Brown Concert 6G12a trem circuit - Some qns

[tubeswell]

Hi there

I am trying to understand how the 6G12a trem circuit works (and what it does - is it just pure trem, or some kind of vibrato as well). I have hi-lighted the bits I think I can discern, and those bits I can't, and I have qns as per the attached .pdf. Any takers please?

[mr.jetski]

hi pete,

wild looking thing you got there. enzo will fix it he fixes everything.

i can't get anybody to help me so i'm stuck on mine. i did see a 6g16 i liked the look of. and may just mod that into what i want. btw the tube that has a pent/triode in the same bottle what's that # again?

[Enzo]

1. yep

2. low pass filter - lets that 1Hz or thereabouts LFO through but shunts off unwanted signal like noise and 60Hz. You don;t want them modulating the amp.

3. yep

4. see #2

5. in my opinion, see #2. In general, and especially on old Fenders, when I see extra caps slapped onto something, it seems to usually be to rid the amp of some spurious signal of higher freq than the signal of interest freq.

6. I'd have to say they join with the two in #7 to form a pair of voltage dividers to knock the LFO level down for the following stage.

7. They are the grid return resistors for the following triode, but I personally don;t call them "grid leak" unless their primary function is to provide bias (usually with the cathode grounded). COnsidering the 4700 ohm cathode resistor and the healthy 3.5v it puts on the cathode, I don;t think they are counting on grid leak. But many guys DO call that a grid leak resistor.

8. Sorta. More like a crossover. The phase inverter split the LFO into two opposite polarity LFOs, one for each half of the 7025 tube. The signal from the input stages comes into the image from above. Note the .00025 cap to the upper grid. That is 250pf, so only the treble part of hte signal goes through that triode. The signal also feeds the lower triode grid through the larger .005 cap with the .01 as a rolloff, so the lower triode gets the lower freq signals. Since they each get the trem pulse out of phase, one will be at the lowest level when the other is highest. So the trem works at alternating times on the highs and lows. This to make a more interesting trem sound. I forget their name for it.

9. Neither. It is a mixer. the 7025 gives the LFO a place to work, then the high and low freq signals are mixed together coming off those plates by the two 470k resistors.

And from there, the normal channel also mixes in through its own 220k. And things then continue through the amp.

That is just my off the top of my head thoughts on it, if there are dissenting views, I am interested.

[tubeswell]

Hi Enzo

Very interesting. So that last stage sort of mixes two signals back together and with slightly varying freq (and possibly varying harmonic distortion?) The 4k7 cathode resistor must make for a cold-biased stage (some +ve signal plate cuttoff - is that the idea?).

Hmmm... if one was to put a pot at the junction of the two 470k with the wiper going to the output to the amp, then one could tweek the amount of hi-vs-lo freq LFO from the 'mixer' stage getting through to the PI?? (or better still replace both 470k with 270k- 390K and have a 100k-250k pot in between them with the wiper going back to the PI?)

Cheers



Hi Mr Jetski

a couple spring to mind - ECL82/6BM8 (in production again), ECL86/6GW8 (similar to ECL82 but more powerful) Check out the specs at the National Valve Museum site

[Enzo]

7199, 6AN8.


I don't think the 4.7k is doing anything fancy, I think it justy provides room for a large signal. This isn;t the input stage, you don;t want to provide 1v bias when the signal can be larger than that - and thus clip.

You could slap a balance pot coming out of those tubes - I'd have to be convinced it would do something useful first, I think overall it would just act like a weak tone control.

[rathmann]

Hmmm... if one was to put a pot at the junction of the two 470k with the wiper going to the output to the amp, then one could tweek the amount of hi-vs-lo freq LFO from the 'mixer' stage getting through to the PI?? (or better still replace both 470k with 270k- 390K and have a 100k-250k pot in between them with the wiper going back to the PI?)

I think this might be the right idea but the wrong location. I would probably try this at 7) in your diagram. Replace these with 500k and put a 1M linear pot in between with the CT to ground. This would truly affect the balance of the LFO reaching the hi/lo audio signal components. I've been meaning to experiment with something like this for a long time now but haven't gotten around to it. The three potential cons I see happening are a) little to nothing; waste of a 1M pot b) the aforementioned "glorified" (yet weak) tone control and c) crazy-ass thumping of the LFO.

In my experience building one of these the caps at 4) had the most effect at reducing thumping without drastically affecting shape/contour of the LFO - not to mention that, hanging right off the intensity pot, they're hecka easy to swap out.

[gbono]

I have a Brown Concert where the vibrato circuit will "thump" when you turn up the speed or intensity.

Would a lower gain 12AX7 help? Should I look at changing the LPF caps in section 4 of the previous attachment from tubeswell?

Also noticed that the intensity control needs to be above 4 before you hear the vibrato come on - is this normal?

[tubeswell]

How old is the amp? (40 +?) Try a couple of new tubes first. The ultimate resort is having to change at least all the caps (and possibly all the resistors) in the LFO circuit (keeping to - or close to - original values)

[gbono]

Yes, its a 61-62 (6G12-A) - I'll try and lower gain tube and if not that then it's fiddle with the capacitors to see if I can filter it out there.

Funny, this is a "true" vibrato design vs the tremelo that was in the tweeds and then later the blackface designs. I wonder why Fender didn't continue building this vibrato in later designs. Or should I have said CBS

[tubeswell]

In that case one or more of the LFO caps are quite possibly worn out. Try popping new tubes in first - old worn out tubes won't make the LFO cycle work as well as it should. If that doesn't work, I'd just replace all the caps and resistors in the LFO circuit (and chuck in a couple of new tubes).

I think Fender decided that vibrato and tremolo tube driven circuits were getting expensive to service, so they went the way of the tube and LDR circuits. (Personally I like the FX from the earlier circuits myself).

Also - how old are the filter caps and bypass caps? (Are they original?) It might pay to get them all changed as well

[gbono]

This amp comes from a touring musican's quiver. He complained that the tubes were noisy and the amp needed to be "re-tubed". Visual inspection revealed that one of the original loudspeakers was full of holes (quarter sized no less - of course it still "worked"). The other adjacent driver had an open voice coil. Not bad, 2.5/4 original drivers still working after 47 years.

The controls and jacks really needed a good cleaning and that solved the awful intermitent noise and crackle issue. The tubes looked fairly new and several caps had been replaced - this amplifier is really very quiet with a lack of 60/120 Hz hum.. As with most working musicans, no money to fix things right so I'm waiting to hear what he wants to do.

[rathmann]

FWIW - this still isn't "true" vibrato. It's just a more fancy-schmancy tremolo. It oscillates high and low frequencies alternatively. It certainly sounds a bit gooier than the LDR trem but still isn't doing any pitch shifting.

My understanding of the evolution of Fender amps is that users valued reverb over tremolo. This was totally a sales/return-on-investment based decision. The "harmonic" trem of the brown amps used up to five triode stages - two and a half tubes with a touchy complex circuit, whereas the blackface config managed a passable tremolo AND reverb with a grand total of two tubes and relatively simple designs. Note that this topology preceded CBS by MANY years.

Anyway - as suggested I would recommend a retubing, possibly try a 12AT7 in there. It's conceivable that higher line levels are allowing too much gain in the oscillator. I find it more likely though that some of the carbon comp resistor have drifted off value. Any electrolytics in there are probably near end-of-life as well. I would be surprised if the ceramics and film caps have drifted much. Failure with those would more likely be a short causing the trem to not work at all. Still, a few bucks worth of ceramics and new resistors could go a long way.

best of luck!

[Alf]

Hi Rathman,

What is the best sounding tremelo in your opinion ?

Imho the brownface trem sounds the best of all trems but have I heard them all ?


Here's a clip:

Real Brownface harmonic vibrato - eSnips, share anything

Alf

[tubeswell]

IMHO the 5G9 has the best sounding trem by miles (but it is a function of the rest of the amps topology to some degree - so its not necessarily a matter of dropping a trem circuit from here into an amp from there - so to speak).

[GibsonLover]

5G9 tubeswell? You haven't heard this one yet.... ;-)
The duplication of components between your boxes 5 and 6 (.05 ,1M, 1M, .1) also serve to balance the PI amplitudes, to add to Enzo's comments.

The operation of the two trems are quite different, as you see, so I don't think it's too fair to judge the side by side. For amplitude modulation, I'll take the output bias modulation any day - give me a Vibrolux. But this circuit is different. The Hi Pass/Low Pass section before box 8 is the key. Remember the modulation is s l o w. While the HP is being swept up, the LP is being swept down. That kind of modulation does not have a 'swell' like the AM version. There is a phase shift going on there and when it's mixed at point 9, you get a little Fender magic (that Magnatone, Gibson and Vox have already done).

Build a Vibrasonic!! With this trem + 15" JBL = heaven.