Bogen AP-60 found in the dust

[GrandDesigns]

I found a Bogen AP-60 integrated amplifer that's been in storage for a while. It's filthy and I've been cleaning off the accumulated dust and grit. I have not attempted to plug it in or turn it on. I saw this post about another AP-60 and what steps to take to see if it had any life, and it mentions using a variac and deoxit. I can clean the potentiometers and switches on this one, but my main questions are more specific:

1. This looks to have tubes of: 2x 7247, 3x 12AV7/ECC83, and 4x 7355. Replacing those new could cost around $300. How likely is it the tubes are bad?
2. Should I attempt to get the tubes tested locally before powering them? There is a local electronics store near me that has a tube tester.
2. How do I remove the large capacitor cans? They're riveted in. Drill out the rivets?
3. Should I just go ahead and replace all of the components that are likely to fail? This ebay seller sells a $150 kit of capacitors, diodes, thermistor, fuse, and directions. I suspect this is $50 of parts but it would save the hasssle of sourcing everything piece by piece. I don't mind the time and expense of replacing these although I realize there's no guarantee the thing will work trouble-free even with new parts. So it's a risk.
4. Is this Bogen amplifier worth $450 in tubes, capacitors, and other components? I realize that is somewhat subjective, however Bogen is not one of the more well known tube amp brands.

I've never worked on anything of this vintage before. Wow, the assembly of things is super sketchy. No wonder circuit boards became the norm!

 

[TriodeLuvr]

I don't have first hand knowledge regarding performance of that specific amp, but the output transformers look pretty healthy. Also, Bogen's early tube products (prior to focussing on PA systems) were well regarded. A few comments on the amp and your questions...

First, I doubt you could replace all those tubes with NOS for $300. A matched quad of 7355 alone (if you could find them) is likely to cost $200 to $300. Regarding current condition, the photo you posted appears to show three of those tubes with the silvering turned white. If that's the case, they're defective and not useable. The least costly means of replacing them will be conversion to 6P3S-E tubes (a 6L6 variant).

Second, there have been many complaints posted in the various audio forums about eBay sellers with kits of restoration parts. Sometimes the quality is sub-par, sometimes the parts are the wrong value, and sometimes they don't physically fit. The sellers have also frequently been called out as being unhelpful, if not downright arrogant and condescending, when problems with the parts were pointed out. Caution is advised.

Finally (and this is why I seldom attempt to restore vintage preamps or integrated amps), 60-year-old controls and switches often fail from age and use, but the original parts are no longer available. Modern hand-made replacements have appeared on eBay from time to time, but those have also garnered their fair share of complaints.

Yes, this amplifier is worth restoring, but you might need to compromise when it comes to fully duplicating its original functionality. For example, a defective volume control might need to be replaced with one that only has three terminals (no tap), but that will usually mean losing the functionality of the loudness contour switch. Bass and treble controls can be difficult too, due to their typically high resistance values (lower values are more common in solid state gear) and physical dimensions. All this aside, you won't know for sure just what's involved until critical capacitors have been replaced and the unit is more or less operational. Sometimes you get lucky, and it turns out the unit saw little use before it was stored in favor of a new solid state amplifier. A unit like that can reward you with decades of excellent quality audio.

Jack

 

[w3jn]

1, unlikely
2, I'd try the thing out first
2a, they're twist-lock. There are four little terminals on the outside perimeter that are twisted to lock the cap to the phenolic. Unsolder any wires attached to them, twist the terminals so they line up with the slots in the phenolic, then just pull them out.
3, no. The cans and the orange paper-encased electrolytics should be replaced but beyond that, standard troubleshooting applies. You risk damage by wholesale replacing parts that aren't bad,
4, in my experience, Bogen stuff sounds great. I have a RP40 receiver (same tube complement, it just has a tuner added) and a DB230 - great amps.

 

[zackthedog]

I have to agree with Jack. Three of the output tubes look DOA to me. The last thing you want to do is damage the output transformers with shorted tubes. The preamp tubes are probably acceptable, they tended to last a long time, but ultimately the amp will sound much better if they are replaced. I think the amp is worth restoring, but I would pro-forma replace all can and coupling caps, and check as many resistor values as you can.

And as Jack also mentions, that eBay seller has a very poor reputation. He charges a lot of money for a few dollars worth of parts, and often supplies the wrong ones to boot. If you want to take on this project, get your hands on a schematic and order the parts yourself. In the end you'll have a nice vintage integrated.

 

[gadget73]

I wouldn't spend money on tubes without testing them. The outputs might be done for but the rest stand a high chance of being good.

The twist lock caps have some options. Some folks re-stuff, I personally hate everything about the process and have given up on it. For me there are two practical options on this one. Hayseed Hamfest can make you new cans that drop right in. If you aren't picky about it looking original, you can get a printed board that will replace that oval mounting washer the existing cans are on that let you mount modern caps on. They are fairly cheap and it lets you use modern inexpensive caps but it looks like a circuit board with caps stuck to it.




to give you an idea.

I generally will replace any wax dipped caps. I can't quite tell if these are waxed paper caps or the epoxy sealed type though. The ones with hard epoxy ends in that ceramic tube are typically a mylar film type. They rarely go leaky, but sometimes they will fail open circuit if physically disturbed. If you can carefully poke at the end and see if its hard or mushy that should answer the question. If its mushy, they go. If its hard I'd maybe leave them initially and change if needed.

and yeah Bogen made good stuff. They aren't so well known in the hifi world but it wasn't crap by any stretch and they had capable engineers building stuff. Its often a little weird in some way but not bad-weird.

 

[GrandDesigns]

Thanks everyone for the replies. Here's what I got from the info:

1. The 7355 tubes are the big expense, and mine are questionable and should be replaced. All 4 of these are different makes. The glass pulled off the base on one of them. The health of output transformers should not be risked by using the existing tubes. I see that the 7355's are out of stock for most sellers. A matched quad could cost as much as $300. I did find this place offering a $29 price per tube, however these are not value 'matched', if that matters as it does with transistors:

https://www.radiodaze.com/new-old-stock-amperex-7355-vacuum-tube-item-rdw-293

2. The preamp tubes could be replaced for better audio fidelity.

3. It is cheaper and safer to source and replace the capacitor cans and orange paper-encased electrolytic capacitors and avoid the ebay seller. Diodes, resistors, and other capacitors can be tested and replaced if needed. I have a Fluke 115 multimeter that will test capacitance, diodes, and resistance, and since I don't have to desolder things from a circuit board, I'm thinking it shouldn't take as long to test things that require removal.

4. This AP-60 would be worth working on.

 

[mrwood200]

4. Is this Bogen amplifier ....

Is this missing the left 'speaker indicator' on the front panel?
For me, I would look for another project. Bogen offers quality equipment & this face plate looks good, but rust/3 power tubes?/no case/I don't need another receiver/$$ to fix/etc. would have me looking elsewhere.

 

[GrandDesigns]

Is this missing the left 'speaker indicator' on the front panel?
For me, I would look for another project. Bogen offers quality equipment & this face plate looks good, but rust/3 power tubes?/no case/I don't need another receiver/$$ to fix/etc. would have me looking elsewhere.

The plastic speaker indicator fell off and was inside the case. When I removed the front panel, the other matching plastic piece also fell off. They were glued to the front aluminum panel and it seems to be common that they fall off. These would be easy to glue the back on; probably the easiest repair of the project

I do have the outer metal/top cover. Yes, the steel body does have surface rust. This may have been stored indoors, but not in a clean or climate controlled place, and it shows. The cosmetics on the inside are not great.

Yes, it looks like I'd have to put $300 to $400 into this thing at least, and my electonics troubleshooting skills are not good. That adds to the risk that I'd dump money into this with little to show for it.

 

[mrwood200]

Great - glad you got the part & case. Rust is not a deal breaker, especially if you already own it!
Of course, your call on $$ fix. If this is a hobby for you and your going have some fun & learn, price is not so bad.
I was mistaken earlier; this is an integrate amp, not a receiver. Should be an easier fix. If you take your time & pay attention to safety, you can be successful.

 

[TriodeLuvr]

Finally (and this is why I seldom attempt to restore vintage preamps or integrated amps), 60-year-old controls and switches often fail from age and use, but the original parts are no longer available. Modern hand-made replacements have appeared on eBay from time to time, but those have also garnered their fair share of complaints.

There's a new post here on AK about this exact problem:

Replacement Control - Bad Seller

Jack

 

[Dswankey]

1. This looks to have tubes of: 2x 7247, 3x 12AV7/ECC83, and 4x 7355. Replacing those new could cost around $300. How likely is it the tubes are bad?

Those would be 12AX7/ECC83 not 12V7 and there should be 6 of them not 3

Amplifier AP-60 Ampl/Mixer Bogen -Presto, David Co., Inc.; New York |Radiomuseum.org

Amplifier AP-60 Ampl/Mixer Bogen -Presto, David Co., Inc.; New York, NY /, build 1963 ?, 1 pictures, 1 schematics, 12 tubes, United States of America

www.radiomuseum.org

2. Should I attempt to get the tubes tested locally, There is a local electronics store near me that has a tube tester..... The preamp tubes could be replaced for better audio fidelity.

If you have access to a tube tester by all means test all of those 12AX7's, as @Gadget mentioned they do stand a high chance of being good and usable and I'd wager that if so their audio fidelity would be equal if not better then new tubes.

This would save you a good amount of money as opposed to buying either NOS or new 12AX7's if you were to fully restore the amp if not then the worst case scenario is you would have 6 desirable pre amp tubes for the future.

1. The 7355 tubes are the big expense, and mine are questionable and should be replaced.

Not questionable, 3 of them look to be shot, that whiteness you're seeing on the tops indicate there is no vacuum left IE they've leaked

 

[WopOnTour]

That's a cool amp with LOTS of tubes, I'd be interested in how it's laid out (at least 3 preamp gain stages, likely a phono-stage, a 7247 driver and pair of 7355 per channel it appears) Plus some pretty beeft iron!
Bogen had some nice topographies for 6L6 based pentodes throughout the 50-60s, these being just a bit later at a time when the 7355s appeared, that were considered less expensive and more efficient option. Too bad they are so spendy these days.
Looks like there is a downloadable SAMS available for the AP60
https://www.samswebsite.com/en/photofact/details/index/id/30621
Looks to me like a great project that will likely sound great! Please keep the posts coming!
WOT

 

[nerdorama]

I have an AP-60. Check the output transformers for continuity before spending any money. They are a high impedance at 10K primary AND they have a cathode feedback winding. On my unit both OPT's have an open half primary. Maybe it isn't a generic failure but it could be that the transformers are somewhat fragile and I think the 7355's get run pretty hard in this amp. If the outputs are bad at least you can make a decision before spending money. I have a copy of the SAM's if you want it. Send me a PM and we can arrange for it.
John

 

[gadget73]

I have a DS-225 with similar type transformers, one half of the primary is open on one. Not sure if its the same transformer though. Whenever I get around to it, the plan is to pull the dead and the still OK transformer off, install some 6.6k transformers and re-wire it to take a 6L6 family tube. The voltages are nearly perfect for datasheet 6L6 operation and the front end should have plenty of voltage swing capacity. I really don't know what else to do with what is basically just a chassis and a power supply.

 

[nerdorama]

Whenever I get around to it, the plan is to pull the dead and the still OK transformer off, install some 6.6k transformers and re-wire it to take a 6L6 family tube.

That's what I've been thinking and I have some outputs that would work but probably not the same bolt footprint. Oh well what are you supposed to do if it's an anchor otherwise. It's a cool looking amp so would be a shame to part it.

 

[gadget73]

well, thats half the plan anyway. The full plan is take my good transformers off the crusty DS-225 and put them on the nice chassis with bad iron. Make that go as what it is. Take the crusty one and make it into a 6L6 amp since I would then be able to paint it without losing anything important. Just need to find something that fits physically.

 

[AlexHempel]

NOS 7355s are $23 at vacuumtubes.net and $30 at vacuumtubesinc.com. I wouldn't convert it.

Looking at the schematic, doesn't it have fixed bias with balance adjustment? No need to waste money on some overpriced matched quad.

 

[TriodeLuvr]

NOS 7355s are $23 at vacuumtubes.net and $30 at vacuumtubesinc.com.

FWIW, vacuumtubes.net has tubes listed which I know are not actually available. In one case, I bought them all (not 7355). The other vendor might be in the same situation for this particular tube. It's always necessary to call first. I agree though that availability for $30 each or less renders conversion unnecessary.

Jack

 

[AlexHempel]

That's a good point -- I have run into that situation with both those vendors, so worth checking. It's the price you pay for their larger selection and lower prices vs. the more consumer-oriented tube sellers.

 

[GrandDesigns]

I have tested the output transformers.

On the secondary side of the transformers, I'm getting good continuity and under 1 ohm between each of the 4 taps (green, orange, yellow, and black-yellow) and the black ground. The concern is the black-yellow is 0.2 ohms and the schematic shows 2.9. Both transformers are reading the same. Is this is a concern?

On the primary side, things are less clear because the wire colors are very faded. I can make out the striped ones, but not the solid colors. I am getting continuity and some readings in the ~120 ohm range and also some in the 7 ohm range which do correspond with the listed values on the schematic. .

If an output transformer is bad, what should I be looking for other than an open circuit or a short, which I assume would be very low resistance where there should be 6.5 or 115 ohms?

One output transformer schematic image attached. The wire colors are the same on both tranformers.