Skylark Tremolo PT

[shaun]

Heyo. Hope all are well.

I've built a Skylark Tremolo, apparently also known as a Kalamazoo 2. I added a bypass cap to the EL84 cathode, and one to the preamp input cathode, and it is one of the sweetest little amps I ever plugged a guitar into. But the PT has a bad buzz which I can feel physically and also comes through the speaker. The PT is from a Columbia radio and was an excellent fit for a bunch of reasons. But I'd like to replace it with the correct tranny, which is proving difficult to find.

I seem unable to find anything labelled for this model. So I'm trying to figure out how to order a replacement. While Fender, Vox, and Marshall trannies are well covered, it seems nobody stocks much in the way of Gibson trannies. So I may have to order according to specs, and this is where my knowledge becomes cloudy. I'd appreciate any clarity that can be brought to bear.

In understanding the PT parameters, I know a few things. First, I only need a 6.3v winding (no 5v needed). The current tranny puts out 275 per plate at 120vac, which gives me a B+ of 310v coming off pin 7 of the 6X4 rectifier, and that couldn't be better.

I know I'd need to know the current draw.
For heaters: 6X4 = 0.6A.  For two 12ax7s = 0.6A.   For the EL84 = 0.760A. The total of which = 1.96A.
So on the 6.3v tap, I'd need at least a two amp heater draw, probably two and a half, if I consider the pilot light and safety margin.

But there is also the plate current to consider. Is it simply a matter of adding plate currents of each tube, as with the heater draw? Also, I see PTs with attributes such as "300mA," but I'm not sure what that refers to. Is that the combined plate current draw, perhaps?

I have a secondary question regarding the tremolo. It works fine, but while it has a speed pot, it is set to only one fixed intensity level. And it is quite intense. I think - but I'm not sure - that a resistor or two could be adjusted in the tremolo section which would make the trem a little smoother - less intense. Any suggestions?

As always, thanks for this great forum and to the people who give of their knowledge so freely.

[shooter]

the "300mA" is the High Volt side, you cranked out the filaments, the schematic gives the values you need (hint 48mA:), so a good engineer doubles so they don't have to do math.  Round up and 100mA at 250vdc + your calcs, walla
He's a schematic that calls out a part number, but I'm watching hail 

[shaun]

Yes, thanks Shooter. Thanks for explaining the mAmps. I saw that TF-5A-P. But it's running two 6AQ5 power tubes, and I'm running a single 6BQ5, so I assumed they'd be different trannies.

But it is matched to a 6X4, so maybe it would be a good fit. They don't come cheap, so I wanted to feel confident I was buying the right one. Still, I haven't felt confident in quite a while, so maybe I'm asking too much....;).

[tubeswell]

The trouble with a 6X4 rectifier running off the same winding as the heaters for the other tubes, is the 6X4 will be running a high heater-to-cathode voltage, and these are known to be somewhat less than reliable rectifiers in that situation.


So perhaps consider SS rectification, or if you still want a rectifier tube, get an auxiliary PT to run the rectifier tube off a separate filament winding


(The hum could just be inadequate ripple filtering on the power rail - seeing as how its SE - or it could be a bad rectifier tube, or something else)

[shooter]

+1

SE really wants, a stiff(big caps) SS rectifier, to quiet hum. 

I posted the part# more in jest, Gibson usually bought stuff off the streets, made in basements by out of work burger workers 
unless you're doing a restoration, source an equivalent like you're doing

[tubenit]

Perhaps this would help some.  6AQ5 is 80ma for push/pull.  IF you found a transformer that is 275-0-275 with a 5v wiring for a 5Y3GT and at least 90ma or more in current handling then you'd be fine.  Just change the 6X4 for a 5Y3GT. 

With respect, Tubenit

[ac427v]

I've had really good results with the Hammond 290AX. It has two sets of high voltage taps. The red-white ones are 275-0-275 at 100ma. Also has a 5 volt winding in case you want to change to a 5Y3 rectifier. But it may be too large to fit your chassis.I really like the idea of making the existing transformer work properly. Shooter and Tubeswell's suggestion of using silicon diode rectifiers and adding another stage of filtering is worth trying. That would involve another 22uf-450volt cap and 100ohm-10 watt resistor.

[shooter]

I like this guy for small tube builds, (using one now), Into a full wave bride I get about 330vdc at 1st tap.  you do have to add an artificial CT for filaments

https://www.edcorusa.com/xpwr050

upside, it's small n cute
downside it takes 6weeks to get 

[shaun]

Lots of great feedback top consider. Thanks folks!

(The hum could just be inadequate ripple filtering on the power rail - seeing as how its SE - or it could be a bad rectifier tube, or something else)

Good idea! I think I'll consider better filtering; I already traded out the 6X4 and it made no difference. The current PT has two 6v windings, one separate for the 6X4. The 6X4 winding is at 6.6vac, the other tubes' winding is 6.8vac, so a little high. If filtering doesn't do it, I'll consider going for the 290AX PT and a 5Y3.

Thanks for the Edcor link, Shooter - good to know they're there, even if a six week wait.

[shooter]

I think I'll consider better filtering;

I don't use tube rectifiers, but they "typically" have a "max cap" spec listed in the datasheets, make sure you check that IF the 1st thing after the tube is a cap.

[shaun]

Will do! Thanks.

[PRR]

> order according to specs,

Nah. Count the tubes. Find a PT for a similar line-up.

At first glance this is a Champ. Except an extra tube, and 6BQ5 sucks more heater than 6V6 (or 6AQ5). And the rectifier is on the 6V circuit. (I have not seen 6X4 failures...) So how about the next-up, the DeLuxe? Two small tubes, two big 6V tubes, a big 5V tube. I think it will be loaf with a Skylark lineup. BUT most Fenderish PTs have been way over-volted.

> .....simply a matter of adding plate currents of each tube,...., I see PTs with attributes such as "300mA," but I'm not sure what that refers to....

I have a "3/4 ton truck". That's actually a historical class not a drop-dead rating. But on the face of it the track can carry 1,500 Pounds. So I figure two 180 pound helpers, 300 pounds tools, 130 pounds nails, 7 pounds of chips, 47 pounds beer...

Since a 6BQ5 can't pass even 100mA, and the little tubes much less, clearly a 300mA PT is over-kill like a 3/4ton truck for just me, chips, and beer.

Can you find pictures/specs of the Original? A replacement should be very near the same size and weight.

[DummyLoad]

Hammond 270CAX should get you in the ballpark. improved, less buzzy-hummy PS attached. don't know if you have room for a choke, if you do not, use a 120R 5 watt resistor in place of the choke - will be more buzzy though.


--pete

[shaun]

Thanks, PRR.

"Can you find pictures/specs of the Original? A replacement should be very near the same size and weight." No, unfortunately. I guess Shooter was right - it is hard to track who made some of these. The schem calls it T1:).

I pulled the trigger and ordered a 290AX. It is priced a smidge better than some others, and it offers two HV taps. I'll probably use the 275 per tap. It also has a 5v winding, and I'll be interested to see how the 5Y3 effects the tone.

Thanks for the schem, DL. I'll probably use it to add more filtering if the new tranny doesn't do the job.

Many thanks for all the feedback.

[shooter]

the Original

I'd have to look, believe I have 4-6 original skylark flavors

keep going, it's a nice amp