Help with National Tremo Tone

[jukelemon]

Hi all.

It has been a looooong time since I have posted.  Hope everyone is well.

I used to have a pretty good working knowledge of things.  But my hobby went back to restoring/building Harleys so please excuse my memory loss.

I have a friend in his 70's that bought an amp/guitar combo in 1956 (his memory).  Guitar is a Epi Regent (really nice/blonde) and the amp is a National Tremo-Tone.

He told me that he was letting his grandson use the amp and it just stopped working.

So, he brought it over last night.

The amp is 100% bone stock.  Still has the original, wooden Trem footswitch.  I mean bone stock.

So, first thing I see are several rub marks in the 2 wire AC cord.  Which is a shame because the rest of the cord is mint.  Not hard.  Looks new.  But that had to be taped up.

Second thing I see is plastic tape over the fuse cap 

My friend says it is because his grandson could not get the cap to stay.  So, I showed him how the cap works...push in and turn. 

Plug it in and bam, things comes alive.  I have never played one of these but I have played many others.  I was expecting a VERY quiet amp-something like an older Alamo.

Not this.  Damn...Loud and sounds amazing. 

I try the Trem and nothing.  And here lies my question..

http://api.ning.com/files/71rkbAiyQyLp11iu04xznkmadPGQcwct1GnHeaTKG91GpAKK6fshYV23a8ROJ0ZnDrVQidnQ0Vwtc3riwFl29gQa-UkPHCfN/Tremotone.jpg

When I hit the Trem button, you can hear something happening because the Volume reduces by ~30%.  There is an On/Off and then Trem intensity (for lack of better words).  It does nothing other than activate what seems to be some failing parts/tube i.e. no changes when you turn the knob/just when you turn it on.

I don't have any of these octals to sub in unfortunately.  That would have been my first approach.

Looks like it is NOT a bias trem on the power tubes.  Correct?

This amp is so clean on the inside it is ridiculous. Like I said..bone stock.

Thanks all.

[terminalgs]

I don't have any of these octals to sub in unfortunately.  That would have been my first approach.

You do have an octal to sub,,,..  the other 6SQ7 in the amp!  swap'em,  if the trem 6SQ7 is dead, it'll be dead in the preamp too.

If the trem 6SQ7 works, then start checking out the oscillator circuit.  The 25/25 under the cathodes (both 6SQ7s share a common cathode) needs to work properly, or the trem 6SQ7 probably won't oscillate. I'd check that one first if you have a spare 25uf +/- cap.  failing that, check resistance to ground in each of the oscillator C/R segments,   failing that,, check out the caps (.05, .05, .02) in those segments..  also, there should be no DC voltage between any of those caps...

[eleventeen]

One characteristic of a phase shift oscillator (note the three caps in series, from plate to grid of the trem 6SQ7) is that it is not a particularly reliable starter. The operation of the oscillator is quite dependent upon good gain from the triode. I have recently proven this to myself by building one with some of the guts from a CONN organ I salvaged. The same circuit will NOT work with a 12AU7 or 12AT7; it needs the high gain of a 12AX7. It takes several seconds to start past the point when the tubes are fully lit up. (allow 30 seconds for the tube to light, won't start for maybe 8-10 more seconds) So *anything* that robs gain from that circuit could conceivably prevent it from starting up. Leaky cap(s), old tube, drifted resistors, everything will fight you.

You might try turning on and off the footswitch a few times, just as a crude "goosing" of the circuit, but you're probably looking at some old, drifted components, perhaps and probably including the tube, as things you're gonna have to replace to have that ckt start reliably.

[jukelemon]

Thanks all.

The amp sounds so good and powerful it is hard to believe something has drifted.

One note...one of my preamps is actually a 12ax7.  Phase inverter?

I am thinking it is v1 and the other 2 are trem and phase?

Either way still the same approach minus the subbing.

I'm betting on a failed cap.

[sluckey]

One note...one of my preamps is actually a 12ax7.

That can't be the right schematic then.

Tubes has to be at the top of the list of suspects. Especially in a tremolo oscillator circuit.

[jukelemon]

Right.  I cannot find a schematic with the 12ax7.  For that matter, I cannot find much on this amp at all.

I am guessing the trem circuit is probably the same and just a different v1 circuit is being used?

I don't know.

I hate to even touch this with an iron.  It is so clean.

[terminalgs]

Right.  I cannot find a schematic with the 12ax7.  For that matter, I cannot find much on this amp at all.

I am guessing the trem circuit is probably the same and just a different v1 circuit is being used?

I don't know.

I hate to even touch this with an iron.  It is so clean.

so, what do you have for tubes?  two 6SQ7's, a 12AX7, two 6V6's, and a 5Y3?

maybe post some pictures, insides and outsides...

[PRR]

> hit the Trem button, you can hear something happening because the Volume reduces by ~30%.

Replace all three of the point-oh trem caps. One or all of them are leaking. Use HI-voltage caps, at least 400V, and 600V is not over-kill for long-term reliability.

Verify all three resistors in trem grid circuit. Don't have to be spot-on, but if "470K" has drifted to 5Meg it won't be happy.

Replace the tube. This type trem just-works with a healthy full-spec 12AX7. A sick tube, or a few new-made tubes of slightly different construction, won't work. If it's supposed to be 6SQ7, be sure it IS a 6SQ7. If there's two 6SQ7 in the amp, swap them: a little-low gain won't hurt the preamp function much, but could make all the difference in the trem socket. There are thirteen+ tube types which "replace 6SQ7" in all the usual *radio* sockets, but most of them will be short on gain and here it matters. New 6SQ7 are $7-$8 at TubeDepot.com or TheTubeStore.com

Replace the 25/25 cap under the common cathodes (trem and preamp). Try to get very close to 25uFd, like 22 or 27. Voltage does not matter, 5V or higher.

> one of my preamps is actually a 12ax7

When 6SC7 went out of style, 12AX7 was the logical re-design.

Trace the functions. Input jack goes to first stage. Trem pot/switch goes to three capacitors on the trem oscillator. OTOH the driver (PI) has caps going to 6V6 grids.

[jukelemon]

Thanks guys.

I will get a new tube and try that first.  I have confirmed it is the right tube.  Probably the original.

If that does not do it, then I will look to the 3 caps in the rem circuit.

Thank you for the help!

[jukelemon]

Well, new tubes came in.  No change.

Can I do the LED trick with an octal?

[PRR]

> Can I do the LED trick with an octal?

You can.

Except I'm not sure how to do it with THIS preamp cathode modulated circuit.

The amp DID work 20 years before LEDs were common. It does not "need" an LED, IMHO.