Gibson GA-5 T Skylark 61 (3 tube) - Value of cathode resisor on Oscillator

[kagliostro]

Tere are at least a pair of different schematics about the Gibson GA-5 T Skylark 61 (3 Tube -- 5Y3 - 12AX7 - ECL82)

that you can see

This is on the web



and this is the version on Doug's archive



No one is perfectly readable, so, starting from the GA-1 RT schematic, with a bit of image editing I've redrawn a more readable version

Problem is that in one GA-5 T schematic the cathode resistor value on the oscillator is 4.7K and a .05 cap is missing, and in the other schematic

the cathode resistor value on the oscillator is 2.2K and there is a .05 cap connected between plate and ground

I would like to send to Doug the new more readable version of the schematic, but don't know which are the correct values for the resistor and if is correct to have the .05 cap

This is the GA-1 RT schematic I used as a base



and this is the version of the GA-5 T Skylark  that I obtained



Someone has an idea about ?

Thanks

Franco

[shooter]

FWIW, the last one I worked on had cathode directly to ground.  it was a GA5-T, I believe I have worked on at least 3 variations, this particular one was the cleanest, best sounding, of the 3 variations.

[HotBluePlates]

... I have worked on at least 3 variations, this particular one was the cleanest, best sounding, of the 3 variations.

The problem is that there is a push-pull output stage.  Therefore the trem signal cancels and doesn't yield the pumping that is normal in Kagliostro's single-ended version.

[sluckey]

I have two more GA-5 Skylark schematics that could be added to the crazy Gibson collection. But seems you are interested in that 3 tube version.

I would draw the schematic with the .05 plate cap to ground and the 2.2K cathode resistor. Then make a couple notes on the schematic...

Note 1.   Some schematics omit this cap.
Note 2.   Some schematics show 4.7K.

[kagliostro]

Thanks Shooter - HotBluePlates - Steve

If someone is able to guess something

I've this





If we don't find any sure value, I'll do as per Steve council

Franco

[sluckey]

This is one of the best looking Gibson amps I've ever seen!

You have a 2.2K cathode resistor and you have the .05µF from plate to ground. See pic.

[kagliostro]

Thanks Steve

I arrived to your conclusions about the resistor, but didn't catch the capacitor

Grazie

Franco

[shooter]

Kagliostro's single-ended version

missed the SE version

This last one was the cleanest I've ever seen, I just did the PS section, the board n wiring was either someone before me, or the best builder Gibson had, that day

[kagliostro]

I put my eyes on this 3 tube version because ..... I've just recovered parts from an old italian reel-reel recorder

that used 1 x 12ax7 (ecc83) - 1 x ecl82 - 1 x em81 tube (magic eye)

so I've PT - OT - Preamp Tube - Oscillator/Power Tube

Only one thing is odd to me ..... the very HIGH value of the plate resistors of the 12ax7

V1A = 470K

V1B = 330K
really odd to me 

If I go on a GA-5T replica (SS rectify) what do you think about this resistors values ?

Franco


p.s.: The schematic was already sent to Doug

[PRR]

2nd image in the first post:

The "4.7" is not drawn in the same hand as the other numbers on the plan. Everything else is done with a Lettering Guide or by a highly skilled hand lettering draftsman. This "4.7K" is less precise.

I "know" the guide he used (or trained on) but can't recall the brand. The RapidDesign below is similar but the "4" is different. There were several brands.

It could of course be a factory change. Or some later technician. Does it work better? or was it a Band-Aid for some other problem in a specific amp being fixed?

[PRR]

Leroy!

http://www.tpub.com/engbas/3.htm78.gif

https://www.youtube.com/watch?v=5t1YLxb2hbY

http://www.calligraphynut.com/keuffel-esser-leroy-lettering-guides/

http://www.mccoys-kecatalogs.com/KELeroy/LeRoy_Production/LeRoy_Soper.pdf

A circuit-drawing template (not a great one):

https://www.hearlihy.com/drafting_instruments/Circuit_Aid_Template

[DummyLoad]

i used to draw mud-logs with those...


--pete

[tubeswell]

FWIW, the last one I worked on had cathode directly to ground.  it was a GA5-T, I believe I have worked on at least 3 variations, this particular one was the cleanest, best sounding, of the 3 variations.

Just to add to the confusion, I found another 'GA5T' schematic with this grid leak bias for the LFO stage (and it's a PP amp  with no tone control, and - they use 22k grid stoppers on the 6AQ5s)

[shooter]

really odd to me 

I think you have a unique version, so you have unique values.  I'd expect you will have more than enough gain to abuse the power tube  

[sluckey]

Leroy!
A circuit-drawing template (not a great one):

Bah! No spell check?   

[PRR]

> Bah! No spell check?

Potrzebie is spelled correctly.

[kagliostro]

BTW, the GA-5 T Skylark of 67 (that has a completely different schematic and uses 4 tubes with an unused triode) uses a 4.7K resistor on the oscillator cathode

---

In Italy we call that Normografo




and ..... I was never very skilled using such a thing, also if I've used sometime

one of the latest thing I did was to write the labels with the names of the soldiers for the cots bunk, during my military service

in 1976, then I think I've never more used the normografo; near my town there was a small industry that produced transferable letters

and those were much easier to use for a small writing


there were also version devoted to PCB




Franco

[frankenxtein]

I should build this into my little Japanese Mayfair FT501 tape deck amp. The only thing different is it has a 6X4 rectifier. I built a Kalamazoo with it pretty much so not too many changes.
        I just put away the tubes that were in it , due to lack of wanting to venture too far from making square boxes. The 6BM8 was "different" and now I see a use for it. Probably better suited for the OT that came in the FT501.

Is the 500k pot audio or linear?
Thanks for the idea.

[kagliostro]

I asked about one other oscillator and Sluckey told me Linear or Reverse Log

(http://el34world.com/Forum/index.php?topic=20998.msg222462#msg222462)

Franco

[frankenxtein]

I asked about one other oscillator and Sluckey told me Linear or Reverse Log

(http://el34world.com/Forum/index.php?topic=20998.msg222462#msg222462)

Franco

Reverse Audio ? lol this is why I built square boxes. I guess it's not like hooking up the controls in reverse ..... done that ,  here's Kalamazoo 2

PS: 6BM8  $18.95 I'm not sure I want to get that out of the box. I guess it's good to know they still make em

[kagliostro]

If you have the occasion, I would like to know the weight and/or measures of the FT501 OT

--

6BM8  $18.95

You must remember that is only one bottle but with a preamp triode and a power pentode inside

Franco

[pompeiisneaks]

Oh man I remember those days, in my Civil Engineering classes I had to do some drafting and learned to use those as well.  They also expected us to free hand a pretty readable form of that as well, I never got 'great' at it like my dad does :) (he's a real Civil Engineer, retired, I'm just a wannabe that ended up failing out and moving on to a degree in software engineering later in life)

~Phil

[frankenxtein]

Sure , looking to put one in the Esteban G10  so it might be this Skylark.
        What I have on the  Mayfair FT501 $32.88 total w/shipping eBay
I felt bad tearing it apart but I got over it 
OT 2-3/8" mount  2" wide  1-5/8" tall 3/4" lamination's , 1.5" winding.  8ohm 
Really skinny leads , it looks neat but , I can warm it up with a Sovtek 6BQ5. Thought about a 6AQ5 but , using it's 6BM8 might be better. Never was a big fan of tremolo but , it might be cool for a electric violin. My son plays Bass , Guitar , Flute , Piano and now Violin.     

PT 2-7/16" x 3-1/16" x 1" lamination's  250-0-250 50mA 6.3v @ 0.7A , 6.3 @ 2.A
"6BM8" would mess up my square box lol
Everything I've built so far is 2 tubes not counting rectifiers. I still have the same .500mA test fuse I started with and last two amps worked on first power up.   

I don't have a scale since the 80's , most folks eyeball them sacks here in Memphis. 

OT about the same size and weight  as the Merit A2901 , the V-M 733 , but slightly larger than the Silvertone 2230 (have Sams pdf) which is about the same as the Silvertone 7072 (need a better schematic) All of them are probably sub 5watt more like 3-4watt. Unlike buying parts to fit , I fit parts to designs. People that use all these pedals should take a look at the Silvertone 1471-1481 simple and clean. Got to find a friend in construction to get some "C" channel scraps to make chassis to fit in a couple of my SS amps.

[HotBluePlates]

... I have ... the  Mayfair FT501 ...
I felt bad tearing it apart but I got over it 
OT 2-3/8" mount  2" wide  1-5/8" tall 3/4" lamination's , 1.5" winding.  8ohm 
Really skinny leads , it looks neat but , I can warm it up with a Sovtek 6BQ5. Thought about a 6AQ5 but , using it's 6BM8 might be better. ...

PT 2-7/16" x 3-1/16" x 1" lamination's  250-0-250 50mA 6.3v @ 0.7A , 6.3 @ 2.A
"6BM8" would mess up my square box lol
...

I keep telling people they need to look at tweaking the preamps in these kinds of things, rather than completely disassemble & try to build something new.  The power supply, OT and output tube are are compatible to each other, may not be compatible with whatever else you try to transform it into.

If you go in a junkyard & get a Mini Cooper, tearing it down will not enable you to make a half-ton pickup from the parts...

[frankenxtein]

  Where's the fun in that?

Seriously ..... your right HBP.

I try to keep the iron together and stick close to the original tubes , usually leaving out a small tube. Such as my Silvertone 2230 amp 12AX7-6AQ5-6X4 all original tubes. Original caps were 40-40-40 , using 33-20-10 caps the OT is small but it is a great sounding amp another Deluxe/Kalamazoo.

The FT501
6BQ5 (760mA heater) 5.7watts
6BM8 (780mA heater) 3.5 watts The output is higher on the 6BQ5 and yes the design is different.

The other 2 , 6BQ5 amps I have built seem fine. I try to stay below what the tubes max is but , that 6BQ5 is a bit much for the FT501 OT.
   
A mystery amp had a 6CM6 I was told but , no markings at all , the PT & OT Midwest Coil & Transformer .... anyone got info on that company?
It's a beefy amp and does not get warm. I wonder why the 6CM6 is not more popular. I might order one just for fun. That is a great sounding amp.

The V-M733 had a 6BQ5 and I'm well within parameters with the Merit P3047 and the V-M733's OT in my Deluxe/Kalamazoo

The V-M733's PT is in my 12BH7 Deluxe Micro. The highest voltage amp here so far lol 2 watts. 

[kagliostro]

Ciao Frankenxtein

OT 2-3/8" mount  2" wide  1-5/8" tall 3/4" lamination's , 1.5" winding.

thanks for the measures

Franco

p.s.: A proposal for your avatar ? 

[PRR]

> I remember those days

There's rub-on letters. There's the templates you stick the pen through. There is the LeRoy where the guide is offset so it does not get in the ink.

But pen and ink is easy. What if you need your name on a wrench (spanner)? What if you need the company name on the wrenches you make? Wrenches are stamped in hard steel by even harder steel stamping dies. You won't easily or neatly hand-letter that tough stuff.

Watch from 7:34 to 8:01 --

=454

There's big pre-cut numbers and the company name. The worker traces a stylus on a pantograph to a milling machine, which cuts the hard steel. There's more good stuff, if you care how wrenches are made (from large steel bar to sales).

If you need a new song for the bar the mechanics drink at, try YouTube video AY4upxZ_zOs

[pompeiisneaks]

> I remember those days

There's rub-on letters. There's the templates you stick the pen through. There is the LeRoy where the guide is offset so it does not get in the ink.

But pen and ink is easy. What if you need your name on a wrench (spanner)? What if you need the company name on the wrenches you make? Wrenches are stamped in hard steel by even harder steel stamping dies. You won't easily or neatly hand-letter that tough stuff.

Watch from 7:34 to 8:01 --

=454

There's big pre-cut numbers and the company name. The worker traces a stylus on a pantograph to a milling machine, which cuts the hard steel. There's more good stuff, if you care how wrenches are made (from large steel bar to sales).

If you need a new song for the bar the mechanics drink at, try YouTube video AY4upxZ_zOs

Sorry to derail the thread more, but man I love that old voice of the 50's 60's its lost now pretty much, but great! ;)

~Phil

[kagliostro]

Ciao Phil

Sorry to derail the thread more, but .............

Absolutely NO PROBLEM 

Franco

[frankenxtein]

Ciao Frankenxtein

OT 2-3/8" mount  2" wide  1-5/8" tall 3/4" lamination's , 1.5" winding.

thanks for the measures

Franco

p.s.: A proposal for your avatar ? 

Thank you for the avatar , the old one was kinda plain. I'd heard of amps being built into cake pans and thought ....lol.

[kagliostro]

  

[murrayatuptown]

I know, 3 year old question...

Midwest Coil & Transformer (EIA mfr. code 549) was in Chicago in early (1961?) EIA listings, then Gray's Lake, Illinois.
They made many power transformers for Heathkit, and I have seen a few on Dynaco chasses. I don't know about output transformers...I've just never crossed paths with them.

Someone at another Illinois transformer answered my question about what happened to Midwest...I had inquired with local chamber of commerce, thinking maybe they were acquired by another company & just had a different name...no response.

I just had to ask another nearby company...

Midwest moved to Mexico in the mid-1990's and was out of business by the end of the 1990's. I assume, perhaps, an early NAFTA casualty?

I do know the 70's 'Midwest' transformers do not disassembly easily...they are potted with a hard, brittle, black material that looks glossy against end bells but almost porous or foamy when not constrained. I gave up finding a solvent for it...very brittle...shatters if pried or whacked with a hammer.

I gave a piece to a friend who works in a analysis lab...said it was a vinyl alkyd (he told me long or short-chain, but I forgot that part...didn't mean anything). It had the thermal conductivity of mineral-filled transformer potting materials...about as thermally conductive as granite...not much...so why use that instead of normal transformer varnish? Dunno, maybe trade secret or to keep secrets. Or good for vibration/magnetostriction damping...

Old Heathkit transformer blueprints spec'ed Perma-Fill or equivalent. Lab told me there are/were several Perma-Fill recipes the Midwest black-stuff was not similar to. He said it was probably a valid material despite seeming weird to me...an amateur transformer butcher.

On the few I tried to take apart to salvage laminations & end bells, it was much more difficult to loosen anything up...bolt heads sheared off, chips flew all over when the potting fractured.

Very small anecdotal sample data, but I never got the impression Midwest transformers were failure-prone.

The old Heath Williamson (W-x) series suffered power transformers failures with enough frequency Heath published a bulletin with recommended universal replacements. Some of the originals were from Chicago & Peerless. To be fair, nothing lasts forever. Some come closer, making us judgemental about the disappointing ones.

I have a lot of 'excess inventory' Heathkit power transformers I bought from surplus dealers in the 90's. 549 and 60 (V&F Transformer, Bartlett, IL) are more common than any others...suggesting replacements from those suppliers did not need replacement as often as they expected. So maybe yours is a nice reliable thing.

Oh, good luck finding any data...they seemed to build custom for manufacturersand not supply standard items to distributors.

TMI, I'll bet.

Where's the fun in that?

Seriously ..... your right HBP.

I try to keep the iron together and stick close to the original tubes , usually leaving out a small tube. Such as my Silvertone 2230 amp 12AX7-6AQ5-6X4 all original tubes. Original caps were 40-40-40 , using 33-20-10 caps the OT is small but it is a great sounding amp another Deluxe/Kalamazoo.

The FT501
6BQ5 (760mA heater) 5.7watts
6BM8 (780mA heater) 3.5 watts The output is higher on the 6BQ5 and yes the design is different.

The other 2 , 6BQ5 amps I have built seem fine. I try to stay below what the tubes max is but , that 6BQ5 is a bit much for the FT501 OT.
   
A mystery amp had a 6CM6 I was told but , no markings at all , the PT & OT Midwest Coil & Transformer .... anyone got info on that company?
It's a beefy amp and does not get warm. I wonder why the 6CM6 is not more popular. I might order one just for fun. That is a great sounding amp.

The V-M733 had a 6BQ5 and I'm well within parameters with the Merit P3047 and the V-M733's OT in my Deluxe/Kalamazoo

The V-M733's PT is in my 12BH7 Deluxe Micro. The highest voltage amp here so far lol 2 watts.

[tubeswell]

Then there is this '67 GA5T Skylark schematic   


This is clearly from a different year, but begs the question - how many versions of the GA-5T did they make?


(LFO Rk = 4k7, so 4k7 could've been what they ended up using, and 2k2 could've been what they started out using back in 1961, and then switch to 4k7 later in 1961 and kept that, because having a centre-biased LFO stage might work better?)

[helloraph]

Hi all
I understand that the pot changes only frequency and volume blends tremolo right ?
I wonder how to blend tremolo and preamp for the 67... is a pot possible to install . I want o have Intensity & freq pots on the amp;
I have EL82 and ECC83 + transformers ready, and I wonder about soldering like the Skylark67...Àny suggestion ? (alternative schem ?)
Is the V1 Cam 1µ ??? I'm more used to the 25/25 + 1500k fender to be honest.
Thank's for your opinion.