Planning single channel 6g4a

[bullkelp]

I’ve ordered a blonde bassman chassis and undrilled face plate, hanmond iron and have my usual 2mm g10 and turrets.

My plan is a 6g4a harmonic trem single channel head in brown with my usual tiki grill emblem etc

My question for the gurus here:


Given they I’ve built quite a few fender amps to schematic spec , and I have a couple original browns , if I was to make such an amp, what specific mods would you recommend?

I like to play surf music (clean and fairly loud) but also like to noodle
With p90s and jazz boxes with warm smooth tone. I like to use an attenuator and get smooth drive. One thing fenders can suffer from is fizz. So some advice about minimising that would be great.

Otherwise I’m happy to build it to stock (sans one channel).

Many thanks

[dogburn]

I'm far from a guru, but I am a fan of playing surf, so your plan sounds great. If I was building that amp, the things I would do are maybe add a mid pot and a negative feedback switch.

[tubeswell]

My plan is a 6g4a harmonic trem single channel head in brown with my usual tiki grill emblem etc

Sounds like a 6G4A without the normal channel. No other changes needed I'd say. Once you get it running, you could just mess around with the V1 preamp gain a bit if it's not clean enough.


(FWIW, a mid pot would just entail replacing the 6k8 resistor with a 10k pot)

[Esquirefreak]

Here's how I replaced the CF-triode with a mosfet in my last harmonic trem build. That way you could get away with 4 preamp tubes.

/Max

EDIT: I think I drew the intensity pot backwards.

[bullkelp]

My plan is a 6g4a harmonic trem single channel head in brown with my usual tiki grill emblem etc

Sounds like a 6G4A without the normal channel. No other changes needed I'd say. Once you get it running, you could just mess around with the V1 preamp gain a bit if it's not clean enough.


(FWIW, a mid pot would just entail replacing the 6k8 resistor with a 10k pot)

Could you have a glance at the schematic when you get a chance
Im reading all over the net how different soudning the normal and vibrato channels are. And this is certainly the case o nthe 6g13 and 6g11 I have. Also the 6g16 Ive built.

But they look identical uo the plate voltage of the second triad on each (160v for vibrato and 120 for normal)
the vibrato channel has a voltage splitter (100k / 6k8 ) then injects prior to the third triad of the vibrato circuit. the normal enters the phase inverter after this.

Where are the major sonic differences coming from ? And if I wanted the vibrato channel to be voiced more akin to the normal what  would be the best way to execute that? Alternatively, given I have front plate real estate, is there some switching I could add to toggle between a couple different voices?

I so far have the 6g6b chassis abd am building a brown aesthetic head cab for that.

Oh and 6g4 vs 6g4a? Is that extra tube just operating in the tremolo circuit ?

Oh and thridly: Given im using a 6g6b chassis Im sort of stuck with 6g4 rather than 6g4a unless I can squeeze in anotehr tube...
Any advice on fitting a 5th pre-amp tube in a 6g6b ?

Any disadvantage to the 6g4 vs 6g4a?

[Esquirefreak]

Making the vibrato channel sound like the normal is very doable, but you will lose the warble in the tremolo. The vibrato channel sounding a little thin is mostly because of the hi/lo filters.

I modded my tonestack to make up for the loss of mids, while still retaining the warble effect in the trem. I also shunted some of the highest frequencies to ground in the hi filter section.

If I remember correctly, this is what my tonestack looks like now. However, this is based on the topology of the 6G4.

/Max

[bullkelp]

Thanks Max
Yes that looks liek 6g4 to me
I think thats teh way im going to go, as it saves me drilling the chassis...
the only thing that might steer me is if the extra tube is felt to increase the usable gain in the vibrato channel
I like the idea of the mods youve done
Can you post the schematic of your build please ?

[Esquirefreak]

The filters are pretty lossy, so the last gain stages are probably needed to make up for that loss. The LFO also wiggles the grids of those triodes, so without them there will be no trem at all. The normal channel is hotter than the vibrato channel.

But you can save 1½ triode if you convert the LFO, CF and PI in the trem section to mosfet.

I have yet to draw a full schematic of the amp. I don't have as much time on hand like I used to.

The amp is (roughly) a 6G4 vibrato channel, with the hi/lo filters from the 6G4-A (slightly altered). Normal channel ia based on the 6G8 (James TS). 6G5 power amp and SS rectification.

I've built a full 6G4 clone as well and I favoured that tonestack over the later brown tonestack.

[bullkelp]

Thanks again
I do have tapped 350k pots on hand also.
Will give this some more thought across the weekend

[tubeswell]

Could you have a glance at the schematic when you get a chance

What schematic?

[bullkelp]

Sorry tube swell I just meant the 6g4 and 6g4a.

I do have questions about placement of the extra preamp socket.
Is it ok to squeeze the phase inverter close to the first out put tube ( I .e on a bassman chassis adding a v5 in the gap between v4 and the first output tube. Or am i better of placing it elsewhere? )

[bullkelp]

Here is the tube socket position I am considering

[bullkelp]

Or would something like this be better

[Esquirefreak]

Like I said earlier, if you make a mosfet tremolo section, you get away with 4 tubes = no need for any drilling.

/Max

[bullkelp]

I havent discounted that, im just not overly confident altering the schematic to install the FET.
Im ok with drilling. I just installed the IEC socekt easy enough and I have step drills and a press. Im just a little uncertain about the best spot for it

[Esquirefreak]

I'd probably go with the second drilling alternative 👍

[tubeswell]

I havent discounted that, im just not overly confident altering the schematic to install the FET.
Im ok with drilling. I just installed the IEC socekt easy enough and I have step drills and a press. Im just a little uncertain about the best spot for it

A enhancement mode mosfet like an IRF820 is pretty much a 'drop-in' replacement for a triode - especially a CF or a cathodyne. You just take the triode out and put the mosfet in. Drain = Plate, Gate = Grid, Source = Cathode. Bonus = no heater required, no H-K insulation to worry about. An IRF820 in a  TO220 package without a heat sink is good for 500V and about 1A. (And a 12V zener between gate and source is additional protection against over-voltage)

[bullkelp]

I think ive settled on running the full valve compliment...

What bias circuit would you choose for fixed adjustable.
I note the 6g4a schematic has a fixed 56k and two 8uF/150 along with a 15k.
Given im building from scratch and using a hammond 290DEX would I be ok just using the standard cirucit per this weber layout for the brown 6g13?
https://www.tedweber.com/diy-kits/

[tubeswell]

Depends on the bias winding voltage on your PT. As there is almost zilch current draw on a bias supply, you can count on a half-wave SS diode rectifier to rectify to 1.1412 x the secondary VAC at the reservoir cap. Whatever voltage divider you put in there after the reservoir cap will knock that down further. You'd be safe with a voltage divider output that gives a bias voltage range that's about +/-15V either side of 1/10th of the plate voltage of the 6L6s. So say your plate voltage is 450; that's a ball-park bias voltage mid-point of -45VDC with the range being -30 to -60. That's what I'd do.

[bullkelp]

Thanks
I think the bias tap is 52v
Why does fender use 2 x 8uf caps in the brown amp bias circuit?
Would I be ok just using my usual go to 50uf /150 v, And start with 470 ohm, a diode and a 10k bias pot / 27k tail and adjust from there?

[bullkelp]

Here is the schematic bias circuit with proposed trim pot

[bullkelp]

And here is the bias propsed by weber from the concert kitset  (please ignore the SS rec)

Note the weber PT specs show a 48v bias tap
The hammond DEX shows 52.8v

[tubeswell]

8uF was a standard size cap that was available at the time Leo designed the amp, and for bias supply it’s all you need - a Pi filter is a ‘belts and braces’ approach to eliminating supply ripple in a bias supply (as there isn’t much current anyway). 50uF will also work fine. The bias supply circuit you suggest will work. Yes play around with the tail (27k) if the output voltage isn’t right.

[bullkelp]

8uF was a standard size cap that was available at the time Leo designed the amp, and for bias supply it’s all you need - a Pi filter is a ‘belts and braces’ approach to eliminating supply ripple in a bias supply (as there isn’t much current anyway). 50uF will also work fine. The bias supply circuit you suggest will work. Yes play around with the tail (27k) if the output voltage isn’t right.

Thanks man
I figured it was one of those historic things and that a bit of over engineering wouldn’t do any harm.
Can over filtering the bias supply effect tone / response at all?

[tubeswell]

Can over filtering the bias supply effect tone / response at all?

No, that won’t affect tone

[bullkelp]

Is there a pentode/triode switch circuit that is favoured in these circles?

[Esquirefreak]

I'd recommend building the amp stock before modding, as it's a pretty finicky machine already.

/Max

[bullkelp]

I’ve built 5 or 6 brown face amps this year and they’ve come out great. Tho I have tried harmonic trem yet ….

As this is single channel I have some room on the face plate so I’d like to include some sensible switching options.
Bright
Half power / triode ?

I take your point tho and you are of course right :)

[dogburn]

As this is single channel I have some room on the face plate so I’d like to include some sensible switching options.
Bright
Half power / triode ?

I'm a fan of Negative Feedback switches.