Princeton Vibro Champ Reverb

[theunrulychef]

Long time lurker here, starting my first thread. Thanks for all the info & tips I've already gleaned from so many of you.

I'd like to build a Vibro Champ with Reverb in a fancy cab to keep in my wife's living room. I was planning on chopping off the front end of a PR & mating it with the vibrato/output of a VC. I know I'll likely want to tweak the power supply to get my values once I figure out what the PT will be putting out.

I've built a couple amps, an 18 watt clone, and a 5F11 Tweed Vibrolux clone. Both came out great, but I went mostly by the schematics & didn't have to use too much brain power to get them running well. My question about this is, will mating these two amps really be as easy as plugging the front end of the PR into the backend of the VC? I keep feeling like I'm probably missing something that will keep the amp from working correctly.

I've done some searching for a ready made schematic, but only found the non vibro champ versions previously posted. I mocked up my own from the fender schematics. Any help, concerns or reassurances would be greatly appreciated.

Thanks,
Jay in Philly

[6267]

Where the two amps are "grafted", there is a 220k resistor from signal to ground, and on the other amp there is a 100k resistor from signal to ground. You only need one here, I'd recommend a temporary 250k pot wired as a rheostat to find the value that provides the correct gain.

Otherwise, it looks like it'll work to me, but you may have to adjust for different preamp voltages. If you google tubecad, and find the "grounded cathode gain stage" article, there are good equations for figuring out the proper class A cathode bias resistor for a preamp stage about 4 pages or so in. I find the values that are produced via this equation to sound very good. If the voltages are close though, might as well leave it as is.

The worst case is going to be too much gain, most likely, so you may have to throw in a voltage divider somewhere to cut the gain down, or if you have WAY too much gain, I'd almost try using the "last" 12ax7 stage as a cathode follower and drive the hell out of the power tube. That'd be a class A2 champ, and probably would put out 10 watts or more at the possible expense of running the power tubes hard. If you have the filament power, you could probably get even more with a 6L6G/GB or a 6P3S russian tube ($40 for 8 or so...).

Good luck and I imagine that will be a neat sounding amp...I am building one out of old furniture for that "nicer" look than the ripped tolex & beer stain thing. Get an old record cabinet stereo from a thrift store, the wood is great for building amps and they sometimes come with lotsa tubes

[theunrulychef]

Thanks for the info. I hadn't seen the Tube CAD Journal yet. Looks like there's a lot of good stuff - most of it way way above my head at this point but interesting nonetheless. I keep reading & re-reading articles on tube amp design, so hopefully some of it will seep into this dense head of mine.

"...there is a 220k resistor from signal to ground, and on the other amp there is a 100k resistor...temporary 250k pot wired as a rheostat..." That's a good tip. I'll have to try that out. I've actually got a Princeton Reverb & an old Silvertone SE6v6 amp, so I might try experimenting with the two of them. The Silvertone's a bit lower powered than a champ, but the circuit design is close. At least I should be able to fiddle with some components to see the effects.

I think I'll likely end up using the Weber PT - 330v(100mA), 6.3v(4A), 5v(3A) - looks like it's got pretty decent filament ratings all around so I should be able to mess around with some different outputs & rectifiers. It's been a while since I had anything other than 6V6s around.

Thanks again for the response,
Jay

[tubeswell]

6267 said "Where the two amps are "grafted", there is a 220k resistor from signal to ground, and on the other amp there is a 100k resistor from signal to ground."

Hi 6267 - I can't see what you are talking about. The 220k is a grid load resistor for the 6V6. The 100k is the driver plate resistor (to the B+ rail).

Hi Jay FWIW, what I did notice is the supply voltages aren't quite sorted out yet. (You might want to try a PT/that enables you to experiment with 5Y3GT, 5U4G and/or 5AR4 rectifiers, as well as different oupt tubes, so you can get 6L6 in the output stage. (This might mean going to 150mA HT, and more on the heater (see the reverb driver suggestion below)). Other options which you're probably aware of include increasing the reservoir cap to between 30uF - 50uF to get the voltage up a bit, and then experiment with the plate supply resistors. If you want only a 2V drop on the 6V6 Plate to screen, you might have to try a small choke in the filter instead of a 1k resistor, and think about adding a 470R - 1k5 screen grid resistor. Also if you can't get 400V for the reverb driver (as your highest B+ voltage there is 355), you might consider going to a pentode driver and different reverb tranny (like a stand alone reverb setup), which might run better off a lower voltage. In addition, if you can't get the trem plate voltage high enough you might want to look at experimenting with the speed and depth pot/taper values in the oscillator circuit).

[Bruce / Mission Amps]

It might be a little gainy but there large signal loss across the 3M||10pF mixing resistor cap combo (you might find a 4m7 with a 15pF works better) and there is no phase inverter after the reverb mixer recovery tube so I'm thinking a split load anode resistor here instead of the 100K.
Maybe a pair of 47K resistors in series with the coupling cap inserted between the two.
Or just use a 100K trim pot and connect one of the outside legs to the anode and the other outside leg to the B+ rail and the coupling cap to the wiper.
Then just adjust the pot to suit your needs.
After you have it dialed in, measure the resistance form the wiper to each outside leg and use two discrete resistors instead of the pot.

[theunrulychef]

I see what you're saying about that grid resistor after going back to the PR schematic. I was thinking the 100k was something I had chopped off & left the 220k in it's place.

I know I'll still have to sort out the power supply - the one on that temporary schematic is mostly just the champ side. Halfway through, I realized it would need a reworking so I stopped drawing it where I was. If I use a PR power supply with the 5U4G, I could then calculate out the correct dropping resistors to get 355v going to the OT & 340 to the screen, correct? That would allow me to get the 400v to the reverb driver? I need to download the Duncan software. I do most of my work on a Mac, so I haven't been able to use it yet. I guess I should try to get the power worked out before I do anything else.

Thanks for the help,
Jay

[tubeswell]

If you want to run a 5U4G type rectifier, your rectifier winding needs to be rated at 3A.

The voltage needs to be at least 400 VDC at the B+ to be able to get 400VDC at the plates of teh reverb driver.

[theunrulychef]

If you want to run a 5U4G type rectifier, your rectifier winding needs to be rated at 3A.

The PT I was thinking of using is a Weber (champ / vibro champ / Princeton Reverb etc) rated at 330v(100mA), 6.3v(4A), 5v(3A). You mentioned I might want to think about one rated at 150mA if I want to mess around with other tubes. I was under the impression I'd have some extra leeway because I'm only running 1 power tube, no?

Also, I was curious if I'd run into any troubles tapping the OT lower down in the power supply - since I'll need the 400v for the reverb. Seems like every schematic I'm familiar with has it connected to B+ before all the secondary caps. I'll be the first to admit that I'm just feeling my way around all of this (or stumbling), so I don't know if that will make a difference or not. Would it be better to run the OT from B+ like "normal" and put the drop down resistor after before it goes to the plate or does it make no difference?

Thanks for everyone's responses. They are giving me some good things to look out for before I start the build.
-Jay in Philly

[tubeswell]

The reason I was suggesting 150mA (well 120 would probably do as well but I was being cautious) on the HT winding is if you were going to use a pentode driver for the reverb as well as having the option of a more powerful output tube (e.g.; 6V6 reverb driver Class A 37mA + 6L6 Class A ~50mA + 3 x 12AX7s @ 4mA = 99mA). But if you want to go ahead with a 12AT7 driver, 100mA should be adequate.

No reason not to tap the B+ for the reverb driver at the first filter cap (along with the OT)

[6267]

sorry, I saw both resistors going to ground somehow... at least I can't get electrocuted from here.