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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Wed 21, 2018 12:41 am 
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john8750 wrote:
I did some more testing and the resistors are still too hot to touch.
I used two 220ohm 1watt resistors. The actual resistance is 441.6.
Test voltage is 21.68. 41ma. Or 1.06watts.
Are you sure your resistors are 1watt?
Attachment:
The attachment 20180320_125724.jpg is no longer available
At 50% power a carbon film power resistor (2 x 1 Watt) would rise about 30 C above ambient which, if it's 25 C, would be about 130 F, or roughly the temp of 'hot water' out of the tap. (note, when you hold it in your hand/fingers you're partly thermally insulating the resistor so it's overall temp will increase).

Power resistors get hot; that's how they dissipate the energy. If there was no temperature rise they couldn't dissipate anything.

You guys need to look at some datasheets.

For carbon film resistors the larger power capacity is comes by increasing the allowed heat rise (rating) of the resistor so it isn't surprising that, for the same load, a 3 or 5 Watt resistor get's as (or almost) as hot as a 1 Watt resistor.

Attachment:
Power Rating.jpg
Power Rating.jpg [ 89.07 KiB | Viewed 1568 times ]


This may seem counter intuitive but it's due to the interior to exterior thermal resistance of the overall package. I.e. the 'small' package has more thermal resistance so the inside is much hotter than the outside. With the lower thermal resistance of the larger package the outside can be hotter and keep the inside just as cool (which is what really matters).


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Wed 21, 2018 7:08 pm 
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This is interesting.
I found the exact 470ohm 5watt resistor that was in the original circuit. I had already discarded the original.
This test has a higher wattage through the resistor than the original. And it is much cooler than the original.
The only difference I can think of would be that the original, in the amp, was in a more confined space. This one
is in open air.
Attachment:
20180321_104229.jpg
20180321_104229.jpg [ 132.7 KiB | Viewed 1561 times ]

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Wed 21, 2018 7:12 pm 
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I didn't have room inside the amp for this volume remote control. So planning on building a separate plug-in housing for it.
Attachment:
Remote volume control.jpg
Remote volume control.jpg [ 111.68 KiB | Viewed 1561 times ]

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Wed 21, 2018 10:02 pm 
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john8750 wrote:
This is interesting.
I found the exact 470ohm 5watt resistor that was in the original circuit. I had already discarded the original.
This test has a higher wattage through the resistor than the original. And it is much cooler than the original.
The only difference I can think of would be that the original, in the amp, was in a more confined space. This one
is in open air.
Attachment:
20180321_104229.jpg



But, 1.38watts would make more heat.

I know this is boring to you guys. Well worth it to me.
Teaching this old hillbilly a lot. Thank you :lol:

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Wed 21, 2018 10:29 pm 
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Men- looking forward, I want to rebuild my 6V6 amp. Thinking about this schematic. But with the same B+ supply I have, silicone rectifiers. I would like to use the tube preamp/inverter though.
Can you suggest a better tube than the 6SL7? I know we talked about this before. 6BR8-12AX7, or? Thanks Men.
Attachment:
6SL7-6V6-Push-Pull-Tube-Amp-Schematic.png
6SL7-6V6-Push-Pull-Tube-Amp-Schematic.png [ 36.15 KiB | Viewed 1558 times ]

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Wed 21, 2018 10:42 pm 
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john8750 wrote:
john8750 wrote:
This is interesting.
I found the exact 470ohm 5watt resistor that was in the original circuit. I had already discarded the original.
This test has a higher wattage through the resistor than the original. And it is much cooler than the original.
The only difference I can think of would be that the original, in the amp, was in a more confined space. This one
is in open air.
Attachment:
20180321_104229.jpg



But, 1.38watts would make more heat.

I know this is boring to you guys. Well worth it to me.
Teaching this old hillbilly a lot. Thank you :lol:

Huh?
What are you saying?

Your original hot problem was with a 5watt 470 ohm resistor dropping 25vDC
25/470 =53ma
53ma x 25v =1.329 watts.

And yes a 5 watt resistor should not get hot with only 1.33 watts.

But I guess it depends on what kind of material the resistor is made from and how it dissipates that heat.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Wed 21, 2018 10:50 pm 
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john8750 wrote:
Men- looking forward, I want to rebuild my 6V6 amp. Thinking about this schematic. But with the same B+ supply I have, silicone rectifiers. I would like to use the tube preamp/inverter though.
Can you suggest a better tube than the 6SL7? I know we talked about this before. 6BR8-12AX7, or? Thanks Men.
Attachment:
6SL7-6V6-Push-Pull-Tube-Amp-Schematic.png

Those are the same tubes that I'm using in my SE "lacewood" amp.
Except not PP.
Image

I'm noticing that volume-wise the input sensitivity or gain of the 6SL7 does pretty darn well compared to the op-275 but not quite as good. The volume on the mp3player needs to be cranked up twice as high... but then it's fine. Of course if I put the passive amplifier made with two Rat-shack transformers in front of the input ... then ... ok .. I get a nice boost and can lower the Mp3 players vol. But it works ok either way.

Why do you want to abandon the op-275 this time?

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Wed 21, 2018 11:54 pm 
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Mike Toon wrote:
22.3V/60ma still too hot. 139° after 1 minute.
Spec sheet wise 139 F is not "too hot." It may be warmer than you would like but (for 1 Watt carbon film resistors) maximum body temp is 212 F @ max power rating and a 25 C ambient. It can handle max power at 75 C as well, which brings abs max body temp to 300 F.

Now, for a 1.34 Watt power dissipation I'd pick a minimum of a 3 Watt resistor (the ole 2x rule) but, as noted above, body temp would get just as 'hot' (if it's a carbon film).


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 12:03 am 
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john8750 wrote:
This is interesting.
I found the exact 470ohm 5watt resistor that was in the original circuit. I had already discarded the original.
This test has a higher wattage through the resistor than the original. And it is much cooler than the original.
The only difference I can think of would be that the original, in the amp, was in a more confined space. This one
is in open air.
Attachment:
20180321_104229.jpg
Yes, ambient can make a huge difference depending on the circulation, or lack thereof. As I said, I could easily see 'inside' ambient being 50 C so add 45 F to your "much cooler" temp and imagine what that feels like. A "not bad" 100 F becomes a scorching "too hot to handle" 145 F. And that's assuming the ambient around the resistor stays that low but with no air circulation it could be even higher.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 12:46 am 
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Flipperhome wrote:
john8750 wrote:
This is interesting.
I found the exact 470ohm 5watt resistor that was in the original circuit. I had already discarded the original.
This test has a higher wattage through the resistor than the original. And it is much cooler than the original.
The only difference I can think of would be that the original, in the amp, was in a more confined space. This one
is in open air.
Attachment:
20180321_104229.jpg
Yes, ambient can make a huge difference depending on the circulation, or lack thereof. As I said, I could easily see 'inside' ambient being 50 C so add 45 F to your "much cooler" temp and imagine what that feels like. A "not bad" 100 F becomes a scorching "too hot to handle" 145 F. And that's assuming the ambient around the resistor stays that low but with no air circulation it could be even higher.




Well, that explains it. I now that the sand resistor don't get as hot as the carbon one. It looks like it can handle the heat better, and would be a better choice for power supplies.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 12:50 am 
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john8750 wrote:
Men- looking forward, I want to rebuild my 6V6 amp. Thinking about this schematic. But with the same B+ supply I have, silicone rectifiers. I would like to use the tube preamp/inverter though.
Can you suggest a better tube than the 6SL7? I know we talked about this before. 6BR8-12AX7, or? Thanks Men.
Attachment:
6SL7-6V6-Push-Pull-Tube-Amp-Schematic.png
It all boils down to gain. The 6SL7 has a mu of 70. The 12AX7 has a mu of 100. In audio terms that's not much more (it's only 3 dB).

In practice you only get about 60% of the theoretical mu in actual voltage gain (depending on the circuit). I mention that because pentodes, because the cathode is shielded from the plate by the screen, aren't spec'd with a "mu." At any rate, I picked the 6BR8 because it was a 'dollar days' tube and the SPICE measured gain was 160 (other pentode could get more, say 300), which corresponds to a 'mu' of, say, 270. That corresponds to an increase of 12 dB over the 6SL7 (and almost 9 dB over the 12AX7).

Then again, the power tubes enter into the gain picture and the 6V6 has only modest gain. The 6BQ5 has 10 dB more gain (and the 6HB6 almost another 3 dB over that). You can see why the 6BQ5 was instantly the tube of choice for medium power Hi-Fi amps when it came out because it takes around 14-20 dB of overall feedback to get "Hi-Fi."


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 12:54 am 
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Pbpix wrote:
john8750 wrote:
Men- looking forward, I want to rebuild my 6V6 amp. Thinking about this schematic. But with the same B+ supply I have, silicone rectifiers. I would like to use the tube preamp/inverter though.
Can you suggest a better tube than the 6SL7? I know we talked about this before. 6BR8-12AX7, or? Thanks Men.
Attachment:
6SL7-6V6-Push-Pull-Tube-Amp-Schematic.png

Those are the same tubes that I'm using in my SE "lacewood" amp.
Except not PP.
Image

I'm noticing that volume-wise the input sensitivity or gain of the 6SL7 does pretty darn well compared to the op-275 but not quite as good. The volume on the mp3player needs to be cranked up twice as high... but then it's fine. Of course if I put the passive amplifier made with two Rat-shack transformers in front of the input ... then ... ok .. I get a nice boost and can lower the Mp3 players vol. But it works ok either way.

Why do you want to abandon the op-275 this time?



I like the IC preamp just fine. I want to try the tube version next. If I cant get high enough volume from the 6SL7, I will use an IC preamp anyway. This amp will be only a power amp. I will build a separate preamp with selection, and tone controls. But, is there a better and more modern preamp tube other than the 6SL7? I want to use the same 6V6G tubes and the same OT's. And use the same B+ supply.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 1:05 am 
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Flipperhome wrote:
john8750 wrote:
Men- looking forward, I want to rebuild my 6V6 amp. Thinking about this schematic. But with the same B+ supply I have, silicone rectifiers. I would like to use the tube preamp/inverter though.
Can you suggest a better tube than the 6SL7? I know we talked about this before. 6BR8-12AX7, or? Thanks Men.
Attachment:
6SL7-6V6-Push-Pull-Tube-Amp-Schematic.png
It all boils down to gain. The 6SL7 has a mu of 70. The 12AX7 has a mu of 100. In audio terms that's not much more (it's only 3 dB).

In practice you only get about 60% of the theoretical mu in actual voltage gain (depending on the circuit). I mention that because pentodes, because the cathode is shielded from the plate by the screen, aren't spec'd with a "mu." At any rate, I picked the 6BR8 because it was a 'dollar days' tube and the SPICE measured gain was 160 (other pentode could get more, say 300), which corresponds to a 'mu' of, say, 270. That corresponds to an increase of 12 dB over the 6SL7 (and almost 9 dB over the 12AX7).

Then again, the power tubes enter into the gain picture and the 6V6 has only modest gain. The 6BQ5 has 10 dB more gain (and the 6HB6 almost another 3 dB over that). You can see why the 6BQ5 was instantly the tube of choice for medium power Hi-Fi amps when it came out because it takes around 14-20 dB of overall feedback to get "Hi-Fi."



IC. That is very evident in the performance of the Prince John. Its remarkable. Is there a better matchup than the 6BR8/6HB6 at reasonable cost? Don't want to get too far on this topic. Will start a new thread.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 1:07 am 
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A new thread about repairing my Heathkit FM-4
viewtopic.php?f=13&t=337644&p=2829007#p2829007

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 1:33 am 
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john8750 wrote:
Flipperhome wrote:
It all boils down to gain. The 6SL7 has a mu of 70. The 12AX7 has a mu of 100. In audio terms that's not much more (it's only 3 dB).

In practice you only get about 60% of the theoretical mu in actual voltage gain (depending on the circuit). I mention that because pentodes, because the cathode is shielded from the plate by the screen, aren't spec'd with a "mu." At any rate, I picked the 6BR8 because it was a 'dollar days' tube and the SPICE measured gain was 160 (other pentode could get more, say 300), which corresponds to a 'mu' of, say, 270. That corresponds to an increase of 12 dB over the 6SL7 (and almost 9 dB over the 12AX7).

Then again, the power tubes enter into the gain picture and the 6V6 has only modest gain. The 6BQ5 has 10 dB more gain (and the 6HB6 almost another 3 dB over that). You can see why the 6BQ5 was instantly the tube of choice for medium power Hi-Fi amps when it came out because it takes around 14-20 dB of overall feedback to get "Hi-Fi."

IC. That is very evident in the performance of the Prince John. Its remarkable. Is there a better matchup than the 6BR8/6HB6 at reasonable cost? Don't want to get too far on this topic. Will start a new thread.
Well, I certainly don't think so, or else I'd have used those instead. :wink: That doesn't mean it's absolutely the 'best amp' possible but I think it's pretty close to, if not, the best 'low cost' all tube amp you can make.

I haven't simulated it but see no reason why you couldn't use the 6BR8 with the 6V6s too. It would just have less gain and, perhaps, need a 'preamp' of some sort to use with MP3 players/cellphones.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 3:27 am 
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Flipperhome wrote:
john8750 wrote:
Flipperhome wrote:
It all boils down to gain. The 6SL7 has a mu of 70. The 12AX7 has a mu of 100. In audio terms that's not much more (it's only 3 dB).

In practice you only get about 60% of the theoretical mu in actual voltage gain (depending on the circuit). I mention that because pentodes, because the cathode is shielded from the plate by the screen, aren't spec'd with a "mu." At any rate, I picked the 6BR8 because it was a 'dollar days' tube and the SPICE measured gain was 160 (other pentode could get more, say 300), which corresponds to a 'mu' of, say, 270. That corresponds to an increase of 12 dB over the 6SL7 (and almost 9 dB over the 12AX7).

Then again, the power tubes enter into the gain picture and the 6V6 has only modest gain. The 6BQ5 has 10 dB more gain (and the 6HB6 almost another 3 dB over that). You can see why the 6BQ5 was instantly the tube of choice for medium power Hi-Fi amps when it came out because it takes around 14-20 dB of overall feedback to get "Hi-Fi."

IC. That is very evident in the performance of the Prince John. Its remarkable. Is there a better matchup than the 6BR8/6HB6 at reasonable cost? Don't want to get too far on this topic. Will start a new thread.
Well, I certainly don't think so, or else I'd have used those instead. :wink: That doesn't mean it's absolutely the 'best amp' possible but I think it's pretty close to, if not, the best 'low cost' all tube amp you can make.

I haven't simulated it but see no reason why you couldn't use the 6BR8 with the 6V6s too. It would just have less gain and, perhaps, need a 'preamp' of some sort to use with MP3 players/cellphones.


Thanks again Flip. Gives me some things to consider. I want to build another amp. I already have a 6V6, 5902, and the Prince John. And the Blumoon. Not that I need another. Maybe a SE like Peter is working on. He has a neat little power supply idea.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 3:47 am 
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john8750 wrote:
Thanks again Flip. Gives me some things to consider. I want to build another amp. I already have a 6V6, 5902, and the Prince John. And the Blumoon. Not that I need another. Maybe a SE like Peter is working on. He has a neat little power supply idea.

Yeah John... this tiny little 300vDC boost supply is fantastic... and only $5

I feed it 32vDC @530ma and it makes 300vDC @ 100ma (to a max of 390v @200ma)

https://www.ebay.com/itm/8-32V-to-45-39 ... SwVqlaEjKr

The 12V6 filament in this schematic will be replaced with two 6V6s in series.
Image

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 5:51 am 
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I like it Peter and just bought one. Could take up till June 1st to get it.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 6:05 am 
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john8750 wrote:
I like it Peter and just bought one. Could take up till June 1st to get it.

There are tons more there on Ebay... look... perhaps some with speedier delivery.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Mar Thu 22, 2018 6:15 am 
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Pbpix wrote:
john8750 wrote:
I like it Peter and just bought one. Could take up till June 1st to get it.

There are tons more there on Ebay... look... perhaps some with speedier delivery.



I already ordered but not a big deal. Next project for me is the Heathkit FM-4.
And some speakers for the 300b amp, Blumoon. Next Month.

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