Noise problems - Silverface Deluxe Reverb

[David Vinson]

Hi there,

I'm new to this forum--the experience here looks impressive! Could anyone please make suggestions for the following questions/issues? My amp is a Fender Silverface Deluxe Reverb ~1977 to 79. It has a mix of tubes - The rectifier, 6V6s and some of the 12's are stamped original Fender, one of the 12's is Electro-harmonix, and one named "Made in China".

The amp has a couple of noise issues:
A) There's a constant "ticking" or "popping" noise; it comes on as soon as the amp warms up. It happens ~4 times a second, and seems to get a little louder over time (or maybe I'm just turning the amp up ;-). There's no apparent line noise on the 120VAC power from my house.

B) The amp just doesn't have that Fender "sparkle" - tone seems a little muffled, or muddy; just not the high-end/chime I'm expecting.

C) There is some constant (slightly annoying) pink noise along with the ticking.

D) Is a little bit of 60-cycle hum normal? It's not so loud to be distracting, but a low background noise when idling. (If it had a "line-out" it would be unusable).

E) one of the 6V6 power tubes is glowing brighter than the other - the metal disc inside the tube (near the top) is glowing, almost red.


Does this mean the power tubes need to be biased? is there a difference between biasing and balancing? Could any of these symptoms be related? Is there a step-by-step online (or book) toot on how to find/replace the balance pot with a bias pot? tapping the tubes w/ a pencil doesn't create any change (bad or good) to the noise.

I'm a newbie to these type of tube amps, but know which end of a soldering iron to hold, to use one hand when poking around plate voltages, and the importance of discharging filter caps.

Thanks very much for any advice!

[Jag]

As many others would probably say, read RG Keen's Tube Amplifier Debugging Page.

Regarding A and C it sounds like noisy preamp plate resistors to me but it could also worn tubes. It could also be due to dirty tube sockets, control pots, jacks or some other worn component. The ticking could also be coming from the vibrato circuit. Does the speed of the ticking change with the speed setting of the vibrato?

Regarding B, it could be due to worn out tubes. Lack of sparkle is also one of the issues sited with the Silverface amps and is the main reason for having them "Blackfaced". Those 1200pf caps going to ground from pin 5 of the output tubes are real tone suckers.

Regarding D, a little hum is normal but I bet your amp could benefit from a cap job. This involves replacing all of the electrolytic caps in the amp. This will likely reduce the hum.

The metal disc should not be glowing. There's probably a bias issue there but you probably could use a new set of tubes as well.

Yes there's a difference between bias balance and adjustable fixed bias. Your amp has a bias balance control. The bias balance merely supplies a fixed negative voltage to the tubes through a pot which is adjusted one way or the other to balance the current draw between each power tube. An adjustable fixed bias supply allows adjustment of the negative supply to allow precise setting of the current draw of the two tubes. The adjustable fixed bias generally does not allow for the balancing of the draw between the tubes, although there are mods which allow adjustment of both the balance and level. It would be preferable to have the bias balance at least converted to an adjustable fixed bias supply.

A google search (or search of this site) for "Fender Blackfacing" and "tube amp maintenance" will give you some things to go on regarding these issues.

[David Vinson]

Yup, that's part of the problems...

Thanks Mucho, Jag

The ticking tracks the vibrato rate. Can I pull that tube (V5) and still run the amp?

For the pink noise, the only tube common to both channels is the phase inverter. It's a 12AT7, so I'll start swapping some tubes.

The link was useful for starters, thanks. Anyone know some resources on *how* to convert the bias balance to an adjustable fixed bias?


DV

[Jag]

My mistake! Your amp is already fixed bias. I've been helping afriend Blackface his Super Reveb and I've got that circuit on the brain. Sorry!

[David Vinson]

You may be right, actually

I think the blackface are fixed Bias, and the CBS (1970s?) use the pot for balancing the bias. I'll find the reference in a book I have lying around here somewhere...

Any online info sources on step-by-step for modding the pot to fixed bias?

Thx

[Jag]

No, I was wrong in my first post. If your amp is stock to the schematic, it will already be fixed bias.

It is true that one of the major differences between most Blackface and Silverface Fender amps is the bias circuit but, the SF Deluxe Reverb is an exception. At least according the the schematics I have seen and my own '79.

You can run the amp with V5 pulled but, how loud is the ticking? The ticking could be due to poor lead dress and there are a couple of ways to deal with it. Here's one way.

As for the "pink noise", swapping tubes will probably not resolve that. Pulling the phase inverter just shuts off everyting before the power amp. While this does give you evidence that the noise is coming from the preamp, it does not neccesarily suggest that the tube is the problem. I'd be more inclined to suspect the 100k resistors connected to pins 1 and 6 of V1 and 2 and pin6 of V4. Before even doing that though make sure the pins of the tubes and the tube socket contacts are clean as well as the pots.

As I said, your amp should already be fixed bias. If you want to see what the difference is though download the AB763 and AA270 versions of the Super Reverb schematics from here. The fixed bias circuit, on the AB763 SR, is everything from the 10K-L pot back to the power transformer and forward to the 2 220K resistors on the circuit card. Compare that to the same area on the AA270 schematic and you'll see the differences. The adjustable fixed bias sends a fixed negative voltage to the pot which can then be adjusted up or down to change the voltage sent to the grids of both tubes simultaneously. The bias balance takes the fixed negative voltage sent to the pot and balances the voltage sent to the grids of the output tubes but does not allow the voltage to be adjusted up or down to affect the operating point of the tubes. Remember, your amp is already fixed bias.

The only real changes I made to my DR, other than the recap and other general maintenance I've already mentioned, was to change the resistors on the Phase Inverter to AB763 specs, remove the capacitors going to ground from pin 5 of each output tube and replace the 2.2k resistor between the screen supply and preamp supply to the AB763 value.

[Jag]

Even more important than all of the above, you need to resolve that red tube before doing anything else.

You've already said you know how to stay safe but I'll still say, "be very careful, you're going to be working around high DC voltages".

Start by taking some DC voltage readings with all the tubes out except the rectifier. What voltages are you reading, to ground, on pins 3, 5 and 6 of the output tubes? What about the AC voltage between the two connections on the pilot lamp?

I'm thinking you probably just need new tubes and/or a rebias but, it would be better to check these voltages first. In other words, don't do anything else until you check those voltages and post them back here.

[David Vinson]

OK, here's the voltages...

Jag, thanks – these are all GREAT suggestions, and are addressing the questions that keep coming up as I dig deeper into this

Here’s the voltages, taken w/ only the rectifier tube inserted:

6V6’s

Pin……..To..……...V8……….V7

3……….Plate……+493…… +490

5……… Grid 1…..-44.1… -44.2

6……… Grid 2…..+489... +490

BTW, it’s V8 that appears to glow more red than V7.

Heater (Was read across the pilot lamp): 6.7 VAC

[David Vinson]

Correction -it's tube, not tube position

When I swap the glowing tube in socket V8 to socket V7, it's now V7 that glows...Looks like I need some new tubes.

I also now see how to perform a bias: I see the points where I can solder in 1watt, 1-ohm resistors between the cathode & ground. But a Bias Plug seems to me a better solution. Any recommendations for where to get inexpensive versions?

um, will I need two? (I need to measure voltage drops from both 6V6's per each tweak of the pot, right?)

Thanks for your patience,

DV

[MWJB]

Hi David,

The plate (pin 3) voltages are not really any help without the 6V6 tubes loading down the power supply. Pin 6 is just a mounting point for the 470ohm screen grid resistor, it's pin 4 at the other end of this resistor that is the reading you want.

V8 might glow more that V7 but that doesn't mean that V7 is particularly happy, likely both are getting hammered. Once you've converted the bias pot to bias adjust, get some bias probes then you can bias with the probes & a screwdriver without having to keep sliding out the chassis, if you use current production, higher voltage handling 6V6s. With NOS types you should really try to keep plate voltage below 425v.

[Jag]

My next suggestion was going to be to take the same measurements with the tubes in. I wanted to be sure he was getting somewhere in the neighborhood of -45v on the grids before re-installing the tubes in case he had a problem with the bias supply or a tranny short, etc. Those voltages will, of course drop with the tubes in. I'm thinking the plate voltages might still be a little on the high side with the tubes in. As MWJB said, NOS tubes won't be very happy if the plate voltage is too high.

You're quite right MWJB, it's pin 4 we want to measure. My mistake, typing too fast.

A bias probe works fine, I find it easier to just have the resistors installed at the cathodes YMMV.

Once again, that amp should already be fixed bias.

[MWJB]

Sorry Jag, good plan, I just skipped through the thread and missed your earlier post.

[Jag]

No problem. I probably should have posted the next step, as well, when I asked him to do the tubes out measurements.

BTW David, while you have the tubes out, check the value of the resistors across pins 4 - 6 on each output tube with your meter. If one or both are open, shorted or have drifted far off spec, they'll need to be replaced. I'd probably replace them anyway.


Those voltages look okay although, as said before, the plate voltages might still be a little high with the tubes in. JJ 6V6s are pretty decent tubes and can handle higher plate voltages than NOS 6V6s. I'd suggest you try a pair of these or another modern 6V6 that is spec'd for higher plate voltages. Your original tubes are already suspect due to that red plate.

[David Vinson]

B+ voltages

The resistors from pins 4 to 6 are marked 470 ohm, and measure 505 ohm each. Carbon comp, fatter than 1/4 watt (one watt on the schematic). DC Voltages w/ 6V6's in sockets are:

Pin……..To..……...V8……….V7

4……….grid 2……+467……+467

6……… B+....…..+469... +460

The schematic and layout drawings call for the B+ at pins 6 to be +415vdc.

Should I increase the 470 ohm resistors to drop the +469 to 415? or ~425?
Trial-and-error to find a sweet spot?

And if so, what value should I use? Or, how do I tell what current is present to calculate the resistance to use?

Thanks for all the help, this is awesome!

DV

[Jag]

505 is close enough but I would still replace them with new 470 2w flameproof resistors.

What we need to know now are the voltages at pins 3, 4 and 5 on both output tubes with the tubes in.

Increasing the value of the screen resistors will not do alot to drop the voltage at pin 4. There are other ways to deal with that. For now, get a new matched pair of JJ's, they'll be able to handle the extra voltage.

You could also drop the voltages by a bit overall by using a 5Y3 rectifier in place of the 5U4GB. You'll still be over 425 at the plates though.