DYNAKIT 70 STERE0 RUMBLE

by tookie00 Fri Nov 26, 2010 4:39 pm

tookie00: Posts : 2
Join date : 2010-11-26

Post n°1

DYNAKIT 70 STERE0 RUMBLE

by tookie00 Fri Nov 26, 2010 4:39 pm

HI ALL, THIS IS MY FIRST POSTING. I HOPE SOMEONE HAS AN ANSWER TO MY PROBLEM. I RECENTLY PURCHASED AN EARLY SIXTIES DYNAKIT 70 THAT NEEDED SOME WORK. THE LEFT
SIDE WAS NOT WOKING. I BROUGHT IT TO MY TECH WHO SPECIALIZES IN RESTORING VINTAGE
AUDIO AND HE REPLACED WHAT WAS NEEDED WHICH INCLUDED 4 JJ EL34 OUTPUT TUBES. THE
PRE-AMP I HAVE IS A MARANTZ MODEL 7 TUBE AMP. HERE IS THE PROBLEM. WHEN I TURN BOTH
UNITS ON AND PLAY A RECORD OR CD AFTER ABOUT 10 MINUTES THE DYNAKIT 70 STARTS TO
RUMBLE AND YOU SEE THE OUTPUT TUBES BLINKING.A FLUTTERING SOUND CAN BE HEARD THROUGH THE SPEAKERS. I CONNECTED THE DYNAKIT TO A SOLID STATE PRE-AMP AND THERE
WAS NO NOISE AT ALL. I ALSO CONNECTED THE MARANTZ MODEL 7 TO A SOLID STATE POWER
AMP AND THERE WAS NO NOISE. IS IT POSSIBLE THAT THE DYNAKIT AND MARANTZ ARE
NOT COMPATIBLE ? HOW CAN THIS BE THEY ARE BOTH TUBE AMPS ?? IT WOULD REALLY BE A
SHAME IF I CANNOT USE THESE TWO UNITS TOGETHER. THE SOUND IS INCREDIBLE. ANY HELP
WOULD BE GREATLY APPRECIATED. THANK YOU IN ADVANCE

by **Bob Latino** Fri Nov 26, 2010 6:38 pm

Bob Latino

Bob Latino: Admin; Posts : 3277
Join date : 2008-11-26
Location : Massachusetts

Post n°2

Re: DYNAKIT 70 STERE0 RUMBLE

by Bob Latino Fri Nov 26, 2010 6:38 pm

Hi Tookie00 and welcome to the Dynaco Tube Audio Forum,

It sounds like something in the high voltage system is not quite right on your amp. The solution that will probably fix the problem is > a new quad cap and a new driver board with all new parts. Sometimes an issue in the amp that keeps the amp on the ragged edge of stability may cause the issue not to show up with one preamp/amp combination. When driven by another preamp, the issue "goes over the edge" and the amp becomes unstable. The rumbing sound that you hear is usually called "motorboating" and is usually a capacitor related issue. The amp is "cycling" and going in and out of stability.

You said that your tech "replaced what was necessary" but probably what the amp needed was more than what he did. Those old capacitors and carbon composition resistors on the driver board are nearing 50 years old and parts don't last forever. Those carbon composition resistors tend to "go high" and the actual resistance slowly rises with time. Eventually the voltages on the driver board go way out of whack and the capacitors sometimes start to leak. If you do get a new driver board be it a stock replacement driver board (Check at Dynakitparts) or an all triode board like the VTA driver board, try to get the board with metal film resistors. Metal film resistors are quieter than the original carbon composition resistors, they don't drift and the can be had as 1% precision resistors. Both Roy Mottram and I use 1% precision metal film or metal oxide resistors on the VTA driver board.

Bob