mwl168

Very neatly done modification. I had to look up the original T1 photos to spot the modification

All international-destined packages have been sent too. I'll PM each with the postage. Happy building!

You would want your trafo to be capable of delivering more than 120mA current per rail. I would personally spec at least 250mA per rail for Megatron use. I am sure there are others that would prefer going much higher. Also, base on my own experience, 20V drop across the regulator may not be sufficient for 450V regulated output under load.

All packages with US shipping address have been posted to USPS Priority Mail. Please PayPal me $7.2 US for the postage. Overseas packages will be posted next week. I'll PM each with the postage as it will be based on destination and weight.

If the valves are lit but do not heat up, I wonder if the filament supply is on but not the high voltage supply? I am not familiar with the BHSE, does it have separate fuse for the high voltage supply?

What's the small board with the terminal blocks for?

On my builds, the Blue Hawaii draws just over 80mA on the positive rail but close to 150mA on the negative rail. The Caron draws around 100mA on both rails. I use the same PSU for Carbon, KGSSHV and Blue Hawaii and will be for the GG as well. The 5.1R current limiter is good for about 120mA and works fine for the Carbon but not for the negative rail of the Blue Hawaii. I paralleled a 15R with the 5.1R on the negative rail to get about 3.8R to support the Blue Hawaii. I have not completed my GG build so don't know what the current draw is. Maybe someone will chime in.

The GRHV boards arrived today. The boards look good! As promised, here is a quick photo. Will be packing and start shipping next week.

The amp boards were delivered tonight. Too late for me to take photos but the board is nice and heavy as can be expected given the 4oz copper. The GRHV boards are supposed to arrive tomorrow. Will take some photos if I get home in time. By the way, please PM me your shipping address if you did not include it in PM or PayPal note to me. Thanks!

It dawned on me while I was trouble shooting my GRHV to put a reminder here. The 3W 5.1 R current limiter on the GRHV board is good for 120mA per rail which is a bit low for the Megatron and not enough for the Blue Hawaii 's negative rail. May need to go to 3R or lower.

I got damned short memory. Just checked some PM with congo5 who provided some really good information for the Megatron. The HV current draw on the positive rail is about 122mA for the front end and output stage together. The current draw for the negative rail is about 110mA. mypasswordis's recommendation of the 200VA 350V Antek is spot on - the 350vac secondaries are rate at 290mA. This is the transformer I will use for my build.

Yes, and I've read and heard arguments that the post-reg caps have more effect to the resulting sound if you believe that kind of thing...

I typically over-spec my transformers plus the Antek 50VA ones are not much bigger and pricier than the 25VA ones. But, yes, the 25VA ones should work too just won't provide as much headroom. The post-reg capacitors only see the desired regulated voltage so 550VDC rated caps are sufficient for those positions.

Thanks for correcting the filament current, it's 150mA for 12.6v so 600mA total, not 1.2A which is good news. I also agree that 550VDC rated caps would not be enough for pre-reg position using 360vac secondaries where they will see well over 500VDC. Mouser has 600VDC rated electrolytic caps but they only go up to 220uf.

The GRHV has a very low drop-out voltage so 360vac secondaries should be good enough for the 450VDC regulated output. And yes, changing those two 390K to 442K on GRHV to adjust for 450VDC output. I am planning to do precisely what you said for the +/-12VDC supplies needed. The 12VDC output needs to supply 1.2A 600mA current for the 4 front end tubes so the 7812 needs to be properly heatsinked. The -12VDC should work for the -15VDC current sink. I am also contemplating using the +12VDC to drive a timer logic for the HV delay. I am hoping someone(s) with technical knowledge can chime in if this is a good idea. I plan to use the Antek transformers too although I may use the 50VA ones for the filament and LV supply. By the way, the CCS EL34 tubes need their filament supply wires very short (can someone help me understand why?) so those are the ones I plan to supply with the 50VA trafo. I will use the 6.3v secondaries on the 400VA/360v trafo for the output EL34. If I traced the board correctly, the CCS EL34 tubes are the pairs on the outside edges of the amp board.

Great! Took a quick look. Don't think we need 1W resistors in most locations. In fact, you may have problem fitting them. Many builders here use either Vishay RN60 or Koa Speer MF resistors. Both are 1/2W and fits the board well. Also, you can use KSA1156 or STN9360 to substitute the 2SA1968. Just need to mount them observing the pintout difference. EDIT: the CMF60 resistor is pretty compact for its 1W rating at 8.74mm length so should fit the board fine.

GRHV board order has been placed with Seeed. Will provide update when boards arrive. I will be putting together a BOM on Mouser for the Megatron amp when I have time. I'll share the BOM when it's done. There should be GRHV BOM already in the forum. I remember building one for the dual board so the difference will be in the connector blocks.

A brief discussion about Megatron on the Stax T8000 clone thread got me reading the Megtron schematic more closely. There are total of 8 .22uf coupling caps between stages that are in the signal path. 4 of them are rated 1000V and another 4 rated 630V. These caps will be critical to the sound of the amp. Any suggestions on which caps to use?

Quick update: Amp board order has been placed with PCBnet. Estimated ship date is 5/23.

Should have looked at the schematic before I spoke - yes, .22uf coupling capacitors between the stages.

i thought Megatron is already DC coupled?

No worries. Will add you to the GB.

Also, I suggest you test the power supplies first before hooking them up to the amp boards. As sorenb suggested, get a variac to gradually bring up the PSU while monitoring the regulated output. Once the PSUs are tested and working then hook them up to the amp board one at a time, again using the variac, to test each amp board.

Please explain why the URL link is a .com and not .edu domain?

That's the right part. Thought they were unobtainium many years ago. I stand corrected. Regardless, I am happy to provide them if just to save shipping charge from having to place additional parts order.