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Old 12th January 2010, 06:52 AM   #301
iko is online now iko  Canada
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Then you might find another improvement using the Salas HV shunt regulator.

Simplistic Mosfet HV Shunt Regs
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Old 12th January 2010, 09:53 AM   #302
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Quote:
Originally Posted by ikoflexer View Post
Then you might find another improvement using the Salas HV shunt regulator.

Simplistic Mosfet HV Shunt Regs
Thanks. I'll try this too. I was reading the same thread just before I saw your post.
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Old 17th February 2010, 09:46 AM   #303
Richard is offline Richard  Australia
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G’ Day All,

It has been a while since anyone posted updates on their 26 project, I suspect many are busy hunting Arcturus 126 tubes on ebay, see the prices the last few went for. Don’t listen to Coolzero, the blue globe 126 are not that special.

I managed to finally finish my filament power supply, I’m using a 60mH choke input (followed by 32000uf & common mode choke) all sealed up in a steel chassis. The output voltage looked really nice on a scope as a neat sine wave. I wired up the filament with a ccs and switch that allows me to use the 26 and 01A. The net results of the choke input filament supply was a more defined bass, I’m still yet to add 0.22uF snubber caps and lower the Q as per Morgan Jones’ suggestion.

I found that shielding the plate chokes in aluminium boxes also helped and that if shielding the tubes with Al or Cu, holding it down with rubber “O” rings can help a lot. The pearl tube coolers also work well to reduce microphonics.

I have a question in regard to the Russian FT-3 teflon caps, did anyone ground the metal can/shield or is it better to just remove it?

Has anyone that has experimented with other output caps, have results to share?

Cheers,

Rich
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Last edited by Richard; 17th February 2010 at 09:52 AM.
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Old 17th February 2010, 10:01 AM   #304
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Hi - nice work there. I have a couple of 26 preamps to experiment with. I've settled on choke loaded operation, with FT-2 output caps - the smaller 200v versions. Ixys cascode was a bit sharp, Holco resistor a shade muffled and weaker bass, though nice sound. Plate chokes just about right. I'm satisfied enough by now to be able to put my energies into my 300b SET. That's now sounding pretty good. VT-67 into 31 into 300b. Separate power supply with a nice old Cossor 53KU (GZ33) rectifier. All DHT system - bliss! I have to say, building that lot gave me a few grey hairs - 8 DC filament supplies for starters and two power supplies - separate one for the preamp. Plenty of stuff to go wrong there. But it really sounds nice, as of course it should!!!!

andy
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Old 17th February 2010, 10:30 AM   #305
Richard is offline Richard  Australia
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Hi Andy,

Thanks for the reply. Did you notice a difference with the different voltage rated caps?

After hearing what a 26 preamp can do I’m moving towards an all DHT set up as well. I have all the parts for a 31 interstage 46 amp, just need to start the build, the 26 pre should provide just enough gain. I also reckon separate filament supplies are the way to go, more over choke input supplies.

Rich
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Old 17th February 2010, 10:42 AM   #306
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Hi - 31 and 46 are both very nice, but you're dropping the gain a bit. Why not use something like 01A in the input of the amp to get a bit more gain - or a VT-67 is nice too, or indeed another 26? 10Y is a lovely driver - possibly the best. 46 is nice and has a nice tone and neutral sound but 10Y is more vivid and dynamic. 31 is good and saves on the filament requirements

I found the FT-2 and 3 pretty similar and just a fraction ahead of the K72 versions. They are all pretty good.

I'm lazy with filaments - just use LM1084 instead of chokes. But you do need heatsinks so you can't escape metal parts!

andy
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Old 17th February 2010, 10:54 AM   #307
Richard is offline Richard  Australia
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Andy,

I’ll give the 10Y a shot as an input, I was saving them as an output tube, but at 1.6 watts there wouldn’t be much of a difference compared to the 46.

In your previous posts on an all DHT 300B amp, you’ve indicated that the 300B does not sound slow and like a typical 300B, could you elaborate on the sound compared to the 10Y or 801A as output tubes (if you’ve tried them).

I’d also like to hear your thoughts/comments on an 01A vs 26 pre, driving that 300b amp

Rich
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Old 17th February 2010, 06:18 PM   #308
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I think as a preamp tube I'd go 10Y, 26, 01A in that order, but it's pretty tight. I never used the 10y as an output - I like the 300b sound when it's driven by a 10Y so that would be my ideal. If I'm feeling cheap and lazy I use VT-67 and 31 just because I have a lot of small 6v transformers and 01A and 10Y really should have 9v secondaries for use with a LM1086/5/4. The 01A may work with a 6v secondary - it's a bit marginal. It's certainly a better sound than the VT-67 and no more expensive these days. But it takes a lot to beat a 26 and they're cheap enough.

andy
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Old 18th February 2010, 11:05 AM   #309
Richard is offline Richard  Australia
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Hi Andy,

Thanks for the feedback, I'll build with a modular approach so I can try different drivers.

Rich
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Old 18th February 2010, 12:41 PM   #310
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I think a modular approach works great. For my amps (and indeed preamps) I use a variety of different top plates. All 275mm deep, and 100mm, 70mm or 50mm wide. That enables me to use 2U rackmount parts, which end up 420mm wide allowing for the joins in the plates. OPTs go on 100mm, output tubes on 70mm and the rest on 50mm or 100mm as desired. You basically just need to use UX4 sockets unless you use 46 or other 5 pin tubes.

I use a separate power supply. In fact I'm thinking of two smaller boxes for the PSU - one for filaments and one for the HT - that allows for more experiments by using different filament supplies. It's been a long time designing all this but beginning to get good results!

andy
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