Dynaco MK VI question

I just obtained a pair of Dyna MK VIs in very good condition. In the Forum's Dynaco Photos section I just posted my completed collection.

A few questions for the Forum regarding the MK VI. Does anyone have the original factory engineering notes explaining the design criteria behind the use of the 6N030T time delay relay? It only controls the power supply feed for the 7199 tube. Lacking the factory engineering notes did Dynaco ever publish a comprehensive description of the reasoning for the relay?

Mk VIs... What a curious amplifier. As you know that relay is there to (try to) protect your speakers from that quartet of 8417s. Disconnecting the B+ this way allows R8, R9, and R101 to simultaneously discharge the differential outputs from the phase splitter. This prevents the pull up and pull down output pairs from receiving signal ac until (ostensibly) things stabilize. When those relays close on my Mk VIs it is a little bit of an event: the audio snaps on and there is a bit of violet flash visible through the clear glass of the relay. They are lively sounding and I like them. I haven't decided whether I should keep them stock or convert them to a more conventional topology. Have you tried running yours with a pair of 8417s per side rather the full quad?

Interesting view of the operation of the relay. They would be normally open during cold startup. The 7199 would warm up but the output tubes would be last to turn on. A few seconds later the relay would close and power the 7199. After they pick up they will not drop out until the power transformer is turned off. Outside of a few seconds I do not see what protection the relay gives. Please let me know more about the particulars of the operation.

My pair were converted to 6550s by a previous owner. On mine R102 is 18 kohms. I have not opened them to track down R104 but they may have just reversed the two resistors. I do measure -60VDC on the adjustment pot going to the output tubes. I will give some thought to trying 8417s but I will need to revise the bias circuit back to original to make that work.

These are beasts even compared to other large vintage amps. So far they sound fine but I doubt I need 120 watts to drive my Altec 615As. My other Dyna amps drive them just fine.

Thank you for your response.

I expect that converting to 6550 would require more than a change to the bias circuit. The 8417 was a very high gain device which facilitated the minimalist design. Your Mk VIs feature a single 7199 driving a quad set of 6550?

As you have probably discovered, the chassis design is also unconventional. Give the odd geometries and mass, I actually had to read the assembly manual to figure out how to disassemble them without doing damage--to me or the amps!

My amps use the 7199 driving a quad of 6550s. Yes.

I am still reading the manual as it is very different from the usual Dyna amps. I suspect you are right about the changes for the 6550. The manual calls out for 1.6 VAC to drive the 8417 to full output. They have only -26 VDC on the grids while the 6550 have -60VDC. To drive the 6550s to full output would require a higher AC input. They still may have left the drive circuit alone and just accepted a higher AC signal for full output. The 7199 circuit topology is just like all other Dynas except for specific resistor/capacitor values. It probably has the ability to drive any output tube but just uses more AC in.

It looks like the meter connections all remain while the front face is disconnected from the chassis. I have to read further. Once I track all the differences out I will send you my findings. I may just send you that info via the Forum messenger feature rather than posting. I may not get to the insides of the amps for a while though.

Why not consider converting these to M-125s?

I wanted these so I purchased the MK VIs to complete my vintage Dynaco amp collection. I stated that in my opening comment. See the photo section for more info.

I did look into the VTA M125s. Nice amps but I decided I did not want them. If I did I would just buy them.

Why would I modify amps that I do want into amps that I do not want?

Simply because it may very well sound better. That is the point, after all IMHO.