Hi DIYAudio'ers!
Had a new baby hit my bench today. A 60's era Packard Bell DPA-30-rev4.
I'll be rejuvenating this over the next couple weeks. Currently it turns on and produces sound after a complete tube replacement. The "Packard Bell" branded 6BQ5's and 7521's are currently spending some quality time in my Vox AC30 while the DPA has some gooch-apathethic JJ's holding their place.
I've never really worked on a stereo amp before so I though it might be appropriate to open an account so I can pick some internet brains.
What a mess!
Phase 1: Add a grounded plug and a fuse.
Phase 2: Replace the caps. Which brings up a question....
I'm going to order a direct replacement for the canister-multicap and cathode bias cap. When I look at the PI->Grid coupling caps, I see....
I was going to order some SoZo blue-molded reissues, but then saw the thread "Packard Bell Tube Amp" , where it appears that they were replaced with orange-drops. Is there any reason to favor one over the other?
The next thing I noticed when looking at the output....
1: I'm not reading a vdrop from pin-8 on the rectifier to the plate of two of the tubes. On the two tubes that I do get a reading on, I see a power dissipation of 10 & 11 watts, which seems extremely high... I kind of expected 6-7 watts. I checked the 120ohm/5w/10% cathode bias resistor and it's reading 129ohm. Everything looks right except the dissipation.
Am I measuring incorrectly, or am I somehow seeing the vdrop for both tubes on the plate of a single? I had assumed that each side was like a push/pull guitar amp + negative feedback.
I've attached a very close schem (DPA-30-rev3) in case anyone needs it.
Had a new baby hit my bench today. A 60's era Packard Bell DPA-30-rev4.
I'll be rejuvenating this over the next couple weeks. Currently it turns on and produces sound after a complete tube replacement. The "Packard Bell" branded 6BQ5's and 7521's are currently spending some quality time in my Vox AC30 while the DPA has some gooch-apathethic JJ's holding their place.
I've never really worked on a stereo amp before so I though it might be appropriate to open an account so I can pick some internet brains.
What a mess!
Phase 1: Add a grounded plug and a fuse.
Phase 2: Replace the caps. Which brings up a question....
I'm going to order a direct replacement for the canister-multicap and cathode bias cap. When I look at the PI->Grid coupling caps, I see....
I was going to order some SoZo blue-molded reissues, but then saw the thread "Packard Bell Tube Amp" , where it appears that they were replaced with orange-drops. Is there any reason to favor one over the other?
The next thing I noticed when looking at the output....
1: I'm not reading a vdrop from pin-8 on the rectifier to the plate of two of the tubes. On the two tubes that I do get a reading on, I see a power dissipation of 10 & 11 watts, which seems extremely high... I kind of expected 6-7 watts. I checked the 120ohm/5w/10% cathode bias resistor and it's reading 129ohm. Everything looks right except the dissipation.
Am I measuring incorrectly, or am I somehow seeing the vdrop for both tubes on the plate of a single? I had assumed that each side was like a push/pull guitar amp + negative feedback.
I've attached a very close schem (DPA-30-rev3) in case anyone needs it.
Attachments
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The old Black Beauty style coupling caps became leaky over time (really bad in that kind of application).
It may be that those 'Blue Beauty' coupling caps have the same problem because they probably use the same kind of dielectric . . . paper.
Use a modern plastic dielectric cap in that case, and it is a really good idea to use 600V or 630V rated ones.
It may be that those 'Blue Beauty' coupling caps have the same problem because they probably use the same kind of dielectric . . . paper.
Use a modern plastic dielectric cap in that case, and it is a really good idea to use 600V or 630V rated ones.
Thanks for the tip. Parts are ordered. Just waiting on the mail man.
Side note, it does not appear that Sozo makes a >=600v blue-molded reissue. I only see <=500v, and am guessing they're mainly for fixing vintage Fender/Guitar amps.
The replacement parts are starting to roll in...
First question:
The 40/40/20/20 @ 450v multicap replacement arrived. It appears to be rated at 525v. My limited education and intuition tells me it's almost always OK to use a higher-rated voltage capacitor in power filtering applications.
Is that correct?
First question:
The 40/40/20/20 @ 450v multicap replacement arrived. It appears to be rated at 525v. My limited education and intuition tells me it's almost always OK to use a higher-rated voltage capacitor in power filtering applications.
Is that correct?
This is the kind of amplifier, or one very similar to it, that Bob Carver (audioshopper on eBay) would buy there. Do a little work on it along with a sloppy fire engine red paint job and sell it on eBay. He'd hype it up big time like only Carver can, even invoking Stewart Hegeman's name to intimate he had something to do with it, and then get stupid money for it from gullible buyers. I witnessed this at least twice on eBay several years ago. Now I'm not saying the amp isn't good when fixed up. Only that there's nothing special about it, even from BC.
Indeed.
As a guitarist, the old 60's era tubes that came with it were infinitely more interesting to me than the amp itself. Finding vintage parts for my (metaphorical) time machines has been hard lately, and I've had a great deal of fun working with them this past month.
This amp rejuvenation is strictly for swank-points, casual listening, and self interest. I'll probably round out the mediocrity with some Dayton drivers.
When I thirst for true aural fidelity, I plug my 76' Les Paul into my AC30 or JMP50 so I can drink from the tap.
Funny aside... The only Carver amp I ever owned was a P-1250. Bought it used in ~95'. The only two notable things that I remember is that it would smoke underrated extension cords and that it advertised a "Magenetic Field Transformer"..... As opposed to all of those transformers out there that don't leverage the properties of electromagnetism. Aside from that, it drove our subs without issue for several years.
;-)
Next question... The PI->Power coupling caps arrived. They do not appear to be polar like the Blue Beauties. How do I go about orienting them? Is it:
1: It doesn't matter?
2: Looking at the lettering, the right is +, the left is -?
3: They're not polarized, but the outer foil is on the left of the lettering?
4: Dig out to oscilloscope, set it to < 5mv/div, and figure it by touching the cap and looking for hum?
1: It doesn't matter?
2: Looking at the lettering, the right is +, the left is -?
3: They're not polarized, but the outer foil is on the left of the lettering?
4: Dig out to oscilloscope, set it to < 5mv/div, and figure it by touching the cap and looking for hum?
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Indeed. It appears that the labeling is inconsistent.
Moments ago a had a Macgyver moment..
Rather than dig up the oscilloscope, I attached the caps via test leads to one of my guitar amps and pinched the cap with my fingers, then switched orientation. When I found the orientation with the loudest hum during the pinch, I know that the outer foil is attached to the tip of the guitar lead. I used a Sharpe to mark the orientation.
Moments ago a had a Macgyver moment..
Rather than dig up the oscilloscope, I attached the caps via test leads to one of my guitar amps and pinched the cap with my fingers, then switched orientation. When I found the orientation with the loudest hum during the pinch, I know that the outer foil is attached to the tip of the guitar lead. I used a Sharpe to mark the orientation.
I remember those! I always thought it weird how arbitrarily some parts were removed for painting and others were not. I also found it slightly disturbing that those phenolic terminal strips at (what I assume to be) the outputs were covered in metallic paint. Not to diss his efforts, but considering the source I expected a bit more class - even if only in the form of masking tape.
KamaK, you're getting some good advice and appear to be doing a good job. Thanks for posting; it's inspiring me to get off the couch and build!
Attachments
Quick update...
Caps are in. Fuse holder replaces aux AC socket. 3-prong AC cord is temp'ed in till I find a suitable strain-relief.
Everything seems to work well. Still lots to clean up.
On a funny note... I noticed that the 5v heater lines run past two auxiliary "lamp" ports...
Hilariously, the ports are RCA-looking-type with a tiny size difference, which could result in someone that doesn't know better plugging their audio device (or speakers) into them.... I'll be removing these.
As if that's not bad enough, the external speaker connectors look like this....
Will be replacing them with banana posts.
Caps are in. Fuse holder replaces aux AC socket. 3-prong AC cord is temp'ed in till I find a suitable strain-relief.
Everything seems to work well. Still lots to clean up.
On a funny note... I noticed that the 5v heater lines run past two auxiliary "lamp" ports...
Hilariously, the ports are RCA-looking-type with a tiny size difference, which could result in someone that doesn't know better plugging their audio device (or speakers) into them.... I'll be removing these.
As if that's not bad enough, the external speaker connectors look like this....
Will be replacing them with banana posts.
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