Help with 30A5 tube amp schematic

Whiting · 13th July 2009, 10:27 AM

Hi, I've recently picked up an amp/tuner which has 2 x 30A5 & 1x 12AX7 tubes in the amp section.

I plan on using these parts to build my first p2p amp and was looking for some help if anyone has any easy to follow schematics or component layout plan. I've seached on the net, however have not come up with anything.

Cheers

leadbelly · 13th July 2009, 06:06 PM

If you search on "50C5" you will find many examples; the tubes are analogous, so you can just use the 50C5 schematic for your 30A5's. Of course you will need to revise your heater string arrangement for the different heater voltage. Note that these tubes were in what was all likelihood a transformerless stereo. YOU MUST USE a power transformer, even if just an inexpensive isolation transformer, for safety reasons in your project.

paulb · 13th July 2009, 06:26 PM

I would bet that the 30A5s' filaments are wired in series with a number of other tubes' filaments (which must therefore all have the same current spec) to get to 120 volts total - maybe the 2 30A5s and 5 12AX7 (or other 12-volt tubes). If so, you could keep this arrangement, wiring up their filaments in series but otherwise not necessarily using all the 12-volt tubes. If not wired in series you must come up with sources of both 30 volts and 12 volts for the various tubes' filaments.

If using the series arrangement don't be tempted to wire them directly across the 120VAC input power. As leadbelly wisely points out you will need an isolation transformer. Danger, Will Robinson!

p.s. filament = heater, at least for vacuum tubes.

Whiting · 14th July 2009, 12:48 PM

Hi Guys

Thanks for your replies, the amp was also fitted with an am/fm radio section which has been removed.

It originally had 2 x 30M-P27 - 1 x 12AX7 - 1 x 12AU6 & 2 x other unknown 7 pin tubes.

The original 2 x 30M-P27 pentodes appear to be relaceable with 30A5.

Ive done a search as suggested and found the attached schematic for the 50C5. Could you guys please advise if I could use the original power transformer which has 240v primary, 100v 0.4a secondary along with a 0.9a 6.3v tap , if iwas to use a 30v transformer for the filaments?

Also, apart from the power supply section of the attached schematic, could I use the same value caps and resistors ?

Cheers

leadbelly · 14th July 2009, 03:33 PM

Well, I couldn't find a 30MP27 datasheet, but I found this:

Yes, you can use a 30V transformer for the filament, and you can wire up the 12AX7 to the 6.3VAC secondary. I would be tempted to try the schematic as is, subbing the 5670 with the 12AX7, and then if that didn't work, try bypassing the cathode resistor with a cap.

singh santa · 15th July 2009, 02:10 PM

Hello,
I find these,might help

singh santa · 15th July 2009, 02:11 PM

Hi,some more

singh santa · 15th July 2009, 02:14 PM

and more

Whiting · 16th July 2009, 08:49 AM

Hi Guys

I'm stoked with all of your replies.

What can I say but thank you all for your help.

All the provided info will give me a good starting point to make someting of the small amount of parts which i have.

If theres more just keep them coming.

Again thanks all and Cheers

13th July 2009, 10:27 AM	#1
Whiting diyAudio Member Join Date: Mar 2008	Help with 30A5 tube amp schematic Hi, I've recently picked up an amp/tuner which has 2 x 30A5 & 1x 12AX7 tubes in the amp section. I plan on using these parts to build my first p2p amp and was looking for some help if anyone has any easy to follow schematics or component layout plan. I've seached on the net, however have not come up with anything. Cheers

13th July 2009, 06:06 PM	#2
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	If you search on "50C5" you will find many examples; the tubes are analogous, so you can just use the 50C5 schematic for your 30A5's. Of course you will need to revise your heater string arrangement for the different heater voltage. Note that these tubes were in what was all likelihood a transformerless stereo. YOU MUST USE a power transformer, even if just an inexpensive isolation transformer, for safety reasons in your project. __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

14th July 2009, 03:33 PM	#5
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	Well, I couldn't find a 30MP27 datasheet, but I found this: Yes, you can use a 30V transformer for the filament, and you can wire up the 12AX7 to the 6.3VAC secondary. I would be tempted to try the schematic as is, subbing the 5670 with the 12AX7, and then if that didn't work, try bypassing the cathode resistor with a cap. __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

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13th July 2009, 06:26 PM	#3
paulb diyAudio Member Join Date: Jun 2001 Location: Calgary	I would bet that the 30A5s' filaments are wired in series with a number of other tubes' filaments (which must therefore all have the same current spec) to get to 120 volts total - maybe the 2 30A5s and 5 12AX7 (or other 12-volt tubes). If so, you could keep this arrangement, wiring up their filaments in series but otherwise not necessarily using all the 12-volt tubes. If not wired in series you must come up with sources of both 30 volts and 12 volts for the various tubes' filaments. If using the series arrangement don't be tempted to wire them directly across the 120VAC input power. As leadbelly wisely points out you will need an isolation transformer. Danger, Will Robinson! p.s. filament = heater, at least for vacuum tubes.

16th July 2009, 08:49 AM	#9
Whiting diyAudio Member Join Date: Mar 2008	Hi Guys I'm stoked with all of your replies. What can I say but thank you all for your help. All the provided info will give me a good starting point to make someting of the small amount of parts which i have. If theres more just keep them coming. Again thanks all and Cheers

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