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    5u4gb for MKIII



    Posts : 20
    Join date : 2011-05-18

    5u4gb for MKIII

    Post by snav on Wed May 18, 2011 2:50 am

    I have a pair of rescued MKIIIs that I'm slowly rebuilding. One worked from the beginning and I've been listening to it since march in between recapping, bias kit etc. However one thing that has bugged me since the first time starting it,after reforming the multisection cap, is the voltage it runs at. The first section before the choke runs at a nominal 503v. I pondered how to get a suitable reduction, including using a buck transformer in the primary. Last week I was reading the labels of tubes from boxes my granddad left me and found some 5U4GB NOS. Once I researched these, I thought I had found the solution as the PT is rated 3a and it was mentioned that the mark II used that as the rectifier and had the same PT. I put some in(tried different ones measuring the terminal voltage)and found that I had reduced the voltage to 478 which is golden as regards the factory drawing. Of course my joy was shortlived as I read today that the effect of using that tube, besides the greater loss, was that the main cap will get jolted on startup due to no load when voltage is applied. So I get out my trusty recording voltmeter and sure enough, the 5ar4(Mullard) peaks at 512v before falling back to nominal 503 but the 5u4gb peaks at 562 before falling back to 478. I also measured the power drawn by the amp with each tube and after rebiasing the difference in the power was less than 0.5%.

    So, I need to know how to either reduce the operating voltage of the 5ar4 or reduce the startup surge for the 5u4gb.

    Anyone have a suggestion? I'm thinking Amperite delay timer if I can find one. When I was younger I remember some high voltage rectifier stacks for TVs that where 4 inches long and maybe 3/16 diameter and were filled with solid state diodes, probably had a 59v drop.

    Seems a little like a house of cards. I was reading another bit of info that says high voltage problems could be solved by reducing the input cap to .47-5uf range on the 5AR4. And after puzzling over what looks like a voltage doubler for the filaments of 12ax7s, on the boards in a pas2, that are wired in series it just makes me want to put my head down.javascript:emoticonp('scratch') On the bright side, opinions about reforming that multisection cap and the dire predictions of an imminent gooey explosion have had new light shed on them. I probably tried 6-7 different 5U4GB tubes looking for the highest voltage.
    Bob Latino

    Posts : 2634
    Join date : 2008-11-26
    Location : Massachusetts

    Re: 5u4gb for MKIII

    Post by Bob Latino on Wed May 18, 2011 6:15 am


    Your analysis of the situation right on. What I would do is not the cheapest solution, but from the standpoint of long term durability of your two amps, is probably the best.

    Replace the quad caps on each amp with the German made 550 volt (continuous) rated "Authenticap" 80, 40, 30, 20 cap. This cap has a turn on surge rating of 600 volts and is the same cap that I use on the VTA ST-120 and M-125 amp kits. I have used well over 100 of these caps in the past 3 years using both GZ34 and solid state rectifiers. Not ONE of the Authenticaps has ever failed. These caps are not cheap ($60 retail). They are available from Dynakitparts at the link below.

    550 volt rated quad cap at Dynakitparts



    Posts : 790
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: 5u4gb for MKIII

    Post by GP49 on Wed May 18, 2011 2:20 pm

    You might also consider a 5V4.

    5U4: 3 amp heater, Irect 275ma
    5V4: 2 amp heater, Irect 175ma
    5AR4: 2 amp heater, Irect 160ma

    (Source; Duncan Amps TDSL)

    The 5V4's lower Irect than the 5U4 could well be sufficient in a monoblock, and it has the further benefit of soft start. Its series voltage drop is slightly higher than that of the 5AR4. They are also widely available.


    New Here

    Post by we on Wed May 18, 2011 7:57 pm

    I to have a pr of MK3s,Stock with the 6AN8 but with all new caps,I to have look,et for ways to get the 500v down.MOD 1 what i have done is use 3 1mf 630v caps at the b+ comeing off the GZ34 where the stock 30mf would be take it off,this drops the B+ to 470-480 then you can bias up to a more class A Zone, an it well drop to 450-460B+ on the output tubes,KT88s.I have to say some of the best sound i have had with Dyanco Amps.
    Mod 2 if you use the 6AN8 put a good old AB carbon 250ohm 1/2w grid stoper from the input to pin 8,this makes the old 6AN8 sound like a good 12Au7. keeps the RF out.
    I am Driveing a pr of MartinLogan SL3,i have a MOD for Highoutput panels,sweet sound,Big bass,

    An i would like to thank Bob Latino For all the great info on the site


    Posts : 148
    Join date : 2010-08-30

    Re: 5u4gb for MKIII

    Post by j4570 on Wed May 18, 2011 8:14 pm


    Buying a new cap will be "cheap" compared to toasting a vintage Mullard 5AR4 and whatever else when it fails..........


    I have a friend that swears by MartinLogans, I guess you would concur. He says you can't go back. Of course, he used to sell them.




    Post by we on Thu May 19, 2011 6:54 am

    As for the ESLs I have Acoustats Xs with the Acoustat tube amps, 2+2s,1+1,4s, An have had SoundLab,but all hard to drive.I Like Big Sound i have a room that is 18'w by 40'L by 14'H all open,Stock Logans are fair thay well draw you in,but there a panel co. Make big bucks off saleing panels,Well the panels would last for ever an sound the way thay should if thay were in to that but we all need $$,ML only feed the HIgh Bias to the panel on one side,the Right side this means that the Right speaker well all ways putout less than the left,fix thay could have put a Bias feed,er ring all the way around the panel cost would have been 50cent,The BIG deal is the panels would have had 3-5db more output.

    Fix.Up grade partes in crossover,put the Ascent i crossover in the SL3s. Pull the panels apart put a Bias feed ring all the way around now you have a Great speaker an you can drive them with 60w tube amp.
    I have had some Big pushpull tube amps, OTLs, But two output tubes
    have allways sounded RIGHT to me,More in my case is not better

    Back to the B+ Drop mod the 3mf cap is at pin 8,


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    Re: 5u4gb for MKIII

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