Pair of Dynaco Mark IV buzz. SOLVED

[Fasturd]

New to the Dynaco world. My new to me Mark IV amps have an identical buzz that is too loud to ignore.

They were partially updated prior to my ownership and did not buzz at the sellers home. I do not know the make of the boards yet but they are currently running 1 Sylvania 6GH8A on each board.

Both amps exhibit the same exact buzz through their respective speaker. This is not influenced by the preamp/volume it stays consistent.

I have done the following:
Set bias to 1.56 however one amp goes no lower than 1.56.

I have pulled the power plugs and flipped them, no change.

I have changed the outlets, power conditioner and shut off every device in the house. No change.

If I pull the RCA plugs they also get a hum but the buzz stay the same.

If I pull each 6GH8A tube everything is dead quiet. I read this is a potential telltale of what the issue might be but I can't find the article to find out what that is...lol


( I have since swapped in a vintage GE tube that lowers the noise significantly, I am assuming something about the Sylvania 6GH8 is causing the noise, new tubes will be on order shortly.)

I don't have a spare set of 6GH8A tubes to swap out yet. (Found an untested GE that seems to solve the problem.)

Any suggested trouble shooting tips would be appreciated however I am really new to the tube world and diagnostics.

For what it's worth other tube amps in this room and with this speaker set up do not have this issue.

Speakers are JBL C-37s with the 010 load. 15" woofer, bullet tweeter and N2400 network all 16 ohm.
The buzz is the same on the 8 ohm or 16 ohm taps.

Any tips would be appreciated.

 

[gadget73]

Heater-cathode leakage in the 6gh8 would produce 60hz hum. Quite possibly thats what you've got happening. Have any 6U8's ? Those will sub for a 6GH8 for testing at least. 6BL8 should also work.

 

[Fasturd]

Upon taking off the bottom cover I see the board says DYNACO PC-4 and 6GH8.

 

[Fasturd]

Heater-cathode leakage in the 6gh8 would produce 60hz hum. Quite possibly thats what you've got happening. Have any 6U8's ? Those will sub for a 6GH8 for testing at least. 6BL8 should also work.

I am going to go try and find my stash. I am in the middle of a move so most of my stuff is packed...
However both amps show exactly the same symptoms. Two tubes go bad at the same time?

 

[RWood]

“Did not buzz at the seller’s house”

What is different at your house? Fluorescent lights? Dimmers? Extraterrestrial activity?

Different preamp our signal source? When I hear “buzz” my thoughts go to either grounding issues or interference.

 

[Fasturd]

I found one sad old GE 6GH8 laying in a box of untested tubes.
It didn't completely eliminate the issue but it was much better!

I guess I need to find a new pair of 6GH8s!

 

[TriodeLuvr]

I am goung to go try and find my stash. I am in the middle of a move so most of my stuff is packed...
However both amp show exactly the same symptoms. Two tubes go bad at the same time?

Not all tubes are created equal. They might both be from a sleeve of hummers. Take your time, don't get discouraged, those are nice amps. There's a lots of expertise here, and this will undoubtedly turn out to be a relatively minor problem.

Jack

 

[TriodeLuvr]

Set bias to 1.56 however one amp goes no lower than 1.56.

I haven't worked on Dynaco amps in many years. However, when bias is at the edge of the window in any amp, the components of interest would be the output tubes and the coupling capacitors. You can identify the former by swapping both tubes from one amp to the other (turn bias to minimum current in both amps before the swap, then reset). If the high bias follows the tubes, you've found the culprit. The next thing to do would be to ask other MK IV owners whether this is common. The range of adjustment might be somewhat narrow on these amps. I don't remember, it's been too long.

Jack

 

[Fasturd]

“Did not buzz at the seller’s house”

What is different at your house? Fluorescent lights? Dimmers? Extraterrestrial activity?

Different preamp our signal source? When I hear “buzz” my thoughts go to either grounding issues or interference.

Good questions. I have been through the house and turned every thing off but my fridge...lol.
Maybe his EV speakers weren't as efficient as my JBLs?
Swapping in my lone spare tube dropped the noise sigificantly.
Other vintage tube amps didn't have this problem in this room so I don't think it's the house..

 

[Fasturd]

Not all tubes are created equal. They might both be from a sleeve of hummers. Take your time, don't get discouraged, those are nice amps. There's a lots of expertise here, and this will undoubtedly turn out to be a relatively minor problem.

Jack

I am totally willing to listen to experts in order to solve this! Thanks for the encouragement.

 

[Fasturd]

I haven't worked on Dynaco amps in many years. However, when bias is at the edge of the window in any amp, the components of interest would be the output tubes and the coupling capacitors. You can identify the former by swapping both tubes from one amp to the other (turn bias to minimum current in both amps before the swap, then reset). If the high bias follows the tubes, you've found the culprit. The next thing to do would be to ask other MK IV owners whether this is common. The range of adjustment might be somewhat narrow on these amps. I don't remember, it's been too long.

Jack

Good advice, regarding the bias!

 

[Fasturd]

Heater-cathode leakage in the 6gh8 would produce 60hz hum. Quite possibly thats what you've got happening. Have any 6U8's ? Those will sub for a 6GH8 for testing at least. 6BL8 should also work.

It looks like you may be right a random GE 6GH8 I had laying around lowered the noise floor significantly.

 

[gadget73]

yeah, it happens. Luckily these tubes are pretty cheap. If you have any hamfests or other things where tubes are common you can usually get them for a couple bucks each. These things got used by the boxload in tv sets.

 

[ramjet]

Have you tried placing them in another location that doesn't have them on top of other electronics?

 

[Fasturd]

Have you tried placing them in another location that doesn't have them on top of other electronics?

Yes, I moved them to another outlet and off the top of the Macs. No change. Pretty sure the noise is coming from the Sylvania 6GH8 tubes.

 

[Fasturd]

yeah, it happens. Luckily these tubes are pretty cheap. If you have any hamfests or other things where tubes are common you can usually get them for a couple bucks each. These things got used by the boxload in tv sets.

After some research I see there world is filled with variants of these tubes.

 

[Fasturd]

Tried 3 variations of new, matched 6GH8 tubes and the sound is still there. I tried two less efficient sets of speakers and the sounds drops to a point where you have to be directly in front of the tweeter within inches to hear it. This may be an unfortunate trait of the JBL C-37s however I am not convinced the amps are perfect. I think I have maxed out my trouble shooting level and I may need to have a tech check these out for me just to make sure there are no issues.

 

[Fasturd]

So I finally got into the new house. A year later I get to playing with some old gear and I remember my Dynaco MK IV issues had not magically solved themselves. I know a bit more and have taken the following measurements based off the voltages listed on page 8 of the Dynaco MK IV manual.




I have a pair of Dynakit MK IVs. They were upgraded prior to my ownership. They have a Dynakit Multi-cap and 6GH8 PC-4 board. They bias fine. They sound great when running however, they both share the same high end buzz/sizzle. This noise if heard more in the mids and tweeters than the woofer. Its loud on efficient speakers and less on inefficient speakers.

The noise is there whether or not the inputs are grounded.


If I pull the 6GH8 there is no buzz on the speakers.


The new Dynakit co. confirmed that I should check voltages against the Dynaco manual spec on page 8 of the original manual.


This lists voltages for the GZ34, the 2, EL34 tubes and what was originally a 7199 but is now a 6GH8.


The bias is at 1.56.

The EL34s Measure very close to what the spec states.


Spec Actual tube 1 and 2

1. 1.56 1.55 and 1.55
2. -
3. 432 422 and 423
4. 432 426 and 426
5. -34 -33.3 and -33.4
6. -
7. -
8. 1.56 1.55 and 1.55
9. -

The AC between 2 and 7 should be 6.3 ac they measure 6.34 ac and 6.35 ac.


The GZ34 rectifier is as follows.


Spec Actual

1. –
2. 440 436
3. –
4. 370 -00.5 mv
5. –
6. 370 -00.3 mv
7. –
8. 440 436

The AC from 2 to 8 spec is 5 AC actual is 5 AC.


Both amps share similar numbers on the GZ34.

The measurements at #4 and #6 seem to be an issue?

What should I learn from that?


The 6GH8 numbers are.

1. 290 280
2. 140 -71.2 mv
3. 42 34.3
4. –
5. –
6. 1.5 108.5
7. –
8. 145 115
9. 140 108

The AC from pin 4 to pin 5 spec is 6.3 actual is 4.77. Pin 2 and 6 are way off?

Do the incorrect measurements on the 6GH8 have anything to do with the incorrect measurements on the GZ34 rectifier?


One thing to mention is when I probe pin 4 or 5 on the 6GH8 the noise drops considerably. Seems like that may be a clue? Maybe a ground issue?


A member of another board had me check the legs of the two large caps on the PC-4 board.

The leg closest to the 6GH8 read 284 DC on one and 116.6 on the other. The leg closer to the power tubes was -33.9 mv on the first one and -33.9 on the other.


I did the tests with the tubes in place as I did not see any reference to pulling the tubes prior to testing all the pins. Some clarity on that would be appreciated.


I know enough to not hurt myself but I am still learning testing processes. Any trouble shooting expertise would be awesome.




Thanks in advance,


Bill

 

[gadget73]

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.

 

[TriodeLuvr]

The GZ34 rectifier is as follows.

Spec Actual

1. –
2. 440 436
3. –
4. 370 -00.5 mv
5. –
6. 370 -00.3 mv
7. –
8. 440 436

The measurements at #4 and #6 seem to be an issue?

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 6GH8 numbers are. (etc.)

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:



Jack