Hallo, one my mk iv I hissed when intermittent and hot , what can be
2 posters
Dynaco mkiv problem
Trilogy 901- Posts : 3
Join date : 2015-05-26
Bob Latino- Admin
- Posts : 3276
Join date : 2008-11-26
Location : Massachusetts
Trilogy 901 wrote:Hallo, one my mk iv I hissed when intermittent and hot , what can be
Sometimes this can be a bad driver tube .. Swap the driver tubes between the two amps and see if the hiss moves over to the other amp.
Bob
Trilogy 901- Posts : 3
Join date : 2015-05-26
changed the driver , still remains a slight hiss in the same hand, it could be the choke I've never changed .
Bob Latino- Admin
- Posts : 3276
Join date : 2008-11-26
Location : Massachusetts
Trilogy 901 wrote:changed the driver , still remains a slight hiss in the same hand, it could be the choke I've never changed .
If you can measure about 50 to 65 ohms across the choke, then the choke is probably OK. If you have the original driver board in that amp, you should consider replacing the driver board. The original Dynaco amps used fairly cheap driver boards with open solder traces. The fact that it hisses when hot could also be a sign of a bad solder trace on the bottom of the board OR a bad part on the board.
Another idea to trace down the problem would be to do a voltage check on the good amp and then do the same voltage check on the amp that hisses. Compare them and see if there are any major differences.
Personally, if you are going to use these amps on a daily basis AND they have the original driver boards, I would just replace the driver boards on BOTH amps. The Mark IV came out in 1960, the year after the ST-70. Some of these amps are now 50+ years old and who knows how many thousands of hours the amps had on them before you acquired them ? Parts don't last forever ...
Bob
Trilogy 901- Posts : 3
Join date : 2015-05-26
thanks , in wekend do the tests then the lighthouse know thank you very much courtesy.
Cristiano
Cristiano