1973 Fender Twin Reverb Bias question

Lefty · From: Grayson, Ga.

I bought this Twin Reverb for my wife for Christmas.
My question is: I assume this amp has adjustable bias. Two of the power tubes(of a matched quad)have bulbs that are loose on the tube base. I am surprised because I tried the amp out, and it sounded really lush and good.
Shame on me for not checking the tubes more carefully. These tubes are in the 2 and 4 position from the backside left. If I change these out will there be a problem using the amp sparingly until I have a new quad installed and biased?

This amp has a cracked cabinet from a fall. That may have something to do with the damaged tubes.
The good part is I did not pay a lot for it, and it has a pristine pair of JBL k-120 speakers in it.
The amp was so heavy that I could barely pick it up.
I am putting some lighter speakers in after I repair the cabinet. The amp looks like it was taken care of otherwise, and the input filter caps have been changed. Transformers are good and the wiring has not been tamperd with.
regards,
Lefty

Lane Gray · From: Topeka, KS

You mean the bottles are loose on the base but the tubes still work? That sounds odd.
Sure it's not loose socket?
I got no answer, but the question makes me scratch my head.
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Stephen Cowell · From: Round Rock, Texas, USA

Most important: the amp has seen a big fall. You are taking your (and your wife's) lives in your hands playing with an amp that hasn't had a complete checkout by an experienced tech. A new-to-you used tube amp should also be seen by a tech... at least examine your fuse and make sure it's a 2A slo-blo and not a 20A automotive fuse. If your amp doesn't have newer filter caps and a three-wire conversion it's time for that too.

Next point: if you're going to pull two tubes, there must be one of the big tubes on each side. Inside, outside, doesn't matter... as long as one is on each side of the four sockets. You can cut one speaker wire and have matched impedance... not really that important unless you run the amp flat out for long periods this way, or unless the bias is not set right... see below.

Next point: pulling tubes will cause the B+ voltage to climb some... your amp will run hotter, since the bias voltage will stay the same. If you see the tubes 'cherry up' (glow dull red inside) then the bias is too low for the finals... and failure is imminent.

Main Point: techs need to eat too. Your new amp should be looked over by a technician. They can sell you a new set of tubes, install/bias them properly, and warranty their work and the tubes.
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Stephen Cowell · From: Round Rock, Texas, USA

Steven Paris · From: Los Angeles

Here's what you can use to set your own bias:
http://www.tubedepot.com/bt-bias-scout.html
A very handy device!
Now, how much do ya want for the two JBL D-120s?

Lefty · From: Grayson, Ga.

Thanks for the input guys.
Yes the tube bottles are loose.
I will change those two tubes before and check the fuse before using the amp. The chassis is really clean without any damage, but I will have my amp tech check it out. I paid 475.00 for the amp, but plan to keep the K-120's as a Christmas present Very Happy

.
The amp was so heavy with those speakers that my back is a wreck today just moving it from the car to my basement. I have some period Jensens to put in for my wife.
thanks,
Lefty

Michael Brebes · From: Northridge CA

I am pretty sure that the Twin from that era as a Balance pot instead of a Bias pot. It balances the two upper tubes against the two lower tubes (schematic-wise) while leaving the bias in a fixed position. Many of these amps have been converted to regular Bias adjust, but only a good tech could tell you.
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Tim Marcus · From: San Francisco, CA

yes - if it has a master volume, its likely also got the bias balance pot which is useless for setting the bias.

One way to avoid needing bias adjustment is to replace the tubes with an exact plate current match

other than that, you should take it to a proper tech to set up
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Jim Rossen · From: Iowa, USA

"Next point: pulling tubes will cause the B+ voltage to climb some... your amp will run hotter, since the bias voltage will stay the same. If you see the tubes 'cherry up' (glow dull red inside) then the bias is too low for the finals... and failure is imminent."

My Vibrosonic Reverb (Twin Reverb but with a 15) has a switch that lifts the cathode of a tube from each pair. This results in a minimal increase in idle current, about 2 ma IIRC. Therefore, seems like bias adjustment is not needed when running two instead of four power tubes.

Jim

Lefty · From: Grayson, Ga.

Well I completed work on this amplifier, and it turned out pretty nice. I got the cabinet reglued, and the tolex glued back in place. I put a quad of "Fender" branded Sovtek tubes, and it runs nice and cool. I put a pair of speakers from my 1968 Pro-reverb in there for now, and it sounds really good.
I will put a picture or two up.
The bad part is one of the JBL K-120 speakers is not working, maybe a blown voice coil. Even with that, I still did not get a bad deal for the money.
I hope my wife will feel the same way.
I am such a nerd that I would like to pull the chassis, and show her the hand wiring.
regards,
Lefty

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