5F6A Gain boost

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5F6A Gain boost

Postby ThisLifeILead on Wed Feb 26, 2014 7:48 pm

Hi Guys, Im converting an old Dukane chassis into a 59' 5F6A bassman for a friend but he wants a footswitchable gain boost. The amp will have 1 input with a bright switch on the bassman channel volume. The input triode on the bassman channel will be using a 12ay7 to keep things clean.
My question is since the rest of the circuit is close to a M*rshall anyways, I was thinking of using a simple switching relay kit from MojoTone to change the input stage of the amp. to say a 2.7K/0.68uf ALA M*rshall.
Im thinking the relay would be switching after the volume controls/before the 12ax7 for both channels. Maybe even cascading both 12ax7 triodes for a bigger gain boost. This way both channels share the tone controls and master volume.
The only issue i see in doing this is maintaining level volumes. So i was thinking of including both a gain and volume control for the gain channel.

Looking at the schematic
Image

Any suggestions? Would a simple marshall input stage be enough of a gain boost or should i tailor in a better circuit?
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Re: 5F6A Gain boost

Postby Papa Dog on Wed Feb 26, 2014 9:54 pm

you do not need a relay, and it aint that complicated.
were I building that I would go ahead and use a 12ax7 for the 1st stage and a 12at7 for the phase inverter.
Just lift the cathode resister and by-pass cap from ground and wire that to a passive footswitch to ground.
a SPDT switch will allow you to chose between two sets of values for a boost or to amke it crunch, depending on how other things are set up.
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Re: 5F6A Gain boost

Postby ThisLifeILead on Thu Feb 27, 2014 2:52 pm

Papadog, so if I understand correctly you would set the switch to crunch and use a large cap and small cathode resistor. But then the footswitch would ground the cap/resistor engaging it. What about when its set for clean and the input stage has no cathode ground reference?
I guess I need to find out from my buddy exactly what kind of boost/gain he wants.
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Re: 5F6A Gain boost

Postby Papa Dog on Fri Feb 28, 2014 4:05 pm

ThisLifeILead wrote:Papadog, so if I understand correctly you would set the switch to crunch and use a large cap and small cathode resistor. But then the footswitch would ground the cap/resistor engaging it. What about when its set for clean and the input stage has no cathode ground reference?
I guess I need to find out from my buddy exactly what kind of boost/gain he wants.


SPDT switch.
one side has a small resistor and the other bigger. either different bypass caps or just one to ground. Depenmds on what ya want from it.
switching to the smaller resistor would give a gain boost and make it a bit more "crunchy".
wire the resistors to the tube socket and then to a trs jack. ground the sleave. use 2 pair telephone wire to a heavy duty pushbutton switch. You could even wire in an LED on either side, so that it would indicate which choice had been made.
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Re: 5F6A Gain boost

Postby ThisLifeILead on Fri Feb 28, 2014 5:21 pm

Ok now i see what you mean Papadog thats a clever passive wiring scheme.
With this amp amp there is tons of available current and real estate so a relay wouldnt hurt the space and or current. it draws 6.3vac from heaters.
My friend wants some crunch and sustain mostly for lead work.
He also wants the 5F6A to be as closest as possible to the original circuit so a 12ay7 will be required for the input stage on the clean channel. The relay kit is only 27$ and it includes an Led and only requires a cheap single 1/4" footswitch to activate the relay. Im sure its probably overkill for what it does but this way i can really add a marshall flavour for lead work.
Im just not sure if i should be doing just a marshall input stage or a full blown dual triode boost.
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Re: 5F6A Gain boost

Postby Papa Dog on Fri Feb 28, 2014 5:50 pm

well, there ya have it. It all depends on what you want things to do.
However, even channel switching can be done passively a'la the SMF tour series. That amp grounds the channel not wanted and lift the ground from the channel desired.
Image
SMF pre-amp

there are choices which are good, choices which are cheap, and choices which are fast...we can usually only have 2 of these.

My issue with relays is that they contain coils and have to be run DC to be quiet. Making a functional channel switching replay system can be a daunting prospect and could lead to excess noise in the circuit,
But, far be it from me to discourage inovation and exploration.
Let us know how this works out for you.
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Re: 5F6A Gain boost

Postby ThisLifeILead on Mon Mar 03, 2014 5:13 pm

Now that is an interesting schematic. whats the purpose of the T3 transformer? Looks like its part of the Dirty channel.
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Re: 5F6A Gain boost

Postby PeterS on Mon Mar 03, 2014 11:22 pm

one side has a small resistor and the other bigger. either different bypass caps or just one to ground.


If you switch the cathode resistor does it make a big bang because you changed the operating point of the tube suddenly?
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Re: 5F6A Gain boost

Postby Papa Dog on Mon Mar 03, 2014 11:56 pm

ThisLifeILead wrote:Now that is an interesting schematic. whats the purpose of the T3 transformer? Looks like its part of the Dirty channel.


yes, T3 is an interstage transformer on the dirty channel. I wish someone with one of those amps would grace me with a part number, as I would like to know.
Anyway, as I understannd it, the idea there was to get symetrical overdrive as one might find in the output of a massivley overdirven smaller amp.
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Re: 5F6A Gain boost

Postby PeterS on Tue Mar 04, 2014 2:39 am

Interesting, I've never seen that feature in an amp before, there's a split load PI and a push-pull amp driving the transformer...
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Re: 5F6A Gain boost

Postby Papa Dog on Tue Mar 04, 2014 3:33 pm

PeterS wrote:Interesting, I've never seen that feature in an amp before, there's a split load PI and a push-pull amp driving the transformer...


I have never seen it anywhere else either. As I understand it, that is supposed to give "output tube" type distortion from a 12ax7 (ecc83). Simple, cleaver, and should allow a player to sound like they are playing "balls out" at a reasonable volume level. Or with that amp, which uses a very clean output stage, to sound like a massivley overdriven power amp, and too loud to stand in front of.
Of course it aint a Marshall, and despite being used by such luminaries as Blue Oyster Cult they just never caught on. Or maybe it was that DMI went under...
There Ain't No Such Thing As A Free Lunch.

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Of cabbages--and kings--"
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Re: 5F6A Gain boost

Postby ThisLifeILead on Tue Mar 04, 2014 7:28 pm

We have decided to simply use the switching relay to switch between two front ends. grounding out the other opossing channel on the 2nd pole on the relay.

The gain front end will be based around the all too familiar cascaded 18watt preamp. This way we can have a gain dial and a volume dial. i imagine i will have to dial the stages down a tad but should make it quite a versatile amp. :jam:
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