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Old 25th January 2006, 01:10 PM   #11
Giaime is offline Giaime  Italy
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Gingertube,

I love you.

Your extremely clever desing, wonderfully thought to overcome on cheap parts limits (EL84 tubes aren't NOS 300B, and with low rp you can drive cheap OTs) is really really clever. I really appreciate your work, and I can't avoid to tell you that one day my dream is to desing such clever amps.

This obviously will be my next project: like probably you noticed from the other thread, I was interested in a PP EL84 amp and I thougt about triode mode, getting me 5W. Now I will absolutely build this amp.

I have a 300-0-300VAC - 6.3V - 5V tranny on hand that seems to be suitable (it's big!), HT will be a bit on the high side but I think under 350V. Output transformers will be from www.elettronicanovarria.it a local italian winder.

Some question, if you want to answer me I will be extremely pleased and grateful.

1) how do I set the bias for the output tubes? I understand that they have CCS under them, so you set the proper voltage by tweaking the resistors near the transistor. But is there a pot somere to overcome maybe components tollerances? Or should I build the circuit this way and don't worry about it?

2) Could you please post some suggestion on transistor replacing? I can get BC547 (it could be replaced by most of the BC*** series, like BC109, BC337, BC549, am I wrong?) but I'm not sure I can get MJE340. Maybe a BD141? I have plenty... or TIP110, BD139...

Thanks very much in advance
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Old 25th January 2006, 01:11 PM   #12
Joel is offline Joel  United States
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I really like it.

Have you done any testing? I'd love to see the following before I try to slap one together myself:
  • frequency response at 1W
  • distortion at 20Hz, mid-band, and 20kHz, at rated wattage
  • 10kHz square wave image
  • Vin for rated wattage

Joel
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Old 25th January 2006, 02:15 PM   #13
Yvesm is offline Yvesm  France
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Quote:
Originally posted by Joel
I really like it.

Have you done any testing? I'd love to see the following before I try to slap one together myself:
  • frequency response at 1W
  • distortion at 20Hz, mid-band, and 20kHz, at rated wattage
  • 10kHz square wave image
  • Vin for rated wattage

Joel
Here is what I measured on the original version with ECL86.
Not as many numbers as you expected

www.dissident-audio.com/PP_ECL86/Spectra.jpg
www.dissident-audio.com/PP_ECL86//Squares.jpg

Of course this may be different with another OPT.

Yves.
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Old 25th January 2006, 06:56 PM   #14
navin is offline navin  India
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Quote:
Originally posted by gingertube

The main reason for using Triode Mode is to get low rp (tubes internal impedance) which reflects to the output transformer secondary as reduced output impedance (better speaker damping) and a more linear response (less distortion) from the EL84. ... Its one of the things I like about it. So NOT RECOMMENDED.

"Can I increase Output Power by using Parallel Push Pull"
If I were going to try to scale it up for higher power I would probably use EL34 instead of EL84 and proceed from there. The EL34 has the required parameters (high gm and tolerance of high Rg1 values when cathode biased) required by the circuit BUT I would be worried about the increased signal voltage drive required to be delivered by the front end and the increased grid capacitance of the EL34s compared to EL84s. To pinch an idea from SY above, EL34 with pentode diffamp may well do the trick and deliver something like 25 to 30W. Any takers???
1. the reason i asked about triode is that i got a couple of OPTs that will work with EL84PP but they dont have UL taps.

2. funny you mentioned EL34. I am working on an amp for just this purpose. It is driven by 1/2 6SL7 triode and 2 6SN7 phase splitters. The reason why I am using 1/2 6SL7 per amp is because I dont have enough 6SL7. Even the 6SL7 I am using is gifted.

SY is helping me with my amp (See "EL34 schematic confusion" theread). If you read my posts there you will realise how much of an ultra newbie I am.

I live in India where these tubes and OPTs are hard to come by. I import mine on my trips to Singapore.
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Old 27th January 2006, 02:40 PM   #15
Giaime is offline Giaime  Italy
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I've simulated and resimulated the PSU with PSU Designer, and I seem that I can't get under 350VDC with my 300VCT PT...

Any thoughts on what I could do? I'm alreading feeding 220V from the highest primary (it has international primaries)?

The only configuration that works is attached. I don't know if it's useful or not... surely a choke is less expensive than a whole new PT, but...
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Old 27th January 2006, 02:56 PM   #16
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How about a bigger voltage dropping resistor..
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Old 27th January 2006, 03:00 PM   #17
EC8010 is offline EC8010  United Kingdom
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There should be no problem with replacing MJE340 in the ring-of-two CCS with a lower voltage device (it only sees 10V, or so) - I expect that gingertube just happened to have a lot of them handy. Similarly BC549 will work just fine in the cascode CCS (and BC549C would be even better).
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Old 27th January 2006, 05:34 PM   #18
Giaime is offline Giaime  Italy
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Quote:
Originally posted by Bas Horneman
How about a bigger voltage dropping resistor..
Hello Bas,

thanks for your reply.

But in your schematic I see that you've connected the PT in bridge mode, thus left unconnected the CT of the HV winding. But doing so, wouldn't you see 600V instead of 300 across the entire primary? In fact doing so I have something like 768V that's not so good for those EL84.

Could this be more likely? Please go easy on me Sorry for the low quality of the pic but the forum software didn't allow me to upload a bigger image.

Also many thanks to EC8010 for his helpful reply. So I can replace the transistors with pretty anything, since voltages and currents are low.
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Old 27th January 2006, 05:54 PM   #19
EC8010 is offline EC8010  United Kingdom
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You're welcome, Giame, but do bear in mind that the MJE340 dissipates about half a Watt, so it will need to be replaced by a device capable of dissipating that power comfortably.
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Old 27th January 2006, 06:23 PM   #20
Giaime is offline Giaime  Italy
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Thanks EC8010, in fact I meant that, mantaining same "critical" parameters (max V, max I, max P) I undestood from you reply that the model of transistor is not critical. Of course whatever transistor I will use in place of the MJE340 I will mount a small heatsink on it.

Sorry, I'm not very good with transistors. I still can't understand why you can replace a MJE340 with a BD140 but not an ECC82 with an ECC83
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