Filter caps for vintage OR-120

[fogwolf]

I have a graphics OR-120 and quite a bit of technical skill (I added a bias adjustment to the amp myself and have done lots of non-amp electrical work) and know I am capable of replacing the filter caps if I needed to, but have never worked with caps that are in a metal canister mounted directly to a chassis before. A few questions I have are

1) I’m guessing the actual cap comes out of the silver metal canister? They haven’t needed replacing so have not mucked with popping them off to see.

2) Finding the exact size here obviously matters more than usual even due to needing to fit in the canisters. Could someone please point me to an exact drop-in replacement in terms of size and with solder terminals rather than leads? These are 100uf and rated for 450V.

Speaking of which like most vintage Orange amps on modern US voltages, I measure closer to 560V in my amp when it’s just plugged directly into the wall. I have a variac to try to keep these old caps alive longer and to save wear on my tubes but am wondering if bumping the ratings of all the caps in the circuit up closer to 550-600V would work and if the filter caps would still fit in the canisters in that case.

3) Are the filter caps plain old aluminum elecs or are they paper in wax, paper in oil, etc and for power supply filtering in a tube amp does it really matter anyway?

4) In terms of the other elecs in the amp that I’m assuming are in the audio signal path (at least some of them) any recommendations for brands that are good substitutes in terms of the effect on the sound of the amp? Know this question is more subjective but any thoughts appreciated.

Thanks!

[bclaire]

Well.....

Some of your questions do not indicate enough knowledge of electronics for me to feel comfortable giving advice.

The whole thing, metal and all, is the filter cap.

The voltage contained within will seriously injure you. Let me repeat that:

THE VOLTAGE CONTAINED WITHIN CAN HARM YOU!

If you do not know the safe procedures for discharging filter caps, stop and bring it to an amp tech for repair, which is what you probably should do anyways - to have them thoroughly check the entire amp circuitry for any issues.

[fogwolf]

I promise I know how to work on the amp, have a snuffer stick for bleeding the voltage off the caps, how to verify the voltage has gone down etc. and I understand the entire thing can be the cap. Like I said I just haven’t dealt with a cap that mounts to a chassis like this. I don’t think any of my questions involve you walking me through anything dangerous - I’m not asking how to replace the cap, for example. If someone could at least just point me to where I’d be able to buy my own filter caps that would fit that would be great. Also any suggestions for elec caps people like in the audio signal. I think those are reasonable and safe questions for someone to answer here. Thanks.

[fogwolf]

For clarity, since I’ve never specifically worked with this exact form factor of cap I wasn’t sure if the metal canisters in this case were like pre-amp tube covers or something, i.e. there to cover and mount the cap, with the cap tucked inside.

[fogwolf]

I think the answer to my main question is I use a normal can-style electrolytic cap and use the bracket that's there to mount it onto the chassis. I was just thrown by the silver metal appearance of the cap itself, and hung up on it looking the same, so it still looked "vintage". Doing more digging online it seems that's not going to happen, and it's not the end of the world.

Also the 1 site I had seen before posting was talking about dielectrics I hadn't used before, like oil and paper. I realized after posting it was a vintage radio shop. I've built my own power supply in 2 modular synths I also built myself, so it's not new territory for me, but I've never built a guitar amp, so didn't know especially for a vintage one if there was something different and again, the look of these caps is new to me. And just because I've worked with power supplies before, I know and understand very well the crazy high voltages and more importantly current in my guitar tube amp and that it's very dangerous.

Anyway, if anyone knows the diameter of cap used in these amps I'd be much obliged. Also again any thoughts on what brands people like for replacing the electrolytic caps in the audio signal.

Thanks.

[Mr Mustard]

You should check with the guys on this antique radio forum. They're big on maintaining originality. I've seen where they remove the old caps from the metal cans and stuff them with modern caps so they still look original. Those old radio amps are similar to tube guitar amps so they can probably help.
http://antiqueradios.com/forums/index.php

And here's a page that has everything you wanted to know but were afraid to ask...
https://antiqueradio.org/recap.htm

[fogwolf]

Thanks I’ll check that out. Appreciate it.

[fiveightandten]

I have a graphics OR-120 and quite a bit of technical skill (I added a bias adjustment to the amp myself and have done lots of non-amp electrical work) and know I am capable of replacing the filter caps if I needed to, but have never worked with caps that are in a metal canister mounted directly to a chassis before. A few questions I have are

1) I’m guessing the actual cap comes out of the silver metal canister? They haven’t needed replacing so have not mucked with popping them off to see.

2) Finding the exact size here obviously matters more than usual even due to needing to fit in the canisters. Could someone please point me to an exact drop-in replacement in terms of size and with solder terminals rather than leads? These are 100uf and rated for 450V.

Speaking of which like most vintage Orange amps on modern US voltages, I measure closer to 560V in my amp when it’s just plugged directly into the wall. I have a variac to try to keep these old caps alive longer and to save wear on my tubes but am wondering if bumping the ratings of all the caps in the circuit up closer to 550-600V would work and if the filter caps would still fit in the canisters in that case.

3) Are the filter caps plain old aluminum elecs or are they paper in wax, paper in oil, etc and for power supply filtering in a tube amp does it really matter anyway?

4) In terms of the other elecs in the amp that I’m assuming are in the audio signal path (at least some of them) any recommendations for brands that are good substitutes in terms of the effect on the sound of the amp? Know this question is more subjective but any thoughts appreciated.

Thanks!

As Billy mentioned, the can IS the cap.

Go to a site like www.tubesandmore.com or something similar. Get some new can caps in 100mfd, in a similar diameter. If they are not the same diameter, you have to buy new mounting clamps that fit the caps you purchase.

They do indeed see over their rated voltage. You'd ideally want them rated over 600VDC. But passives are forgiving to being run over spec, and the amp likely has resistors bridging the 2 caps, so the first one in the line isn't getting slammed.

These caps are only 2 of a number of electrolytics in the amp. You should replace all of them if you're recapping the amp, and that includes the 8 mfd in the bias supply. Those are important. I like Sprague Atoms to replace the ones mounted on the board. Be advised that some of the Orange PCB's have very small holes in the PCB. My '71 head would not accept modern high quality electrolytics. The leads were too thick to fit through the holes in the PCB. I *carefully* used a small drill bit to open them up a little. You have to be extremely careful doing this, as to not damage the PCB traces. Just FYI, in case you get in there and find out that your new caps can't be mounted on the PCB...that's why.

As Billy said, be EXTREMELY careful when playing around with high voltage electrolytics. The stored voltage can kill you. I've even seen these short to the external casing and give a jolt if you touch the exterior. This is dangerous stuff.

Speaking of which like most vintage Orange amps on modern US voltages, I measure closer to 560V in my amp when it’s just plugged directly into the wall. I have a variac to try to keep these old caps alive longer and to save wear on my tubes but am wondering if bumping the ratings of all the caps in the circuit up closer to 550-600V would work and if the filter caps would still fit in the canisters in that case.

It sounds like you're not familiar with how these B+ rails work. There are dropping resistors for each stage, so the majority of the caps are seeing nowhere near the 560V output at the rectifier/ OT CT.

3) Are the filter caps plain old aluminum elecs or are they paper in wax, paper in oil, etc and for power supply filtering in a tube amp does it really matter anyway?

Yes it matters. You need regular polarized electrolytics.

4) In terms of the other elecs in the amp that I’m assuming are in the audio signal path (at least some of them) any recommendations for brands that are good substitutes in terms of the effect on the sound of the amp? Know this question is more subjective but any thoughts appreciated.

Thanks!

There are no electrolytics in the audio path. As mentioned above, my favorite are Sprague Atoms. F&T and JJ are also nice quality. You could potentially have lead fitment issues with any of these, as mentioned above.

-Nick

[fogwolf]

Super helpful info thank you!

[bclaire]

Thanks, Nick.

[fogwolf]

And the reason I assumed that I’d need to bump the voltage rating of all the larger electro caps is most of the stock ones already there are rated for high voltage (450V). When I biased the amp yesterday the caps bled voltage off pretty quickly and I didn’t measure voltage at any of them while the amp was powered on, but from the last time I replaced the tubes and added the bias adjustment several years ago I swear I recall measuring ~560V not just at the tubes but also at at least some of the caps. Could misremembering though. Thanks again.

[fiveightandten]

Thanks, Nick.

No problem Billy.

And the reason I assumed that I’d need to bump the voltage rating of all the larger electro caps is most of the stock ones already there are rated for high voltage (450V).

...As they should be, when they're supposed to see in the 300V range, though IIRC the early preamp stages are probably in the 200s. I'd imagine the cost of a 100mfd can cap in excess of 500VDC wasn't exactly low, as it isn't low now, hence the under spec caps at the beginning of the B+ rail. Physical size may have also been a limitation.

When I biased the amp yesterday the caps bled voltage off pretty quickly and I didn’t measure voltage at any of them while the amp was powered on, but from the last time I replaced the tubes and added the bias adjustment several years ago I swear I recall measuring ~560V not just at the tubes but also at at least some of the caps. Could misremembering though. Thanks again.

That's correct. Voltage at or near the full B+ will be present at the first 2 filtering stages. Though, I hope you are misremembering the 560V figure, as the amp shouldn't have voltage that high anywhere in it. It's much too high, and you won't find EL-34's that can take screen voltage that high in the long term, reliably. Svetlana EL34's are the cheapest readily available option with robust screen grids.

Make sure the PT primary is wired correctly and set to the right input voltage. Check the heater voltage as a secondary reference. The amp most likely has a 117VAC primary for US wall voltage and will be seeing loaded secondary voltages around 7% higher than spec. If it's more than that, I would put the brakes on things to figure out why.

My '71 head runs around 518VDC on the plates and around 6.7VAC loaded on the heater legs and I consider this just barely acceptable.

[fogwolf]

Yeah I’m certain it was that high which does (and did at the time) seem extreme, which is why I’ve been running it exclusively through a variac to keep that down to 450V DC. But I’m gonna to dig into this more.

EDIT - I do have the input voltage set for US voltages, but do you have any recommendations on finding a datasheet to determine if the secondary on the PT is wired correctly?

Also are the voltages at the secondary on this schematic correct? 190V, then 315V to the heaters and 36V for the bias? So ~7% higher than these voltages you’re saying? http://www.orangefieldguide.com/OFG_SCH ... hem_72.gif

[fiveightandten]

Yeah I’m certain it was that high which does (and did at the time) seem extreme, which is why I’ve been running it exclusively through a variac to keep that down to 450V DC. But I’m gonna to dig into this more.

What input voltage are you running the amp at, and what PT primary tap is it running on?

but do you have any recommendations on finding a datasheet to determine if the secondary on the PT is wired correctly?

If any of the secondary connections were wired incorrectly, you'd see smoke very fast. What you should be concerned with is the primary wiring.

There's no data sheet needed, you have the schematic. There's a neutral connection and 4 primary hot taps.

Also are the voltages at the secondary on this schematic correct? 190V, then 315V to the heaters and 36V for the bias? So ~7% higher than these voltages you’re saying? http://www.orangefieldguide.com/OFG_SCH ... hem_72.gif

The voltages on the schematic are correct. Have you worked on a guitar amp before? It's 3.15 VAC supplied from each heater leg, not 315V. Pretty much every guitar amp in existence has 6.3VAC heaters. I want to stress again that this amp can kill you if you touch the wrong thing, even when unplugged.

190VAC on each leg of the HT secondary for 380VAC, through the rectifier 380VAC x 1.3 rectifier multiplier = 494 VDC

115VAC input ramped to 380VAC = 3.3 PT multiplier. Your wall voltage is probably more like 123VAC.
123VAC x 3.3 = 406VAC x 1.3 = 528VDC, or about 7% higher than spec.

Though, we have no idea what input voltage you're running at with the amp hooked up to a Variac. Only you know that.

-Nick

[fogwolf]

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.

I thought you meant the secondary, yes the primary wiring is clear from the schematic, thanks.

And I have not done much work on amps, just this one. I’ve built several modular synths from scratch including the power supplies and distribution. Also a few rackmount effects and power supplies in those as well. So I definitely get it, the voltages and current in a tube amp are a whole other ballgame and to make sure the caps are discharged before poking around even if it’s been off a while. But no, tube amp circuits are not what I know the most about but I know a lot about electronics and know enough to respect what’s needed to juice the tubes in this amp.

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.

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.

Thanks again I really appreciate it.