beatnik

Ampeg Reverberocket humming
« on: September 12, 2017, 05:48:58 PM »
I have just finished restoring an old 1962 Ampeg Reverberocket. This is one is version R-12-RA with 5Y3 rectifier and 7591 power tubes. I bought knowing it had a fried power transformer which I managed to get re wound by a local company (Transformer Equipment LTD in Kent, great people)

I have replaced the tubes and all electrolytic capacitors, the amp is working nicely and voltages seem to be in the ballpark, but I am  experiencing too much hum. The hum disappears if the preamp tubes are removed, but is present with the phase inverter in the circuit. I have tried three different ones so I would exclude a defective tube

I am thinking more of a grounding problem. The amp had been serviced before, so I am not entirely sure what the original ground arrangement was. From what I have found V1 power supply filter capacitor and cathode resistors were grounded at the input jack sleeve, while the rest of the circuit went to chassis at the filter can capacitor, and some of the potentiometers were grounded to their own shells

I have tried grounding the filter capacitor section at the input jack sleeve, but that made no improvement

I have read that these Ampegs aren't the quietest amps, but I would really like to improve the situation

Would you recommend to modify the grounding arrangement or rather look into something else ?


beatnik

Re: Ampeg Reverberocket humming
« Reply #1 on: September 12, 2017, 05:54:39 PM »
Since I couldn't find the actual schematic of this version, I had taken a picture of the drawing on the bak of the amp. The quality is not great, but should be of some help



Re: Ampeg Reverberocket humming
« Reply #2 on: September 12, 2017, 06:53:42 PM »
Have you tried grounding the V1 filter cap back to where the other caps meet ground ,leave the cathode cap ground close to the input jack . You might have some kind of ground loop issue with the pot shells going on , Id suggest trying a toothed washer between the pots and the inside of the chassis, then take the lower end of each pot back to close too the grounded end of the cathode resistor/cap of the appropriate valve , avoid loops at all costs, can be tricky to spot and undo a funky ground connection put in when someone replaced a pot , if you can spot a replacement pot different to the others it could be a good place to start looking

beatnik

Re: Ampeg Reverberocket humming
« Reply #3 on: September 12, 2017, 07:20:27 PM »
thanks for your reply. i've just had a closer look at the pots and found that also the volume control is grounded on its shell, and it seems the soldering is newer than the others... it seems very likely this had not been done at the factory.

i will try this and all the other tips you suggested and see if I can improve the situation

abbey road d enfer

Re: Ampeg Reverberocket humming
« Reply #4 on: September 13, 2017, 06:12:01 AM »
Have you tried grounding the V1 filter cap back to where the other caps meet ground ,leave the cathode cap ground close to the input jack . You might have some kind of ground loop issue with the pot shells going on , Id suggest trying a toothed washer between the pots and the inside of the chassis, then take the lower end of each pot back to close too the grounded end of the cathode resistor/cap of the appropriate valve , avoid loops at all costs, can be tricky to spot and undo a funky ground connection put in when someone replaced a pot , if you can spot a replacement pot different to the others it could be a good place to start looking
+1. Grounding the filter caps at the input is heretic. I see that done too often because of misunderstanding the star-ground concept.
Now, one must be very cautious with rewinding a 60Hz xfmr for 50 Hz. Many power xfmrs are dimensioned so that they don't tolerate the increased flux due to the change from 60 to 50. The only solution, short of changing for a larger core, is to dial in some power reduction.
« Last Edit: September 13, 2017, 06:15:42 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

beatnik

Re: Ampeg Reverberocket humming
« Reply #5 on: September 13, 2017, 07:52:08 AM »
luckily the transformer was already a 230V model, so that shouldn't be a problem

i am gonna try changing the ground point of the 10uF filter cap and will also clean up the surface beneath pots and add toothed washers if not present

beatnik

Re: Ampeg Reverberocket humming
« Reply #6 on: September 13, 2017, 10:25:37 AM »
grounding V1 filter cap  together with the other filter caps reduced the hum a little bit, but in my opinion is still too high, I would really like to improve it further

right now there are 5 different points where the circuit is grounded to chassis :

1) the section that is grounded to chassis at the input jack sleeve is shared between V1 first and second triodes - should i try only the first triode on this point ? (the second triode cathode is grounded at point 5 described below)

2) tone capacitor grounds on the potentiometer shell - this is definitely stock wiring, but maybe would be more correct moving this at the input ground ?

3) tremolo speed control grounds at the potentiometer shell

4) tremolo intensity control grounds at the potentiometer shell

5) the rest of the circuit ground signals are joined together and tied to chassis at the multi section filter capacitor negative terminal, this includes the h.t. and filament centre taps

do you think this layout could be improved ?

I forgot to mention that instead of 60uF for the first filter capacitor I have used a 20uF, but this was used on the previous revisions of this amp, with 6V6 power tubes, so I had assumed it would have been fine

abbey road d enfer

Re: Ampeg Reverberocket humming
« Reply #7 on: September 13, 2017, 10:56:40 AM »
1) the section that is grounded to chassis at the input jack sleeve is shared between V1 first and second triodes - should i try only the first triode on this point ? (the second triode cathode is grounded at point 5 described below)
I don't think it's a good idea for now.

Quote
2) tone capacitor grounds on the potentiometer shell - this is definitely stock wiring, but maybe would be more correct moving this at the input ground ?
Yes, more correct, but I'm not sure it would make much difference. Does the hum content changes significantly when moving the tone pot?

Quote
5) the rest of the circuit ground signals are joined together and tied to chassis at the multi section filter capacitor negative terminal, this includes the h.t. and filament centre taps

do you think this layout could be improved ?
I Would disconnect the filter caps from the chassis in order to have only one junction between "ground" and chassis (at the input since the jacks are not insulated IIRC).

Quote
I forgot to mention that instead of 60uF for the first filter capacitor I have used a 20uF, but this was used on the previous revisions of this amp, with 6V6 power tubes, so I had assumed it would have been fine
60uF is necessary with SS rectifiers.
BTW, have you disconnected the meat-puppet killer cap (the 0.05 uF between fuse and chassis) and installed a three-prong mains cord?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

beatnik

Re: Ampeg Reverberocket humming
« Reply #8 on: September 13, 2017, 11:18:52 AM »
Does the hum content changes significantly when moving the tone pot?

hum is not affected by the tone pot at all. it does react to the tremolo though

BTW, have you disconnected the meat-puppet killer cap (the 0.05 uF between fuse and chassis) and installed a three-prong mains cord?

yes, this had been done by the previous repair man

I Would disconnect the filter caps from the chassis in order to have only one junction between "ground" and chassis (at the input since the jacks are not insulated IIRC).

right now the portion at point 5) is not physically joined to part 1). they connect through the chassis. if I remove the ground bond at the filter capacitors basically all the circuit that is on section 5) is left without a reference to ground. Is this what you mean ?

abbey road d enfer

Re: Ampeg Reverberocket humming
« Reply #9 on: September 13, 2017, 11:55:17 AM »
right now the portion at point 5) is not physically joined to part 1). they connect through the chassis. if I remove the ground bond at the filter capacitors basically all the circuit that is on section 5) is left without a reference to ground. Is this what you mean ?
No, you need to connect it via a strong wire to the cathode resistors of the output stage.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.


walter

Re: Ampeg Reverberocket humming
« Reply #10 on: September 13, 2017, 04:01:48 PM »
Is the reverb disabled? the Dimension pot turned all the way down? Have you tried moving wires with a chopstick to determine if the hum is related to wiring? Have you checked the grounds with an ohm meter?
Blown like a fuse

beatnik

Re: Ampeg Reverberocket humming
« Reply #11 on: September 14, 2017, 07:39:17 AM »
I have modified the grounding scheme by keeping only the first triode cathode resistor and bypass capacitor at the input jack sleeve. the rest of the circuit grounds at the power supply filter capacitors

Almost all the low frequency component of the hum has disappeared and there is only a slight buzz in the background, which is totally acceptable. I am very happy with the improvement

Now I noticed another problem though. I am not sure wether this appeared after the modification or was present even before, because I had been paying attention just to the hum.

Basically even when the reverb is deactivated via the footswitch, there is still some slight reverb sound in the background. In order to completely cut off the reverb sound, the dimension pot (aka reverb level) needs to be turned down.

I am wondering what could be causing this, and if it could be somehow related to the changes in the grounding path ?




andyfromdenver

Re: Ampeg Reverberocket humming
« Reply #12 on: September 14, 2017, 09:53:14 AM »
don't have much to add, just interested that they would list 60mfd off a 5y3.
was gonna say, perhaps you could go SS and increase filtering. drop some B+ with a zener; or added pi filter.
also, if from a cold start, the instantaneous hum is too much, perhaps the rewind is talking to your OT more than you would like, or perhaps a rotation of the verb tank could help.  As an experiment, can you unbolt the OT and listen if any rotation hurts/heals?

In my little tracking studio, a deadly quiet verb and trem ~15W tube amp is part of my signature recording sound.

andy
2017 current fav movie: Slums of Beverly Hills
2017 current fav book: Foundation ( I. Asimov)
1997-2017 fav artist: Erik Satie

abbey road d enfer

Re: Ampeg Reverberocket humming
« Reply #13 on: September 14, 2017, 10:08:17 AM »
don't have much to add, just interested that they would list 60mfd off a 5y3.
60uF is for SS versions, 20uF for vacuum state.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

abbey road d enfer

Re: Ampeg Reverberocket humming
« Reply #14 on: September 14, 2017, 10:12:50 AM »
Now I noticed another problem though. I am not sure wether this appeared after the modification or was present even before, because I had been paying attention just to the hum.

Basically even when the reverb is deactivated via the footswitch, there is still some slight reverb sound in the background. In order to completely cut off the reverb sound, the dimension pot (aka reverb level) needs to be turned down.

I am wondering what could be causing this, and if it could be somehow related to the changes in the grounding path ?
The reverb "kill" switch shorts the input of the tank, which leaves the recovery active. The spring is excited by the mechanical vibrations. You may try increasing the mechanical insulation or try connecting the kill switch on the recovery side, with associated risk of increased hum.
« Last Edit: September 14, 2017, 10:16:46 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

beatnik

Re: Ampeg Reverberocket humming
« Reply #15 on: September 14, 2017, 10:19:00 AM »
don't have much to add, just interested that they would list 60mfd off a 5y3.

i have been wondering that as well, the tube datasheet says 20uF is the typical value, I also remember reading too much capacitance can damage the rectifier tube. Actually the power transformer got shot because the rectifier went shorted. Maybe all these facts were linked together ? i think this model with 5Y3 and 7591 was in fact a transition model, and basically they wanted to clear out their 5Y3 stock before moving on the ss rectifier, but ended up using the new can capacitors. The capacitance values for the 5Y3 / 6V6 version are different.

Regarding the reverb sound, I came to your same conclusion. I will try moving the footswitch on the output side and see if it works fine

Thank you very much for your help

Re: Ampeg Reverberocket humming
« Reply #16 on: September 14, 2017, 01:42:34 PM »
Theres a bit of wizardry involved in getting lowest hum , sometimes hum can partially cancel in subsequent stages due to phasing.
Reverb tanks are notorious for picking up induced hum , orientation and proximity of the output transducer to the mains transformer will certainly make a difference ,some residual noise is inevitable though. Mounting the tank on rubber stand offs/gromets might be worthwhile if its not done already ,this will help prevent mechanical feedback from the speakers which could be the cause of the small output from the tank even with the drive level is down. 

In reference to Andys post ,I agree ,low powered tube amps with special attention to low noise are great in studio situations ,you can still get reasonable amounts of drive without too much spill or causing the window panes to rattle .

andyfromdenver

Re: Ampeg Reverberocket humming
« Reply #17 on: September 14, 2017, 02:13:07 PM »
In reference to Andys post ,I agree ,low powered tube amps with special attention to low noise are great in studio situations ,you can still get reasonable amounts of drive without too much spill or causing the window panes to rattle .

oh awesome!

@Abbey, thank you, that makes more sense.
@ beatnik, good job on improving the ground and reducing the hum.
2017 current fav movie: Slums of Beverly Hills
2017 current fav book: Foundation ( I. Asimov)
1997-2017 fav artist: Erik Satie

Re: Ampeg Reverberocket humming
« Reply #18 on: September 14, 2017, 03:04:24 PM »
Converting the input tube to dc heaters on your amp can make a good reduction in amount of hum  , doesnt need to be anything fancy , just rectify the 6.3volt ac line ,then CRC smoothing to bring the dc back to around 6 volts then 2x100 ohms as ground reference , if done right should bring hum levels well below hiss or johnson noise .

beatnik

Re: Ampeg Reverberocket humming
« Reply #19 on: September 14, 2017, 03:27:25 PM »
Would you recommend this specific to this amp or is in general a good practice ?

The heater wiring is actually not twisted, as far as i know the heaters must be tightly twisted together in order to minimise hum leaking into the audio path, maybe redoing the heater wiring would actually make a further improvement ?