Vintage Fender experts: couple of questions...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by neteraser, Dec 7, 2016.

  1. neteraser

    neteraser Member

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    1. Could you tell me, what was the exact primary impedance of the Deluxe Reverb OT ? Hammond/ClassicTone say 6.6k, but was it closer to 6k in the original? For certain reasons, I find this information important as I try to spec a perfect transformer for my build (Fender BF Deluxe as well, but I use a different output tube. I currently use a universal OT from Hammond, which works quite well, but probably leaves some space for future research).
    2. Unfortunately, I've never had a chance to inspect a Fender chassis. So could you point out for me, just where the brass plate connects to the chassis and how? I haven't managed to find any info on the net yet.
    3. Everything else seems working in my prototype :) Be it an authentic OT, or just a working one, I did it. I'm very proud. Long story short, what I'm probably gonna do is to take a very cheap-n-crappy amp and convert it to a normal channel of a BF Deluxe by gutting it and rebuilding from scratch while also replacing the OT and 2 of 3 tubes. Customer pays about $300 and gets THAT poor-man's BF Deluxe sound. $300 is yet possible in Russia. Of course, this is going to be a local thing, at least until I decide to start something on my own. That's the plan right now. If you don't feel like helping someone who could strat a business in future, you know you shouldn't post. At least I've honestly stated my intentions. EDIT: and I expect no more than 10 orders by my calculations, the reality is tough in Russia...
     
    Last edited: Dec 7, 2016
  2. neteraser

    neteraser Member

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    Right now with the current hook up of the universal OT it sounds more like a Reissue. I.e. like it was a 8k OT in a real DR. I believe a more authentic tone could be found by further adjustments. Hope you understand.
     
  3. jviss

    jviss Member

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    Just a nit, but transformers really don't have an impedance, they have a turns ratio, and the secondary load impedance is reflected into the primary according to the square of the turns ratio. The primary impedance you see depends on the load, i.e, the impedance of the speaker. Impedance of speakers varies over frequency, but the nominal value given for a speaker is O.K. to use. My guess is that the difference between 6.6k and 6k is negligible; it's a turns ratio difference of 29:1 v. 27:1 for an 8Ω load. For a 4Ω load it's only 41:1 v. 39:1.
     
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  4. neteraser

    neteraser Member

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    I'm trying to analyze why a certain value was picked. I can understand the reason one would pick 8k and/or 6k, but everything is not so clear with 6.6k... It just looks like a step-away, so I wonder was this step-away a part of the original way of thinking or not. I'm sure it'll make a little difference. Of course, everything between 6-8.5k would work just fine, but we're talking "what's going to be perfect/more authentic".
    On the other hand, maybe ya right. For the first time in my life I feel like I can ass on something and just use what works. I can. But what if there was another option... ;)
     
  5. neteraser

    neteraser Member

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    That's fine :)

    Yeah, I understand. But what about different losses for example? Unfortunately, I'm not a specialist on transformers... I can't do the winding/balance. So can't fully participate in that discussion.
     
  6. RiftAmps

    RiftAmps Member

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    I have a poor man's theory that the DR was originally designed for 6L6s, the higher B+ and 6.6k OT certainly point to that and are consistent with his other 2x6L6 designs from the Tweed & Brownface era. What I'm not sure of however, is why he switched to 6V6s.

    One day I'll investigate further, but the soldering iron waits for no man.
     
  7. Roe

    Roe Member

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    try 6k6 or higher if you want to avoid redplating
     
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  8. jviss

    jviss Member

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    That's a good question. Loss would be more a matter of the quality of the transformer, I think. I wouldn't worry about it. As long as you have the correct power handling capacity and correct turns ratio you should be fine.

    Update: following up on this, the turns ratio thing is really not complicated. Here's an example.

    The transformer has a turns ratio of 25:1, meaning 25 primary turns per one secondary turns.
    A 4Ω speaker is connected to the secondary.
    The secondary is the anode load of the output stage of a single-ended amp.
    The load impedance is according to the square of the turns ratio, so in this case it is 4Ω x 25^2 = 4 x 625 = 2500Ω.
    Assume an output voltage (primary) of 100V. This voltage will appear at the secondary according to the turns ratio, so 100V ÷ 25 = 4V.
    The load current is I = 100V ÷ 2500Ω = 0.040A = 40mA.
    The current is reflected to the secondary as the inverse of the turns ratio, so Is = Ip X 25 = .040 x 25 = 1A.

    Note that the power is the same on both sides! Pp = 100V x .040A = 40W = 4V x 1A = Ps.
     
    Last edited: Dec 8, 2016
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  9. Advisable Owl

    Advisable Owl Member

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    Don't even worry with the brass faceplate. I've never heard anyone say anything good about it.
     
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  10. neteraser

    neteraser Member

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    Thanks for the detailed info! Appreciated! I like when the theory is put into simple terms which everyone including even me can understand. Thank you. :)

    Multiplied by the efficiency, right? Or in other words, what I know is everything isn't too easy - Why? - Because you have to optimize the transformer for minimum loss achieving the right musical balance for a particular amp. That's not as obvious as just the turns ratio. I don't know how they do it, no idea at all right now. And I've never seen any core size/thickness calculation methodology. Another fact further increases my confusion, iron has different magnetic properties, so you have to know what's you're winding on or go home. I can imagine myself taking _inhuman_ efforts to experimentally pick the right values for a semi-professional power transformer. But an OT? _QUIT_. I'll leave that to pros. Some guys just can figure out everything.

    So your point is the original already has 6.6k? Because we don't see too many people complaining about redplating...

    I don't think so, ma friend... Didn't Leo use different power supply for 6L6 amps? If you try to convert a DR to those specs, you'll probably see it's not going to work that way. 100% it won't work that way.

    Yep. :)

    I'm afraid this won't please Fender fans... :)
     
  11. RiftAmps

    RiftAmps Member

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    A DR will quite happily run a pair of 6L6s with only a rebias, I've done this many times and so have many other builders. The power supply is more than capable of supplying the required current for 6L6s at a B+ of 410v or so and the amp will put out 25w. A few quick calculations will confirm this, ma friend ;)
     
  12. neteraser

    neteraser Member

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    Yes, but my point was the contrary: that you can't built a 6L6 circuit with a pair of 6V6's. Meaning those design changes were probably made specially for this tube.
     
  13. makerdp

    makerdp Member

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    You will not hear a difference between a 6k6 and 6k output transformer. If you do, it's probably more to do with quality of the transformers you are comparing and nothing to do with the spec.

    The brass grounding plate is a really really bad grounding scheme. There is a reason nobody does that anymore. While there is more than one viable grounding scheme, the one I prefer and have never ever had any issues with is this one: http://www.valvewizard.co.uk/Grounding.html

    As far as putting a 6L6 in a 6V6 amp... you had better be darn sure your power transformer can handle it and you better be clear on which variant of 6L6 you are using as not all of them have the same current draw specs. It is not as simple as a rebias and go. You have to make sure your transformer is up to the task or you will eventually burn it out early.

    Rule of thumb is:
    6L6 amp to 6V6... YES it will work.
    6V6 to 6L6... it depends on the power transformer spec. If in doubt, don't do it!
     
  14. jviss

    jviss Member

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    I should have said my post assumed an ideal transformer, i.e., a perfect on that doesn't exist in reality. Otherwise you have a quite complex analysis. Transformers, magnetics, can be quite complex. Just buy the correct turns transformer with the best reputation, I guess.
     
  15. neteraser

    neteraser Member

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    Are there any real arguments on that being a bad scheme? Maybe it's a part of the tone? Like running a wire on the pots in a Marshall. That's also considered "bad" on TGP, however I know it's not true.

    Anyway, I just wanna know how it connects to the chassis. If anyone is willing to help, I'd be very grateful.

    I understood.

    Yes, that's what I wanted to say.

    Yes, sir, that's what I'm gonna do. Or will just use a Hammond I already have, which works well, so I can call this Reissue-type sound a satisfying result for the first time. I'll think about it. I'm fighting between people in Russia wanting cheap (b/c the economy sucks) and yet doing best I can if it's still not too expensive. You really have to impress someone for it to work, that's what I still believe. My prototype seems OK, it's only 2 watt and 8" speaker, but has lots of bass and quite captivative tone, just like any more mature Fender build would.

    I just hoped someone would be interested to help. You guys in the US can pick up a DRRI for $650 on CL. Here it costs about $2200 new and about $1200-2000 used, but they pop up really really rarely. So my guess is that nobody here really knows what Fender is. So I strive for the most correct interpretation possible for the money. OK, you get the idea...
     
  16. Kurzman

    Kurzman Supporting Member

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    I'm not a tech.

    The brass grounding plate is connected to the chassis with star washers and the nuts holding the pots on. It's "L" shaped. In old ('60's - '70's) Fenders sometimes you can get some type of growth or corrosion in between the plate and the chassis causing all kinds of mysterious ground related problems.
    You remove all of the pots and pull them away, pull away the brass plate and clean up the inside face of the chassis as well as the outside face of the brass plate and then reinstall making sure the washers have been turned (or new) to get a good ground connection to the chassis. It's a hassle.

    I can't imagine this being a problem already on a re-issue.
     
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  17. tele_player

    tele_player Member

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    The reissues don't use the brass plate.
     
  18. Roe

    Roe Member

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    My point is that low primary impendance (below 6.6k) and high voltages (400v or higher at the anode) will easily result in redplating. DRs often run the 6v6s very hard, bordering on redplating sometimes (depending on wall voltage, bias, tubes etc)
     
  19. jviss

    jviss Member

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    I don't get that at all. Redplating is overheating. This is caused, 99% of the time by a bias issue: incorrectly adjusted bias, or a bad grid resistor, or something like that. If an amp is designed for 6kΩ, it should probably work fine that way, assuming all other things working properly.
     
  20. Roe

    Roe Member

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    it is not designed for 6k at 415v with 6v6s. look at the load lines
     
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