What the difference between 7591a & 7868

[hoyhoysum]

Sorry newbie question...

What is the difference between this two tubes? Which is the better sounding? I have a Pioneer SA810, i know it is not a transistor amp but a tube one. The one came with the amp is a 7868 and can i substitute a 7591a? I havent tried yet. Care anyone to enlighten me.

[Bob, W7ETA]

http://scottbecker.net/tube/sheets/049/7/7868.pdf
Noval, 9 pin

http://scottbecker.net/tube/sheets/127/7/7591.pdf
Octal, 8 pin

And, difference in circuits.

Bob

[Tom Bavis]

They have identical characteristics but different bases - so unless you like to change tube sockets... you'll be sticking with 7868s. There are new 7868s available from ElectroHarmonix - but they have oversize pins (.050" vs. .040"), so it's a one-way trip if you use them - the sockets won't hold the old version once you use the new ones.

[hoyhoysum]

Which sound better the 7591a or the 7868? I better ask first before making the plunge...There is no turning back then...

[kevinkr]

As pointed out previously the 7591A and 7868 are not directly interchangeable - to use the 7591A you would have to change out all of the power stage tube sockets.

For a given brand and vintage the innards (plate structure and internal construction) are virtually identical so you should expect no difference provided they were made at the same time in the same plant. Lot to lot and brand to brand variations would be typical of that experienced with other types. Therefore, IMHO I would not expect anyone to be able to give you a concrete answer to this question.

I think it comes down more to cost/availability and your comfort wrt to replacing sockets and doing the required rewiring.

FWIW I would probably just stick with 7868 and save the significant effort involved in a socket change for no concrete gains. The one exception I would make is if the sockets were in poor condition and needed to be replaced - then I would probably choose the octal option.

[Eli Duttman]

IF the large bottle of the ElectroHarmonix 7591 will fit in the available space, change to Octal sockets and install EH 7591s. An EH 7591 and an Octal socket costs less than an EH 7868.

[hoyhoysum]

Thanks for your helpful comment. As for now it is easier for me to get EH in my place. I do not mind changing the sockets as the sockets is already 30+ years. Will the EH7591 be better sounding than EH7868? Please bear with me as i have not seen nor heard either of the tubes before. If i want the tubes i need to place an order with my local agent and have it imported in from US. This is a costly affair for me. The tubes in my existing amp is the original Toshiba 7868.
I have read somewhere in the net that the 7868 is a newer version of 7591a, is it true?
BTW I know of 7591a thru Sansui AU70 also toshiba tubes.

[kevinkr]

Originally Posted by hoyhoysum

Thanks for your helpful comment. As for now it is easier for me to get EH in my place. I do not mind changing the sockets as the sockets is already 30+ years. Will the EH7591 be better sounding than EH7868? Please bear with me as i have not seen nor heard either of the tubes before. If i want the tubes i need to place an order with my local agent and have it imported in from US. This is a costly affair for me. The tubes in my existing amp is the original Toshiba 7868.
I have read somewhere in the net that the 7868 is a newer version of 7591a, is it true?
BTW I know of 7591a thru Sansui AU70 also toshiba tubes.

The 7868 really is just a rebased version of the 7591A internal differences are pretty insignificant. The only thing that is slightly newer about the 7868 is the envelope it was stuck into, and that is only by a couple of years at most.

[Jim McShane]

Originally Posted by kevinkr

As pointed out previously the 7591A and 7868 are not directly interchangeable - to use the 7591A you would have to change out all of the power stage tube sockets.

For a given brand and vintage the innards (plate structure and internal construction) are virtually identical so you should expect no difference provided they were made at the same time in the same plant. Lot to lot and brand to brand variations would be typical of that experienced with other types. Therefore, IMHO I would not expect anyone to be able to give you a concrete answer to this question.

I think it comes down more to cost/availability and your comfort wrt to replacing sockets and doing the required rewiring.

FWIW I would probably just stick with 7868 and save the significant effort involved in a socket change for no concrete gains. The one exception I would make is if the sockets were in poor condition and needed to be replaced - then I would probably choose the octal option.

One nice thing about the larger pinned 7868 EH - if the original sockets are tired the larger pins solve that problem for you with no work at all! If the original sockets aren't cracked or damaged but just have loose contact with the 7868 socket - the EH is a no-brainer.

I think old stock 7868 is pretty much gone too. What's left is what's in private hands and not for sale. Not many will be able to "go back".

[mb-de]

My two Eurocents:

Stay with the Novar sockets, get yourself a decent matched quad of EH 7868, clean the sockets, and plug the EH7868 - their base pins diameters are between Magnoval and Novar base diameter, so the fit will be rather tight, but then, you shall have lower contact resistance than with the Novar-based 7868.

I know the EH7868 as both reliable and electrically well behaved - the sound should then be ok, too...

Best regards,

Micha
===
PS: Be very weary with 'old stock' 7868 sold on eBay - there is a lot of junk out there waiting for people to be fooled... many are worn out despite the fact that they are sold as "NOS". Real NOS 7868 in pristine state are quite rare, and frequently expensive if offered.