Fender DRRI and SRRI: Not enough headroom

[stratomaster]

Hello,

I have both a DRRI and SRRI that are distorting at around 2.5 on the volume knob. I have late break-up power tubes in both amps, and I have not substituted any of the preamp tubes for ones of a different type (no 12AX7s where 12AT7s should be, etc.) And I'm running single-coil pickups into it (no distortion pick-ups or humbuckers).

This would be desirable for most, I guess. But not for me. It tends to be a muddy distortion that does not handle low notes very well. It sounds a lot like pre-amp distortion.

I have the 6V6s at around 8.5W idle power (14 max) and the 6L6s at around 19W idle (30 max). So there's nothing fishy happening there.

Of note, I have disconnected the Vibrato Intensity pot from both amps, as well as removed the V1 tube. I understand this acts to increase distortion, but it also fattened up my cleans (at apt. listening levels). I would think I'd still be able to get up to 4 or 5 on the volume knob before I get significant breakup. Could this have caused the complete loss of headroom?

I feel like I'm missing out on much louder sweet Fender clean sounds. Thanks.

[ajeffcote]

Tried putting them back to the way they were?

[chipaudette]

If you don't want the early break-up, you have to switch your amp back to stock.

The mods you did will indeed "fatten up" the sound. But, the fatten up the sound by increasing the gain within the pre-amp section. Increasing the gain is going to cause distortion in the pre-amp somewhere. Sure, at low settings on the volume knob, you're only driving the pre-amp a little hard, thereby making it compress just a bit and fatten up. At higher settings (like 2-3), though, you're likely getting clipping in one of the pre-amp stages...clipping that you don't like the sound of.

I think that the biggest offender is that you lifted the vibrato intensity pot. On the DRRI, this is a 50K pot, whose presence (even when the vibrato is off) provides a 50K path to ground. That's a little hard for the previous tube to support, so the signal voltage drops off a bit (ie, you've reduced the gain). By lifting that path to ground, you've made it easier for the tube to keep the signal level high (ie, you've increased the gain). Since the amp was designed around the drooping voltage by having the 50K in place, the higher signal level caused by removing that 50K pot increases the likelihood of overdriving the following stage. You've got to switch it back to stock.

However, If you like the no-pot sound at low volumes but don't like the break-up seen at high volumes, you could put the pot on a switch. Switch out the pot at low volumes for "fat" and switch in the pot at high volumes for "clean". I'd recommend leaving the pot in and using other methods to fatten the sound.

For a fatter sound, my biggest recommendation for the DRRI is to put a switch on the "bright cap" that is wired into the vibrato channel on the DRRI. The "bright cap" is the 47pF cap jumping around the volume pot. Put it on a switch and then you can toggle between the standard blackface Fender sound heard in non-DRRI Fender amps (round and scooped) or switch to the stock DRRI sound (bright...really bright).

Good luck.

Chip

[chipaudette]

<Message Deleted Due to Idiocy>

Chip

[stratomaster]

I think that the biggest offender is that you lifted the vibrato intensity pot. On the DRRI, this is a 50K pot, whose presence (even when the vibrato is off) provides a 50K path to ground. That's a little hard for the previous tube to support, so the signal voltage drops off a bit (ie, you've reduced the gain). By lifting that path to ground, you've made it easier for the tube to keep the signal level high (ie, you've increased the gain). Since the amp was designed around the drooping voltage by having the 50K in place, the higher signal level caused by removing that 50K pot increases the likelihood of overdriving the following stage. You've got to switch it back to stock.

If this is true, then the normal channel on a stock amp should be breaking up at around 2.5 on the volume knob. I don't recall that being the case on my or any other DRRI I've used. This also seems to have come on all of a sudden.

For a fatter sound, my biggest recommendation for the DRRI is to put a switch on the "bright cap" that is wired into the vibrato channel on the DRRI. The "bright cap" is the 47pF cap jumping around the volume pot. Put it on a switch and then you can toggle between the standard blackface Fender sound heard in non-DRRI Fender amps (round and scooped) or switch to the stock DRRI sound (bright...really bright).

I already have used the dummy ground switch to make the vibrato intensity pot switchable. It happens even with the switch set to stock, but just not as much. The break-up is still there at around 2.5 to 3 on the volume knob. Also, my "bright cap" is now at 10pF already. The 47pF stock value was a bit too much for my maple neck strats and low capacitance cables.

[MWJB]

I'd check the PI plate resistors, what voltage do you have at pins 1 & 6 of V6?

Blown screen grid resistor?

A voltage check might highlight an issue.

[chipaudette]

You didn't say that "This also seems to have come on all of a sudden.". That changes everything. So, with your mods in place, the amp sounded fine...then at some point later it sounded different? That definately implies that something went south and that it's not a circuit design / configuration / modification problem.

Could you describe the circumstances when the amp changed tone?

Chip

[stratomaster]

You didn't say that "This also seems to have come on all of a sudden.". That changes everything. So, with your mods in place, the amp sounded fine...then at some point later it sounded different? That definately implies that something went south and that it's not a circuit design / configuration / modification problem.

Could you describe the circumstances when the amp changed tone?

Chip

What I should have said was, I hadn't noticed it before recently. I never really turn my amps up past 2 or 3, so when I did, it was quite a surprise to hear how muddy they got. I especially noticed that my OD pedals sounded bad through it. It was not like that before (from the few times I had turned it up before the mods). The low volume cleans never really sounded bad (they were improved by the mods), and that's mostly how I use the amp.

I have tried various cables and different power sources (at gigs, jams, or get togethers) with no change in performance. Changing V2 to 5751s or 12AY7s helped some, but not all that much. I have the mods switchable, and turning them off helped some, but not much either.

When I get a chance I will take those voltage measurements off the PI. Any rough values on what I should be expecting to see if the amp is running normally? Thanks.

-Omar

[ajeffcote]

Two amps involved with the same mods?Unlikely to have a bad resistor on both amps causing the same problems.I'll bet there is something else going on.
When you get this straightened out,you can really fatten up the tone with a better output transformer and speaker.I also have a DRRI.Now modded to the point of no return.

[txstrat]

The second halfs of the Normal and Vibrato channels tubes share one cathode resistor. The value is 820 ohms. In case you pull V1 the remaining V2 will have it's second triode running with half the cathode resistors value than before. This causes much more gain.
This is part of the Cesar Diaz mod of the Stevie Ray Vaughan Vibroverb amp made by Fender.

[chipaudette]

Hmmm...very interesting point. I hadn't considered all the implications of the "pulled V1" mod. This is a really good one.

I might have phrased its effect a little differently, though. With only half the value of the normal cathode resistor but with only one tube's quiescent current draw, the DC voltage at the cathode wll only be half the normal Fender value. This means that the bias point (the voltage of the grid relative to the voltages at the cathode and anode) of that gain stage is going to be very different.

If the traditional Fender cathode value (1500K) is assumed to result in the correct bias point (ie, right in the middle of the linear performance range), then any other bias point (such as with the pulled V1) will result in an off-center bias. At low volumes, the off-center bias won't cause any clipping because there's enough space for your small signal to wiggle around unimpeded. But, at higher volume settings, one side of the waveform is going to run out of headroom due to the off-center bias. It'll compress and then clip. Viola, early distortion.

My only quibble with the previous explanation is that, since the cathode resistor has a cathode bypass cap, the actual "gain" (the boost in signal voltage) should be the same with either the 820R or a standard 1500R. It is by the shifting of the bias that the 820R could see premature distortion.

Looking at the schematic again, it is possible that the pulled V1 does cause an increase in gain (increase in signal voltage)...but the increase in gain is not via the effect on the cathode voltage. Instead, the missing V1 will cause less quiescent current to be drawn through the power supply (ie, the part that provides high voltages to the tubes). Less current draw means less droop in the voltage being provided to the tube anodes (plates). So, by pulling V1, the plate voltage on V2 in the DRRI might go up from 260V to 280+V. Since, gain is related to the DC voltage difference across the tube, increasing the plate voltage will increase the gain a little. The other tubes will also see a voltage increase, but those at the end of the line (V2 and V4) should see the biggest change. Still a ~20V increase isn't much relative to 260V, so I expect that the increase in gain (and the resulting potential for overdriving/distorting later stages) to be small.

So, we've got two things (at least) going on when pulling V1 -- (1) shifting bias causing pre-mature distortion and (2) missing current draw causing an increase in the voltage to the other tubes. Cool! Awesome catch, txstrat, on thinkng about the implications of the pulled V1. I wonder if either effect is the cause of the disortion sound that he's hearing?

Chip

[MWJB]

The voltage on V2 pin 1 may rise slightly with V1 pulled (+/- 20vdc is an audible difference even at 260v) but at the same time voltage at pin 6 will DROP due to the change in bias point with a cathode resistor value of 820ohms at pin 8. It's really the change of the cathode resistor that has the biggest effect...as you can judge for yourself by splitting the cathodes at V1 & V2 pin and giving each tube it's own cathode resistor, 1x 1500 & 1x820.

You don't have to remove any tubes with regard to the Diaz/Vibroverb mod and you still get the change in tone.

[stratomaster]

Awesome!

I think you guys may have hit it with the V1 pulled tube causing that nasty early distortion. I've got a practice session tomorrow where I can turn up some. We'll hear how it reacts with a 12AX7 back in the V1 position.

If it acts up again with the tube in and the intensity pot switched back into the circuit, then I'll open it up and get some voltages.

Thanks again!

[stratomaster]

No such luck. Still crapping out at low volume levels on the bass notes. I'll get some voltage readings as soon as I can. Hope to fix this soon!

[stratomaster]

I'd check the PI plate resistors, what voltage do you have at pins 1 & 6 of V6?

Blown screen grid resistor?

A voltage check might highlight an issue.

For DRRI

Stock setting (Vibrato pot and V1 in)
V6:
Pin 1: 178.2
Pin 6: 183.3

Mod setting (Vibrato pot and V1 out)
V6:
Pin 1: 186.6
Pin 6: 190.7

I haven't opened the SRRI up yet, but these are the voltages I'm getting. The nastiness happens mostly when I play low notes. They just fart out and distort. There should be at least *some* clean headroom at volumes higher than 2.