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previous: Ray Ivers Bill,You're running... -- 6/8/2000 1:13 PM View Thread

Re: Help!Weird prob with "Super" DR clone!

6/10/2000 12:51 AM
Wild Bill
Re: Help!Weird prob with "Super" DR clone!
Thanks for the reply, Ray! Actually, I'm running 6 tubes in push-pull, 3 on a side.  
Since my initial plea for help I think I've solved my problem. It was bothering me that the cathode rsistance I had calculated seemed to be too low - the tubes got a little red in the plates after a while. So I put a 'scope on the output and found a full-power oscillation!  
What I was hearing on startup as a low-freq "putt" was not disappearing but rather jumping up in frequency above the audible range. It was right in the phase inverter so it was at full "invisible" volume!  
I put a 47 pf between the PI plates and that killed most of it. Changing lead dress did the rest. I can make it come back at about 2 watts of power if I run an 8 ohm spkr on the 16 ohm position - the OT is about 2k at 8 ohms so I guess if the switch is in the wrong position the load at 1k is just too low even for parallel tubes. At least now if a mistake is made about the load position the oscillation will be weak enough not to damage the tubes.  
Your idea about transconductance during warmup probably explains why the oscillation jumped up to an ultrasonic frequency. I'm using 1k5 grid stoppers on each and every tube and RG-174 shielded cable feeding each grid but with that many tubes I guess it's easy for an oscillation to happen.  
Thank heavens for 'scopes!  
Anyhow, thanks again for being kind enough to give some input. The amp sounds great - the punch of a DR with the power of a Bassman!  
Wild Bill

pumpstein This sounds like a *really* cool am... -- 6/13/2000 2:09 AM