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Old 30th March 2006, 07:16 PM   #1
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Default Idea for a 2Tube 6080 PP amp.

I am about to recieve some 6080's and some 6AS7's and I was just starting to think what I might do with them.
I had a crazy idea to build a 2 tube PP amp. The idea would be to have a beefy single pentode driver stage (choke/CCS loaded) and then to do the phase splitting by taking the signal for one of the 6080 triodes from the cathode of the other. In this way it would be possable to build a complete stereo PP amp with just 2 driver tubes and two 6080's. If there wasn't a suitable driver pentode, how about using a ECL86 as the driver and maintaining the two tube approach.

What do you think of my chances of success. I understand that the 6080 and the 6AS7 can be very variable, even within the same envelope. I also understand that cathode bias is mandatory and that these tubes can bias up at wildly different voltages - sounds like a bit of a long shot to get this working, but what do you guys think of my minimalist idea?

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Old 30th March 2006, 08:34 PM   #2
Fuling is offline Fuling  Sweden
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Nice idea, but I believe self splitting PP output stages works best with tubes that has a little higher µ that 6AS7
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Old 31st March 2006, 05:31 PM   #3
kevinkr is offline kevinkr  United States
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Yes, that would be my experience with the 6080/6AS7 as well. Might try an interstage transformer and cathode bias the output tubes independently of each other. Some means of trimming the operating point of one against the other is advisable. A single 6080 in pp should be capable of at least 7wrms..

Fixed bias is much more efficient than cathode bias in terms of required HT (HV) but is risky due to warm up issues and the variability of the tube's transconductance making it very sensitive to minute variations in grid bias voltage. A mix of both might be a good compromise.

I believe James, Sowter, and several others make suitable driver/splitter transformers.

Aim for a pk-pk swing of at least 300V to assure that you can drive them to full power.

Several of my friends have built pp amplifiers with these tubes using talema, ulveco or similar toroidal power transformers for the output transformer. As I recall a model with two 120V primaries and two 18V secondaries works well to match to an 8 ohm load, and they're cheap.

Connect the primaries in series, with center tap connection, and connect the secondaries in parallel. If you want to match a 4 ohm load I think a 12V secondaries is close enough.

This equates to a primary Z of about 1.42K ohms plate to plate to an 8 ohm load. This trick probably won't work as well with EI types due to the much higher leakage inductance so use a toroid.

Kevin
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Old 31st March 2006, 06:18 PM   #4
Yvesm is offline Yvesm  France
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Just an idiot idea not yet checked :

Click the image to open in full size.

Almost no DC in the OPT so toroīds should be usable.

Yves.
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Old 31st March 2006, 08:42 PM   #5
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I was toying with the idea of using toroidals as outputs, but was a little concerned about DC with such variable tubes. Seperate cathode resistors would seem a must.
I saw listed on a datasheet a completely different operating point for these tubes. 135V anode, 250R cathode resistor, giving a current of 125mA. This means that the grid is at -35V which presumably would lessen the variability of bias. Since my speakers are 4ohm, I would need a 800ohm load which would mean about an 18V:240V which coincidentally is exactly the toroidals I have. The driver design would presumably be much more straighforward at these voltages, how about an improved futterman style driver with the cathode returned to the output.

Good to get some feedback.

Shoog
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Old 31st March 2006, 08:57 PM   #6
Yvesm is offline Yvesm  France
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Hey Shoog,

I'm not so far from that you suggest.

It's designed so that each half 6AS7 draw 100mA and has 115V Vak, bias being 40V.
This already set the plate dissipation at near 12W.

This sleeps in cartons for some month, I plan to build a preamp based on the same topology but with smaller 6n6p triodes before, but I can't tell when

Yves.
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Old 1st April 2006, 07:20 AM   #7
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It looks interesting and sort of the same, but isn't it running as something like a SEPP. Are the phase splitter and first stage two halves of ECC83, as this maintains the 2 tube philosophy.
I like the idea of running mine at just 135V because I have a big toroidal power transformer which would probably do power supply duty. Mine would be running at just over 0.5A in total.
When the output tubes grid is at -35V, whats the drive voltage which the phase splitter need to supply (70V PP maybe) ? This would mean that the phase splitter would need at least double that across it, so at least 140V.
How would I go about biasing this into more class AB territory. I have never done PP before so don't know about things like this.

Shoog
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Old 1st April 2006, 11:21 AM   #8
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How about using a mini power pentode to do current amplification, putting it through a toroidal interstage transformer with a 1:10 Voltage ratio. This would do the voltage amplification and phase splitting and would drive 6as7 directly. Simple and elegant.The only issue that would arise would be stability issues if global feedback was needed.

Shoog
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Old 1st April 2006, 01:32 PM   #9
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Heres a plan;
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File Type: jpg 6080 pp with interstage transformer.jpg (36.7 KB, 2795 views)
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Old 1st April 2006, 02:42 PM   #10
kevinkr is offline kevinkr  United States
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I wouldn't use a 1:10 step up unless the rp of the driving tube was extremely low as this equates to a 25 fold increase in source impedance to each of the driven grids.. (say 3K rp to 75K effectively at each grid) Mr. Miller would come to town and you would be lucky to have any response above 10kHz.

The designs I have seen all use the toroid as a conventional pp output transformer. Bear in mind the low voltage configuration is going to give you somewhat fewer watts as well. Some means of adjusting dc balance may be advisable.
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