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 Post subject: control amp for a 300B
PostPosted: Dec Sat 25, 2021 8:26 pm 
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I have a home made 300B, built a few years ago. Never used it. I plan to set up a listening room.
Will use a pair of 8ohm 8" full range speakers.
I need a control amp because of a hearing deficiency, so can tune tones.
The only input will be an MP3 player.
Am planning on 3 12ax7's. About 150v B+ or?

I need to dig up old schematics for planning
Will post a photo of the 300B.

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Sat 25, 2021 11:53 pm 
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Here is the amp I will build. Only need the tone stack with a little boost.
Peter and Flip designed it.
viewtopic.php?f=12&t=355519

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Sun 26, 2021 12:16 am 
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And here is the 300B.
Remarkable, using correct type speakers.
viewtopic.php?f=12&t=325429

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Sun 26, 2021 4:14 am 
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What speakers are you going to use and what frequency range is your hearing loss in . If you bought something like EV aristocrats , regencies or certain Klipsch speakers they have level controls on the midrange and tweeters .This way you can adjust the speakers to compensate or your hearing loss .

https://frank.pocnet.net/sheets/084/3/300B.pdf

A single ended 300B amp will put out about 5 true watts of power and you will need very efficient speakers for a good room filling volume , if a preamp with a tone control is used , the system would need at least 10 watts( most likely a lot more ) to keep the amp from clipping, when a lot of bass or treble is added .

I use a pair of 6B4G push pull amps that put out 8 true watts of power , WE300B PP will put out between 10 and 20 watts push pull .

Built my sound system around my hearing at the age 69 with a lot of bass loss due to the work environment and high frequency loss due to age .

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Sun 26, 2021 8:49 pm 
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Thanks Mark.
My 300B uses aftermarket tubes. Would be great to afford an original pair. It is single ended. If it was a PP, I could use a two way speaker system.
I like the full range speakers I use right now. I mounted them in an old Magnavox consul. Dont know any spec's. 8".
My hearing gets very dull right at 1100hz. But, can hear below 20hz. Weird.
So I need the control amp or maybe an equalizer. Notice the buffer stage in my Little Prince preamp.
The 300B sounds remarkable. But I must say Many smaller amps sound very well.
I will post a photo soon.

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Mon 27, 2021 7:11 pm 
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What is a control amp?


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 Post subject: Re: control amp for a 300B
PostPosted: Dec Mon 27, 2021 7:46 pm 
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I think he means a preamp.

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Mon 27, 2021 8:59 pm 
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I would call a preamp with a tone stack a control amp. 8)
BUT, that's in my world so sorry for the confusion.
My preamp will have on/off volume balance bass treble.
Will need three tubes, including a buffer with gain.
To keep with the vintage, might use a tube rectifier,?
B+ might need to be around 250v, instead of 150.

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Mon 27, 2021 9:01 pm 
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Here she is. Been is storage so needs dusting.
But has that vintage look.

Attachment:
20211226_134115.jpg
20211226_134115.jpg [ 369.76 KiB | Viewed 2577 times ]

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Fri 31, 2021 1:18 am 
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If I use this power supply, how would I convert it for a tube rectifier? It must be an octal base type tube.
download/file.php?id=246310

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Fri 31, 2021 2:01 am 
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john8750 wrote:
If I use this power supply, how would I convert it for a tube rectifier? It must be an octal base type tube.
download/file.php?id=246310

Don't do it John... lol
....Why would you want to?
....I don't see any benefit.
Lots of extra stuff like filament transformers and tube sockets even if you use just two dual rectifier tubes.
Or needing a higher voltage CT transformer. For Full WAVE using single dual rectifier tube.
Tubes have larger voltage drop across them and SS diodes have almost none.
REF
( https://www.rfcafe.com/references/elect ... 1-3-20.htm )

Attachment:
full wave bridge with tubes.png
full wave bridge with tubes.png [ 67.75 KiB | Viewed 2503 times ]

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Last edited by Pbpix on Dec Fri 31, 2021 2:47 am, edited 1 time in total.

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Fri 31, 2021 2:06 am 
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https://groups.io/g/duncanampspsud




Use this program above .

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Fri 31, 2021 6:04 am 
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Don't use the schematics posted above exactly; using a transformer with 500 V on each side of the center tap will give about 700 V B+. That's WAY too much. Use a transformer with about 200 to 250 V on each side of the center tap. That will give B+ in the 250 to 300 V range. Even that is more than is needed for a preamp unless your power amp is unusually insensitive (with the number of tubes shown in your photo it better not be since then something would be wrong). Most power amps need about 1 V of signal for full output. A preamp with 125 to 150 V of B+ can easily do this.

If you want an octal rectifier, your best choices are a 5Y3 if the transformer has a 5 V winding or a 6X5 if it only has a 6.3 V winding. If you insist on a 12.6 V winding your only easily available choice is a 12X4 which is a miniature tube.

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 Post subject: Re: control amp for a 300B
PostPosted: Dec Fri 31, 2021 8:37 pm 
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Note:
John... as concerns the tube circuit above:
"If directly heated diodes are used in a bridge rectifier, three separate filament transformers are required. This is due to the different potentials existing at the filaments of the diodes. The filaments of V2 and V3 in figure 3-14 are at the same potential, but the filament of V1 is at a different potential from either V2 or V4. The three filament transformers must be well insulated from each other, and from ground, because of the high potentials to which they are subjected. The use of indirectly heated diodes would solve the filament transformer problem, but the high potential difference between cathode and heater would be likely to result in arcing. "

So to avoid 3 filament transformers, maybe 6X5 would work w/o arcing at your B+

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 Post subject: Re: control amp for a 300B
PostPosted: Jan Sat 01, 2022 12:15 am 
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I just looked at the posted schematics again. The bridge rectifier is worse than what I wrote above. With the transformer shown the B+ would be about 1400V! For a bridge circuit, the total secondary voltage should be about 200 to 250V.

If you want to use the bridge circuit, you could use two 6BY5 tubes.

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 Post subject: Re: control amp for a 300B
PostPosted: Jan Sat 01, 2022 4:25 am 
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In the early days of HI-Fi, some power amplifiers had a socket to power an external preamplifier. As the current requirements of a preamp are small, most power amps could supply it with no problems; as a bonus, it kept the hum-producing power transformer a fair distance from the low level stages. If you choose to do this, make sure to add a decoupling network to the B+ feed to the preamp, to guard against motorboating. A tapped bleeder resistor across the output of the power supply would do; choose the resistor values to supply the correct B+ voltage.

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 Post subject: Re: control amp for a 300B
PostPosted: Jan Sat 01, 2022 4:46 am 
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Jim Mueller wrote:
I just looked at the posted schematics again. The bridge rectifier is worse than what I wrote above. With the transformer shown the B+ would be about 1400V! For a bridge circuit, the total secondary voltage should be about 200 to 250V.

If you want to use the bridge circuit, you could use two 6BY5 tubes.

Jim... that schematic I posted was only pro-forma for sample tube-type bridge rectifier circuits in general... Certainly not anything for his particular voltages.
I only wanted to show him the variations involved w/and w/o a CT.
So I thought it was obvious that those particular transformer values were NOT for him... esp since I put the reference link there and that schematic was just a sample from the link info about tube type bridges.
The reference was from a tutorial entitled:
"Introduction to Electronic Emission, Tubes, and Power Supplies Navy Electricity and Electronics Training Series"

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 Post subject: Re: control amp for a 300B
PostPosted: Jan Sat 01, 2022 6:30 am 
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Pbpix wrote:
john8750 wrote:
If I use this power supply, how would I convert it for a tube rectifier? It must be an octal base type tube.
https://antiqueradios.com/forums/downlo ... ?id=246310

Don't do it John... lol
....Why would you want to?
....I don't see any benefit.
Lots of extra stuff like filament transformers and tube sockets even if you use just two dual rectifier tubes.
Or needing a higher voltage CT transformer. For Full WAVE using single dual rectifier tube.
Tubes have larger voltage drop across them and SS diodes have almost none.
REF
( https://www.rfcafe.com/references/elect ... 1-3-20.htm )

Attachment:
full wave bridge with tubes.png



No good reason Peter.
The 300B uses a tube rectifier, so I would like to keep the vintage.
But not absolutely sure.

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 Post subject: Re: control amp for a 300B
PostPosted: Jan Sat 01, 2022 6:43 am 
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Jim Mueller wrote:
I just looked at the posted schematics again. The bridge rectifier is worse than what I wrote above. With the transformer shown the B+ would be about 1400V! For a bridge circuit, the total secondary voltage should be about 200 to 250V.

If you want to use the bridge circuit, you could use two 6BY5 tubes.



I believe about 250v on the plate will work.
So, maybe a 500v secondary, half wave rectifier. And a 5v tap, 12.6v filament with center tap.
Remember, the schematic is times two. Left and right channel.
Thanks Jim.

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 Post subject: Re: control amp for a 300B
PostPosted: Jan Sat 01, 2022 6:46 am 
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Tim Tress wrote:
In the early days of HI-Fi, some power amplifiers had a socket to power an external preamplifier. As the current requirements of a preamp are small, most power amps could supply it with no problems; as a bonus, it kept the hum-producing power transformer a fair distance from the low level stages. If you choose to do this, make sure to add a decoupling network to the B+ feed to the preamp, to guard against motorboating. A tapped bleeder resistor across the output of the power supply would do; choose the resistor values to supply the correct B+ voltage.



I like that Tim. And its very doable.
The control amp will match the 300B casing. So why not install a preamp plug.

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