Simple mods for earlier breakup- 6g2 Princeton

Uncle Daddy

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Anyone suggest some simple mods that might give me more/earlier breakup on my 6g2 build? With the 12" driver fitted, it doesn't really break up till almost at the end of the volume sweep, with Strat single coils. Currently running a JAN 5 y3 and a Weber Sig 12A smoothcone. Gold Lion in v1? Cap/resistor swaps? NFB mods? V1 cathode cap?
The build has a stock circuit apart from 120k plate resistors on v1 instead of the usual 100k.

Thanks.
 

ruger9

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A different 12AX7 in V1 won't help you. Other than that, just replying to keep up with this thread, as I am also interested. Methinks the V1 cathode cap is likely the answer.
 

Uncle Daddy

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I was looking at the 5e3, which has the same value cathode cap on v1, but an 820r resistor rather than the 1.5k on the Princeton.

A different 12AX7 in V1 won't help you. Other than that, just replying to keep up with this thread, as I am also interested. Methinks the V1 cathode cap is likely the answer.
 

sds1

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I was looking at the 5e3, which has the same value cathode cap on v1, but an 820r resistor rather than the 1.5k on the Princeton.
Casual observer here, I don't really know these circuits by experience.

The 820Ω is because it's a shared cathode, this is effectively same as 1.64kΩ per triode if the cathode were split.

What are you suggesting with the V1 cathode bypassed cap? I see V1A is already bypassed but V1B is not. Were you talking about changing the cap at V1A, or adding one to V1B?
 

NTC

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Raise the value of the feedback resistor. Try putting a 100k pot in series with the 56K and adjust to taste. Then replace the pot with a fixed resistor that measures close to the pot value or replace the 56k with a resistor with a value close to 56k plus the pot value.

82k will get you close to the amount of feedback in a later Princeton; 100k makes for early break up. I have two 56k resistors in series in my 5F11.

What is your voltage at the OT?
 

Badside

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Anyone suggest some simple mods that might give me more/earlier breakup on my 6g2 build? With the 12" driver fitted, it doesn't really break up till almost at the end of the volume sweep, with Strat single coils. Currently running a JAN 5 y3 and a Weber Sig 12A smoothcone. Gold Lion in v1? Cap/resistor swaps? NFB mods? V1 cathode cap?
The build has a stock circuit apart from 120k plate resistors on v1 instead of the usual 100k.

Thanks.

6G2 does not have a bypass cap on 2nd stage (V1b?) so that's the first thing I'd try. It's cheap and it's easy gain!
(Forgot that the 6G2 has the NFB feeding the cathode directly, if you add a bypass cap you lose NFB which makes it a very different amp. Would have to modify it to be more like AA964, meaning a 47R resistor between the 1.5k Rk and ground and feed NFB at the junction of these, with a 1k8 NFB resistor to keep the same ratio, of 2k7 like the AA964)

I do like the idea of reduced NFB (by way of a larger resistor) if what you want is dirt. The NFB helps with the clean tone, but it also lowers the gain of the power amp.
 
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Powdog

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By adding a bypass cap to the second gain stage you get the double whammy of the bypassed resistors and also effectively removing NFB. If you put the bypass cap on a footswitch it acts as a boost pedal. You’d need to add a 1/4” or RCA jack to the chassis.

You might also consider cathode biasing the power tubes.
 

Phrygian77

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6G2 does not have a bypass cap on 2nd stage (V1b?) so that's the first thing I'd try. It's cheap and it's easy gain!

I do like the idea of reduced NFB (by way of a larger resistor) if what you want is dirt. The NFB helps with the clean tone, but it also lowers the gain of the power amp.

You cannot add a bypass cap without changing the NFB scheme to one similar to an AA964/1164 Princeton. Otherwise, all of the NFB signal is effectively grounded through the bypass cap.
 
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King Fan

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Simplest way? Unlike my other amps, an NFB cut (to none) doesn’t sound nasty, and gives some really nice breakup on my 6g2. If it's a bit too much, then the pot to tune it. If not enough, then maybe the V1b bypass.
 

Uncle Daddy

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What exactly are you looking for when you say earlier break up? Lower volume or more gain?
I don't need any more volume as this is a house amp, plus I built a 6g3 for more louds. What I'd like is to hit some breakup earlier on the volume pot instead of at the end. More break up at a lower volume.

The switchable nfb looks useful; I can use the ground switch hole.
 

Badside

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You cannot add a bypass cap without changing the NFB scheme to one similar to an AA964/1164 Princeton. Otherwise, all of the NFB signal is effectively grounded through the bypass cap.

Oh wow yeah I looked too quickly and had the AA964 schem in my head. Forgot the 6G2 has the NFB feeding the cathode directly.

So it's doable but a bit more involved.
 

Badside

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I don't need any more volume as this is a house amp, plus I built a 6g3 for more louds. What I'd like is to hit some breakup earlier on the volume pot instead of at the end. More break up at a lower volume.

The switchable nfb looks useful; I can use the ground switch hole.

"More break up at a lower volume."

Do you mean more break up at a lower "physical" volume, or just lower on the volume pot? Because that's two very different thing. Trying to get more break up at a given volume setting will not make the amp quieter, just gainier.

If you enjoy the break up as it is but want it at a lower perceived volume, that is a whole different thing. You can try a lower efficiency speaker, using 6K6GT power tubes, reducing voltage to the power amp (while keeping preamp at same level), an attenuator between the amp and the speaker, etc. All of these will affect the sound of the amp to some extent.
 

cobaltu

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This amp is REALLY tricky to coax more gain out of. The problem is that the key to more gain is in the second gain stage. But it is also where the NFB is injected into the circuit, so any mod there to get more gain will also affect the NFB loop.

Also cathodyne PIs creates no net gain, so they are not good for driving power tube in to gain either. ON top of that, cathodynes don't like too much gain being driven into them either. They create a rather nasty OD. This also leads back to difficulty in modding the second gain stage for more gain. For example, adding a cathode bypass cap to the second gain stage will produce a ton more gain - as well as disabling the NFB - but it might also be waaaay to much for the PI.

I suppose the easiest way to get more gain IS to lower the NFB. However be wary. People who first get into modding often tweak NFB a lot. It's easy, cheap, and the effects are very noticeable, and there is little to no calculation of risk in doing it. The problem however, is that NFB affects like 6 things about an amp. The OD, texture, feel, frequency extension, etc. You might find that you get the increased OD you want, but 4 other things about the amp don't feel quite right. Like the amp might get to loose or rough sounding for example.

I have 4 fender amps, I have played with the NFB in all of them. In almost every case, save one, I have returned to the stock value. The one I did change was a very small decrease in NFB - I went from 22K to 27K.

If you do play with NFB, be subtle! I would try a 68K resistor to start. again, you'd be surprised at what a small difference can make.
 

Uncle Daddy

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As it stands there's not much breakup at all, what little there is is at the end of the volume sweep. More break up, earlier on the dial.
 

Dennyf

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A) pedal or B) different amp. Princetons are so good at what they do, it's almost a sin to try to make them into something else. Voice of experience. My '68 was a great amp until I tried to "improve" it.
 

arlum

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This may seem too simple but have you considered a different speaker. Speaker breakup plays a role in what you're talking about. Example .... running a 12 to 17 watt amp into a single 15 watt Celestion Alnico Blue will offer far more breakup than running the same amp into any speaker rated to handle 20 to 25 watts or higher. The greater the difference the less the speaker contributes to amp breakup. A higher wattage speaker will "clean up" the amps voice. If you're going for breakup the closer you can match the amp wattage to what the speaker can handle when pushed to the edge will provide the best result.
 

cobaltu

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Another thought, try changing the FIRST gain stage's cathode resistor to 1K or 820R and it's plate resistor back to 100K. It probably won't do much, but you'll get a bit more juice outa that first gain stage without altering the NFB.
 

King Fan

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Agree with those noting *lower-SPL* dirt doesn’t come from more gain by itself. I don’t know how *efficient* your Sig 12A smoothie is, or if a 10” would be a bit quieter by nature, but mine sounds sweetest with a *low-efficiency* 10”.

OTOH I'd still try a total NFB cut. And I’ll say it again -- I don’t like *total cut* on my other NFB amps. Here it doesn’t make my amp much louder, tho it's a bit 'noisier.' It does let me get enough dirt I can drop the volume knob a tad. Easy to try, easy to reverse, easy to put on a switch.

Then… Since the amp already has a 5Y3, a B+ solution I liked was a bucking transformer. Warmer, more brownface, at a schematic 315V B+.
 




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