A casting would be nice way for you to reproduce a badge.
I've had sucess polishing and repainting the metal ones.
If you can get a scan, a 3D printer can produce one.
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Ampeg V4 Club
- Thread starter birdman14
- Start date
A casting would be nice way for you to reproduce a badge.
I've had sucess polishing and repainting the metal ones.
If you can get a scan, a 3D printer can produce one.
I use the kit that hoffmanamps.com sells, mojo also sells it. I like it because it measures current with your meter set for mA. this can be more accurate than measuring a low voltage with a handheld voltmeter.
Hoffman Bias checker
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The SVT uses 1 ohm 1W, the V4B reissue uses 10 ohm 1W for the cathode resistors. So 1W is their standard. Typically wire wound is used which has low inductance and low noise. Flameproof is a good idea. Often you see cement block resistors for this position.
What is important is that the resistors be matched. When measuring the push and pull side, you want the balance to be as close as possible. This will minimize harmonic distortion. Buy 1% and buy extra, then use your meter to select the closest pair. The readings across the pair of cathode resistors will tell you how close each trio is to one another. Sometimes you can swap around tubes to balance them better so that the cathode currents are better matched. It depends on how good your matched set is, it is rare that they are dead on all the same. The SVT goes a step above this, it has a balance pot that allows you to set the balance between each power tube trio.
daveobieone
Providing the "musical foundation" since 1966
Wow Beans...thanks VERY much! That's a lot clearer than the schematic I've been working from. The one on the top plate of mine is badly deteriorated. And a SM too!...I'm in tube-tech heaven!
I have noticed that my V4B seemed likely to oscillate on the bench with the gain way up, and treble turned up. That oscillation was around 45khz. It showed up on the clipped edges of the waveform too. When I swept the freq resp, there was a big peak/resonance around 45khz. First, I replaced the cap across the feedback resistor, and it remained. I then doubled-up the .001 cap across the feedback resistor, and it went away. I also lost about 1db at 10khz, but I don't think anyone is likely to notice.
I'm wondering if anyone else has ever noticed this combination of oddness with a V4B.?.
I've changed all the coupling caps. (most were leaking) One filter cap (screen supply) was shorted. Changed that. The other filters seem to be in good shape. I'm not sure what else might cause it to get wacky up at 45khz.
Thanks for the guidance on the cathode resistors! I may put in 10 ohm 1 watters because matching would be easier and closer. I guess they might also act like a fuse if a output tube goes red-plate. I'll probably also put a bias pot on it.
Thanks again!
Dave O.
Wow Beans...thanks VERY much! That's a lot clearer than the schematic I've been working from. The one on the top plate of mine is badly deteriorated. And a SM too!...I'm in tube-tech heaven!
I have noticed that my V4B seemed likely to oscillate on the bench with the gain way up, and treble turned up. That oscillation was around 45khz. It showed up on the clipped edges of the waveform too. When I swept the freq resp, there was a big peak/resonance around 45khz. First, I replaced the cap across the feedback resistor, and it remained. I then doubled-up the .001 cap across the feedback resistor, and it went away. I also lost about 1db at 10khz, but I don't think anyone is likely to notice.
I'm wondering if anyone else has ever noticed this combination of oddness with a V4B.?.
I've changed all the coupling caps. (most were leaking) One filter cap (screen supply) was shorted. Changed that. The other filters seem to be in good shape. I'm not sure what else might cause it to get wacky up at 45khz.
Thanks for the guidance on the cathode resistors! I may put in 10 ohm 1 watters because matching would be easier and closer. I guess they might also act like a fuse if a output tube goes red-plate. I'll probably also put a bias pot on it.
Thanks again!
Dave O.
Experiment, get to know the amp, and find what works.
Hard to say what the cause of the oscillation is without some testing. There are so many possibilities: a tube, wire layout, a bad component, etc.
The current Ampeg amps such as the SVT, have a ferrite bead on the plate of each power tube. This can help suppress high frequency oscillation. I have them in my ’74 and have no issues with oscillation. I install them inline with the wire at the tube socket, leads as short as possible, cover the lead-wire splice with heatshrink tubing.
This is the part: Murata BL01RN1A1F1J, available from Mouser
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An adjustable bias makes life easier. There are many examples on the net. One way is with a tiny circuit board. There is one in the TB Portaflex wiki:
Bias Pot Technical - Amplifier | TalkBass.com
I’ve never tried to retrofit this board into a V4B but it should work. The values of the pot and resistor would have to be changed to suit the V4B circuit. The important thing is no not just replace the bias resistor with a pot, add a series resistor to the pot. You don’t want to adjust the pot down zero ohms and have no resistance present.
You might want to check out the PEC tone board as well if yours is out of spec, also in the wiki. It is a pain to get to in the V4B.
Tone board Technical - Amplifier | TalkBass.com
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daveobieone
Providing the "musical foundation" since 1966
Thanks Beans! I'll order some of those beads with my next Mouser order.
I also note that there are no "grid stopper" resistors on the power tube sockets of my V4B at all...aside from the 47K's on the circuit board. None on the input stage tubes either. I may add a few of those while I'm in there soldering. Perhaps the combination will help, and I can get rid of the extra cap on the feedback.
Mine does not have the extended HF response quoted on the Service Manual you sent. +/-1db out to 20khz...no way on mine. Mine is a few db down at 10khz...and that was before the extra cap across the feedback resistor to tame that 45khz peak. Not that response at 10k means much of anything to a bass player. I wish I had another classic V4B I could put on the bench to compare. Maybe someday.
Everything checks well on the tone (echo) board. I just put in a new (NOS) 6K11, and ordered another NOS one for a spare.
Lead-dress is rather sloppy in mine. Surprisingly long wires where they don't need to be long. Very little twist on the filament leads, etc. Nothing like my B-15n from a few years earlier. I'll make some of that more neat and tidy next time.
Dave O.
daveobieone
Providing the "musical foundation" since 1966
If you guys get some horizontal logo plates made-up, I'd be in for one as well. Mine is missing too.
Dave O.
You could try moving the 47K power tube grid stopper resistors onto the tube socket but I doubt that it would make a difference.
I've found that bad solder joints are common in these amps. The amps run so hot and over time it takes a toll. Reflowing all the joints on the boards helps. The tube sockets can have gunk between the terminals under the sockets. I use pipe cleaners soaked in a cleaner to get under there and clean.
I did install twisted heater wires because it irked me the way that they did it. Same goes for vintage SVT's.
I don't know how accurate the service manual bandwidth specs are. They seem more than a little optimistic.
Here’s what the template one looks like....the scribbling shouldn’t turn up in the castings.
Kids and markers......
Some 99% isopropyl alcohol (pharmacy) or vodka on a Q-tip will remove the markey marks.
I like vodka.
I used some alcohol to remove the ink, but it started removing the white paint as well
I used some alcohol to remove the ink, but it started removing the white paint as well
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I got into using an Ampeg V-4B along with the Ampeg SVT-21AV, short time ago. I'd like a number, if you have any more left.
daveobieone
Providing the "musical foundation" since 1966
Hey...that's the bass I play!!! I have two SG's. One is red, the other a "limited edition" with the flame maple top. I love them both!
daveobieone
Providing the "musical foundation" since 1966
The Tung Sol 7027A's arrived today. Very quickly from Canada. A few pics are attached. All were within a smidge of 7000 micromhos in my Hickok tester. According to Hickok, average new is 5900. Even that is quite a bit hotter then the average 6L6GC at 5000. These are some very hot tubes!
It's becoming clear that I will need to install some cathode resistors, and probably adjustable bias to keep them from roasting. Kudos to The Tube Store dot com for some right-on matching.
You can see the extra radiators on the plates. They do look like 35 watt tubes.
It's becoming clear that I will need to install some cathode resistors, and probably adjustable bias to keep them from roasting. Kudos to The Tube Store dot com for some right-on matching.
You can see the extra radiators on the plates. They do look like 35 watt tubes.
Sounds good. I’ve also found the matching at the tube store to be very good.
Your tester looks like it is in very nice condition. I have a Hickok made one as well. They are a work of art.
A couple of considerations. The best way to measure the tubes is at the plate voltage and bias voltage of the amp. The Hickok plate and bias settings are lower, intended for a relative comparison of tubes to the standard that they devised. The calibration of the Hickok tester is another consideration.
As I mentioned earlier, you don’t have to bias 7027A’s at 70% of the maximum plate dissipation for them to sound good. Unless you are trying to get all you can out of the amp, I find that more conservative bias works well sonically and extends tube life.
Your tester looks like it is in very nice condition. I have a Hickok made one as well. They are a work of art.
A couple of considerations. The best way to measure the tubes is at the plate voltage and bias voltage of the amp. The Hickok plate and bias settings are lower, intended for a relative comparison of tubes to the standard that they devised. The calibration of the Hickok tester is another consideration.
As I mentioned earlier, you don’t have to bias 7027A’s at 70% of the maximum plate dissipation for them to sound good. Unless you are trying to get all you can out of the amp, I find that more conservative bias works well sonically and extends tube life.
daveobieone
Providing the "musical foundation" since 1966
Thanks Beans!
My plan was to initially set them at 60-70% of the normal 30 watt rating of 6L6GC's. As long as the crossover distortion is low.
I'm about to pull the chassis out now, and do the cathode resistors etc.
Funny story about that Hickok... I stumbled across that about 20 years ago at a pawn shop in the tiny town of Anna Illinois. It was like-new physically, but dead electrically. I gave $75 for it. A fuse (which is really just a light bulb) was open. Changed the bulb/fuse, and it's been perfect ever since. I'm guessing someone stuck a shorted tube in it when it was new, blew that, and then put it away in a closet somewhere for 40 years. It's still like new. My best pawn shop find ever!
daveobieone
Providing the "musical foundation" since 1966
Oh, and BTW, there's nothing on pins 1 and 6 on these new reissue 7072A tubes, which would have been connected to the control and screen grids on the originals...
The missing pins make things easier in Ampeg amps, they were not used. The unused socket terminals can be used as tie points.
On the original 7027A, pin-1 was internally connected to pin-4, and pin-5 and pin-6 was also connected internally. So pin-1 and pin-6 are not needed. Some amps have to be rewired if those pins are used in the circuit.
Original 7027A data sheet:
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