The type of instability is uncertain I think.
Did you confirm no sign of instability when tertiary feedback winding was disconnected?
If instability requires a feedback connection then perhaps a few simple tests could be:
Did you confirm no sign of instability when tertiary feedback winding was disconnected?
If instability requires a feedback connection then perhaps a few simple tests could be:
- With feedback connected, and a low signal level on output, then measure Vrms on output (across load resistance) and then measure again when feedback disconnected (the voltage should increase by up to about 10x, so confirm that is not pushing output power above its rating). The ratio of output voltages is then the feedback level (convert to dB) that people report.
- add in another 2k2 in series with R11, and check if that still makes the amp unstable. Can also measure the level of feedback that provides.
- this line of testing indicates if level of feedback is the main cause, but doesn't identify if it is low or high frequency related, or what remedies are available.
Went to 4k7/500pF NFB.
With about 100mvac input to grid, I get around 20mvac of feedback as measured from tertiary winding.
Other mods still implemented:
-10k series input grid resistor
-10k/220pf in series as snubber across V1 plate R (R4)
I will goose input signal and see if it lights up, and starts thumping.
With about 100mvac input to grid, I get around 20mvac of feedback as measured from tertiary winding.
Other mods still implemented:
-10k series input grid resistor
-10k/220pf in series as snubber across V1 plate R (R4)
I will goose input signal and see if it lights up, and starts thumping.
Still doing it. Tried a different DMM, and got about 50mv Input
400mv NFB.
Bias to power tubes around 25ma
no input signal stays stable.
With signal applied right around, 70mvac if I give it any more it starts thumping and bias and voltage start swing around. (measuring input vac, reservoir cap dc voltage, and one power tube bias.
All tubes I’ve tried are matched sets and sound fine in other amps.
I don’t recall implementing all these osc fixes (grid leak, series grid R, v1 snubber, NFB lessened) and still having onset appear at about the same conditions.
hmmm
I guess I’ll unhook NFB and see what it does. Should be way earlier me thinks.
I haven’t tried the ac voltage measurements on OT secondary. But I’m thinking it’s gotta be low, with the bias in at??
400mv NFB.
Bias to power tubes around 25ma
no input signal stays stable.
With signal applied right around, 70mvac if I give it any more it starts thumping and bias and voltage start swing around. (measuring input vac, reservoir cap dc voltage, and one power tube bias.
All tubes I’ve tried are matched sets and sound fine in other amps.
I don’t recall implementing all these osc fixes (grid leak, series grid R, v1 snubber, NFB lessened) and still having onset appear at about the same conditions.
hmmm
I guess I’ll unhook NFB and see what it does. Should be way earlier me thinks.
I haven’t tried the ac voltage measurements on OT secondary. But I’m thinking it’s gotta be low, with the bias in at??
If you have a dual trace scope you might want to pull the power tubes and measure behavior at the control grids of the output tubes. Need either a signal generator or an app on a PC or phone.
I have seen an Altec amp look fine and then have the drive stages go bad at higher levels.
You should see at least 50V RMS on each grid with them 180 degrees out of phase without things looking bad.
I have seen an Altec amp look fine and then have the drive stages go bad at higher levels.
You should see at least 50V RMS on each grid with them 180 degrees out of phase without things looking bad.
Thanks, yep that’s where I was headed too!
Was gonna pull power, variac up to near power tube loaded HT. Confirm filaments and see where this preamp swamps! Will give that a try.
Saddest part, is I have another right behind it doing the same ills.
Was gonna pull power, variac up to near power tube loaded HT. Confirm filaments and see where this preamp swamps! Will give that a try.
Saddest part, is I have another right behind it doing the same ills.
Okay, back in this thing today. And after sitting on the ”throne”, realized that my speaker output on this unit is floating. In other words, on output terminal block there is a “chassis ground”, and a speaker “COM”.
they are not jumped…….
So, output is floating As of now.
perhaps that’s all it is. Output needs a chassis reference to avoid swinging around floating?????
Will update.
they are not jumped…….
So, output is floating As of now.
perhaps that’s all it is. Output needs a chassis reference to avoid swinging around floating?????
Will update.
I'd suggest the amp should be stable with the speaker line floating - as per the schematic showing it as floating, and as per typical PA application where it would be floating. Stability may well be compromised if the floating speaker taps were not adequately loaded, although even then I'd anticipate a large margin.
Hmmm…. I’m accustomed to valve amps, single ended and push pull where the OT secondary is usually ground referenced. I can see the flexibility it adds the user to control potential referencing. allowing a floated secondary.
Even an ST70 chassis grounds the common.
Perhaps just the way the input and output cables were laying, it always started to oscillate.
Also, happened for their owner, when auditioning.
Well, I guess the next question is, is there any reasonable detriment to running a stereo pair of these and all have a shared ground??? Wouldn’t think so, leaving em secondary of OT Com and Chassis strapped.
Even an ST70 chassis grounds the common.
Perhaps just the way the input and output cables were laying, it always started to oscillate.
Also, happened for their owner, when auditioning.
Well, I guess the next question is, is there any reasonable detriment to running a stereo pair of these and all have a shared ground??? Wouldn’t think so, leaving em secondary of OT Com and Chassis strapped.
PA operators run long speaker lines, and from mains or battery or ... power sources. Having a floating speaker line allows one point of failure (eg. to some ground) to not stop the performance.
For restored PA amps used in diy/home/hi-fi/guitar applications then yes grounding one point of the speaker taps is safer and imho preferred. The only uncertainty is that if the amp uses global feedback, then where the speaker taps are grounded may have some influence on high frequency instability due to parasitic capacitance within the output transformer.
If one monobloc is misbehaving, then perhaps best to chase down that issue before complicating the setup with stereo.
Dual monoblocs used for stereo has been achieved for decades, but removing all unforeseen issues for that setup is a separate issue and when you progress to that then perhaps search and read up on what others have experienced.
For restored PA amps used in diy/home/hi-fi/guitar applications then yes grounding one point of the speaker taps is safer and imho preferred. The only uncertainty is that if the amp uses global feedback, then where the speaker taps are grounded may have some influence on high frequency instability due to parasitic capacitance within the output transformer.
If one monobloc is misbehaving, then perhaps best to chase down that issue before complicating the setup with stereo.
Dual monoblocs used for stereo has been achieved for decades, but removing all unforeseen issues for that setup is a separate issue and when you progress to that then perhaps search and read up on what others have experienced.
Excellent input, thanks so much.
It hasn’t burst into oscillation since grounding secondary. So I’m keen to believe.
I pulled power tubes and scoped grids and preamp stayed stable.
only with power glass loaded and secondary floating, did it have fail condition. And did so every time power was brought up. Even with 5w output per tube (low bias).
Secondary chassis referenced has not oscillated at all, and I was able to bias to schematic.
It hasn’t burst into oscillation since grounding secondary. So I’m keen to believe.
I pulled power tubes and scoped grids and preamp stayed stable.
only with power glass loaded and secondary floating, did it have fail condition. And did so every time power was brought up. Even with 5w output per tube (low bias).
Secondary chassis referenced has not oscillated at all, and I was able to bias to schematic.
It's really not safe enough, in a modern context, to float the speakers' secondary winding. Any leakage from primary to secondary (including a not-impossible short) appears between speaker wiring and chassis, where someone is very likely to bridge hand-to-hand. Not worth the danger; just say no.
All good fortune,
Chris
All good fortune,
Chris
So after finding both these altec units to be solid, I started to try the different plug-in transformers.
15k:15k
This the unit it came in with.
I think they were using a 10k source, and wanted isolation.
???
Sounds ok.
15095
600:15k (I think)
Sounds good. Big signal step.
get to a certain point and I heard the power supply thump. Quickly voltaged down.
hmmmm
So how do you run these things for home audio power amps?
just input to volume lug 3, direct to grid kinda setup?
Just bridge em in????
Any thoughts considering input staging, and what AC to keep in mind. Seems the manual states .9v for unbalanced???
thanks everyone.
-j
http://www.dvq.com/hifi/images/1569a.pdf
15k:15k
This the unit it came in with.
I think they were using a 10k source, and wanted isolation.
???
Sounds ok.
15095
600:15k (I think)
Sounds good. Big signal step.
get to a certain point and I heard the power supply thump. Quickly voltaged down.
hmmmm
So how do you run these things for home audio power amps?
just input to volume lug 3, direct to grid kinda setup?
Just bridge em in????
Any thoughts considering input staging, and what AC to keep in mind. Seems the manual states .9v for unbalanced???
thanks everyone.
-j
http://www.dvq.com/hifi/images/1569a.pdf
Input level transformers should only be operated at very low voltages - like microphone transformers, unless you have specs for the transformers.
You should be testing them yourself to confirm input and output signal levels and when your output starts clipping. This is basic info you need to go through a learning curve on, as it only gets more detailed and involved.
Imho you need to do you homework on what you will drive them with and how that type of source signal will interface with your amp - google is your friend.
You should be testing them yourself to confirm input and output signal levels and when your output starts clipping. This is basic info you need to go through a learning curve on, as it only gets more detailed and involved.
Imho you need to do you homework on what you will drive them with and how that type of source signal will interface with your amp - google is your friend.
Thanks, I agree. But, as you well know, the performance of a system is the sum of the parts. And I don’t have the rest of the owners gear.
I‘ve been interfacing them with every audio send (balanced and unbalanced) that I have. And trying all the plugin iron I have at hand, to try and nail down likely performance.
I got the numbers and nothing is misplaced to me. But the amp had this odd oscillation issue that started the post, and by grounding the secondary (not floating) was able to get it stable under signal condition biased around 20w per tube. Totally odd solve
Im trying to get some insight into source/load relationships, to see if it sheds a light on what I was experiencing, and what to expect when interfaced with other gear.
I‘ve been interfacing them with every audio send (balanced and unbalanced) that I have. And trying all the plugin iron I have at hand, to try and nail down likely performance.
I got the numbers and nothing is misplaced to me. But the amp had this odd oscillation issue that started the post, and by grounding the secondary (not floating) was able to get it stable under signal condition biased around 20w per tube. Totally odd solve
Im trying to get some insight into source/load relationships, to see if it sheds a light on what I was experiencing, and what to expect when interfaced with other gear.
You should be able to run them just fine without an input transformer.
Just check what happens to exist in the input side as far as C1 and C2. Are they shorted or not?
The input transformer was used to break any ground loops and also allow the use of a low impedance feed allowing you to locate the amplifier hundreds of feet from the source.
Strapping the 15095 transformer for 150 ohms instead of 600 ohms would get you around a football stadium without high frequency loss. 600 ohms might not make it.
Just check what happens to exist in the input side as far as C1 and C2. Are they shorted or not?
The input transformer was used to break any ground loops and also allow the use of a low impedance feed allowing you to locate the amplifier hundreds of feet from the source.
Strapping the 15095 transformer for 150 ohms instead of 600 ohms would get you around a football stadium without high frequency loss. 600 ohms might not make it.
Right on. thanks
yep, input is still strapped.
Do you think that the reflection from iron created ring because of termination bridging via transformer?
Further testing
yep, input is still strapped.
Do you think that the reflection from iron created ring because of termination bridging via transformer?
Further testing
Not following what you mean be reflection from iron ring above. Do you mean having an input transformer versus directly connected?
As far as the floating output goes, I can say I have seen a place where it is useful.
In the case of a drive in theater with pole mounted speakers with a 70 volt transformer in each speaker. The largest lot may have 800 speakers powered by three Altec 1570s. They have a zone switching configuration of maybe 20 zones so you could assign an amplifier to a zone. Also it was helpful to disable a zone if a short exists. Shorts happen often either due to an old frayed cord, poles getting hit by either tired or drunk visitors. The person in the projection booth would know a short existed by the grunting sound of the amplifier and the orange plate of the 811a tubes. Orange plating may happen weekly and might be helpful in chasing away the gas inside the tube.
Having floating outputs avoids ground loops when dealing with more than one amplifier sharing a common grounding point.
You still want to be sure the floating output dues not float too much though. I am fairly sure the outputs of the amplifiers have seen a slight kiss from the lightning strikes in their time. A damaged output transformer could also put plate voltage on the outputs in some cases.
As far as the floating output goes, I can say I have seen a place where it is useful.
In the case of a drive in theater with pole mounted speakers with a 70 volt transformer in each speaker. The largest lot may have 800 speakers powered by three Altec 1570s. They have a zone switching configuration of maybe 20 zones so you could assign an amplifier to a zone. Also it was helpful to disable a zone if a short exists. Shorts happen often either due to an old frayed cord, poles getting hit by either tired or drunk visitors. The person in the projection booth would know a short existed by the grunting sound of the amplifier and the orange plate of the 811a tubes. Orange plating may happen weekly and might be helpful in chasing away the gas inside the tube.
Having floating outputs avoids ground loops when dealing with more than one amplifier sharing a common grounding point.
You still want to be sure the floating output dues not float too much though. I am fairly sure the outputs of the amplifiers have seen a slight kiss from the lightning strikes in their time. A damaged output transformer could also put plate voltage on the outputs in some cases.
Thanks again for the input.
Reflection meaning, that due to source/load impedance relationships, the signal in is prone to oscillation brought about by ringing in the transformer that proper load termination, or zoebel would null out.
In short. I have the units working direct coupled to input pot, and they seem to do ok with a 15k:15K Input iron, but when switching to a 15095, and bridging low z into it, the problem I was facing originally, oscillation or cyclical interruption starts, and some weird thump is audible.
This is after it was stable without the 15095. But since the unit is spec'd for that input iron. It's got me hesitant to call it solved.
Reflection meaning, that due to source/load impedance relationships, the signal in is prone to oscillation brought about by ringing in the transformer that proper load termination, or zoebel would null out.
In short. I have the units working direct coupled to input pot, and they seem to do ok with a 15k:15K Input iron, but when switching to a 15095, and bridging low z into it, the problem I was facing originally, oscillation or cyclical interruption starts, and some weird thump is audible.
This is after it was stable without the 15095. But since the unit is spec'd for that input iron. It's got me hesitant to call it solved.