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Old 11th January 2013, 11:51 PM   #71
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Quote:
Originally Posted by m145mcc View Post
very cool, and I probably will tweak the "attenuator". you should post a few pics when ya get every thing done. I would be very interested to see and maybe hear it.
Well, here's a pic, but it's far from done

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Old 12th January 2013, 12:00 AM   #72
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Originally Posted by m145mcc View Post
Since everybody been so helpful I was wondering if anybody has an psu advice. I using a Avel Y236652 250VA 25V+25V Toroidal Transformer 122-625 and a 262F6 Hammond Manufacturing | 262F6-ND | DigiKey . I'm 65% sure this will give me what I need With "ok" ripple rejection. I'm most worried about the way I'm going to wire the tube filaments. sorry no neat simulations just pen and graph paper this time.
your +30/-30 is wrong

you have paralleled the secondaries. you either make a centre tap and use 1 bridge, or use two bridges, one for each secondary. And it will probably come out at around +/- 35 or something
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Old 12th January 2013, 02:15 AM   #73
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You'll find it expensive to get 2200uF caps in 250V. Newark are showing 2, the cheapest is ~U$13. 160V is a bit too close to 150V for comfort, and you won't find those any easier to get. You don't need such big caps anyway. Newark show a 10H 50mA choke for U$8.33. If you don't want to use a choke use a CRC filter. 220uF or smaller will be OK, depending on the current and the exact configuration of CRC filter. If you google it, you'll find a CRC ripple calculator. Or you can use a regulator, but either way you're going to lose some voltage.

If you rectify the heater supply and follow it with a capacitor, you'll get an output DC voltage ~8.9V. This is too much. You need to use an AC heater supply or regulate the DC (or use a dropper, but this will need to dissipate a bit of heat). 9V isn't much overhead over 6.3, so you'll need to be careful with your choice of regulator. It's an instrument amp, you should be OK for hum with AC.

You might need some kind of hum-dinger though, when you join the grounds with an AC filament supply.

I'd be tempted to use a 12V transformer back-to-back with a 9V transformer and run the (DC) heaters in series with an LM317 constant current sink (or source). You'll get 17V rectified from the 12V, this should be enough overhead to run the CCS. From 110V you'll get ~147VAC from the 9V Tx, slightly more from 115V.
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Old 12th January 2013, 02:17 AM   #74
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Looking good there Robert, I could do with something like that for tube prototyping.
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Old 12th January 2013, 05:16 AM   #75
m145mcc is offline m145mcc  United States
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Mr. Kesh, love the proto board is that custom? May I ask how you calculated the heat-sink size, Its something I have always wanted to know how to do. Here is my B+ and heater supply. Used MIC29150-12WT Micrel Inc | 576-2206-ND | DigiKey and 2 of these VPP12-800 Triad Magnetics | 237-1054-ND | DigiKey I ran the heaters in series as told (hope It's right) and regulated it for 12vdc. Now the CRC filter seems a bit funny to me. I used the amp-books calc and got a -64.4 ripple rejection. I chose the transformers based an a GE datasheet for the 12ax7. heaters series 12.6v @ 150mA, 150v @ .8mA (per half I think) total current needed is 301.6mA which is less than the max for the transformer in series. Let me know if I missed something, Ill have my chip-amp supply up for approval in a day or so. Thanks in advance.
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Old 12th January 2013, 08:45 AM   #76
JMFahey is offline JMFahey  Argentina
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Sorry but:
1) you wired your T1 tube transformer for 230V primary.
2) you shorted its secondary. (you grounded the secondary "center tap" *and* the bridge "-" leg)
3) you committed the same sin on T2's secondary.
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Old 12th January 2013, 10:43 AM   #77
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Quote:
Originally Posted by m145mcc View Post
Mr. Kesh, love the proto board is that custom? May I ask how you calculated the heat-sink size, Its something I have always wanted to know how to do.
Thanks.

It's a standard bread board from Maplin (UK's equivalent of radio shack) with back panel, that had two holes that were almost the right size for valve bases, so out came the file, and in went the bases.

My hybrid has B+ of 73V (seems ok for ECC88 types in a guitar amp. The idea was to have a single trafo for B+ and chip amp). Not sure how I'd feel about using higher voltages on a bread board.

The heat sink info on the LM3875 datasheet is very thorough, there's a useful table. I followed it, but I think the sink is vastly overrated. The chip stays totally cool, and I haven't even put in any thermal paste yet. Maybe it's because I followed a lot of advice from the chip amp forum about snubbers and decoupling, and maybe it;s because not much below 80Hz is going through it.

One time I left the input not connected, so it was open circuit as the resistor to ground is not on the board (it will be the master volume, and attenuator to line level, as per your schematic). The amp got a little hot, but the zobel resistor, a 400mW, started smoking. Worked ok after, just the paint burning, but I replaced it with a 2W one.

Last edited by Robert Kesh; 12th January 2013 at 11:02 AM.
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Old 12th January 2013, 10:55 AM   #78
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Originally Posted by counter culture View Post
You'll find it expensive to get 2200uF caps in 250V. Newark are showing 2, the cheapest is ~U$13. 160V is a bit too close to 150V for comfort, and you won't find those any easier to get. You don't need such big caps anyway. Newark show a 10H 50mA choke for U$8.33. If you don't want to use a choke use a CRC filter. 220uF or smaller will be OK, depending on the current and the exact configuration of CRC filter. If you google it, you'll find a CRC ripple calculator. Or you can use a regulator, but either way you're going to lose some voltage.
Agreed.

I used a CRCRC, but ECC88s are fairly high current. The 12AX7 at 150V with 330k plate resistors will have current in the microamps. You could use some pretty big resistors and lose very little Vs.

Last edited by Robert Kesh; 12th January 2013 at 11:04 AM.
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Old 12th January 2013, 08:15 PM   #79
m145mcc is offline m145mcc  United States
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Ok with the help of my eraser I think I managed to prevent electronic blasphemy. on T1 I change the primary to parallel and removed the GND on the center tap, I did the exact same thing on T2. Thanks for the advice.
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Old 13th January 2013, 01:13 AM   #80
JMFahey is offline JMFahey  Argentina
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Cool, now it can work.
Last detail is making the low voltage first capacitor after the bridge much larger, at least 1000uF and preferrably 2200 x 25.
Otherwise ripple there will have too much amplitude and the regulator won't handle it.
But now we are getting close
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