Discussion in 'Tube Audio' started by ferninando, Mar 26, 2015.
whats the diff between a DPA30 and DPA 20 Packead Bell consol amp?
I have seen them both but can't remember which is which. I think one runs four 6BQ5 tubes push/pull each channel, and the other one runs only two 6BQ5's single ended each channel.
Don't quote me on that 'cause now I'm going to have to look them up.
its 10 better obviously
honestly, don't know.
The DPA 20 & 30 have the same tube line-up. Four 6BQ5 tubes, two for each channel in push pull. I own the DPA 20 and it has the tuner preamp which has to be powered from the power amplifier via umbilical connection.
Both pics are from e-bay. No affiliation!
First pic shows the DPA 30. It looks to be the same as the DPA 20 but has a different power transformer. Possibly this model didn't come with a tuner/preamp.
2nd pic is the DPA 20. Not my pic but I have the exact same amplifier.
one is ultralinear the other is not, is all I know. What makes it ultralinear.?
If you are referring to the amps on the bay, the one listed as ultralinea is actually not ultralinear. The guy probably doesn't know what he's talking about so be aware. You can look at the underside pics and see that it uses regular push/pull, plate to plate with center tap output transformers.
Packard Bell never made an ultralinear amplifier that I am aware of.
From Wikipedai. No affiliation!
"A pentode or tetrode vacuum-tube (valve) configured as a common-cathode amplifier (where the output signal appears on the plate) may be operated as:
a pentode or tetrode, in which the screen-grid is connected to a stable DC voltage so there are no signal variations on the screen-grid (i.e. the screen-grid has 0% of the plate's output signal impressed on it), or
a triode, in which the screen-grid is connected to the plate (i.e. the screen-grid has 100% of the plate's output signal voltage impressed on it), or
a blend of triode and pentode, in which the screen-grid has a percentage (between 0% and 100%) of the plate's output signal impressed on it. This is the basis of the distributed load circuit, and is usually achieved by incorporating a suitable "tap" on the primary winding of the output transformer that the vacuum-tube (valve) is connected to.
The impression of any portion of the output signal onto the screen-grid can be seen as a form of feedback, which alters the behaviour of the electron stream passing from cathode to anode".
yes cadman, those are the ones. only visable diff is the pwr trannys.
Thanks for clearing that up.
As long as we're on the subject, has anyone deciphered Bob Carver's mod on these that makes them $uper good?
I have a friend who has one , will try to get info from his.
Those look like Mullard output tubes, with very low hours. Not much nickel on the glass yet, just enough to tell the tale.
Have you or anyone else done anymore digging into the DPA-20, the DPA-30, or the Baby Silver Seven?
I have read the thread on "DC restoration" and it kind goes all over.... without a conclusion.....
Bump I am curious about these...
I have been learning alot... I have 4 of these, and I have a pair that have potted transformers that are a delight! I have only got them "limping along" but they sound as good as the Dennis Had amps I've owned...
Only difference in the 20vs30 is the 20 has another winding on the power transformer for the tunerpreamp. The 20 power transformer is potted and when not used to power the tuner, stays a lot cooler... you know the possibilities
so 3 years later
Lets drop some schematics in here just for simplicity. Images linked from here
Help me pick an amp design: P-P EL84 with 12ax7 driver
stock schematic (the 5w zener at the lower part of the schematic is NOT stock)
Bob Carver version
The only significant change is the bias system. Stock is a cathode resistor, Bob used a zener diode to effectively make it fixed bias. It should have better distortion performance since the bias point will not shift as you pass more signal through the amp, and it should bump the power output as well. Using EFB instead of the zener diode will do very similar things and make it a little easier to adjust the bias. I see its running higher voltages, guessing it has a different rectifier but I've not seen that portion of the schematic. WAG, its probably now a solid state rectifier and originally 5U4GB ? That would be about the 70v difference.
The HF stability stuff is tweaked, I see bigger coupling caps, the bias point on the voltage amp section is changed. Overall its a good amp design to begin with, and as long as the transformers are capable it should perform well. Direct coupled split load inverters were used on a ton of things.
**edit** fixed the image
I copied a Silver Seven Auction....back when. I'll try to post these screen shot in order...but not guaranteeing anything. I've got 2 of the 30's. Plus the 30s have those OPT terminals sticking out of the top of the transformers don't they? (and the 20s don't?) So, I think different OPTs.
I hope this doesn't break any rules. Mods please take this down if it is a problem.
And if Bob Carver sees this, Thanks for the information on this amp.
He never really tells you what you want to know. Maybe a trade mark infringement.........I don't know.
Also, I think there is a 30-1, 30-2 and a 30-3. But don't know the differences.
Here's a couple more schematics.
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.
Thanks for the screen shots!
I have a 30 also. The outputs on the 20s just have the wires tucked into the last layer of lacquered paper and ran under chassis to the term strip.... then each side (R&L) has a feed ran to that center screw term.
Then on my pair the two screws are jumped (coupled) to result in paralleling the two channels into a double power (and additive THD) . My Dpa20s were found in the same Rpc3 console, effectively being used as mono- blocks . I also have the tags advertising it as a 60watt unit...
I haven't ohmed out the 20 vs 30 opts yet but they appear the same and have similar stampings (ink not pressed) .
Having a 20 next to a 30, just by appearance and inference, the 20s look to be the higher cost build.
Separate names with a comma.