Deluxe Reverb to Class A?

[chipaudette]

Hi All,

I've been having fun butchering, I mean "tweaking" my Fender Deluxe Reverb Reissue (2006). I've been having some fun modding the pre-amp to slam the power amp thereby getting some massive overdrive out of the power section.

I've got a THD hotplate to help bring the volume down when I'm driving it so hard, but I'm always setting the Hotplate to high amounts of attenuation when more moderate attenuation settings on the Hotplate tend to sound better.

So, I thought about dropping the power of my amp. Fender rates the amp around 22 W. It's got two 6V6s in Class AB operation. At the moment, it's biased to about 22 mA at idle and I measured the plate voltage at about 415 V. I guess that makes 9.1W of dissipation per tube when idling.

If I wanted to pull and tube and rework it to use a single 6V6 in class A operation, this should pull the power down to 5 -7W or so, right? That sounds good to me! This 4-6 dB drop sounds perfect because it would let me drop back a notch or two on my hotplate.

What steps would i need to take to mod my amp to class A with one tube? Is it feasible? Would it sound decent?

Thanks,

Chip

[Chuck H]

Ideally you would want a lower plate voltage for class A operation. But there are plenty of old SF Champs out there with A Vp around 420. The one I rebuilt 2 years ago is still working fine anyhow. Other than that you would need a different OT. You can get away with just unplugging a tube and rebiasing using the stock PP OT, but only at low volume settings. Since your cranking the amp, that would be a bad idea. Class A OTs are designed to avoid saturating when operated in one polarity. They are generally much larger for a given wattage and incorporate a "gap" in the laminations. If you do try to crank your amp with the stock OT and one tube pulled, you could damage some expensive parts. You could try using a lower wattage output tube and rebias. IIRC a 6k6 will plug right into a 6V6 socket.

Chuck

[chipaudette]

Thanks for the info. If I understand your message, I think that you offered two solutions: (1) swap out the OT for one designed for SE operation, or (2) swap the 6V6 for another tube like a 6K6.

On the first option, I've never bought another OT before. What are the criteria that I need to look for to make sure that it satisfies my needs?

On the second option, I've never heard of a 6K6. Assuming that I can find one on the web (the mother of all things), what would the power output be when operating in class A? To what idle current should I bias it? Will it tolerate the 415V plate voltage?

Thanks heaps. This is really interesting.

Chip

[MWJB]

Re (2) or fit 1x6L6/5881 & rebias that single tube to a higher current, you could probably get away with 40mA with the stock OT? This would be a better impedance match and the bigger tube will easily handle the plate voltage. You might like to experiment with lower mu preamp tubes (12AY/12AT7) to tame distortion?

(1) if you go this route (a dedicated SE OT) there's no easy way to switch back and forth...unless you try an OT in the style of the Victoria Regal...this runs SE or PP, with 6V6 or 6L6. Drop them a line, they might sell you one?

[chipaudette]

Thanks for the comments. The 6L6 route worries me because the point of this exercise is to lower the power of my amp. I want to keep sound of an seriously overdriven power amp, just with 4-6 dB less sound.

What is the power output of a 6L6 in classA? With two 6v6s in class AB, the amp is 20W or so. Isn't a 6L6 class A good for 10W or so? That would be a 3dB reduction, which wouldn't be bad. I'd like a little more reduction, but 3dB is a good start.

Would 10W in SE operation still saturate my currently-installed PP output transformer?

Chip

[DirtyGeorge]

Chip,
You could start by killing the signal at the grid of one 6v6. Move the grid stopper connection from the top of the reference resistor to the bottom. This will ground that grid (DRRI is cathode biased?).

The idle current through the non-signal 6v6 will prevent OT core saturation so you can use your PP OT. You should double you load Z since you are cutting the AC current in half. 8 ohm speaker on 4 ohm tap etc.

With the 6v6 still biased for class AB operation you should start to distort very early. To move closer to class A you need to increase idle current (through both 6v6s). To do this safely would require plate voltage reduction.

For true class A (whatever that means) you would need to switch your power supply to a choke input type to get the voltage down. Then bias to about 45mA per tube (if that doesn't overstress the OT) to get to the center of the loadline.

DG

[chipaudette]

Hi DirtyGeorge. Thanks for the recommendations.

Regarding grounding the grid of the 6v6, the DRRI is not cathode biased. Will your recommendation still work to prevent saturation of the OT even though I'm not cathode biased?

Then, to go to Class A, I hear here from you and from others outside of this forum that dropping my plate voltage is probably required. How low should I go? 350V? 300V? Switching out major power supply components (like the power transformer) is probably more than I want to do. Or, is there an easier way to drop the voltage?

Thanks!

Chip

[chipaudette]

Well, as an experiment, I tried pulling one 6v6 and then biasing up the remaining 6v6. To push he remaining tube into class A, I attempted to raise the bias point. I pushed it to the max of the bias pot on my DRRI, but it topped out at 27 mA. Even at that level, it sounded like it was clipping improperly when driven hard. It still sounded like it wasn't biased high enough for proper class A.

So, I modded the bias circuitry so that I could go higher. I pushed it to 50mA and still got no red plating on my stock Groove Tube 6v6s. The amp didn't really sound that much better, though.

So, either I don't like the sound of overdriven class A, or I didn't really achieve full class A with my bias level, or I was hearing the nasty sounds of a saturated OT. I'm not sure what was going on but it wasn't what I'm looking for.

Maybe going to the lower power tube (6K6) is the right route or maybe dropping the plate voltage is the right route.

Dropping the plate voltage sounds like a pain. I'm going to check with a buddy of mine to see if there's a 6K6 to borrow. Do I need to change the plate voltage for the 6K6?

Chip

[MWJB]

"So, I modded the bias circuitry so that I could go higher. I pushed it to 50mA and still got no red plating on my stock Groove Tube 6v6s. The amp didn't really sound that much better, though." By running SE you are in class A anyway, 50mA is perhaps more than I would want to see on a fixed bias 6V6 at 420-ish volts.

Can you describe "clipping improperly"?

"So, either I don't like the sound of overdriven class A, or I didn't really achieve full class A with my bias level, or I was hearing the nasty sounds of a saturated OT. I'm not sure what was going on but it wasn't what I'm looking for" Indeed, maybe it's not what you are looking for, but then you were looking for a different class of operation rather than a specific sound.

A SE amp will have more crunchy/fuzzy harmonics compared top push pull, one reason why I suggested trying a lower mu preamp tube to tame distortion.

I doubt that your 6K6 is going to last very long at 6V6 voltages with lots of current, that's why I suggested a 5881/6L6, which will also straighten out your impedance mismatch.

You might get 10W-ish, but it won't be a clean 10W and you won't have the headroom that a 3db drop would suggest. Forget the numbers (W & db) and listen first, consider the theory after, W & db don't reflect envelope, tone & voicing.

Also listen to the difference between which socket you use? I would pull the 6V6 that sits next to the rectifier.

A lower voltage rectifier will drop the B+ voltage, try a 5Y3GT. Bear in mind that Fender typically ran their tubes at higher voltages than the data sheets suggested, Champs typically run 400v on a 6V6.

[scole]

this from aikens web page q&a section:

Q: Is there any way to turn a push-pull amplifier into a single-ended amplifier, like pulling all the tubes on one side?
A: The output transformer in a single-ended amplifier must be gapped to prevent DC offset in the primary from saturating the core. A push-pull output transformer is not gapped, so if you try removing the tubes on one side, there will be an offset DC current that will saturate the core and cause a loss of low end (or worse!). The easy way to do this is to leave both sets of tubes in, but remove the signal going to one side, but leave it biased the same current as the other side. This will cause the primary DC currents to cancel out, so you don't get transformer saturation. This will effectively result in single-ended operation of the push-pull output stage. Note that this will only work if you bias the amp into class A, because you will only be running one side, and it will clip if biased at class AB or class B. You will also have to reduce the signal level going to the output tubes compared to the level at class AB, otherwise it will clip too soon. The best candidate for this type of mode switching is a true class A push-pull amplifier, because then you can easily switch from push-pull to single-ended simply by removing the signal drive to one of the sides.

[Satamax]

Hi everybody!

Well Chip. Have you thought about the use of a lower sensitivity speaker? My be your speaker is rated 98/99db, you get a 92/93 one, and you've dropped the 6db you wanted! It's rather simple and doesn't mean extensive modding. Another thing you could try is the zener mod to get in real Class A PP if your output trany copes with it. http://music-electronics-forum.com/s...ht=zener+trick

Here on page 2 you have some intresting numbers about A1 and AB1PP triode

http://www.mif.pg.gda.pl/homepages/f...84/6/6V6GT.pdf

HTH.

Max.