Ampeg V4/V4B hybrid & DIY 2X15 project

Picked up a working 1973-ish Ampeg VT-22 that had been taken out of the combo cabinet for $200. Whoever did the surgery was not precise. Gigged with it a few times then decided to make it proper. At the same time I am prototyping a 2x15 cab with budget speakers from MCM Electronics:

http://www.mcmelectronics.com/product/MCM-AUDIO-SELECT-55-2983-/55-2983

Got some help with the cab modeling in this thread:

http://www.talkbass.com/forum/f15/winisd-plot-bass-cab-exp-speaker-builders-please-review-988687/

My goal is to end up with a nice looking head and cab that will be alternates to my SVT-CL and SVT-410HLF. So far I have gotten my tech to refurb the amp. I will attach an invoice for the work he did. The neatest thing is we converted channel one to V4B specs and left channel two for guitar. Reverb stayed too so it's a V4B AND a V4. I am restoring the headbox to damned-near factory including a new faceplate from Fliptops.net.

As far as the speaker cabinet goes, I wanted plenty of fundamental through the 5-string range. I am not real concerned about weight or size. Nor efficiency so much. I am mainly building this rig so I can get some tube tone going at smaller/quieter shows.

So I bought the aforementioned driver and put it in a thrown-together, appx 5.5 cu ft box tuned to 35 Hz. This gives me roll-off starting at 70 Hz, F3 @ 37Hz and F6 @ 31Hz. It sounds good. Don't need a horn or mid and my open B is holding up. I was able to make it fart just a tad at almost full volume and that's prolly around 250 watts. Vents are a bit noisy, but I new that....just used some 3" ports I already had (the piece of 2x4 is covering a port/horn hole). I am going to A/B it with my 410 soon.

I am encouraged enough to proceed with a proper box for two 15's. Think I will change the Fb to 43 Hz and make sure my ports won't bark. This will give me F10 at 31 Hz, a louder cab, more room to work around Xmax, more overall power handling, and a little more clean headroom for the country gigs.

See attached images for what was done to the amp. More pics to come as progress is made.

 

Here's the head box with the bottom from the combo reattached and the "window" that goes behind the grill cloth frame installed. Made mine bigger than stock because I don't have a head logo and these are prone to run warm anyway.

 

Here's a photo of the logo from the VT-22. Nice piece of original grill cloth left underneath it. A previous owner apparently ripped the entire grill off when they chopped the combo cabinet. I am going to shamelessly put this logo on the new 215 cabinet. The other photo is of the head box getting skimmed with putty to fill 40 years worth of dings so I can get a nice result with the new tolex.

 

Thanks, Will. Great input! I will explore all your suggestions, as well as anyone else's. Having a throw-away box to prototype with is a first for me. Gonna try to get it right this way with a single driver before I bust out my cabinetry skills on the final 215.

 

So I put a third port in the box. Each is 3" in diameter and has an effective length of apprx 4.25 inches. WinIsd shows it to be tuned at 40 Hz. I amplified it with a board and power amp (240 watts at 8 Ohms) this time.

I tried it with my bass first. I was very pleased with the loudness of the open B string. You could really feel the fundamental sitting under the overtones. Not a lot of noticeable drop in volume over the open E.

Next I used the tone generator in WinIsd to feed it a 31 Hz tone and then a 40 Hz tone. I could hear that the 31 Hz was not as loud as the 40 Hz tone but it was not dramatic and was actually just what I would expect from the modeling showing -9 dB at 31 Hz. The speaker happily took all the power, assuming I was getting all or most of the 240 watts out of the amp. I had the laptop turned up all the way, plugged into the tape in jacks, the main fader at unity and the gain on the amp all the way up.

I will give it another listen with the tone generator. Try out some more frequencies, maybe a sweep. Then hook it back up to the V4/B and jam a bit. Wish I could hook it up to my SVT-CL and A/B it with my 410. I certainly know how that rig sounds which would be great for comparison.

So far, so good. Am I missing anything or can I move forward with building a proper box for two 15's and go for the 43 Hz tuning frequency and a -10 dB point of 31 Hz?

 

OK.....why? Are you taking into account this has 6550's not 7027's in it? What value should the grid resistors be?

 

Modding this amp for 6550's significantly changed a lot according to my tech, George - Especially with the bias circuit. The reason the individual 47k resistors were changed to 5.6k is there was a change in value of a shared resistor in that circuit which altered the whole balance. He assured me it's right. He got the mod design from that amp designer at Marshall, whose name I cannot recall right now.

 

George emailed me a page out of Aspen Pittman's The Tube Amp Book. The amp designers' names are not familiar to me and not whom I thought George was speaking of. No wonder I couldn't remember the name. This info makes me feel comfortable about changing my max power in WinIsd to 200 watts. This will give me a little more room with my tuning to avoid excursion past xmax.

 

Having been satisfied with how the cabinet sounded through a PA and power amp, I hooked up my Ampeg V4B head to it. Really loose and distorted at all except the lowest volume. I have just had that head completely rebuilt but have not replaced the tired, horribly mismatched output tubes yet. Then I accidentally plugged the speaker into the "amp in" jack instead of the speaker jack and turned the amp on for a few seconds. Now it just makes a deep and loud hum. Oops. Going back to my tech. In the mean time, here's another bit of modification to the head that has not been mentioned. Quoting George, the amp tech, from this morning:

"I Changed the plate R for the input preamp tube and took out the bypass(swamper- makes distortion) cap. Ideally you want the preamp B+ at 1/2 power amp B+ for clean. I'll check the voltages and now that I have a cab that can take the watts ,test it . Bring your extra tube box."

 

Final incarnation of this project can be seen in this thread:

http://www.talkbass.com/forum/f15/rat-rod-ampeg-v4b-1024644/