Vibro Champ to Vibro Princeton

[alerich]

I have a seventies Vibro Champ that is not pristine or unmolested by any means. Fun little amp but that's about it. The PT is the same as the SF Princeton so it will support 2x6V6. I have an Allen/BillM TO20 OT from an aborted PRRI upgrade. My plan is to install solid state rectification and use the rectifier socket for the other 6V6. It was recently recapped with a 525VDC cap can. It will be cathode bias. It has a Weber VST C8SS 8" 15W speaker. Under powered a little but I won't be cranking it.

This should work, should it not?

[eleventeen]

Potentially a nice little package! Take some pix as you build it. Any thots of going to a 10" speaker?

[alerich]

The 10" speaker is a thought. I actually really dig the Jensen C10R in my PRRI. I threw the post up in a hurry. Obviously, I will need a phase splitter. Since I want to retain the trem I will add a tube and drop a cathodyne in it between the second gain stage and the power tube(s). If i don't like that I can always use the whole tube and build a LTPI. I know the voltage is gonna jump up with diodes instead of the 5Y3 but that'll get pulled down a bit by the second 6V6 and the extra 12AX7.

I bought the VC carcass on eBay years ago. Someone removed the trem for more gain (supposedly). They butchered the circuit. It worked but very poorly. They also re tolexed it in blond tolex with black grill cloth. I bought it to rebuild it. It was my first amp build/rebuild. It was a great project but it has sat collecting dust since then for the last ten years. I am itching to build something but I need another amp like I need the proverbial hole in my head. I'm hoping I can re-purpose this amp into something I can actually play and enjoy.

 

[eleventeen]

Heh. I got a few of those.

I bot a Deluxe 90 which a SS single-12" just because I wanted to see if a SS amp could work for me. It was $100 used in nice shape so no big deal, and the cabinet and Celestion speaker are worth $100 to build a toober in if it dies. Meanwhile, it works OK in a small room but sounds like sheer crud in a larger room.

Than I found a free Peavey Bandit, allegedly not working. Got it, drove it home, and...it works! So I am a little hesitant to just tear it apart.

It would be nice if one could build all manner of amps with stuff lying around, and in general...I can, as far as tubes and trannies go. But if you want them quiet, then you pretty much have to buy a pile of jacks and pots and it gets to be a small investment of bucks but a large investment of time.

[alerich]

I got the Vibro Champ -> Vibro Princeton put together today. I basically removed the rectifier and replaced it with diodes and a standby switch, added another power supply node with an unused section of the cap can, rewired the rectifier socket for the second power tube and added a preamp tube which consists of an additional basic gain stage followed by a cathodyne phase splitter. The schematic is below. I left the trem intact. The trem injects into the second gain stage as does the NFB (which I have switchable) as they did in the VC. I inserted the additional gain stage and the cathodyne between that former second gain stage and the power tube(s).

As you might surmise, going from a 5Y3 to diodes has bumped voltages up quite a bit. B+ is about 470VDC. The other nodes run rather high as well due to the small droppng VC resistors. I don't have the parts on hand at the moment to install larger dropping resistors similar to the PRRI. I have a Weber Signature ceramic 4Ω 15W. Not a perfect match but it will have to do for now. I'll try it plugged into my PRRI Jensen speaker tomorrow.

The amp sounds pretty good. Nice and full - much better than the VC. The switchable NFB was a subtle change in gain previously. Now it makes a huge difference. Likewise, the trem is like trem on steroids. It's only really usable If you keep the amp on the low side (3-4) and keep the intensity of the trem on the low side as well. I think I am going to switch the gain stages around so that the trem and NFB will be injected into the gain stage just before the cathodyne to see if that improves their performance. I think they are too early in the gain path. I may remove the added gain stage altogether. I had originally only intended to drop in the cathodyne and leave the other triode unused.

I got the OT leads wired correctly on the first try and Auburn beat Alabama. It was a good day.