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    6CA7EH or 6P3C-E in Mark III?

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    kygeezer

    Posts: 51
    Join date: 2012-11-30

    6CA7EH or 6P3C-E in Mark III?

    Post by kygeezer on Tue Jun 18, 2013 9:06 am

    Just curious if anyone has tried either of these tubes in a Mark III, perhaps with a 5R4 to drop the B+ voltage? I guess the reason is the same as why climb a mountain - because it's there!

    kygeezer

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    Join date: 2012-11-30

    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by kygeezer on Tue Jun 18, 2013 9:10 am

    That should be 6P3S-E

    hawaii.ken

    Posts: 157
    Join date: 2012-01-31

    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by hawaii.ken on Tue Jun 18, 2013 3:12 pm

    6CA7EH should work with either a 5R4 or a 5AR4 if the bias circuit has enough range to set the bias to 40-45ma per tube.

    The 6P3S-E max anode voltage is only 250V so should not be used.

    Bob Latino
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    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by Bob Latino on Tue Jun 18, 2013 4:27 pm


    This is just my opinion but .... If you use any modern made 6CA7's in a Mark III, they will probably have a short life. The B+ with a GZ34 tube rectifier in a Mark III is about 475 to 500 VDC on today's 120 volt mains. Using a 5R4 or a 5U4 rectifier will drop the B+ to *maybe* a range that a 6CA7 can deal with ?
     
    As to the bias "range" that hawaii.ken mentioned, he is correct that a Mark III that is normally set up to bias KT88 or 6550 tubes may not have enough range to bias a 6CA7. You can, however, alter the bias range on a Mark III to more favorable bias range for a 6CA7 by pulling the 1000 ohm main bias resistor and replacing it with a larger value like a 5K or 7.5K.
     
    Bob

    Jim McShane

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    Location: South Suburban Chicago

    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by Jim McShane on Tue Jun 18, 2013 9:47 pm

    "The 6P3S-E max anode voltage is only 250V so should not be used."

    I wouldn't pay much attention to that spec, it's doubtful that it's a maximum spec anyway. You can use the 6P3S-E anywhere you'd use a 6L6GC. I burn in the 6P3S-Es I sell at 465 volts on the plate and screen. I've sent hundreds to people who use them far above 250 volts. There have been zero failures.

    The 250 volt number is found on a Cyrillic language data sheet, and I suspect it's a translation error or such.

    hawaii.ken

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    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by hawaii.ken on Tue Jun 18, 2013 10:18 pm

    Thanks for the info, Jim.

    BNR_1

    Posts: 61
    Join date: 2013-06-11

    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by BNR_1 on Wed Jun 19, 2013 8:42 am

    Bob Latino wrote:
    This is just my opinion but .... If you use any modern made 6CA7's in a Mark III, they will probably have a short life. The B+ with a GZ34 tube rectifier in a Mark III is about 475 to 500 VDC on today's 120 volt mains. Using a 5R4 or a 5U4 rectifier will drop the B+ to *maybe* a range that a 6CA7 can deal with ?
     
    As to the bias "range" that hawaii.ken mentioned, he is correct that a Mark III that is normally set up to bias KT88 or 6550 tubes may not have enough range to bias a 6CA7. You can, however, alter the bias range on a Mark III to more favorable bias range for a 6CA7 by pulling the 1000 ohm main bias resistor and replacing it with a larger value like a 5K or 7.5K.
     
    Bob


    Hi Bob:

    Can the stock (factory issued) PT handle the additional current draw from a 5U4 rectifier considering the higher voltages at today's AC outlets?  Isn't the 5U4 rated at 3 amps while the 5AR4 and 5R4 rated at 2 amps.

    Thanks you

    Bob Latino
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    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by Bob Latino on Wed Jun 19, 2013 11:14 am


    I have used 5U4's in stock ST-70's with no issues. The B+ is lowered somewhat and you might lose a watt or two
     but they appear to work just fine in an original ST-70 even at today's somewhat higher line voltages ..
     
    Bob

    GP49

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    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by GP49 on Wed Jun 19, 2013 11:23 am

    The 5U4 was the stock rectifier in the Mark II, with the same power transformer and EL34 tubes.  The added filament draw is a non-issue.

    BNR_1

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    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by BNR_1 on Wed Jun 19, 2013 4:13 pm

    GP49 wrote:The 5U4 was the stock rectifier in the Mark II, with the same power transformer and EL34 tubes.  The added filament draw is a non-issue.

    Hi GPA:

    Far from an expert on the subject matter but doesn't the Mark II have 3 less tubes than the ST70?  Would that factor into the equation at least from the filament draw?

    Thank you


    Last edited by BNR_1 on Wed Jun 19, 2013 4:15 pm; edited 1 time in total (Reason for editing : My comment appeared to be written by GPA49 and not me.)

    GP49

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    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by GP49 on Wed Jun 19, 2013 6:49 pm

    BNR1 wrote:

    Far from an expert on the subject matter but doesn't the Mark II have 3 less tubes than the ST70?  Would that factor into the equation at least from the filament draw?



    =======================================


    Yes, but the thread is entitled, "6CA7EH or 6P3C-E in Mark III?"  So my reply was regarding the Mark III.  However the Dynaco manual for the Stereo 70 does say "The 5U4 or 5U4B can be used in place of the GZ34."  This is mentioned as an emergency substitution which implies that the amplifier will not perform to its full specifications with the substitution.  But you won't do any damage.

    BNR_1

    Posts: 61
    Join date: 2013-06-11

    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by BNR_1 on Wed Jun 19, 2013 7:13 pm

    Thanks for clarifying GP

    I missed the part of the Mark III in the original posting.  My comments to Bob and you were meant for the ST70 amp.  My bad.  Sorry for any confusion.

    tubes4hifi
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    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by tubes4hifi on Thu Jun 20, 2013 12:29 pm

    6CA7 - in a MK3 or in an ST70 (or any other amp that could use an EL34) definitely yes, they sound great!   Not as much power as a KT88 though.
    They will bias like an EL34, so keep the bias around 40 - 45 ma per tube.

    GP49

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    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by GP49 on Thu Jun 20, 2013 1:53 pm

    New old stock 6CA7s are virtually certain to be closer to the original EL34s, than what is manufactured today as "EL34."

    Jim McShane

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    Re: 6CA7EH or 6P3C-E in Mark III?

    Post by Jim McShane on Thu Jun 20, 2013 3:52 pm

    New old stock 6CA7s are virtually certain to be closer to the original EL34s, than what is manufactured today as "EL34."
    With all due respect, how did you reach that conclusion?

    As a matter of fact, most EL34 old stock tubes were true pentodes, and 6CA7s were beam tetrodes. So no, NOS 6CA7s are not closer to today's EL34s. While the 6CA7 and EL34 could interchange, they certainly don't sound the same, at least in part due to the internal construction differences. 

    So what do you think is manufactured today that is so different than the old stock? There is an excellent 6CA7 available in the Electro-Harmonix line (and old Ei 6CA7s were terrific too, but they're gone); and there are a number of very good EL34 tubes out there as well - as well as the outstanding KT-77 Genalex

    I'm interested to find out what you meant by your statement, and what you based it on.

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