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PostPosted: 08 Jun 2010, 20:17 
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This project has been one of the most educational experiences I've had in quite a long time.

Does anybody know what voltage Edcor uses to hi-pot their tube output transformers? I forgot to ask today.

I got an email from Edcor today. Bad news, sort of. They say the wire they would need to use to get about 20K primary impedance would be too small using their standard 100 watt bobbin and core. Not a surprising answer. I now understand a lot more than I did just a week ago after a lot of reading.

They also said the CXPP100-MS-10K may work by using the 4 ohm tap with an 8 ohm speaker, they aren't sure. I gave them all the details including a load line for one tube across half the primary as well as the circuit schematic so they know what I want to do and the voltages involved.

By my calculatons 6 ohms would be tapped at 61.2% of a 16 ohm winding, so it seems to me that by eliminating the 16 and 8 ohm portion of the secondary winding and just winding a 6 ohm secondary they could reduce the size of the secondary winding by 39%. It also seems like the extra room could be used for beefed up insulation and/or more primary winding turns.

I called Edcor today after getting that email and ended up talking to a different guy. I asked him if it would be possible to build a transformer with a 15K primary and a 6 ohm secondary. I emailed him all the info and he's looking into that now.

What is the failure mode of an internally arcing transformer - does appreciable voltage develop on the secondary if it's grounded? If the amp is fused properly and the secondary is well grounded would it be dangerous?

To insure safety of the amp when constructed, I also have a microwave oven transformer that I could put on a variac to create whatever voltage potential I wish to up to about 4000 VRMS. I could use that to hi-pot the transformers myself to make sure they don't arc at the voltage I wish to use them. I'd current limit it with a very high value resistor so that an arc couldn't be sustained in the event it did fail and do it from a long distance with the case of the variac solidly grounded and attached to a GFCI.

I’ll call tomorrow again and ask, but does anybody know what voltage Edcor uses to hi-pot their tube output transformers?

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PostPosted: 13 Jun 2010, 03:27 
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More developments to add to my monologue on the GM70 oddwatt. I had a major success in testing one of the critical stages.

The BUX85 450 volt transistor in the CCS works great. I installed one in my KT88 oddwatt to test. At first it was very noisy, mostly hiss, but bypassing the base to the emitter with a small tantalum cap completely eliminated any percepable noise. It's now operating flawlessly as far as my ears and instruments can tell. The response looks and sounds the same. The cathodes can have up to 450 volts on them without damaging the CCS, more than enough for the GM70 tube.

The trannys are 3 weeks out and the case is now drilled for 4 GM70 and 2 6H8C (6SN7) tubes.

If the LM317 is used as a voltage regulator there are very specific recommendations for caps to improve the response and reduce the noise. Is there a way to improve the CCS circuit with a cap similar to the voltage regulator circuit? I have a tiny bit of hiss in both channels, and after hearing how much the cap on the transistor reduced the hiss I have to wonder what a small cap across the LM317 (across the resistors?) would do.

The bias on the BUX85 is temporarily provided by a 9 volt battery to supply the base current.

Image

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PostPosted: 13 Jun 2010, 14:17 
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I'm afraid that for driving GM70, your dr. tube doesn't have enough gain
/gain 6SN7 is 20, in SRPP will be no more 25-30. If Your input signal is 0,6-0,8 v,
you must have gain about 150, and You can use penthode 6E5P, E180F, E280F, EF861...!/.

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 13 Jun 2010, 16:16 
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azazello wrote:
I'm afraid that for driving GM70, your dr. tube doesn't have enough gain
/gain 6SN7 is 20, in SRPP will be no more 25-30. If Your input signal is 0,6-0,8 v,
you must have gain about 150, and You can use penthode 6E5P, E180F, E280F, EF861...!/.


I think output impedance and output voltage of the driver stage are the two important considerations, not gain.

I own the 6E5P. It's capable of driving a GM70 when run at a relatively high plate voltage, but it's pushing it. The 6SN7 SRPP is a low distortion, medium gain circuit with an relatively low output impedance that can easily provide the voltage output needed (240 volts peak to peak). I will need and expect to require a preamp for this amp so I don't care about the driver gain so much. The 6H8C appears to work well in my 300B amp and may be able to replace the 6SN7 in the GM70 amp depending on the voltage it can handle.

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PostPosted: 13 Jun 2010, 16:42 
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300B has bias 30-50v and for driving it is need mu 40-50. 6SN7 is suitable in SRPP only!
But GM70 has bias more 100v and mu must be 150. I'm afraid that 6SN7 isn't suitable.
/Aprox. You can choose tube with mu = Ubias next power tube divide Uinput ....
P.E. mu in.tube must be = 100v GM70 divide 0,6-0,8v = 166-125 ....You can use for driving GM70 only penthodes with current 30-50 ma/.

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 14 Jun 2010, 01:59 
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azazello wrote:
300B has bias 30-50v and for driving it is need mu 40-50. 6SN7 is suitable in SRPP only!
But GM70 has bias more 100v and mu must be 150. I'm afraid that 6SN7 isn't suitable.
/Aprox. You can choose tube with mu = Ubias next power tube divide Uinput ....
P.E. mu in.tube must be = 100v GM70 divide 0,6-0,8v = 166-125 ....You can use for driving GM70 only penthodes with current 30-50 ma/.

??? :confused:

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PostPosted: 14 Jun 2010, 02:52 
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sampleaccurate wrote:
azazello wrote:
300B has bias 30-50v and for driving it is need mu 40-50. 6SN7 is suitable in SRPP only!
But GM70 has bias more 100v and mu must be 150. I'm afraid that 6SN7 isn't suitable.
/Aprox. You can choose tube with mu = Ubias next power tube divide Uinput ....
P.E. mu in.tube must be = 100v GM70 divide 0,6-0,8v = 166-125 ....You can use for driving GM70 only penthodes with current 30-50 ma/.

??? :confused:

Or You can use 2 stage preampl. 6SN7 /mu = 20 x20= 400, but it current is small/ and for more current You can use
one more stage cathode follover 6V6, 6L6, KT80. But Your prestage will be 3.....more elements, low quality IMO. I'd advice You only 6E5P /amazing tube with 40 ma current and low Ri, I used for my 2 stage ampl SE 6S33S/.....See OddBlock with perfect quality....only 7-8 passive elements. This is only one PP that I like! Your Oddblock GM70 is in right direction!

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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PostPosted: 15 Jun 2010, 17:18 
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azazello wrote:
300B has bias 30-50v and for driving it is need mu 40-50. 6SN7 is suitable in SRPP only!
But GM70 has bias more 100v and mu must be 150. I'm afraid that 6SN7 isn't suitable.
/Aprox. You can choose tube with mu = Ubias next power tube divide Uinput ....
P.E. mu in.tube must be = 100v GM70 divide 0,6-0,8v = 166-125 ....You can use for driving GM70 only penthodes with current 30-50 ma/.

I don't quite understand how you compute the input power needed to drive the GM70.

How would you model the GM70 as a load? A capacitor? A capacitor in parallel with a resistor?

Take a look at the data sheet, tell me what the input impedance of the GM70 is, and I'll tell you if the 6SN7 SRPP will be able to drive it. I admit my ignorance here, but certainly a prerequisite for a driver stage is voltage swing. I repeat, the gain, the current and the power don't mean a thing if the driver tube can't handle the voltage. Conversely, the voltage swing doesn't mean much if the driver output impedance is too high to drive the power stage.

My problem is computing the input impedance of the GM70. The input capacitance on the grid is pretty clearly defined. I guess my sticking point is "how can a tube transfer power to another tube if tubes draw no appreciable grid current?"

The models I've run in PSPICE indicate the 6SN7 SRPP could drive at least several times the input capacitance of the GM70 without any ill effects.

I think there's too much of a language barrier here. I have trouble understanding your posts. Nobody's fault - it is what it is.

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PostPosted: 15 Jun 2010, 20:59 
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Hi, I also would not use the 6SN7 because the gain is a bit low. A 6SL7 is a better choice and should be sufficient with most typical inputs. The outputs should ideally not require a significant amount of current (just voltage) drive. If more drive is wanted a design with a 12AT7 SRPP should work. I would suggest an idle current in the 12AT7 of about 6-7 ma with a B+ on it of around 220. Remember to watch out for the heater to cathode ratings in both sections of the tube.

Good listening
Bruce

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PostPosted: 16 Jun 2010, 00:30 
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gofar99 wrote:
Hi, I also would not use the 6SN7 because the gain is a bit low. A 6SL7 is a better choice and should be sufficient with most typical inputs. The outputs should ideally not require a significant amount of current (just voltage) drive. If more drive is wanted a design with a 12AT7 SRPP should work. I would suggest an idle current in the 12AT7 of about 6-7 ma with a B+ on it of around 220. Remember to watch out for the heater to cathode ratings in both sections of the tube.

Thanks for the suggestions. I think I need at least 240 volts peak to peak voltage to drive the GM70, 250 to be safe. A B+ of 220 won't hack it. Therein lies my problem.

I need a tube that can handle a high plate voltage. If I use a lower gain tube like the 6SN7, it should work fine if my preamp can supply enough input voltage to the 6SN7, correct? If so I'm inclined to go with the 6SN7 because of the higher B+ and the higher cathode to heater voltages - the heater can swing 300 volts with respect to the cathode on the 6SN7 - it has great voltage specs - max rated DC plate voltage is 450V. I've also seen a lot of 300B tube circuits with 6SN7 drivers. My 300B SE has a 6SN7 driver and the sensitivity of the amp is great - plenty of overall gain.

I'll also probably put a high quality SS gain stage right at the input to boost and buffer the signal (OPA134s make great preamps IMHO and there are those that agree - read the review of the Oatley headphone amp and the comments by the author (Mark) about the OPA2134 and the headphone driver chip). I agree with him. Such a board could be placed extremely close to the inputs and buffer the signal before it hits the input pot and the 6SN7. This would almost surely reduce induced noise in the signal after it enters the amp before it is amplified by the first tube stage as well as increase the sensitivity of the amp. I know a lot of people will cringe at this but until you've tried the latest op-amps specifically designed for audio and made a fair, objective comparison don't knock it.

BTW, the high voltage CCS is working great. I've tested it for several days now and it works like a charm. 450 volts!!! :hot:

A high wattage volt dropping resistor will be needed for high cathode voltages to reduce the power dissipation of the transistor unless a massive heatsink is used. I put a 100 ohm resistor in series with the LM317s in my oddwatt and it reduced the heat significantly on the regulator by dropping the input from 40 volts to 20 volts. The extra heat is now dissipated by the resistor.

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