Help with tweaking Fender 68 Custom Deluxe Reverb Reissue

[Urumiko]

First off a big thanks to Pedro Vecino and others for this thread.
I took a punt on a brand new Fender 68 Custom Deluxe Reverb Reissue that was being sold as not working for £499 based on the fact it was making terrible noise with nothing plugged in.

After replacing the modelling circuitry in my amp with a basic optocoupler its gone from sounding like a bag of snakes on the radio to air traffic control to lovely sounding amp, all be it with a fairly high noise floor.
I should point out I left the grid stopper resistors in place after performing this mod as i figured they were generally a good thing.

My amp seems free from any obvious 50 cycle hum, and any noise still there sounds like stable thermal/white noise.

I'm now looking towards basic mods and further noise reduction and noticed Paul Rivera did some good mods.

The first thing I would like to do and would like to ask for help on is simply replacing the volume pots.
My understanding is that the stock parts use a J taper, I understand if I were to replace them with a more traditional audio taper it might lead to a subtly more gentle ramp up in volume at low levels.
I note that the stock part is PCB mounted and has some kind of integral buttress design? Part no:0037596000 CONTROL SNAPIN 1M 30A TAPER
Do audio taper pots of this type exist or do i just need to go with a generic through hole mount.
I thought tacking a small resistor in parallel might be another way to go but wasn't sure what a sensible value would be?

Looking past that I'd like to look to what would be the most effective mods to reduce hiss?
I made the assumption most poor wiring issues etc would cause hum as opposed to hiss?
I was looking at increasing negative feedback, and or introducing it within the preamp. and possibly a master volume mod, or simply swapping the preamp tubes for 12AT7

but I'm eager to take advice as to where my efforts are best placed to reduce hiss in particular.

Any advice greatly appreciated!

[Mick Bailey]

I've had quite a few of these in with noise issues. Sometimes the reverb circuit can be noisy and hiss - does turning the reverb to zero cut the noise? What about the controls - what makes the most difference? Getting a clear indication of where the noise is coming from will help in resolving it. the main fix has been with tube swaps and reducing the output tube current draw which can reduce noise dramatically and also prolong tube life. The 12AT7 to my ears isn't the best preamp tube and I prefer a 5751 to reduce the gain. I've also used this tube to replace V4 where the reverb has a lot of hiss. Make sure though you don't have a noisy tube that's the problem.

If you do change the pots, you're better sticking with the Fender snap-in type as they're far more robust and they are grounded off the buttress in many Fender amps.

[vintagekiki]

To reduce hiss only quality new (not as new) preamp tubes.

https://www.mesaboogie.com/tubes/pre-amp-tubes/SPAX7.html

[g1]

I've had quite a few of these in with noise issues.

I've seen a lot of complaints about them being more hissy and noisy than the standard BF type re-issue, and the reply is often that this is normal.
Have you found them to be as quiet as the non-custom circuits when in good repair?

[Mick Bailey]

As they come from the factory, they're have more noise than the BF reissues that I've had - these usually come here for tremolo ticking or just a tube replacement. I get brand-new '68 reissues that are under warranty for remedial work. when I say brand new I mean they come to me within a week or so of purchase. One was for use by a guitarist in a West End show. She turned on her brand-new amp at sound check and the hiss and hum through the PA was way too loud and unusable. In common with others I've worked on, the reverb tray was installed 180 degrees out, the reverb cables cheap and noisy, preamp tube hiss, reverb recovery stage too noisy.

All the '68 reissues sold here have the piggyback trem circuit board and loads of additional wiring that picks up noise. They also have the 10k grid stoppers on each preamp tube. Tube quality from the factory seems to be very poor in the amps I've worked on and auditioning preamp tubes is needed to give the best results. I did one late last year where it was sold cheap because of the noise. The new owner bought it to me and I removed the SS trem module and all the associated wiring, installed an LED/CDs 'roach', rotated the reverb tray, changed the reverb cables, reduced the 6V6 idle current and changed the preamp tubes. It's now so quiet all you hear at gig volume (he mics up) is the normal mechanical transformer hum. The grid stoppers are all left in place. The reverb is lush and the trem is superb, with no signal bleed. He's really happy with it.

[pdf64]

That’s interesting, when this topic cropped up a while ago, I recall that the grid stoppers got the blame for the hiss.

[Urumiko]

Thanks all for your responses. Here is some more info.

what makes the most difference? Getting a clear indication of where the noise is coming from will help in resolving it. the main fix has been with tube swaps and reducing the output tube current draw which can reduce noise dramatically and also prolong tube life. The 12AT7 to my ears isn't the best preamp tube and I prefer a 5751 to reduce the gain. I've also used this tube to replace V4 where the reverb has a lot of hiss. Make sure though you don't have a noisy tube that's the problem.

So prior to any mods I tried the amp with just the power tubes and phase inverter in place and it was whisper quiet if the vibrato intensity was turned to 0, but turning the VIB up introduced the main noise. Introducing V5 also introduced lots of noise. I tried swapping v6 with the valve from V3 but I had no spares. I tried swapping v5 with a good mallard I had and that didn't make any difference.
I'm not sure if this test proved the power section to be quiet or if id have to apply a test signal to he grid of the phase inverter to know that?

Since the mods and with all tubes back in place, there is a base line hiss with all controls at 0, reverb does introduce a little 50cycle but it is not severe, either preamp volume with nothing plugged in does cause more hiss but not loads, possibly double what it is with everything on zero.

This is my first amp with a tube vibe, but I notice its not disengaging fully on minimum setting.

I do have an oscilloscope but I'm not proficient with it. I was considering trying to use it to try and trace the static noise in the circuit?

If you do change the pots, you're better sticking with the Fender snap-in type as they're far more robust and they are grounded off the buttress in many Fender amps.

I've noticed the 2 volume pots and the vibe intensity pot all seem a bit noisy, the vibe pot seems to click as you move it off zero.
Id be happy to stick with the fender pots but I'm guessing they don't come with a proper audio taper? would adding a resistor in parallel help ease the taper?

I'd be equally happy removing the pot PCB and hand wiring pots in instead if there's not much else on that PCB.

That’s interesting, when this topic cropped up a while ago, I recall that the grid stoppers got the blame for the hiss.

yes I have heard similar, after doing a bit of reading, (designing tube preamps for guitar and bass - Merlin Blencowe, is my go to reference)I realise in path resistors, especially carbon based can add thermal noise. I was considering removing all but the ones at the input and replacing them with 10k metal film.

Increasing negative feedback has been said to help too.



So my current " I wonder if that would help" list.

• increase negative feedback
• check biasing on power tubes
• Quality 12at7 instead of 12ax7 in preamp
• Removing grid stoppers /replacing key resistors with metal film/wire wound

[Helmholtz]

Merlin Blencowe, is my go to reference)I realise in path resistors, especially carbon based can add thermal noise. I was considering removing all but the ones at the input and replacing them with 10k metal film.

You misread.
Thermal noise is the same for all resistor types of same value.
CC resistors can add considerable current distribution noise - but only if they carry current. Grid stoppers don't, so resistor type doesn't matter.
Even the contact noise sometimes found with poor quality resistor requires a current to develop.

[Urumiko]

You misread.
Thermal noise is the same for all resistor types of same value.
CC resistors can add considerable current distribution noise - but only if they carry current. Grid stoppers don't, so resistor type doesn't matter.
Even the contact noise sometimes found with poor quality resistor requires a current to develop.

oh. thanks. Yes come to think of it I think the example discussed referred to the anode resistor. Do you think it's worth upgrading the Anode/Cathode resistors in this context?

[Helmholtz]

oh. thanks. Yes come to think of it I think the example discussed referred to the anode resistor. Do you think it's worth upgrading the Anode/Cathode resistors in this context?

It may make sense to change anode resistors to metal film. But I wouldn't expect too much. Generally the difference between good quality carbon film and metal film resistors isn't large.
As cathode resistors are typically lower in value, their influence is smaller.
Generally resistors in the input section contribute more to noise.

But all this theory only applies to flawless/good quality resistors.

[Mick Bailey]

That’s interesting, when this topic cropped up a while ago, I recall that the grid stoppers got the blame for the hiss.

That's what I read a while back. I did remove them in the first amp I got in with them fitted as part of an overall approach to combating noise, but never noticed any difference. Since then I've left them in place because I've been able to get the overall noise down to an acceptable level without removing them. Maybe the experience of other people is different to my own.

In reply to the OP, I would firstly eliminate any tube issues. It appears that your power section is largely noise free and perhaps not an issue, but it's my preference to reduce the idle current as much as possible anyhow for reliability. Maybe change the NFB resistor to 820R and see if that helps. Again, a 12AT7 doesn't sound as good (to me) as a 5751 for lowering the gain. Your pots may just need some Deoxit.

[Urumiko]

Thanks for the advice.

I intend to take another look and experiment with some of the things suggested. I've uploaded a video to YouTube with my experience so far if it helps.

[DrGonz78]

Working the graveyard shift tonight and came across the video. Nice job!

[Mick Bailey]

Yes, nicely produced and clearly shows the extensive cable run to and from the satellite board. Aside from removing it, I chopstick the remaining leads to get the best dress as there is little care taken with how they come from the factory. Your amp is very noisy and I'm inclined to think you either have a noisy tube or some other component issue that needs to be resolved - does removing V1 and/or V2 kill the noise?

Changing the gain by using an alternative preamp tube type usually won't make much a great deal of difference with noise because with a lower gain preamp tube you end up turning the volume up more to get to the same volume level. It's just that with the volume control set the same as it was before it will given the impression of being quieter. You could experimentally bridge the grid stoppers to see if there's any change, but first try to pinpoint where the noise is being generated.

[Urumiko]

DrGonz78[/B];n925566]Working the graveyard shift tonight and came across the video. Nice job!

Cheers buddy. I don't really know what I'm doing but hopefully it helps a few people and points them in the right direction faster than I found answers.

Your amp is very noisy and I'm inclined to think you either have a noisy tube or some other component issue that needs to be resolved - does removing V1 and/or V2 kill the noise?

I don't recall all configurations but i will check on the next outing. i seem to recall 4 5 & 6 was noisy with no preamp tubes. but there is a sense its everywhere.

Im going to check voltages, remove stoppers, swap tubes, not nescesarily in that order, and if it comes to it replace the anode resistors with lower noise ones.