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#1 2012-01-23 16:36:19

battlestarcackalactica
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From: Carrboro, NC
Registered: 2012-01-22
Posts: 3
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Pulling Power tubes and running the amp as 50 Watt

I have been running my 69 v-4 as 50 watts pulling the outside power tubes so that the tubes are pushed harder and it overdrives at lower volume levels. I recently had some resistors in the power section burn up and maybe it was their time but was wondering if the keeping it 100 watts would be less stress on old components?


"I came here to do two things: chew bubblegum until its gets gross then take out another piece of gum"

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#2 2012-01-24 13:49:34

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Pulling Power tubes and running the amp as 50 Watt

Good question.  What resistors in the power section blew out of curiosity?


Matthew

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#3 2012-02-07 22:37:24

battlestarcackalactica
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From: Carrboro, NC
Registered: 2012-01-22
Posts: 3
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Re: Pulling Power tubes and running the amp as 50 Watt

I opened it up and the sand-ceramic resistors all got damaged. It was

the four rectangular resistors that drop the voltage on pin 3 of each

power tubes and the grid resistors got cooked as well. Is there any good

photos of the circuit board on this Site or elsewhere?

The board got a little melty and I had to solder in some jumpers where

the traces and pad were damaged. Luckily I had a friend that repairs

amps guide me through the process. Still don't know if failing tubes

would do this or if it was a chain reaction of failing resistors.


"I came here to do two things: chew bubblegum until its gets gross then take out another piece of gum"

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#4 2012-02-08 01:06:58

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Pulling Power tubes and running the amp as 50 Watt

All those 5 watt wirewound can be mounted right to the tube pins, and often are on other amps.  Plenty of pictures to be had on this very website, if you go to google.com and type (or cut and paste) this:

site:ampegv4.com   

Then, choose 'images' it will show you a lot of the images that have been posted on this board.  You can also surf posts in the workshop section, this stuff is discussed from time to time.

Last edited by Liquids (2012-02-08 01:07:38)


Matthew

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#5 2012-02-21 22:24:15

battlestarcackalactica
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From: Carrboro, NC
Registered: 2012-01-22
Posts: 3
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Re: Pulling Power tubes and running the amp as 50 Watt

thanks that's a good tip. the pcb is pretty well jumpered and next time something burns out that might be the best solution. thanks again


"I came here to do two things: chew bubblegum until its gets gross then take out another piece of gum"

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#6 2012-09-28 18:47:30

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Pulling Power tubes and running the amp as 50 Watt

A thought regarding pulling two power tubes - which I do with my V4.
It affects the output transformer impedance...

If the 8 ohm tap effectively becomes a 16 ohm tap when two power tubes are pulled - following, if one want's to maintain the amp's negative feedback as designed - shouldn't the feedback tap be moved from the 8 ohm tap to the 4 ohm tap, since the 4 ohm tap is now functioning as an 8 ohm tap?


Matthew

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#7 2012-09-28 20:42:34

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

Re: Pulling Power tubes and running the amp as 50 Watt

Correct,  that has been remarked upon before... But it only really makes sense to change it if you are leaving it that way.   

Either the resistor value to 6.8k or change the tap... Either would work.

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#8 2012-10-02 16:16:58

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Pulling Power tubes and running the amp as 50 Watt

I've got a great idea!  I could drill a hole, and put in a switch that either parallels a resistor in the feedback loop, or switches the stock feedback tap between the '4 and 8 ohm taps' as labeled, so it can be switched on the fly depending on how many tubes I run!!!   big_smile


Matthew

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#9 2012-10-02 17:50:57

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

Re: Pulling Power tubes and running the amp as 50 Watt

You know how much I love hearing about people drilling holes in v4s

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#10 2012-10-02 20:17:29

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Pulling Power tubes and running the amp as 50 Watt

I was thinking a little more and thought that I might add switches and two holes so that both options would be available to me...

smile


Matthew

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#11 2012-10-03 17:48:57

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

Re: Pulling Power tubes and running the amp as 50 Watt

You're killin' me

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#12 2012-10-03 17:50:41

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Pulling Power tubes and running the amp as 50 Watt

I hope you know I am kidding, strictly to tease you.  I was awaiting a comment about a mesa/boogie!  smile


Matthew

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#13 2012-10-03 19:44:37

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

Re: Pulling Power tubes and running the amp as 50 Watt

Of course.  I am just playing along!

I like some Mesa boogies...  The original rectifier is a great amp.   The f series and mark lll are alright too.

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#14 2012-10-09 04:06:21

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Pulling Power tubes and running the amp as 50 Watt

On a more serious note - I actually had my amp on the bench tonight...it's been a while.

I switched the transformer tap since I have no reason to use the amp with 4 tubes.  60 or whatever watts I get clean has always been enough. I'd rather eat/blow 2 tubes at a time, thank you.  smile 

But I've been thinking and looking.  The KT88s fit in the V2 nicely, because they look to effectively only utilize the 2 'spots' for power tubes away from the transformer, as compared to a V4.

Typically, tube pairs are inner/outer, or power tubes 2&3 are a pair, 1&4 are a pair, if you follow.
I swapped the wires that feed pins 3/4/5 of tube sockets 1&3. Now, tube pairs are 1&2, 3&4.

I am thinking that now  I can more easily run 2 large bottle tubes like KT88s.  Again, I only need 2.  These can go in the now-paired power tube sockets away from the transformer, and I can re-install spring clamps for those sockets permanently.  I can run a pair of 6L6GCs like I have now in slows 1&2 right next to the transformer, and keep the bear clamps for those two sockets since they fit just fine.

Lastly, I may be able to make use of the standby switch as a way to parallel a resistor in order to bias for either 6L6GC pair or KT88 pair as desired.  No irreversable mods, and everything was marked with p-touch labels so I don't forget.

I had a quad of 6550s in there at some point that I eventually sent back, so I'm all but certain I'll have no issue with running KT88s in the 'far' sockets away from the transformers.

Lastly, this also gives an option to choose EL34/6CA7s instead of KT88s and re-wire those sockets as needed, if I ever wanted to.  All reversable...not that I'm anticipating selling the amp, especially now!

Last edited by Liquids (2012-10-09 04:28:16)


Matthew

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#15 2012-10-10 18:52:39

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Pulling Power tubes and running the amp as 50 Watt

what resistance range (and corresponding % of idle bias, for reference) do you usually end up in for KT88s?  I'll be fine to measure/calculate bias once I have the tubes, but I am clueless about what bias resistor I'll want...I want to make sure I have a handful in the range (or at least a starting point of resitance/yielded idle bias) for reference as a starting point, knowing full well that each pair of tubes is going to draw a different amount of current with the same bias resistor...


Matthew

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