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New Tubes (6CA7)--now bias too high

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Posted on February 8, 2016 at 17:15:07
daveap
Audiophile

Posts: 13
Location: nj
Joined: February 13, 2005
Hello:

I have an ST-70 with Joe's restoration board and quad-cap replacements that has been serving me well for several years. I currently have an original (or old) set of Mullard EL34s and a newer set of EH EL34s, both of which function fine and are easily biased somewhere in the middle of the bias pot's rotation. Wanting to try some different tubes, I ordered a matched set of 6CA7s from Antique Electronic supply. I eagerly popped them in this evening and went through the process of setting bias; however, after several seconds the voltage reading at the "biaset" point climbs well above 2.5vdc even with the bias pot turned all the way counterclockwise. I haven't pulled the bottom cover yet to take measurements, but verified that the 15.6 ohm resistors read 1.57 ohms (within reasonable tolerance). Anyone have any recommendations? Does this seem like it could be remedied through Joe's bias expansion mod or are these tubes pulling too much current? Your suggestions are greatly appreciated.

-David

 

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RE: New Tubes (6CA7)--now bias too high, posted on February 9, 2016 at 15:49:23
petercapo
Audiophile

Posts: 460
Joined: December 29, 2012
You should be able to bias those without any difficulties.

Does the bias reading come down after the several seconds have passed?

Bias reading problems could have different causes. The first thing I think of is cleaning and retensioning socket contact #5 for the four power tubes. I suggest 99% isopropyl, interdental brush of suitable size, and compressed air to promptly blow the alcohol out of the contact after scrubbing. To retension the socket contacts, press on the top and bottom of the "C," not directly opposite the split in the "C." If you have a look at the contact, I think you'll see what I am referring to (assuming you have the original sockets).

Of course, this might not be the cause, but bias problems are often enough caused by contact issues with #5.

 

RE: New Tubes (6CA7)--now bias too high, posted on February 10, 2016 at 17:04:47
daveap
Audiophile

Posts: 13
Location: nj
Joined: February 13, 2005
Thanks, Peter. I didn't think there should be any bias issues either as these are touted as a drop-in replacement for EL-34's. Unfortunately, the reading does not drop after a few seconds and continues to rise slowly. I haven't kept the amp powered on for more than a few minutes with these for fear of damaging something. I replaced the tube sockets a couple of years ago with ceramic ones. When I swap EL34s back in I am able to bias them fine. I'll try you re-tensioning trick.

 

RE: New Tubes (6CA7)--now bias too high, posted on February 10, 2016 at 17:19:10
petercapo
Audiophile

Posts: 460
Joined: December 29, 2012
Cleaning and retensioning contacts #5 is still worth a try.

But, the problem might be that the tubes are just drawing too much current. Antique Electronic Supply serves a lot of musicians who have different kinds of qualifications for their tubes vs. for audiophile use. Did you mention to them it was for a hi-fi amp and not a musician's amp? Hopefully AES will do right by you in this, if it ends up being the case.

 

RE: New Tubes (6CA7)--now bias too high, posted on February 11, 2016 at 13:19:56
daveap
Audiophile

Posts: 13
Location: nj
Joined: February 13, 2005
Thanks again, Peter. I cleaned and re-tensioned #5 to no avail. In doing a little research, it appears that a subset of the current production EH tubes that pass factory spec draw a high amount of plate current and cannot be biased in most EL34 amps. Tubestore.com apparently culls these and sells them at a discount. (see here: http://www.thetubestore.com/Tubes/EL34-6CA7-Tube-Types/Electro-Harmonix-EH6CA7-HPC ) The only thing I can think of is that I have a set that draws high current. If this is the case and, if I get stuck with the set, would it be possible to decrease the current by adding resistance on the B+ side?

 

RE: New Tubes (6CA7)--now bias too high, posted on February 11, 2016 at 14:15:10
petercapo
Audiophile

Posts: 460
Joined: December 29, 2012
Not something I have delved into for tubes having this issue. However, if you were to somehow make adjustments to the circuit in order to bias the 6CA7s, you might not then be able to bias your other EL34s. Not sure if there is a way to have a wide enough biasing range to accommodate all of them. Don't know if Joe's bias expansion mod or his bias control module can do it or not.

 

RE: New Tubes (6CA7)--now bias too high, posted on February 15, 2016 at 14:24:10
xaudiomanx
Audiophile

Posts: 2947
Joined: August 16, 2004
My suggestion would be to put back a set of the old tubes and see what your readings are. If they read fine you can still use your new tubes but you might have to change the bias resistor to a lower value on the bias power supply to let more bias voltage into the tubes. I've had to do that on various occasions with different EL-34's.

I don't know if Joe's bias mod will correct the issue. He would know more than me on the subject but I would start with a simple thing first and work from there.

 

RE: New Tubes (6CA7)--now bias too high, posted on February 15, 2016 at 17:49:02
xaudiomanx
Audiophile

Posts: 2947
Joined: August 16, 2004
I know I am not in this discussion but as I wrote before. I've had NOS tubes that didn't bias correctly in some amps. The fact that your older set does bias properly just tells me that these particular set of tubes might need something as simple as a smaller bias resistor on the power supply. The resistor after the diode that has a cap on either side. Usually they are about 10 or 18K depending on the amp.

Also, if you can, check the bias voltage at pin 5 on each set of tubes. If they are all in the same range then it could just be the particular tubes they sent.

As far as what Peter said. I never heard of a particular set of tubes for musicians and a different set for regular music listening. I have ordered many sets of tubes for either type of use and I just use whatever was sent as long as they were matched. My particular source for all my tube needs is Jimmy McShane, especially when I need a closely matched set of tubes, like for the Dynaco. He would also be able to tell you if your tubes might be somewhat defective.

Also, individual bias pots is a great addition to any amp that is now using paired tube biasing.

A good friend/tech also told me that as long as the plates don't glow cherry red you aren't damaging the tubes and Dynaco is know for running tubes at the point of destruction with no conservation at all in an effort to get as much power output as it can. I don't know if that's true or not but I have to go with what others tell me when I am not sure. The only thing I can think of that might be compromised is the power transformer going way past its limitations as it is the weak link of any Dynaco amp.

Also, if you really want to hear the amp sing get yourself another one and use one channel from each(not bridged). Just use either the right channel or the left from each amp(doesn't make a difference). In essence you decreased the demand on the amp and doubleing the power supply without doing a thing, and that makes the power transformer work less. You can pull the power tubes out of the non-used side but if you do that make sure it is the side that doesn't power up the 7199 tube. That will help the power tranny to run a bit cooler as well. But again, not bridged.

And also remember, these amps were designed when the power source was 115AC. We now have generally 125AC. So other power supply issues arise because of the higher source voltage.

Sorry for butting in but I wanted to try and see what others say on the subject. Another good place to get others to give advice that I rely on heavily is DIY Tubes. Jimmy McShane, as well as very smart others are more than glad to help. I use them all the time for advice and they are usually correct when I present myself correctly.

Just my "0" cents!

 

RE: New Tubes (6CA7)--now bias too high, posted on February 16, 2016 at 15:04:04
daveap
Audiophile

Posts: 13
Location: nj
Joined: February 13, 2005
Thanks, xaudiomanx. I think your initial suggestion was where I am headed, i.e., as I understand it, the positive bias supply has a resistor after the rectifier so, perhaps, increasing this value would reduce the current on the positive side allowing these higher-current-drawing tubes to be biased correctly. I thought, maybe someone had a similar issue and performed a similar fix. I definitely appreciate the feedback and suggestions. I think I will see if I can return these tubes and purchase another set (hopefully they will take them back) elsewhere before performing any changes to the bias circuit.

-David

 

RE: New Tubes (6CA7)--now bias too high, posted on February 16, 2016 at 17:32:24
xaudiomanx
Audiophile

Posts: 2947
Joined: August 16, 2004
If I read you correctly you mentioned to use a larger resistor. You have to use a smaller one to let more negative voltage go to pin 5 of the EL-34's. After the bias diode the voltage them becomes negative. The orientation of the resistor is not an issue but putting the caps in properly is. You have to remember that it is negative DC voltage after the diode so the negative of the cap goes to the proper pins and the positive goes to ground.

I would also try and contact someone from DIY Tubes and see if they have a handle of why this happens. All the guys there are very smart and are great to correspond with.

 

RE: New Tubes (6CA7)--now bias too high, posted on May 5, 2016 at 04:56:04
jsblock
Audiophile

Posts: 16
Joined: November 28, 2001
One other thing to check is your line voltage. Ours recently changed from 117 to 125, and I had to change the bias resistor to compensate.
Your EL34's bias ok, so maybe this is not the issue, but it's worth checking.
I just made a bucking transformer to bring the line voltage back down to 110. Now the bias voltage is a lot lower.

 

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