Mullard 6v6's in my Marshall 1987

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Jimi Ray, Jan 15, 2004.


  1. Jimi Ray

    Jimi Ray Member

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    My amp came with 6550's. I had rebiased with el-34's and they sounded pretty good. However last night I poped in two mullard big bottle nos 6v6's (not 6v6gc) and holy moley I didn't stop playin for quite a while. They sound fantastic in there. Its like a totally new amp. Any of you guy's ever tried running 6v6's in an older non-master four input marshall??? Much to my suprise the cleans and the crunch tones are excellent. Let me know what you think.
     
  2. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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    My understanding is to never put 6V6s into an amp that wants to see EL34s or 6L6s. Seriously - be careful.
     
  3. Swarty

    Swarty Member

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    6V6s require about exactly the same bias voltage as EL34s, so it is possible to switch. Many '70s 1987s run under 400v, so they actually run cooler than in a Deluxe Reverb. I've been swapping them for years (after looking at ads for the Trainwreck Express, where this is a feature). You will get a better impedence match if you set the switch to half the rating of the cab... IE, if the cab is 16ohm, set the impedence selector to 8ohm.

    BTW, I believe a 6V6 would be a metal tube?
     
  4. Jimi Ray

    Jimi Ray Member

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    Your right the plain 6v6 is metal, I have somme. This is the 6v6g thanks for correcting me. Serves me right for doing that by memory.
     
  5. Swarty

    Swarty Member

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    Also, I'm curious, are the Mullards all black w/ a badge logo? I have seen these that were in reallity the cheapo Russian variety (counterfeit?). If they are these, I would not trust them.
     
  6. Jimi Ray

    Jimi Ray Member

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    By the way, I will change the impedance to half as you say. Is this because of current draw through the trannys? I was only able to adjust the bias to 25ma due to limitation on the trimmer. I have also put 6l6gc's in my Shiva that came with el-34 tubes. This was suggested on Euro tubes website under the tube substitution area. Very nice results as well for me.Thanks again for any info you can contribute.
     
  7. Swarty

    Swarty Member

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    The output transformer's function is to couple the high impedence tubes w/ the low impedence speakers. EL34s and 6V6s have dffererent plate resistance (one of the bzillion tube parameters). Switching down a notch will optimize throughput/power.

    As for the current draw at 25mA, if the plates are not glowing red you should be OK (look at them w/ the lights dimmed. What is the plate voltage?
     
  8. Jimi Ray

    Jimi Ray Member

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    I paid dearly for those tubes, I bought them from Lorenze Tang from Wi Wi tubes. I have bought lots of tubes from him, I doubt these are anything but the real deal. His reputation is top notch. I'm at work right now so I can't give you a real accurate discription on those yet. They came in the blue boxes with the mullard name all over them, and they have the mullard name and I'm pretty sure they have shield logos. I will check my voltage@plates too when I can. I think they are running approx. 375 volts. By the way I stuck them in my Shiva for about 5-10 mins. but one of the tubes started to look redish (lights on) so I had a moment of common sense and pulled them. I'll get back to you once I get home and confirm.
     
  9. teleharmonium

    teleharmonium Member

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    Swarty wrote: "Also, I'm curious, are the Mullards all black w/ a badge logo? I have seen these that were in reallity the cheapo Russian variety (counterfeit?). If they are these, I would not trust them."

    I've seen plenty of Russian made Mullard branded 6V6GT - these may well be legitimately branded by Mullard - but Jimi Ray is talking about 6V6G. I have never heard of a Russian or Chinese 6V6g, so even if they are not made in England as they are marked, what's the worst they could be ? There were definitely 6V6G tubes produced in England in the past, I have some ex military ones made by Genelec, and the French made some great ones too.

    I've heard about the impedance mismatch trick and indeed ran one of my amps that way for a while, but after learning more about impedance recently, I'm confused as to which direction is the right way to mismatch. Somebody correct me if I'm wrong, but a higher "impedance" actually means the load is easier to drive, contrary to what you would think based on the non technical meaning of the word impedance. So, when you set your output transformer for 8 and run it with a 16 ohm load, you're actually making it easier for the tubes to drive the load. I thought the point of the mismatch was to make it harder for the tubes to drive the load, so that they would use up more of the otherwise excessive plate voltage going through them in an EL34 or 6L6 type amp.
     
  10. John Phillips

    John Phillips Member

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    The speaker load has no effect on the plate voltage (certainly not at idle or low power). You need to make sure that the plate voltage is not above what the tubes will take, period. Sometimes this is higher (occasionally a lot higher) than the tube manufacturer intended, but unless you know what you're doing, don't try it. You could potentially wreck not just the tubes, but if you're unlucky other parts inside the amp as well that will mean a trip to the tech.

    The purpose is setting the output impedance differently from the speaker is to correctly match the impedance of the tubes to that of the speaker, as Swarty said, because 6V6s have a different impedance to EL34s (or 6L6s), and the transformer ratios will be designed for those tubes. The tubes are least stressed overall when the match is correct.
     
  11. teleharmonium

    teleharmonium Member

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    John Phillips wrote: "The purpose is setting the output impedance differently from the speaker is to correctly match the impedance of the tubes to that of the speaker, as Swarty said, because 6V6s have a different impedance to EL34s (or 6L6s), and the transformer ratios will be designed for those tubes. The tubes are least stressed overall when the match is correct."

    ok , so assuming you have 6V6 that can handle the voltage - I have some GEC/Marconi that I'm sure can take a lot - which direction of mismatch is correct ? Would it be setting the OT to 8 and connecting a 16 ohm speaker load ? I hope so, as that is what I was doing.
     
  12. DreamTone7

    DreamTone7 Guest

    teleharmonium, what you are doing is correct to get closer to a correct match for 6V6s in an amp w/EL34 designed OT.
     
  13. teleharmonium

    teleharmonium Member

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    thanks DreamTone7 and John
     
  14. Swarty

    Swarty Member

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    ...and I'll maintain that a 6V6 will be happier in a 370V Marshall than a 420-440V Deluxe Reverb. I do agree that the EH tubes are not up to task (either actually).
     
  15. DreamTone7

    DreamTone7 Guest

    "Doug says dont do it..."

    Making the change I and others spoke to teleharmonium about will help to relieve the problem Doug mentions.

    I do agree that NOS is the way to go if you are going to do this. New production tubes are not up to par, yet.
     

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