Packard Bell DPA30 vs DPA20?

I wondered about that strip on the front between the 2 OPTs. Only the 20s have that my 30s don't. Interesting, so you can strap the two channels together?!
 
I wondered about that strip on the front between the 2 OPTs. Only the 20s have that my 30s don't. Interesting, so you can strap the two channels together?!

Yeah. The first time I fired one up (variac) I had just one channel hooked up and then removed the jumper and returned it and heard/saw the double ...

I first thought It was a three channel... but now I am sure it is a "doubler option" ... weird but cool.
 
Are the revisions on Sams photo-facts ? Where did you get those prints? I have an original Photo fact, without any revisions... hmm

I copied em from the web somewhere...don't remember.
Probably more threads here on AK about these amps too.
 
honestly I'm more than a little tempted to try the zener diode thing on my Fisher 600. I can't EFB it since it uses two tubes as the cathode resistor, but I can strap a zener across it to keep the bias fixed. Maybe I'll throw a diode on my next parts order and clip it in as an experiment. At worst, it will be a horrible failure and we'll never speak of this again.
What is the zener thing? The resolution is bad and i can't make out the details.
 
What is the zener thing? The resolution is bad and i can't make out the details.

As I recall, it's Bob Carver's version of a DC Restoration circuit which was featured in his modded version of the Packard Bell amp, examples of which sold for silly prices some years back. Carver's claims and description of the amp would have made P.T. Barnum proud! Although there seems to be no direct evidence that Barnum is actually the source of the famous phrase attributed to him. ("There's a sucker born every minute.")

Retrovert posted a wonderfully detailed investigation of DC Restoration a while back. It was a group effort with other posters, Gadget in particular, measuring its effects. Here's the link: http://www.audiokarma.org/forums/in...ng-distortion-simulation-and-analysis.742318/

The last few pages veer off topic as these things often do.

I'm not a techie at all but after seeing this current thread I scanned through Retrovert's thread and my takeaway was that:

1. any positive effects are most evident in a situation where the amp is being pushed to its maximum output limit, as is most often the case with a guitar or bass (instrument) amp. Since audio amps are rarely, if ever, operated at these extremes, the mod is essentially useless in these applications.

2. the diode mod had essentially no effect on a well designed audio amp. Measurements were, in fact, slightly worse with the mod than without it.

3. in one case, the use of a slightly larger than normal value of grid stopper on the output tubes (1k, rather than a more typical value of a couple hundred ohms) not only eliminated blocking distortion but offered additional benefits which the diode mod did not provide.

Since I'm not very technically oriented, it's entirely possible that I've misinterpreted the results. If so, I'm sure someone will correct me.
 
On this particular amp its not for DC restoration though, it forms a fixed bias arrangement. DC restoration is a different thing, you'd see a diode on the grid rather than on the cathode. Basically what the zener does is sub for the cathode resistor. Instead of voltage being variable based on how much current flow is happening, voltage ends up fixed to whatever voltage the zener diode is.

Honestly I don;t think its the worst idea, but its not something I've ever measured the performance of. I have suspicions of what it would do, and at some point I mean to test it out. The amp I want to test it on needs some TLC first though. It forgot the words so its about time to replace the original filter caps.
 
Well, it was Carver who called it DC Restoration. Perhaps he didn't understand what DC Restoration actually is. I was so astounded by the prices people were paying for these things, as well as Carver's preposterous claims (others would probably call it "marketing prowess") that I saved screenshots of the auctions.

Here's an answer he wrote to a question about why he used a zener with this amp rather than a 6AL5 as he apparently had done on some other amp:

"The 6AL5 is best used for fixed bias amplifiers such as my big high power amps. On the other hand, the zener diode is is best for an auto-biased amplifier because it is the increased voltage drop across the cathode resistor during hi-drive that generates the need for DC restoration. As long as the shift is smooth and has no induced non-linearities, the sound will be the same. I carefully made certain of that when I designed the DC restorer for this amplifier . . ."

And, in response to another question, he wrote: "the DC restorer circuit works to keep the DC value of the drive signal at exactly the correct value over the entire input signal. In all auto-bias circuits, the Achilles heel is that the grid bias increase substantially when the output stage is driven hard. For example, suppose the grid bias is -12 volts at no signal. When driven close to its maximum output, the bias will change several volts, becoming more negative by roughly four to six volts. This drives the tubes closer to cut-off and causes distortion during high output demands. In this DC restorer circuit, a 12 volt zener is connected to the cathode bias resistor and fed back to the grids such that the grid bias is held at a constant 12 volts, thereby restoring the lost DC component of the drive signal. This is regardless of the cathode current that causes a voltage drop across that cathode resistor! 'viola! No crossover distortion."

I think the key phrase here is "when driven close to its maximum output". So, unless you run the piss out of the amp, it would likely have little, if any, effect.

I would also note that he is apparently biasing the amp differently from stock. The Sams says the cathode voltage is 10v and he is biasing his at 12v or a bit higher if there is also a resistor involved. Might that, in itself, account for the lower distortion which he claims? I don't know, as I'm not much of a techie.

It would seem you could also use combination bias to accomplish the same thing by using a 9v lithium battery on the grid and a low value resistor on the cathode. Correct? The battery would be more expensive and take up more space, however. I guess the only advantage would be if there are any reliability issues with zeners. I know nothing about them so I don't know. I do use battery grid bias in one amp I built.
 
potato, potahto eh? :) Hey if it works it works. Its certainly easy enough to try. I just don't happen to have a fully working cathode bias amp and a suitable zener diode hanging around to try it with right now or I'd do so.

Battery on the grid would just be fixed bias. Effectively the same as a grid supply, but with zero noise from the supply. Probably better on SE designs where you don't have the advantage of common mode noise cancellation. With the near-zero current drawn the battery would last basically forever too. Mixing grid and cathode bias is also not anything terribly unusual. Hafler did research on that idea, which is part of why the Dynaco amps have those small value cathode resistors. The fact that it makes a convenient way of measuring current through a tube is another bonus but his logic was that it reduced distortion by a measurable amount.

Notice the plate voltage is higher as well, so the cathode voltage being higher isn't too unusual. ~300v on the plates and ~12v of bias is basically datasheet conditions. Looks like there is a resistor there across the diodes to carry some of the load, which is sensible. If i ever do this with my Fisher it will be basically the same arrangement. The tube heaters will form the resistor, the zener would simply sit across them. I'll give it a shot one of these days, at worst it will be a failed experiment and I'll learn something for the buck or so the diodes will cost.
 
Great info. I restored one of these a while back and was wondering about the "DC Restoration" circuit. I will say that this is a sweet sounding little amp when everything is in spec. I think I had almost $300 in mine when I was done though, so I might just WANT it to sound good ;)
 
Great info. I restored one of these a while back and was wondering about the "DC Restoration" circuit. I will say that this is a sweet sounding little amp when everything is in spec. I think I had almost $300 in mine when I was done though, so I might just WANT it to sound good ;)

Did you do the so-called "DC restoration mod"?

I have one of these as well, a DP-20. I rebuilt it but I can't say I've listened to it much. It's been sitting for years so I should put it on the variac and then give it a fresh listen.

Looking at Carver's auction pics it seems like most of the original parts were left in place. This is the pic of the guts that I saved from his auction. Clicking on it will get you more detail.

Carver Modded Packard Bell Guts.jpg

The original can cap is still in place. All he did was to parallel a larger value cap with one of the sections. Only a few of the original CC resistors were replaced. The original dull brown "dog turd" coupling caps are still there. The big red tubular electrolytic is still there. I don't know about your experience, but I've never once found any of the tubular lytics to be in spec and many are totally dead, especially the low voltage ones. The view of his "DC restorer" is unclear.

This amp sold for $1075 plus shipping!
 
If I'm not mistaken some of those units came with the covers?. I saw one at my buddies shop a few years back and it had them too.
I own 3, and have seen dozens online . I haven't seen any with opt covers... not sayin they don't exist... just haven't seen one.
 
well its a good circuit, so as long as the transformers aren't junk it should sound good.

Oh, the output transformers are the most magical part of the amp! According to Bob, their bandwidth is "so good it was difficult to believe".

In his auction description he mentions that his heroes are Albert Einstein, Mahatma Gandhi and Stu Hegeman. And, after much investigation, he concludes that one of them designed those transformers and, perhaps the whole amp.

Carver describes how he spent hours studying schematics and talking to people at Packard Bell. They had no info on the amp but they gave him the name of a guy who, when he was young, once worked there briefly as an apprentice. The guy told Bob that he vaguely remembered some old guy who was supposedly an outside consultant of some kind who designed amplifiers. This old guy would visit the factory on occasion. That was all he remembered. Bob asked him if he had ever heard of Citation amps. The former apprentice said the name sounded familiar, but that was all.

Carver then states, "That was enough for me! Taking all the evidence together, I am now virtually certain this output transformer, if not the whole amplifier, WAS designed by Stu."

I bet the kid had heard about the Theory of Relativity too so I think it's just as likely that it was Albert who designed them. I'm ruling Gandhi out, though.

He included answers to some questions about the OTs in the auction. Again, click to enlarge.

Carver on P-B OTs.jpg

If anyone has the ability to measure them I'd be really interested in seeing if their results are similar to his and, especially, how they compare to transformers used in similar console amps. It's hard to believe that there could be that much difference in their construction or performance compared to other similar transformers.

The Sams lists a Packard-Bell part number and an equivalent replacement made by Ram. Since I doubt P-B made their own transformers, I speculate that Ram was the OEM. A quick search indicates that there is a Ram Transformer Co. located in New Rochelle, N.Y. but that they were founded in 1995. Perhaps they are a re-organized version of an older company??

The Sams lists the Primary as being 5.5k CT and the Secondary as 3-4 ohms. Carver states that "the ohm rating on this amp ranges anywhere between 4 to 16 ohms" - so as not to discourage potential buyers, I'm sure. He does not specify the load that was used to take measurements but using 8 or 16 ohm speakers would affect the amp's performance, I assume. Perhaps someone with more technical knowledge could comment.

Carver is clearly a marketing genius. Take a fairly typical console amp, replace a few parts leaving most of the original ones in place, do an "exotic mod" that might cost a dollar or two, spray a couple coats of bright red paint on it and sell it for $1075.00! That shouldn't surprise us at all considering that it was designed by Stu Hegeman and the few parts that were replaced were actually soldered into place by the famous Bob Carver himself.

Never mind that the mod might not actually do much of anything unless you turn the amp up all the way. You DO listen to your amps with them turned up all the way don't you? No? Me neither. I'd be interested in seeing if there is any measurable difference at, say 3/4 power, which is still probably higher than most people typically use.

OK, he does say he made a few other changes, although some of them are difficult to confirm in the pic, which I'll post again for easier reference:

Carver Modded Packard Bell Guts.jpg

"I increased the open loop gain". Is that the gray cathode bypass cap he apparently added to the 12AX7s on pin 8?

"I changed the feedback". Hard to tell.

"I have regulated the screen voltage". I see absolutely no evidence of that. There is a (1000pf) ceramic cap between the plate (pin 7) and the screen (pin 9) of the 6BQ5s, exactly as shown on the Sams schematic. Nothing more that I can see.

"Beyond that I've given it every de-lux circuit feature known to Man, Woman, or Minor Wizard."

Perhaps some of the Minor Wizards in residence here will weigh in on this. I have not ascended to that status yet and I doubt I ever will.
 
Oh, the output transformers are the most magical part of the amp! According to Bob, their bandwidth is "so good it was difficult to believe".

In his auction description he mentions that his heroes are Albert Einstein, Mahatma Gandhi and Stu Hegeman. And, after much investigation, he concludes that one of them designed those transformers and, perhaps the whole amp.

Carver describes how he spent hours studying schematics and talking to people at Packard Bell. They had no info on the amp but they gave him the name of a guy who, when he was young, once worked there briefly as an apprentice. The guy told Bob that he vaguely remembered some old guy who was supposedly an outside consultant of some kind who designed amplifiers. This old guy would visit the factory on occasion. That was all he remembered. Bob asked him if he had ever heard of Citation amps. The former apprentice said the name sounded familiar, but that was all.

Carver then states, "That was enough for me! Taking all the evidence together, I am now virtually certain this output transformer, if not the whole amplifier, WAS designed by Stu."

I bet the kid had heard about the Theory of Relativity too so I think it's just as likely that it was Albert who designed them. I'm ruling Gandhi out, though.

He included answers to some questions about the OTs in the auction. Again, click to enlarge.

View attachment 1267360

If anyone has the ability to measure them I'd be really interested in seeing if their results are similar to his and, especially, how they compare to transformers used in similar console amps. It's hard to believe that there could be that much difference in their construction or performance compared to other similar transformers.

The Sams lists a Packard-Bell part number and an equivalent replacement made by Ram. Since I doubt P-B made their own transformers, I speculate that Ram was the OEM. A quick search indicates that there is a Ram Transformer Co. located in New Rochelle, N.Y. but that they were founded in 1995. Perhaps they are a re-organized version of an older company??

The Sams lists the Primary as being 5.5k CT and the Secondary as 3-4 ohms. Carver states that "the ohm rating on this amp ranges anywhere between 4 to 16 ohms" - so as not to discourage potential buyers, I'm sure. He does not specify the load that was used to take measurements but using 8 or 16 ohm speakers would affect the amp's performance, I assume. Perhaps someone with more technical knowledge could comment.

Carver is clearly a marketing genius. Take a fairly typical console amp, replace a few parts leaving most of the original ones in place, do an "exotic mod" that might cost a dollar or two, spray a couple coats of bright red paint on it and sell it for $1075.00! That shouldn't surprise us at all considering that it was designed by Stu Hegeman and the few parts that were replaced were actually soldered into place by the famous Bob Carver himself.

Never mind that the mod might not actually do much of anything unless you turn the amp up all the way. You DO listen to your amps with them turned up all the way don't you? No? Me neither. I'd be interested in seeing if there is any measurable difference at, say 3/4 power, which is still probably higher than most people typically use.

OK, he does say he made a few other changes, although some of them are difficult to confirm in the pic, which I'll post again for easier reference:

View attachment 1267369

"I increased the open loop gain". Is that the gray cathode bypass cap he apparently added to the 12AX7s on pin 8?

"I changed the feedback". Hard to tell.

"I have regulated the screen voltage". I see absolutely no evidence of that. There is a (1000pf) ceramic cap between the plate (pin 7) and the screen (pin 9) of the 6BQ5s, exactly as shown on the Sams schematic. Nothing more that I can see.

"Beyond that I've given it every de-lux circuit feature known to Man, Woman, or Minor Wizard."

Perhaps some of the Minor Wizards in residence here will weigh in on this. I have not ascended to that status yet and I doubt I ever will.

I like this evidence-less, speculation of marketing nefariousness, snake oil suspicion...
I think that the truth is a moving target on this one. Perhaps the proof is in the pudding, but like so much in audio, proof and opinion are blended together in taste.

One thing that keeps creeping into my tiny little sleuthing mind though... is why would a prominent and wealthy audio engineer of famed status stoop to "huckster" levels for a thousand bucks?

I admit that I have not measured distortion of either of my Dpa-20 or Dpa-30, but mostly because I don't expect to get better metrics than a similar Maggie or any other console amp. Maybe it was designed by Stu... don't know for sure. But it is certainly a good design and easy to work on. I personally am impressed with the build quality of the Dpa-20 and the potted power tranny . I am impressed with the old PB caps, and the layout. I know that I'd rather play with these instead of any of the 300-400 dollar tube amps being slapped together overseas and being touted as high quality online .

Question Carvers claims? Sure, just like today's specs and measuring techniques. Is advertising a dark art ? Yep. And audio is realm of grey area opinions and colorful terminology. It's great! :biggrin:
 
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