Tube Amps / Music Electronics
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|6/7/2000 5:21 AM|
|Wild Bill||Help!Weird prob with "Super" DR clone!|
Finished my latest project - a Deluxe Reverb style clone with just the reverb channel. The output stage is a little different - I'm using 6 5V6GT's with a 500 ohm cathode resistor for each tube but all cathodes strapped together to establish a cathode baseline, putting them in parallel.
Now I'm into the debugging stage. I got rid of almost all the hum by star-grounding everything back to the filter/HV centre-tap. Although I used a Hammond PT 300-0-300 the B+ was a little high - 410 volts at idle. This was making the 5V6 plates glow just a little bit so I disconnected two of the 500 ohm/33uf cathode resistors to raise the bias a bit - I'm running about 35 ma at rest on each tube.
My problem? When the amp is first turned on there's a bit of "putt-putt" at low volume settings. It goes away after 30 seconds or so but it's bothering me. If I immediately crank the volume up it stops. Lower the volume and it comes back. A few seconds later it's gone for good.
I'm not using a standby switch 'cuz I thought I'd wind up with less B+ and have little danger of cathode stripping. Now in a way I'm glad I didn't -I can always do something like a delayed B+ with a relay with a cap across the coil but I might never have noticed this warmup "putt-putt".
It's not immediate - the tubes take 11 seconds to warm up and then it starts. It's a slow putt. When it's gone it's truly gone - I can't make it come back by adjusting any controls.
I thought it might be a decoupling problem. I bumped up the filters - I'm using 47uf-choke-33uf-22uf-22uf. The reverb 6AQ5 runs from a tap at the 33uf through 3k3 to its own 10uf filter. No change.
One factor is that it didn't do this until I got rid of all the chassis gnds. At first I star'd everything to the common points at the tie strip gnd lugs near each section. I lifted these with nylon screws and nuts and ran gnd wires back to the filter/CT point. The hum dropped "big time" except there's still some at high settings of the reverb mixer pot. Now I've got this other problem instead.
Any thoughts would be appreciated.
Oh yeah - how does it sound? LOUD AND AWESOME!
Thanks in advance -
|Peter S Sounds to me like a preamp tube. C... -- 6/7/2000 5:58 AM|
Ray Ivers Bill,