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Old 15th January 2009, 01:15 AM   #1
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Default PP EL34 or 6L6 circuits?

Hello everyone, I'm looking for a simple push pull circuit to use with the Dynaco A470 OPT. The pri. Z is 4.3k and it has screen taps. I have a hefty 400v B+ power supply, and would prefer to use all octal tubes. I'd like to find something very basic and uncomplicated. Any ideas would be greatly appreciated. Thanks, Jay
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Old 15th January 2009, 01:51 AM   #2
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What are you planing on driving?

http://www.audiotropic.net/Projects/ampEL34.html

There is a newer version of this circuit that has a 6GK5 LTP driving the EL34's. I would be more than happy to post it but I would like Poinz's permission first. I know he is very kind to the DIY community being a DIYer himself, I just don't know how liberal he has been with this design.

http://www.audiofanatic.it/Schemi/Ti..._Dyna_Mull.jpg

I know, a different output transformer, but just a thought. I had an idea to try this circuit in the future but with a 6J7 input and 6SN7 splitter/driver. Hard to get simpler than Poinz's EL34 amp (linked), add uses octals as requested. 400v B+ is a tad high but with a tube rectifier or choke input that can be tamed.

Good luck and happy listening!

Cheers

James
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Old 15th January 2009, 02:46 AM   #3
anatech is offline anatech  Canada
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Hi jay,
Your voltage is well within the range of 6L6GC designs. With a higher plate dissipation rating than EL34 types, these might be what you are looking for. It would be nice to stay with normally accepted tubes (nothing odd) to make things simpler. Your primary impedance will work with both 6L6GC types and also 6CA7 / EL-34 types. I find the 6CA7 "fat bottle"is more robust than the EL34 thin bottles. You could even use 6550A / KT-88 types, just don't push the outputs too hard, the core might saturate. This is more of an issue at higher powers with lower frequency response.

The simplicity of a design will not change much between tube types. A fixed bias would allow higher power, but that comes at a price. You'll have to set the bias currents periodically.

Understand too, a small increase in complexity can really pay dividends when it comes to sound quality - or not. Depends on what you do. You have decent iron, so make a good circuit worthy of that iron.

Do you have any idea how much power you'd like? What speakers are you planning to drive, and to what levels? How large is your room? Do you expect any major changes in your living quarters or sound equipment? If so, aim for your new circumstances. If you have to trade power for low end response, go for the lower frequency response.

One thing for sure. Make your chassis large enough so that you can space your tubes out, and transformers. Too small a chassis will bring trouble with heat and probably supply noise as well. A stereo chassis is reasonable for this power class.

-Chris
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Old 15th January 2009, 03:46 AM   #4
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Jay,

Your O/P "iron" was intended for use with EL34s. As 6.6 K trafos are "routine" with 6L6s, I'd stay with the EL34/6CA7/KT77 group. A 400 V. B+ rail is not particularly "tall". Use "fixed" bias on the O/P tubes.

Simple and all Octal you say. How about a Mullard style circuit with a 6SJ7/5693 as the voltage gain tube and a 6SN7/5692 as the LTP? CCS load the splitter tail with a 10M45S, instead of a resistor. If push comes to shove, you can always buffer the splitter with IRFBC20 MOSFET source followers.
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Old 15th January 2009, 03:51 PM   #5
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Interesting ideas guys, thanks. I'm still struggling to learn this stuff, so some of that went over my head. I've built several amps, but pretty much have to follow a schematic, for the most part. I was looking at an early Sherwood design (the S1000), which is certainly simple enough, but the OPT Pri. Z is 5500, compared to the Dynaco's 4300 (it's a 6L6 design). I certainly wouldn't object to a more complicated circuit, but it would have to be in the form of a schematic. I hate coming right out and asking for schematics, but untill I get a better grasp of things, I'm sort of stuck.
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Old 15th January 2009, 04:33 PM   #6
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You could consider the Williamson with EL34's too. I would try a few things things though when you settle on a design. First would be to wire the EL's at triodes, unless you need more than 14-16 watts. The second would be to try it free of NFB, then if you need feedback slowly add it little by little.

OK I did some searching and I found the other schematic I was talking about posted around here so I will go ahead and slap it up. ...Poinz, if you read this and don't want it up I will have a mod take it down...

It doesn't use all octals like you requested but it is a simple and elegant way to get about 16watts from the minimum amount of gain stages. I talked with Poinz through email about this one, I have since deleted that email, but what I got was he liked this one better than his first EL34 amplifier (the one I linked).
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Old 15th January 2009, 04:34 PM   #7
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Same thing but with a 5AR4 in the power supply.

Good luck and happy listening!

Cheers

James
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Old 15th January 2009, 05:11 PM   #8
anatech is offline anatech  Canada
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Hi Eli,
Quote:
As 6.6 K trafos are "routine" with 6L6s
True, classic for 6L6 types. However, you can also use 4.3K for 6L6 types also. This is probably more common at higher powers and a quad of 6L6s. I wonder what a quad of 6BQ5s would sound like with this transformer.

-Chris
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Old 15th January 2009, 05:16 PM   #9
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hey-Hey!!!,
I'd recomend the E-Linear 6L6GC. 6AC7 pentode front end LTP/diff amp will keep you all-Octal. Micah heard this one built into my Heathkit W6m amps and can give you an opinion if you ask. It is simple, two stage and does not require loop NFB. It can take SE or balanced input( I do mine balanced in the Heath, and SE on the Dyna Mk.iii ).
cheers,
Douglas
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Old 15th January 2009, 08:43 PM   #10
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I am presently breadboarding Poindexter's 6GK5 amp shown above, and I was considering using a 5AR4 rectifier since my power transformer voltage was higher than 310V or so, but I only had the SS rectification PS schematic provided by Poinz.

Thank you for the 5AR4 PS schematic!

I'm sure I'll also have lots of newbie questions once the build is close to being finished. I'm using Hashimoto HW-60-5 output transformers and a one electron Power transformer.
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