Like to "open up" '51 TV Pro.

Hiro Keitora · Post by **Hiro Keitora** » 22 Mar 2001 2:04 am

I know field coil speaker isn't the way to go today, but this was it back then. I fooled around by changing coupling caps to lower value( .1 to .o2) and it started to open up. Any other "tricks" to let it lose more?

Jay Ganz · Post by **Jay Ganz** » 22 Mar 2001 8:24 am

So this is a model 5C3, right? It's got
the metal 6SC7 preamp tubes instead of the
12AX7 & 12AY7. I know what you mean about
"opening up" the tone on it. I used to
have TV front 5B3 for awhile. The main
thing about 'em compared to the later
models is the really primitive phase inverter
circuit. I don't think a field coil speaker
was stock back then to be honest. They
usually had a Jensen P12R in them. I have
one of those in my '57 Deluxe & it sounds
outrageous. Of course I use that amp mainly
for Slide Guitar (not steel).

Blake Hawkins · Post by **Blake Hawkins** » 22 Mar 2001 8:39 am

I have a Fender 5B3 which I use frequently.
The stock coupling caps are .05 mfd @600v.
When you change to a lower value, you'll probably use some low end.
Mine has the Jensen P12R which is the original speaker.
Never saw one with a field coil speaker and the service material I have does not show it either.
I plug the lap steel into the Microphone input. That seems to be a better match for
my Magnatone.
Blake

Bill Crook · Post by **Bill Crook** » 22 Mar 2001 10:00 pm

I must be missin' something here....

You guys are stating that the early (50's) model used field coil speakers, While this IS true,the field coil had nothing to do with the ability to "Open Up".

A short lesson here:

In the early days of radio, The magnet on the back of speakers was almost non-existent. The way the magnet field was produced was a steel cone was placed behind the voicecoil with the voicecoil extending over the front part of the steel cone. Now, a coil of wire was placed around the back half of the cone. This coil was feed from the DC power supply at the rated voltage of the DC power supply. (about 450 volts) This coil had a two-fold function. 1st, it served as a "choke" in the power supply to filter the ac ripple rideing on the dc voltage, (as a finely tuned choke can filter much better than a cap, which at that tine wasn't too good.) and 2nd, It did make one heck of a magnet source for the speaker.

Now, with that out of the way, Changing the coupling caps is whats making your amp sound better, The value of the caps (.02, .05, .1) can be trimmed to suite your perferred taste. Only the voltage rateing of the caps MUST be keep higher than the plate voltage of the tube. In general, the speaker had little to do with the dynamnics of the sound, as High-Fedelity wasn't even a desired part of the radio/amp back then.

I will say this tho,, Up untill the advent of the new foam surround built into speakers to allow them more "throw" using less power, the old "Field Magnet" speakers did produce a much better sound at low and high volumnes than the later cobolt magnets used later. This was partly due to the fact that as the volumne got louder, the tubes conducted more, thus causeing a current drain on the DC power supply, which lowered the avalable DC voltage to the tube. The results was a self-made compression system. No distortion, speakers seldom bottomed out, a pure tone 100% of the time.

Too bad we can't have that sound again. The old tube sets always had a more warm tone and sound than the stuff on the market nowadays.

Hiro Keitora · Post by **Hiro Keitora** » 23 Mar 2001 4:18 am

Goodletsville? Then you must know Woody Woodell of Benchcraft Electronics! By the way folks, this model is 5A5. And the suspicion of the field coiled speaker not being an original, can be explained well when you don't see no trace of output transformer on chasis. Sorry, it's stock al'right.

Its input has so called "grid leak bias" with .1 cap and 5 meg resistor unlike more common cathode-bias style with 1 meg resistor and no cap. I'm wondering I should convert it or not.Would this conversion help to "open up" the tone because I'm getting rid of that .1 cap?

Bill Crook · Post by **Bill Crook** » 23 Mar 2001 9:03 am

I would not suggest gettin' rid of the .1 cap. It's function is to "ac bypass". These early circuits were not too stable at best so I wouldn't get too far removed from the original cap. These early amps were of the class "A" type and had a tendency to "RF" a bit. Removeing the cap is a sure way to cause this effect. Plays hell with tubes and your neighbors proberly wouldn't like it either.

Yes, I know Woody, He is a super person and has done work for me in the past. I would recommend him for all your speaker and case needs.

Jack Stoner · Post by **Jack Stoner** » 23 Mar 2001 9:23 am

I've got a lot of older Fender schematics, but not that one. I even have one old Princeton Amp schematic that doesn't even have a chassis number. It has a 5Y3 rectifier, 6SL7 preamp and one 6V6 output tube.

Jay Ganz · Post by **Jay Ganz** » 23 Mar 2001 3:02 pm

Hiro,
The amp that I had also had the output
transformer mounted on the speaker. But
that wasn't a Field Coil speaker.
If you'll notice on many early 50's
Jensens, they have a flat area on the
frame where a small output transformer can
be mounted. Some amps just mounted them
there instead of the on the chassis itself.

Bill Crook · Post by **Bill Crook** » 26 Mar 2001 6:13 am

The "Fender TV Pro" era 1953 as shown in the "Fender" web-site used a 2 tube 6V6 as a output section. Unfortunatly tho,the chart dosen't go back any farther that 1953.

Any audio device that uses a field coil to drive(make) the speaker magnet will not be a 2 tube output circuit. These early speakers were used in the 1920-40's before the advent of push-pull circuits and output txmers and such.

I have seen several radios and other audio equipment with the output xmer mounted on the speaker frame. All this was early(1950) stuff tho. Usally,even then there was only one(1) output tube involved. The audio technogily(sp) at that timeframe wasn't up to speed. In fact,that it worked at all was a feat in itself. [This message was edited by Bill Crook on 26 March 2001 at 06:23 AM.]

Hiro Keitora · Post by **Hiro Keitora** » 26 Mar 2001 6:00 pm

F15N Jensen was quite common for early "Pro". Output tubes were alway 6L6s and not 6V6s. These paraphase inverter style is a "love and hate" and it's far from more efficient "push-pull". Tho,it's premitive, but it sounds right on money when you plug in early "Dual Professional" or something.

Blake Hawkins · Post by **Blake Hawkins** » 27 Mar 2001 7:20 am

Bill, as a long time radio restorer specializing in 1930's radios I must respectfully disagree with your statement about no two tube output circuits with field coil speakers.
Field coils were the first dynamic speakers, long before permanent magnets were developed.
I have worked on '30s PA amplifiers, Gibson Guitar Amps, radios, and juke boxes all with two to four tubes in the output stage and employing field coil speakers.
In many cases, the speaker field is also employed as a power supply filter choke, however the earliest of radios used separate power supplies for the field. In later units, if the current demands were too great for the field coil, and additional choke would be used.
Also, in later speakers when the field is used as a choke, the pulsating current in it would cause a 60 cycle hum in the speaker. The solution to this was to provide an additional winding on the voice coil, connected out of phase to cancel the hum.
Many radios and amps had the output transformer mounted on the speaker as well.
In the days of field coil speakers, there were some very high quality units made for broadcast monitoring and for use in juke boxes.
Blake

Bill Crook · Post by **Bill Crook** » 27 Mar 2001 5:48 pm

Blake...

My experience with these early day radios and such has been limited to the floor model systems (home units, not the hi-end broadcast stuff) I cannot remember seeing a push-pull output in these sets. The second output tube was simply parelled with the 1st tube.

Your post in general, (about the speaker field) matches mine. I am sure that you doing restoreation on these vintage radios, are much more knowledgeble about this. My experience with them has been repair in my early years of electronics. (Consummer gear only,My Industrial Electronics era begin in the late 1970's)

Good thread here tho, Really brings back the early days when I generally distroyed far more that I fixed. [This message was edited by Bill Crook on 27 March 2001 at 05:49 PM.]