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Motorboating!! But with NEW caps!!!! HELP!

3/1/2003 10:18 AM
Motorboating!! But with NEW caps!!!! HELP!
Hello, i'm an spanish guy and i'm finishing my first hybrid amp, tube preamp/mosfet poweramp.  
The B+ voltage must be 360v, but the transformer i own only give 200V. If i use very low volumes it works fine, but when the volume is slightly increased it starts to "motorboat". I've find that it's told to be caused by the caps, but they're news !!  
I'm starting to think that the trouble can be the low voltage of the B+, but i'm not sure about this, the lower voltage, the lower gain, but it's high enough to keep it working anyway.  
I have 100nF caps in each stage and around 400uF in the power supply. I tried just to put 47 + 47 uF, it still motorboat, I've used an old choke, I've used an RC filter, none of this works, and I'm starting to get frustrated about this. I thought the trouble could be in the preamp - poweramp interphase, but i've put a cathode follower between the preamp and poweramp, it doesn't solve the problem, but gives a better tone.  
Can anyone help me??
3/1/2003 12:01 PM
HELP! I've solve all posible problems.
Solved the problem of B+, now it's the correct one. It's 333V, but it still motorboat.  
HELP !!!!!!
3/1/2003 8:00 PM
John Culp

Well, I don't know what your schematic looks like. A few things I can think of that can cause motorboating in tube amps are: Powering more than 2 stages off of a single power supply node, or with insufficient decoupling resistance between nodes; Big coupling capacitors between stages; High grid resistance or a poor connection somewhere between grid and ground; A failing rectifier tube.
3/1/2003 11:34 PM

The Schematics is a Fender Blackface preamp.  
About powering more than 2 stages, You say the problem can be solved taking 2 differents wires to each tube? Or maybe changing my decoupling resistance and use it between the gain stages and the cathode follower. I'll try tomorrow. About the coupling capacitors i'm using 0.047uF (as told in the schematics) but in the cathode follower I've used 1uF, i shall change it tomorrow too.  
I do not use tubes in the rectifier, no mistakes here.  
I'll try all of this tomorrow, anyway it's a curious thing than when i turn off the amp (not the filaments) the amp keeps working PERFECTLY for a few seconds (just to discharge all the caps).
3/2/2003 2:18 AM
Dai Hirokawa

I don't know if this is your problem, but once when I forgot to hook up the ground side of a cathode bypass cap, that gave me motorboating.  
3/2/2003 9:22 AM

Well, I don't know what exactly you mean with "hook up". I think you could mean not to leave wire beyond the soldering?  
I'm very interested on this, because i had no idea this kind of things could make motorboat any amp!  
Thanks, i'll try it too. (in 2 hours i'll tell how it goes)
3/2/2003 11:13 AM
Dai Hirokawa

Sorry. I mean forgot to connect (solder).  

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