My Dynaco st35 just died!

[Kent Kassler.]

I was listening to some music and the right channel started distorting shortly followed by the left,no snap,crackle or pop,just straight out distortion......what now chaps?all 3 transformers run extremely hot ,i want to jump!

[Kent Kassler.]

...surely it cant be the valves....both channels would surely not fail at the same time?is there a common component to both channels that could cause this?The big metal can cap perhaps?Oom Johan ek kannie meer nie jy moet hierna toe trek asseblief.

[mahleu]

Weird coincidence, my Leak started humming yesterday... I think mine is a dodgy earth or old joint.

Maybe we need to get some solid state 

[GearSlave]

Maybe the tube rectifier?

[Kent Kassler.]

diode rectified these are sir.

[GearSlave]

Fair enough, those rectifiers might still be busted. That might explain the transformers heating up that way.

[handsome]

indeed probably something common to both channels which would mean the psu either transformer, diodes, caps etc...dynaco ran their transformers very hot so that is not much of an indication. my rebuilt sca35 power tranny runs too hot to touch and has always been that way. get a multimeter and measure the supply rail it should be about 350V to the power valves. a failed psu cap is usually accompanied  by a lot of hum. but fear not st35s are simple amps and easy to repair or rebuild. as long as the excellent output transformers aren't cooked you will get yourself up and running again.

[LAV]

Hmm, also sounds like PSU problems. Diodes / diode bridges like to fail in short-circuit mode.

[Kent Kassler.]

thanks G i will investigate....i see two red wires running to two tag strips....two 5401 diodes run from those strips and join together...from that point they run to the big metal quad can cap....could the cap cause that?

[Kent Kassler.]

Thanks gents i will have to get her seen to....no humming occured and the amp was re-capped(excluding the big metal one)all resistors and caps on the boards were replaced.The diodes look original though....a 5w resistor where the diodes run to the cap also seems to have been replaced...a black mark under it indicates that the previous one died horribly.The only non replaced items i can spot are the two diodes and the metal quadcap.No humming occured at all just sudden distortion.Many thanks.

[Kent Kassler.]

...been pushing the amp harder than usual with the Scanspeaks instead of the Fostex fullrangers.....i wonder?

[Kent Kassler.]

....one other thing that i've been noticing is when powering up the unit the front right tiny 7247 (dual valve?)would burn bright with the inrush for a second and then settle.....only on that side though?

[Steerpike]

Some Dynacos used a single resistor to set the cathode bias on all four output pentodes. If one valve fails, it will pull the bias completely out, which can do harm to the other valves. That same resistor has one bypass capacitor, if that fails it wrecks the bias on all 4 valves too.

My preference is to modify this layout and give each valve its own bias resistorr and capacitor.

[charles]

Dear Peter,

If I am correct you have a SC35 Dynaco. The sudden glowing of the 7247 valve is of no concern - sometimes NOS valves that are > 100% do it - I observed this on my AVO valve tester, when testing valves.

My SC35 does not have a 250V mains transformer but it uses a 110V one - on some of the kits that were imported to RSA a dropper resistor was used to "step it down" to 110V.  This is not the correct way to do it, one has to use a 250/110 step down transformer. If yours is like this, check for a blown dropper resistor- I hope this is not so, as this may damage the mains transformer.

Hope it will help.

Kind Regards

Charles

   

[Kent Kassler.]

thanks for the assistance gents.....i see a single black wire running off a pin on one of the el 84's it goes to a resistor and cap that are soldered together,one end goes to an earth type lug on the quad can cap and the other to the corresponding valves pin on the second board.....so i reckon this is the biassing arrangement for all 4?And so one valve could be killing all 4?The cap on the biasing resistor line looks brand new and appears to have been changed...its a 100uf Rec cap?....change the valves and test?