Need Help Getting Bias Right For KT66 On An Original ST70

First time posting here, long time Dynaco owner. I just got a good deal on some gold lion kt66's and did the modifications to get the bias to adjust properly while running them, so I thought, something is wrong. Started up great, super quiet, Bias all the way down, then got a quick flash in the rectifier tube and hit the power immediately.

I'll try and include all relevant information, I apologize if I leave anything out, feel free to tell me! Early st70, original wiring, Ive replaced the selenium with a diode, quad cap, maybe a few other things. I read that I should replace the 4 270k resistors on the far right and left side of the board with 100k, did that. Did I need to replace the other 2 270k resistors as well? Also read that I should wire two 15k resistors parallel to the ten k's on the terminal strip, did that and replaced the caps while I was at it.

I will open it up and check my work tonight, take pictures etc. All the joints looked and felt great. When I first got the tubes I tried them with the original wiring, bias all the way down and could only get them to about 1.9 vdc, the tubes seemed fine but after about five minutes the rectifier wasn't looking so good, or sounding so good. Any chance its a bad rectifier?, seemed to be working okay with the old mismatched el34's. I'd love to get my amp back up and running, listened to it almost every day for the last fifteen years, any help is greatly appreciated. I'll post more tonight if I discover anything.

evangerardo wrote:    First time posting here, long time Dynaco owner. I just got a good deal on some gold lion kt66's and did the modifications to get the bias to adjust properly while running them, so I thought, something is wrong. Started up great, super quiet, Bias all the way down, then got a quick flash in the rectifier tube and hit the power immediately.

   I'll try and include all relevant information, I apologize if I leave anything out, feel free to tell me! Early st70, original wiring, Ive replaced the selenium with a diode, quad cap, maybe a few other things. I read that I should replace the 4 270k resistors on the far right and left side of the board with 100k, did that. Did I need to replace the other 2 270k resistors as well? Also read that I should wire two 15k resistors parallel to the ten k's on the terminal strip, did that and replaced the caps while I was at it.
   
   I will open it up and check my work tonight, take pictures etc. All the joints looked and felt great. When I first got the tubes I tried them with the original wiring, bias all the way down and could only get them to about 1.9 vdc, the tubes seemed fine but after about five minutes the rectifier wasn't looking so good, or sounding so good. Any chance its a bad rectifier?, seemed to be working okay with the old mismatched el34's. I'd love to get my amp back up and running, listened to it almost every day for the last fifteen years, any help is greatly appreciated. I'll post more tonight if I discover anything.

The 270k to be replaced is the two that connects to eyelet '6' or '21' on the board, these are grid resistors.
Don't replace the plate resistor (that is connected to the eyelet '19' )

Okay, took out the 15k that goes to ground, left the one between caps, checked all connections, all good. put in old el34's bias all the way down blue arcing in rectifier and static at warm up. My next move would be to switch the 100k's back to 270 and see if its better, I don't have a spare rectifier tube, I'll try and get one. what do you guys'd think about using a 5r4 with original wiring, probably would have been easier than the troubleshooting.

Thanks Peter, I replaced all 4 on the sides of the board but not the other twostill getting blue flashing, should I try and ride it out and see what happens, audible static too. don't want to replace the rectifier if its not that, maybe ill try and go basck to the original wiring, the new caps are 60 uf 100v, thanks for your response.

Well made another bad call, decided it must be the 5ar4, replaced it, powered on, got a little static then a blue flat and pop from the underside of the tube, forty bucks down the drain just like that. The new bias caps I put in are 60 uf 100v, thought that should be good but maybe not. only thing I can think of is rebuilding the bias circuit from the ground up, I haven't replace the multi cap on it yet, and going back to the original wiring, maybe that will help me figure it out.

Okay, first successful troubleshooting so far. Removed the new caps put in old ones, powered on with only one channel, old tubes, old rectifier, no flashing. Was 60uf 100v to much? I would definitely appreciate any expert guidance still. I'm going tot try the other channel, the new rectifier then Finlay the new tubes next.

If you're using the original resistors I would suggest measuring them ( or replace with new).
That's all I've got , sorry.

When the rectifier went pop , did it leave a carbon trace on the tube socket? The rectifier is being asked to flow too much current , have to find out why. Is it capable of supplying what the KT66 are asking for?
Maybe measure the current coming from the rectifier. Do have access to a tube tester?

Check the first filter cap after rectifier.
If the capacity max is 60 uf if  if this exceeds the rating The rectifier will arc.

Swapped out new caps for old ones and it's fine. Haven't had a chance to test the new capacitors but from what I understand they should have worked fine. Biases perfectly now, Kt66 working great. Any ideas on what could cause this beside bad caps. I saw on audio karma someone had the same problem but everyone assumed he had the diode in backwards. Also I have some transformer hum, it's real old so... but what components should I troubleshoot to try and eliminate, Filter caps and bias caps, anything else? Is there room under the hood for individual filter caps to make trouble shooting clearer? Thanks!

Thank you guys, no carbon traces and rectifiers still good(ish). Need to take it apart and do some proper testing tonight. What values should be used for bias filter caps and the small disc cap in the bias circuit? Getting all kinds of conflicting stuff from the net

The "yellow sheet" diode mod for the rectifier in the Dynaco ST70 is supposed to reduce the voltage that the rectifier tube gets.  I have put this mod in 3 Dynaco ST70s and have not had any issues with the Flash over in the rectifier tube.  I originally did the mod because of a flashover in my personal Dynaco ST70 and have not had any issues since.



I found a link on this forum with the image as well:

https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-yellow-sheet-diode-mod

IMO, this is wrong.

The purpose of the diode is to prevent current flow from the capacitors through the transformer in the event of a valve rectifier failure.

There should be one forward biased diode between pin 8 and the capacitors.

I disagree and so does Bob. If you look at the link, he even has a better looking diagram of the same circuit.

Thanks, I had already done the yellow sheet mod.

arremondo wrote:I disagree and so does Bob.  If you look at the link, he even has a better looking diagram of the same circuit.  

On what electrical basis do you disagree?

Each time the part count increases, so does the failure probability.

Also, IMO, if one is going do it, it could be a 1n5408 3A/1000PIV which has 200A peak surge forward current as opposed to 30A for 1n4007

I have a question about biasing a tube amp - such as the Dynaco. Given that the bias will fluctuate with an audio signal going through it due to changes in incoming voltage and changes in the perceived load presented by the speakers, I have two questions:

1. Is it better to bias while connected to speakers or by placing a suitable resistor across the speaker posts of the amp?

2. What size / value (ohms / watts) should that resistor be?

Thanks in advance.

Thx1326 wrote:I have a question about biasing a tube amp - such as the Dynaco.  Given that the bias will fluctuate with an audio signal going through it due to changes in incoming voltage and changes in the perceived load presented by the speakers,  I have two questions:

1. Is it better to bias while connected to speakers or by placing a suitable resistor across the speaker posts of the amp?

2. What size / value (ohms / watts) should that resistor be?

Thanks in advance.

Bias in this type of amp is done at idle. Speakers or resistors as load makes no difference.

Multiple sources have stated that you should NOT run a tube amp with no load.

So... now I'm confused. All state that damage to the output transformers or the tubes could occur.

I recently did the resistor mod and I had a dummy load hooked up to the speaker outputs (you could also connect the speakers) when I set the bias; just don't put anything on the inputs when you set the bias.  

The resistors that I used are 15K ohm but you could use 10K ohm also (installed in parallel).  I couldn't find a wattage so I used a one watt resistor.  They are smaller than those that were already in there so if I did it again I would use a larger one.  Below is a pic of the mod I did:

Thx1326 wrote:Multiple sources have stated that you should NOT run a tube amp with no load.

So... now I'm confused.  All state that damage to the output transformers or the tubes could occur.

This is correct.

To check bias (prior to using an auto bias board) I played my ST 120 for about half an hour to warm everything up. Then with my source still on and nothing playing, I checked the bias.

I have done it with large wire wound load resistors connected to the amp outputs and shorting plugs on the inputs. It’s a lot less convenient. I’m not sure of the resistor value as they’re put away now.

Found them - maybe you can look these up on the web. A former forum member gave them to me

The pic is in the next post-

BIAS controls the DC IDLE current through the tubes. The music signal adds / subtracts to the bias to produce AC which the transformer changes from x00VDC to something the speakers can handle

Transformers DO NOT PASS DC. Hence there is no need to use a load resistor in place of the speakers.

A dummy load may be a good idea on first start or after tube replacement in the unlikely event a tube is bad, burns out the OPT and puts B+ on the speaker.


As simple as possible and no simpler!

SEAMUS: A quick search and I find that several sources disagree with you.  Tube amp are different than solid state amps.  You can damage the transformers, tubes or circuits if you run it without a load (speakers or a dummy load).  Solid state amps don't care if there is a load or not but tube amps do.  

EVANGERADO: I purchased some dunny loads from eBay for a reasonable price also.