What do the tubes in my 70's v4b do?

[ampegfuzz]

I just got a 70's v4b and had to replace a bad 12ax7 and I don't even know what purpose it serves in the amp. Can anyone tell me what all the preamp tubes do in the amp. EQ, phase, driver, etc.

[KramerBassFan]

Be more specific about the tube in question.... my V4 (not B) has six or seven preamp tubes, and a 6K11 as the reverb driver.

[fdeck]

http://www.unofficialampeg.com/schematics/v4bpream.gif

[ampegfuzz]

I can't read that ^^^^^ but I want to know what all the pre amp tubes do. It has two 12ax7, a 6k11, a 12au7, and 12dw7 what do these all do?

[staindbass]

the 12dw7, 12ax7 and 12au7 are the preamplifier tubes. they take the tiny signal from your guitar and each one amplifies it thousands of times, then passes the signal to the next tube and amplifies it again, over and over through each tube until it gets to the large power tubes and then to the speakers. johnny a.

[MAMMOTHvolume]

there is one 83 per channel providing input gain etc, they 6k11 is for the mid range circuit, the 12dw7 provides drive (is set up as a cathode follower) and the 82 is the splitter

[MAMMOTHvolume]

Originally Posted by staindbass

the 12dw7, 12ax7 and 12au7 are the preamplifier tubes. they take the tiny signal from your guitar and each one amplifies it thousands of times, then passes the signal to the next tube and amplifies it again, over and over through each tube until it gets to the large power tubes and then to the speakers. johnny a.

unfortunatly thats not how it works at all, those valves will not get anywhere near a gain of a thousand unless they were in a totem stack config

[fdeck]

Originally Posted by ampegfuzz

I can't read that ^^^^^ but I want to know what all the pre amp tubes do. It has two 12ax7, a 6k11, a 12au7, and 12dw7 what do these all do?

That's OK. Here's the power amp schematic as well.

Also http://www.unofficialampeg.com/schematics/v4Bpowra.gif

V1a = gain stage
V1b = gain stage
V2a and b, likewise
V201 = cathode feedback stage for the midrange control
V3a = cathode follower to drive line output
V3b = input and cathode feedback stage for power amp
V4 = phase splitter in power amp
V5,6,7,8 = Push pull final amplifier

The gain stages are pretty generic. Some of the overall gain built up by those stages is to overcome the insertion loss of the passive tone controls. You can think of V201 like an op amp -- a unique circuit that I have seen nowhere but in Ampeg amps. A cathode follower has no gain, but a relatively low output impedance suitable for driving a cable. V3b is where the power amp gets its first stage of gain, and also where it takes the feedback signal that linearizes and flattens its response.

[ampegfuzz]

THANKS EVERYONE! I looked for a solid hour or better with no luck last night. Like I said had v1 12ax7 go bad didnt know what it did. Weather it was gain or a tube controlling one of my eq points or what. I popped a new JJ in their on advice from my friend and so far no problems. It would run for a bit then you would get this crazy buzz sound. Really loud like you were touching the instrument cord to your hand with the amp on. No probs yet ( crossing fingers) amp sounds cool as hell like half my svt with some break up at higher and manageable volumes. I play pretty loud and to people who think this amp won't be enough, tell the rest of their band they are WAY too loud