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Blueline SVT Power Transformer Need Advice/Trade

Discussion in 'Amps and Cabs [BG]' started by Wasnex, Mar 10, 2013.


  1. Wasnex

    Wasnex

    Dec 25, 2011
    What voltage do unmodified SVTs running 6146Bs typically have on their screens.

    I bought a blueline with 6146B tubes awhile back. I was specifically looking for an amp with 6146s.

    The good. The amp sounds awesome! Much better than my MTI era SVT.

    The bad. The amp has a replacement power transformer (PT) and the screen supply is tapped for 6550s. Voltage is 350 volts at Test Point E rather than 220 volts. The screens have 250 volts on them which is the absolute "do not exceed spec" for 6146B.

    I am sure I am not the only blueline owner with an aftermarket PT. Is there an accepted technical solution, such as inserting a resistive L section to drop the voltage to the screens...If yes, would a bypass capacitor be recommended.

    Anyone have luck getting a transformer with the correct screen voltage? Both Heyboer and Mercury Magnetics vintage SVT power transfomers appear to be tapped for 350 volt screen supplies.

    If you have converted your blueline to 6550s, let me know if you would be interested in trading out tranformers so you can set your amp up to run the correct screen voltage rather than tapping into the plate supply.

    v/r


    Monty
     
  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    There was a recent posting (http://www.talkbass.com/forum/f15/a...-68-early-70s-688690/index8.html#post13974521) by a 6146 SVT owner saying that he addressed the issue in his amp by adding a filter section. He should be able to help you if you send a personal message.

    The RCA 6146A and B data sheets have some interesting comments on the plate and screen voltages: 1 2. On the plus side, what they don't tell you in the data sheet is that their maximum limits can be exceeded. They are specified for maximum service life of the tube. In this case 100V higher is too much.
     
  3. Wasnex

    Wasnex

    Dec 25, 2011
    Thanks for the response.

    Looks like dmartinsmith is running his 6146 screens at 260 volts. Mine is sitting right at 250 volts so maybe I am okay. I suspect the tech who installed the transformer added a compensation circuit to knock the voltage down a bit. Looks like an extra 15K resistor and 220uF capacitor.

    Many musical instrument amplifiers exceed the tube design parameters. However, there seems to be a general concern on ham radio and audio related forums that suggests the screens are unusually fragile on 6146s compared to other tube types, and a general suggestion to keep them down around 200 volts as failure rates rapidly increase as the screens approach 250. This concern is expressed in the context of running the tubes within rated power specifications rather than conditions of overload such as is often the case with musical instrument amps.

    Although this opinion is prevalent, I have no way of knowing if it is ignorant paranoi are factual. It does appear that Ampeg originally designed their circuit to run the screens at around 200 volts and if the stories about the Stone's tour are true, it sounds like the original designer's experience was the screens will fry if the amps are pushed too hard (to the point where the screen voltage exceeds the plate voltage). I am sure I would never push this amp into continued output saturation, but clipping transients is entirely likely, so adding a bit of safety margin would be preferred.

    I'll contact dmartinsmith to see what approach he took as it sounds like his mod was extremely successfull with 400 watts output. Of course only time will tell how reliable his amp operates at that extreme.

    v/r

    Monty
     
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I think that you are fine with 250V. Radio engineers take these specs as challenges to meet so they strive to adhere to them.

    When you power up this amp, there are surges in the power supply voltages while the capacitors are charging that exceed operating conditions. In the 6146A RCA data sheet they mention not letting the screen voltage exceed 400V in key down conditions. Clearly this is momentary but indicates that the tube can take more than their specified maximums.

    Please post whatever you learn from dmartinsmith.
     
  5. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    IS that 250v measured unloaded? Probably drop a bit when its got valves in if so.
     
  6. Wasnex

    Wasnex

    Dec 25, 2011
    250 volts with 5 of 6 tubes functioning properly.

    Since my initial post, I took a few readings and it looks like one of the screen resistors is open. Probably explains the extra 60 hz hum as this would knock the output section out of balance a bit.

    v/r

    Monty
     
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    In an amp of that age, I usually change the plate, screen, and cathode resistors with matched sets, especially the 1 ohm cathode ones, as a matter of course.


    David
     
  8. Wasnex

    Wasnex

    Dec 25, 2011
    David,

    Although I am competent to do the repairs msyelf, I think I will leave it to my tech as he has a voltage limiting variac and such so the components can be replaced with less risk.

    My main goal is/was to try and determine whether I should put in a properly rated power transformer, and if yes get it on order. I am still a bit concerned about running the screens at 250V but it sounds like you would recommend against replacing the transformer. Hopefully dmartinsmith will send me a email with his solution and experience to further ease my concerns. My tech is about to close up shop and move across town so I have awhile to research and gather opinions.

    To your suggestion on the resistors...I agree. Here are the conditions in my amp:
    Screen, control and plate resistors are all relatively new. The cathode sense resistors, surprisingly, are not. Plate resistors are 15 ohms instead of 10 per the schematic. Everything else looks original, or at least very old and is covered with a not-so-thin layer of sludge.

    The open screen resistor is a different brand than the others, so this is probably not a new problem for the amp. I would guess that tube is either shot or possibly having some hf oscillation problems. Might be necessary to increase the value of screen resistors a bit to quell that. Also, when I received the amp, I checked the bias. The amp was set within 2mV of spec (meaning two tubes were pulling the current specified for all three). Given the two tubes pulling 75 milivolts across the 1 ohm sensing resistor, do you think it would be best to replace all three tubes on the side with the fault?

    v/r

    Monty
     
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Monty, it's possible that dmartinsmith didn't notice the private message. He posted some details yesterday of the inductor that he used in the thread that I referenced above.

    In terms of changing your transformer, that's a tough call. On one hand finding a 1969 or 70 transformer is going to be very difficult. Old transformers can have issues such as excessively rusted plates or cracks in the varnish on the wires used to make the coils. A new replacement doesn't have all the potential problems that an old one has. Often they are made to higher spec. No reason why you can't lower the voltage on the one that you have, as has been done, just a little more if you are uncomfortable with 250V. Although you are at the published limit for the screen voltage, you are still within the limit. If you are blowing tubes I would be concerned.

    See if your wall voltage higher than 120 VAC. Some areas have a higher voltage which causes the voltages inside the amp to be higher.
     
  10. Wasnex

    Wasnex

    Dec 25, 2011
    David,

    The wall voltage is 120 volts.

    Having all six tubes in the circuit may pull the voltage down another 10-20 volts.

    Realistically I think using the existing transformer is probably the best coarse of action. Amps often tap multiple voltages off the same winding. No reason to think that is not going to work here...just that I am not an electrical engineer. Easy enough to roll in a couple of resistors until I get the desired voltage but no idea how this effects the intended function of the circuit or if it will have any impact at all.

    Scaling the voltage with a series resistor in no way guarantees the screens will get an appropriate amount of current. But then again with a 15K resistor in series they are probably not going to get too much. Might be why the amp sounds so great a lower volumes but farts out a bit when pushed...more likely this is occuring because one tube is not working.

    The ideal trade would be someone else with a 6146 based amp that wants to convert or has converted to 6550s (but as you have said there is a certain degree of risk with that, and I am not holding my breath). Other choice is to call Heyboer and Mercury to see what they can do. The transformer they list on their websites appear to be set up for 6550s so it would probably have to be a custom job. Sounds expensive.

    I sent dmartinsmith an email rather than a PM. Maybe his account is not set up to send to an account he uses. I'll check out the thread again as I am curious what he is doing.

    Got some bad news today...my tech emailed me and said he moved to Nashvilled over the weekend. Wow! He just finished repair on one of my amps last Tue.

    v/r

    Monty
     

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