3 lead Cap across vol ctrl on gibson skylark - what is its purpose?

[Enzo]

Don't be a pane, I can see right through that argument.

[Chuck H]

Am I that transparent?

[Enzo]

Don't give me no sash.

And I will claim being double-hung before you do.....

[J M Fahey]

Dear wagdog, your experiments were very useful for us all.
Even if you practically got back to square one, now it's for a reason.
Many complain about these amps being from "too thin" to "unbearably bright" but you can't argue physics laws.
Truth is, they are very weak (a 6AQ5 is a classic "table radio" power amp) , the transformer is, as you say, tiny, and the speaker must have been quite cheap too.
The only way to let it have reasonable volume was to cut bass a lot, where power would be wasted anyway, besides being farty, and boosting highs, which would seem loud even if by virtue of annoyance.
The typical mid cut was Gibson's trademark.
I'm sure it cuts "ugly" mids allowing some extra perceived headroom.
I wonder who would have been the typical user of these amps, what instruments and music would he have played.
Anybody has some data on that?
For once, I don't see it driven by big box guitars or used by Jazz cats, but I may be totally wrong, of course.

[km6xz]

JM, you need to put the performance in context of the time. There was no bass as if known today, popular music just did not have it. A big sound was from AM car radios, running 2-3 watts into 6X9 speakers either in the dash pointed to the windshield or in the rear deck. The presence of lower notes or kick drum(kick was often tuned high or not even used in pop music), and it was filtered out due to FCC bandwidth regulation from AM radio. Even large concerts halls had very low power to work with but used high efficiency speakers with multicellular horns for mids and up. Listening to old classic rock for example or the Motown early crossover hits did not deter the enjoyment even when the lower octaves were missing....no one had every heard them or cared. The harmonic distortion was high in the process so fundamentals were revealed in the second harmonics. The brain is really good at inferring a fundamental if a harmonic series is heard. So a 2 watt system with nothing below 80hz still gave the impression of the fundamentals....no one missed hearing the Jamerson bass lines because the brain filled in the perception based on the harmonics present.
The average livingroom today has more total power than the largest concert of the early 60s. But the living room and clubs, in trying to reproduce the lowest octaves that never really was part of music....ever... requires gigantic increases in power. Speakers and ears are both less sensitive to those sought after lowest frequencies that have never been used in music until the last 40 years. Even into the late 60s, places like Winterland or Filmore had less total power than a typical car stereo now yet we never thought the music was missing anything or too soft. People have tried in vein to reproduce those sounds and performance for decades and fall short.
Expectations of sound characteristics changes with time and fade, just as standards of beauty or art. My theory is that concerts are not so popular today, in part, because of the irritating assault on the senses that FOH deaf mixers create with stomach churning bass and bleeding ear induced high frequencies, proving their manliness by having more power and lower frequencies than the next concert mixer. It is not music they want, but special effects of physical responses. Also, each lowering of the frequency at which 140db is achievable costs exponentially more than an octave above. A $10,000 system that will fit in a small truck will produce the same acoustic power at 80hz as a $500,000 system will at 25 hz and requires 4 tractor/trailer rigs. No wonder tickets are $200.

[Enzo]

Amen, when I was touring as a soundman in 1969 with a regional bar band, we had a 100 watt PA system. COuld have been a 200 watt system, but we wanted stage monitors - bye bye second 100 watts. We drooled at the cool bands who had Shure Vocal Masters, those had reverbs. Built right in!!! And if I recall, 120 watts, not a mere 100.

And just one man's opinion, but I just HATE the sound of those old Gibson amps. Fender made amps in the same era, and under teh same constraints, and they sounded a hell of a lot better.

[km6xz]

Sure, by the 60s a new sound became a fad partly due to Leo Fender but that was later history with Gibson making amps since 1935. Guitar was not even a lead instrument until a couple decades later, mostly with the work of Charlie Christian's work. Most of the Gibson's in use by the time Fender came out were of a older generation aimed at jazz and accompaniment. Gibson did modernize and were more common in the 60s than the Fenders, but never really caught on with the rock musicians and they stopped production in 1967. They had their place and can be heard on a lot of 50s classic records.
Ah, yes, the EV Vocal Master....every school gymnasium had one.
Even as late as 1970, amps over 60 watts a channel were very rare. When the DC300 came out it changed the concept of power...a whole 150/cha. I built and toured with a sound system for the 1970 US Aqualung Tour, all home made as the first touring active xcover 3/4 way system with dozens of highly modified Dyna 120 power amps, and a home made mixer with the first fully balances transformerless inputs(because we could not afford $54/ea mic xformers), switchable meter ballistics(because we could not afford $60 real VU meters, or PPM) etc. Each channel was no more than 60 watts with 24 cabinets. It really sounded good for the day but would not be enough power for a car system now....but no one complained of low sound pressure level or bass. In fact it got rave reviews. We got the contract because we won a sound audition among commercial sound companies of the day, not even a sound company. We were just 4 hippies building on the anticipation of winning, two of us were engineer types and other 2 were friends who did not mind grunt work. I ran into Ian Anderson a year ago here in St Petersburg and we both had fond memories of that sound of the concerts.

[J M Fahey]

Dear km6, let's start with a self-quote:

you need to put the performance in context of the time

Oh , but I did !! I was not criticizing them but, on the contrary, justifying them.
I was also referring to the futility of what many posters attempt (even here in music-electronics) trying to give it "more bass".
This experiment just proved it, at least working with the basic original parts.
EQ won't supply what speakers (and in this case tiny output transformers) won't pass undistorted.
More bass than that translates into instant fart.
As of the general sound possibility in that era, for us mere mortals was quite limited indeed.
To start somewhat earlier, in the 50's when my Father's friends wanted to dance with some gals (in a way I was the after product of one of those dances, ahem !!), they used *the radio* as both a program source and sound system .
He told me there were special "Dance " programs, non-stop music with little talk, not too different from being at some dance hall with the house DJ mixing and talking. Go figure.
When I was a teen, in the 60's, the regular Party machine, which *everybody* used, was the Wincofon record player, "almost" 2W, a tiny 5x7" speaker mounted in the base of the record player, I very much doubt it reproduced anything below 200 or 250 Hz.
The beast in all its glory:

In fact, this was the "modern" (1962) version, but I remember having repaired in my tender early teen days, an earlier one, which used the classic 35W4/50C5/50L6 trio.
They used one very clever (or cheap) trick: they did not have a power transformer by itself, nor were straight ACDC line powered (in a 220V Country that means instant death) but used *a tap* on the turntable motor winding.
When I became a "PA Engineer" somewhat later, I was proud of my Made in Italy Geloso PA amplifier: 2 Mic+1Aux inputs, separate Bass and Treble controls (wow!), 40 powerful watts courtesy of a couple unknown tubes with caps on top-
They were not EL34, not 6DQ6, I suspect they might have been ¿EL36?.
Looked like EL34 with a cap connection.
It drove a *single* homemade column with 4x8" LEEA speakers (the local Altec Lansing licensed builder, they were *very* good).
What did the band use? They plugged *everything* into a similar-to-blackface-bandmaster amp: both guitars into one channel, the bass and one mic into the other.
No wonder my Geloso was considered (by them) the loudest baddest PA system they could hire .... for the competitive price of all the Pizza and Coke I could get into my (considerable) frame.
Oh well.

PS: back to small Gibsons: my own very first guitar amp build was a ¿GA-4? , the one with a 12AX7 and a 6V6 , single volume, no tone control, straight from Jack Darr's book.
All solid wire, bent at 90º, looked like a poor man's Hiwatt , he he.
Boy , was I proud !!

So, *me* criticizing a small Gibson amp? ...... no way José !!

[dkevin]

Sprague Network

Hello all!
I have several of these Crestline Gibson Skylarks and they all had this sealed network in them. I also found the Paleo-Electronics information helpful when I encountered them. Removing them was only the start of seeking a heartier tone for this amp.
When I checked all the schematic components and verified their values in my amp, I discovered that even with the "correct" components in the amp, it sounded thin and piercing. When I found Paleo's article about the second schematic, it explained why I was finding extra components (like the Sprague network) in the amp. When I compared the two schematics, I discovered in the second schematic, a cathode bypass cap added to V1B that doesn't appear in the first version. It is a .1uf cap. When I removed it, the amp breathed a sigh of relief. It went from thin and piercing to full and lush. I do not have the technical understanding to explain why this was so, but it was. This amp has a 10 inch speaker and sounds great! It no longer sounds like a cheap radio.
I would like to thank Paleo-Electronics for digging deeper than the surface. I had plenty of advice like "stop butchering vintage amps" and "if you don't like the sound, sell it and buy another amp". They said that "all 60's Gibson amps sounded this way from the git-go and that was the way it was". This amp had/has the potential to sound great and it took a bit of digging to get it there.

[Chuck H]

Right on. This is a better method IMHO. Take out what sounds wrong rather than trying to add what seems lacking. The amps are small. You won't get a big sound from them. But you can take out the harsh treble and just let them be small.

[dkevin]

I have seen and read advice about drastically upsizing the interstage caps, etc. I like to start with what the manufacturer provided and get it operating at its best. I have tried speakers and output transformer replacements when searching for the lost chord. But straying far from the original schematic is too easy to do.
After a while you forget what the original amp sounded like and open the door to a whole new set of issues. These small amps will never become club amps or such. But there is a lot to learn from the simple ones that are designed with a sense of balance. One of my favorite amps is a Supro 6606. It has a 12AX7, 6V6 and a 5Y3 rectifier. This tube lineup is very, very common. But this amp has absolutely fabulous tone from 1 to 10. It has such thoughtful structuring in the selection of components. It may or may not be your favorite amp to service but if it was a major league hitter, we would say it had no holes in its swing. The most amazing thing is to find an amp like this and open it up to discover disk capacitors and a small OT. It kind of gives you an appreciation for well-thought-out design. I feel the same way about some of my Gibsons and Silvertones too!

[wagdog]

Dear wagdog, your experiments were very useful for us all.
Even if you practically got back to square one, now it's for a reason.
Many complain about these amps being from "too thin" to "unbearably bright" but you can't argue physics laws.
Truth is, they are very weak (a 6AQ5 is a classic "table radio" power amp) , the transformer is, as you say, tiny, and the speaker must have been quite cheap too.

What I basically ended up with was a slightly larger cap for the v1b cathode bypass (.47uf vs .1uf), a little hotter bias on v1b w/the 1.5k cathode resistor vs the 2.2k and the added tone control.

I did try a 2.2k and 1.5k without the bypass and the amp sounded ok, but sort of lifeless. There is a definite sweet spot on the "loudness" dial, and it's about 3/4 of a way up. I don't think I really changed the character of it, however the tone control does let me dial out some of the brightness without increasing any muddiness.

I'm using a 10" weber signature speaker, which is still a pretty cheap, but functional speaker - nicer sounding then the original gibson speaker, which seemed pretty brittle sounding.

Hello all!
I have several of these Crestline Gibson Skylarks and they all had this sealed network in them. I also found the Paleo-Electronics information helpful when I encountered them. Removing them was only the start of seeking a heartier tone for this amp.

I'm curious; did you replace the sprague network with like parts, or just remove it? I tried the amp without it and it sounded pretty splatty and undefined. For mine, I ultimately decided it sounded better with it in.

I have seen and read advice about drastically upsizing the interstage caps, etc. I like to start with what the manufacturer provided and get it operating at its best. I have tried speakers and output transformer replacements when searching for the lost chord. But straying far from the original schematic is too easy to do.
After a while you forget what the original amp sounded like and open the door to a whole new set of issues.

Yeah, that's what I started to find when I was experimenting and ultimately decided not to veer too far off the original design.

This amp is definitely a one-trick pony, but I think it sounds pretty good at what it does. I like that it's not that loud and has a nice sweet spot. It's a cool little amp.

BTW, really have enjoyed reading the discussion so far!

[dkevin]

GA-5 Crestline Skylark info

I see where you tried a 1.5k cathode resistor on V1A and a 2.2k cathode resistor on V1B without any bypass caps on either stage. You called it "Lifeless". I have a 1.5k bypassed with a 20uf cap on V1A and a 2.2k with no bypass cap on V1B and I love the tone! As I mentioned earlier, I do not know the term for a small cap (.1uf) placed on the cathode of V1B but it definitely causes a harsh, brittle trebly tone.
I removed the Sprague network and substituted the interstage caps shown in the EA-50 Epiphone Pacemaker amp. This amp is the Epiphone version of the GA-5 Crestline Skylark. The much smaller caps of the "missing" GA-5 schematic seemed to be another place for tone to be lost. If you kept the original .0047uf caps I have no doubt the tone would still be shrill and somewhat disappointing.
It is difficult to harmonize the two schematics into one coherent amp. It should be noted that the layouts are quite different as well. Having worked on both of them, I can say that the Epiphone Pacemaker seems the most logical. I have chosen to use a Fender blue alnico 10 inch speaker in place of the Gibson Ultrasonic. The extra thump of the 10 inch speaker is a real bonus with this amp.
All-in-all, this is a very cool amp to cut your teeth on. Some people gripe about point-to-point flying leads and bird's nest of components. I don't believe that is the case with this amp. Due to its simple design and lack of bells and whistles, it is relatively easy to translate the schematic to the actual layout. I cannot say this about some of the larger Gibson amps with 'verb and tremelo. The Gibson Scout is an adventure you will never forget!

[wagdog]

I see where you tried a 1.5k cathode resistor on V1A and a 2.2k cathode resistor on V1B without any bypass caps on either stage. You called it "Lifeless". I have a 1.5k bypassed with a 20uf cap on V1A and a 2.2k with no bypass cap on V1B and I love the tone! As I mentioned earlier, I do not know the term for a small cap (.1uf) placed on the cathode of V1B but it definitely causes a harsh, brittle trebly tone.

The .1uf on the cathode of v1b is a cathode bypass cap as well. Merlin explains it fully in chapter 1 of his book, which is downloadable here: The Valve Wizard - the full book is definitely worth buying though!

Just out of curiosity yesterday, I tried removing the sprague network AND removing the .1 cathode bypass cap at the same time - and will say that I do like the tone. It seems like removing that tone network resulted in more signal to v1b (which makes sense), and by removing the .1uf bypass cap, reduces the gain of v1b, somewhat smoothing things out. I still kept my .01/250k tone control though - like that.

I removed the Sprague network and substituted the interstage caps shown in the EA-50 Epiphone Pacemaker amp. This amp is the Epiphone version of the GA-5 Crestline Skylark. The much smaller caps of the "missing" GA-5 schematic seemed to be another place for tone to be lost. If you kept the original .0047uf caps I have no doubt the tone would still be shrill and somewhat disappointing.

I'll have to have a look at that amp as well. The .0047 coupling caps are somewhat small, usually you will see .02 or .047 caps used as coupling caps (or other coupling circuits, including a cap and resistor in parallel if some tone shaping is desired), but I sort of figured the designers were cutting the bass out to reduce the splatty tone that would result from overloading the OT and speaker.

All-in-all, this is a very cool amp to cut your teeth on. Some people gripe about point-to-point flying leads and bird's nest of components. I don't believe that is the case with this amp.

It's a neat little amp to work on. I like point-to-point wiring, makes it easy to see what's going on. That type of construction lends itself to experimentation too.

[nickvivid]

Hey guys great thread and tips. My sprague network was attached via pins 2 and 3 to the volume control and via pin 1 to a .47uf bypass cap. I put that .47uf directly to where pin 3 of the sprague network was on the volume control (pin 1 of the volume control). Everyone else had theirs like this? Seemed like the sprague network would be in different spots according to which skylark people had.

Also had that undocumented .1uf 200vdc cap in parallel with the 2.2k resistor that's attached to v1b. I left the 2.2k in but changed the .1uf to a .47uf as suggested. Without it there was lots of headroom but it was flubby as hell and no punch in the bottom end. Not tight at all. It needs a load on it of some kind to get that balance between headroom and tightness.

The original Gibson speaker had a tear in it so it was opportunity to add a a fender blue alnico 10" and gotta say the results are stellar. I love the sound of this amp now. Creamy and rich with a sweet midrange and great breakup when cranked.

Thanks again for the tips. Those 3 mods made the amp for me.