Noisy MK IV amps. SOLVED Trouble shooting insight appreciated.

Hello, new member here just keeping my head above water learning how to care for my tube (addiction) collection.

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 is heard more on the tweeters and mids than the woofer. It's loud on efficient speakers and less so on inefficient speakers.

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

The new Dynakit co. online 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.

Pin 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.

Pin 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.

Pin    Spec   Actual
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

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?

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 also.

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

Unfortunately I can’t post pictures yet because I am a newbie on this forum.

Thanks in advance,

Bill

You did not mention if the amps is more quiet if a shorting plug is connected at the input.
If the amps is quiet with shorted input the hum comes from the interconnection ( ground loops
or similar)

You did mention that it was more quiet if the filament wires were probed. This is a clue.
According to the manual the filament winding are grounded in the center. Check if
it still is. If not and the center tap is not available you could easily connect the filament
to a suitable grounding point, or, possibly create an elevated ground by tapping the B+
to lets say 50-70V. This will make life easier for the 6GH8 tube where some leaking
between the filament and cathode will create hum that leaks.
Replacing the 6GH8 might solve the problem, a lot of tubes could be used ( ecf82/6u8,
ecf80/6bl8 or just another 6GH8) Get a new one and test !

Measuring voltages is with all tubes in place, some load connected on the output ( speaker or
power resistor) and nominal input voltage.Give a few minutes for voltages to stabilize.

peterh wrote:You did not mention if the amps is more quiet if a shorting plug is connected at the input.
If the amps is quiet with shorted input the hum comes from the interconnection ( ground loops
or similar)

You did mention that it was more quiet if the filament wires were probed. This is a clue.
According to the manual the filament winding are grounded in the center. Check if
it still is. If not and the center tap is not available you could easily connect the filament
to a suitable grounding point, or, possibly create an elevated ground by tapping the B+
to lets say 50-70V. This will make life easier for the 6GH8 tube where some leaking
between the filament and cathode will create hum that leaks.
Replacing the 6GH8 might solve the problem, a lot of tubes could be used ( ecf82/6u8,
ecf80/6bl8 or just another 6GH8) Get a new one and test !

Measuring voltages is with all tubes in place, some load connected on the output ( speaker or
power resistor) and nominal input voltage.Give a few minutes for voltages to stabilize.

Thanks for your response Peter.

The sound output is the same with the input shorted or with no input.

Specifically #4 and #5 pin when probed individually dropped the noise floor considerably. I will look into your grounding suggestion.
I did try multiple sets of "matched" 6GH8s with no improvement on either amplifier.

I did all the testing with the tubes in place, fully warmed up.

The measurements on that concern me are the pin 4 and 6 on the rectifier that should be 370 DC and I get -00.5mv and -00.3mv as well as the 6GH8 tube which should be 140 DC on pin 2 and 1.5 DC on pin 6. I end up with -71.2 MV on pin 2 and 108.5 DC on pin 6...

I will check through the grounds again and look into adding as you suggested.

Thanks again.

The GZ34 rectifier is as follows.

Pin Spec Actual
1. –
2. 440DC 436
3. –
4. 370AC -00.5 mv
5. –
6. 370AC -00.3 mv
7. –
8. 440DC 436
The AC from 2 to 8 spec is 5 AC actual is 5 AC.

I read the chart wrong MULTIPLE TIMES... pin 4 and 6 are AC not DC as kindly pointed out by Kevin from DKP.

So I probably have a few less problems than I previously thought! I will re-measure tonight and hopefully move forward!

After reading the voltage chart again The measurements for the GZ34 and the El34s are right on target.
I do still have weird reading on pin 2 and 6 of the 6GH8 tube.
Pin 2 reads-71.2 mv and it should be 140 DC.
Pin 6 reads 108.5 DC and it should read 1.5 DC.

Kevin from Dynakit confirmed a suspicion that the green/yellow wire from the transformer to the 5 lug terminal strip of the bias circuit needed to be grounded. I saw photos of other MK IVs on line and they all had this wire jumped to the ground lug or soldered directly to the grounded lug. I confirmed with a jumper over lunch and both amps are now quiet! Problem solved.

Thanks for the input it was appreciated.

I will probably be back as I am taking on a stock PAS 3 next before I tackle a sad pair of MKIIIs.

Cheers!