Extra gain

[gui_tarzan]

Looking at the JTM45, there is an extra gain stage between the volume and tone stack. My new 5F6-A build doesn't have much crunch at all, I'd like to add this extra half-triode if it makes sense. The schematic is what I have now minus the areas inside the red boxes. I want an adjustable bias, does this one look right? I also noticed the 100K resistors on the pair of triodes after the volume control are on opposite sides. Is this a phase thing and why isn't there a decoupling cap between the plate and grid of the halves? The original JTM45 doesn't have one to block DC going into the second half. What am I missing?

[Justin Thomas]

It looks like as built, you have a more Blackface preamp with the tone stack in a different place - 2 gain stages, tone, PI. The 5F6A has he first gain stage, then the second, which drives the cathode follower, then the tone stack, then the PI. So you'd have to add the 2nd "in the box" stage, and change the third triode to a cathode follower, then the tone stack, then the PI. See if adding the CF makes a difference. Right now, it's not a 5F6A. It's closer to a BF. Gimme a few minutes and I'll lost a 5F6A & a JTM45.

Fender: Prowess Amplifiers - Fender - Schematics - Bassman 5f6a - Schematic

Marshall: Prowess Amplifiers - Marshall - Schematics - Jtm 45

HTH,

Justin

[Leo_Gnardo]

Looking at the JTM45, there is an extra gain stage between the volume and tone stack. My new 5F6-A build doesn't have much crunch at all, I'd like to add this extra half-triode if it makes sense. The schematic is what I have now minus the areas inside the red boxes. I want an adjustable bias, does this one look right? I also noticed the 100K resistors on the pair of triodes after the volume control are on opposite sides. Is this a phase thing and why isn't there a decoupling cap between the plate and grid of the halves? The original JTM45 doesn't have one to block DC going into the second half. What am I missing?

There's something hinky about the schematic you put up. Ordinarily V2 is tasked as EQ driver, the voltage gain triode directly driving the next triode in cathode follower mode (that's why there's no DC blocking cap) with typically a 100K resistor on the cathode and signal taken from the cathode driving the EQ. You're showing signal taken off the plate, driving the EQ, and 820Rin the cathode, this is not the typical circuit at all. And I can't imagine what kind of signal passes thru that triode with its grid biased at @ +200V . . . Yes I think it's a good idea to add your extra triode, also alter the second triode in this pair to proper cathode-follower like in standard 5F6A or JTM45 schemos. BTW it's a very good idea to use a 1 watt or larger 100K cathode resistor. Develops a lot of voltage across it & frequently challenges a 1W rating. Many commercial manufacturers (Marshall, Fender and others who should know better) neglect this and I often find toasted half-watt 100K resistors here.

Half-triode? No it's a triode. Half a dual triode = one triode. Can't much amplify with anything less.

Once you get that sorted out here's an option for a dab more gain: It's an old amp-tech trick to add a cap in parallel with the EQ driver's gain triode cathode resistor typically 820R. You could add a pinch of highs with say 0.22 uF, or honk up the mids with just a single microfarad, or go whole hog and boost wideband with 10 or 20 uF. It's a good place to "voice" the amp. I offer a mod that puts a 10K pot in series with a cap here, "dial-a-boost", cheap and effective.

[loudthud]

Look on Merlin's site. I'm not a fan so I don't have it book marked. The second stage just before the tone stack is a cathode follower. It doesn't need a coupling cap if the previous stage is setup correctly. The 100K is connected to the cathode. It doesn't provide any voltage gain but it lowers the impedance driving the tone stack.

Your bias supply needs to be connected the the 325VAC line. It won't work the way you have it drawn.

[gui_tarzan]

There's something hinky about the schematic you put up. Ordinarily V2 is tasked as EQ driver, the voltage gain triode directly driving the next triode in cathode follower mode (that's why there's no DC blocking cap) with typically a 100K resistor on the cathode and signal taken from the cathode driving the EQ. You're showing signal taken off the plate, driving the EQ, and 820Rin the cathode, this is not the typical circuit at all.

You are correct, that was just a copy/paste to add the second triode.

And I can't imagine what kind of signal passes thru that triode with its grid biased at @ +200V . . . Yes I think it's a good idea to add your extra triode, also alter the second triode in this pair to proper cathode-follower like in standard 5F6A or JTM45 schemos. BTW it's a very good idea to use a 1 watt or larger 100K cathode resistor. Develops a lot of voltage across it & frequently challenges a 1W rating. Many commercial manufacturers (Marshall, Fender and others who should know better) neglect this and I often find toasted half-watt 100K resistors here.

That makes sense.

Half-triode? No it's a triode. Half a dual triode = one triode. Can't much amplify with anything less.

Again, correct. I typed that shortly after getting home from work and it was one of those days.

Once you get that sorted out here's an option for a dab more gain: It's an old amp-tech trick to add a cap in parallel with the EQ driver's gain triode cathode resistor typically 820R. You could add a pinch of highs with say 0.22 uF, or honk up the mids with just a single microfarad, or go whole hog and boost wideband with 10 or 20 uF. It's a good place to "voice" the amp. I offer a mod that puts a 10K pot in series with a cap here, "dial-a-boost", cheap and effective.

And yes, I had the bias coming off the wrong tap, I corrected it on my schematic.

[big_teee]

No cathode follower.
Here's the regular JTM45, weber version.
https://taweber.powweb.com/store/6m45_schem.jpg
You can add the other stage like a JCM800 2204.
Like the preamp on this one, but this is the 100w version, but the preamp is the same.
https://taweber.powweb.com/store/8cm100_schem.jpg

[gui_tarzan]

More like this?

[J M Fahey]

Looking at the JTM45, there is an extra gain stage between the volume and tone stack. My new 5F6-A build doesn't have much crunch at all, I'd like to add this extra half-triode if it makes sense. The schematic is what I have now minus the areas inside the red boxes. I want an adjustable bias, does this one look right? I also noticed the 100K resistors on the pair of triodes after the volume control are on opposite sides. Is this a phase thing and why isn't there a decoupling cap between the plate and grid of the halves? The original JTM45 doesn't have one to block DC going into the second half. What am I missing?

What are you talking about?

They are the same, have exactly the same amount of gain stages, since the JTM is a British clone of 5F6

There are minuscule tweaks, nothing else.

What you are missing is that you did not build a 5F6 .

[dai h.]

in my estimation, if:

1st) you build it correctly(!), then

2) use 470k for the "mix R" instead of 270k

3) turn the bass control to zero

4) use a hot humbucker...

this will probably give you decent distortion (though probably not the real modern metal kind of distortion)

[gui_tarzan]

What are you talking about?

They are the same, have exactly the same amount of gain stages, since the JTM is a British clone of 5F6

There are minuscule tweaks, nothing else.

What you are missing is that you did not build a 5F6 .

Actually I did start with the 5F6A schematic. I simply removed the "bright" channel and associated parts and revised the bias controls. And shifted a couple of wires around. It's still the same circuit. HOWEVER, I see on another copy I just downloaded there is the additional gain stage in this one so my other copy must have been modified at some point, removing the extra gain stage.

Holy crap.

*sigh*

Ok, forget the extra gain part, that question seems to have been answered. Does the bias setup in the most recent schematic look correct? I compared it to a few others and I just want to make sure. I don't like the idea of having a non-adjustable bias so I added the pot.

[loudthud]

At last years Dallas Guitar show I saw an interesting Traynor amp. The YBA-1 was an amp that was similar to the 5F6A first made in Canada in the mid 60's.. The version I saw was a YBA-1MOD1. It has a switch that allows you to run the input triodes in parallel like normal or in series like the Marshall 2204.

Link to Service Manual: http://traynoramps.com/downloads/ser...myba-1mod1.pdf

I tried the idea on an amp and it has silly amounts of gain. Layout is tricky with such high gain.

[pdf64]

For a 'free' dollop of extra gain, see the bootstrapping idea at the bottom of The Valve Wizard

The bias circuit should work in principle now, but the values seem rather different than those used by Marshall for something similar http://schems.com/manu/marshall/blue...__45w_1962.pdf

[gui_tarzan]

Ok, it's almost working normally now, it's got plenty of gain - a little too much. I am going to try an AT7 in the PI, right now I have an AX7 there. Here's the schem as it's built today. The only glitch I have at the moment is a "bup bup bup bup bup bup bup" sound (cap?) and some hum that gets louder as the volume goes up, especially when the NFB is turned up high. My wiring is not optimal yet, so it could just be a placement issue. The goal right now is to get it where I want sound-wise, then tidy up the wiring, etc. as I put it in the chassis it's going to end up in.