SF Twin Reverb on the bench

[JohnFender]

MV appeared in '72. MV w/switch appeared part way thru '73. UL 135W appeared part way thru '77.

Yeah you're right I thought about that right after I wrote the msg, I got mixed up with my 72 which does not have a boost but has a mv, after getting addicted and buying 3 SF twin in about a year span has got me spending alot of time figuring out how to read a schematic, had to learn everything from scratch and I still have a lot to learn, I think it's harder to be an expert at fixing/building amps than to play guitar so if you understand everything in a Twin Reverb you got my respect and can we hang out?

 

[Lynxtrap]

After fixing the shady solder joint in the bias supply, I did a new test last night, monitoring the output tubes with the bias probes.

Over about 30 minutes, the plate current slowly crept up about 2 mA, then it seemed to settle. No voltages changed during that time. Well, actually the whole B+ fluctuated a little through it all, and the plate current with it, but I wonder if that is due to fluctuating wall voltage.

I decided to think that the current increase is perfectly normal as long as it's not more than 2 mA. With the amp hot, I adjusted the bias a bit cooler so that it should stay safely below 70% at all times.

I then let the amp cool down and turned it on again. Sure enough, the current was down about 2 mA.

 

[Lowerleftcoast]

The OT is a couple of coils of wire. In use it will warm up. For those of us that use the resistance of the coil to determine the bias numbers, we want to know the resistance of the coil primary at operating temp. If you have ever checked the DC resistance of guitar pickups you will notice even the temperature of your hands handling the pickup will result in a different measurement. It should be no wonder that the bias numbers will change as an amp warms up.

 

[Lowerleftcoast]

I have a pull boost/master volume on mine and it still works good, I only it used a dozen times still fun to wonder how music was at in 76

In the '70s there was a pedal effects *race* going on. Silicon transistors were taking over but germanium were still used.
The Crown DC 300A was from '74.
Most club musicians were still using Shure style column speakers.
Rhodes pianos were the mainstay for keyboards.
Echo EFX were from tape machines.
MXR Phase 90 was from 1972.
The red and black Big Muff was from 1977.
Boss OD-1 was from 1978.
Groove Tubes was from 1979.

 

[Wally]

In the '70s there was a pedal effects *race* going on. Silicon transistors were taking over but germanium were still used.
The Crown DC 300A was from '74.
Most club musicians were still using Shure style column speakers.
Rhodes pianos were the mainstay for keyboards.
Echo EFX were from tape machines.
MXR Phase 90 was from 1972.
The red and black Big Muff was from 1977.
Boss OD-1 was from 1978.
Groove Tubes was from 1979.

of those, I have an Echoplex II and the Ram’s head Big Muff. Those were elemental to the sounds that David Gilmore was creating.

 

[Lynxtrap]

The OT is a couple of coils of wire. In use it will warm up. For those of us that use the resistance of the coil to determine the bias numbers, we want to know the resistance of the coil primary at operating temp. If you have ever checked the DC resistance of guitar pickups you will notice even the temperature of your hands handling the pickup will result in a different measurement. It should be no wonder that the bias numbers will change as an amp warms up.

Thanks, you are probably right, this is probably it.
I did calculate the bias using primary resistance to begin with.
But since plate voltage did not really change in any consistent way, I deemed it pointless and used the bias meter.

What I should have done is measure the primary resistance with the current draw at its peak after the amp had been on for >45 minutes.

I took one reading of it when I started working on the amp, and it had only been on for about 10 minutes by then. Not enough time for the OT to warm up I guess.

 

[Lynxtrap]

Can I see a picture of the amp?
I have a 76 too if you need pictures of the inside for any reference
I think mine is all original on the inside

Not the best pics, and taken after I had already done all the work on it. Most of the white Mallory electrolytics are gone for instance.