Curcuit ideas for scratch build needed

[Kegger]

AHH yes I did miss the first 4uf cap going to the choke, man they musta been torturing
those original 6L6G tubes, 6L6GC no problem but an old G, WOW they probably did gets
to be replaced often I'd have to think. (but 4uf will keep the voltage down a bit though)

A choke in the ground leg is still choke input filter if it wasn't for that first 4UF cap, I am
not up on how the rest plays out for the choke but it's suppose to be a little easier on it
as it doesn't see all the dc voltage it would in the positive leg of the supply.

 

[centervolume]

Baldwin Amp, Model PC

Hello

I am a newcomer to this forum. I have recently acquired a pair of these model PC Baldwin organ amps with 6L6s and 6sl7 tubes, just like Kegger's.

I dont think anything has been done to them and I am also interested in keeping the 6L6s in there since they are blackplates... will still need to check them out.

I was considering turning one into an instrument amp, but have been persuaded to use them to create true stereo setup via 2 mono signals.

Kegger, how did your effort turn out? I am uncertain how to address the need for a preamp section. It seems that the rest is rather straightforward in terms of inputs, speaker outs and power considerations. Am I correct in this assumption?

thanks!

Paul

 

[vinylkid58]

Gordon, I have a question.

Fixed bias would be just fine! If you do want protection, just use some 10 ohm 1/2 watt cathode resistors (makes a good bias test point, too), one on each output tube... they'll act as "fuses" before anything gets enough out of hand to damage the tube...

Why would an amp manufacturer use 1R 5W wirewound resisters for cathode test points when the amp is fixed bias? The amp in question is my Jolida 302b.

Thanks,
Jeff

P.S. Sorry for the OT post.

 

[battradio]

Why would an amp manufacturer use 1R 5W wirewound resisters for cathode test points when the amp is fixed bias? The amp in question is my Jolida 302b.

Thanks,
Jeff

P.S. Sorry for the OT post.

To measure cathode current ,when you measure the voltage across the one ohm resistor ,devide the voltage by the resistance to get the cathode current .

 

[vinylkid58]

To measure cathode current ,when you measure the voltage across the one ohm resistor ,devide the voltage by the resistance to get the cathode current .

Thanks for the post Mark, but not the answer I was looking for, based on my poorly worded question.

My question should be; why use a huge resistor that will allow the tube to fail, when a smaller wattage value would work just as well, and potentially save a tube by acting as a fuse as Gordon suggested?

Thanks,
Jeff

 

[battradio]

There are two thoughts on that mater I prefer the large wattage cathode resistor. If the cathode resistor opens the cathode voltage can go as high as 400 volt before the fuse blows ,the cathode could short to the fillament , causing a failure of the tube the output transformer and the power transformer along with all the fiaments of the other tubes .On a 6L6gc the maximum is 200 volts between the cathode and filament ,with an average of 100 volts .It would be beter to put a fuse or a 10 ohm 1/4 resistor in the B+ line than in series with the cathode

 

[vinylkid58]

Ok, thanks.

Jeff

 

[Kegger]

I still have not done anything with mine, but my original idea is probably what I'd go
with, and use 4 triode strapped 6l6GC, 2 parallel, for say 35 - 45 watts pretty easy.

 

[centervolume]

is there a benefit from running 2 as opposed to 4 6L6 tubes? Can I refer to your schematic that you posted for my mod/update?

thanks

 

[battradio]

If your not running them hard ,one 6L6GC will do the work of two 6L6 metal or two 6L6G and as long as the cathode resistor is correct . A pair of 7027A would be perfect for the 3800 ohm transformer and is what baldwin used in later models of that amp .

 

[Kegger]

is there a benefit from running 2 as opposed to 4 6L6 tubes? Can I refer to your schematic that you posted for my mod/update?

thanks

I would not run a single pair of 6L6 tubes on that 3.8k primary, an the schematic I had posted was
just a throw up idea an so was not a real fleshed out set of operating parameters. (just for ideas)
(4 tubes obviously more power and parallel matches the output transformer better)

I took some time (to much time that is) to put together the schematic of what I was contemplating
on building and am attaching it, voltages are not exact, just ballparks to get things close and tweak
from there if need be, but should be a working setup and is what I will do when I get time to do mine.

If someone wants to build this I'll help with any tweaking that may need to be done once ran/built up.

(updated schematic, 12-15-2010)

 

[centervolume]

battradio, kegger

OK, I see that the 4 6L6s as is looks like it wont cause any grief at least. Although 6L6 G is what's written on the chassis, these two amps came with RCA blackplate 6L6GC, and so I am really happy about that (given they check out alright).

It looks like it may be preferable to do something with the 6SL7 rather than leaving that in there. The 6EM7 is not a tube with which I am familiar, whereas the 7027s are (given that I am coming from a gtr amp background). So my questions are this:

1. Looks like the 6EM7 and 6SL7 share tube socket/octal base design, and if my tube manual is correct, the EM7 can just go right in there?! I'll check Kegger's schematic for changes to cap or resistor values, but a heads up there would be great.

2. The 7027 approach: I am guessing this would require at least one extra hole to be drilled in each chassis? Would this design eliminate the need for an outboard or extra preamp circuit? And, how great is the need for separate preamp circuitry?

Other than that, my questions are just a bit routine in terms of procedure for changing from the organ-socket input/output style to update to something more friendly for home stereo use in terms of rca/phono jacks, location, AC power socket wiring, location, safety considerations; speaker output types, location; is the gain pot sufficient as is to act as the volume control for these units... that type of thing.

Many thanks in advance for your invaluable assistance on this. I am really excited to have this kind of system on the horizon.

pl

 

[Tinkerbelle]

Well, I'd say you got lucky with getting RCA 6L6GC blackplates in there.:thmbsp:

The pin-outs for the 6SL7 and 6EM7 are the same but the characteristics are different. The 6SL7 is a twin triode with both sections equal; the 6EM7 has two dissimilar triode sections, so be sure to follow Keggers changes if you plan to use this tube.

The 6EM7 draws a lot more heater current than the 6SL7; also the 6EM7 plate dissipations are 1.5W/10W for the two sections vs. 1W/1W for the 6SL7. Actually, with those dissimilar characteristics, the 6EM7 tube is used in some circuits to make a simple single-ended triode amp.

The changes for RCA, speaker connectors, power etc are fairly straightforward and there are a number of options. You'll need to consider how you plan to orient these when using them, as well as wire layout, to help decide on placement (short side, long side, top, etc.).

Good luck with the build!

 

[battradio]

Hi Center ,

The 7027A are heavy duty 6L6's, no holes necessary. Later versions of this amp used a 12AX7/7025 instead of the 6SL7.
The 6EA7 has less gain than a 6SL7 or 12AX7.

 

[Kegger]

My ramblings on why I was changing the circuit and why this particular tube/setup.

The original circuit was either the 12ax7 or 6sl7 in a paraphase alignment, where triode
1 feeds 1 half/1 side of the output tubes an a portion of that signal is fed to the other
triode's grid which in turn feeds the other half of the output tubes.

I'm not going to say that it's a bad splitter/driver/circuit, BUT does have some "technical"
drawbacks or limitations, in the original design I'd say that it was barely capable of driving
the outputs properly, sure good gain but no current and no drive, now factor in I am triode
strapping the outputs and putting even more strain on the driver circuit.

The original type circuit with the 6SL7 or a 12AX7 is fine for driving say EL84's or 6V6's but
really more gain then needed so a decent amount of feedback is used, I've redone some of
those amps using a 12AU7 or 12AT7 and the circuits work pretty well for those tubes.

But 12AX7 or 6SL7 are high plate resistance low current tube that needs a high plate load
an does not have the "drive" that is needed for larger output tubes, you really wanna use
low plate resistance higher current tubes that can work well on low plate loads to do your
grunt or shall we say "driving" of the output tubes plus the phase splitter.

Also a paraphase like many other splitters depends on matched sections per bottle an the
phase splitters will be off when changing tubes or as they age so for best use should have
an adjustment pot and tweaked from time to time.

-------------

So after factoring all that in I decided the 6EM7 could be a good choice as we already have
the single octal socket with it's heaters wired up, 1/2 of the 6EM7 is basically a 6SL7 so we
can use that as the voltage gain amp getting the same gain we had originally, then go direct
couple to the other half which is kinda like say a mini 300B or 2A3 low plate resistance, good
solid current to drive those outputs quite easily, and we use it as an concertina splitter here
to which as long as we have even plate and cathode resistors splits evenly even with aging.
...(not saying concertina PI's are the best splitter or have no drawbacks, but works here)...

So basically we get the voltage gain we want with the high mu section, an the drive we want
with that low mu section, we use the same socket we had originally, those tubes are plentiful
and cheap, also 6EA7 an 6GL7 are the same type tubes that will plop in there for some rolling.

Extra heater current draw in my mind is negligible when only using 1 tube, then also from what
Battradio had to say about the power tranny's and using KT88''s, this setup and 6l6GC's should
be just loafing along. :thmbsp:

 

[Tinkerbelle]

Nice ramblings...:yes:

In fact, as inferred in the other thread, I may give this a try. I already have the amps up and working fine with the 6SL7s but I have plenty of 6EM7s and 6EA7s and it shouldn't be a big job to change over. Also, I am intrigued anyway, as I was looking at eventually trying a triode strapped arrangement.

I have a third unit that I'm not using; I would do that one first and A/B them, except that is the 12AX7 flavor rather than the 6SL7 type that I have as a pair.

Thanks for sharing the circuit and ideas.

 

[Kegger]

You are welcome and thanks, I'd like to see someone try it..

I believe there is a 9-pin or 2 equivalent to the 6EM7, I will see what I can dig up..:thmbsp:

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I'm curious Tinkerbelle, how is your power supply configured, an do you get the same voltages as Battradio?

 

[Tinkerbelle]

You are welcome and thanks, I'd like to see someone try it..

I believe there is a 9-pin or 2 equivalent to the 6EM7, I will see what I can dig up..:thmbsp:

-----------

I'm curious Tinkerbelle, how is your power supply configured, an do you get the same voltages as Battradio?

I guess I could look for something along the lines of a 6DE7 9-pin in place of the 12AX7...

I pretty much used the stock PS, except that I upped the capacitance values from 4uF up to 20uF (I think) etc. IIRC voltages were pretty close to original cct. I'll have to check for you.

 

[Kegger]

Ahh don't worry about the voltages, I was just curious.

A quick search reveals the 6DR7, 6FD7, 6FR7, would be 9-pin equivalents to the 6EM7.
(all have the same pin-out as one another and should work in this circuit)

Yah 6DE7 has a medium mu triode and low mu triode, the 6CY7 is the same/equivalent.
(the 6CY7 has a different pin-out then the above, I'd use one/all of the other 3 to roll)

--------

EDIT:

Actually just took a closer look, the 6CY7 is the same pin-out as 6DR7, 6FD7, 6FR7 but
it doesn't tie pin 2 and 3 together, only uses 3, so just use pin 3 and all will work there. :thmbsp:

 

[Tinkerbelle]

Great, thanks. The 6EW7, 6DA7 too...and no doubt there'll be others

I'll have to see what I've got handy