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Old 29th April 2003, 11:27 AM   #1
SHiFTY is offline SHiFTY  New Zealand
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Question A plug'n'play EL34/KT66/6B4 SE amp

Well I just got a "big ole box" of valves, in them was a pair of GEC KT66, a pair of Mullard EL34 and a couple of 6L6, as well as all sorts of other bits and pieces (finally i have a GZ34!)

At the moment I am using a (sovtek) 6B4G triode SE amp. Apparently these triodes are capable of at least 25W dissipation without problems; so.... that got me to thinking, would it be possible to build a single ended amp which would be capable of just plugging in 6B4/EL34/KT66/6L6 with no changes necessary?

The reason I ask is this: looking at the pinout for EL34 and 6B4 below, it would be quite easy to rig the socket so that pin 8 was connected to its own cathode resistor & cap, and connected to pin 1 (both unused on 6B4)

Pin 4 could be connected via a 100 ohm resistor to pin 3, to give triode mode EL34.

I am probably going to buy a pair of Hammond 1627SE, so I am guessing the ultralinear taps could also be used to give yet another option.

Anyway, the major headache is the driver stage, would the JE LABS driver circuit be adequate for this?

Anyone tried this?! It seems like a neat idea!!!
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Old 29th April 2003, 01:17 PM   #2
Joel is offline Joel  United States
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Default Re: A plug'n'play EL34/KT66/6B4 SE amp

Quote:
Originally posted by ShiFtY
...the major headache is the driver stage, would the JE LABS driver circuit be adequate for this?
Sure, for higher line level signals. The cathode-degenerative amplifier in the first position has very little gain.
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Old 4th May 2003, 05:43 AM   #3
SHiFTY is offline SHiFTY  New Zealand
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Well I have given up this idea: one of the KT66 had a loose base and seems to have a broken internal connection and the EL34s test pretty averagely. So yeah, too much hassle.

Oh well, have to stick with the triodes. Not such a bad thing!




Oh and also I got a 240V mains shock from a dodgy old switch It was just rigged up temporarily and i guess it had some sort of internal short...

I guess messing with anything electrical carries a small risk of shock but its still a bit of a surprise when it actually happens!!!
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Old 4th May 2003, 12:43 PM   #4
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Default Re: A plug'n'play EL34/KT66/6B4 SE amp

Quote:
Originally posted by ShiFtY
Anyway, the major headache is the driver stage, would the JE LABS driver circuit be adequate for this?
This would be a doable amp if you change your mind and decide to pursue it. Pity about your tubes being stuffed, but they're not exactly expensive types. I just bought a new quad of Svet EL34's for $US20, and 6L6's are as common as muck, so I bet you could turn up some cheap, if not now then soon. 'If you build it they will come'

The difficulty would be choosing a PSU voltage that gives you a good compromise for all the tubes to give you satisfactory op points. Personally I'd either optimise it for the 6B4G as I think it's the best tube you mentioned (though I like the others too) which means about a 310V B+ for the 'classic' 250V/60mA/-45 2A3/6B4G, however Frank and others have run the Sovteks a lot hotter, so maybe a higher B+ would do for that. The EL34 like voltage so that's not an issue, but I find a lot of the 6L6 tubes sound better run lower and hotter but YMMV.

Here's how I'd do it ideally;
- set your B+ voltages for the various tubes. A selectable supply would be great if you have a PT with various secondary taps, but if not, chosse a compromise that's not too far from the op points selected for each tube.
- use two octal sockets for the output on each channel to simplify the wiring a bit; one for the 6B4G, one for the multi-grids. Have the driverstage wired to both, and only have one socket filled at a time.
- install a selector for the various tubes on the multi-grid socket that switches in the appropriate cathode RC combo, or a voltage preset if fixed bias. If you use a multi deck switch you could even have it set up to do triode / UL connections and bias adjustments in one, perhaps even with different biasses for some tubes depending on how you like the sound. 101 flavours in one amp!
- once you know what you various bias voltages are for the range of tubes, select a driver with adequate swing, headroom and distortion performance.

JE Labs driver.
As I haven't done all the calcs on bias points for your tubes tonight, it's hard to say whether it's adequate for your use. <i>I,</i> think it has too much gain, around 200, for what you'd probably need. My guess is you're not doing to need more than about 50V of bias (300B in JE is about -73V) and the JE driver is already giving full power with 250mVrms/350mVpk in. My experience is that you seldom need more than 1Vrms sensitivity in an amp, even directly driving a CDP in through an attenuator. However you know your system's gain requirements best, but a gain of 50-ish opens up a lot of possibilities, for both single and two stage drivers: 6C45, C3m/o/g in pentode, E180F/8532 trioded, 12B4A -> 6BX7, cascodes, DHT's etc. Lots of options and could be very cheap or expensive depending on what you decide.

These are just my random thoughts on the subject whilst I'm waiting for the PC to finish another project.
I sketched some IDHT/DHT combo amps a while back (KT88/300B) to get around the cost of using 300B's all the time, but never built it as I moved on to bright emitter tubes run at modest voltages which should last forever. Plus I have plenty of spares. My gain stages were all <30 as my pre can put out big voltages cleanly and I set sensitivity at about 2Vrms to fit in with my system's gain structure.

I still think the combo amp is a great idea.

Quote:
Anyone tried this?! It seems like a neat idea!!!
Pete Millett did a 'universal' amp for 6L6/EL34/KT88/2A3 that might give you some ideas.
http://www.pmillett.addr.com/push-pu...driver_pcb.htm

Cheers
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Old 5th May 2003, 06:54 AM   #5
SHiFTY is offline SHiFTY  New Zealand
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Well I ended up building a "mini" 300B amp, using the unchanged JE Labs schematic, except with B+ of 400V and the Sovtek 6B4s dissipating 25W.

It seems to be handling things nicely, I have been running it for about an hour so not sure if there will be any problems yet. Going to wait and see if anything gets too hot!

Also I tried the 5K:8ohm connection on the 125ESE, seems to be a bit cleaner but not much.

Gain is a little high, about half volume brings clipping. However, I plan to use a little negtive feedback, so it will come in handy.

It has noticeably more power than before.

:edit: seems the GZ34 is getting a little bit hot, faint glow on the plates... I guess its asking a lot to be run at 160mA... Its a NOS miniwatt one though so i guess it will be tough enough?

:edit again: the plates aren't glowing, its just the reflection of the heater... I think it will be ok!
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Old 5th May 2003, 07:34 AM   #6
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I'll be curious to find out if you have any reduction in lifetime or performance over time, running the tubes at 25W. Please let us know to give another data point on them.

Cheers
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Old 11th May 2003, 11:33 AM   #7
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hi,

Many date pass, My muliiple purpose Se amp kit was funish.
Pls see the layout of the kits.
any Suggestion to improve was welcome.

thanks

thomas

www.diyaudiocraft.com
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