my dynaco's one tube not lighted up

Hi all
I'm totally noob to tube amp.
And my first one is dynaco st-70 - rebuilt by will vincent.
When I powered up the amp, all tubes except one (probably el34)
tube located on the right of the amp not lighted up att all.
However, it still is playing wonderfull sound as well.
I just want to confirm that the tube that not lighted up is not problem?
or is that a problem?
Thank you very much.

A quick test you can do to prove the tube is bad is to swap the unlit EL34 with a lit one. If the filure stays with the tube then it is the tube. If the failure stays on the socket you will need to figure out what is wrong on the socket.

Thank Bob and ruffian. You really helped me a lot.

ruffian wrote:A quick test you can do to prove the tube is bad is to swap the unlit EL34 with a lit one. If the filure stays with the tube then it is the tube. If the failure stays on the socket you will need to figure out what is wrong on the socket.

I already checked as you mentioned. The result is the tube is bad, not the socket.

Just to make sure: If I replace the bad tube with the new one on my dynaco st-70. I must do bias for the new tube, right? Not just plug it in then play, right?
Maybe i'm wrong but I heared that will vincent's restored st-70 amp have the auto bias feature.
thanks you very much

Thank you, Bob. You are very kind.

Bob Latino wrote:Yes - If you replace one output tube you will have to rebias BOTH channels.
Bob

Butthe strange problem is that when I opened my front cover of my dynaso st 70, the bias pin on front socket, to which I will connect the wire of my multimeter, is not there. And the bias pin it not in the back or in the upper chassis also.
My dynaco st 70 is not stock one but rebuilt by Will Vincent. So how do I measure volt of my newly replaced kt77 tube?
You can see the image below (it's not mine but very as same as mine).
http://cdn.head-fi.org/7/74/7462d91c_dyna70.jpeg
Please help me. I need your help. Thank you.

Well I have one of Will's amps as well...


Looking at your picture, the 2 white jacks in front of the power transformer(big one in the middle) are where you will place the red lead of your meter to measure the bias. The black lead of the meter, you will place somewhere on the amp chassis for ground.

Next to each of the white jacks, you will see a slotted knob. Those are what you turn to adjust the bias.(Turning left lowers the bias, turning right raises bias)

To start, I would turn each one a very little to the left to lower the bias some.
Turn the amp on and let it sit for about 10-15 mins to warm up some.
After the 15 mins, check the bias voltage(make sure your meter is set to DC voltage) and adjust it accordingly.
IMO, for that amp, 1.45-1.5v is good.

The left jack and knob are for the left power tubes and the right jack and knob are for the right power tubes.
Make sure to have speakers plugged into the amp but do not have anything playing at the time of adjustment.


Just an FIY and IMHO....I find that the EH 6ca7 Fat boys work and sound GREAT in that amp!!

Hope this helps you out!

-Vic

thevic24 wrote:Well I have one of Will's amps as well...


Looking at your picture, the 2 white jacks in front of the power transformer(big one in the middle) are where you will place the red lead of your meter to measure the bias. The black lead of the meter, you will place somewhere on the amp chassis for ground.

Next to each of the white jacks, you will see a slotted knob. Those are what you turn to adjust the bias.(Turning left lowers the bias, turning right raises bias)

To start, I would turn each one a very little to the left to lower the bias some.
Turn the amp on and let it sit for about 10-15 mins to warm up some.
After the 15 mins, check the bias voltage(make sure your meter is set to DC voltage) and adjust it accordingly.
IMO, for that amp, 1.45-1.5v is good.

The left jack and knob are for the left power tubes and the right jack and knob are for the right power tubes.
Make sure to have speakers plugged into the amp but do not have anything playing at the time of adjustment.


Just an FIY and IMHO....I find that the EH 6ca7 Fat boys work and sound GREAT in that amp!!

Hope this helps you out!

-Vic

You made my day. Your explanation is very clear and easily understandable.

vuthecuongjapan wrote:

thevic24 wrote:Well I have one of Will's amps as well...


Looking at your picture, the 2 white jacks in front of the power transformer(big one in the middle) are where you will place the red lead of your meter to measure the bias. The black lead of the meter, you will place somewhere on the amp chassis for ground.

Next to each of the white jacks, you will see a slotted knob. Those are what you turn to adjust the bias.(Turning left lowers the bias, turning right raises bias)

To start, I would turn each one a very little to the left to lower the bias some.
Turn the amp on and let it sit for about 10-15 mins to warm up some.
After the 15 mins, check the bias voltage(make sure your meter is set to DC voltage) and adjust it accordingly.
IMO, for that amp, 1.45-1.5v is good.

The left jack and knob are for the left power tubes and the right jack and knob are for the right power tubes.
Make sure to have speakers plugged into the amp but do not have anything playing at the time of adjustment.


Just an FIY and IMHO....I find that the EH 6ca7 Fat boys work and sound GREAT in that amp!!

Hope this helps you out!

-Vic

You made my day. Your explanation is very clear and easily understandable.

No worries!! Just take your time. The adjustment knobs are sensitive so go slow when you adjust the bias.

May I ask what tubes you are using in the amp?

I have EH 6ca7's for power tubes, Philips GZ34 rectifier and Sylvania 7199's.
I sometimes swap out the 7199's for 6gh8a's though...

-Vic

>May I ask what tubes you are using in the amp?
I use 4 jj EL34 for power tubes , and GZ34 rectifier and NOS 7199.

vuthecuongjapan wrote:>May I ask what tubes you are using in the amp?
I use 4 jj EL34 for power tubes , and GZ34 rectifier and NOS 7199.

This is just my opinion but it your GZ34 is a new one and not NOS, I would be thinking of replacing it soon.

Most of the new GZ34's are not that great and can cause problems, that and they just do not seem to last very long.
NOS GZ34's can be quite expensive but are worth it. IMO, you should look into getting a Weber Copper Cap, IIRC the WZ68 in place of the GZ34 and it will work perfect in that amp.

Of course, if you can get your hands on a nice, NOS GZ34(Mullard, Philips, ect)it should last a lifetime!

Good luck and enjoy your amp!!

Let us know if you have any other questions!

-Vic

thevic24 wrote:

vuthecuongjapan wrote:>May I ask what tubes you are using in the amp?
I use 4 jj EL34 for power tubes , and GZ34 rectifier and NOS 7199.

This is just my opinion but it your GZ34 is a new one and not NOS, I would be thinking of replacing it soon.

Most of the new GZ34's are not that great and can cause problems, that and they just do not seem to last very long.
NOS GZ34's can be quite expensive but are worth it. IMO, you should look into getting a Weber Copper Cap, IIRC the WZ68 in place of the GZ34 and it will work perfect in that amp.

Of course, if you can get your hands on a nice, NOS GZ34(Mullard, Philips, ect)it should last a lifetime!

Good luck and enjoy your amp!!

Let us know if you have any other questions!

-Vic

thank you for this info. by the way if I replace the GZ34, for example with new NOS GZ34, do I still need to bias the new gz34, or adjust the bias is just for power tube only?
thank you.

Yes, you will need to check and most likely readjust the bias after changing the GZ34.

And with new power tubes, you will want to check the bias after about 20-50 hours. As the new tubes "burn in" the bias may drift a little. After abut 50-100 hours, they should be good to go.

After that, you should check the bias every month or 2.

Is it just me, or is anyone thinking that a restorer as well known as Will Vincent should perhaps provide his customers with a bit of support or at the least an instruction manual covering something as basic as a power tube failure? Anybody searching "Dynaco" on Audiogon will run into his amps and it's been that way for years now. His restorations are high quality and his powder coated finishes are top notch, but he makes minor changes to chassis test points and general circuit that will cause his amps to differ from standard Dynaco instruction manual. If your going to sell $1000 ST-70's, the least that you can do is teach your customers how to work them. Just my two bits.

He, (OP) may have bought the amp used...

When I had mine built by Will, he went over everything with me that he did to the amp. He did include a diagram of where the test points were and what to set them to.

So I am thinking the OP got the amp used...


But hey...thats is what we are here for if all else fails!!

-Vic