Gibson GA-5T Skylark '61 (12ax7 + 6bm8 + 5Y3) - Question about Plate resistors

[kagliostro]

This is the schematic of the amp to which I refer



I'm astonisched about the really high value of the plate resistors of the 12ax7 tube

a high value gives a high gain amplification stage, but I'm confused about, didn't remember to have seen so high values on a 12ax7 tube

Someone has an idea about this unusual choice ? Which can be the reason for that on this circuit ??

Thanks

Franco



p.s.: If someone has a link to a demo of THIS model of GA-5T with the 6bm8 tube I'll be glad to have it, thanks again


(I was thinking I listened to some demo, but now I'm not so sure, Gibson build more than one version of the GA-5T)

[sluckey]

There are plenty of 12AX7 circuits that use 470K plate resistors. Look at the tremolo circuit of this 6G9 amp...

     http://el34world.com/charts/Schematics/files/fender/Fender_TREMOLUX_6G9.pdf

Or look at V3A of this Magnatone M10A...

     http://sluckeyamps.com/magnatone/Magnatone_M10A.pdf

[Greg]

In one of the RCA recipe book, they suggest 5200 ohms cathode with an Rg of 1meg. On this Gibson circuit, they biased a bit on the «hot» side at 0.9 volt... But this measurement include the analog meter load by a small amount. Anyways, a hot bias can give you a compressed sound with earlier saturation reminiscent of the grid leak method of biasing.       

[kagliostro]

Many Thanks Steve and Greg

@ Steve

I completely missed the use of those high value resistors

@ Greg

What you say give a sense to the choice of those high values

Thanks again friends

Franco

[Greg]

I think it is pretty much consistent in some Gibson amplifiers. I've seen pentodes with 470k plate load. The GA-19RVT uses a plate load of 470k but have the bias more centered (4k7 cathode resistance instead of 2k2, more like the RCA suggestion). But... If you ever listen to some clips of the GA-19RVT you will rather hear the huge mid scoop effect of the notch filter, which is probably why they had to compensate for more gain, I don't know for sure.

Merry Christmas!           

[kagliostro]

Thanks

Merry Xmas also to you

Franco

[PRR]

AM radios mostly used 470K to maximize gain, allowing the treble to fall short of bass-mids. You don't want extended treble in an AM radio (anything over 3KHz is mostly static hiss/crash), you do want (in the US*) to bring-up weak stations so the buyer thinks it is a good radio.

Gibson apparently took this line of design.

(*) I might want to listen to Boston or NYC, 400-500 miles away. In Europe, long-range is less important, because so many different countries and languages are so close together.

[kagliostro]

Merry Xmas PRR

This expected low level on highs joined with the fact that I'm not sure if I really listened to a demo of this amp

or to a demo of one other version of GA-5T

keeps me in doubt about which version of the GA preamp plan to use (ecl82 SE will be used for sure because of the PT-OT & tube set I've)

Franco

[sluckey]

(ecl82 SE will be used for sure because of the PT-OT & tube set I've)

In that case I would just build according to the schematic you posted above.

[Greg]

I remember the tube radio set of my grandparents. The loss of highs and distortion added warmth, especially from speech. Luther Perkins guitar sound was jumping out of the set.

If you are worried about high frequency loss, you can follow Gibson idea of not following their schematic and it might just sound like a Gibson. Maybe bring down the value of both 470k plate resistors to 220k and bypass the volume pot with 150pf. If you have an ECL82 with transformers, you could always euro-gibsonize with a single EF86 up front and keep the tremolo circuit as is without negative feedback.

[kagliostro]

On the forum archive there was a modded version of this amp (with 220K plate resistors and other mods)



I've also seen the GA5 version with 6V6 power tube (and no tremolo) that uses 220K plate resistors



and a version (the medalist) that has also a tone control + a tone filter (but this uses a 6BQ5  tube that is way more sensible (11mA/V) than  6V6 & 6BM8 which are on the range of 4.1-6.4mA/V, so I doubt it will be a good choice to use that preamp schematic



Following the 6v6 version of the schematic Greg's suggestion seems a good chance (only seems I must lower  the 22K resistor on the PS as to have a bit higher voltage on the plate of the 12ax7 tube)

at least I can give a try to both solution (Greg's & Steve's), about the other mods on the first schematic (of this post) I think to avoid them

Thoughts about ?

Thanks

Franco

[VMS]

This could be good place to do a little test. Build it with 470k plate resistor and put another 470k resistor parallel with a switch, this way you can easily hear if there is a difference in sounds.

[Greg]

How do you guys insert a quote in a middle of a post?

At 220k/2k2 you can estimate the plate current to be around a milliamp for both sections so, 10k instead of 22k will only bring up your available preamp supply around 10 volts. You might be happy with both 220k/2k2 sections. If you end-up with too much white noise, you can bring it down to 100k/1k5 like a Fender Champ or reduce the feedback resistor a little. There's a possibility that the 0.05uF bypass capacitor in the LFO circuit might not be enough to prevent speaker thumping if you are using a bigger speaker than a 8 inch.       

[kagliostro]

@ VMS

Yes, something like that

@ Greg

How do you guys insert a quote in a middle of a post?

Use the quote TAG it is the button under the Font Size option that appears when you are posting or editing a post

this one

--

On the GA5 there are 165V on plates on the GA-5T there are 110v and 115v, I need an increase of around 50v, I think I can have it, don't you ?

The pair of SS amp that I can use for case and speaker has both only a 6-6.5" speaker ......

Thanks

Franco

[Greg]

Thanks!

Ok, so...

On the GA5 there are 165V on plates on the GA-5T there are 110v and 115v, I need an increase of around 50v, I think I can have it, don't you ?

It depends on your available B+ supply... What is your available supply? All the versions you mentioned have different conditions.

[kagliostro]

The PT I've is from an old rell to reel recorder

on that circuit there was one EM81, one ecc83 and one ecl82

AC voltage is around 190v so 190 x 1.4 I think a B+ around 266v  with Solid State rectify (at least 256v)

on the GA-5T B+ is around 278v

Franco

[Greg]

I was wondering why aiming for a specific preamp plate voltage regardless of the plate load... But...In case you accept your first posted schematic with a 220k/2k2 change in both 12ax7 sections, I would expect a voltage drop around 100-110V across the 220k plate load. A voltage drop of around 20 volts across the 22k and 12 volts across 1k.

So, if you start with 256V B+ you will end-up with 224v for your 12ax7 preamp supply and about 110V on the plates. This is the average «first stage» preamp supply for 50's Gibson amps and many others. Maybe darker than Fender tone-wise, but with a 150 picofarad bypass volume cap and some 6 inches speakers I would not worry so much about darkness.

However, if you insist, you can gain about 20 volts for your preamp supply and better overall filtering if you insert a choke BEFORE the plate supply. I'm talking about a small 3-4H, 75ma, 100 ohm choke. Same type used to filter the screens in a Twin, bandmaster etc... but it will be used to filter the whole amp. First node: no connection. After the choke, you feed the OPT and the LFO. Then, you will still have that 1k resistor that will feed the screen and both 12AX7 sections. 3 filter capacitors, all 40µF caps. This will also get rid of the majority of single-ended hum. However, single-ended hum may not be a big problem with 6" speakers. For this reason and an economical one, I would return to the first option. 

[PRR]

> I think a B+ around 266v ..(at least 256v) ....... on the GA-5T B+ is around 278v

?? 278V, 256V--- what is the difference?

Yes, 22V or 8.5% on voltage and 18% on power. So?

If a GA-5T would do the job, a 256V GA-5T should do the same job, or close-enough that the audience won't laugh you off the stage. I don't think the tone will be very different (and could maybe improve). Tube life is slightly better. You can't get an exact copy of the GA-5T's speaker, and even they varied 8.5% unit-to-unit and over the decades (even Alnico fades, slowly). Put a "hot" speaker in it (or rig a Full Stack) and you will never miss the 22 Volts.

[kagliostro]

@ Greg

I'm sure I've some small choke to experiment with, the space on chassis is small but may be I can find one that is enough small

One other option may be is to divide the PS in separated branches, like in the VOX AC30 PS

@ PRR

May be I expressed me badly, I agree with what you say about % difference in voltage of B+ for the whole amp

As in the GA5 (220K 12ax7 plate resistors) the preamp tube are feed with a higher voltage I was thinking that lowering to 220K the value of

the 470K & 330K resistors of the GA-5T, it will be better to increase the voltage on plates (that on the GA-5T is "only" 115v & 110v)

but all this within the limits given by the components I've and without the intention that it is strictly necessary to do it


BTW .... please, what you mean ? >>> or rig a Full Stack

Do you refer to speakers ?

Franco

[Greg]

Deadpan humor for you to consider the negligibility of your gain Generally speaking, rigging a full stack is to assemble a 4X12" speaker cabinet on top of a another 4X12" speaker cabinet.

[kagliostro]

Ah, OK, 4x12" x 2

I was in doubt about 

Franco

[kagliostro]

I'm sure I've some other small ones, but at the moment I'm not able to find it

However I've find a pair that are small enough

one is 52mm x 31mm x 42mm plus attacks = 4.8H - 215ohm

the other is 45mm x 36mm x 38mm plus attacks = 7H - 175ohm



so I can experiment also with choke and see what happen

Franco

[Greg]

That's nice if you already have it. What will happen? Single-ended ripple reduction and maybe 20 volts more for you preamp supply which is really negligible. You will drop around 7 volts for your total current of about 40ma across the 175 ohm choke and 12 volt across 1k for 12ma screen & preamp supply.   

[kagliostro]

Thanks Greg

There is only one incognito

I don't know the max current that those choke can afford

it is recovered material that comes from incomplete circuits I collected during a long time

And if I use it only for G2 and the preamp section ?

Like in some old Geloso



Franco

[DummyLoad]

use the 7H - it's likely rated around 75mA. this is based on the DCR & inductance & size values compared to other manufacturers specs.


--pete

[kagliostro]

Oh, Thanks Prete

That is a good news

I controlled the weight, it is 0.57 lbs

looking to Hammond choke the 155G (7H - 40mA) is 0.3 lbs and the 156LA (7H - 75mA) is 1.1 lbs

so, may be something between 40mA and 75mA

but the Hammond are way higher in resistance (??)

Franco

[Greg]

To make you feel good even more, the heat through it will be like 1/3 of a watt.  If you install the choke in the preamp, you will not get the ripple reduction for the power tube plate. In SE operation you do not get ripple cancellation like in push-pull.   

[DummyLoad]

Oh, Thanks Prete

That is a good news

I controlled the weight, it is 0.57 lbs

looking to Hammond choke the 155G (7H - 40mA) is 0.3 lbs and the 156LA (7H - 75mA) is 1.1 lbs

so, may be something between 40mA and 75mA

but the Hammond are way higher in resistance (??)

Franco

thinner wire will yield higher DCR for same number of turns. your 7H choke will work, unless it is not already defective.


--pete

[DummyLoad]

should one wish to build with available transformers. see attached. i spun one up using a compactron. voltages indicated are from simulation - YMMV.


--pete

[kagliostro]

Cool Pete, Thanks for Sharing

.... your 7H choke will work, unless it is not already defective.

I think it is OK, resistance and impedance were measured not copied from a label on the choke

EDIT: I noticed also R12 ... it is not present on original, is it there because of the higher voltage or for other reasons ?

FRanco

[DummyLoad]

Cool Pete, Thanks for Sharing

.... your 7H choke will work, unless it is not already defective.

I think it is OK, resistance and impedance were measured not copied from a label on the choke

EDIT: I noticed also R12 ... it is not present on original, is it there because of the higher voltage or for other reasons ?

FRanco

it is optional and for G2 protection. power output will be negligibly lower with it. 


--pete

[kagliostro]

Protection ... OK, Thanks

Franco