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#1 2009-06-29 05:19:10

cn9c
New member
Registered: 2009-06-29
Posts: 2

6550 "mod"

Hello.  I'm interested in switching to 6550's in my v-4 (~74).  My particular amp was moded to include a potentiometer for biasing.  I have looked at a number of different sources, eg.
   http://www.triodeel.com/7027.htm
   http://www.talkbass.com/forum/showthread.php?t=73472
   http://www.ampegv4.com/7027.php (your site),
to get a clear idea of what I need to do in order to use these tubes.  Unfortunately, there seem to be some conflicting opinions.  Some people (credible and not) suggest that you have to do little to nothing, may be remove the tube cages.  Others suggest replacing some resistors.  On the 7027 section of this site "The Axeist" suggests the most extensive alteration I have seen.  He says that "the screen resistors [should be] changed from 470 ohms. 1 watt to 1K 5 watts," and that "R50 has to be changed to 82K."
    I would like to do as little as possible while ensuring that the amp will operate safely.  In particular I would rather not change the resistor R50.  Apparently you can change the screen resistors without having an adverse affect even if you want to use 6L6 type tubes, so I don't mind that so much.
   In a previous thread the moderator told the author that the amp does indeed need to be moded in order to use 6550's.  Can anyone decompress this for me and, if your feeling generous, give me a suggestion of which modifications should be made to operate safely.

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#2 2009-06-29 20:36:37

cn9c
New member
Registered: 2009-06-29
Posts: 2

Re: 6550 "mod"

From the ampeg heads section:
"The screen resistors are bumped up to 1k from 470ohm  these you can leave alone,  the amp will probably be happier with the 1k screen resistors.
the bias circuit is also modified to provide correct bias voltage for the 6550s.  usually its R49 but I've seen people do R50 too."
That's good enough for me.  If anyone has anything to add let me know.  Sorry for asking a question that was answered elsewhere.

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#3 2009-06-30 11:04:23

rob
Member
Registered: 2008-10-28
Posts: 171

Re: 6550 "mod"

Hi cn9, Probably the biggest deal with this swap is the tube retainers. I am not sure what is in the 74 model, but they  are in the earlier amps. The "bear trap" style retainers have to have thier rivets carefully drilled out, and replaced with the marshall style spring type. The 6550 base is too large to fit the stock retainers. The bias would need to be a bit less negative for the 6550. I would check the bias for sure after this type of change. The 6550 is an excellant choice for these amps for bass, and will allow the most flexability in setting the bias because none of its ratings are exceeded.

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#4 2009-07-01 11:56:11

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 6550 "mod"

The tube retainers must be removed but I suggest you change the screen resistors too.  R49 and r50 are both in the bias circuit as I recall so you need not worry since you've got the adjustable bias. 
Truth be told I change the screen resistors even if I keep the 7027s  the amp seems happier with the 1k screen resistors.

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#5 2011-11-23 11:09:57

beedoola
Member
Registered: 2009-05-25
Posts: 58

Re: 6550 "mod"

was doing some research and figured I'd bump this instead of starting another. R53 was lifted at the spot where it meets the screen resistors in my v4. I recalled in the past that his had lifted and there is a burn mark at the pcb point where they all meet. My v4 uses 6550s - it was modded that way when I got it. 41, 42, 47 and 48 had been changed to 1.2k - the schematic was updated too on the chassis cover. I replaced those with new 1.2ks but after seeing this - should I use 1k. Does it matter that much if I don't? Could that have been what cause the heating?? causing 53 to lift?

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#6 2011-11-23 15:02:01

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 6550 "mod"

1.2k is fine.  it really just needs to be higher than 470ohm.   I think 2K is a bit too high,  but 1.2 is just fine.  was it R55 or R53 that lifted?

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#7 2011-11-23 21:46:16

beedoola
Member
Registered: 2009-05-25
Posts: 58

Re: 6550 "mod"

Cool. I should correct - it was R48 that was lifted. That hole junction where the 41,42,47,48, 55 and B meet was burned and the trace had be lifted previously - completely gone connecting 48 with the rest, so the leg of R48 is a little long to meet the rest of the trace and the group of resistors.

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