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Help!Weird prob with "Super" DR clone!


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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 -  
Wild Bill
 
6/7/2000 5:58 AM
Peter S

Sounds to me like a preamp tube. Could just be a coincidence that it started after changing the grounds. At first I thought motorboating but I doubt it based on what you wrote about your grounds and decoupling caps. Good luck!  
 
PS  
Kimerik Amplifiers
 
6/8/2000 1:13 PM
Ray Ivers

Bill,  
 
You're running all the output tubes in single-ended parallel? What value are you using for the output tube grid stopper resistors? I'm thinking this could be instability caused by large differences in output tube transconductance during the warmup period, and if so might be eliminated by increasing the size of the grid stoppers.  
 
Ray Ivers
 
6/10/2000 12:51 AM
Wild Bill

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
 
6/13/2000 2:09 AM
pumpstein

This sounds like a *really* cool amp, WB. Inspiring that you are trying something off the beaten path...  
 
- P
 
6/14/2000 3:32 AM
Wild Bill

Well Mr. Pumpstein, it just sorta "grew" that way!  
It started with a junkbox OT that was heavy enough for at least 60 watts if not more that measured kinda low on the primary - 2100 ohms. That low value got me thinking about parallel tubes as a better match.  
I have just "wussed out" and abandoned the 5V6's. They worked but I had got 5V6GT's rather than 'GTA. They were lower on plate dissipation and had a tinge of blue inside from the high B+. I went with 6V6EH's by Electro Harmonix so's I could run them the way I wanted with reasonable tube life. I'm fortunate that I live in the same town as "thetubestore.com" so I could just drive down and buy them. So much for all the money I saved on the less popular 5V6's but I figured it was worth it rather than screwing around to dial down the power.  
Not trying to brag - I'm just so pleased with the way the amp has turned out! I built it on a Hammond 17x10x3 chassis with a "cage" on top, a la some old Stromberg Carlson's. The chassis is painted gold, the top is a dark Christmas green and I put black suitcase-style equipment handles on the sides of the cage. I had done something similar before to a smaller amp that was promptly dubbed the "Toaster". This one is now the "SuperToaster"!  
I figure I'm running between 50-60 watts - I haven't measured it yet but it sounds as loud as it should to my ears. It's even more "punchy" than a regular DR - probably the cathode bias.  
If anyone has a similar low primary value OT in their junkbox I'd sure recommend giving this approach a try. It's not cost-effective for a manufacturer but as a hobby using up scrounged parts the cost is not relevant.  
The only remaining problem is some hum creeping in as you advance the reverb pot. I've tried all the standard things - this may take me a while.  
 
Wild Bill
 
6/15/2000 2:10 AM
pumpstein

Thanks for the detailed post describing your amp, WB. It was fun to read. Too bad about the 5V6's, but not such a big deal...I'd love to hear this amp. the "punch" factor is intriguing.  
 
I would love to get a look at the schematic if you ever post it.  
 
The paint job sounds pretty cool, too. Keep letting us know about your amps; I love hearing about other folks' experiments.  
 
- P
 

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