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7027's in a Blonde Bassman


 :
6/2/2000 9:23 AM
Rich P.
7027's in a Blonde Bassman
I'd like to get your opinions/ideas/technical details regarding the use of these tubes. I have four NOS RCA 7027's "brand new" (probably mid 70's) and while the ancient Tung-Sol 5881's in the Bassman still test & work fine, I would like to use these when and if replacement of the 5881's is necessary.  
I tried a pair of Groove Tube 6L6, grade 5 tubes in the amp, but didn't like the sound.  
Pros? Cons? Biasing? Sound? Please lend your expert advice. Thanks in advance, Rich  
BTW, the Bassman has the 7025 phase inverter / driver, solid state rectifier, and front-panel prescense control. Tolex is blonde, and the front panel is brown in color.
 
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6/2/2000 2:13 PM
Jim S.

The main issue here is the differences in pinout between 7027 and 6L6/5881. 7027's cannot be used in most 6L6 amps without modifying the output tube socket wiring. Specifically, any amp that uses pins 6 and/or 1 as tie-down points (such as many Fenders do for mounting screen and control grid resistors directly on the socket terminals) will need modification. I believe the blonde Bassman falls into this category, at least as far as the screen resistors are concerned.  
 
If it were my amp, I would save the 7027s for something else or sell them to someone who has an Ampeg, rather than doing the mods. If you do decide to mod the output sockets, I'm sure those tubes would work and sound fine -- especially if they are identical to the legendary RCA "blackplate" 6L6GCs except for the basing. Whatever you wind up using, I recommend biasing for 35-40 ma idle current per tube.
 
6/2/2000 3:48 PM
Rich P.

Thanks! I should probably find someone to trade the 7027's for NOS 5581's. I'll bet an NOS dube dealer would even do it, as the price on these are going way up.  
What can I expect to pay for a good set of NOS 5881's?  
I miss the days of working on an old 1950's Western Electric (M-33) radar system used for instrunmentation on a test range. We had spare parts & tubes out the kazoo, and I would pick up JAN 5881's and JAN 6L6WGB's in little cartons of six tubes. The 5881's were all 1950's Tung-Sols, and the 6L6WGB's were mostly GE and Sylvanias, but of various years of manufacture. I put four of those in a Super Reverb I tuned up once, stuck 'em in, biased the amp, and it just came alive. Same thing on a late 70's Mesa Boogie MKIIC amp; two NOS 5881's made it sound better than it ever had in 15 years.
 
6/5/2000 2:47 PM
Graywater

Rich,  
 
Much as I would hate to part with them I may be willing to trade my NOS-Milspec Tung-Sol 5881s for 7027s - these were spares for a surplus db meter - the 5881s being used as the pass tube in a series voltage regulator. E-mail me  
 
GW
 

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