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4xEL34 cathode bias ZNFB - done before?


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9/15/2000 3:26 PM
lion
4xEL34 cathode bias ZNFB - done before?
I'm planning my next DIY amp project. My main goal is to build a backup for my 2xEL34 stage amp, which is also a DIY project. But, instead of making a identical copy of this amp, I've been thinking about building something a little different. (The never ending quest for my ultimate signature tone).  
 
I'm building the amp on the remainings of a old 4xEL34 amp, (chassis, PT, OT, choke, ect.) which got me thinking about the possibility of doing a cathode biased 4xEL34 amp with zero negative feedback. Thinking about, I can't recall that I've have seen this done before.  
 
Anyones got any experiences with this. How would it turn out. Bad idea?  
Comments, opinions, suggestions or advise will be highly appreciated.  
 
Regards  
Lion
 
9/16/2000 11:28 AM
EDGAR

Hello Lion.  
 
There was an English amp brand, called IMPACT, that obviously had a short life somewhere around 1970-71.  
They had two 60W amp heads, one stright two channels, one with tremolo and reverb. They also had a 100W head and a mixer-amp and some speakers.  
 
I recently got hold on a 60W stright head, and have tried to get some information about theese amps, but with poor result. Not many have ever heard of Impact!  
One guy in England who has one after his father, had heard that they were built from excessed parts from Sound City. Anyone knows for sure out there?  
 
Never the less, a tube lover friend of mine, have a 100W head, and that one is built exactly like what You are asking for! Four EL34's with cathode bias and zero feedback!  
 
I don't think he have tested it very much, but I've got a simple shematic on it, which I can send to You, or put out on the web, if others are interested. I have also drawn up the shematic for my 60W amp.  
I'm just working with it, but it doesn't sound good, and I can't find out really why! Something strange happens in the p.i. stage. But the work continous!  
 
I would be happy to here from anyone who knows anything about Impact amps!  
 
I will put out what I have about these amps on this adress: http://www.edgaraudio.se/tubes/impact.htm.  
(Nothing there yet!)  
 
Regards  
"Edgar"
 
9/16/2000 2:32 PM
EDGAR

"I will put out what I have about these amps on this adress: http://www.edgaraudio.se/tubes/impact.htm.  
(Nothing there yet!)"  
 
Now it's there!
 
9/18/2000 7:04 PM
lion

Edgar! Thanks for the comments.  
 
I have never heard of IMPACT amp.  
I'll have a look at the schems at your site.  
 
Lion
 
9/17/2000 5:47 PM
Wild Bill

Hi Lion!  
Sounds like a great experiment! I'd love to hear how it turns out. Still, before you heat up the soldering iron I have a caution for you, FWIW. Watch out for oscillations, particularly ultrasonic!  
I've built a few parallel/PP amps and I found that lead dress, especially with the grids can be really touchy. After all, you've got more wire feeding more tube elements! Cathode bias actually seems to help - a bit of degenerative feedback. Using an existing chassis also helps by forcing you into a certain parts layout.  
NFB swamps out a lot of oscillation problems. Without it grid stoppers and such become even more mandatory. I used shielded wire star-gnded to feed the grids and plates and still had some problems.  
Ultrasonics can only be seen with a scope but will surely bite you if ignored. Tubes tend to fail early when running fullout with wasted power. To the ear there's no way to tell - the amp can sound great at guitar frequencies!  
Don't mean to scare you - it can be done! You may even get lucky first try. I've got an amp running 6 6V6's (3 on each side) sounding great and stable as a rock. It did take some work however and without a scope I could never have done it. Mr. Marshall had the luxury of trying layout after layout until he worked out all the bugs. We homebrewers have a harder time of it - you can't keep punching new holes for the tube sockets when chassis is so expensive!  
Despite my warnings - I bet it sounds awesome when you finish!  
 
---Wild Bill
 
9/18/2000 2:34 PM
Ray Ivers

Lion,  
 
The Vox AC80/100 amp from the early/mid sixties uses this design, and is supposedly 'Class A' as well, although I never did the math on the one I worked on. Talk about heat! Sounded pretty good, though - loud and fairly clean, as you'd expect, with that Vox HF chime. It was real hard finding a set of new output tubes whose plates didn't glow red, as the owner wanted to keep it 100 percent original.  
 
Ray Ivers  
R.A.G.E. Electronics
 
9/18/2000 7:41 PM
lion

Wild Bill.  
Thanks for the warning. I'll keep it in mind.  
 
Ray Ivers.  
Are you sure about the AC80/100 - the schem I've seen is fixed bias (AC100).  
Wouldn't 4xEL34 cathode biased be more like 60 watts?  
 
I haven't made up my mind about this project yet.  
I could also go for a 4xEL84 type of amp. Anyway I'm considering a EF86 preamp.  
 
I'll report back when I decide - and post the results of the project if anyones interessted.  
 
 
 
Lion
 

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