My latest project, a Hammond AO-44 reverb amp rebuilt into a Vox AC15 with a built in boost. Amazingly on the normal setting it is quiet, the whole time I was building it I had doubts given how cramped it is! It is however unusable on the boost side, suffers badly from oscillation which I need to track down.
I should explain that the idea behind this amp was to build it as cheaply as possible, hence the wooden faceplate and mismash of parts inside. There is a hole where the indicator will go but before I do that my partner is going to paint the faceplate for me.
Control wise it’s volume, bass and treble. Bass is same as the songwriter, 500pf and .047uf caps separated by a 3Meg pot, treble is just the Vox cut control but the opposite way round, turn it up for more treble.
Rebuilt Hammond AO-44 into Vox
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Littlewyan
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Rebuilt Hammond AO-44 into Vox
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Stevem
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Re: Rebuilt Hammond AO-44 into Vox
Try removing the output jack, spinning it to twist those wires and then for now sit it on the other side of that power switch.
I have fixed a ton of oscillating issues in those Tiny Terror amps by making a sheild for the input jack .
On these amps the input jack sits 3 inches across from the 3 output jacks that the amp has and even when stock or for certain when really strong testing preamp tubes are used the amp would go into oscillating above 6 on the volume pot.
I am pretty darn sure you have the same issue with that tight chassis.
I have fixed a ton of oscillating issues in those Tiny Terror amps by making a sheild for the input jack .
On these amps the input jack sits 3 inches across from the 3 output jacks that the amp has and even when stock or for certain when really strong testing preamp tubes are used the amp would go into oscillating above 6 on the volume pot.
I am pretty darn sure you have the same issue with that tight chassis.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Littlewyan
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Re: Rebuilt Hammond AO-44 into Vox
It mostly occurs with the amp volume above halfway. It stops if the guitar volume is on 0, if I have it on 1 or 2 it will be quiet and if I strum the guitar sound is clean with a load of fuzz on top. I can mitigate it a bit if I sit really far back from the amp, but as soon as the amp volume is above halfway it doesn't matter how far back I am.
Tried moving grounds, moving wires and fitting the shield on the bottom, none of which made a difference. The high gain sound was very muddy at first switch on, moved the raised .0022uF coupling cap away from the EL84 grid wire and that solved that.
Absolute worst case I'll get rid of the cascaded stage, but it does sound good when it's not oscillating.
Tried moving grounds, moving wires and fitting the shield on the bottom, none of which made a difference. The high gain sound was very muddy at first switch on, moved the raised .0022uF coupling cap away from the EL84 grid wire and that solved that.
Absolute worst case I'll get rid of the cascaded stage, but it does sound good when it's not oscillating.
Re: Rebuilt Hammond AO-44 into Vox
Finding the outside foil of the coupling caps and attaching it to the plate side can help on the margins, also keeping the plate side of the cap wire longer might improve things slightly. You might also try adding a cap from plate to plate on the PI or across one of the plates to see if that helps. Start with something big to test if it gets rid of the oscillation, then knock down to the smallest cap you can get away with. I built a few AO-44 el84 amps a while ago, some of the power transformers seemed to handle things better than others. Looks like you might have one of the beefier transformers, but make sure it isn't getting too hot. You might also want to use a smaller value resistor on the power tube cathodes, mine were really cold at 150r.
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Littlewyan
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Re: Rebuilt Hammond AO-44 into Vox
Well I've been measuring the amp with a scope and it oscillates without anything plugged into the input jack. I can measure the oscillation (Seems to start at 18khz but also appears below 6Khz) from the first stage all the way to the output jack. Removing any of the valves individually stops it and I can measure it on the anode of the first stage whilst the input of that stage is grounded, so it seems it's the output stage coupling with the preamp as Stevem predicted. I've tried the following:
- A 250pf cap across the 220k anode resistor on the first boosted stag.
- Shortened EL84 grid wires and increased grid stoppers to 2.7K (Minor increase I know, they were values I had on hand).
- Twisting the Cut control wires.
- Moving the speaker jack around whilst measuring the oscillation.
- Swapping valves.
- Adding a .68uF in parallel with the filter cap for the preamp.
I can vary the lower frequency oscillations by moving any of the wires. I think it's looking as if I'm just asking for too much gain in such a small chassis with the preamp so close to the poweramp.
And you're right, the EL84s are running at 87% dissipation at the moment. I have a 150R sag resistor after the PT as I thought the HT would be higher without the EZ81 but it came out at 290V. Will either remove the sag resistor or just lower the 150R cathode resistor.
- A 250pf cap across the 220k anode resistor on the first boosted stag.
- Shortened EL84 grid wires and increased grid stoppers to 2.7K (Minor increase I know, they were values I had on hand).
- Twisting the Cut control wires.
- Moving the speaker jack around whilst measuring the oscillation.
- Swapping valves.
- Adding a .68uF in parallel with the filter cap for the preamp.
I can vary the lower frequency oscillations by moving any of the wires. I think it's looking as if I'm just asking for too much gain in such a small chassis with the preamp so close to the poweramp.
And you're right, the EL84s are running at 87% dissipation at the moment. I have a 150R sag resistor after the PT as I thought the HT would be higher without the EZ81 but it came out at 290V. Will either remove the sag resistor or just lower the 150R cathode resistor.
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Littlewyan
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Re: Rebuilt Hammond AO-44 into Vox
With the changes I made
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Littlewyan
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Re: Rebuilt Hammond AO-44 into Vox
Done a whole bunch more testing and getting confusing results.
- With nothing plugged into the input jack if I turn the volume up above halfway I start reading a sine wave on the speaker jack. Quite a large sine wave at that. I can trace this back to the first gain stage. As I vary the volume the frequency varies but it's large enough to develop harmonics.
- With an unwired jack plugged into the input jack the amp will squeal even with the volume all the way off.
- With an unwired jack plugged into the input jack and my signal generator attached it will behave perfectly. No squealing, I can crank the volume no problem and I can hammer the hell out of it with a large signal no issues.
I can stop the oscillation if I turn the cut control all the way up and on single gain stage mode it behaves perfectly and is very quiet. I'm at a loss, maybe a dodgy connection somewhere is doing this?
- With nothing plugged into the input jack if I turn the volume up above halfway I start reading a sine wave on the speaker jack. Quite a large sine wave at that. I can trace this back to the first gain stage. As I vary the volume the frequency varies but it's large enough to develop harmonics.
- With an unwired jack plugged into the input jack the amp will squeal even with the volume all the way off.
- With an unwired jack plugged into the input jack and my signal generator attached it will behave perfectly. No squealing, I can crank the volume no problem and I can hammer the hell out of it with a large signal no issues.
I can stop the oscillation if I turn the cut control all the way up and on single gain stage mode it behaves perfectly and is very quiet. I'm at a loss, maybe a dodgy connection somewhere is doing this?
Last edited by Littlewyan on Thu Jun 05, 2025 10:25 pm, edited 1 time in total.
Re: Rebuilt Hammond AO-44 into Vox
Have you tried a cap from plate to plate on the PI or across one of the PI plates? If the cut control gets rid of the oscillation, theres a good chance that would work. Could also try a conjunctive filter, cap and resistor from plate to plate on the power tubes.
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Littlewyan
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Re: Rebuilt Hammond AO-44 into Vox
I haven't yet, I just tried a cap across the plate resistor of the first stage. I haven't bothered with the PI cap as the cut control only kills the oscillation when it's killing all treble essentially. So for the PI cap to work it'll have to do the same and would render the amp useless.
I've just found that the oscillation stops if I touch my scope probe on the x1 setting to earlier points in the amp. If I change it to x10 it doesn't load it down and the oscillation continues.
I've just found that the oscillation stops if I touch my scope probe on the x1 setting to earlier points in the amp. If I change it to x10 it doesn't load it down and the oscillation continues.
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Littlewyan
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Re: Rebuilt Hammond AO-44 into Vox
Came to the conclusion the amp is just too small to be high gain. So I’ve rebuilt it into a single stage preamp with a one knob tone control, same as the Marshall 18W Lite2B and now it sounds fantastic. Plenty of gain for my needs.
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Stevem
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Re: Rebuilt Hammond AO-44 into Vox
Space and layout are as you have seen very important.
Every coupling or de-coupling cap that a signal passed thru heading down stream and in series so to speak can produce up to a 90 degree shift in phase, and once that phase shift exceeds 180 degrees then you will start to have issues if things are not right.
So even with only two gain stages going and with crappy caps you can be right at that 180 degree going off the edge point.
There are a few things one can try to eliminate the oscillation.
The first thing to look for are any two parts of the circuit that are in phase with each other and within 3 inches of each other
Of course this is what takes place with any triode since what comes out of the plate is 180 degrees out from what went into the grid.
1)
The least harmful to tone is move the coupling cap right to the grid pin of the down stream gain stage, yes you will make the plate wire longer and the grid wire shorter which is always better.
2)
Add series grid resistance right at the tube grid.
3)
Place a 10pf to 500 pfcap across the plate resistor, note the voltage needed for that cap!
4)
Place a 5pf to 500 pf cap from plate to grid, once again note voltage rating needed.
5)
Increase power supply node uf.
6)
During a build or mod in a high gain amp only have 1 preamp gain stage per power supply node.
7)
Put in alow pass filter consisting of a 10k resistor in series and a 10 pf cap to ground
7A)
Cap orinentation .
Solder a short lenght of wire off the plate a make 3 turns with that wire around the input grid wire.
Shrink tube over the end of the wire .
This mod will for sure have aeffect on the high end responce and should only be used as a flat out last resort, however it can be helpful in tracking which which gain stage is producing the issue.
Of course if you want to get a high roll off with a mid boost this is one way to get close with out having a active preamp section.
I don't know how the face with the sunglasses got in here, but I can't seem to delete it.
Every coupling or de-coupling cap that a signal passed thru heading down stream and in series so to speak can produce up to a 90 degree shift in phase, and once that phase shift exceeds 180 degrees then you will start to have issues if things are not right.
So even with only two gain stages going and with crappy caps you can be right at that 180 degree going off the edge point.
There are a few things one can try to eliminate the oscillation.
The first thing to look for are any two parts of the circuit that are in phase with each other and within 3 inches of each other
Of course this is what takes place with any triode since what comes out of the plate is 180 degrees out from what went into the grid.
1)
The least harmful to tone is move the coupling cap right to the grid pin of the down stream gain stage, yes you will make the plate wire longer and the grid wire shorter which is always better.
2)
Add series grid resistance right at the tube grid.
3)
Place a 10pf to 500 pfcap across the plate resistor, note the voltage needed for that cap!
4)
Place a 5pf to 500 pf cap from plate to grid, once again note voltage rating needed.
5)
Increase power supply node uf.
6)
During a build or mod in a high gain amp only have 1 preamp gain stage per power supply node.
7)
Put in alow pass filter consisting of a 10k resistor in series and a 10 pf cap to ground
7A)
Cap orinentation .
Solder a short lenght of wire off the plate a make 3 turns with that wire around the input grid wire.
Shrink tube over the end of the wire .
This mod will for sure have aeffect on the high end responce and should only be used as a flat out last resort, however it can be helpful in tracking which which gain stage is producing the issue.
Of course if you want to get a high roll off with a mid boost this is one way to get close with out having a active preamp section.
I don't know how the face with the sunglasses got in here, but I can't seem to delete it.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Rebuilt Hammond AO-44 into Vox
Ok, but I did not mention the name, just the technique employed.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
-
Littlewyan
- Posts: 1944
- Joined: Thu Sep 12, 2013 6:50 pm
- Location: UK
Re: Rebuilt Hammond AO-44 into Vox
Thanks for all the suggestions but it's rebuilt now and I'm pretty pleased with it. I had already tried increasing the EL84 grid stoppers to 10K, adding an anode bypass capacitor, paralleling 0.68uF capacitor with the filter caps, disconnecting the cut control, chopsticking around the amp and physically moving preamp components around and nothing helped. I think the ECC83 socket is just too close to the EL84 socket. The oscillation could be seen on the first stage anode and removing valves, turning down volume or even turning the cut control all the way down seemed to break the chain and stop it.