, I don't used the filter choke, but I used a big value filter capacitor.

 

[rayma]

He is still using the 5AR4, with series diodes for improved reverse voltage blocking.

Your line voltage must be high. The normal Stereo 70 amplifer's B+ after the choke should be
about 435VDC if you have an older power transformer designed for 115VAC (used on 120VAC),
or else 415VDC if you have a newer power transformer designed for 120VAC.

Otherwise the plate currents could be set too low, which will also raise the B+ up.
They should be 50mA per plate.

 

[6A3sUMMER]

You put in a 1N4007 in place of the selenium rectifier.
A 1V increase in bias, will reduce each EL34 current by about 10mA.
The B+ will go up.
You need to re-adjust the bias to the EL34s.

Are you going to check the Individual cathode currents of all 4 EL34 tubes?
A push pull transformer will 'love' you if you match the push and pull quiescent currents.

If you do not use the amp to power a Dyna preamp, then remove the wiring to the Octal Socket that is on the front panel.
Replace the single 15.6 Ohm resistor that goes to both cathodes in one channel.
Do the same for the other channel.
Install Four 10 Ohm resistors, one for each EL34 cathode to ground.
Connect those cathode/10 Ohm junctions to two pins on each of the front panel Octal Sockets.
Connect your DMM from ground to those 4 pins, one at a time.
Using a DMM (not an old VOM, unless it is accurate), Set the bias for 500mV (0.5V) on each cathode.
If the currents do not match, get matched tubes.
Reset the bias.
(Gone away is the Bass distortion, and gone away is the intermodulation of the Bass onto the other music notes/tones.
Global Negative Feedback can not correct for early saturation of the output transformer core that is caused by un-matched quiescent DC current in the push and pull leads).
Done.

If you still have the Black Beauty coupling caps that go to the EL34 grids, then replace them with a good 600V or 630V coupling cap.
Re-check your EL34 cathode currents. They may be unbalanced due to leaky Black Beauty (if not now, then they will in the future).

Now, sit back and relax. Listen, Enjoy!

 

[Andrew7dg]

You put in a 1N4007 in place of the selenium rectifier.
A 1V increase in bias, will reduce each EL34 current by about 10mA.
The B+ will go up.
You need to re-adjust the bias to the EL34s.

Are you going to check the Individual cathode currents of all 4 EL34 tubes?
A push pull transformer will 'love' you if you match the push and pull quiescent currents.

If you do not use the amp to power a Dyna preamp, then remove the wiring to the Octal Socket that is on the front panel.
Replace the single 15.6 Ohm resistor that goes to both cathodes in one channel.
Do the same for the other channel.
Install Four 10 Ohm resistors, one for each EL34 cathode to ground.
Connect those cathode/10 Ohm junctions to two pins on each of the front panel Octal Sockets.
Connect your DMM from ground to those 4 pins, one at a time.
Using a DMM (not an old VOM, unless it is accurate), Set the bias for 500mV (0.5V) on each cathode.
If the currents do not match, get matched tubes.
Reset the bias.
(Gone away is the Bass distortion, and gone away is the intermodulation of the Bass onto the other music notes/tones.
Global Negative Feedback can not correct for early saturation of the output transformer core that is caused by un-matched quiescent DC current in the push and pull leads).
Done.

If you still have the Black Beauty coupling caps that go to the EL34 grids, then replace them with a good 600V or 630V coupling cap.
Re-check your EL34 cathode currents. They may be unbalanced due to leaky Black Beauty (if not now, then they will in the future).

Now, sit back and relax. Listen, Enjoy!

I haven’t done that mod yet. I do have the resistors and matching tubes
Also I don’t have the black beauties but the black cats

I am going between two options
One replace the Black Cats
Replace the entire board and keep the original board in a box to preserve it

The 10ohm resistors are 1/4w
Will that be enogh?

He is still using the 5AR4, with series diodes for improved reverse voltage blocking.

Your line voltage must be high. The normal Stereo 70 amplifer's B+ after the choke should be
about 435VDC if you have an older power transformer designed for 115VAC (used on 120VAC),
or else 415VDC if you have a newer power transformer designed for 120VAC.

Otherwise the plate currents could be set too low, which will also raise the B+ up.
They should be 50mA per plate.

Good to know. I will raise the bias up to the 1.56 and see where the volts are after.

I do have the older transformer rated for 117 ac. It is the small stack os anything i can do to take a little stress of of it.

That is normal for solid state rectify, My ST-70 EL34 pin 3/4 even higher it is about 485 to 488 Volt with 40ma bias, I don't used the filter choke, but I used a big value filter capacitor.

Good to know
I have the 6ca7 EH tubes that are pretty tough IMO
I have settled down more and am more comfortable with the voltage.

 

[rayma]

I will raise the bias up to the 1.56 and see where the volts are after.
I do have the older transformer rated for 117 ac. It is the small stack os
anything i can do to take a little stress of of it.

The best thing is to set the plate currents at the proper level of 50mA.

 

[6A3sUMMER]

I thought you said you already replaced the Selenium Rectifier with a 1N4007.
That causes increased bias voltage, and will reduce the current in the output tubes.
Time to re-adjust the bias.

At 50 mA, 10 Ohms, and 0.5V
P = E squared / R
0.5 squared / 10 = 0.025 Watts.

But I do not like the wimpy body and wimpy wires of a 1/4 watt resistor.
Just wrapping the wires, soldering, etc. might stress the resistor.

I like 1 Watt resistors for cathode sense resistor service.

Black Beauties or Black Cats, I do not know the difference.
If I doubt about an old coupling capacitor, I replace it.

If you replace the B+ tube rectifier with solid state diodes, the B+ will reach Peak voltage (1.414 x RMS), before the EL34/6CA7 warms up.
That high B+ voltage can destroy 400V coupling caps.
Use 600V or 630V coupling caps.

 

[rayma]

Some output tubes will not bias properly (not enough current) when a silicon diode is used
to replace the selenium diode. The bias voltage becomes too negative. If this is the case,
add a 1k resistor in series with the silicon diode.