Report on VTA Mark III Driver Board

I completed the build of my first Dynaco Mark III using the VTA driver board. I had to fuss a bit more than I expected, but the final results are excellent. The completed amp sounded excellent from the beginning, but I had a higher level of residual hum than I liked.

Part of this hum arises from the fact that the grounding scheme on the Mark III is cruder than the star grounding scheme on the ST70. I ended up moving the power ground and the filament center tap to the driver board ground and this helped a lot. In addition, Roy supplies the board with the triodes in the input 12AT7 paralleled. When I removed the traces that tied the two triodes together, and used a single triode, the hum dropped to an inaudible level. I had also increased the feedback a bit, which drops the gain and noise. After all this, the gain is still plenty high.

The 1 watt square waves at 1 khz and 10 kHz are near perfect. I get ~55 watts of output power between 30 Hz and 20 khz at less than 1% THD, dropping to 28 watts at 20 Hz (power line at 117V AC). The frequency response is flat to +/-0.2 db between 20 Hz and 20 kHz.

The amp sounds excellent, as good as the best I've built. The tested performance and sound is a notch above what I got with the Triode driver board on a previous Mark III, although some of this might be due to the separate bias controls on the VTA board. Proper balance has a big effect on distortion.

The VTA driver circuit has a minimum of parts and complexity, but it has the essentials including a two triode phase splitter and individual bias. Definitely recommended for someone willing to fuss a bit. It may change your mind about the Mark III.

much thanks to DanF for his comments and testing on the MK3 version of the VTA driver.
I don't sell alot of these, maybe half dozen a year, but I always appreciate any customer comments good or bad, as this is what drives improvements on future sales to future customers.
Roy www.tubes4hifi.com

Hi Danf I also completed the VTA install on my MKIII's and have the low level hum also. I am by no means technical about these amps but know some basics. Here are some things I encountered with these amps after the upgrade. The input sensitivity was hugely increased also when I moved my speaker connections to the 8ohm tap the hum was less and disappeared totally at the 16ohm output. I replaced the quad cap with the Authenticap 30 20 20 20 and get about 540 volts on each lug and the output tubes biased easily at 50 volts. I use a pair of A/D/S 710's speakers rated at 4 ohm minimumal 6 ohms nominal.
I,m lost for now maybe you can make some sense out of this. Thanks
Best regards Scott

Hi Scott,

I think that you may be drawing too little bias current because the voltage is so high (540V). If you want to discuss this, please describe your bias resistors on the output tubes and how you measured the bias. I would address this issue before doing anything else. It is possible that this high voltage appears because your AC line is rather high. I would expect more like 510 V after the choke with 117 VAC in.

After this issue is resolved, I'll describe the steps to eliminate the hum. By the way, is your chassis grounded with a three wire cord?

Dan

Hi Dan
Thanks for the swift reply.
I"m using the 10 ohm resisters that were supplied with the kit and I am testing it at pin 8 where these resisters are wired to ground with a fluke 23III multimeter. My AC current from the wall is a bit high at 121.9ac. Tomorrow I intend to replace the power cord and ground the third wire to the chassis. I knew I should have done that first. sorry...Thanks again for your help.
Best regards Scott

Hi Scott,

Ok, with nearly 122 VAC in, something like 535-540 VDC is reasonable with the original transformers. The filter capacitor will have to earn its keep at that voltage!

I am assuming that you have remembered to hook up the feedback from the 16 ohm tap of the output transformer.

Here are some things to try to reduce the hum. Try these in order until you're happy with the hum level. Start with the first two steps.

(1) Move the filament center tap to the ground of the VTA board. Disconnect the filament leads from the octal preamp power socket for safety.

(2) If you change the power cord to a grounded three wire setup I would recommend attaching the ground lead to a solder lug attached to one of the grounded mounting holes of the VTA board (left side viewed from underneath). Make sure that this lug is solidly bolted to the chassis. This grounded power cord is for your safety; it doesn't decrease the hum. If I grounded it elsewhere, I found that the hum level went up.

(3) Decrease the feedback resistor R7 from 7.5K to 3.9K. This will increase feedback, decrease gain and hum.

(4) This is harder. Remove the socket for the input 12AT7 and cut the traces joining the two triodes. Roy has these paralleled for more gain (not needed). You will have to jumper the anode of the active triode (pin 6) over to R11. Cut the connection to the unused anode (pin 1). Reinstall the socket.

(5) I ran a ground wire from the VTA board ground to the ground lug of the filter cap, and grounded the HV center tap and the output transformer ground to this lug, but the last two are probably not necessary.

After this work, I can only hear a very slight hum if I plaster my ear to my speaker.

Dan

Dan,
thanks for posting this very useful info
Roy www.tubes4hifi.com

Hi Dan
I decreased the feedback resistor R7 from 7.5K to 3.9K. I found a 10k on my pre-assembled board at R7 so I removed it. The gain is less but just fine. I also grounded per your instructions and things settled out great.
Thanks for taking the time.
Best regards Scott

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Hi Peter,

What I describe above, and have tested is not really a star ground. A real star ground puts all of the grounds, including the output tube cathodes, the transformer center tap, the filter capacitors, the board ground, etc., at a common point typically near the filter caps. The AC ground is typically placed where the power cord enters the chassis. The construction of the VTA board makes this difficult because the board is grounded at its mounting holes. Yes, you can isolate the board with nylon screws and implement a true star ground, but I didn't find this necessary or advantageous.

I don't like four things about the original Mark III grounding scheme: (1) grounding the transformer HV center tap to the chassis remotely from the filter caps, (2) isolating the filament CT from ground through a capacitor (done in case the preamp has a hum pot on the external filament leads), (3) running the output transformer ground to the chassis remote from the VTA board, and (4) not grounding the chassis with a three wire cord. The last is a safety, not a hum reduction issue.

I spent a fair amount of time chasing a ground loop caused by the fact that I grounded the Mark III chassis with a three wire cord and that my preamp and second Mark III all are grounded. I think that this practice is absolutely vital for safety, and I would never consider lifting this ground despite web advice to the contrary. Placing the chassis ground at the VTA board mounting screws seems to reduce the ground loop hum. In addition, it is necessary to plug all components into the same power strip.

Regards,
Dan

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Hi Peter,

The goal of good grounding practice is to keep large currents from flowing in the ground path used by sensitive, low-level circuitry. This is particularly important if the ground is a somewhat resistive steel chassis. This goal can be achieved in more than one way, but star grounding is conceptually the simplest. http://www.aikenamps.com/StarGround.html gives a good explanation.

The stock Mark II and Mark III gave a reasonably low noise floor according to old test reports, so even if not theoretically ideal, their scheme worked with the original circuit. If your amp is complete, I would try it before changing the ground scheme. If you are building from scratch, you might want to put in a ground scheme like the ST-70's.

In my case, the most annoying hum that I had to eliminate was a ground loop set up by multiple chassis grounds, not really a Mark III problem. These external ground loops are still an issue with modern home theater equipment.

Regards,
Dan

http://www.graniteaudio.com/zero/page2.html I have never used one of these but have heard good things, it would be nice if someone developed a DIY version of something like this as this is way overpriced.

Hi,

I just wanted to report that in the course of renewing my quad caps in my Mark 3 that has the VTA driver board I also replaced the power cable in order to earth the chasis. I followed danf's advice given in this thread namely:

(1) Move the filament center tap to the ground of the VTA board. Disconnect the filament leads from the octal preamp power socket for safety.

(2) If you change the power cord to a grounded three wire setup I would recommend attaching the ground lead to a solder lug attached to one of the grounded mounting holes of the VTA board (left side viewed from underneath). Make sure that this lug is solidly bolted to the chassis. This grounded power cord is for your safety; it doesn't decrease the hum. If I grounded it elsewhere, I found that the hum level went up.

Those two things above is all I did and I'm pleased to report that I have to plant my ear right against the speaker fabric to hear the slightest of hums and then when no music is playing.

The music sounds so much better having changed the power cable to a grounded three wire setup. Only because I feel more relaxed when listening as there is now a safety factor that previously did not exist.

Thanks Dan for providing the information some time ago and I highly recommend that others also consider earthing their Dynaco chassis if not already earthed