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.
 
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.
 
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
 
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.
 
freefallingegg said:
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
An externally hosted image should be here but it was not working when we last tested it.
)

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.
 
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
 
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.
 
Hi,

You can use many different types of valves for guitar (and HiFi). Too many builders get stuck with 6AQ/BQ5-itis and it is really sad that ideas get so narrowed that the world is full of "just another EL84-ish" amps.

How about a list of what you do have. Valves or way of iron. I assume you have PP iron on hand? What's your power transformer?

Cheers!
 
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Look for old copies of Alfred Morgan's "The Boys 1st/2nd/3rd Book of Radio & Electronics " - there are 3 volumes and IIRC the third one has a pushpull 50C5 amplifier design in it. It is very close I think to what you are looking for. I had these books as a boy and built some of the projects.

There is also a 4th book in the series which was published around 1969 dealing with solid state electronics, but I have not seen that one.

http://en.wikipedia.org/wiki/Alfred_Powell_Morgan

These books turn up quite regularly in used book shops and on eBay.
 
kevinkr said:
Look for old copies of Alfred Morgan's "The Boys 1st/2nd/3rd Book of Radio & Electronics " - there are 3 volumes and IIRC the third one has a pushpull 50C5 amplifier design in it.

The one that appears in the third volume is identical to the Lafayette KT-92. It used 35C5s, a different type. Had a xfmrless PS, and was basically a really, really bad design. I would avoid that thing like the plague.

There's a reason why the design appeared in a book entitled: The Boys Third Book of Radio and Electronics, and it wasn't sonic excellence. Cheap, difficult to mess up completely, and a likely initial success, yes, but sonics, no.
 
Thanks again guys.

I think I'm just going to build one SE 50c5 amp...since a PP 50c5 amp obviously seems to be considered such a major taboo.

My junk box is limited to a couple of AA5's, a couple of cheap pre's, several 12A*7's, several 6L6's and a moderately larger amount of solid state things.

I have a couple PT's with 100v primaries and 33vdc secondaries, a few PT's that I have no idea what they are, one 12v filament tranny, and no OT's other than the AA5 ones and a significantly larger one cannibalized from from a Peavey Heritage...I'd have to look up the specs on that one.

I've checked out Nachbaur's stuff and the AX84 site...learned alot from reading and the amps all look nice...but like I said before, I don't have the proper funds to sink into something like that.

I was hoping to spend on an OT and a PT and that's about it.
 
Hi,

freefallingegg said:
Thanks again guys.

I think I'm just going to build one SE 50c5 amp...since a PP 50c5 amp obviously seems to be considered such a major taboo.

It's not. Fred Nachbuar knew that, so do experienced DIYer's :)

You can use most any tube. You don't even need to build it at the correct phase of the moon while the chassis is held by virgins, like the audiophiles would have you believe ;)


My junk box is limited to a couple of AA5's, a couple of cheap pre's, several 12A*7's, several 6L6's and a moderately larger amount of solid state things.

I have a couple PT's with 100v primaries and 33vdc secondaries, a few PT's that I have no idea what they are, one 12v filament tranny, and no OT's other than the AA5 ones and a significantly larger one cannibalized from from a Peavey Heritage...I'd have to look up the specs on that one.

Unless you want bar room filling power, which you have 6L6's for, a single ended 50C5 will do nice into that AA5 OPT. You'll get about a watt and one amp I have here that pushes a watt into a Weber 8" AlNiCo, that's plenty for practise or a family/friends jam.


I was hoping to spend on an OT and a PT and that's about it.

Power transformer..... the 50C5 was designed to run off rectified line voltage, so it's a low voltage tube (160-180V). You just need an isolation for safety. If you do the 50C5 SE with your AA5 OPT, a 15VA bathroom shaver isolation transformer can work. They were ripped out of houses upgrading to the GFCI type and can be found in some thrist stores for a buck.

Use it to power the heater too and use a dropping resistor in your filament string.

Can't find a little isolation tranny? PM me and I'll find one.



leadbelly said:


What's with all the formality? You never call me Mr. Leadbelly.

Mr. Leadbelly... there :cool:

Cheers!
 
freefallingegg said:
I think I'm just going to build one SE 50c5 amp...since a PP 50c5 amp obviously seems to be considered such a major taboo.

Well, not by me. It's something I'm planning for a sometime project. I have a very preliminary design idea. Just because the designs using 50C5s back in "the day" were uniformly horrible, that doesn't mean that you can't do better. Of course, this project'll probably require a consult with Jack over at Electraprint for decent OPTs for this. Of course, what it'll do open loop, and what corrections will be required as to overcoming whatever sonic deficiencies may exist remains to be seen.

I like the 6GK5 here: gain to burn, excellent linearity, and since this is a UHF small signal triode, it doesn't have the Cmiller that you'd get with types like 6AV6s, 6SF5s, or 12AX7s, so doesn't interact with the volume control in undesireable ways. Has more than enough current capability to drive the 50C5 control grids (not that it's all that difficult of a load anyway).

By using the CCSs in the 50C5 cathodes, the final stage can do double duty as a phase splitter and PA, and you can adjust for balance.
 
I was hoping to spend on an OT and a PT and that's about it.


Egg,

If you follow the advice I previously gave, your "iron" purchases will be limited to: a Triad N-68X isolation trafo, a Hammond 125E O/P trafo, and possibly a Triad C-7X filter choke. CLC B+ filtration is better. Allied Electronics charges $10.36 for the C-7X choke and $11.41 for the N-68X trafo. An Edcor XPP15-8-5K O/P trafo, which costs $20.00, should be suitable in this guitar application. Take advantage of the O/P trafo's UL taps by connecting the 50C5 screen grids to them by 1 KOhm resistors.

A 12AT7, a 12AX7, 2X 35W4s, and 2X 50C5s you seem to already have. Jim McShane charges $12.50 for a current production EH 12BH7. Jim also sells UF4007 low noise SS diodes at a fair price.

BTW, you might be better off with a 12AU6 or 12SJ7 pentode in the preamp, instead of a 12AX7, as the modest B+ rail voltage in this amp will not allow large plate load resistors.

I've attached the Mullard 5-20 schematic. Study the topology, as it works for the types we've been discussing along with EL34s, etc.
 

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