Dynaco Mark III "Restoration" Gone Wrong - Let's fix it (Part 1)

Join me as I assess and repair a Dynaco Mark III Vacuum Tube Power Amplifier that was left broken by another technician. This video also serves as a cautionary tale that just because someone 'restored' your vintage HiFi gear, it doesn't mean they did a good job.

[@Stelios.Posantzis]

4:12 I am a bit sceptical about these thermal camera photos: could it be that on shiny, reflective objects they merely read the reflected radiation off these objects instead of the emitted radiation? Have you ever tested that by measuring the object's temperature with a thermometer/probe? My reasoning is that if the object was truly 125 degrees, then all of it would show up red. Instead it shows up with many colours resembling shadowing in a photograph's film negative which to me this says the object is not that hot but instead reflects the radiation emitted from the tubes nearby.

[@philsanalogbasement]

You make a valid point. I could try that. I haven’t put much more thought into it since better capacitors/solutions are available for roughly the same price(or less), so why bother using the inferior option.

[@Stelios.Posantzis]

@@philsanalogbasement I've seen the same thing happen with guys using these devices to measure the case temperature of solar water heater water tanks made of polished stainless steel. I reckon that the surface must be rough, not polished, in order to be able to measure its temperature with one of these devices. I'm wondering if the same effect could compromise a reading from an IR temperature meter. Good multi-section electrolytic caps can be pricy but they are worth it if you are after maintaining originality. They probably also shape the sound of these amps to some extent.

[@herbward5240]

Yes I quite agree , Polystyrenes in a tube amp are a no no. COG MLCC are a better choice. The F&T made quad can caps are much better. Have had bad luck with CE distribution can caps in the past. I always use inrush current limiters in every build. JJ tubes … nah. Film caps are voltage derated in higher temps . I always use .22 uf at 800 volts. This insures a less leaky output tube coupling cap when hot . Yes the UL tap screen resistor is a valid mod . NO mechanical connections for ground returns . Add a star ground off the PT HV center tap , run all grounds with soldered connections to a 5 lug terminal strip . Better yet , use silicon carbide rectifiers to replace the rectifier tube with 30 ohm TO220 slugging resistors . A modern 5AR4 is poor compared to the original 1960’s Mullard UK made rectifier tubes. The analyzer indicates healthy tubes . Call Kevin at Dynakitparts if interested in the Waters / Ward PCB upgrades.

[@philsanalogbasement]

Thanks for the feedback, Herb! I'm still relatively new to working on tube gear so appreciate any advice, trying to soak up as much as I can. I have a strong Mullard 5AR4 here, so maybe I'll test that out in place the JJ. I was also thinking about trying a solid state rectifier, but was not familiar with the SiC rectifiers, so I'll look into that. I've been doing some testing tonight, and I'm able to get 47W(8 ohms) of clean power (0.4% THD+N) before the supply voltage(measured after the 6.8k resistor) drops to around 415 and the THD+N creeps up beyond 1% at 49W. Is this in line with what you'd expect to see with a tube rectifier?

[@philsanalogbasement]

I found the document that included revised power ratings for these amps after the FTC ruling from 1974, 50W at 1% THD... I'm happy. I swapped the Nestone 6550's for the LinLai KT88's he provided and tested again, they eke out 50W at 0.4% THD+N (non a-weighted). I'm happy!

[@herbward5240]

@ Hi Phil, Yeah the modern 5AR4 tend to have more internal resistance than the Early ( 1960’s ) issues. Some years ago I re rebuilt a pair of Fairchild mono blocks. Each amp had a pair of 5AR4 to replace the original pair of 5U4 . They had no trouble producing 70 watts RMS , and power supply stiffness was fantastic. As so many have pointed out, Dynaco amps suffered from being built to meet a price point. Have built a number of ST-70 and MK III using Ray Waters driver design that I assisted Ray with. All the amps get a SS rectified power supply and are similar in performance to the Citation II.

[@Stelios.Posantzis]

Have you got any explanation why Dynaco decided to have this huge square hole on the chassis exposing the circuit board? I cannot think of any reason for it other than it being a marketing gimmick. If I owned a pair of these, I would ensure this hole would be covered by a sheet of metal and the circuit board lowered in the case as it is a safety hazard.

[@philsanalogbasement]

There’s normally a cage over the entire chassis so not an issue. No clue otherwise.

[@Stelios.Posantzis]

@@philsanalogbasement Yes of course. It's just weird and I've always wondered about it. Also, there are some people who may prefer their amps without cages either occasionally or all the time. Personally, if the amp has no balloon-shaped power triodes in it I see no reason for it not to have a cage, especially if it came with one. I think cages add to the allure to be honest.

[@marks5816]

the ac line cord is wired wrong

[@philsanalogbasement]

It wasn’t wired wrong, it was wired per the original dynaco manual (except for it being swapped to a polarized cord). Check out the 2nd video I released a few days ago where I cover some improvements I made to the AC cord wiring. In the new setup, the hot goes to the fuse, then the switch, then a cl-80 inrush current limiter, and the neutral from the wall straight into the other lead of the transformer.

[@marks5816]

you have wired it right. with the polarized cord you do need the black wire to the fuse. the none polarized cord was not as safe so you have it right now. .it was wrong when you got it. good job