• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Pair of Dynaco Mark IV buzz. SOLVED

The Dynaco is supposed to float the heaters to whatever they want to be at based on leakage in each tube. Have to measure it to see what it actually is, but if the leakage is at the very high resistance it ought to be, even a 10M meter will load it enough to skew the measurement significantly.
 
gadget73 said:
The numbers at 2 and 6 seem awfully different and that doesn't make a lot of sense. 6 is the pentode plate, 2 is the pentode grid. Pentode grid ought to be zero volts or near enough to it. Voltage at the grid will screw with voltage at the plate, and problems with the pentode plate voltage will show up at 9, the triode grid, since that is direct coupled. That will also mess with voltages at 1 and 6.

on the 5AR4, voltage at 4 and 6 are AC volts. If you have the proper DC out, the AC input has to be correct.
Click to expand...
I just had this pointed out to me and I see now that I read the voltage chart wrong approximately eleventeen times... Total newbie mistake. I will verify that the voltages are correct AC but as you state they must be if the other voltages are correct. So my main issue now is the funky numbers at the 6GH8. Maybe I am not identifying the pins correctly?
Thanks for finding the flaw in my reading ability! LOL
 
TriodeLuvr said:
What @gadget73 said. This is a measurement error. Pins 4 and 6 are AC voltage. If they were zero volts, there would be no B+ voltage on pins 2 and 8.


The specified voltages are for a 7199. The pin numbers are different for a 6GH8. I think you're OK here.

None of this is the problem. The buzz probably has nothing to do with moving the amplifiers or where you've located them. It's more likely the previous owner's speakers were simply less efficient, so you didn't hear it. Based on your descriptions, I'm pretty sure this is heater-cathode leakage. The 6GH8 wasn't designed for hi-fi like the 7199, so this is not a tightly controlled characteristic. The first thing I would do if these were mine would be to lift the green-yel center tap wire off ground and elevate the heaters to about +40VDC. You'll need to add a resistive divider for this, but it's not too difficult.

Here's an excerpt from the Valve Wizard site:

View attachment 2704968

Jack
Click to expand...

Hey Jack thanks for that insight. I will do a bit of studying on this topic. Does doing the lift act similar to when I contact either the 4 or 5 pin on the 6GH8 with my volt meter and the noise drops significantly?

Thanks for you rinsight.
 
Fasturd said:
Does doing the lift act similar to when I contact either the 4 or 5 pin on the 6GH8 with my volt meter and the noise drops significantly?
Click to expand...
I don't think touching the filament pins with your voltmeter should do that. You might want to verify that the center tap of the filament winding is connected to ground. In any event, elevating the winding is the next step.

Jack
 
Unless I am reading something incorrectly "again" I still have funky readings on the 6GH8 at pin 2 which should be 140 DC and I get a reading of -71.2 MV and pin 6 which should be 1.5 DC and I get 108.5 DC...

SERENITY NOW! lol.
 
TriodeLuvr said:
I don't think touching the filament pins with your voltmeter should do that. You might want to verify that the center tap of the filament winding is connected to ground. In any event, elevating the winding is the next step.

Jack
Click to expand...
Jack I apologize, I am REALLY new to this and I am not following the idea of the "center tap". Does that refer to a specific pin on the tube or are we discussing how the tube relates to the power take off socket? As I stated earlier I have to go read up on the whole "lift" thing.
 
Yes, you're misreading it. Pin 2 of the 6GH8 is the grid of the input section. Pin 6 is the anode.

6GH8 pinout.jpg
 
Fasturd said:
Jack I apologize, I am REALLY new to this and I am not following the idea of the "center tap". Does that refer to a specific pin on the tube or are we discussing how the tube relates to the power take off socket? As I stated earlier I have to go read up on the whole "lift" thing.
Click to expand...
The center tap is the green-yellow wire of the power transformer. You might want to see if someone local can provide some hands-on help with this.
 
Jack, Another illustration /reading comprehension issue gone bad? . I was looking at a chart that went clockwise from 7PM as #1 to 5 PM being #9. Does your chart show the order of the pins or just a reference to what they are tied to? I love learning this stuff but I don't want to wear out my welcome!
 
Okay I am reading them in the correct order. I am going to check the values again and check the grounds.
 
TriodeLuvr said:
I have a silly question. Do these amplifiers work?
Click to expand...
Yes they work and sound great other than the noise!
I have vintage JBL C37s, a Peerless 3 way set up and other vintage efficient speaker sets and the buzz at low volume is too distracting. The fact that both amps do it makes me think something was missed by the previous owner.
 
As other has said, the DC level of the filements could affect this. AC leakage between filaments and cathode
could create a "buzz" ( not a low frequency hum but a noice that comes back 50 or 60 times per second)
The filaments in MK IV is ( should be according to the manual) grounded at the center tap of the filament winding.

This problem might be "solved" by using other rubes ( 6U8, 6GH8 or similar) , it could also be fixed by
raising the filament to 40 - 80 V DC, this thread has a description about how to do this. ( post #20) One could also
feed the board (only) with DC but that involves more.

7199 was specifically made to supress this buzz.
 
Last edited:
TriodeLuvr said:
The center tap is the green-yellow wire of the power transformer. You might want to see if someone local can provide some hands-on help with this.
Click to expand...
I get where you are going with the local help but zI really need to learn this! I am following safety rules and I read as much as zI can before any random poking!

The green and yellow tap from the transformer currently goes to its own lug on the 5 lug bar where all the bias bits are.
 
peterh said:
As other has said, the DC level of the filements could affect this. AC leakage between filaments and cathode
could create a "buzz" ( not a low frequency hum but a noice that comes back 50 or 60 times per second)
The filaments in MK IV is ( should be according to the manual) grounded at the center tap of the filament winding.

This problem might be "solved" by using other rubes ( 6U8, 6GH8 or similar) , it could also be fixed by
raising the filament to 40 - 80 V DC, this thread has a description about how to do this. ( post #20) One could also
feed the board (only) with DC but that involves more.

7199 was specifically made to supress this buzz.
Click to expand...

Just an FYI multiple matched sets of 6GH8 exhibit the same symptom. Do it looks like corrective action will be needed. And this info goes for the other amp as well.

As you described it is definitely a buzz not a hum.
 
So after being enlightened on the fact that I mid read the voltage chart it looks like my GZ34 and the EL34s are doing what they are supposed to be doing. All voltages are correct.

The issue now seems to be pin 2 and 6 of the 6GH8. Pin 2 should be 140 DC and I get -71.2 Mc and a loud buzz. Pin 6 should be 1.5 DC and I get 108.5 DC
 
You must log in or register to reply here.
Back
Top Bottom