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Post by BradPaisleyFan (Pat) on Jan 20, 2014 11:41:56 GMT -7
I was just looking around on the Dr Z website and was wondering what the difference was between the two??? I understand what biasing an amp is and what it does but I'm not familiar with cathode and fixed or any other term like that. Can someone explain?
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Post by Paul (TRANE) on Jan 20, 2014 12:42:39 GMT -7
Fixed bias amps must be biased to have the power tubes in the correct range for current. Cathode bias amps are plug and play. No need to bias a cathode based amp.
Hope that helps.
Sent from my SM-N900V using proboards
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Post by jesslm02 on Jan 20, 2014 14:09:28 GMT -7
What he said. But just to add to it - fixed biased amps usually have an small variable resistor inside that is used adjust the bias, if needed, upon installing new or different power tubes.
Neither is necessarily better, but they each have their own character.
Sent from my Galaxy S4 using Proboards
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Post by zpilot on Jan 20, 2014 14:15:37 GMT -7
Check out "Inside Tube Amps" by Dan Torres for a good explanation of the circuitry. That's for technicians. As a player what you need to know is cathode biasing an amp will tend to make it compress a bit more than fixed biasing.
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Post by BradPaisleyFan (Pat) on Jan 20, 2014 14:43:50 GMT -7
So cathode biased amps basically put "bias themselves" automatically??? While fixed are the ones you have to adjust yourself?
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Post by Deleted on Jan 20, 2014 14:52:18 GMT -7
A cathode biased amp will have more harmonic content. Where a cathode biased amp has a more focused sound.
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Post by BradPaisleyFan (Pat) on Jan 20, 2014 15:00:28 GMT -7
A cathode biased amp will have more harmonic content. Where a cathode biased amp has a more focused sound. Ugh run that by me one more time lol
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Post by Deleted on Jan 20, 2014 15:21:32 GMT -7
This is one of the reasons I love my Mazerati. Closely matched set of el84s and you're good to go.
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Post by southmusic70 on Jan 20, 2014 16:14:54 GMT -7
Yes, I re-tubed my Rati v2 last month, and it really sings now.
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Post by zpilot on Jan 20, 2014 19:33:02 GMT -7
A cathode biased amp will have more harmonic content. Where a cathode biased amp has a more focused sound. Ugh run that by me one more time lol Me too.
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Post by zpilot on Jan 20, 2014 19:48:09 GMT -7
So cathode biased amps basically put "bias themselves" automatically??? While fixed are the ones you have to adjust yourself? Generally, that is correct. As in all things there are exceptions so don't put a set of tubes in your cathode-biased amp and just assume they will work. When initially powering up you need to monitor the amp for a while to make sure they don't redplate. It's rare but it happens.
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Post by Deleted on Jan 21, 2014 8:03:59 GMT -7
A cathode biased amp will have more harmonic content. Where a cathode biased amp has a more focused sound. Ugh run that by me one more time lol Must have been stoned when I wrote that. The "fixed" biased amp will be more focused.
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Post by greenblues58 on Jan 21, 2014 8:43:26 GMT -7
have recently had experience of this. Purchased a set of reissue Tungsol EL84 for my 1974x they ran for about 30 mins then amp was intermittent and on checking one valve was redplating.. I let them cool down then swapped positions and the same valve redplated after about 10 mins. Returned the valve to the supplier and it tests fine on all three of their test rigs so they sent me a new valve matched to the original one. Again after a slightly longer period ,about an hour,it redplates and amp goes intermittent. In case there was a problem with the amp,though it has run fine on JJ,s and has run for three months on TAD STR,s,I put them in my MAZ. This time I put the amp on standby for a few minutes then observed the valves as I switched the power on. They both immediately redplated then arced . I couldn,t switch it off any quicker than I did. Popped the EH,s back in the Maz and back to normal. I have contacted the supplier who is in disbelief that there should be a problem again. I can,t believe these Tungsols are incompatible with both amps. The valves test slightly higher on PC from the supplier than I normally have ,but the same TC, but its minimal in terms of the range that they test.
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Post by myles on Jan 26, 2014 11:17:24 GMT -7
There are many explanations but here is one - mesaboogie.com/US/Smith/ClassA-WebVersion.htmKeep in mind that there is a difference between true class A, single ended, and cathode biased class A push pull. Just because an amp is cathode biased it does not mean it is a class A amplifier circuit.
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