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Old 29th September 2004, 08:49 AM   #1
Hamish is offline Hamish  Australia
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Default 6AQ5 Line stage?

I've just found some 6aq5 tubes lying around, and looking at the gain of these things they have just about the right gain in triode mode for a (much needed) line stage. They should also have more than enough grunt

Problem is, i have no experience designing tube circuits, and this is far from the normal application for these things. I've searched everywhere but can't find a thing.

If anyone could help me out with component values, i'd be most appreciative

Thanks,
Hamish
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Old 29th September 2004, 11:11 AM   #2
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What do you have available for a PS, and at what voltage? That'll be your starting point to determine what you can easily use for the B+, unless you're willing to get something made (muchos pesos in Oz) or use surplus.

It's unlikely many people have used this tube in this application, but I can't see why it won't sound good. Trouble is, you're going to have to find the op point yourself (which is why I asked about the PS).

An OTTOMH suggestion to start would be B+ = 400V, Ik = 30mA, Va = 150V, Rl = 8k (20W), or 300V B+ / 5k.
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Old 29th September 2004, 11:21 AM   #3
Hamish is offline Hamish  Australia
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Thanks for the reply, Brett. I've been meaning to build your 12B4A line stage but got side tracked with silicon

It's good to get a little validation on the design. Seeing noone had used it before had me worried, but you've gotta try everything at least once (except folk dancing).

As for power, i've got a 250-0-250 little transformer (rather poor quality) that was in the AWA receiver that i got the tubes from. I also have a 350-0-350 R-core moster i bought a while back for the 12B4A that never was.......yet.

I figure if i can get this working with some junk stuff, then i can splash out a bit more on some real hardware.

Thanks again,

Hamish
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Old 29th September 2004, 12:25 PM   #4
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Quote:
Originally posted by Hamish
Thanks for the reply, Brett. I've been meaning to build your 12B4A line stage but got side tracked with silicon
It's not a bad little design. If you get around to it, I hope you like it.
Quote:
It's good to get a little validation on the design. Seeing noone had used it before had me worried, but you've gotta try everything at least once (except folk dancing).
I don't know that it'll sound, good, but I suspect it will. The 6AQ5's I've tried recently in another project were very good, and as a lot of people don't use them, cheap too.
Quote:
As for power, i've got a 250-0-250 little transformer (rather poor quality) that was in the AWA receiver that i got the tubes from. I also have a 350-0-350 R-core moster i bought a while back for the 12B4A that never was.......yet.
I threw some estimate numbers into PSUD for both transformers and for simple SS FW rectification, got 420 and 320V for both channels off a single supply @ 30mA/tube.
Measure the transformer secondaries and futz with the numbers to see what you get. Even with the big one, and a couple of RC stages the 300V op point I mentioned is viable, if you don't mind a few extra watts of heat.
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I figure if i can get this working with some junk stuff, then i can splash out a bit more on some real hardware.
Everything I've suggested is available cheaply through local suppliers. If you like it this way, then I'd suggest using a FW with a 5AS4 or similar, LC (+LC if affordable; Hammonds are cheap) off the 700Vct transformer. Then by changing a couple of resistors and sockets, you can try the 12B4A too.
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Old 29th September 2004, 12:55 PM   #5
Hamish is offline Hamish  Australia
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Thanks for your replies. I've modelled a few in PSUDesigner, and for now trying to get an CRCRC filter to work with R values of a sensible wattage. I want to try and use the cheap transformer, as i have the big one still planned for the 12B4A. I see Evatco have some reasonable open style chokes for around $30 a piece.

I'm just working my way through a tube design book now, so hopefully i'll be not so much in the dark. I found the sticky thread for newbies, so i'm devouring everything in there for now.

I do have to admit the 6AQ5's 8% THD is a little disturbing.....but this is a first project

Thanks again for your help and guidance, Brett

Cheers.
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Old 29th September 2004, 01:41 PM   #6
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Quote:
Originally posted by Hamish
Thanks for your replies. I've modelled a few in PSUDesigner, and for now trying to get an CRCRC filter to work with R values of a sensible wattage. I want to try and use the cheap transformer, as i have the big one still planned for the 12B4A. I see Evatco have some reasonable open style chokes for around $30 a piece.
If you're going to use a SS rectifier, simply add a 100u cap at the first position. That got me the 320V I mentioned earlier. I like to use the constant current option for the load (even though I know it's not CC); in this example 60mA. Look at the voltage at the CCS, which is 20V high for the 300V op point. The R should be 20/60mA = 330R. I'd probably split the supplies after the first cap and run a seperate RC for each tube; say 650R and 220u. Not the best supply in the world, but it'll work and give you an idea whether it's worth refining the design.

The open frame Hammonds are quite good, and the 193's good for poweramps.

As I mentioned in the last post, you can build this using the bigger trans, and with a 4 resistor modification and some new sockets, use the 12B4A's later in the same setup. Just don't build it all fancy until you finalise it
Quote:
I do have to admit the 6AQ5's 8% THD is a little disturbing.....but this is a first project
You won't get but a tiny fraction of that trioded, working into a modest load and swinging the huge 1-2V a linestage will give.
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Thanks again for your help and guidance, Brett
My pleasure.
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Old 29th September 2004, 05:33 PM   #7
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The 6AQ5 is similar to the 6BQ5/EL84 which i have seen mentioned for use in triode mode in a line stage.

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Old 29th September 2004, 11:42 PM   #8
jlsem is offline jlsem  United States
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Quote:
The 6AQ5 is similar to the 6BQ5/EL84 which i have seen mentioned for use in triode mode in a line stage.
Actually, the 6AQ5 is a 6V6 in a seven-pin miniature envelope, so trode-connected it should sound fine. Any triode that makes a good driver for a DHTSE output will usually make a good line out with suitable transformer when using longggg cales.

John
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Old 29th September 2004, 11:57 PM   #9
Hamish is offline Hamish  Australia
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I actually wanted to try and avoid using an output transformer, and just go with a cap. I've found a couple of schematics using similar tubes in triode mode for headphone output (also some for the 6BQ5 (thanks for the tip, Dave), which i seem to remember are lurking inside my Akai R2R )
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Old 30th September 2004, 02:17 AM   #10
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Quote:
Originally posted by jlsem
Actually, the 6AQ5 is a 6V6 in a seven-pin miniature envelope, so trode-connected it should sound fine. Any triode that makes a good driver for a DHTSE output will usually make a good line out with suitable transformer when using longggg cales.
Yep. And even better in {real} amps, ie PP's too.
Quote:
Originally posted by Hamish
I actually wanted to try and avoid using an output transformer, and just go with a cap. I've found a couple of schematics using similar tubes in triode mode for headphone output (also some for the 6BQ5 (thanks for the tip, Dave), which i seem to remember are lurking inside my Akai R2R )
The 6BQ5 is an excellent tube, but it generally has too much gain for a linestage, unless you Tx couple the output, which I think is a good idea anyway. Use Lundahls.
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