T-Rocket 5879 & 5881

[10thTx]

Thought I'd toss a DRAFT idea out for someone to consider. I've built a few amps with 5879 and like the fat (somewhat chimey) tone of that pentode. I have not built this.

The active FX level makes a great master volume and allows a reverb or delay pedal in the mix.

The 220p cap across the LTPI entrance triode plate resistor is an "enhance cap" and allows the higher frequencies to be smoother, IMO.

The 2.2k 5881 screen resistors add some more compression.

With respect, 10thtx

[RJ Guitars]

I like this idea... Not that I need another project but I could use on of my Rockster reverb chassis and not have to drill any new holes for all the tubes. Have you looked into a layout yet? Seems like the standard sized preamp board would work well to make it even simpler.

I've always thought it would be a good idea to build a pentode entry into a Rocket and I already like the Octals in the Rockster amp (KT-66 is my favorite but 5881's can sound quite nice). Do you know how the 5879 compares to the EF86 tube?

Let us know if you build it, cool idea!

[10thTx]

My experience with the 5879 is that it has less gain then the EF86 and I have had very few microphonic problems with the 5879. It's a really very warm and FAT tone, IMO. The tube seems to be very touch sensitive.

DaGeezer on the Hoffman forum designed a HoSo56 using this tube in V1. I've built that amp and some others with 5879 tubes. It also works well in an overdrive circuit like in the Tweed Overdrive Special, Tweed BlueMeister light, and the D'Mars Overdrive Special.

I think one could easily change from 5881's to KT66's for the design.

I don't have a layout but it would be reasonably easy to draw up one IF you have some genuine interest in this idea?

With respect,10thtx

[Richie]

Gibson GA40, Divide by 13 and many others have used the 5879. They are not too expensive to find, and you have to go through some to find a good less michrophonic one. I've built a few amps using the 5879 sort of Orange or vox type click tone control. I think too, you can tweak the values of the caps and resistors to get it to what you like.

[Tone Lover]

I really like the 5879 tube with a gain stage in front driving it. it gets a great tone, sweet creamy sustain.
Bill

[dorrisant]

Great topic... I have a bunch of 5879s. I have been debating on using donor chassis ant PT from a Gibson GA-RVT Minuteman to build a GA-20T Ranger... They sound like heaven!

http://youtu.be/pco3AAvBW6w

Brad, the dude in the video, brought this one in for repair quite a while back. I love that amp... Magical!

Tony

[Richie]

Great topic... I have a bunch of 5879s. I have been debating on using donor chassis ant PT from a Gibson GA-RVT Minuteman to build a GA-20T Ranger... They sound like heaven!

http://youtu.be/pco3AAvBW6w

Brad, the dude in the video, brought this one in for repair quite a while back. I love that amp... Magical!

Tony

Yep, one of my favorite amps.. I like it over the GA40 because the 20T has the 5879 and a 12AY[orX ]7 channel. A friend of mine has a bunch of Gibson amps that I service. It does sound really good.

[10thTx]

OK, here is a DRAFT layout. CHECK for errors! Compare with schematic.
If there is a discrepancy, go with schematic values, etc......

Only thing I might change is adding a 22k grid resistor on V2-2 and perhaps changing the grid resistor on the 5881's from 1.5k to 5.6k.

I have NO plans to build this but thought someone might be interested in it.

With respect, 10thtx

[dorrisant]

Now I will have to put this build on the list.

Tony

[matt h]

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[10thTx]

Swap the first two stages.

I have had a 12A_7 triode into a 5879 with 12A_7 cathode follower. I do like this arrangement quite a bit in an OD channel like on the Tweed Overdrive Special, but not so much on the clean channel.

I also tried a 12A_7 with a 5879 gain stage that could be switched in and then into another 12A_7 triode into LTPI.

I thought they sounded OK, but prefer the 5879 in V1 with this design. Just a matter of personal preference.

With respect, 10thtx

[matt h]

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[Decko]

The active FX level makes a great master volume and allows a reverb or delay pedal in the mix.

Hi 10thTX, your post caught my eye as I recently built an Express-like circuit with 5881's on the output using the same mv circuit that you have drawn up. I am very pleased with the tone. It is a very versatile amplifier. I can crank the MV to 10 and set the volume around 1:00 and you get that classic Express tone. If I reverse the setting, the amp goes into overdrive insanity.

Could you expound with more detail on your active effects circuit? I have 3 extra triodes and would consider implementing. My concern is how the express circuit would respond to your effects loop implementation.

Thanks for your post. I hope this doesn't detract too much from the 5879 V1 discussion.

Regards,
Decko

[jazbo8]

You've got to optimize the operating point of the tube. Plate, screen, and cathode voltages matter a lot--especially, especially, especially (thrice for emphaphaphasis) if the stage is being overdriven. As shown, that's a very cold bias and I can't imagine it sounding good in a secondary spot.

Why is V1 considered in a "secondary spot"? Even if it has a cold bias, the chance of it being overdriven isn't that high, unless you use a very hot pickup. I did question 10thTx about the "pentode tone" as it has nearly the same gain as the 12AX7 used in the Rocket but he said there was definitely a difference when he swap the tubes.

[matt h]

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