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Old 27th November 2005, 11:46 PM   #1
Bryan is offline Bryan  United States
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Default 845 A2 Monoblock Project

Hey Guys,

I've decided to share my new project with everyone on the BB, in hopes to develop a great sounding pair of SET amps. I've been listening to my DIY 845's (Grover Gardner's design) for over a year now, and I have been quite happy. Recently, I've decided to tear them down and rebuild the amps in a proper enclosure with safety in mind, as I have a girlfriend living with me now. Having things torn apart, the DIY bug has hit me and now I am about to change the design quite a bit...

The overall design goals of the project are to provide me with a pair of monos to drive my ProAc D15's (89.5 db/w/m). I need about 15-20 watts, but would like to have ample headroom for transients (near 30-35 watts). This leaves me with a few options...

I will drive the monos with my DIY 12b4 preamp, which has a gain of 4.5.

I have most of the bits and pieces on hand for the amps, which include, but are not limited to:

Input Tubes: 6J5 and similar (specifically 7193), 76, 27, 6N7, 6SN7
Driver Tubes: el34 (triode), 2a3, 300B. I don't have the 2A3 or 300B's, but they are options for the design.
Output Tube: Chinese 845 (will use alternate types of 845 after the design process is complete).
IT: Electraprint 3634
Output Tranny: Tamura F-2013
PS Trannies: Electraprint 1500CVT/150mA
Hammond 750VCT/350mA
Several 10h 100mA chokes.
Various Hammond trannies for bias, rectifiers, heaters, etc.
I have a TON of motor run caps, with a few custom made 1200VDC 100uF pieces from ASC. I also have several HUGE 88uF 1500V caps, 2200uF defibrilator caps, etc.
I was thinking about building two B+ PS, the first for the input and driver which will be fed from an 83. The second will power only the 845 and for rectification I will use a string of Cree 1000V schottkies.

I plan to use relays as a means to provide warm-up and bias to come on before the HV is applied.

The construction will be a tower similar to the Cary 805, with the PS below, and the audio circuitry above.

Other goals include no feedback, no electrolytics, dynamics, and speed, and of course SAFETY!

I am completely open to suggestions and criticism, and I really hope to learn as I progress through the project. I have no set time-frame for completion (as long as the gf can put up with the mess in our small apt).

Before I begin drawing up diagrams, I would like to hear ideas about the topologies, dual PS, input and driver combinations, and chassis concerns. Once these "issues" are clarified, I will begin actual circuit design.

Thoughts?

Bryan
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Old 28th November 2005, 12:59 AM   #2
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Bryan,

According to Cree, the high PIV SiC Schottky diodes should not be stacked. PERHAPS you can get away with stacking, if voltage equalizing resistors are used. Stacking FREDs is an option, but there is a LITTLE noise. Yet another option is a pair of 6AU4 damper diodes. 6AU4s have a LOW forward voltage drop and a HIGH PIV. Obviously, switching noise is not an issue with 6AU4s. Also, B+ rise is SLOW when dampers are used. Your 845s will be nice and hot before being hit by the B+.

FWIW, I'd steer clear of Mercury vapor rectifiers for RF noise and toxicity reasons. JJ makes a nice 5AR4 that's adequate, and then some, for the driver circuitry needs.
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Old 28th November 2005, 01:38 AM   #3
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I have built an A2 845SE amp and I am quite happy with it. I get just over 40 watts per channel. The circuit is on my web site. I don't use an interstage transformer, I use a CCS - mosfet follower that I call PowerDrive. This allows serious grid current, and frequency response beyond 300 KHz. Other members of this forum have found it useful also.

I have found that good sound requires a good power supply. I am now building my fourth power supply. The current one uses a voltage doubler powered by a 480 volt industrial transformer. Rectification is done with 5AR4's. I get some strange interaction between the 2 channels with this power supply when both channels are driven into light clipping.

The power supply under construction uses a full wave CT circuit with 3B28 rectifiers. I may try the HV schottkeys.

The dual power supply is a good idea. The first one used the circuit (and the Audio Note transformer) from the Ongaku. I got all kinds of interaction using this setup.

The PowerDrive circuit takes care of the grid current and power requirements, so that the driver just provides voltage gain. I have tried 45's, 2A3's and 300B's in my amp and I get the best sound with a 45.

The Chinese 845's work pretty good. I use them for most listening, since I can mistreat them without worry. I don't dare run 100 watts of dissipation through the RCA's.
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Old 28th November 2005, 12:24 PM   #4
Bryan is offline Bryan  United States
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Eli:

Thanks for the input. I've always heard mixed reviews with respect to the MV's and noise. I'm not too concerned about toxicity, being a graduate student in Cancer Biology, I'm ALWAYS in contact with messy stuff. I'll probably be long gone from all the ethiduim bromide well before any mercury can have its effect on me . I guess I could scrounge up some 5AR4, but I did want to use up the 83's I have lying around. I'll have to think about this one.

I looked at the Cree datasheet, and I didnt see anywhere mentioning not stacking them up to avoid passing the max PIV. I currently have a string of 4 without equalising resistors on each leg of the HV secondary. It hasn't given me any noticable problems, but I'm not throwing my scope on the HV rail to get a close peak... Never the less, I have thought about damper diodes as a posibility. I want the amp to last a long time, so over-spec'ing and reliability of components is an issue here. Where do you source your damper diodes?

Tubelab:

I've seen your PowerDrive circuit. Nice stuff. For this project I want to stick to valves. I'm not against CCS's at all, and will probably implement one in the input, but I think it will be tube based. Dont ask why, just a preference here. I've used the Pimm CCS's before with great results, but I've always likes the looks of tubes! I'm curious to see what response I will get out of the 3634 IT. I've heard goog things from those I've spoken with. This will be the first avenue I go down, and if things need adjustment or swaping out, I'm happy to know alternate options exist.

I've heard a few negatives about the dual PS, and many positives. Given that my HV tranny is only rated at 150mA, it is stretching it using only one PS. Also, I have the other Hammonds on hand, and this would work well for my application. I really want to go the 45 route, but the cost and availibility is a concern of mine. I cant seem to find any 45's for a good price (nor 300B for that matter). I still have some time to look, as I'm in no rush with this project. Maybe Santa's little helper will catch on the the many hints I've dropped her way latley...

I didn't mention it earlier, but the target is to run the 845 at ~1000Vdc and ~90mA. I don't think I'll get up to 40 watts, but probably close. How have the 845's held up in your amps. Do they mind the abuse? Hopefully, once the project is finished, the newer 845's will be consistant, and this amp wont over-tax them. That's the plan anyway!


Bryan
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Old 28th November 2005, 02:09 PM   #5
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I bought two of the original Chinese 845's when they first came out. They are from the first run, the word "China" is spelled wrong. When I saw that I wondered about the quality of the tube. I got them with the idea that I would use them for initial design and test, then switch to the RCA's. That was over two years ago, those tubes are still in the amp. I have run them as hard as 1100 volts and 110 mA with no ill effects, although I usually run them at 80 to 90 mA. I have even plugged my guitar preamp into this amp and abused it. This will find flaws in any design. No problems were encountered.

I have had good luck with the Chinese 300B's also.
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Old 28th November 2005, 02:17 PM   #6
dhaen is offline dhaen  Europe
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I'm using Chinese 845's and they seem fine. The alternatives, KR and NOS are so overpriced as to make them unreachable by me, also the KR does not deliver the ratings.

Edit: I'm still using the original pair that I did the A2 exhaustive reliability tests on (watching to see if I could see the grid wires move at peaks.... )...
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Old 28th November 2005, 03:10 PM   #7
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Bryan
Eli:

Thanks for the input. I've always heard mixed reviews with respect to the MV's and noise. I'm not too concerned about toxicity, being a graduate student in Cancer Biology, I'm ALWAYS in contact with messy stuff. I'll probably be long gone from all the ethiduim bromide well before any mercury can have its effect on me . I guess I could scrounge up some 5AR4, but I did want to use up the 83's I have lying around. I'll have to think about this one.

I looked at the Cree datasheet, and I didnt see anywhere mentioning not stacking them up to avoid passing the max PIV. I currently have a string of 4 without equalising resistors on each leg of the HV secondary. It hasn't given me any noticable problems, but I'm not throwing my scope on the HV rail to get a close peak... Never the less, I have thought about damper diodes as a posibility. I want the amp to last a long time, so over-spec'ing and reliability of components is an issue here. Where do you source your damper diodes?

Bryan
If you don't mind switching to a bridge, you could use a hybrid bridge. With a bridge your PIV ratings need only be half of what they are for a CT. Or you could make a different hybrid rectifier and just hang one of the damper diodes after the ss rectifier. See here (although this shows a ss rectifier bridge, don't see why the same principle wouldn't work with full wave): http://tinpan.fortunecity.com/saints...mer/211-se.gif

Sheldon
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Old 28th November 2005, 11:18 PM   #8
2wo is offline 2wo  United States
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How about a 211 verses 845, a quick look at the data sheets look like the 211 might be easier to drive?

But I have no experience with transmitter tubes…John
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Old 29th November 2005, 12:50 AM   #9
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I designed my amp to work with both tubes. I have tried both types in both vintage RCA and new production Chinese. The 211 is definitely easier to drive, and requires much less bias. However I, and most of the people that have heard my amp, think that the 845 sounds better.

If you design your amp so that the bias is adjustable from -40 to -150 volts and you can use both tubes.
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Old 29th November 2005, 02:06 AM   #10
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AES carries 6AU4s. Check with tubelab and other dealers. Perhaps you'll get a better price.
AES

If you make use of Hg vapor rectifiers, be sure to keep either powdered Sulfur or powdered Zinc on hand. Either powder will sequester Hg that's gotten loose from a broken envelope. Even if you never accidentally break a "bottle", you need a Hg sequestering agent. You can't throw a used up 83 down the garbage chute. The "dead soldier" has to be carefully broken into a thick walled plastic bag that contains sequestering powder. Don't turn TOXIC Hg loose in the environment.

Don't feel too bad about the organic chemicals you work with. When I was in school, we used Gooch crucibles for filtering. The filter medium in a Gooch crucible is long fiber asbestos. It's been more than 40 years; so, I think asbestos related problems are not in cards for me. Whew!
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