Filter caps for vintage OR-120

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fiveightandten
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Re: Filter caps for vintage OR-120

Post by fiveightandten » Mon Jun 18, 2018 12:25 pm

fogwolf wrote:
Mon Jun 18, 2018 3:53 am
Thanks for the info. To get the plate voltage down to 450V I’m running the variac at ~110V, probably closer to what US voltages were in the 70s.
Heater voltage is a good indicator. Generally, if you're getting 6.3VAC loaded, the input voltage is where they intended it to be. 110V is probably low.
fogwolf wrote:
Mon Jun 18, 2018 3:53 am
Don’t worry I’m actually more concerned about doing something to damage the amp so I’m not planning to do major surgery. To be honest getting the PCB up and in a position where I could replace the on-board non-filter caps seems like hassle enough (any tips for that?) in terms of not disturbing or mucking with the tight wiring any more than I need to that I’ll likely take it to a tech if/when I decide to change those.
Personally, I wouldn't touch any of the non-filtering caps. They're probably old WIMA caps and are part of the sound of the amp. To get to the electrolytics, work with the power tubes closest to you and lift the power tube side of the board. There are more flying leads on the pre-amp side, so I wouldn't recommend lifting from that end. Be sure to note the caps' orientation so you don't have any "kaboom" moments on power up.
fogwolf wrote:
Mon Jun 18, 2018 3:53 am
I’ll measure what AC voltage is coming out of the wall in my studio and check the voltages in the amp again without the variac to see what I’m getting.
Be sure to check the heater voltage for reference as well.

-Nick
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fogwolf
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Re: Filter caps for vintage OR-120

Post by fogwolf » Tue Jun 19, 2018 10:02 pm

Ok, the voltage coming out of my wall is almost 128V AC so doing the math that does put it around 550V DC (if I did it right).

Also I measure 5.83V AC at the heaters. So it’s close but a little low. I don’t know enough about tube amps to know if that’s acceptable or what the impact could be or if it would effect the sound, performance or burden of the amp at all. I’m thinking it’s probably preferable to keep that a little low and keep the voltage at the plates to 450V since these are still the original caps and that’s what they’re rated at. Would you agree?

Thanks again so much for the info. I know there’s the serious danger aspect with poking around tube amps but it’s so much better to own vintage gear in general if you know your way around them, since chances are good issues will arise, and being qualified enough to address them yourself safely is rewarding and time and cost effective. I definitely feel more informed now which is exactly what I was hoping for with this post. Really appreciate it.

fiveightandten
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Re: Filter caps for vintage OR-120

Post by fiveightandten » Tue Jun 19, 2018 10:59 pm

fogwolf wrote:
Tue Jun 19, 2018 10:02 pm
Ok, the voltage coming out of my wall is almost 128V AC so doing the math that does put it around 550V DC (if I did it right).
Your math is correct, and your wall voltage is scorching high. Wow. Bear in mind that cold bias will yield high B+ voltage. With that kind of input voltage, I can see how you might get up in the 560's, as you indicated previously.
fogwolf wrote:
Tue Jun 19, 2018 10:02 pm
Also I measure 5.83V AC at the heaters. So it’s close but a little low. I don’t know enough about tube amps to know if that’s acceptable or what the impact could be or if it would effect the sound, performance or burden of the amp at all. I’m thinking it’s probably preferable to keep that a little low and keep the voltage at the plates to 450V since these are still the original caps and that’s what they’re rated at. Would you agree?
5.83VAC is quite low. As far as I know, the only real danger there is that if it's low enough, the filaments will not light and you could do damage to the tubes. There's a small potential to do damage to the amp if some of the power tube filaments light and others don't. At under 6V, I'd recommend visually verifying that when you power the amp up.
fogwolf wrote:
Tue Jun 19, 2018 10:02 pm
I’m thinking it’s probably preferable to keep that a little low and keep the voltage at the plates to 450V since these are still the original caps and that’s what they’re rated at. Would you agree?
Not necessarily. 450V is well under what the amp was designed to run at. Van Halen fans might tell you that this sounds good, but I'm personally not a fan of under running Oranges. As I mentioned earlier, if you want the amp to run at spec, set your variac to the point where the heaters are running at 6.3VAC.

Think of it like this;

The B+ voltage shifts a bit with bias, different tube sets, component values, ect. The dropping resistors and plate resistors drift over time. There are a lot of factors that can contribute to B+ being off spec. And even then, you're talking about a 3.3 PT multiplier, than a 1.3 rectifier multiplier. Small changes turn into big ones.

However, you know the heater voltage. They're all designed to run on 6.3VAC. So if you're using a variac to hunt for how the amp is supposed to be run, that's your best indicator. If it were me, I'd use that as my calibration point and bias the amp accordingly from there.

That being said, I don't particularly enjoy carrying around a Variac. They're heavy, and they also carry risk with them. If that knob gets bumped in the wrong direction, you can have an expensive problem on your hands.

FYI, there are other options if you want to lower the input voltage. The Brown Box has less adjustability, but it removes the possibility of over running the amp.

https://www.amprx.net/product.html

And all that being said, I run my '71 head right from the wall. It's being over run. But I'm vigilant with tube replacements, I bias it somewhat conservatively, and I know what's going on under the hood. I prefer this to carrying around a complicated variac setup. I already to have to lug an attenuator around with it. That's enough!
fogwolf wrote:
Tue Jun 19, 2018 10:02 pm
Thanks again so much for the info. I know there’s the serious danger aspect with poking around tube amps but it’s so much better to own vintage gear in general if you know your way around them, since chances are good issues will arise, and being qualified enough to address them yourself safely is rewarding and time and cost effective. I definitely feel more informed now which is exactly what I was hoping for with this post. Really appreciate it.
No problem. Be safe. As someone who has been bit by an OR-120, I can tell you it's not a pleasant experience and I was lucky my heart got through it. You make a mistake like that exactly once. Either you're dead or you never ever do it again.

-Nick
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fogwolf
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Re: Filter caps for vintage OR-120

Post by fogwolf » Wed Jun 20, 2018 1:23 am

Yikes sorry to hear that happened but glad you were ok. That’s terrifying!

I just got the heater voltage to ~6.25V AC and the variac is outputting ~120V AC, so where the wall voltage ideally would be. This also puts the plate voltage at ~480V DC so not terribly over what the caps are rated for.

However this brought back memories of the fact that now with the voltages bumped up I can’t quite bias it low enough. I added the bias circuit myself years ago, replacing the 22K resistor with a 25K linear trimmer in series with a 10K fixed resistor. I think I need to tweak the value for the fixed resistor a bit if I’m going to run it like this because I can only get the tubes “cool” enough to be just under 70% (17W) for the hottest one and just over 60% for the coolest one. I’ll see how it sounds to me when I get it over to my drummer’s house and can play through my proper cabinet (been testing it with a small bass cabinet here) to hear how it sounds to me. Any thoughts on a value to try for the fixed resistor for the bias circuit? I figure I don’t want to bump the trimmer up or it will have be too touchy/have too much sweep.

Oh and I’ve never had trouble with any of the power tubes not lighting up and they’ve always gotten nice and hot even with the heater voltages at 5.8V DC.

Thanks!

fiveightandten
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Re: Filter caps for vintage OR-120

Post by fiveightandten » Wed Jun 20, 2018 2:43 am

fogwolf wrote:
Wed Jun 20, 2018 1:23 am
Yikes sorry to hear that happened but glad you were ok. That’s terrifying!
Yeah. DC bites. 500 volts DC is a different experience from getting zapped by 120V household AC, which a lot of people have experienced.
fogwolf wrote:
Wed Jun 20, 2018 1:23 am
I just got the heater voltage to ~6.25V AC and the variac is outputting ~120V AC, so where the wall voltage ideally would be. This also puts the plate voltage at ~480V DC so not terribly over what the caps are rated for.
That sounds like a good place to be. You can easily find 500V 100 mfd can caps, and the rest of the caps will probably be within spec. Shoot for 500V on the 32 mfd 2nd stage pair. Sprague makes a 30 mfd 500V Atom.
fogwolf wrote:
Wed Jun 20, 2018 1:23 am
However this brought back memories of the fact that now with the voltages bumped up I can’t quite bias it low enough. I added the bias circuit myself years ago, replacing the 22K resistor with a 25K linear trimmer in series with a 10K fixed resistor. I think I need to tweak the value for the fixed resistor a bit if I’m going to run it like this because I can only get the tubes “cool” enough to be just under 70% (17W) for the hottest one and just over 60% for the coolest one. I’ll see how it sounds to me when I get it over to my drummer’s house and can play through my proper cabinet (been testing it with a small bass cabinet here) to hear how it sounds to me. Any thoughts on a value to try for the fixed resistor for the bias circuit? I figure I don’t want to bump the trimmer up or it will have be too touchy/have too much sweep.

Oh and I’ve never had trouble with any of the power tubes not lighting up and they’ve always gotten nice and hot even with the heater voltages at 5.8V DC.

Thanks!
There was no trim pot from the factory? It sounds like this is one of the 1973 or so amps with PCB mounted pre-tubes and laydown transformers (the only ones I've seen without a bias pot). That's a nice sounding circuit. Though my previous advice for removing the PCB does not hold true if you have this more rare PCB layout.

Your mod should be getting you in the right range. But if it's not, raise the resistor value. You can try a 15K resistor (have a 5K on hand in case my recollection is wrong!, but I think you need to go up). But what kind of negative voltage range is the bias circuit putting out? If you're going to recap the amp, now would be the time to replace those 8 mfd in the bias circuit to be sure it's stable.

A 2W or so variance between tubes isn't small. I'd be sure to stagger them so you don't have the 2 coldest running on one side and the 2 hottest on the other side.

Personally, I'd be fine with running them at 17W with 480V on the plates. That's completely reasonable. I'd recommend Svetlana EL-34s, simply because of the screen voltage rating. But have a look at what your tubes are rated for on the screens. If you were running off your 128VAC wall and running at 17W with 564VDC on the plates, I'd be less inclined to recommend it. But with the Variac, your numbers are fine. Still, it's best to have the adjustment range you need.

Let it heat up real well before you unhook your test equipment and assume all is good. Things tend to drift up after it's been working for a while. I like to run it up high, but silent, through the attenuator for a good 20 minutes or so, then double check for any drift. I usually play my iPod through it and come back in a while to have a look. Drift is usually good for another watt or so. Sometimes I back it down, and sometimes I say, "that's where they want to be" and leave it.

-Nick
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fogwolf
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Re: Filter caps for vintage OR-120

Post by fogwolf » Wed Jun 20, 2018 7:33 pm

fiveightandten wrote:
Wed Jun 20, 2018 2:43 am
There was no trim pot from the factory? It sounds like this is one of the 1973 or so amps with PCB mounted pre-tubes and laydown transformers (the only ones I've seen without a bias pot). That's a nice sounding circuit. Though my previous advice for removing the PCB does not hold true if you have this more rare PCB layout.
Nope, no trimpot. And based on the SN I've always thought it was ~1973 (definitely pre-1974). And yes, everything else as you describe. And it's an incredible sounding amp. I love it so much.

fiveightandten wrote:
Wed Jun 20, 2018 2:43 am
Your mod should be getting you in the right range. But if it's not, raise the resistor value. You can try a 15K resistor (have a 5K on hand in case my recollection is wrong!, but I think you need to go up).
Sounds good thanks.

fiveightandten wrote:
Wed Jun 20, 2018 2:43 am
But what kind of negative voltage range is the bias circuit putting out?
With my trimpot mod the range I'm currently getting from the bias circuit is from around -44V DC to -38V DC. Will try bumping that resistor value a bit at some point to see how much I can extend this, but since you say it sounds like it's not too hot where I have it now, might leave it for the moment. Will see what it sounds like through my proper cabinet, to see if it's still crisp enough biased that hot.

fiveightandten wrote:
Wed Jun 20, 2018 2:43 am
If you're going to recap the amp, now would be the time to replace those 8 mfd in the bias circuit to be sure it's stable.
Here's where I'm a little confused. I don't see 8 mfd caps in the schematic here http://www.orangefieldguide.com/OFG_SCH ... hem_72.gif.

Is this the wrong schematic? If so any chance you know where I could find the correct one for my amp? I see 2 22uf caps in that part of the circuit. do you mean those? Or the 2 0.068uf caps further down the line from the DC filtering part of the bias circuit?

fiveightandten wrote:
Wed Jun 20, 2018 2:43 am
A 2W or so variance between tubes isn't small. I'd be sure to stagger them so you don't have the 2 coldest running on one side and the 2 hottest on the other side.
Ok.

fiveightandten wrote:
Wed Jun 20, 2018 2:43 am
Personally, I'd be fine with running them at 17W with 480V on the plates. That's completely reasonable. I'd recommend Svetlana EL-34s, simply because of the screen voltage rating. But have a look at what your tubes are rated for on the screens.
I'm trying the Electro-Harmonix EL34s this time around. I've heard they're decent and break up more easily. Since it's hard to get my amp turned up much at all it's so earth shatteringly loud thought I'd see if I could get a sound with a little more dirt at a lower volume. However I just read the datasheet and looks like the max screen voltage rating is 425V. Am I pushing that too much at 480V? Will it just mean that the tubes will burn out faster or will it make them not sound so great or potentially damage the amp? If the former, how fast - i.e. instantly? If not I might just use them for a bit and plan on swapping them out with Svetlana's a little later.

Also I got EHX pre-amp tubes. Any voltage concerns there? Assuming that voltages there would be much lower but should they be ok?

Thanks again!

LD50
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Re: Filter caps for vintage OR-120

Post by LD50 » Wed Jun 20, 2018 9:04 pm

Remember your mains cans are in series 100@450v in series with 100@450v (=50uF @ 900v handling - so you double the voltage handling on those and the screens (32uF@450v in series is 16uF@900v). It is the first preamp can in the chain at the PI that can be a bit marginal if they are 15 or 16uF@ 450v but still ok.

The usual bias arrangement is a 100k in parallel for bias.

Oh and the screens should be fine at that, you cam always drop the screen voltages a smidge with bigger screen resistors of 1k5, 2k2 or even 3k3 although that can add a little compression when the amp is pushed.

fiveightandten
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Re: Filter caps for vintage OR-120

Post by fiveightandten » Thu Jun 21, 2018 2:39 am

fogwolf wrote:
Wed Jun 20, 2018 7:33 pm
With my trimpot mod the range I'm currently getting from the bias circuit is from around -44V DC to -38V DC. Will try bumping that resistor value a bit at some point to see how much I can extend this, but since you say it sounds like it's not too hot where I have it now, might leave it for the moment. Will see what it sounds like through my proper cabinet, to see if it's still crisp enough biased that hot.
Yeah, that's probably not enough negative voltage. -44V isn't a lot. Ideally, you'd like to get from the low 30's to 60V or so. For more conservative settings, you can adjust the resistor as discussed previously. You can also try www.mouser.com for a suitable pot in a wider range. You're right that 50K could have the potential to be touchy, but maybe you can find something in-between.
fogwolf wrote:
Wed Jun 20, 2018 7:33 pm
Here's where I'm a little confused. I don't see 8 mfd caps in the schematic here http://www.orangefieldguide.com/OFG_SCH ... hem_72.gif.

Is this the wrong schematic? If so any chance you know where I could find the correct one for my amp? I see 2 22uf caps in that part of the circuit. do you mean those? Or the 2 0.068uf caps further down the line from the DC filtering part of the bias circuit?
My mistake. I had assumed, wrongly, that you have the later more common circuit, which has either 8 mfd or 10 mfd caps in the bias supply. If your amp is consistent with the '72 schematic, you have 22 mfd caps in the bias supply. It's a different bias circuit.

All the available OR-120 schematics are on the Field Guide website. Your amp should match one of them (most likely the one you linked). But there are no guarantees. You need to go off what you see in the amp in front of you. Replace parts based on what you have in your amp, if it differs from the schematic and is obviously factory installed.

Leave the .068 mfd caps. Those are coupling caps for the PI circuit and they're in the signal path. It's part of the sound of the amp. The only ones I would replace are the electrolytic caps in the B+ rail and those two 22 mfd in the bias circuit.
fogwolf wrote:
Wed Jun 20, 2018 7:33 pm
I'm trying the Electro-Harmonix EL34s this time around. I've heard they're decent and break up more easily. Since it's hard to get my amp turned up much at all it's so earth shatteringly loud thought I'd see if I could get a sound with a little more dirt at a lower volume. However I just read the datasheet and looks like the max screen voltage rating is 425V. Am I pushing that too much at 480V? Will it just mean that the tubes will burn out faster or will it make them not sound so great or potentially damage the amp? If the former, how fast - i.e. instantly? If not I might just use them for a bit and plan on swapping them out with Svetlana's a little later.

Also I got EHX pre-amp tubes. Any voltage concerns there? Assuming that voltages there would be much lower but should they be ok?

Thanks again!
I've used EH's quite a bit in OR-120's and rarely had an issue with them. Though you are exceeding the manufacturer's recommendations by running them in the amp. Will you have a problem with them? Probably not. Theoretically, there's a greater risk of the tubes failing if you're running over the voltage spec they recommend. With the variac and 480V (screens will be slightly lower), I think you'll be fine. If you were running straight from the wall and showing 560V+ to EH screens, I might start to worry.

I have residual habits from working on customers' amps, where I'd always take things conservatively. Though, in my own amps I run things pretty hard; exceed tube ratings and exceed speaker ratings. There's some risk involved, and if you're informed and ok with it, that's fine. I just want you to be aware of the specs. If you have them, I'd run them.

-Nick
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fogwolf
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Re: Filter caps for vintage OR-120

Post by fogwolf » Fri Jun 22, 2018 3:06 am

Awesome, thanks again for all the helpful info! Think I’m set, for the moment at least.

fogwolf
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Re: Filter caps for vintage OR-120

Post by fogwolf » Fri Jun 22, 2018 3:10 am

Thanks!
LD50 wrote:
Wed Jun 20, 2018 9:04 pm
Remember your mains cans are in series 100@450v in series with 100@450v (=50uF @ 900v handling - so you double the voltage handling on those and the screens (32uF@450v in series is 16uF@900v). It is the first preamp can in the chain at the PI that can be a bit marginal if they are 15 or 16uF@ 450v but still ok.

The usual bias arrangement is a 100k in parallel for bias.

Oh and the screens should be fine at that, you cam always drop the screen voltages a smidge with bigger screen resistors of 1k5, 2k2 or even 3k3 although that can add a little compression when the amp is pushed.

fiveightandten
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Re: Filter caps for vintage OR-120

Post by fiveightandten » Sat Jun 23, 2018 7:12 pm

fogwolf wrote:
Fri Jun 22, 2018 3:06 am
Awesome, thanks again for all the helpful info! Think I’m set, for the moment at least.
No problem, let us know how it goes! And post some pics of this amp if you get the chance!

-Nick
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'71 GRO100 || '96 OR-80 || AD30 || '64 AC-50 || AC-30TBX || Hiwatt DR504 || HI-TONE HT30
LP Standard || LP Studio || LP Custom Lite || Ric 620 || Ric 360 || MIA Tele || SG 61 RI

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