Tube amp report - Dynaco MK IV

From my previous musings I promised a sound check for the recently acquired Dynakit/Dynaco MK IV I recently snagged - 40 W mono tube power vs. 75 W of mono Dynaco ST-150 transistor power. I retubed the MK IV with 2 Siemens EL-34's and a Mullard GZ-34/5AR4 rectifier tube, kept the original Dynaco/Mullard 7199. Got the unit warmed up, checked/reset the bias (got rid of the Sovtek EL-34's and Chinese 5AR4 that came with the amp). After some warm up time with me in the shower and cleaning up, the following happened: I massively need another MK IV!!!!!! The midrange is so much smoother compared to the ST-150, the highs twinkle, and the bass notes are more distinct when moving the balance from the tubed side to the transistor side. I listened to all sortsa stuff: Sun rockabilly, Van Halen, Buddy Holly FTOMT, Paul Revere and the Raiders, Sniffin' the Tears, Lonnie Mack, Elvis 24K, Creedence Willie and Green River (whoa dad, so much better with tubes...), the Cramps, the Sonics and they ALL sounded better through the tube amp. There was also another interesting occurrence: the area in the middle of my speakers filled in when listening to mono sources - no "2 speakers are playing in 2 distinct areas" sound, but everything smack in the middle. Folks, find some way to tube up your system - I have about $300 invested in NOS tubes and the 1 amp, another $200 or so will get another MK IV and while I won't be buying any new discs for awhile, I'll get to hear all the stuff I already have in a more satisfying manner.

 

Try this again with the ST-150 only being drive on one side and with the "mono" output of that one side wired to your speakers exactly the same way you have wired the MK IV.

I built a number of both products and even had a hand with the power supply design changes adopted for these units. Both are good clean products and I have never heard the MK IV paired with the tubes you used, but I found that some speakers tended to respond better to one or the other. Differences in the power factor of these amps and different dampening factors are the likely cause of subtle differences in sound. With small component changes one could alter these characteristics and likely flip your preference. Mr. Carver offered several sold state amps with power factors matched to better tube designs and it is widely reported that he hit the nail on the head. With proper design one can design a sold state amp to sound like a well designed tube amp. Don't know if those Carver designs are still on the market. Mr. Carver knows what he is doing. (Not getting into which sound is better, but there clearly is a difference in the sound produced by different amp designs, even with nearly identical basic amp specs. The Power Factor is the likely explination)

 

Richard,
I have a Carver Receiver 2000 which I bought back in the mid-eighties. Sounds pretty nice to me but, I've never had, or even heard, a decent a decent tube rig.So I can't really compare.

 

Hello Richard! You're a Dynaco man? Boy, does our friend sckott need to hook up with you! I've been trying to slowly get tubes into my system - 1st step was a tubed buffer stage between my SS Adcom preamp and my Marantz CD67SE disc player; more natural highs resulted. Egged on/encouraged by Steve I've been looking for skinny tube amps to fit cramped quarters (and be affordable) and the MK IV fit the bill (I grew up in a St-70/PAS-2 house, so I was familiar with the Dyna stuff). Listening to just the tubed amp with mono sources, the presentation of reverb/echo is better defined and seems to be more distictive, highs are more natural sounding. Cranking up the SS ST-150 leads to a definite stopping point as things get harsh and sizzly up top; the tube amp just lets the bass/woofer control get flabby (damping factor at work???), but no harshness creeps in. Any chance you're familiar with the MK II? What's the value/tolerance of the red rectangular resistor on the circuit board? My MK II reclaimation project is on pause as that resistor was filthy and even cleaned it's colored dot code is unrecognizable, and of course it's fried...

 

Heck, I don't remember what I had for breakfast. Bet you can get your hands on a Sam's Photofact sheet on this amp which will disclose all of the components and their values. Sorry to hear it got fried. Problem with tube hardware is that the heat causes value changes that change the performance of the unit. The thermal stability of components today is superior to what we had to work with 40 years ago. I'll look to see if I have any of the kit booklets around but I doubt it after all this time. Also, do a web search, I am sure some old timers are still offering to work on these units and would have the information you seek. These guys love to talk about this stuff and are often free with their advise. Heck, some old AR guys are still offering parts and service for the AR3a and old Dahlquist guys still offer parts and help. The company may be gone but the staff is still kicking. I'm sure the same is true for Dynaco stuff.

 

Thanks - if you do dig up any old documentation I'd sure appreciate it - copies of manuals are floating around, but having the genuine article is part of the fun!