Tung-Sol 7581A Questions

[Old Fart]

I'm hoping Beans-On-Toast or another tech can advise me. My 7581As arrived today for my Ampeg V-4. Questions:

Are these a direct, drop-in replacement? That is.....can I just install them and go?

By the way, the 7027A tubes I am removing from the amp have eight pins. The 7581A tubes have six pins. I guess this is Question 1B: Are they really direct replacement, having two less pins than the 7027As?

Assuming I can just install them and fire it up, I hear it is a good idea to have an amp tech check the bias. I will do this, but I wonder if I can play this amp between now and the time I get it to my tech.

Finally: This is a matched quad. I did not ask for any special information about them or paperwork, but there is the number "19" written on each box. What is the significance of the "19"?

Thanks

 

[B-string]

Have your tech do it. Pin 4 screen and 5 control grid are repeated on the 7027A, pin 1 and 6 respectively. If there are connections at pin 1 or 6 they MUST be moved. The 7027A used all pins by repeating the screen and control grids. The "re-issue" 7581A has those pins removed like the 6L6 family.

 

[Old Fart]

I don't get it. Numbering the pins clockwise, starting at 1:00, with the "key" facing upward (12:00), it is indeed pins 1 and 6 that are missing on the 7581A's. This amp came with 6L6 tubes in it, which I did not like. 6L6 and 7581A tubes are, reportedly, direct replacements. If this amp needs to be rewired each time I switch from one kind of tube to another.....I guess that's not what I was hoping for and not what I thought I was getting into.

Okay, I'm done whining. Bottom line: These are not just direct replacements as if I were using a new set of 7027A tubes, right? I really do need to have my amplifier modified for these to work, right?

 

[Old Fart]

Wait a minute. My amp tech said something about V-4 amps having factory-installed jumpers that allowed use of either 6L6 or 7027A tubes without modification. Is this what he was talking about? When you say "If there are connections at pin 1 or 6 they MUST be moved," are you and my amp tech saying the same thing? Were V-4s wired from the factory to accept 8 or 6-pin (7027/6L6) tubes?

 

[B-string]

I don't recall off hand if 1&4 and 5&6 were jumpered on the V4/V4-B. Mine is long gone and would not trust my memory on your amp. If they are jumpered no problem that way with plugging them in. Most if not all re-issue Tung Sol (and EH) 6L6 family tubes I have installed have had a much different bias requirement. I would not be comfortable saying "plug and play" for that reason also.

The 19 may not mean anything other than how they ID their matching grades, you would have to ask them.

 

[Old Fart]

Thanks so much, B-string. That's very good, clear, responsive information. Which I appreciate.

 

[Eric_Allard]

Thanks so much, B-string. That's very good, clear, responsive information. Which I appreciate.

Report back once you get it sorted. I recently had to buy new tubes for my Vt-22. I specifically asked about these and was told they wouldn't last long with the plate voltages in those Ampegs. I'd like to hear if they work out for you.

 

[beans-on-toast]

It isn't a problem that two of the pins are missing.

The reissue tung-sol 7581A only has six pins as shown below. On the original 7581, pin-1 was internally tied to pin-4, and pin-5 was tied to pin-6. On the reissue tung-sol pin-1 and pin-6 are missing and are not needed.

Why did they do this? It helps with reliability and production cost. Each pin has a thin wire that runs from inside the glass bottle to the outside where they are soldered to the tube base. By not having the two additional wires coming through the glass, there are two less chances of an air leak where the wires emerge. Not having to connect and test two additional pins also allows them to save on cost.

Looking at the V4B schematic, pin-1 and pin-6 are not used.

The V4B does not use those pins and the terminal are not used. As you can see in the image below, wires are not connected to either pin-1 or pin-6.

As was mentioned, the tubes need to have the bias checked. You can try them but be very careful to look for any signs of red plating. You risk damaging them if you don't know what to look for.


Every seller has their own code when matching tubes. Often the cathode current is recorded, sometimes the transconductance is there as well, other times it is simply a scale or a color that they use to group them. You'd have to ask the seller what the number means. It would be good for you to know. If you repurchase a new set down the road, and you bought a set of 19's, then you shouldn't need to re-bias. TH-hey should be plug and play because the bias is set for 19's. Ask them about this as well. Despite this, I would still verify the bias if I were changing the tubes. That's just me though because I'm set up to check them.