Simple P-P Capacitors

[akimmet]

I used CXPP25-MS-7.6K output transformers as well.
In my case 680pF was best with those transformers. I used a scope to determine the value.

[Einric]

Thank you Akimmet, did you use 500V or 1000V Mica Caps?

[boywonder]

Originally Posted by Einric

Would 500V be sufficient or would I be better off going with the 1KV?

They are on the cathode of the driver tube so either of those voltages are way overkill. Just pick up a small assortment of pairs between say 270pf and 680 pf or so (assuming that you have access to a scope and a function generator).

Use an 8 ohm resistor for testing connected to the speaker terminals, as tube amps shouldn't be powered up without a load.

[Einric]

My scope is out of commission right now, the traces are all fat and fuzzy.
My function generator is in great shape though.
My communication service monitor has a software defined O-Scope onboard but I have my reservations about testing a home made amp on a $20,000 service monitor.

Would there be a benefit to testing this amp with more than just Sine & Square wave?
Maybe sawtooth and others?

[boywonder]

Originally Posted by Einric

Would there be a benefit to testing this amp with more than just Sine & Square wave?
Maybe sawtooth and others?

I usually check for noise at the speaker terminals using the scope with an 8 ohm load resistor before connecting any speakers, and I usually bring up the mains voltage slowly with a variac and keep an eye on the B+ voltage. If your B+ voltage does not look reasonable or the amp is popping fuses, power down and triple check/troubleshoot. Make sure the mains fuse is the appropriate size. Although it's somewhat difficult to do, I have let the smoke out of a power transformer power up my first amp build. I've also fried a speaker with an amp at first power-up.

In addition to B+ voltage, you can also check the output tube idle current with a DC voltmeter across the output tube cathode R's. If your various B+ voltages are reasonable and the output tubes are drawing the correct idle current, you are mostly there.

If you don't have a variac, there is a series lightbulb trick to powering an amp up for the first time; do a search here if you are interested.

You can check freq response with a sine wave, and power output as well. If you have a basic scope, you'll have to do some basic ohm's law calcs for output power.

Having a few multimeters (even $6 cheap Harbor Freight ones) makes bringing up a new amp less stressful.

In general, I check the various DC voltages with meters (using clip leads-no hands req'd) and use the scope and func generator to look at sine waves at various places in the circuit if req'd.

I've never used a triangle wave but I'm still a semi-newbie so who knows.....

[Einric]

C2 is calling for a 150uf 450V capacitor.
Would this be where a Motor Run would be used?
If so what Motor Run should I use?

Would something like this be suitable?

Thanks

[boywonder]

Originally Posted by Einric

C2 is calling for a 150uf 450V capacitor.
Would this be where a Motor Run would be used?
If so what Motor Run should I use?

Would something like this be suitable?

Thanks

Can you post a schematic? (so that everybody is on the same page?). Yes, the final cap in the PS is typically where the motor run cap would be used. You can use a motor run cap, or a motor run in parallel with a low ESR electrolytic to get more uf and the low ESR benefit of a motor run cap.

The cap you posted in the Ebay link should be what you want, although the title says "run" and "start". You don't want a motor start cap, you need a motor run cap.

The idea is to have a low output impedance power supply, and these large motor run caps have a lower ESR than a comparable electrolytic. Running say a 47uf motor run in parallel (bypassing) with say a 100 uf or 120uf electrolytic would also be fine, since the MR cap gives you the low ESR and you still have plenty of uf. If there is a spot on the PCB for C2, then it should be easy enough to do both, since the MR cap will be mounted off-board due to it's size.

If you want a good quality low ESR electrolytic to run in parallel with the MR cap, consider the panasonic TS-HA, TS-HB, TS-UP series or Cornell Dublier caps. Nichicon and others make appropriate caps as well. These are usually "snap-in" style and are designed for PS use.

[Davec113]

Originally Posted by Einric

C2 is calling for a 150uf 450V capacitor.
Would this be where a Motor Run would be used?
If so what Motor Run should I use?

Would something like this be suitable?

Thanks

No, that's noit what you're looking for. It is an aluminum electrolytic, you want an oil filled motor run cap.

In my SSE, I have 2 ~100uF motor run caps and just replaced an 150 uF electrolytic with a 100 uF Mundorf M-Tube Cap in the C2 position:

Mundorf MTube Series Film Capacitor

The clarity cap may be even better (lower esr) but they were out of stock when I ordered:

ClarityCap TC Series Film Capacitor

The difference in clarity and dynamics with the Mundorf cap is pretty big. If you're going to spend some extra cash on good parts, this is a very good place to do it!

[Einric]

I had my eye on the TS-HA because of the 300hrs@105C.
Maybe I could look at the ClarityCap for a bypass.

[boywonder]

Originally Posted by Einric

I had my eye on the TS-HA because of the 300hrs@105C.
Maybe I could look at the ClarityCap for a bypass.

The TS-HA's max out at 400V, the TS-HB's at 450V, and the TS-UP's at 500V. The capacitor voltage rating needs to be at least 141% (sqrt 2) of your power transformer secondary voltage to live a long happy life.

There are other low ESR type snap-ins that will work fine as well, although some get rather pricey.

Bypassing any of these with a film cap would work fine, or bypassing with a motor run cap.

Good catch by Davec113.....you don't want an electrolytic for a motor run, you want oil filled like an Aerovox, ASC or equivalent.