6550 power tubes in an Ampeg V-4B?

[Wasnex]

The 7581A sounds a bit like a 6550, noticeably more solid than a 6L6.

How does 7581A sound pushed into drive?

 

[BassBeginer64]

You would think by now this question would have come up on the reissue V4B 2013 and up, must be the geek in me !

 

[LetItGrowTone]

How does 7581A sound pushed into drive?

I can't remember, partly because that isn't my thing. At some volume level they probably just softened.
I do remember what I said, that they were pretty solid sounding when clean.
This was in an amp with "ultralinear" output, so, cleaner than most.

 

[LetItGrowTone]

My notes say, comparing them to 6550WE, that "7581A sounds about the same but not as loud" (they didn't keep the solidity to as high a volume output).

I haven't looked at the schematic of a V4B, but depending on the power tube grid circuit resistance it could be risky to run 6550s (according to my non-expert understanding), which led me to KT88s, which luckily I like even more.

 

[BassBeginer64]

2013 Ampeg V4B schematic.

 

[BassBeginer64]

OK say that there is a bias pot on the 2013 VB4 - Did not know this, fixed bias.

 

[Wasnex]

Please double check my work. It appears the 7581A pinout is the same as 6L6GC. I believe 7027A has a couple of incompatibilities with the V4BR output tube sockets.


Top row (left to right) 7581A, 7027A, 6L6GC
Bottom row is from the V4BR tube socket wiring.

-Pin 8 is cathode in all
-Pin 7 is heater in all (not shown on V4BR)
-Pin 6 is not connected in 7058A, 6L6GC, and V4BR diagram
-Pin 6 and 5 is control grid in 7027A
-Pin 5 is control grid in all
-Pin 4 is screen in all
-Pin 3 is plate in all.
-Pin 2 is heater in all
-Pin 1 is not connected in 7058A and 6L6GC
-Pin 1 and 4 is screen in 7027A...not compatible with V4BR diagram
-All tube diagrams show suppressor is tied to cathode
-V4BR diagram shows pin 1 tied to pin 8. Also seems to indicate pin 1 is tied to suppressor
-7027A shows pin 1 is tied to screen grid...so this is not compatible if V4BR diagram is accurate

 

[BassBeginer64]

If it works I guess bias at 450 v @ 54ma, love my Euro tube bias tool

 

[Wasnex]

If it works I guess bias at 450 v @ 54ma, love my Euro tube bias tool

IMHO unless you have a sufficient engineering background, I would stick to the factory recommended 40-50mA.

IMHO if you select a tube with higher plate rating, stick with the factory bias setup. The capacity of the transformers and power supply are scaled to a 30W tube.

If you select a tube with a lower plate rating, bias to the new plate rating.

 

[BassBeginer64]

then that would be 450/39

 

[agedhorse]

Do not attempt to run 6550's in this amp. The heater current is beyond the safe value for the power transformer, which is rated for about 6.4 amps continuous.

Using 6550's would result in 8.4 amps RMS which exceeds the 8 amp fuse provided.

The calculations are a little tricky because of the capacitor input FWB configuration on the preamp tube heaters:

For 6L6 tubes:
Heater current = 0.9A x 4 = 3.6A
For preamp tubes (V1-V4): = (300mA x 4) x 1.7 = 2A
Total current = 5.6A, fuse is 8A, 5.6/.8 = 7A which is fine (must be less than 8A)

For 6550 tubes:
Heater current = 1.6A x 4 = 6.4A
For preamp tubes (V1-V4): = (300mA x 4) x 1.7 = 2A
Total current = 8.4A, fuse is 8A, 8.4/.8 = 10.5A which is is the approx. necessary fuse size to support this heater current.

It's crystal clear that this transformer was NOT designed to support anything but 6L6 power tubes.

 

[agedhorse]

then that would be 450/39

That's a lot better, .54mA would cook those tubes pretty quickly, and the transformers will also run warmer (until something fails at least).

 

[agedhorse]

This is why I usually recommend leaving well enough alone.

 

[Wasnex]

Do not attempt to run 6550's in this amp. The heater current is beyond the safe value for the power transformer, which is rated for about 6.4 amps continuous.

Using 6550's would result in 8.4 amps RMS which exceeds the 8 amp fuse provided.

The calculations are a little tricky because of the capacitor input FWB configuration on the preamp tube heaters:

For 6L6 tubes:
Heater current = 0.9A x 4 = 3.6A
For preamp tubes (V1-V4): = (300mA x 4) x 1.7 = 2A
Total current = 5.6A, fuse is 8A, 5.6/.8 = 7A which is fine (must be less than 8A)

For 6550 tubes:
Heater current = 1.6A x 4 = 6.4A
For preamp tubes (V1-V4): = (300mA x 4) x 1.7 = 2A
Total current = 8.4A, fuse is 8A, 8.4/.8 = 10.5A which is is the approx. necessary fuse size to support this heater current.

It's crystal clear that this transformer was NOT designed to support anything but 6L6 power tubes.

AFAIK he's considering 7581A.

 

[Wasnex]

then that would be 450/39

The schematic specifically indicates 40-50mA. The procedure is to measure for 400-500mV across 10 ohm resistors. The target is actually 45mA.

 

[agedhorse]

AFAIK he's considering 7581A.

The title of the thread is 6550 which is why I addressed this.

 

[agedhorse]

AFAIK he's considering 7581A.

The title of the thread is 6550 which is why I addressed this.

 

[BassBeginer64]

It's all good, in fact hopefully what you wrote @agedhorse will stop someone else from making the mistake with 6550 tubes, and yes we are chatting about the use of 7581A tubes. Wont let me change the title............

 

[BassBeginer64]

Nice read https://www.talkbass.com/threads/new-tung-sol-7581a-power-tubes.1149421/

 

[Sunnampeg]

The weird thing is that Ampeg, in the 1970s, was offering the upgrade to 6550s on the V4 and V4b. why?