Vibrolux Reverb AA964 Build

Discussion in 'Glowing Bottle Tube Amp Forum' started by theprofessor, Jan 1, 2021.

  1. Bob Arbogast

    Bob Arbogast Tele-Afflicted Silver Supporter

    Posts:
    1,782
    Joined:
    Mar 16, 2003
    Location:
    Lowell, Michigan USA
    Not sure how that bias probe works, also not sure if you have installed 1-Ohm cathode resistors for bias measurements . . . but if so, it's possible that the bias probe has its own internal 1-Ohm resistor and you could be reading the voltage across both that resistor and your own 1-Ohm resistor. That would give double the voltage, and thus the tube would appear to have twice the cathode current that you would expect.
     
  2. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    A pic of the two bad "Tung-Sol" 12ax7's in the post below.

    You can see that the one on the right actually lost its vacuum. I used these for 1-2 weeks. I spoke with Mike Kropotkin at KCA NOS tubes today when I was purchasing some JAN Philips 6L6WGB's, and he said that the fail rate on these "Tung-Sol" preamp tubes was so high for him that he stopped carrying them about 10 years ago. Interestingly enough, he said that he still carries other New Sensor 12ax7's, like the "Svetlanas" and the "Mullards," and that he hasn't had the problems with those that he had with the "Tung-Sols." That's too bad, because, tone-wise, they're the best new-production tube I've heard. Oh, well. 2 for 2 in terms of bad "Tung-Sol" 12ax7's from Tube Depot is not a good percentage...
     
  3. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    IMG_0254.jpg
     
  4. Axis29

    Axis29 Poster Extraordinaire Silver Supporter

    Posts:
    6,930
    Joined:
    Jan 2, 2007
    Location:
    Beaumont, CA
    Well, that sucks big.... uh... time.

    I had a few of those Tung Sols and liked them. I've had more failures from newer tubes, than any of the old tired used tubes I've picked up in lots. But, i dunno that I've paid attention to what brand most of them were.

    I have a decent size stash of preamp and 6L6 tubes. I hate the idea fo buying new tubes... it's one of the most stressful things in the world to me. All the descriptions begin to sound the same, plus I know it's all just marketing BS. Honestly, that's probably why I just buy lots and try them.
     
    theprofessor likes this.
  5. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    That was an interesting experiment with my AmpRX Brown Box! I decided to put my SF Deluxe Reverb on the bench and see what it would sound like through the Brown Box at around the same level (116-117 VAC) as the Vibrolux Reverb. First, I checked the bias and voltage running on the 123.3 VAC at the line. Plate voltage was at 453, and plate current on the 6V6's was at about 66% and 70%, figuring on a 12W maximum (not 14W, as one might and is well justified to do so). Then I put the DR through the Brown Box with a 3% reduction, which meant it was running on 116-117 VAC, just as I run the Vibrolux Reverb. That brought the plate voltage down to 435 VDC, and so I re-biased the tubes to about the same place: ca. 66% and 70%mpd. The sound is sweeter for sure. And: I measured the heaters when running at this 3% reduction: 6.35 VAC. Perfect!

    So my VR and my DR now run at right about the same plate voltage, with heaters right about in the same place (my Vibrolux Reverb gives about 433 plate volts when running at ca. 116-117VAC, with the heaters at 6.33VAC and the power tubes at ca 60% mpd). That has me thinking that my Hammond 290CX PT may well be spec'd from a SF Vibrolux Reverb, instead of a BF one. Maybe not, but the numbers on these two amps line up remarkably

    Vibrolux Reverb clone @ 116-117VAC
    435 DCV on the plates
    6.35 VAC on the heaters
    ca. 60% mpd on the power tubes

    SF Deluxe Reverb @ 116-117VAC
    433 DCV on the plates
    6.33 VAC on the heaters
    ca. 66-70% mpd on the power tubes (figured on 12W max)
     
    Axis29 likes this.
  6. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    Sound clips are coming. I promise. I hope to try to figure out something to play either tonight or tomorrow.

    In the mean-time, I've been A/B'ing my Vibrolux Reverb and my Deluxe Reverb. I was worried that I wouldn't like my Deluxe Reverb anymore after this Vibrolux, but that's not the case. It has its own thing going on.

    I may be getting hung up on the volume pot taper or something, but this Vibrolux Reverb is not as loud as the Deluxe Reverb. It seems like it should be, which is possibly what has made me a bit confused from the start. Granted, I have different speakers in each amp. The Deluxe Reverb has a newly reconed Oxfort 12T6 in it (which sounds fantastic!). The Vibrolux Reverb has an old CTS ceramic from the 1970's (original cone) and a new Eminence 1028k. I think the Eminence is a 98dB speaker at best, and the CTS isn't even that. But still.

    On both my DR and my VR, nothing happens on the volume dial until 2.5. With the DR, it's loud by 3.5. I have to get up to about 4.5 on the volume pot on the VR to get to a similar volume with a Strat. And the NORMAL channel on the VR is pretty wimpy, while the DR's NORMAL channel is not at all wimpy.

    With the VR, there are no weird sounds or pops or strange voltages -- other than that I thought the voltages would be lower. The tone on the VIBRATO channel is magnificent. I'm just still confused as to why the amp isn't as loud as I expected it to be. It's actually quite serviceable as it is. On the VR, I can dig in and it can get loud. I can play softly -- even on 5 or 6 on the volume dial -- and it can sing softly. The DR is a different beast. It is not as dynamic that way. It's got beef in the middle where the Vibrolux has air, and when it's loud, it's just loud. And when it's quiet, it's just quiet. That dynamic quality is one of the things I'm finding remarkable about the Vibrolux Reverb. But I'm not getting blown out of the room by it. I can easily blow myself out of the room with the Deluxe Reverb.

    I'm starting to wonder whether it's the New Sensor "Tung-Sol" power tubes. They do bias properly, at least according to my readings across the 1 ohm resistor. But when I use my bias probe, they read out at 8 mA or 6 mA or something really weird and low. The probe reads the plate voltage just fine, but the current readings are low. Makes me wonder if something is going on in the power tube area. I've got a pair of known good 6L6's coming from KCA NOS tubes next week, so I can always see how those perform by comparison. Or I can take my matched pair of GE 6L6GC's out of my 5f4 and see how they sound and if they produce more power.

    Another thing: I got a brand-new "Tung-Sol" 12ax7 as a replacement from Tube Depot for the one whose vacuum failed. It definitely produces sound. I auditioned it in my DR, and it was very loud. But the tube is as dark as night. And so were the previous two bad ones... That doesn't make any sense to me.
     
    Last edited: Mar 27, 2021
  7. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    I am happy (and embarrassed) to report an operator error! The bias numbers my Eurotubes Pro One bias probe were giving me were correct. I don't know what I was measuring across that 1R resistor to get my voltage drop. Maybe I was measuring some kind of resistance in Ohms on accident? But my bias was WAY low. That means that my power tubes were barely working, which explains why the amp didn't sound as powerful as it should.

    After my bias probe worked perfectly on all my other amps, I started to question whether it was right and I was wrong. So I went back and pulled the chassis and re-measured everything. I ended up using a 13k bias range resistor, I think, and I swapped out some rectifiers as well. After trying out a JJ GZ34, I ended up putting in a "Fender" labeled Sylvania 5U4GB that I just got used. It's really strong, and at my regular line voltage tonight (not through a Brown Box), I got 410V on the plates. The power tubes aren't as well matched as I'd like. They're at about 40mA (55%) and 46mA (63%) respectively, so I'm straddling the 60% mark. I plugged in and -- there it is: all the power I had been looking for. The tonality is very much the same, just with way more horsepower under the hood.

    So that answers why the PT was "giving me too much voltage" on the plates: I was hardly drawing any current to the power tubes! Once I did that, everything behaved exactly as it was spec'd to behave.
     
  8. NTC

    NTC Tele-Holic

    Posts:
    727
    Joined:
    Mar 17, 2003
    How does it stand up to the DR now?
     
  9. mrfitz98

    mrfitz98 Tele-Meister Silver Supporter

    Posts:
    150
    Joined:
    May 26, 2020
    Location:
    Michigan
    Not to be nit-picky here...

    My math tells me 3% of 123.3 is ~3.7. This would put your voltage after a 3% reduction at ~119.6. Close to what was coming out of the wall in 1970, maybe a little hot for 1964. Are you checking the voltage out on the brown box?
     
  10. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    It's no contest now. The DR is still loud, but the VR has, as @Bob Arbogast said, a kind of authority that the DR doesn't have. The Vibrolux is more articulate and more capable of dynamic range. It is overall clearer, but it has less "body" in the midrange than the DR. I think the VR is what people are going for when they stick 6L6's in a DR, but with 2x10 speakers. As jaded as it sounds, the "voice of angels" comes to mind.
     
    Last edited: Mar 28, 2021
    Bob Arbogast likes this.
  11. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    The way I was reckoning things, I'd measure the wall first. Then it would show the same on the Brown Box. Once I turn the amp on, the voltage comes down a bit because of the load (say, to ~120ACV or so). Then I select that voltage on the top control of the Brown Box and the 3% on the bottom control. The 3% reduction is 3% of that last number (~120ACV), not the initial line voltage number.
     
    mrfitz98 likes this.
  12. NTC

    NTC Tele-Holic

    Posts:
    727
    Joined:
    Mar 17, 2003
    So, Professor, this is an apples and oranges question, or a "Which of your children do you love the most?" question. How is the VR compared to the 5F4? I know the basic sonic differences - I have a Bandmaster Reverb and several tweed like amps.
     
    theprofessor likes this.
  13. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    OK, @Axis29 et al.: so I finally got a sound sample recorded. This may be the only one you get, so I hope it gives you a sense of the tonality and dynamics of the amp!

    This is a sound sample of my newly built Vibrolux Reverb clone (AA964) with a EJ Stratocaster plugged straight into the VIBRATO channel, using the neck pickup only. Guitar vol 10, tone 10. The "Bright" switch is on. The amp volume is on 3.5. Treble 5.5. Bass 7. Reverb 4. The "raw/mid" pot is about 1/4 of the way up. Mic'd using a combination of a Sennheiser E906 (mic'ing the Eminence 1028k only) and the condenser mics in a Zoom H2N for the room. Recorded straight to mini SD card. No post-production.

     
    Last edited: Mar 28, 2021
    screefer likes this.
  14. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    I do have a question, though. Since the amp is working properly, the tremolo noise has become more pronounced. I have a recording of it here. Is this the kind of thing that can be improved by one or all of these "tremolo ticking" fixes noted by Rob, or is this something altogether different? It sounds like the tremolo is picking up signal noise.

    And another thing: I've sometimes had to futz with the tremolo a bit to get it working. It's like there's a fairly long delay between pressing the switch and the tremolo coming on -- like it's got to work its way through the whole signal, until it finally comes on. Once I get it working, it seems to work from then on out, as long as I have the amp on.

    https://robrobinette.com/AB763_Modifications.htm#Tremolo_Ticking

     
    Last edited: Mar 28, 2021
  15. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    I think they sound very, very similar, except, of course, for the integrated reverb and tremolo in the VR. And the VR has a bit more horsepower, even when running both on 6L6GC's. But boy is the 5f4 a sweet amp! I think the cabinet dimensions affect the sound and give it really nice bass response. I'm partial to amps with integrated reverb, so if I had to grab one, it would be the Vibrolux. But that 5f4 is incredibly clear and responsive and dynamic. And I prefer the aesthetics of tweed amps, but I don't like jamming all the parts in the narrow shoebox-like chassis! Then again, I don't like working on amps with tray-style chassis, either. Too much to take apart! But they're sure nice, in that you don't have to worry about the torque created by sideways-hanging transformers in a tweed amp!
     
  16. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    Posting a note here on my experience with the voltages on this Vibrolux Reverb and with the AmpRX BrownBox. The BrownBox tells me not only the initial wall voltage, but the voltage under load. That's the number I use when determining performance.

    My voltages/current on the amp under load at 121 VAC are:

    - 413 DCV to the power tube plates
    - tubes biased at 43.5 mA (60%) and 49.2 mA (68%)

    If I use the BrownBox for a 3% reduction of this number (121 VAC), I land at 116 VAC. Without any re-bias, the amp's voltages/current at 116 VAC are:

    - 401 DCV to the power tube plates
    - tubes biased at 41.2 mA (55%) and 46.2 mA (62%)

    I think the amp sounds best running under load right at 120 VAC. Plate voltage is around 413 DCV and heater voltages are at 6.5 VAC. Of course I can reduce the overall voltages with the BrownBox and re-bias, but I wanted to compare some numbers like this, since I often play at a venue that consistently puts out 126 VAC at the line. There I'd like to reduce the voltage, whereas at home I can sometimes operate under load at about 120 VAC. I don't really want to bias the amp only for home or only for that venue. So I wanted to see what would happen if I left it as-is, and where the numbers would end up with the 3% reduction. At that venue, I'm often running at 117-118 VAC with the 3% reduction, which is better than 116 VAC, in my opinion.
     
  17. theprofessor

    theprofessor Poster Extraordinaire

    Posts:
    5,776
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    Vibrolux Reverb voltages_Page_1.jpg

    So here's my final, corrected voltage chart. The only real outliers are the two cathodes on V5. Running on a wall voltage today of 124.8 VAC, everything else tracks pretty much right between the schematic voltages for the AA964 and the AB568.
     
    Last edited: Apr 8, 2021
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.