[cactusjack]

I have an integrated amplifier that uses 4 KT-88's as finals. The amplifier runs AB and has manual bias adjustment. I happen to have about 5 sets of matched 6550 quads that have never been used. Can I use the 6550s in my amp? If so do I need to make a bias adjustment change? if so, any idea how to calculate?
Old audiophile new to tubes. I have done some research and gotten Yes and No answers. any help would be appreciated.
Thanks

[SteveFord]

What amp is it and do you have an owner's manual?
I had the same deal with my VTLs and according to the manufacturer they are NOT a drop in. 
Others say BS but I listened to VTL.

[cactusjack]

It is a Melody I880. I procured a manual yesterday from fellow audio circle member. It does not say anything about having to use KT-88's nor that the 6550's would not work. Thus my inquiry. The 6550's were developed first and KT-88's came a year later. Most folks claim that the KT-88 is just the European designation for the USA designated 6550. I was just hoping My stash of 6550's could be used in the amplifier.

[SteveFord]

I'm sure that they could be but I would contact the manufacturer tomorrow to get the definitive answer. 
The tube vendors were telling me no problem as it's the same family, VTL said we have a kit which you'll need to install if you want to go from 6550s to KT88s (or vice versa).

[cactusjack]

Steve thanks.
The local distributor said he did not recommend it.  I may just have to get my tubes tested and sell them. Then buy some more Kt-88's.

[FullRangeMan]

I'm sure that they could be but I would contact the manufacturer tomorrow to get the definitive answer. 
The tube vendors were telling me no problem as it's the same family, VTL said we have a kit which you'll need to install if you want to go from 6550s to KT88s (or vice versa).

My 2 cents:
Better do as Steve suggest and emailed the manufacturer to get a writen opinion from the factory.

[Pez]

Steve thanks.
The local distributor said he did not recommend it.  I may just have to get my tubes tested and sell them. Then buy some more Kt-88's.

KT88s sound better anyway! 

[SteveFord]

I'm with Pez - I wish that Ei would get back into the tube biz.

[cactusjack]

Seems too often nobody comes back and concludes a thread. I did not want to leave this one open.
I was unable to get a schematic for my integrated amp so I examined the unit and drew my own. I then checked all operating voltages and found that they were well within the specs of 6550 Tubes. So I decided to try a set. I have been running a set of 6550's in my Melody I880 for about 3 weeks. The tubes are just now breaking in and they seem to be sounding very nice. There seems to be the a difference in the bass with the KT-88's a bit stronger but the mids and highs seem to be a little better with the 6550s. My KT-88s may have quite a bit of time on them. I have no knowledge of their history. Just believe they were the original Melody tubes. I am running the bias slightly higher than manufacturer's specs for the KT-88s on the 6550s.
I will not be back unless there is a 6550 Meltdown. Thanks to all that contributed.
cheers 

[Pez]

look forward to your final conclusions. 

[rpf]

There seems to be the a difference in the bass with the KT-88's a bit stronger but the mids and highs seem to be a little better with the 6550s.
cheers

This is what I have always found the difference between KT88s and 6550s to be. I've always preferred the latter but soon, for the first time, will be able to audition high quality examples of the former (Genalex, Pentas and/or BTs).

[Freo-1]

This is what I have always found the difference between KT88s and 6550s to be. I've always preferred the latter but soon, for the first time, will be able to audition high quality examples of the former (Genalex, Pentas and/or BTs).

Will be looking forward to your feedback.  With the old Tung Sol's, the midrange  and treble were really good.  Having said that, I find the current Penta/Black Treasures sound to be very involving (more so than 6550).

My tech buddy and I both think 6550's are just a bit "too" forward sounding compared to the KT88 (at least with the gear we have on hand).   This was really brought into focus when he built a pair of custom mono blocks with 6SN7 input tubes and KT88 output tubes for a customer.  The customer supplied EAT KT88's as output tubes.  I have to say those sounded absolutely magical in that amp.  The price on those tubes are cost prohibitive, though.

[tubenut]

I'm sure that they could be but I would contact the manufacturer tomorrow to get the definitive answer. 
The tube vendors were telling me no problem as it's the same family, VTL said we have a kit which you'll need to install if you want to go from 6550s to KT88s (or vice versa).

I'd be curious to know what's in the kit. I'm trying to think what it might be and so far the only thing that comes to mind is possibly gridleak resistors, a bias voltage range adjustment or possibly minor feedback compensation. Can't imagine it would be anything too deep. The OP reported no issues wth the change, which is what I would have expected - sonic shift notwithstanduing. I'd guess VTL is more concerned with micro-tweaking rather anything to do with major reliability issues.