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Old 2nd January 2011, 06:57 PM   #1
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Default Rundmaus at work - PP1C without sand

Hi there,

as I am now finding some time to continue work on my version of Allen Wrights PP1C (EL34-PP), I will post the progress and the (surely coming) open questions in this thread instead of starting many new ones.

The original schematic by A. Wright:
Click the image to open in full size.
Click the image to open in full size.

A short list of older posts directly concerning this project:

Suggestions for Tube Amplifier

More Questions about A.Wrights PP1C

All Tube Ccs.

How much ripple on B+ is allowed?

Power supply for PP1C - sanity check

tube CCS below cathodes, startup issues?

The current planning/situation:
* GZ34 power supply reproduces or exceeds specs of original supply (PSUD, not built yet)
* startup issue will be solved by a defined power-up sequence delaying the -HV rail
* first parts arrived

Greetings,
Andreas

Last edited by Rundmaus; 2nd January 2011 at 07:07 PM. Reason: Link to Vacuumstate was dead, replaced by other location.
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Old 2nd January 2011, 07:04 PM   #2
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Default Cathode resistor choice - a matter of taste?

Hi there,

I was planning to use these resistors below the EL34 cathodes: Ohmite 470R, 10W, 1%, wirewound

Click the image to open in full size.

Via a german electronic surplus store, I also got these extremely cheap: Vishay Dale 470R, 10W, 1%, wirewound

Click the image to open in full size.

I guess the Dales can dissipate the 10W only when attached to a heatsink oder chassis, while the first ones might be mounted in free air. What do you think, is it a matter of taste or are the reasons to prefer one of the two?

Cathode bypass capacitors will be these: Sprague 8000F, 50V, axial
Click the image to open in full size.

Greetings,
Andreas
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Old 2nd January 2011, 07:13 PM   #3
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Have you looked at the Sprague 678D series, I think there is an axial version (maybe the one you have) that has a very low impedance as well.

As far as the resistor, look them up and check for the lowest inductance.
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Old 2nd January 2011, 07:41 PM   #4
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Hello Rundmaus,

This is really a great amp!

I suggest that you use a current source instead of the big cathode resistors and the caps.

In the later version, the PP1CS, Allen use a single FET, 9 Volt battery and 3 resistors. You might get a copy of the schematic if you send him an email?
But hey, it's just a very simple ccs.

Be sure to make the PSU choke loaded and the choke should be no less than 8H. You might consider using polypropylene motor run caps in the PSU?

Prepare the amp for a (shunt) voltage regulator for the driver stage.

Choose the OPT and transformer so you can upgrade the amp for 300Bs and 6H30...

Happy building!
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Old 8th February 2011, 10:37 PM   #5
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Hi there,

just finished a first layout idea for the amp. Chassis shown is sized 50cm by 40cm. All components drawn to scale (except connectors and HV caps, only approx.).

Image is a top view, tubes, controls and connectors will be on the upper side, all iron below.

As nothing interesting is expected to happen at the rectifier sockets, they are covered by a subchassis carrying the chokes. Distance will be provided to ensure enough air flow to the rectifiers.

Click the image to open in full size.

Any comments?

Greetings,
Andreas

Last edited by Rundmaus; 8th February 2011 at 10:40 PM.
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Old 9th February 2011, 12:33 AM   #6
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Rundmaus View Post
I was planning to use these resistors below the EL34 cathodes: Ohmite 470R, 10W, 1%, wirewound

Via a german electronic surplus store, I also got these extremely cheap: Vishay Dale 470R, 10W, 1%, wirewound

I guess the Dales can dissipate the 10W only when attached to a heatsink oder chassis, while the first ones might be mounted in free air. What do you think, is it a matter of taste or are the reasons to prefer one of the two?
The Ohmite resistors will allow for 10 W dissipation at an ambient temperature of 25 deg C. At this dissipation, you can expect their temperature to approach 250~300 deg C (yes C!).

The Vishay-Dale RH-10 series can dissipate up to 60 % of its rated power at 25 C ambient without a heat sink. Their 10 W rating is based on them being chassis mounted. See the RH-10 datasheet for more details.

With power resistors in general, I recommend a significant design margin. I wouldn't allow for more than 1/4~1/3 of the rated power to be dissipated in a power resistor. Even under those conditions, the resistors will be over 100 deg C at 25 C ambient!

As long as you can get good air flow around the Ohmite resistor, that's probably what I would go with. I don't like using the chassis for a heat sink on products that the user can touch. But if you're fine with heating up the chassis, it's really up to you which resistor you choose to use. There's no significant advantage one way or the other.

~Tom
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Old 9th February 2011, 02:25 AM   #7
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Interesting amp, I might build one someday

I'm just wondering, why a huge 8000 uF cathode cap??
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Old 9th February 2011, 09:14 AM   #8
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The big cathode bypass cap is a recomendation from AW going way back. Im not sure why but I think it has to do with minimal LF phase shift, and an "on ear" improvement reported by AW.
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Old 9th February 2011, 09:23 AM   #9
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Quote:
Originally Posted by Rundmaus View Post
Hi there,

Any comments?

Greetings,
Andreas
Youve got quite long runs of cable back and forth to the input/ output jacks. A friend of mine is building an amp in this tread: http://www.hifisentralen.no/forum/in...c,47654.0.htmlIts very well planned. Check it out!

I like AWs designs too and have for long been planning to build an amp like the one you are doing.
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Old 10th February 2011, 09:23 PM   #10
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Default Surplus surplus, surprise

Hi there,

the *really* cheap heater transformers from a german electronics surplus have arrived...

Click the image to open in full size.

They smell awfully , deliver 6V/2A per winding at 220V mains, so should be 6,3V at 230V mains. As they look quite rotten, they will probably only serve for testing purposes. They sold for 3,50 Euro per piece, which is ok for a test setup...

For the final amplifier a custom-wound heater transformer will be ordered - unless some forum member knows a source for a transformer with 6.3V/4A, 6.3V/4A, 6.3V/3A secondaries.

Any more comments to the chassis above? The long signal path is gone, I will move the input selector directly to the sockets and fit a mechanical solution to operate it from the front, same for the power switch, which will then be located near the mains transformer.

Greetings,
Andreas
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