Re: How about Class A (or pseudo Class A)?

 :

12/15/1999 7:09 PM

John Stokes

Re: How about Class A (or pseudo Class A)? Yo Steve. Not only would there be a bit of an impedance mismatch (not too terrible though), I think the 6 6V6s drawing a heavy idle current might exceed the current rating of a transformer designed for AB1 operation. Think about it. A 6L6 in AB1 is idling at...what...30 to 40 ma, depending on how hot the player likes? How hot would the hot bias 6V6s be running? Maybe 40-50 ma each, total of 80-100 per side? The primary wire is hair-fine stuff. It would be a shame to burn one open.

 

12/16/1999 6:39 AM
Steve A.	John:         So how about running just a single pair of 6V6's in Class A with a PT and OT designed for two 6L6's? Or alternately, a single pair of EL84's in Class A?     Thanks!     Steve Ahola

 

12/16/1999 6:19 PM
John Stokes	Yo Steve,     In the example you site, I think the transformer would be able to handle the current for a single pair of 6V6 running a hot bias. However, it then comes to an impedance match question. If you put a load on the secondary that would reflect back about an 8k load to the primary, you should be in bid-ness.

 

12/15/1999 10:08 PM

SteveF

Hi, Steve. Laying on my workbench for lo, these   many months is a "sort of" AC-30 4-EL84 that uses   a Bassman tranny. (Thanks to Dave Harris and Bruce Collins for input and advice. Actually, I cribbaged nearly the whole thing from Dave's schematic.)   I sort of ran out of gas on the project, but it works   pretty good as near as I can tell.   It's not in a cab so have not really thrashed it out   thoroughly, but it makes about the right amount of power and the trans. doesn't get hot. It took some experimenting to get the idle current under control (cathode biased), and I don't think it's running full tilt Class A. I'm running them at about 39ma per tube, if I remember right. Anything much more than that and they were glowing! Be that as it may, it sounds not bad at all.     SF

 

12/16/1999 6:41 AM
Steve A.	Steve:         When you get a chance to check your notes, could you post the value of the cathode resistor(s) you used and the B+ going to the plates of your power tubes?     --Thanks!     Steve Ahola

 

12/16/1999 12:47 PM

Pete

I'd like to suggest the use of differnt rectifier tube also.   This has a changing affect on the B+ and thusly affecting the cathode current draw     The 4 x 6V6 amp I built has a power tranny with a dual B+ taps. I installed a switch to select power state. This works well because the power tubes are cathode biased.     I generally use ether a 5AR4, 5V4, or a 5AS4. So combimning the differnt the rectifiers with the the dual b+ taps I can get a B+ of 320V to 405V.     I've settled on a 200ohm/10watt resistor and a 10uf/50volt   cap per pair of power tube ( each side of the push-pull)   because It's the best sounding to my ears.     Some time durring the next week or so I'll post all the numbers.     Pete

 

12/17/1999 2:39 PM

SteveF

Steve:     I'm using a 68 ohm 5W cathode resistor, voltage from   cathode to plates is 340V. I started out using a   50 ohm per the Vox design, but at this voltage my   tubes started glowing. I'm pulling a total of about   44ma (my memory was faulty when I said 39ma before - that   may have been my guess as to how much was actually   going to the plates.)thru each tube at idle with that res., which means I'm dissipating almost 15 watts per tube, well above its maximum rating of 12 watts.     If anyone reading this cares to comment, please help   with this question:     Am I likely to toast these tubes running them at that   high dissipation? I'm assuming a significant portion   of the 15 watts is dissipated by the screen, but   how much? I've had no problems so far but have not   used the amp hard - it's never been off the bench.     Thanks, Steve F.

 

<<First Page

<Prev

Page 2 of 3

Next>

Last Page>>