Replacing Only 2 output tubes of a matched quad

[vintasonic]

Hi

Just some random thinking.  Generally, when I have an amplifier with a single bad output tube, I replace the full set with another matched set.  However, I have an Ampeg V4 on the bench and it has one bad tube out of four from a fairly new set.  I was wondering about replacing only 2 of the 4 might with another matched set of 2.  While not all four tubes would be matched (unless I got really lucky ) the sides would be pretty much matched so current draw would be close.  I understand that there are other factors in matching but it might be a way to extend the life on a set of tubes and save the owner some money.

Again, not my usual MO at all but it's something I have thought about in the past and just looking for some feedback.  I know it's only saving less than $100 but that's still real money to some of the guys I work with and again, this is a fairly new set of tubes.

I can think of several reasons while this is not a good idea but not sure if it's a horrible idea.

Thanks

[PRR]

I would just replace the one tube.

IMHO: The only reason to own a V4 is to play it LOUD. At that volume, a little unbalance can't be noticed.

If that is not good enough: replace all FOUR with Russian 6550. You see on some schematics that 6550 is preferred for professional players. The 6CA7/EL34s are just "starter tubes" for kids. "My" VT-40 lasted way over a decade on the factory 6CA7s before one shorted and burned-up the power transformer.

Yes, I realize that Russian tubes have political implications. I'm just speaking as a technician. The Chinese can make OK EL34s, and everybody can make EL34-grade "6550"s, but only TungSol/GE KenRad and the Russian military industry made great 6550s. (KT88 is an option if the height fits.)

My final solution was to re-PT down to ~~~400VDC and accept less than half rated power for much more than double mean time between flame-outs.

[tubeswell]

Might be okay on a Ampeg V4, but I wouldn’t do it with EL34s on a Marshall 1959 Super Lead (been there, and it blew up the Dagnall OT) unless the new pair of tubes closely matched the existing pair.

[shooter]

closely matched

that's where small resistance in the cathode line comes in handy

[vintasonic]

Thanks for your replies, very much appreciate the input

that's where small resistance in the cathode line comes in handy

Would that be sort of like a cathode bias configuration to compensate for differences in dc bias?  Also, the V4 has small value anode resistors, so would that accomplish a similar function?

I wanted to add, I suspect this tube may have failed due to mechanical issues since the amp is missing the back plate and is a gigging amp - so a good possibility it got knocked at some point; just speculation...  BTW, this amp has a new power transformer, my bigger fear is damage to the output transformer

[rake]

It's funny, some guys will only use tubes that are a perfect Apex matched quad on a midnight blue moon high tide and then you get the guys that scour the interweb to buy every used tube they can get their hands on so they can have an amp full of old Mullards! 

[BrownIsound]

It's funny, some guys will only use tubes that are a perfect Apex matched quad on a midnight blue moon high tide and then you get the guys that scour the interweb to buy every used tube they can get their hands on so they can have an amp full of old Mullards! 

I’m closer to your second example.

[vintasonic]

It's funny, some guys will only use tubes that are a perfect Apex matched quad on a midnight blue moon high tide and then you get the guys that scour the interweb to buy every used tube they can get their hands on so they can have an amp full of old Mullards!

Honestly, this reply was very helpful.  My decision to replace only half of the set was based on "yeah, I guess there are bunches of guys running un-matched sets of vintage tubes" so I don't think it'll be much of a problem 

[tubeswell]

Honestly, this reply was very helpful.  My decision to replace only half of the set was based on "yeah, I guess there are bunches of guys running un-matched sets of vintage tubes" so I don't think it'll be much of a problem 

It honestly depends on the tubes and the amp design. Some amps aren't fussy. Others are.


EL34s are particularly finicky because the signal grid is wound close to the cathode to increase transconductance and gain. This means that if these grids don't have sufficient grid current leakage pathways, the grids will tend to build up charge more easily under heavy signal conditions, which leads to loss of bias, which leads to  more charge build-up, which leads to more loss of bias, which leads to thermal runaway, which can destroy the OT pretty quickly (just as the amp is beginning to sound incredibly amazing while the tubes are positively glowing cherry red). A Marshall 1959 Super Lead has a really dumb grid leak arrangement given the EL34 plate voltage is 500 or so. In these amps, any pair of tubes which encourages more current to flow, will become the 'default' conductors under heavy signal conditions, leading to tube meltdown and OT destruction. You have been warned