Fender Concert - increase headroom

[lowell]

This amp breaks up around 3-4 on the volume. What would be the first couple things someone might do to increase headroom on this amp? I'm aware that power tubes w/ later breakup would be good, and maybe a decrease in the quiescent current.

[tubeswell]

lower the pre-amp gain with lower-gain tubes. Try a 5751 or 12AT7 in the PI

[jbltwin1]

concert break up

Which concert are we talking about? 60's? 80's? Like suggested, lower gain preamp tube may help and don't require anything other than a swap. If the amp is biased up in the greasy zone, you might have to do a little more. Also, are you sure it's the amp and not the speakers breaking up?

[lowell]

thanks guys... yes I've tried other speaker cab that I know is clean sounding.

[Chuck H]

Compared to other similar Fender amps the Concert has a smallish OT. I think Gerald Weber even called it "cheesy" in one of his books. It follows that the PT may also be smallish. Those two componants are paramount to getting good headroom. I've heard this complaint about Concert amps before. It's not a real big sounding amp. You could change out the transformers for a dramatic improvement.

And I don't want to start a whole big thing here, but... Pay no attention to the ideas that lower gain preamp tubes improve headroom. They don't. They only decrease gain which is not the same thing. You will get less breakup at higher volume settings. But the amp won't be any louder when clean than it was before. You just won't have the same ability to overdrive the amp as you did with the higher gain preamp tubes. So it's just the same as turning down the volume.

Without changing the transformers anything else you do will be a waste of time because the amp simply can't reproduce a big sound cleanly. An amp is just a modulated power supply coupled through a transformer. If you have a less than ideal power supply and poor coupling then you can't go beyond those limitations.

Chuck

[MWJB]

"Compared to other similar Fender amps the Concert has a smallish OT. I think Gerald Weber even called it "cheesy" in one of his books. It follows that the PT may also be smallish. Those two componants are paramount to getting good headroom. I've heard this complaint about Concert amps before. It's not a real big sounding amp. You could change out the transformers for a dramatic improvement." - If we are talking about the 60's brown Concerts this is completely untrue! So we really need to identify which amp we are talking about.

For the record ALL brown Concerts used 100W power transformers, as used in Twins & Showmans of the time. OT's in early concerts were the same as used in tweed bassmans, later brown concerts used the Schumacker equivalent. You would need a 2ohm OT for a brown/black 60's Concert, if such an OT existed for a 100W amp it would have to be designed for a 1ohm load with 4x6L6...!

Yes, putting a 12AT7 in the PI will increase headroom (after all, it does it in SRs and AB763 Black Concerts), if you wanted to tweak the circuit to suit the PI you could try the BF 470ohm cathode and 22K tail? It is not the same as "turning down the volume". Plate voltages change, tube's bias point changes, etc, etc...

I have seen A LOT of brown Concerts and the fact is the PI & PI tube is the biggest difference between them and a BF amp if you look at the Normal channel. Other differences relate to the preamp dropping supply, the BFs run the input valve at more volts.

Lowell - 30mA on the power tubes should do you fine.

A 60's Concert should be properly serviced & the first stage filter caps wired as per a Super Reverb, rewiring the screens as per an RI bassman is a good idea too & easy enoughtto fit in the cap pan.

[tubeswell]

And I don't want to start a whole big thing here, but... Pay no attention to the ideas that lower gain preamp tubes improve headroom. They don't. They only decrease gain which is not the same thing. You will get less breakup at higher volume settings. But the amp won't be any louder when clean than it was before. You just won't have the same ability to overdrive the amp as you did with the higher gain preamp tubes. So it's just the same as turning down the volume.

Chuck you are outnumbered at this point. A lower gain PI/driver tube will allow you to increase headroom virtually without any compromise to volume (or at least - the compromise to volume is hardly noticeable). I have done this many times. The amp will play cleaner at louder volume than it does with a higher gain tube in the PI - simply because the signal going into the output stage grid(s) will clip less - ipso facto there is more headroom. (of course whether you like what is does to the tone is another matter and also depends on the tube you sub in). Anyhow - the reason I suggested it is that it is a quick & dirty thing to try before you start pulling the amp apart etc

[Chuck H]

The amp will play cleaner at louder volume than it does with a higher gain tube in the PI - simply because the signal going into the output stage grid(s) will clip less

Ok. This really $h!t the bed, didn't it? I started a "whole big thing".

Tubes, I don't want to argue. But why does a higher gain tube clip more than a lower gain tube? Unless it's because it has more gain. So turn the gain down and you have the same thing. If anything the higher gain tube could clip LESS than the lower gain tube because it could provide a louder clean signal to drive the following stage. If that following stage starts to clip at "X" AC volts in, why would that change between a 12ax7 driving it and a 12at7 driving it? The voltage at the power tube grids that the power tubes can amplify cleanly remains the same. The only difference with the lower gain tube is that you now have less max voltage to drive the power tubes. Thats not the same as headroom.

Or are you saying that the PI tube itself clips less at a higher output voltage with a 12at7 than a 12ax7 due to the operating conditions? I haven't checked that out but that would make sense if that is the case.

Chuck

[tubeswell]

Or are you saying that the PI tube itself clips less at a higher output voltage with a 12at7 than a 12ax7 due to the operating conditions? I haven't checked that out but that would make sense if that is the case.

Chuck

Yep that's what happens. The 12AT7 doesn't amplify the signal as much, and therefore is less likely to go into either grid current limiting or plate cut-off. So the signal going from the PI to the output tube grid is more of a pure sine wave shape, and you as you turn it up, the signal stays cleaner for longer

[lowell]

Guys this is a good thread and thanks for input. I tried different tubes: 12at7, 12ay7, 12au7. I think I'm going w/ the 12ay7. The 12au7 WAS a bit cleaner but the tone suffered w/ that tube in there. It took what little warmth the amp has away. I think, at least in this amp, that changing PI has an effect albeit a very little effect. Looks like I'll need to start w/ a better OT.

[Chuck H]

My bad. Your absolutely right. In fact, the OT used in the 763 Concert was the same as the Super Reverb. I had the Concert mixed up with the Pro. I simply confused the OTHER two Fender amps that you don't see much.

Chuck

[Chuck H]

Lowell,

MWJB was good enough to point out an error in my previous post. I had the Concert mixed up with the Pro in my head. As it turns out the transformers in your amp have been used in many amps that have no issues with headroom. So you shouldn't need to replace them to maximize your amp. We can move on to other possible remedies.

What kind of music do you play? What kind of guitar are you plugging in? How loud do you need to get? And most important, when was the amp last serviced? Tubes, filter caps, etc.

Chuck

[lowell]

This is a '90s Concert. Shall I start w/ rewiring the PI per Brown specs and lowering the C+ for preamp tubes? Or should I start w/ lower gain tubes. Will lower gain tubes REALLY allow for more clean volume or just decrease gain?

[Chuck H]

I don't have a schem for a 90's Concert. But I'll bet that the PI is set up very similar to the old ones. In which case the best PI tube should be a 12at7, not a 12ax7. As tubeswell pointed out, the operating conditions in that part of the circuit are better for a 12at7 so it will give you the best performance. all other preamp tubes should be 12ax7. Although I have used a 12ay7 in the V1 spot when lower gain was Ok just because I like the tone. But it won't provide more headroom.

Looking at the ab763 Concert schem I can see that it's pretty much built for performance. Choke screen supply, diode rectifier, low resistance in the B+ rail and highish preamp voltages all contribute to better clean performance. Other considerations to increase headroom "MAY" include (again, I don't have the schem, but I will try to find one) increase the first filter node uf value and if "farting out" is where your headroom gets lost, decrease the preamp cathode bypass caps. Other than that I can only suggest that you make sure the amp has good filters and tubes and be sure your using a properly matched high efficiency speaker.

If I find the schem and see anything more to contribute I'll pop back in.

Chuck

[Chuck H]

Ok...I can't find a "90's" era Concert schem. Is this a RI type amp from the 90's (looks like a blackface panel, no master vol.) or a late run Rivera type amp (ie: master vol, pull boost, etc.)?

Also, What kind of music are you playing through it? What kind of speaker are you using? Does it "fart out"? When was the last time it was serviced? And does it have excessive hum or "ghost notes"?

Chuck