• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL34 Dynaco Stereo 70 fault

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Dear Forum,

I assume that my tube's gotten out, as out of nothing I heard a hum getting louder from my QUADs right channel.
I immediately turned off my amplifier. Checked the biaset's 1,56, whereas after turning on the amp, the left side stated 1,56 correctly after about 1 min, but the faulty right channel didn't stop at 1,56 but started to rise uncontrable, and around 3,00V, the 'faulty' tube's plate started to glow.

What do you think could be the problem? A friend of mine advised me to oversolder all the legs of the tube, that it could be some kind of miscontact.

Anyway, I am planning to swap all the four power tubes, so ordering a quad of el34s, but I have no idea which four should I buy.
I read pros and cons about the Russian Mullards, JJs and other manufacturers products. I have no coverage now for NOS solutions.

My question is that if you have any requirements, experience with these new EL 34's, and could suggest a manufacturer or a good NOS cost/performance ratio set for me.

Thank you for your help in advance!

101442784_1583673015143447_8440662312137785344_n.jpg
 
First thing to look at is the coupling cap to the glowing EL34.
If it is 'leaking', meaning letting through some DC to the grid of the EL34 it will cause the runaway current.
Take out the EL34s, power it on and monitor the voltage on the grid of that tube. It should stay at the same negative voltage. If it changes or goes positive: replace the cap.
 
Yes, this tube is toast. Any tube that has experienced overheating is damaged and should

be discarded.
Buying a matched quad is a wise idea, that way even if any two tubes fails during the
years of usage you have at least a matched pair left.



A have good experience of JJ EL34, i've distributed several hundreds.
 
We should check the bias voltage (pin 5) of all the EL34 also, too low the bias voltage might be caused by the dead coupling capacitors. To take such measurement, we don’t need to have all the EL34 plugin.
Yeah, the two 50 uf capacitors and the selenium rectifier producing the minus voltage to eyelets 6 & 21 are high failure parts. Particularly after 59 years. Mine failed about 1970 on my 1961 build unit. Electrolytics and selenium recifier are more failure prone than the .1 uf caps. If the .1 are still paper, might be time to move on to plastic film. Paper dielectric is not a forever part either. Selenium rectifier is a dodo, put in a 1n4002,3,4,5,6,or 7. You'll need a cinch solder terminal strip to mount it on, don't parallel a new rectifier to the old rectifier. Selenium fails low resistance backwards.Selenium rectifier is a stack of square plates if you haven't seen one. BTW the new selenium rectifier I put in 1970 was leaky again about 1978, after about 2000 hours use a year.
 
Last edited:
Moderator
Joined 2011
Measure (with power unplugged) the resistance of the current sensing resistor, in case it has been damaged by the
excessive current. If this is a stock ST-70, then it is a 15.6 ohm white resistor soldered to the tube socket tab.
There is one for each channel, shared by each pair of output tubes. You can measure its resistance at the Biaset
test points on the front (ground clip connected to the chassis), if you unplug the amplifier first.
 
Last edited:
Moderator
Joined 2011
All the ST-70s came with Mullard outputs and rectifiers. The later PAS preamps came with Telefunkens,
and before that it was Mullards. I tried to buy 100 pcs of each kit when they were discontinued in 1977,
but it was too late.
 
Last edited:
Measure (with power unplugged) the resistance of the current sensing resistor, in case it has been damaged by the
excessive current. If this is a stock ST-70, then it is a 15.6 ohm white resistor soldered to the tube socket tab.
When my unit red plated 4 new tubes for a minute due to failed selenium rectifier & leaky 50 uf caps, it didn't damage the 15.6 resistors. It also didn't damage the tubes. I think those were GE 6CA7's.
 
Thank you for all your answers, attention and help!


This is a very authentic ST 70, with the factory oilpaper caps, resistors and rectifier.
Only the ,,tower of 4 caps" were changed to a remanufactred one, and the two 50uf/75 at the bottom.
I will check the specs until the tubes will arrive
 
For some reason I also have seen EL34 as they get old they kinda develop insane grid leak..to the point the tubes will drag the bias voltage up. I have mostly seen that class A amplifiers but still could be possible if the issue follows the tube. (I have seen this not only with the EL34s but various tubes after years long service like the EL84 ECL86/PCL86 and on those tubes the bias voltage goes way off course and at best its just damaging itself and not stressing the OTP too much.)
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.