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Old 20th January 2008, 09:30 AM   #1
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Default 50c5 push-pull for guitar?

Hey guys,
I'm new to the forum, but I've learned alot from reading your prior posts.

I'm no stranger to DIY. I've done many guitar pedals from scratch and know schematics and circuits well enough.

I have a moderate amount of tube experience. I know what not to do and I'm pretty familiar with the given sections of an amp.

I've never built a tube amplifier from scratch. That's what I'm hoping to do, however, my design skills are not near what I hope they will be in the near future.

I'm looking to make a guitar amp based roughly on the AA5 design.

I'm hoping to use a push-pull pair of 50c5's, with a 35w4 rectifier, and either a single 12a*7 gain stage or a duel 12**6 gain stage for a relatively low-noise, 2-5 watt output amp.

Now, I DO CERTAINLY KNOW the rules of these things. I WILL BE USING AN ISOLATION TRANSFORMER (I have no desire to die), I know to keep the chassis and ground seperate, etc... So, let's please not have that discussion over and over.

The main thing is, I haven't been able to find an output schematic of a 50c5 push-pull pair. I've found plenty SE designs but I'm looking for a little more output.
In addition, I'm not exactly sure what cheap output tranny would work (to keep cost down) and I'm totally lost on the splitter stage. I'm looking for 2-5 watts output at 8 ohms.
I'm also looking to use parts that I already have (with probable exception of the OT and possible splitter tube.)

Any suggestions?

Thanks.
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Old 20th January 2008, 01:35 PM   #2
engels is offline engels  Israel
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Default 50C5... what???

a very iteresting choise of tubes to start designing a tube amp from scratch I have to say.

you would probably need a some - 8W PP 5000 CT output transformer.

One of the options for substituting cheap trannies for PP output is looking for a dual primary low voltage transformers. I don't quite remember how it goes - I hope someone will say if I'm wrong - but since impedance ratio is equal to the voltage ratio squared you may use something like 220V CT primary - 9V secondary for an output in your case. the primary must be CT (220-110) and the output for 220 must be around 9V (from 220V). You get (220V/9V)squared=(24.44)squared=597.
If you plug the 9V secondary into a 8 ohm speaker, tube anodes to the 0-220 points on the primary and HT to the 110V CT you receive 8X597 = 4776ohms primary with each half = 2388 ohms which looks suitable for the given 50C5.

I'd look for at least 1W rated transformer. 500mA will probably die under the load.

If you plan to use 4 ohm speaker - look for a 6V transformer and if the speaker is 16ohm - for a 12V transformer.

The cathode resistor for the 50C5's should be around 150-200 ohms I guess.

And as for the splitter.... later.
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Old 20th January 2008, 04:08 PM   #3
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There is some very interesting reading on this subject on Fred Nachbaur's site. He has a 2 - 35C5 amplifier that presents about 4 watts here...

http://www.dogstar.dantimax.dk/tubestuf/4x4index.htm

and a 4 - 35C5 amplifier of 8 watts here...

http://www.dogstar.dantimax.dk/tubestuf/dv8index.htm

Some good ideas could come from reading about his experiences in these articles.

Obviously you could use 50C5 in these circuits if you are able to make accommodations for the difference in heater voltage.

Kind regards,

Wade
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Old 20th January 2008, 04:45 PM   #4
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Egg,

When 1/2 wave rectification is employed, a "standing" DC voltage is impressed on the rectifier winding. Available isolation trafos, like the Triad N-68X, are not designed for such service. You can achieve your purpose by employing a hybrid bridge rectifier made from 2X UF4007 SS diodes and 2X 35W4 vacuum rectifiers.

Set up 2 series heater strings, with a 35W4 and a 50C5 in each leg. A 12AX7 goes in 1 string and a 12AT7 goes in the 2nd string. Join the 2 strings together and add a 12BH7 (12.6 V./300 mA.) into the setup. A 5 W./68 Ohm dropping resistor at the "head" of each string completes things.

A Mullard circuit, using a 12BH7 section for voltage gain and a 12AT7 as the LTP, will easily allow you to drive the 50C5 "finals" into clipping. That leaves 2X 12AX7 sections and a 12BH7 section for preamplification and signal shaping circuitry.

A Hammond 125E should be fine as the O/P trafo. There's plenty of magnetic headroom for the NFB error correction signal.
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Old 20th January 2008, 06:11 PM   #5
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Default Re: 50c5 push-pull for guitar?

Quote:
Originally posted by freefallingegg
The main thing is, I haven't been able to find an output schematic of a 50c5 push-pull pair. I've found plenty SE designs but I'm looking for a little more output.
In addition, I'm not exactly sure what cheap output tranny would work (to keep cost down) and I'm totally lost on the splitter stage. I'm looking for 2-5 watts output at 8 ohms.
Yeah, they don't publish PP info for 50C5s. I was thinking about that very thing, since I have a whole bunch of these lying around. Shame not to use 'em, even if the power is rather limited. So I did my own design for that. Now, usually, the distortion figures for this type are pretty horrible (ever heard one of those AA-5's Click the image to open in full size. )

However, if you do some spec-busting, and get that Vsgsg down, the figures aren't half bad. Even if an h(3) estimate of 4.38% seems high, the graphical method of estimation runs a bit high. What also figures into the sonic performance is the harmonic "personality" of the VT (mostly h(3) and less higher order harmonics, or more higher order harmonics?) You can also get the 90Vdc for the screens from either a Zener or a VR90 gas discharge tube.

50C5 PP Loadline

I would ditch the 35W4 for power diodes. If you don't want to go solid state, there are lots of other 6.3V dual diodes for this purpose. For 50C5 heater power, connect a 12V "doorbell" xfmr, secondary to the 5.0V winding of the power xfmr, and you get pretty close to the 50V that the 50C5s require.
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Old 20th January 2008, 06:46 PM   #6
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Not wanting to criticise your choice of valves, but what influenced you to choose 50C5s for this project? If you already have them, then I understand. If you just want something different, I understand. If you haven't purchased anything yet then why not consider 6AQ5/6005 output valves? They would function very similarly and the parameters of these valves are well understood and they are acknowledged by most to sound very good. They are robust and also very inexpensive. Any 6V6 circuit operating at 275 volts or less can use the 6AQ5. Just another idea.

Wade
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Old 20th January 2008, 07:07 PM   #7
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Default Re: Re: 50c5 push-pull for guitar?

Quote:
Originally posted by Miles Prower


50C5 PP Loadline


What software did you use, and where to get so nice software?
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Old 20th January 2008, 07:21 PM   #8
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^^^^

The GIMP. It's a free download.
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Old 20th January 2008, 07:49 PM   #9
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Thanks everyone for your input. I do greatly appreciate it.

As far as my tube choice:

Honestly, I already have them. I was inspired by this POS Cord amp that uses the 35w4 and a single ended 50c5. The sound is rather terrible, but it's the kind of terrible that I personally look at as having a useful and warm personality.

I do ALOT of recording in ALOT of different styles and I'm always looking for new tones. Sometimes, the worst tones are the right one's for the job. And I just like having things that most other people don't (hence the DIY lifestyle.)

Liking this particular tube setup's sound, I've found myself wondering what else it can sound like.

That, and I'm pretty much broke...typical musician's financial situation. So, I'm looking to do something rather inexpensive, with things I already have, that I could still consider useful.
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Old 21st January 2008, 12:27 AM   #10
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You may want to check out the AX84 guitar amp website. There are many step-by-step plans as well as various schematics. I've built the "High Octane", which is a SE 6BQ5/EL84 guitar amp with a ultra-high gain preamp section. It's very versatile. And the best part is, it cost less than 200 rubles!
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