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Old 9th September 2014, 10:32 AM   #1
6V6dude is offline 6V6dude  Australia
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Default Opinion on amp build

Few years ego I've build hifi amp from spare parts I had around. It' been great but since it's somewhat out of specs I'd be interested in an opinion from someone who is experienced in the not so common builds.
Amp is: Single ended 6V6 stereo - basic 6V6 schema with single 6SN7 for preamp. PT is 180V leaving me with 165V on 6V6 plates (bellow specs). To get some power out of it I've biased the cathode at 50mA (11V) using only 22ohm resistor - getting about 3W clean - 5W with some smooth clipping. Audio trafo is 7K/16ohm running 4 or 8 ohm speakers. With everything being somewhat out of specs I took my best guess to put it all together. Amp sound great, is in use daily and tubes aren't wearing out. I'd welcome opinions on how close or far I got to specs on it since my standard tube calculators seem to be useless in this case.
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Old 9th September 2014, 12:02 PM   #2
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A schematic would be helpful.

I take it you're running the 6V6's in pentode mode? What OT primary impedance, what about NFB?

I'm guessing your preamp clips before the power amp. 165 volts with the 6SN7 won't leave any real headroom, unless you're using plate chokes. Therefore - and I'm still guessing because there's no schematic - I'd look into the preamp first as a source of improvement.

Is the PT 180-0-180? If so, current capability allowing, you could run it with a bridge rectifier to get 360. That would leave you with a lot of great opportunities, such as parafeed for the output stage and proper op point and plate loads for the preamp.
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Old 9th September 2014, 12:08 PM   #3
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SE or PP?

You meant 220 ohms for 11V and 50mA?
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Old 9th September 2014, 05:20 PM   #4
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Yes more details are needed, especially the power supply.

Correct me if I am wrong;

1) Single ended 6V6 output 165v B+
2) Single triode 6SN7 gain stage (what are your plate load and cathode resistor values used)
3) Output transformer primary 7k/16R but using 8 or 4 ohm load so reflective primary load is 3.5k with 8 ohm speakers and 1.75k with 4 ohm speakers.
4)6V6 biased to 50mA (you said you had 11v at cathode with a 22R resistor this would give you .5A so I am with MrCurwin in thinking you have a 220R cathode resistor giving you 50mA.

Please post schematic it would help a lot.
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Old 9th September 2014, 07:08 PM   #5
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Quote:
Originally Posted by 6V6dude View Post
Amp is: Single ended 6V6 stereo - basic 6V6 schema with single 6SN7 for preamp. PT is 180V leaving me with 165V on 6V6 plates (bellow specs). To get some power out of it I've biased the cathode at 50mA (11V) using only 22ohm resistor - getting about 3W clean - 5W with some smooth clipping. Audio trafo is 7K/16ohm running 4 or 8 ohm speakers. With everything being somewhat out of specs I took my best guess to put it all together. Amp sound great, is in use daily and tubes aren't wearing out. I'd welcome opinions on how close or far I got to specs on it since my standard tube calculators seem to be useless in this case.
Doanworrybouddit.

Transistors have specs; vacuum tubes have suggestions. Hollow state is a helluvalot more forgiving than solid state. Your 165 VPP isn't that far off from the suggested 180V. Running 50mA static plate current puts the PD well within the PD= 12W spec.

The load impedance is a bit on the high side, but all that means is a bit less power, and lower distortion. The only real concern here is the low supply voltage for the pre. Passively loaded triodes like a bit more VPP for both headroom, and for reducing the plate load (larger plate load rersistor). It should be NBD to add a voltage multiplier to give the pre a bit more headroom. It doesn't look like this a a problem, however.

You didn't mention gNFB (but I'd guess it doesn't use any if the pre is a 6SN7) and it may not be necessary as the 6V6 has good harmonic distortion performance. If it sounds good enough for daily use, then if it ain't broke, don't fix it.
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Old 10th September 2014, 03:33 AM   #6
6V6dude is offline 6V6dude  Australia
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Sorry don't have a schema. I always build amps "as I go". But I just got this of the Net and added my values. http://members.optushome.com.au/myplanet/6v6schema.jpg
The 6SN7 is perfect, it's the 6V6 I'm curious about, specially the 22ohm resistor. It does seem too low to me as with my other builds at over 250V I'd be using 220ohm or so. But the 50mah is what it reads.
Yes I agree with the "if it ain't broke, don't fix it" so I won't, but I'm just very curious if 6V6 running bellow minimum specs would require such low bias resistor to run properly.
I remember trying 220ohm etc when I was building it but the output was way too low like 1-2W.
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Old 10th September 2014, 03:38 AM   #7
6V6dude is offline 6V6dude  Australia
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You can see the amp running video amongst my few other builds. Bad quality vids tho.
https://www.youtube.com/watch?v=J6sL...7lV8UqK5wCVfVQ
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Old 10th September 2014, 04:18 AM   #8
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11 volt drop across a 22 ohm resistor would be 11/22 = .5A not 50mA


Are you using a 16 ohm speaker on the 16 ohm tap?

Sounds like Pretty Lights on the next video

Last edited by famousmockingbird; 10th September 2014 at 04:23 AM.
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Old 10th September 2014, 05:08 AM   #9
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With a 22R resistor you should be getting just over 1V at the cathode for the 6V6.
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Old 10th September 2014, 12:45 PM   #10
6V6dude is offline 6V6dude  Australia
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No, the measured values are with using 4 ohm speakers on the 16 ohm tap. But mostly I use large Advent 8 ohm speakers with the amp and it pushes them just fine. I didn't measure the values them since they're in my room and no in workshop.
And I'm getting 11V on the 22 ohm not 1V. Yeah I know it somehow doesn't make sense but that's what I'm getting. But one thing I've noticed is that when I use modern made 6V6 tubes like Svetlana , the amp sound louder but way too bright. Using EL84 produces even more mower but it's distorted. 6V6 tube normally have in the amp are old AWA ones. They're at about 80% but been the same since I've build the amp. Maybe that's got something to do with the weird readings? The old tubes used to be made out of different materials (apparently) so maybe handle things slightly differently, not really sure. I just know that vintage tubes sound much better to my ear :-)
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