Gibson GA-20 voltages?
Phil_S
tubbes. I've probably got transformers, chassis and other parts. The
question is, what are the appropriate voltages for this old amp? Can you
get any sense from the schematic?
http://www.schematicheaven.com/gibsonamps/ga20.pdf
I only have one 6SJ7, so it will be a one channel build. Probably point to
point.
Thanks.
Phil
WB
>
>
> http://www.schematicheaven.com/gibsonamps/ga20.pdf
>
> I only have one 6SJ7, so it will be a one channel build. Probably point
> to point.
Certainly isn't over 275 due to 6v6 :
http://boozhoundlabs.com/howto/pdf/6V6GT.pdf
el84 have a higher B+ tolerance.
You will likely need another voltage amp ( pre amp/tone stack ) and
12at7/ax7 for a PI ... what else you got lying around ?
Phil Allison
"WB"
> Phil_S wrote:
>
>> http://www.schematicheaven.com/gibsonamps/ga20.pdf
>>
>> I only have one 6SJ7, so it will be a one channel build. Probably point
>> to point.
>
> Certainly isn't over 275 due to 6v6 :
>
> http://boozhoundlabs.com/howto/pdf/6V6GT.pdf
** With cathode resistor bias, the plate supply voltage cannot be more than
about 300 volts DC for good class AB operation. But with grid bias, the
plate supply voltage can go as high as 450 volts.
Note how the 6V6 data sheet gives a max plate (signal) voltage of 1200
volts.
> el84 have a higher B+ tolerance.
** Not strictly true.
The Fender re-issue " Deluxe Reverb Amp " I posted here about two weeks
ago uses 6V6GTs with plate supply of 425 volts and grid bias of -38 volts.
Puts out 22 watts clean.
..... Phil
jh
Hi Phil,
I'd expect it to be in the territory of the 5E3 Deluxe. Similar output
stage. Slightly smaller cathode R. I would expect 320-350 Volts DC.
regards
Jochen
WB
> Note how the 6V6 data sheet gives a max plate (signal) voltage of 1200
> volts.
>
Phil Allison
"WB"
** In any mode, in any circuit - subject to max plate dissipation only.
You stupid, know nothing, fucking context snipper.
.... Phil
Phil_S
"jh" <j...@org.de> wrote in message
news:hcrd11$fui$1...@news.eternal-september.org...
This answer surprises me. It's just an impression, but I would not have
expected a Gibson from this era, 1950-1953, to run that hard. Your
suggestion prompted me to look and think harder.
First, I was reminded to look here: http://www.superiormusic.com/page199.htm
(about 1/6 of the way down) it says this amp was 12-14 watts. The RCA tube
manual only lists class A operation, so I got a copy of the GE spec sheet
which says typical AB operation is at 285V/14W or 250V/10W. 12W would be
somewhere in between...maybe ~267.5V ;~}. I think that WB probably hit this
on the nose when he says 275V as an upper limit.
From here, it isn't difficult to figure the right PT.
Thanks for the spark.
Phil_S
"WB" <sp...@there.com> wrote in message
news:4af0ed9b$0$5324$bbae...@news.suddenlink.net...
> Phil_S wrote:
>>
>>
>> http://www.schematicheaven.com/gibsonamps/ga20.pdf
>>
>> I only have one 6SJ7, so it will be a one channel build. Probably point
>> to point.
>
>
> Certainly isn't over 275 due to 6v6 :
>
> http://boozhoundlabs.com/howto/pdf/6V6GT.pdf
>
LOL, I just obtained this one. Friggin' RCA RC-30 book only lists Class A.
I agree, espeically now that I see this amp was 12-14W.
> el84 have a higher B+ tolerance.
>
> You will likely need another voltage amp ( pre amp/tone stack ) and
> 12at7/ax7 for a PI ... what else you got lying around ?
I don't understand your question. The GA-20 is a one knob amp (well two
knobs if you build both input channels): 6SJ7 > 6SL7 > pair 6V6GT. No tone
stack to speak of. LOL, why would I booger up a perfectly good design with
a tone stack and 2nd gain stage? I've got the 6SL7 for the PI.
I've got lying around 6SL7, 6SN7, 6AQ7 (half of a 6SL7), 12AU7, 12AX7,
12AV7, and probably 12AT7, just in case you think I might need something.
Am I correct in identifying the PI as a paraphase inverter?
jh
My Victoria 5E3 puts out something between 12-15Watts; B+ is about
340-350V, Rk is 250ohms. The finals idle at about 12,5 Watts ;-)
Whether that's pure class A or something else is - at least for me
irrelevant. The thing works and sounds good.
That's just the operating point of an actual working amp of the same
class. The datasheet would apply here as well, but Fender - and I bet
Gibson too - never really cared in detail about the datasheets anyway.
IMHO they took them as a rough estimation and squeezed as much out of
the tubes as possible. Watts were marketing factors.
Some Deluxe Reverbs were at 430V stock - fixed bias. Kelley even went up
to 480V DC there..
my bet would still be about 300-320V.
Anyone out there who has a real GA20 to provide actual readings?
Jochen
WB
>
>
>
> .... Phil
>
>
>
Aren't we a flaming asshole this morning ?
Phil_S
"jh" <jh-audio...@t-online.de> wrote in message
news:hcs5u7$fmd$01$1...@news.t-online.com...
Certainly it's cause for pause....I don't care about A vs AB, I was simply
guessing it is an AB amp, taking that hint from cathode bias. But I don't
really know and it's just a guess.
Phil Allison says 300, you say 300-320, WB says 275. These are all
reasonably close.
It would be nice to know. This amp was made from '50 to '53. They made
about 5,000 of them. The question is, how many have survived for almost 60
years? I'm not holding my breath that anyone has an actual voltage chart
from one of these.
I spent a little time with my bone pile transformers this morning. I've got
one that has no CT and the secondary is only 228VAC unloaded. It is hefty
and made in Germany. I am guessing 1.4*228 less 5% for the load bring this
in at around 300VDC. I've got to bench test it to see if it can meet
current demand of 120mA or so (pair of 6V6 + 6SL7 + 6SJ7). It would be nice
to find a use for this one.
Not sure of the filament winding either; it puts out 8VAC unloaded but I'm
reasonably sure it is a 6.3v winding. I only need 1.5A here.
Lord Valve
I have schemos for the GA-20, GA20-T, and GA-20RVT.
No voltages for the plain 20, but the 20-T is listed as having
340 VDC on the plates, and the 20-RVT shows 289 VDC.
LV
Phil_S
"Lord Valve" <detr...@ix.netcom.com> wrote in message
news:4AF1DF87...@ix.netcom.com...
Hello Willie,
Thanks for the info. I own a GA20-RVT, a totally different animal. I didn't
realize there is a 20-T. The GA20-T tremolo channel is not all that
different from the GA20, but the "regular" channel is all "wrong", and it
went into production in 1954. I see the voltage chart. That's helpful.
Regards,
Phil
TD Madden
TD Madden
Phil_S
"TD Madden" <tdmadd...@spam-comcast.net> wrote in message
news:hcta3p$2dh$2...@news.eternal-september.org...
I looked at a bunch of old Gibson schematics today. There are some that
resemble the target amp and they all seem to be in the low to mid 300's.
I decided to go with a 290-0-290 that I've got. It seems to be one of the
few I've got with a 5V winding. No guessing on the B+ either -- this one is
150mA. With a 5Y3GT, I should get ~320. For a pair of 6V6, that should be
very good. I found I've got an 8K:8ohm output transformer with a bolt
pattern that matches the donor chassis opening for it. I've got a bit of
metal work to do to make the chassis right...holes to open, holes to close.
I'm going to paint it. I've found it is best to leave a painted chassis for
at least 3 days to make sure the paint is fully cured, regardless of what
the can says. I'm going to have to spray it outside and it's in the 50's,
so I hope it makes it inside OK to dry. I need to find some "10 minute"
primer. I used to have a can, but I forget where I bought it.