New Project, But first, I have some questions

[Rob F]

Hi Everyone,

This is what I have going on.....

The other day I scored an old HiFi console with the perfect chasis for a guitar amp conversion. The tube layout is great, it has (4) 6V6s, (2) 6EU7 that I believe are phase inverters (with an extra chasis punch for a third pre amp type tube), and it runs a 6CA4 rectifier tube. Two output transformers.

Questions for you guys;

1. What kind of amp would you suggest to build that would maximize all of what this chasis has to offer?

2. Do I have to run the 6CA4 (9 pin) rectifier tube or can I use one of the 6V6 slots for a more traditional rectifier tube?

I'd like to get at least a 5E3 out of it. Is it possible to run (2) 6V6s and (2) rectifier 5Y3GT tubes?

I plan on getting rid of the cap can and going with a more traditional board layout.

Keep in mind that is will be my first build so I want to keep it somewhat of a simple design and not get too crazy.

Let me know what you guys think. Any and all suggestions welcome.


Thanks.

[d2camero]

Wow, the world is your oyster. If you are a fender type guy, you can build anyone of the fenders that have 4 6v6 or 4 6L6 or just two of each - thats darn near any fender.

As well, you can build almost any type of marshall. Lots to choose from.

Perhaps pick something without a lot of effects, loops or switching just to make it easier for the first build though.

So... what type of amp do you like?


As for the rest tube, wait till you figured out your amp first, cause:
- you can use a variety of 9 pin rect tubes
- you can widen the hole
- you can use SS rects and ignore the hole completely


As for the caps, again, wait till you figure out what kind of amp you like, then decide, as there are a lot of nice chassis caps as well.

Do you plan on using the PT and OT from this chassis? If so, then your options are quite a bit more restricted.

[Rob F]

Yes, I would like to use the power and output transformers. After thinking about it for a while, even though the chasis has a lot of potential, I would like to keep it simple (in the 5E3) area.

What kind of filter cap would be suggested for a 5E3? That would make things a lot easier. Although I have to admit, I would be a bit nervous straying from a layout diagram for the first build.

Could I use a Y53GT rect tube in one of the 8 pin spots or would there be a voltage issue concerning the power transformer?

I took some readings last night, Let me know how they look:

6CA4 rectifier tube pins
1. 6
2. 0
3. 272
4. 16
5. 0
6. 0
7. 6
8. 0
9. 0

6V6 readings. Pin
1. 0
2. 16
3. 256
4. 239
5. 0
6. 0
7. 16
8. 16

6EU7 readings pin
1. 16
2. 16
3. 222
4. 2
5. 0
6. 140
7. 112
8. 0
9. 0

[d2camero]

OK, if you want to use the OT and the PT then you have a few restrictions to work with:

1. the impedance of the OT is fixed so you can either use the output stage from the existing amp (easy), or design one that works with the impedance of your OT (a lot more work).

2. the B+ voltage of your PT is fixed, so you need to live with this as well.

Anything in between you can change. This includes:

- the rest of the power supply (e.g. filtering and rectifiers)
- the preamp
- the phase inverter

I think your idea of the 5E3 is a good idea, and quite straightfoward to put in. The B+ might be a bit low (knowing fender - other please chime in), but would be a good start, then you can modify as you see fit.

So here is how I would attack it:

1. locate the power amp stage on the existing circuit. This includes the power tubes, OT, bias, grid bias resistors and maybe PI coupling caps). Leave this stuff.

2. locate B1 and B2 - leave these alone (as they supply the existing power stage) and then rip out the rest.

3. build you 5E3 preamp and PI, then put the outputs of the PI to the grid resistor inputs.

4. add on your 5E3 filtering for your PI and preamp to the existing B2. Resistor values might have to change a bit to get the right voltages, but first attempt might be OK (do some research to find 5E3 voltages)

5. as for the rectifier, I would first try it with the stock rectifier to see if tone was good (6CA4 drops about 28V, 5Y3GT about 20V) if not, then I would tear it out and put in 5Y3GT. Besides 6CA4/EX81 works fine for a Marshall 18W! Regaurdless you are in good shape because the 5E3 uses a 5Y3GT which does not require a 5V supply (which I don't think you have).



make sense?

[Johnhenry]

I'm OldSchool so, 3 X 12AX7's, 4 X 6V6's = a nice Plexi Preamp, should be about 40 or 50 watt's,
the recto you got run's 6.3 V. on the heater's, you would need a 5 V. tap on your power tranny for a 5Y3, 5AR4 or 5U4.
John

[Rob F]

My other Salvaged HiFi runs a 5U4 rectifier tube. Along with (4) 6V6s and (1) 12ax7 phase inverter.

Should I take that one into consideration given it rectifier tube, even though the chasis layout is a little more cumbersome? Or maybe just use it's Power transformer for the 5E3 in the other chasis?

Thoughts?

[Rob F]

What about keeping the two Output transformers and making a 5E3x2 stereo output?

Like I said...just a thought.

[d2camero]

I think a stereo 5E3 would be GREAT. Stereo really makes quite a difference on stage, especially if you use chorus and stuff like that.

[Rob F]

I think I will stick with capacitors that I can follow a layout diagram more easily.

Thanks for your input. I think I'll order the parts I will need tonight. I fyo think of any other bright ideas please let me know.

[Phil_S]

As other say, you have almost unlimited choices. The 6CA4 is a fairly robust rectifier. I'd look to work with it and probably there is no 5v filament winding. Of course, you can go with diodes and free the 1.0A from the rectifier for something else -- EL34's might draw too much but 6L6 should work. Don't be put off by a possible need to reduce the hole size for a noval. Just screw in a metal plate and redrill it. You'll need a step bit for a clean job.

Add up the filament supply on the tubes. According to what you list, they draw a total of 2.5A. So, that is the assumed safe limit for the tranny and probably caps your project.

If it were me, I'd look at a double 5E3. If you want some headroom, purchase the mod from Bruce at Mission Amps. He figured something out. You also get an easy shot at a Marshall Plexi 36W w 6V6. If you wan't clean headroom, as someone else said, look at the Fender designs.

On these old ones, I usually gut and start over. That way I know I can understand what's in there. I also save anything that might look good, like black beauties.

[Rob F]

Thanks Phil,

Sound advice. I believe I will stick with the existing rectifier and see how it sounds.

What are black beauties?

I'm excited about this project. I'm going to shop around for some parts and I'll be sure to post my progress.

[mlp-mx6]

Black Beauties are old Sprague capacitors with a black jacket and usually orange printing, IIRC.

[Rob F]

NIce. I'll keep my eye out for them.

[Phil_S]

Thanks Phil,
What are black beauties?

Sprague caps, black body, red or orange printing, typically one stripe around it. Also, mustard colored Astron, bumble bee (lot's of stripes) and perhaps other old tone caps are valued, as well. Sometimes they are way out of spec. People pay silly money for them on eBay.

[Rob F]

I took a look last night to see if any were hiding in the HiFis I have. I didn't see any Sprague but there were some interesting black caps that werre labled Pyramid. Ever hear of those?