Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Aug Fri 23, 2019 3:25 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 14 posts ] 
Author Message
 Post subject: Dynaco Mark II Problem
PostPosted: Nov Fri 10, 2006 6:35 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2334
Location: Seattle, WA, USA
Hi,
My Dynaco mark II monophonic amp is showing a definite orange glow on the plates of both output tubes. The unit has been fully recapped and all of the out of spec resistors have been replaced. I have set the bias per the manual. Both cathodes are wired together and are connected to B- through a 12 ohm 1% resistor. The voltage drop across this resistor is measured to set the bias point. I have set the bias, as the manual instructs, for 1.56v across this resistor, which comes out to 130mA cathode current for the two 6AC7 output tubes. Still, the plates of both tubes glow orange at the seam. To make this go away, I have to turn the bias almost all of the way down, so that less the voltage drop is less than 0.5v. Any ideas what is wrong here?

_________________
If it ain't broken, you’re not trying hard enough...


Top
 Profile  
 
 Post subject:
PostPosted: Nov Fri 10, 2006 2:27 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5279
Location: Rochester NY USA
What brand of EL34s are they? Some of the Russian ones aren't really good for 25W. SED (the old Svetlana) are good, I hear.


Top
 Profile  
 
 Post subject:
PostPosted: Nov Fri 10, 2006 2:45 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 564
Location: Wisconsin
It's also possible that you have a runaway oscillation. Put a scope on the output and verify.

You might be able to pull the 6AN8. If there's an oscillation, that might kill it.

Tom


Top
 Profile  
 
 Post subject:
PostPosted: Nov Fri 10, 2006 8:21 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2334
Location: Seattle, WA, USA
Thanks for the replies. I have not tried the 'scope yet, but pulling the 6AN8 made no difference what so ever. As for the tubes, I am still using the original RCA/Mullard 6CA4's. I should note that both fail the gas test on my Hickok 752A. Not very badly, only about twice as much as the cutoff, but it is still there. Could this cause my problem? I also tried another pair of identical RCA/Mullard 6A64's with the same results. When I tested them, they also tested gassy, one much more so than the other, and the one with more gas definitely had brighter glowing plates. Could this be my problem? If so, how would I verify that it is?

Thanks,
Matthew D'Asaro

_________________
If it ain't broken, you’re not trying hard enough...


Top
 Profile  
 
 Post subject:
PostPosted: Nov Fri 10, 2006 8:47 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5279
Location: Rochester NY USA
Well, Mullards should take 25W, no problem. But they might be seeing more... is that 1% resistor STILL 12 Ohms? Gas or grid leakage will often make tubes draw more current- a good test is to ground the grid and see if the current drops...


Top
 Profile  
 
 Post subject:
PostPosted: Nov Fri 10, 2006 9:19 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2334
Location: Seattle, WA, USA
I grounded pin 5 of both EL34's. During warm-up the voltage drop across the resistor reached 6v and was still rising. I shut it down immediately to prevent damage. Just to make sure we are on the same page, the schematic for this thing can be found at http://www.geocities.com/vintageaudio/markii.JPG

Thanks,
Matthew D'Asaro

_________________
If it ain't broken, you’re not trying hard enough...


Top
 Profile  
 
 Post subject:
PostPosted: Nov Fri 10, 2006 10:37 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 369
Matthew,you can't ground pin 5 on the El34's because you
are also killing the bias voltage.The tubes could be bad
but you may have other issues.Start by measuring the
B+ voltage going to the center tap on the output transformer.
What kind of rectifier tube are you using?In that amp it
should be a 5U4GB.Do not use a 5AR4 because you will
get excessive B+ that the EL34's are not happy with.
What is your line voltage?These amps should not be operated
with more than 120 VAC.If it is higher than 120,you need to
use a variac or an autoformer to lower the voltage.The ideal
line voltage for this amp is 117VAC.The B+ voltage at the
output transformer center tap should not be higher than
450 VDC.I have a feeling that you are going above the
25W rating of the EL34's.The slight gas in the tubes could
cause this problem when you are at or over the max. dissipation
rating.Make sure the B+ is correct before you further evaluate
the tubes.You are pulling 65MA through each EL34 even if the
B+ is 450 VDC you are over the dissipation limit.You should
only be pulling no more than 50MA through each tube.You should
set the bias to 1.20VDC.EL34's simply can not handle that level of
power without showing red spots at the plate seams.
Regards,
Swanson


Top
 Profile  
 
 Post subject:
PostPosted: Nov Fri 10, 2006 11:03 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5279
Location: Rochester NY USA
I was think of a cathode biased amp when I suggested grounding the grid... with fixed bias you could short the 100K grid resistor and see if the current drops - if so grid leakage is excessive. But regardless of the cause, dissipation is too high if plates are glowing.

Have you measured that 12 Ohm resistor? I agree that 65 mA is too much - .065 A x 465 V is 30W per tube... 25W is the plate rating. (screen can take another 7W, but you don't know how the cathode current is divided between plate and screen)


Top
 Profile  
 
 Post subject:
PostPosted: Nov Sat 11, 2006 12:21 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 369
I can think of at least one other vintage HI-FI amp
that really beats up EL34's and that is the Eico HF60.
Eico also insisted on 65ma per EL34 but the B+
voltage can routinely go over 460VDC because
of the use of a 5AR4 rectifier and high line voltage.
At those operating parameters you are better
off plugging in some 6550's or KT88's.
Regards,
Swanson


Top
 Profile  
 
 Post subject:
PostPosted: Nov Sat 11, 2006 1:32 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2334
Location: Seattle, WA, USA
B+ is a whopping 540v, even when I run it through a variac set to 117v. What gives? I have replaced all of the power supply capacitors, could that do it? The rect. tube is a plain old 5U4GB. It tests good. And yes, I have checked the 12 ohm resistor - it is within 1%.

Thanks,
Matthew D'Asaro

_________________
If it ain't broken, you’re not trying hard enough...


Last edited by 7jp4-guy on Nov Sat 11, 2006 2:20 am, edited 1 time in total.

Top
 Profile  
 
 Post subject:
PostPosted: Nov Sat 11, 2006 1:40 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2334
Location: Seattle, WA, USA
I take that back. If the bias is turned all of the way down, B+ is 550. If bias is set to exactly 1.56v, as the manual says it should be, B+ is exactly 465v, also exactly what the manual says it should be.

Thanks,
Matthew D'Asaro

_________________
If it ain't broken, you’re not trying hard enough...


Top
 Profile  
 
 Post subject:
PostPosted: Nov Sat 11, 2006 3:54 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 564
Location: Wisconsin
It's possible those output tubes have an unusually high transconductance... but not likely.

Check your bias circuit... make sure that the grids of the tubes are getting clean DC.

You can do that with the tubes pulled, there's no appreciable current in the bias circuitt.

Tom


Top
 Profile  
 
 Post subject:
PostPosted: Nov Sat 11, 2006 5:59 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5279
Location: Rochester NY USA
Well, you could do what Dynaco did - put in a pair of 6550s and a 5AR4 and call it a Mark III... Most people bias the Mark II lower than the factory setting, as it is right at the limit of tube ratings. I don't know if you could hear the difference if it's biased at 50 mA per tube, and Mullard EL34s aren't cheap. The Stereo 70 is biased at 100 mA per pair.


Top
 Profile  
 
 Post subject:
PostPosted: Nov Sun 12, 2006 6:13 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 564
Location: Wisconsin
That's a good recommendation. It doesn't need to be biased that far into class A, especially if you're not playing it loud.

Tom


Top
 Profile  
 
Post New Topic Post Reply  [ 14 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 3 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB