substitute for 6DR4 tube

[Bluefinger]

Hi,

I am considering building me a Gibson GA-40 Les Paul amp clone. It seems like all the components are easily available. mercury Magnetics offers clones of the original transformers and I also found a supplier of NOS 5879 tubes.

What I didn't find is a 6DR4 tube. I never heard of it. It is used as a tremolo driver (that thing wiggles the preamp tube instead of the output tubes!) Here's a layout (I couldn't fins a schematic in good quality): http://www.chasingtone.com/schematic...0%20layout.jpg

Can that tube be substituted for something else? What's the best way to stick as close as possible to the original design?

thanks
BF

[J M Fahey]

I very very much doubt the tube used for the tremolo oscillator has much effect, if any at all, on amp sound.

As of the schematic, Mr Google sent me straight to Gibson itself, which shows this as GA40 Les Paul amp:



It uses an 6SQ7 which is also a very old very obsolete "radio" tube, must have been obsolete already in the 50's and available dirt cheap because of that.

Radiomuseum says that its "sucessor" tube, meaning what was used instead of it when it disappeared, was
6AT6 or 6AV6 , both cheap and easy to find NOS .
6SQ7, Tube 6SQ7; Röhre 6SQ7 ID2986, Double Diode-Triode

But that's not the point.

The amp is just a datasheet application of 6V6 power amp, with a flat straight from datasheet 5879 preamp driving it, so there's nothing special there, lots of similar amps same vintage.

The magic element which will be very difficult to duplicate and which definitely colors its voice with its huge response peaks and dips, is the paper thin cone, paper former coil, nitro lacquer glued Alnico Jensen Concert speaker:





For authenticity I'd worry much more about that than anything else.

Good news is that sometimes such speakers can be found after cannibalizing old organs and such.

That and the cabinet affect voice a lot; the electronics part is the easy part

[Bluefinger]

great info, thanks a lot!

I was not worried about the tremolo oscilator tube changing tone but I don't know which tube can be used as a replacement without changing the circuit.

Those older cabs look like they don't have a baffle board at all. It looks like the speaker is bolted right to the solid pine cab. It's hard toi tell from those pictures.

[Jazz P Bass]

As an aside, I think that the circuit that varies the preamp, as opposed to varying the bias, sucks.

I believe that it was a misguided first attempt, which was later abandoned for the bias vary circuit.

[Bluefinger]

Thanks P Bass,

I thought it would be worth a try. On the other hand, there might be a reason the preamp tremolo never appeared on newer amps anymore. I never built an amp with an octal front end so I thought this might be a good start. MAybe I'll just omit the tremolo circuit. To make a bias tremolo work I guess I would have to go fixed bias which is something I would like to avoid in this amp.

[Pedro Vecino]

For me the tremolo on the preamp is a variant and can be very musical. It also has the advantage of being assigned to a single channel.
The 6DR4 tube seems to be a 6AV6 or half 12AX7. In this circuit shares the cathode resistor of the pentode.
The only drawback I see in this tremolo circuit (that I don´t understand) is that the depth potentiometer handles high voltage (!). The section of capacitors and resistors series/parallel appears to be associated to the voice of the tremolo circuit.
If I had to build it I would include the complete circuit. First making sure that it´s well drawn because the power line to the output transformer is taken after the choke. It can contain more errors.

[EFK]

You don't need to go fixed bias to use a bias shake trem. You can do it easily with half a 12AX7 or a 6SQ7 if you want or whatever. I do it all the time and with a paraphase too (which this amp is). You'll need to add one cap and one resistor to avoid screwing up the divider feeding the second grid of the paraphase. Looking at the schem above, you add a larger cap, like @ .033 or .047, after the upper .02 output cap which feeds the first output tube, then you add a 470K grid leak R after that. The junction of the two 470K power tube grid leak resistors is lifted from ground and fed to the wiper of a 250K trem intensity pot. The lower lug of the pot goes to ground. The upper lug of the pot is fed by the trem oscillator - you build your standalone simple oscillator, then run an additional cap off the trem plate resistor (not one of the 3 oscillator caps that will be feeding the oscillator grid) that feeds the upper lug of the intensity pot. I usually use .1 uF and I don;t run a resistor inline like some amps do, but it all depends on how you design your oscillator and how strong of a signal it puts out. Look at some schematics for the GA16T, GA18T etc. or the Epi EA35T 'Devon' tremolo to see how this is done.

Alternately the preamp cathode shake can also sound great. All Valcos were done this way and the earlier versions (pre @1961) also used what is essentially a variable plate resistor (B+ across pot, like here) to control intensity. Works well. I use PEC 2W pots now for these, though, just to play it safe.