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#26 2010-12-22 18:25:57

gsb
Member
Registered: 2010-02-10
Posts: 58

Re: 6550 Drop-In Compatible Tubes?

Thans. Do you think that the "balance pot" mod will fix low hum?

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#27 2010-12-22 19:27:49

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 6550 Drop-In Compatible Tubes?

if the tubes are not balanced it can certainly cause some hum.

without seeing your amp, i couldn't say with any certainty that the balance pot would fix the low hum.  I think it would help though.

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#28 2011-05-20 19:34:27

Blaster5150
Member
Registered: 2010-04-21
Posts: 12

Re: 6550 Drop-In Compatible Tubes?

Have any of you tried London Powers Master Volume Kits or Bias Adjust Kits, or Power Scaling kits?  No attenuators needed. They have two types of Master Volume Kits, one after the splitter and one imbedded in the splitter. Their power scaling will let you play at any level without losing the distortion you want and without coloring the sound.  Currently I'm working with JFets and MosFets as cathode followers to help dump excess current so a Class A or AB amp will not go into class B operation because the voltages are too close together at the screens, plate and cathodes.  Too hot a bias or out of spec. resistor can cause flashover and eventually arcing in a tube.  The current needs to go somewhere.  Usually through the tube and output transformer.  These mods are not in the signal path so, no sound changes.
Taniguchijap51

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#29 2011-09-27 18:39:01

Blaster5150
Member
Registered: 2010-04-21
Posts: 12

Re: 6550 Drop-In Compatible Tubes?

I have installed London Powers Bias kit in my early 70's V-4 and it allows me to use a variety of octal based tubes, after re-biasing.  I also use it in an amp I'm modifying, an old Bogen PA amp with 6l6's.  In the Bogen I installed a separate raw bias power source and on the V-4 I used London Powers Raw Bias Supply. The bias voltage really needs to be stiff.  When playing, your bias voltage is going to change slightly when playing loud heavy passages.  Your voltage needs to be stable when connecting 4 pots and associated circuitry to the bias section.  On Class A amplifiers/Cathode Biased types, the Resistor and Capacitor on the Cathode of each tube is set up so that a constant current is already running through the tube(s).  So, Cathode biased amps are self-biasing but, run at max current with or without a signal input.  In other words it's running at max all the time.  Consequently they use up tubes more often then fixed-biased amps.  When hit with a signal it appears to be more touch sensitive to most players. Class A amps always run hotter, heat wise than comparable fixed biased types.  Some companies run their Class A amps at 80% current flow. Push/Pull/Class A/B amps run with alternating pairs, one tube or set of tubes is off while the other(s) are on.  Either way I install a fan in all my tube amps to pull the air out and assist in circulation.  However, I also have a 5W,15W,22W,30W and a couple of other 100 watt amps (both switchable from Class A-A/B and Triode/Pentode selectable that give me 100W to around 30W along with other amps, EL84, EL34,6L6's,6550's,6V6's among tubes used. I have modified old tube amps for guitar with a variety of cabinets to match my needs.  They are all tube except for one solid state amp that uses FET's (controls much like a tube) for clean mic'd sounds mixed with a dry signal etc.  50 years of selecting, and trying to hold on to a variety of amps and guitars.  For some, that's what it takes to be able to be ready for as many situations as you can that may arise through your guitar playing career.
Blaster5150

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#30 2011-09-28 07:21:48

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 6550 Drop-In Compatible Tubes?

Blaster5150 wrote:

On Class A amplifiers/Cathode Biased types, the Resistor and Capacitor on the Cathode of each tube is set up so that a constant current is already running through the tube(s).  So, Cathode biased amps are self-biasing but, run at max current with or without a signal input.  In other words it's running at max all the time.  Consequently they use up tubes more often then fixed-biased amps.  When hit with a signal it appears to be more touch sensitive to most players. Class A amps always run hotter, heat wise than comparable fixed biased types.  Some companies run their Class A amps at 80% current flow. Push/Pull/Class A/B amps run with alternating pairs, one tube or set of tubes is off while the other(s) are on.

I think you are mistakenly grouping cathode biased amplifiers and Class A amplifiers.   there are plenty of cathode biased amplifiers that are not class A.  the Vox AC30 is a perfect example.
the AC30 is biased hotter than most Class AB amps, that is why the tubes run so hot and don't last as long,  but it is still class AB.   

if you want a good read about class A:
http://www.aikenamps.com/ClassA.htm

As always Randall Aiken provides an excellent explanation.

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