1996 Ampeg Reverberocket r12r Hum Problem

[RiffRaff]

This is a real puzzler and I hope someone can give me some help.

I have a 1996 Ampeg Reverberocket r12r reissue combo amp. The amp is playable and generally sounds good. However, I have a strange hum issue. It has a fairly loud hum (120 hz?) on the clean channel, but is quieter on the drive channel. The hum's not too intrusive when playing at full volume, but is quite annoying when playing at practice volume.

The amp schematic is #154 Ampeg_r_12r_212r_50h_reverbrocket_ri in EL34 World's amp schematic library.

On the clean channel, the volume control does not affect the hum level. When the volume control gets to 6 some white noise starts to come in, but the hum level doesn't seem to change.

On the drive channel, the amp is dead quiet with the gain control at zero and the master volume control at 3. As expected, hum and white noise increase at the gain control is turned up.

The drive channel hum level starts to equal the clean channel hum level when the drive channel gain control reaches about 3 (master volume control still at 3)

So, I've concluded that the power amp section is fine, and the preamp section is good starting from the gain control input to the second stage amplifier (second half of the first 12ax7;) at least going through the drive channel's signal path.

I must be getting some hum from the first stage amplifier (first half of first 12ax7,) because I start to hear it in the drive channel when the gain control gets to 3. However, if the hum is only in the first stage amplifier, it should be affected by the volume control on the clean channel, and it's not.

I've switched the first 12ax7 with one I know is good, but that did not reduce the hum.

I checked the operation of the clean/drive channel switching relays and the switching circuitry seems to be working properly except for one very puzzling thing: Based on the circuit schematic, a 3.3 M resistor (R62) at the top of the tone control stack should be shorted on the clean channel and not shorted on the drive channel. However, the amp actually operates in the opposite fashion, the 3.3 M resistor is shorted on the drive channel and not shorted on the clean channel. At this point I'm assuming that the schematic is incorrect. If not, it would be a gross design and manufacturing error. However, I don't know enough to understand what this resistor is doing or why it's shorted/not shorted based on the channel selection.

Regarding the hum coming from the first stage amplifier, I'm suspicious or a 0.1 micro farad 50 V coupling capacitor between signal ground and chassis ground. If this capacitor is bad, would that induce power supply noise into the circuit?

Any help would be greatly appreciated!

[PRR]

https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_r_12r_212r_50h_reverbrocket_ri.pdf

It's not about the 3Meg resistor. The relay switches big C40 in parallel to little C3 to massively boost the treble knob range.

The rest of it..... I've pondered long enough. It makes my head hurt. There are no "channels", just one signal path but with gain-loss taken at different points. Clever but not brain-friendly. I sketched the connections for part of it and gave up.

Where would hum come from? If it is buzz, and the amp is 20 years old, and low-price, the B+ filter caps may be past their best-freshness date.

[RiffRaff]

Thanks PRR,

The clean/drive channel switching makes my head hurt too. Thanks for your help sketching the differences between the two channels.

The hum is a buzz (60 hz with 120 hz overtones?.) I can see it on a scope and it definitely looks like power supply noise.

I think I will go ahead and replace all of the B+ power supply filtering capacitors. This won't hurt anything and hopefully will reduce the hum. Do you think it would do any good to increase the filter capacitor values? (I'm pretty reluctant to alter the amp from stock, but if it would help ...)

You are correct about C40. The amp is much brighter on the drive channel - too bright for my taste.

Here's a couple of things that still puzzle me:
- Why does the gain control affect hum on the drive channel, but the volume control does not affect hum on the clean channel. Note that the amp is dead quiet on the drive channel when the gain control is zero.
- What's the purpose of the 220pf capacitor (C6) connected between the high side of the volume control pot to it's wiper? Also, why include the .022 microf capacitor (C7) between the volume control wiper and the grid of V2a? I don't think I've ever seen anything like this before.

Again, thanks for your help.

RiffRaff

[PRR]

> What's the purpose of the 220pf capacitor (C6) connected between the high side of the volume control pot to it's wiper? Also, why include the .022 microf capacitor (C7) between the volume control wiper and the grid of V2a? I don't think I've ever seen anything like this before.

The 220p across the top of the volume pot semi-compensates for treble loss when pot is turned down.

Cap-coupling into the distortion stage changes the distortion type.

Both are very common in guitar amps.

[RiffRaff]

Thanks PRR. I'm going to move ahead replacing the B+ capacitors. I'll report back results in a week or so. RiffRaff

[RiffRaff]

PRR,

I will order new B+ capacitors for my Ampeg this week. I need to pull up the circuit board so I can see the pin style and spacing before ordering them.

In the meantime, I decided to collect some baseline data to compare with the hum level after replacing the B+ capacitors.

Since the amp is dead quiet on the drive channel with the gain control at zero, I concluded that most of the hum is generated in the first preamp stage, V1a. Is this a correct conclusion?

With the inputs shorted, there's about 15mv peak-to-peak noise on the output of V1a, measured at the junction of C1 and R4.

With an input of 200mv peak-to-peak @1000 Hz, the output of V1a is about 10V peak-to-peak.

With the inputs shorted and:
Gain = 3
Volume = 3
Treble, Mid and Bass all =5
Master =3
Reverb =0
(about normal playing settings, except for reverb)

Noise level on the speaker output is:

Clean channel = about 40mv peak-to-peak (Looks like white noise. The hum is audible, but buried in with some high frequency noise.)
Driver channel = about 20mv peak-to-peak (Ditto. The hum is audible, but buried in with high frequencies.)

With 200mv @1000hz on the input

Clean channel = 6v peak-to-peak (Nice sine wave)
Drive Channel = 16v peak-to-peak (Plenty of distortion)

Thanks in advance for any comments you might have. I appreciate your help.
RiffRaff

[RiffRaff]

Here's an update on my Ampeg r12r.

I have the new B+ caps, but have not installed them yet.

I looked at the hum waveform with a scope, and it's definitely a 60 Hz signal somewhere between a sine wave and a saw tooth wave.

I did a little more research and learned that a 60 Hz "hum" indicates heater noise, while a 120 Hz "buzz" indicates power supply filtering noise. So, I'm focusing on heater noise right now.

I replaced all three 12ax7's with new tubes to eliminate any possibility that a bad tube is causing the hum. Note that I had previously replaced the two EL34 power tubes, and that the power amp section is dead quiet when I put a shorting plug into the line in jack. So, I'm pretty sure the power tubes aren't the hum source.

My next steps will be to install a humdinger pot or perhaps convert the heater supply to DC.

I'll post the results.

[RiffRaff]

Further Updates on my Ampeg r12r

I decided to install the new B+ caps as a preventive maintenance measure and to eliminate these caps as a factor in my amp's hum problem.

While I had the PC board out, I checked all the solder connections and re-flowed any that looked questionable. I also checked all of the ground connections to make sure I had a good ground.

I also installed a 100 ohm 5W linear wire-wound potentiometer in the chassis. This will be the humdinger pot if needed.

I put everything back together and powered up the amp. There's no change in the hum. At this point I think I've eliminated the B+ power supply caps, bad solder connections and bad grounds as the source of the hum.

My next step is to wire up the humdinger pot. I'll report back the results.

[RiffRaff]

I wired up the humdinger pot. Adjusting the pot significantly lowered the hum level. It did not remove all of the hum, but knocked it down to something tolerable - more on par with a typical tube amp.

Before installing the humdinger, I took a hum signal measurement at the output of V2b. The hum signal was 7-8 mv peak to peak at 60 HZ. Tomorrow I'll see what the hum level is post humdinger and report back.

[RiffRaff]

After installing and adjusting the humdinger, the hum signal at the output of V2b is 2 mv p-p. This is a -11 to -12 db reduction in the hum signal.

I checked the bias the new 6l6's. Plate dissipation on V4 is 18 w. Plate dissipation on V5 is 18.5 W. This is around 60% of the 30 w maximum plate dissipation - right in the middle of the average bias range.

There seems to be some mechanical resonance in the cabinet that's shaking the chassis and causing some slight tube rattling noise. I'm not sure how much is the tubes and how much is just cabinet rattle. Resonance occurs at 130 hz, 170 hz, 200 hz and 250 hz. Unfortunately, this causes some buzzing in a few low notes on the guitar fretboard. I'm going to look into this a bit more.

[Williamblake]

ThAnks for sharing on cutting down on the hum and what about elevating the heater center tap to 30 or 40V?

[RiffRaff]

WilliamBlake

Thanks for your interest in my Ampeg Reverberocket r12r reissue. Do you own one? If so, I'd be interested in what your experience has been.

Based on what I've read, elevating the heater voltage reference should reduce the hum further. It would be fairly easy to elevate the voltage 15 V or so using the low voltage DC power supply. I'd need to install a voltage divider to be able to elevate the heater voltage by 30 or 40 V, which seems to be the recommended value. Installing the voltage divider could be a little tricky since the Reverberocket reissue is a PC board based amp.

I wonder if lowering the heater voltage reference would work as well. This would be fairly easy to do using the negative bias supply voltage.

[RiffRaff]

Here's where things stand with my Reverberocket reissue.

I've lowered the 60 HZ hum level, but this unmasked some higher frequency noise that seems to randomly raise and lower in volume. Overall, the noise level is still pretty high, higher than a well-maintained vintage Fender. I wonder if my amp's noise level is typical for this amp. Has anyone else had experience with these Reberberocket reissues?

I'm also concerned about the mechanical resonance at low frequencies as I mentioned before. I'm a harmonica player, and the lowest note we typically play in a blues setting is G3, which is about 200 HZ; higher than most of the amp's resonance frequencies. I think the resonance would be objectionable to most guitar players however.

I'm reluctantly putting this project aside for a while. I think it's probably as good as I can get it. Any more ideas anyone?

PS - I'm going to experiment with attaching a noise gate between the line out and line in jacks. I'm hoping this will block the amp's noise without affecting my harp sound. The noise gate will probably cut down on feedback too.

[Willabe]

I'd need to install a voltage divider to be able to elevate the heater voltage by 30 or 40 V, which seems to be the recommended value.

Yes, that's often what people recommend/use.

A number of people here have tried 30dcv to 40dcv and said it didn't help with buzz. But it could have been other problems.

I think the 30dcv to 40dcv comes from amp company's that tied the heaters to the K dcv in K biased output stages. It was a freebie for them to do it, a little insurance, can't hurt kind of thing. 

Kevin O'Connor (London Power amps/books) has always said that's not enough and recommends 70dcv to 80dcv. 

At least 1 member here did a test on an amp he had built and it wasn't until he got up to 70dcv to 80dcv did the noise  -fully-  go away. 

[RiffRaff]

Thanks WilliamBlake. This Ampeg is currently my tinker amp, so I may try elevating the heater voltage reference at some point in the future.

I'll report back regarding my experience with the noise gate.