Sluckey's Rocky Project: Questions

[uki]

Hey guys !!

Recently I was lucky and got a nice deal on an old radio with pickup, with a PP 6v6 amp.

The 1st idea was to convert it to a guitar amp,      After a hard and deep search for the schematic without luck, I have tried to draw a schematic but it doesn't looks like anything that resemble to a guitar amp with weird methods to do things like half of the filament wires going to ground, and besides that lots of caps are done, some the values are unreadable... but after looking around a little more I did found the Rocky amp that Sluckey converted ! (thanks for that)

It does looks like I can build the Rocky with what I have in hand, the major difference is the power tubes(I don't have 6L6s).
There are irons for the 6v6 PP, 5U4G recto tube, 5 octal sockets in line in the chassis, I got the feeling this is the way to revive this one !! 

What have to be done in the circuit to use 6v6 instead of 6L6 in the Rocky amp?

Rock Schematic

[sluckey]

Look at this instead...

     http://sluckeyamps.com/RCA/RCA.htm

[uki]

Hmm I don't really want another amp with vibrato   ,

 is it possible to patch the Rocky preamp into the other one ?   

[kagliostro]

Look to the RCA conversion (the Ampeg Jet), use V1a and V1b with the schematic of the Rocky and V2 - V3 - V4 as in the Ampeg Jet

Something like this



Franco

[sluckey]

There are several ways to approach this project. Here's what I did...

I got the original power amp up and running. Then I simply added a Fender AB763 preamp that connected to the input of the original PI. One additional power supply node was added to power the new preamp.

[PRR]

> resemble to a guitar amp

A radio is a tuner and an audio amplifier.

A guitar amp is an audio amplifier with one more stage than a radio's amp needs.

Find volume control. Put guitar to the top of it. This will play. You would like more gain. Either put a booster pedal in front, or adapt one of the tuner tube sockets to be an audio stage.

[uki]

Hey guys thanks !! And thanks Franco for the schematic !!

What about the recto tube can I use the 5U4G instead since it came with it, probably the PT will do better ?

[kagliostro]

As Sluckey said

There are several ways to approach this project. Here's what I did...

I got the original power amp up and running.

Tube#   - Base - Fvolt - Famp - Vdrop - MaxPmA - MaxPv - notes

5U4-G        - 5T    - 5.0   - 3.0     - 44       - 225      - 450        - octal 5Z3

5Y3-G/GT   - 5T    - 5.0   - 2.0     - 60       - 125      - 350         - octal 80

5U4G has less drop than the 5Y3G/GT and developes the drop at a higher current, but why don't use it and adjust the PS on the values of the resistor feeding the preamp tubes (if required)

Franco

[uki]

Thanks !

5U4G has less drop than the 5Y3G/GT and developes the drop at a higher current, but why don't use it and adjust the PS on the values of the resistor feeding the preamp tubes (if required)

Do you mean to use the 5Y3 ?


In the schematic I catch a resistor without value just before the PI , would that be 1M ?

[kagliostro]

I apologize if you understand so

english isn't my language and I'm not able to menage it as I would like

I mean you can use the original rectifier tube (or one other 5U4G if you have)

Sorry

Franco

[uki]

english isn't my language and I'm not able to menage it as I would like

It is all good my friend , my english isn't that good either !   

What about the resistor ? 1 m ?

[sluckey]

I used 1M on my Rocky amp.

[uki]

Ok thanks !!  Is it possible to use 6sn7s instead of 6sl7s ?

[sluckey]

Yes, but you will have much less gain.

[shooter]

possible to use 6sn7s instead of 6sl7s ?

fwiw, I built this one based originally off Steve's rock-ola.  I strayed pretty far from original though

[uki]

Hey Shooter !!   Thanks for share the schematic !!

[uki]

Chassis and transformers cleaned up , all ready for the assembly !!

Check out !!

[uki]

Did some layout for this amp, check it out, is everything in order ?

File updated.

[kagliostro]

Why you didn't use Sluckey's layout ?

http://sluckeyamps.com/RCA/Ampeg_J12B.pdf

Franco

[sluckey]

You need to put a ground at the junction of those two 270K grid resistors. The 470K and 510K are reversed. I didn't look any farther.

[uki]

Why you didn't use Sluckey's layout ?

Not doing a board this time, it will be true point to point !
Been looking at BCAudio amps , wow that is a very beautiful work!!

http://www.bcaudio.com/BC-Audio_Amplifier-No-8_img51_guts.jpg

You need to put a ground at the junction of those two 270K grid resistors. The 470K and 510K are reversed. I didn't look any farther.

Thanks Sluckey !!

Going to check all over again, I did update the file with the corrections you pointed !

[kagliostro]

BC-Audio .... Those small amps on Ammo can ? I like !

If you want inspiration on PTP give a look to Darryl's Amps at AGGH (www.guitargear.net.au)

he is very skilled on that



Franco

[uki]

If you want inspiration on PTP give a look to Darryl's Amps at AGGH (www.guitargear.net.au)
he is very skilled on that

Oh I didn't know about that website good hint !!

[kagliostro]

Darryl is also one of our friends (like TIMBO and others)

and you can find some of his works also here (do a search)

(here one http://el34world.com/Forum/index.php?topic=20374.msg215472#msg215472)

Ciao

Franco

[uki]

Oh I see !! That is a marvelous work with PTP !! Going to use that as reference to do this building !!

I was checking my layout again and catch a few more things to correct! It is fixed now 

Thanks Franco !!

[uki]

This capacitor connected to pin 6 of V2 doesn't have unit value.
What is the unit value of this capacitor, µF or pF or what ?

Thanks !

[sluckey]

µF

[uki]

What about voltage on electrolitic caps, I know the ones for the filter are 450-500v, what about the others?

Thanks

[sluckey]

What voltages do you typically measure on cathode bypass caps?

[uki]

What voltages do you typically measure on cathode bypass caps?

 

Looking at Fender schematics the ones for preamp are usually 25v. And by the presence pot there is a 0.1 200v , what is that one ? 

[sluckey]

I call it the presence cap.

Which schematic (not layout) are you using for this build?

BTW, I wasn't trying to be a smartass with my last post. I just wanted you to think about the answer because I knew you knew even if you didn't know you knew. 

[uki]

I just wanted you to think about the answer because I knew you knew even if you didn't know you knew. 

I figure that out, thanks I appreciated that (teach a man to fish).

I got big doubts sometimes because my lack of knowledge, but I search and learn as much as I can on my own.

Ok besides the filter caps all others are at least 25v. What about that one 20µF in parallel with 500/10w resistor between pins 8 of the power tubes, is that a filter or cathode cap ?

The schematic I'm using is the one put together by Franco at reply 3 http://el34world.com/Forum/index.php?topic=21003.msg222524#msg222524

[kagliostro]

That is the cathode resistor + bypass cap of the power tubes, this way the amp is a cathode biased amp

Oh, there is a question I've for Steve, I'm not familiar with that kind of PI and I'm in doubt about the junction of the two 270K resistor

(point X in the schematic), I think that the X point is to be connected to ground, is that correct ?

Franco

[sluckey]

That is a paraphase inverter. Don't know which variant.

Yes, "X" should be connected to ground since you eliminated the tremolo.

[kagliostro]

Thanks Steve




Franco

[PRR]

> What voltages do you typically measure on cathode bypass caps?
> Looking at Fender schematics the ones for preamp are usually 25v

He didn't say "look" (though that is good technique). He said "measure".

I've measured a few cathodes. In this crowd, I was gonna say "4V", because "we all" use 12AX7, with under 400V supply, Mu=100, so 400V/100 is 4V to cut-off a 12AX7. Actually we want the plate voltage which is lower, and we don't want to cut-off, so numbers like 2V are typical and a 4V cap covers this with some margin.

But then I looked at what you were talking about. 6SN7 has Mu=20. So by the same logic, we never need more than 400V/20= 20V at cathode. We also tend to run 6SN7 plate at a much lower fraction of full supply.

If you believe Sluckey's meter (in the file you were working from), there's under 5V at these cathodes. A 5V cap would work for months. We like to be generous for long life. 10V would be ample. But 25V caps don't cost much more, and can be used in more places than just cathodes (low voltage bias supplies? Op-amp chips?). 25V is the common small cathode cap.

Big cathode circuits under power tubes may be much higher volts.

[uki]

 

Wow thanks PRR , great explanation !! It did clarified a lot more than just the voltage value for the cap. I got this now !!

For the power tubes cathode bypass cap I should use at least 100v since the voltage there for the amp in question is 27v.

[PRR]

There's reasons a power tube cathode cap will "never" over-volt as much as a small-signal tube.

(Except if the cathode resistor fails, or just isn't really connected (oops), the cathode voltage can be fairly high....)

I would fearlessly use a 50V cap on a "27V" cathode. (I would have a meter on the cathode on first smoke-test, as a general sign of power stage health or distress.)

Caps are getting smaller thus cheaper, and I suspect 100V is today a very reasonable choice.