2. amps-preamps
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Red Plating??

I have a CLASS A KT88 amp that has an autobias circuit.  The company rep recommended SED 6550C Winged C tubes.  I purchased a quad of them.  They bias OK and they sound wonderful but I just noticed that they red plate a bit. If I can figure out how to do it I will attach a picture.

The red plating is just in the inner corner of one of the plates on all 4 tubes.  It looks as though I am looking at the filament through a translucent plate but I know that its red plating. 

https://imgur.com/a/RVLWPcf

Can I operate like this safely?  The bias does hold steady and does not fluctuate. The red plating does not change.  As a Class A amp this status should hold steady.  Of course, it does run hot. If all that it hurts is tube life, thats OK.  These are my best sounding tubes without a doubt.

chinook9
chinook9 OP
68 posts
 
Great information being posted in this thread.  For the time being I am waiting to hear from the company rep but I'm not holding my breath. I'll use the KT88s until I get this resolved. 

Not sure if the bucking transformer will solve the problem but its worth probably worth the investment of $90 if it might work. 

I'd like to hear opinions on potential of the bucking transformer to solve the problem.  They are available in 3% or 7% reduction in voltage and it looks as though I should go for the 7% reduction if I get one.
ieales's avatar
ieales
1,201 posts
 
chinook9, log your voltage over a few days. It is not uncommon to have a higher level for short periods. Base your required drop percentage on average. Note that voltage levels can vary drastically by season dependent on the local requirements. In L.A. summer is the high load, but the Oregon coast winter load is higher.

A device rated for 115v is typically ±5% which is the line voltage standard. However, local utilities may struggle to maintain. This is a good read  Voltage Tolerance Boundary (pge.com)

The eBay link above has no current spec. The box is plastic. Pass...

Build a variable bucker and be done with it!
atmasphere's avatar
atmasphere
12,198 posts
 
That is 5VA, not 5A. This is a low voltage transformer for things like doorbells.

https://www.ebay.com/itm/Edwards-590-Transformer-10V-5A-Load-120V-Line/121034476341?hash=item1c2e378...


Ebay has premade bucking transformers: https://www.ebay.ca/itm/Bucking-Transformer-Voltage-Reducer-For-Fender-Marshall-Gibson-Vox-Ampeg-Etc...

Good catch! - thanks.


I was not aware of the product for guitar amps but its nice to know about! That would be perfect for this sort of thing, if line voltage is all there is to it.
yogiboy's avatar
yogiboy
5,927 posts
 
@atmasphere
It looks like the 6550 is back in production. This place has them listed as new!
https://tubedepot.com/t/tubes/power-tubes/6550-kt88-kt90
atmasphere's avatar
atmasphere
12,198 posts
 
Yes, there are now several varieties. Electric guitar players like them for the distortion they make.
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